Publications

[ 2024 ]
[ 2023 ]
[ 2022 ]
[ 2021 ]
[ 2020 ]
[ 2019 ]
[ 2018 ]
[ 2017 ]
[ 2016 ]
[ 2015 ]
[ 2014 ]
[ 2013 ]
[ 2012 ]
[ 2011 ]
[ 2010 ]
[ 2009 ]
[ 2008 ]
[ 2007 ]
[ 2006 ]
[ 2005 ]
[ 2004 ]
[ 2003 ]
[ 2002 ]
[ 2001 ]
[ 2000 ]
[ 1999 ]
[ 1998 ]
[ 1997 ]
[ 1996 ]
[ 1995 ]
[ 1994 ]
[ 1993 ]
[ 1992 ]

2024[ to top ]

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance. Hermosilla-Palacios, M. Alejandra; Martinez, Marissa; Doud, Evan A.; Hertel, Tobias; Spokoyny, Alexander M.; Cambré, Sofie; Wenseleers, Wim; Kim, Yong-Hyun; Ferguson, Andrew J.; Blackburn, Jeffrey L. In Nanoscale Horiz., p. -. The Royal Society of Chemistry, 2024.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices, due to their low cost, extensive tunability, easy fabrication, and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility, stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general), a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density, and by extension carrier mobility, in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model, and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density, a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.

@article{D3NH00480E, abstract = {High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices{,} due to their low cost{,} extensive tunability{,} easy fabrication{,} and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility{,} stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general){,} a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density{,} and by extension carrier mobility{,} in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model{,} and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density{,} a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.}, author = {Hermosilla-Palacios, M. Alejandra and Martinez, Marissa and Doud, Evan A. and Hertel, Tobias and Spokoyny, Alexander M. and Cambré, Sofie and Wenseleers, Wim and Kim, Yong-Hyun and Ferguson, Andrew J. and Blackburn, Jeffrey L.}, journal = {Nanoscale Horiz.}, keywords = {myown}, pages = {-}, publisher = {The Royal Society of Chemistry}, title = {Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance}, year = 2024 }

2023[ to top ]

Tuning counterion chemistry to reduce carrier localization in doped semiconducting carbon nanotube networks. Murrey, Tucker L.; Aubry, Taylor J.; Ruiz, Omar Leon; Thurman, Kira A.; Eckstein, Klaus H.; Doud, Evan A.; Stauber, Julia M.; Spokoyny, Alexander M.; Schwartz, Benjamin J.; Hertel, Tobias; Blackburn, Jeffrey L.; Ferguson, Andrew J. In Cell Reports Physical Science, p. 101407. Elsevier BV, 2023.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Understanding and controlling the impact that electrostatic interactions have on the transport of injected charge carriers is important for the utilization of pi-conjugated semiconductors in opto-electronic applications. Here, we explore the impact of dopant chemical and electronic structure on the doping efficacy and charge carrier transport in semiconducting single-walled carbon nanotube (s-SWCNT) networks using molecular charge-transfer dopants based on functionalized icosahedral dodecaborane (DDB) clusters. Calculations indicate that localization of electron density on the DDB core reduces the coulombic interactions that contribute to hole localization in the s-SWCNTs, thereby improving charge carrier transport. The enhanced delocalization produces an increase in the electrical conductivity and thermopower at lower charge carrier densities, yielding enhanced thermoelectric transport and a thermoelectric power factor that surpasses the previous best in class for enriched s-SWCNT thin-film networks. This strategy can be applied broadly across pi-conjugated semiconductors to tune and enhance performance in a variety of energy harvesting devices.

@article{Murrey_2023, abstract = {Understanding and controlling the impact that electrostatic interactions have on the transport of injected charge carriers is important for the utilization of pi-conjugated semiconductors in opto-electronic applications. Here, we explore the impact of dopant chemical and electronic structure on the doping efficacy and charge carrier transport in semiconducting single-walled carbon nanotube (s-SWCNT) networks using molecular charge-transfer dopants based on functionalized icosahedral dodecaborane (DDB) clusters. Calculations indicate that localization of electron density on the DDB core reduces the coulombic interactions that contribute to hole localization in the s-SWCNTs, thereby improving charge carrier transport. The enhanced delocalization produces an increase in the electrical conductivity and thermopower at lower charge carrier densities, yielding enhanced thermoelectric transport and a thermoelectric power factor that surpasses the previous best in class for enriched s-SWCNT thin-film networks. This strategy can be applied broadly across pi-conjugated semiconductors to tune and enhance performance in a variety of energy harvesting devices.}, author = {Murrey, Tucker L. and Aubry, Taylor J. and Ruiz, Omar Leon and Thurman, Kira A. and Eckstein, Klaus H. and Doud, Evan A. and Stauber, Julia M. and Spokoyny, Alexander M. and Schwartz, Benjamin J. and Hertel, Tobias and Blackburn, Jeffrey L. and Ferguson, Andrew J.}, journal = {Cell Reports Physical Science}, keywords = {swnt}, month = {may}, pages = 101407, publisher = {Elsevier {BV}}, title = {Tuning counterion chemistry to reduce carrier localization in doped semiconducting carbon nanotube networks}, year = 2023 }
Trions, Exciton Dynamics, and Spectral Modifications in Doped Carbon Nanotubes: A Singular Defect-Driven Mechanism. Eckstein, Klaus H.; Kunkel, Pascal; Voelckel, Markus; Schöppler, Friedrich; Hertel, Tobias. In The Journal of Physical Chemistry C, 127(39), pp. 19659–19667. American Chemical Society (ACS), 2023.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Doping substantially influences the electronic and photophysical properties of semiconducting single-walled carbon nanotubes (s-SWNTs). Although prior studies have noted that surplus charge carriers modify optical spectra and accelerate nonradiative exciton decay in doped s-SWNTs, a direct mechanistic correlation of trion formation, exciton dynamics, and energetics remains elusive. This work examines the influence of doping-induced nonradiative decay and exciton confinement on s-SWNT photophysics. Using photoluminescence, continuous-wave absorption, and pump–probe spectroscopy, we show that localization of and barrier formation by trapped charges can be jointly quantified using diffusive exciton transport and particle-in-the-box models, yielding a one-to-one correlation between charge carrier concentrations derived from these models. The study highlights the multifaceted role of exohedral counterions, which trap charges to create quenching sites, form barriers to exciton movement, and host trion states. This contributes significantly to understanding and optimizing the photophysical properties of doped SWNTs.

@article{Eckstein_2023, abstract = {Doping substantially influences the electronic and photophysical properties of semiconducting single-walled carbon nanotubes (s-SWNTs). Although prior studies have noted that surplus charge carriers modify optical spectra and accelerate nonradiative exciton decay in doped s-SWNTs, a direct mechanistic correlation of trion formation, exciton dynamics, and energetics remains elusive. This work examines the influence of doping-induced nonradiative decay and exciton confinement on s-SWNT photophysics. Using photoluminescence, continuous-wave absorption, and pump–probe spectroscopy, we show that localization of and barrier formation by trapped charges can be jointly quantified using diffusive exciton transport and particle-in-the-box models, yielding a one-to-one correlation between charge carrier concentrations derived from these models. The study highlights the multifaceted role of exohedral counterions, which trap charges to create quenching sites, form barriers to exciton movement, and host trion states. This contributes significantly to understanding and optimizing the photophysical properties of doped SWNTs.}, author = {Eckstein, Klaus H. and Kunkel, Pascal and Voelckel, Markus and Schöppler, Friedrich and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {myown}, month = {sep}, number = 39, pages = {19659–19667}, publisher = {American Chemical Society (ACS)}, title = {Trions, Exciton Dynamics, and Spectral Modifications in Doped Carbon Nanotubes: A Singular Defect-Driven Mechanism}, volume = 127, year = 2023 }

2022[ to top ]

Probing the ultrafast dynamics of excitons in single semiconducting carbon nanotubes. Birkmeier, Konrad; Hertel, Tobias; Hartschuh, Achim. In Nature Communications, 13(1). Springer Science and Business Media LLC, 2022.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Excitonic states govern the optical spectra of low-dimensional semiconductor nanomaterials and their dynamics are key for a wide range of applications, such as in solar energy harvesting and lighting. Semiconducting single-walled carbon nanotubes emerged as particularly rich model systems for one-dimensional nanomaterials and as such have been investigated intensively in the past. The exciton decay dynamics in nanotubes has been studied mainly by transient absorption and time-resolved photoluminescence spectroscopy. Since different transitions are monitored with these two techniques, developing a comprehensive model to reconcile different data sets, however, turned out to be a challenge and remarkably, a uniform description seems to remain elusive. In this work, we investigate the exciton decay dynamics in single carbon nanotubes using transient interferometric scattering and time-resolved photoluminescence microscopy with few-exciton detection sensitivity and formulate a unified microscopic model by combining unimolecular exciton decay and ultrafast exciton-exciton annihilation on a time-scale down to 200 fs.

@article{Birkmeier_2022, abstract = {Excitonic states govern the optical spectra of low-dimensional semiconductor nanomaterials and their dynamics are key for a wide range of applications, such as in solar energy harvesting and lighting. Semiconducting single-walled carbon nanotubes emerged as particularly rich model systems for one-dimensional nanomaterials and as such have been investigated intensively in the past. The exciton decay dynamics in nanotubes has been studied mainly by transient absorption and time-resolved photoluminescence spectroscopy. Since different transitions are monitored with these two techniques, developing a comprehensive model to reconcile different data sets, however, turned out to be a challenge and remarkably, a uniform description seems to remain elusive. In this work, we investigate the exciton decay dynamics in single carbon nanotubes using transient interferometric scattering and time-resolved photoluminescence microscopy with few-exciton detection sensitivity and formulate a unified microscopic model by combining unimolecular exciton decay and ultrafast exciton-exciton annihilation on a time-scale down to 200 fs.}, author = {Birkmeier, Konrad and Hertel, Tobias and Hartschuh, Achim}, journal = {Nature Communications}, keywords = {myown}, month = {oct}, number = 1, publisher = {Springer Science and Business Media LLC}, title = {Probing the ultrafast dynamics of excitons in single semiconducting carbon nanotubes}, volume = 13, year = 2022 }
Single-Walled Carbon Nanotubes as an Additive in Organic Photovoltaics: Effects on Carrier Generation and Recombination Dynamics. Abudulimu, Abasi; Eckstein, Klaus; Gemayel, Mirella El; Namal, Imge; Phillips, Adam B.; Ellingson, Randy J.; Heben, Michael J.; Hertel, Tobias; Meier, Sebastian B.; Lüer, Larry. In Solar RRL, 6(4), p. 2101010. 2022.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Employing single-walled carbon nanotubes (SWNTs) as an additive in the active layer of organic photovoltaics (OPV) to improve charge extraction is gaining weight in the research community. While SWNTs can transport charge carriers orders of magnitudes faster than conventional polymers, they also quench excitons and trap-free charges, reducing device performance. Such influences of SWNTs on OPV device performance remain inadequately explored and incompletely understood. Herein, the impact of SWNTs, enriched in (6,5) chirality, on the charge generation and extraction properties of two different OPV devices is studied by femtosecond-microsecond transient absorption spectroscopy and transient photovoltage/photocurrent techniques. It is shown that depending on the donor material properties (e.g., energetics), SWNTs can reduce carrier generation by quenching the excitons and increase carrier recombination through trapping processes. Then, by performing an analytical calculation of the current–voltage characteristics of the devices using the parameters determined via the transient measurements, it is shown that the increased recombination also relates to morphological changes induced by the SWNTs. These results shed light on the performance limiting factors of the SWNTs when incorporated into the active layer of the solar cells. This work thus can accelerate the blend optimization process for high-performance organic solar cells.

@article{https://doi.org/10.1002/solr.202101010, abstract = {Employing single-walled carbon nanotubes (SWNTs) as an additive in the active layer of organic photovoltaics (OPV) to improve charge extraction is gaining weight in the research community. While SWNTs can transport charge carriers orders of magnitudes faster than conventional polymers, they also quench excitons and trap-free charges, reducing device performance. Such influences of SWNTs on OPV device performance remain inadequately explored and incompletely understood. Herein, the impact of SWNTs, enriched in (6,5) chirality, on the charge generation and extraction properties of two different OPV devices is studied by femtosecond-microsecond transient absorption spectroscopy and transient photovoltage/photocurrent techniques. It is shown that depending on the donor material properties (e.g., energetics), SWNTs can reduce carrier generation by quenching the excitons and increase carrier recombination through trapping processes. Then, by performing an analytical calculation of the current–voltage characteristics of the devices using the parameters determined via the transient measurements, it is shown that the increased recombination also relates to morphological changes induced by the SWNTs. These results shed light on the performance limiting factors of the SWNTs when incorporated into the active layer of the solar cells. This work thus can accelerate the blend optimization process for high-performance organic solar cells.}, author = {Abudulimu, Abasi and Eckstein, Klaus and Gemayel, Mirella El and Namal, Imge and Phillips, Adam B. and Ellingson, Randy J. and Heben, Michael J. and Hertel, Tobias and Meier, Sebastian B. and Lüer, Larry}, journal = {Solar RRL}, keywords = {swnt}, number = 4, pages = 2101010, title = {Single-Walled Carbon Nanotubes as an Additive in Organic Photovoltaics: Effects on Carrier Generation and Recombination Dynamics}, volume = 6, year = 2022 }
Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects. Li, Min-Ken; Riaz, Adnan; Wederhake, Martina; Fink, Karin; Saha, Avishek; Dehm, Simone; He, Xiaowei; Schöppler, Friedrich; Kappes, Manfred M.; Htoon, Han; Popov, Valentin N.; Doorn, Stephen K.; Hertel, Tobias; Hennrich, Frank; Krupke, Ralph. In ACS Nano, p. ASAP. 2022.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions, additional gate-dependent emission lines appear, which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6, 5) nanotubes. We also compare functionalized (7, 5) electroluminescence data with photoluminescence of pristine and functionalized (7, 5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.

@article{doi:10.1021/acsnano.2c03083, abstract = {Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions, additional gate-dependent emission lines appear, which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6, 5) nanotubes. We also compare functionalized (7, 5) electroluminescence data with photoluminescence of pristine and functionalized (7, 5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.}, author = {Li, Min-Ken and Riaz, Adnan and Wederhake, Martina and Fink, Karin and Saha, Avishek and Dehm, Simone and He, Xiaowei and Schöppler, Friedrich and Kappes, Manfred M. and Htoon, Han and Popov, Valentin N. and Doorn, Stephen K. and Hertel, Tobias and Hennrich, Frank and Krupke, Ralph}, journal = {ACS Nano}, keywords = {swnt}, note = {PMID: 35732039}, pages = {ASAP}, title = {Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects}, year = 2022 }

2021[ to top ]

Coherent two-dimensional electronic spectroelectrochemistry. Heitmüller, Julia; Eckstein, Klaus; Renner, Rebecca; Stolte, Matthias; Hertel, Tobias; Würthner, Frank; Brixner, Tobias. In Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 253, p. 119567. 2021.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We report the development of a new spectroscopic scheme, coherent two-dimensional (2D) electronic spectroelectrochemistry. Conventional 2D electronic spectroscopy has become well established to investigate molecular energy transfer, charge transfer, or structural dynamics with femtosecond time resolution following electronic excitation, providing frequency resolution for both the excitation and the detection step. Here we combine this method with electrochemistry in a flow cell. Thus we have established access to the dynamics of various oxidized and reduced molecular species in solution. We investigate the photophysics of a tetraphenoxy-substituted perylene bisimide dye and its reduced species as a proof of principle and find substantially different dynamics for the neutral and the twofold reduced compound. The electrochemical flow cell is furthermore applied in conventional transient absorption spectroscopy and photoluminescence spectroscopies as an application in different setups.

@article{HEITMULLER2021119567, abstract = {We report the development of a new spectroscopic scheme, coherent two-dimensional (2D) electronic spectroelectrochemistry. Conventional 2D electronic spectroscopy has become well established to investigate molecular energy transfer, charge transfer, or structural dynamics with femtosecond time resolution following electronic excitation, providing frequency resolution for both the excitation and the detection step. Here we combine this method with electrochemistry in a flow cell. Thus we have established access to the dynamics of various oxidized and reduced molecular species in solution. We investigate the photophysics of a tetraphenoxy-substituted perylene bisimide dye and its reduced species as a proof of principle and find substantially different dynamics for the neutral and the twofold reduced compound. The electrochemical flow cell is furthermore applied in conventional transient absorption spectroscopy and photoluminescence spectroscopies as an application in different setups.}, author = {Heitmüller, Julia and Eckstein, Klaus and Renner, Rebecca and Stolte, Matthias and Hertel, Tobias and Würthner, Frank and Brixner, Tobias}, journal = {Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy}, keywords = {spectroscopy}, pages = 119567, title = {Coherent two-dimensional electronic spectroelectrochemistry}, volume = 253, year = 2021 }
Diffusive and Unimolecular Nonradiative Decay of Excited States in Doped Carbon Nanotubes. Eckstein, Klaus H.; Kunkel, Pascal; Voelckel, Markus; Schöppler, Friedrich; Abudulimu, Abasi; Lüer, Larry; Hertel, Tobias. 2021.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
Doping can profoundly affect the electronic- and optical-structure of semiconductors. Here we address the effect of surplus charges on non-radiative (NR) exciton and trion decay in doped semiconducting single-wall carbon nanotubes. The dependence of exciton photoluminescence quantum yields and exciton decay on the doping level, with its characteristically stretched-exponential kinetics, is attributed to diffusion-limited NR decay at charged impurity sites. By contrast, trion decay is unimolecular with a rate constant of $$2.0\,\rm ps^-1$$. Our experiments thus show that charged impurities not only trap trions and scavenge mobile excitons but that they also facilitate efficient NR energy dissipation for both.

@preprint{eckstein2021diffusive, abstract = {Doping can profoundly affect the electronic- and optical-structure of semiconductors. Here we address the effect of surplus charges on non-radiative (NR) exciton and trion decay in doped semiconducting single-wall carbon nanotubes. The dependence of exciton photoluminescence quantum yields and exciton decay on the doping level, with its characteristically stretched-exponential kinetics, is attributed to diffusion-limited NR decay at charged impurity sites. By contrast, trion decay is unimolecular with a rate constant of $2.0\,\rm ps^{-1}$. Our experiments thus show that charged impurities not only trap trions and scavenge mobile excitons but that they also facilitate efficient NR energy dissipation for both.}, author = {Eckstein, Klaus H. and Kunkel, Pascal and Voelckel, Markus and Schöppler, Friedrich and Abudulimu, Abasi and Lüer, Larry and Hertel, Tobias}, keywords = {swnt}, note = {cite arxiv:2103.15477}, title = {Diffusive and Unimolecular Nonradiative Decay of Excited States in Doped Carbon Nanotubes}, year = 2021 }
Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes. Eckstein, Klaus H.; Hirsch, Florian; Martel, Richard; Hertel, Tobias. In The Journal of Physical Chemistry C. American Chemical Society (ACS), 2021.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Electronic degrees of freedom and their coupling to lattice vibrations in semiconductors can be strongly modified by doping. Accordingly, the addition of surplus charge carriers to chirality-mixed carbon nanotube samples has previously been found to give rise to a Drude-type plasmon feature as well as Fanotype antiresonances in the far- to mid-infrared spectral range (FIR/MIR). Here we investigate the FIR/MIR response of redox-chemically doped semiconducting (6,5) carbon nanotubes (s-SWNTs). We find that, contrary to expectations, the Drude-type plasmon shifts to lower wavenumbers with increasing doping level. By means of Monte-Carlo simulations of the optical response, we attribute this behavior to the confinement of excess charge carriers at low doping levels and their progressive delocalization when approaching degenerate doping. The coupling of vibrational modes to intraband excitations in the doped s-SWNTs can be probed via a double resonance process similar to that responsible for the Raman D-band. The resulting Fano antiresonances shed new light onto the character and coupling of electronic and vibrational degrees of freedom in these one-dimensional semiconductors.

@article{Eckstein_2021, abstract = {Electronic degrees of freedom and their coupling to lattice vibrations in semiconductors can be strongly modified by doping. Accordingly, the addition of surplus charge carriers to chirality-mixed carbon nanotube samples has previously been found to give rise to a Drude-type plasmon feature as well as Fanotype antiresonances in the far- to mid-infrared spectral range (FIR/MIR). Here we investigate the FIR/MIR response of redox-chemically doped semiconducting (6,5) carbon nanotubes (s-SWNTs). We find that, contrary to expectations, the Drude-type plasmon shifts to lower wavenumbers with increasing doping level. By means of Monte-Carlo simulations of the optical response, we attribute this behavior to the confinement of excess charge carriers at low doping levels and their progressive delocalization when approaching degenerate doping. The coupling of vibrational modes to intraband excitations in the doped s-SWNTs can be probed via a double resonance process similar to that responsible for the Raman D-band. The resulting Fano antiresonances shed new light onto the character and coupling of electronic and vibrational degrees of freedom in these one-dimensional semiconductors.}, author = {Eckstein, Klaus H. and Hirsch, Florian and Martel, Richard and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {swnt}, month = {mar}, publisher = {American Chemical Society ({ACS})}, title = {Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes}, year = 2021 }

2020[ to top ]

Banning carbon nanotubes would be scientifically unjustified and damaging to innovation. Heller, Daniel A.; Jena, Prakrit V.; Pasquali, Matteo; Kostarelos, Kostas; Delogu, Lucia G.; Meidl, Rachel E.; Rotkin, Slava V.; Scheinberg, David A.; Schwartz, Robert E.; Terrones, Mauricio; Wang, YuHuang; Bianco, Alberto; Boghossian, Ardemis A.; Cambré, Sofie; Cognet, Laurent; Corrie, Simon R.; Demokritou, Philip; Giordani, Silvia; Hertel, Tobias; Ignatova, Tetyana; Islam, Mohammad F.; Iverson, Nicole M.; Jagota, Anand; Janas, Dawid; Kono, Junichiro; Kruss, Sebastian; Landry, Markita P.; Li, Yan; Martel, Richard; Maruyama, Shigeo; Naumov, Anton V.; Prato, Maurizio; Quinn, Susan J.; Roxbury, Daniel; Strano, Michael S.; Tour, James M.; Weisman, R. Bruce; Wenseleers, Wim; Yudasaka, Masako. In Nature Nanotechnology, 15(3), pp. 164–166. 2020.
- [ BibTeX ]
- [ URL ]
- [ DOI ]
@article{heller2020banning, author = {Heller, Daniel A. and Jena, Prakrit V. and Pasquali, Matteo and Kostarelos, Kostas and Delogu, Lucia G. and Meidl, Rachel E. and Rotkin, Slava V. and Scheinberg, David A. and Schwartz, Robert E. and Terrones, Mauricio and Wang, YuHuang and Bianco, Alberto and Boghossian, Ardemis A. and Cambré, Sofie and Cognet, Laurent and Corrie, Simon R. and Demokritou, Philip and Giordani, Silvia and Hertel, Tobias and Ignatova, Tetyana and Islam, Mohammad F. and Iverson, Nicole M. and Jagota, Anand and Janas, Dawid and Kono, Junichiro and Kruss, Sebastian and Landry, Markita P. and Li, Yan and Martel, Richard and Maruyama, Shigeo and Naumov, Anton V. and Prato, Maurizio and Quinn, Susan J. and Roxbury, Daniel and Strano, Michael S. and Tour, James M. and Weisman, R. Bruce and Wenseleers, Wim and Yudasaka, Masako}, journal = {Nature Nanotechnology}, keywords = {swnt}, number = 3, pages = {164–166}, title = {Banning carbon nanotubes would be scientifically unjustified and damaging to innovation}, volume = 15, year = 2020 }
Electronic and Ionic Electric Field Screening and Persistent Built--In Electric Field in Carbon Nanotube/PCBM Films. Vidmantas Jašinskas; Oberndorfer, Florian; Hertel, Tobias; Gulbinas, Vidmantas. In physica status solidi (a), 217(6). 2020.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
The application of carbon nanotubes in electronic devices requires detailed knowledge of their electrical properties. Herein, the long‐lasting electric field‐induced polarization of single‐wall carbon nanotube (SWCNT) networks is demonstrated. It is found that electric voltage applied to the films of SWCNTs and their blends with [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) creates persistent polarization that partly screens the external electric field and creates a built‐in electric field of the opposite direction remaining for several days. The built‐in electric field has caused the appearance of an open‐circuit photovoltage and a short‐circuit photocurrent under the sample illumination at zero applied voltage. The built‐in field showed a clearly bicomponential decay. The short tens of microseconds component is attributed to the electronic polarization, while the long‐lived component, which decreases at low temperatures, is attributed to the temperature‐assisted motion of ions.

@article{javsinskas2020electronic, abstract = {The application of carbon nanotubes in electronic devices requires detailed knowledge of their electrical properties. Herein, the long‐lasting electric field‐induced polarization of single‐wall carbon nanotube (SWCNT) networks is demonstrated. It is found that electric voltage applied to the films of SWCNTs and their blends with [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) creates persistent polarization that partly screens the external electric field and creates a built‐in electric field of the opposite direction remaining for several days. The built‐in electric field has caused the appearance of an open‐circuit photovoltage and a short‐circuit photocurrent under the sample illumination at zero applied voltage. The built‐in field showed a clearly bicomponential decay. The short tens of microseconds component is attributed to the electronic polarization, while the long‐lived component, which decreases at low temperatures, is attributed to the temperature‐assisted motion of ions.}, author = {{Vidmantas Ja{\v{s}}inskas} and {Florian Oberndorfer} and {Tobias Hertel} and {Vidmantas Gulbinas}}, journal = {physica status solidi (a)}, keywords = {swnt}, number = 6, title = {Electronic and Ionic Electric Field Screening and Persistent Built–In Electric Field in Carbon Nanotube/PCBM Films}, volume = 217, year = 2020 }
(Invited) Charge-Localization in Doped Semiconducting Carbon Nanotubes Revealed By IR- and EPR Spectroscopy. Eckstein, Klaus H; Auth, Michael; Oberndorfer, Florian; Schoeppler, Friedrich; Sperlich, Andreas; Dyakonov, Vladimir; Hertel, Tobias. In ECS Meeting Abstracts, MA2020-01(7), pp. 717–717. The Electrochemical Society, 2020.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
IR and EPR spectroscopic investigations of redox- p-doped semiconducting (6,5) single-wall carbon nanotubes (s-SWNTs) were used to study carrier localization and delocalization from low- up to degenerate doping levels. At low carrier concentrations, the analysis of IR-intraband transitions as well as the appearance of a doping-induced vibrational D-band anti-resonance, point to charge-localization on the few nm length-scale. Moreover, the rise of a spin 1/2 signal in EPR closely tracks the increase of the doping level, supporting the notion of charge-localization at low carrier concentrations. At doping levels exceeding 0.1 nm-1, EPR data as well as intra- and D-band absorption in the IR point to antiferromagnetic coupling between surplus charge carriers becoming dominant. These results are expected to be relevant for field- or redox-chemical doping of s-SWNTs in any polarizable, heterogeneous environment.

@article{Eckstein_2020, abstract = {IR and EPR spectroscopic investigations of redox- p-doped semiconducting (6,5) single-wall carbon nanotubes (s-SWNTs) were used to study carrier localization and delocalization from low- up to degenerate doping levels. At low carrier concentrations, the analysis of IR-intraband transitions as well as the appearance of a doping-induced vibrational D-band anti-resonance, point to charge-localization on the few nm length-scale. Moreover, the rise of a spin 1/2 signal in EPR closely tracks the increase of the doping level, supporting the notion of charge-localization at low carrier concentrations. At doping levels exceeding 0.1 nm-1, EPR data as well as intra- and D-band absorption in the IR point to antiferromagnetic coupling between surplus charge carriers becoming dominant. These results are expected to be relevant for field- or redox-chemical doping of s-SWNTs in any polarizable, heterogeneous environment.}, author = {Eckstein, Klaus H and Auth, Michael and Oberndorfer, Florian and Schoeppler, Friedrich and Sperlich, Andreas and Dyakonov, Vladimir and Hertel, Tobias}, journal = {{ECS} Meeting Abstracts}, keywords = {swnt}, month = {may}, number = 7, pages = {717–717}, publisher = {The Electrochemical Society}, title = {(Invited) Charge-Localization in Doped Semiconducting Carbon Nanotubes Revealed By {IR}- and {EPR} Spectroscopy}, volume = {{MA}2020-01}, year = 2020 }

2019[ to top ]

Optical Spectroscopy of Doped Carbon Nanotubes. Hertel, Tobias. In Handbook of Carbon Nanomaterials, Vol. 10, pp. 191–236. 2019.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Controlling charges in semiconductors is key for many of the device technologies that enable the modern information and communication age. Doping by chemical or physical means is intended to provide mobile charge carriers which facilitate crucial semiconductor device functionality. This chapter discusses some of the aspects related to the fabrication and optical characterization of reduced or oxidized semiconducting single-wall carbon nanotubes (s-SWCNTs) for the purpose of doping. We focus on redox- and electro-chemical modification to illustrate how far our current understanding allows not only a qualitative but also a quantitative spectroscopic assessment of impurity concentrations. Somewhat surprisingly, we find that commonly used n- or p-doping schemes appear to lead to inhomogeneous carrier distributions with some excess electrons or holes immobilized by deep Coulomb traps. Such carrier localization is believed to be symptomatic of the weak screening and pronounced susceptibility to environmental perturbations of low-dimensional materials such as SWCNTs. These findings may also have broader implications for the fabrication of functional semiconductor devices from nanoscale materials.

@inbook{herteloptical, abstract = {Controlling charges in semiconductors is key for many of the device technologies that enable the modern information and communication age. Doping by chemical or physical means is intended to provide mobile charge carriers which facilitate crucial semiconductor device functionality. This chapter discusses some of the aspects related to the fabrication and optical characterization of reduced or oxidized semiconducting single-wall carbon nanotubes (s-SWCNTs) for the purpose of doping. We focus on redox- and electro-chemical modification to illustrate how far our current understanding allows not only a qualitative but also a quantitative spectroscopic assessment of impurity concentrations. Somewhat surprisingly, we find that commonly used n- or p-doping schemes appear to lead to inhomogeneous carrier distributions with some excess electrons or holes immobilized by deep Coulomb traps. Such carrier localization is believed to be symptomatic of the weak screening and pronounced susceptibility to environmental perturbations of low-dimensional materials such as SWCNTs. These findings may also have broader implications for the fabrication of functional semiconductor devices from nanoscale materials.}, author = {Hertel, Tobias}, booktitle = {Handbook of Carbon Nanomaterials}, chapter = {Chapter 13}, keywords = {swnt}, pages = {191-236}, title = {Optical Spectroscopy of Doped Carbon Nanotubes}, volume = 10, year = 2019 }
Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy. Eckstein, Klaus H.; Oberndorfer, Florian; Achsnich, Melanie M.; Schöppler, Friedrich; Hertel, Tobias. In The Journal of Physical Chemistry C, 123(49), pp. 30001–30006. 2019.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Controlling doping is essential for the successful integration of semiconductor materials into device technologies. However, the assessment of doping levels and the distribution of charge carriers in carbon nanotubes or other nanoscale semiconductor materials is often either limited to a qualitative attribution of being “high” or “low” or entirely absent. Here, we describe efforts toward a quantitative characterization of doping in redox- or electrochemically doped semiconducting single-wall carbon nanotubes (s-SWNTs) using vis–NIR absorption spectroscopy. We discuss how carrier densities up to about 0.5 nm–1 can be quantified with a sensitivity of roughly 1 charge per 104 carbon atoms assuming inhomogeneous or homogeneous carrier distributions.

@article{doi:10.1021/acs.jpcc.9b08663, abstract = {Controlling doping is essential for the successful integration of semiconductor materials into device technologies. However, the assessment of doping levels and the distribution of charge carriers in carbon nanotubes or other nanoscale semiconductor materials is often either limited to a qualitative attribution of being “high” or “low” or entirely absent. Here, we describe efforts toward a quantitative characterization of doping in redox- or electrochemically doped semiconducting single-wall carbon nanotubes (s-SWNTs) using vis–NIR absorption spectroscopy. We discuss how carrier densities up to about 0.5 nm–1 can be quantified with a sensitivity of roughly 1 charge per 104 carbon atoms assuming inhomogeneous or homogeneous carrier distributions.}, author = {Eckstein, Klaus H. and Oberndorfer, Florian and Achsnich, Melanie M. and Schöppler, Friedrich and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {swnt}, number = 49, pages = {30001-30006}, title = {Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy}, volume = 123, year = 2019 }

2018[ to top ]

Direct Tracking of Ultrafast Carrier Motion Dynamics in Semiconducting Single-Wall Carbon Nanotubes. Jašinskas, Vidmantas; Oberndorfer, Florian; Pakštas, Vidas; Hertel, Tobias; Gulbinas, Vidmantas. In The Journal of Physical Chemistry C, 122(28), pp. 16424–16430. American Chemical Society (ACS), 2018.
- [ BibTeX ]
- [ URL ]
- [ DOI ]
@article{Ja_inskas_2018, author = {Ja{\v{s}}inskas, Vidmantas and Oberndorfer, Florian and Pak{\v{s}}tas, Vidas and Hertel, Tobias and Gulbinas, Vidmantas}, journal = {The Journal of Physical Chemistry C}, keywords = {ultrafast}, month = {jun}, number = 28, pages = {16424–16430}, publisher = {American Chemical Society ({ACS})}, title = {Direct Tracking of Ultrafast Carrier Motion Dynamics in Semiconducting Single-Wall Carbon Nanotubes}, volume = 122, year = 2018 }
(Invited) Ultrafast Spectroscopy of Free-Carrier like Dynamics in Heavily Doped Semiconducting Carbon Nanotubes. Eckstein, Klaus H; Achsnich, Melanie M; Schoeppler, Friedrich; Luer, Larry; Hertel, Tobias. In ECS Meeting Abstracts, MA2018-01(7), p. 748. 2018.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We have investigated the photophysical properties of heavily doped semiconducting single-wall carbon nanotubes (s-SWNTs) in the vis and NIR spectral range. Doping is here achieved either by electrochemical gating or by redox chemistry, using AuCl3 as oxidizing agent. Stationary and time-resolved absorption spectroscopy show that the dynamical behaviour of electrochemically- and redox-doped s-SWNT samples are identical, indicating that doping mechanisms and the character of charged states is the same for both doping processes. Spectroscopically, the heavily doped regime is defined by the disappearance of NIR trion and neutral exciton signatures. The remaining absorption band in heavily hole-doped (6,5) s-SWNTs is broad, flat and practically featureless over a very broad spectral range from about 1150 nm to 620 nm. It carries nearly all the oscillator strength of the first subband exciton of intrinsic s-SWNTs. The transient spectral response of this so-called H-band (for heavily doped) is surprisingly weak, suggesting that signal contributions from ground- and excited state absorption and from stimulated emission cancel out nearly perfectly. We discuss these findings also in the context of UV-IR measurements of the optical conductivity which shows an ambiguous response in the low THz regime.

@article{eckstein2018invited, abstract = {We have investigated the photophysical properties of heavily doped semiconducting single-wall carbon nanotubes (s-SWNTs) in the vis and NIR spectral range. Doping is here achieved either by electrochemical gating or by redox chemistry, using AuCl3 as oxidizing agent. Stationary and time-resolved absorption spectroscopy show that the dynamical behaviour of electrochemically- and redox-doped s-SWNT samples are identical, indicating that doping mechanisms and the character of charged states is the same for both doping processes. Spectroscopically, the heavily doped regime is defined by the disappearance of NIR trion and neutral exciton signatures. The remaining absorption band in heavily hole-doped (6,5) s-SWNTs is broad, flat and practically featureless over a very broad spectral range from about 1150 nm to 620 nm. It carries nearly all the oscillator strength of the first subband exciton of intrinsic s-SWNTs. The transient spectral response of this so-called H-band (for heavily doped) is surprisingly weak, suggesting that signal contributions from ground- and excited state absorption and from stimulated emission cancel out nearly perfectly. We discuss these findings also in the context of UV-IR measurements of the optical conductivity which shows an ambiguous response in the low THz regime.}, author = {{Klaus H Eckstein} and {Melanie M Achsnich} and {Friedrich Schoeppler} and {Larry Luer} and {Tobias Hertel}}, journal = {ECS Meeting Abstracts}, keywords = {swnt}, number = 7, pages = 748, title = {(Invited) Ultrafast Spectroscopy of Free-Carrier like Dynamics in Heavily Doped Semiconducting Carbon Nanotubes}, volume = {MA2018-01}, year = 2018 }
Influence of Carbon Nanotube Chirality on Sodium Cholate Adsorption in Aqueous Suspensions. Schoeppler, Friedrich; Vollert, Ivonne; Bergler, Felix; Hertel, Tobias. In ECS Meeting Abstracts, MA2018-01(7), p. 700. 2018.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Established chirality separation techniques for single-wall carbon nanotubes (SWNTs) in aqueous suspensions make use of chirality specific adsorption behavior of surfactants on these nano-carbon surfaces. Here, we investigate the adsorption of sodium cholate on SWNTs of different chirality with diameters ranging from 0.7 to 1 nm. Titration studies are performed at different temperatures by measuring photoluminescence emission energies of the diluted SWNT suspensions. The high sensitivity of the exciton to its environment allows to detect small changes in arrangement and concentration of surfactants on the SWNT surface, as recently shown for the (6,5) chirality by Bergler et al. [1]. Here we report on substrate-induced cooperative adsorption regimes for sodium cholate on five different SWNT chiralities. The critical micelle concentrations of the cholate-SWNT micelles exhibit a linear increase with SWNT diameter, while the thermodynamic stability of the cholate-SWNT micelles decreases. These diameter and temperature dependences may play a key role in the separation of SWNT chiralities by density gradient ultracentrifugation and aqueous two phase extraction and thus may help not only to understand the microscopic sorting mechanisms but also to improve empirical preparation protocols. [1] Bergler et al., Langmuir 2016, 32, 9598-9603, DOI: 10.1021/acs.langmuir.6b02759

@article{schoeppler2018influence, abstract = {Established chirality separation techniques for single-wall carbon nanotubes (SWNTs) in aqueous suspensions make use of chirality specific adsorption behavior of surfactants on these nano-carbon surfaces. Here, we investigate the adsorption of sodium cholate on SWNTs of different chirality with diameters ranging from 0.7 to 1 nm. Titration studies are performed at different temperatures by measuring photoluminescence emission energies of the diluted SWNT suspensions. The high sensitivity of the exciton to its environment allows to detect small changes in arrangement and concentration of surfactants on the SWNT surface, as recently shown for the (6,5) chirality by Bergler et al. [1]. Here we report on substrate-induced cooperative adsorption regimes for sodium cholate on five different SWNT chiralities. The critical micelle concentrations of the cholate-SWNT micelles exhibit a linear increase with SWNT diameter, while the thermodynamic stability of the cholate-SWNT micelles decreases. These diameter and temperature dependences may play a key role in the separation of SWNT chiralities by density gradient ultracentrifugation and aqueous two phase extraction and thus may help not only to understand the microscopic sorting mechanisms but also to improve empirical preparation protocols. [1] Bergler et al., Langmuir 2016, 32, 9598-9603, DOI: 10.1021/acs.langmuir.6b02759}, author = {{Friedrich Schoeppler} and {Ivonne Vollert} and {Felix Bergler} and {Tobias Hertel}}, journal = {ECS Meeting Abstracts}, keywords = {swnt}, number = 7, pages = 700, title = {Influence of Carbon Nanotube Chirality on Sodium Cholate Adsorption in Aqueous Suspensions}, volume = {MA2018-01}, year = 2018 }

2017[ to top ]

Diameter Selective Chemical Doping of Semiconducting Single-Wall Carbon Nanotubes. Achsnich, Melanie; Deutsch, Marian; Muetzel, Carina; Hertel, Tobias. In ECS Meeting Abstracts, MA2017-01(9), p. 661. 2017.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Doping of semiconductors, such as single-wall carbon nanotubes (SWNTs), by the introduction or removal of charges can be used for modification of their electronic and optical structure. This offers unique opportunities for the fabrication of versatile materials with new properties. However, despite a broad body of work,1−3 the control and spectroscopic assessment of the doping of SWNTs is still being developed4,5 in order to be able to reproducibly tailor semiconductor properties. Here, we have studied the photophysical properties of chemically doped SWNTs by absorption and photoluminescence spectroscopy. Polychiral samples of purified HiPCO soot were charged by oxidation with gold chloride. Thanks to very well-resolved and narrow exciton bands in these samples, the trionic signatures induced by charges could be resolved for up to five distinct nanotube species. An investigation of spectral changes in absorption measurements as a function of gold chloride concentration reveals that tubes with smaller bandgaps are doped at lower gold chloride concentrations. The trion binding energy with respect to the dipole-allowed first sub-band exciton transition was also found to increase systematically with decreasing diameter in agreement with previous findings.1,2 In addition, we find that SWNT oxidation potentials decrease nearly linearly with increasing nanotube diameter. Lastly, a surprisingly abrupt change of exciton oscillator strengths at redox potentials near the nanotube band edge provides evidence for band gap renormalization. References 1. R. Matsunaga, K. Matsuda, Y. Kanemitsu, Phys. Rev. Lett. 2011, 106, 037404. 2. J. S. Park, Y. Hirana, S. Mouri, Y. Miyauchi, N. Nakashima, K. Matsuda, JACS 2012, 134, 14461−14466. 3. S. M. Santos, B. Yuma, S. Berciaud, J. Shaver, M. Gallart, P. Gilliot, L. Cognet, B. Lounis, Phys. Rev. Lett. 2011, 107, 187401. 4. H. Hartleb, F. Späth, T. Hertel, ACS Nano 2015, 9, 10461−10470. 5. M. Yoshida, A. Popert, Y. K. Kato, Phys. Rev. B 2016, 93, 041402.

@article{achsnich2017diameter, abstract = {Doping of semiconductors, such as single-wall carbon nanotubes (SWNTs), by the introduction or removal of charges can be used for modification of their electronic and optical structure. This offers unique opportunities for the fabrication of versatile materials with new properties. However, despite a broad body of work,1−3 the control and spectroscopic assessment of the doping of SWNTs is still being developed4,5 in order to be able to reproducibly tailor semiconductor properties. Here, we have studied the photophysical properties of chemically doped SWNTs by absorption and photoluminescence spectroscopy. Polychiral samples of purified HiPCO soot were charged by oxidation with gold chloride. Thanks to very well-resolved and narrow exciton bands in these samples, the trionic signatures induced by charges could be resolved for up to five distinct nanotube species. An investigation of spectral changes in absorption measurements as a function of gold chloride concentration reveals that tubes with smaller bandgaps are doped at lower gold chloride concentrations. The trion binding energy with respect to the dipole-allowed first sub-band exciton transition was also found to increase systematically with decreasing diameter in agreement with previous findings.1,2 In addition, we find that SWNT oxidation potentials decrease nearly linearly with increasing nanotube diameter. Lastly, a surprisingly abrupt change of exciton oscillator strengths at redox potentials near the nanotube band edge provides evidence for band gap renormalization. References 1. R. Matsunaga, K. Matsuda, Y. Kanemitsu, Phys. Rev. Lett. 2011, 106, 037404. 2. J. S. Park, Y. Hirana, S. Mouri, Y. Miyauchi, N. Nakashima, K. Matsuda, JACS 2012, 134, 14461−14466. 3. S. M. Santos, B. Yuma, S. Berciaud, J. Shaver, M. Gallart, P. Gilliot, L. Cognet, B. Lounis, Phys. Rev. Lett. 2011, 107, 187401. 4. H. Hartleb, F. Späth, T. Hertel, ACS Nano 2015, 9, 10461−10470. 5. M. Yoshida, A. Popert, Y. K. Kato, Phys. Rev. B 2016, 93, 041402.}, author = {{Melanie Achsnich} and {Marian Deutsch} and {Carina Muetzel} and {Tobias Hertel}}, journal = {ECS Meeting Abstracts}, keywords = {swnt}, number = 9, pages = 661, title = {Diameter Selective Chemical Doping of Semiconducting Single-Wall Carbon Nanotubes}, volume = {MA2017-01}, year = 2017 }
Carrier photogeneration, drift and recombination in a semiconducting carbon nanotube network. Eckstein, A.; Bertašius, V.; Jašinskas, V.; Namal, I.; Hertel, T.; Gulbinas, V. In Nanoscale, 9(2017-07-26, 34), pp. 12441–12448. The Royal Society of Chemistry, 2017.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Charge carrier photogeneration, drift and recombination in thin film networks of polymer-wrapped (6,5)-single-wall carbon nanotubes (SWNTs) blended with phenyl-C61-butyric acid methyl ester (PCBM) have been investigated by using transient photocurrent and time-delayed collection field (TDCF) techniques. Three distinct transient photocurrent components on the nano- and microsecond timescales have been identified. We attribute the dominant (>50% of total extracted charge) ultrashort photocurrent component with a decay time below our experimental time-resolution of 2 ns to the intratube hole motion. The second component on the few microsecond timescale is attributed to the intertube hole transfer, while the slowest component is assigned to the electron drift within the PCBM phase. The hole drift distance appears to be limited by gaps in the nanotube percolation network rather than by hole trapping or recombination. Photocurrent saturation was observed when excitation densities reached more than one charge pair per nanotube; we attribute this to the local electric field screening.

@article{ecksteincarrier, abstract = {Charge carrier photogeneration, drift and recombination in thin film networks of polymer-wrapped (6,5)-single-wall carbon nanotubes (SWNTs) blended with phenyl-C61-butyric acid methyl ester (PCBM) have been investigated by using transient photocurrent and time-delayed collection field (TDCF) techniques. Three distinct transient photocurrent components on the nano- and microsecond timescales have been identified. We attribute the dominant (>50% of total extracted charge) ultrashort photocurrent component with a decay time below our experimental time-resolution of 2 ns to the intratube hole motion. The second component on the few microsecond timescale is attributed to the intertube hole transfer, while the slowest component is assigned to the electron drift within the PCBM phase. The hole drift distance appears to be limited by gaps in the nanotube percolation network rather than by hole trapping or recombination. Photocurrent saturation was observed when excitation densities reached more than one charge pair per nanotube; we attribute this to the local electric field screening.}, author = {Eckstein, A. and Bertašius, V. and Jašinskas, V. and Namal, I. and Hertel, T. and Gulbinas, V.}, journal = {Nanoscale}, keywords = {swnt}, number = {2017-07-26, 34}, pages = {12441-12448}, publisher = {The Royal Society of Chemistry}, title = {Carrier photogeneration, drift and recombination in a semiconducting carbon nanotube network}, type = {Article}, volume = 9, year = 2017 }
(Invited) Ultrafast Spectroscopy of Doped Carbon Nanotubes. Hertel, Tobias; Eckstein, Klaus H. In 231st ECS Meeting (May 28 - June 1, 2017). 2017.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
We will report on an investigation of exciton and trion dynamics in chemically-...

@article{hertel2017ultrafast, abstract = {We will report on an investigation of exciton and trion dynamics in chemically-...}, author = {{Tobias Hertel} and {Klaus H Eckstein}}, journal = {231st ECS Meeting (May 28 - June 1, 2017)}, keywords = {ultrafast}, title = {(Invited) Ultrafast Spectroscopy of Doped Carbon Nanotubes}, year = 2017 }
Devices and Applications 1. Hertel, Tobias. In 231st ECS Meeting (May 28 - June 1, 2017). 2017.
- [ BibTeX ]
- [ URL ]
@article{hertel2017devices, author = {{Tobias Hertel}}, journal = {231st ECS Meeting (May 28 - June 1, 2017)}, keywords = {devices}, title = {Devices and Applications 1}, year = 2017 }

2016[ to top ]

(Invited) Evidence for Strong Electronic Correlations and Band-Gap Renormalization in Doped Single-Wall Carbon Nanotubes. Hertel, Tobias; Hartleb, Holger; Spaeth, Florian; Eckstein, Klaus H; Achsnich, Melanie M; Schoeppler, Friedrich. In 229th ECS Meeting (May 29 - June 2, 2016). 2016.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
We have investigated the photophysical properties of chemically and gate-doped semiconducting single-wall carbon nanotubes (s-SWNTs). The first subband exciton oscillator strengths as obtained from a global analysis of absorption spectra are used for assessment of carrier densities and provide evidence for band-gap renormalisation (BGR) in (6,5) SWNTs. We predict that BGR of one-dimensional gate doped semiconductors is accompanied by a stepwise increase of the carrier density by Δn=32meff b/(πℏ)2once the electrochemical potential reaches the valence or conduction band offset with b = (0.15 ± 0.05) eV nm (meff - effective carrier mass). Moreover, we show that the width of the spectroelectrochemical window of the first subband exciton of (1.55±0.05) eV corresponds to the fundamental band gap of the undoped (6,5) SWNTs in our samples and not to the renormalized band gap of the doped system. We also compare spectral changes in gate doped with those of chemically doped SWNTs. Femtosecond time-resolved pump-probe spectroscopy of chemically doped SWNTs provides evidence of exciton-trion coupling. These observations as well as a previously unidentified absorption band emerging at high doping levels in the Pauli-blocked region of the single-particle Hartree band structure, provide clear evidence for strong electronic correlations in the optical spectra of SWNTs.

@article{hertel2016evidence, abstract = {We have investigated the photophysical properties of chemically and gate-doped semiconducting single-wall carbon nanotubes (s-SWNTs). The first subband exciton oscillator strengths as obtained from a global analysis of absorption spectra are used for assessment of carrier densities and provide evidence for band-gap renormalisation (BGR) in (6,5) SWNTs. We predict that BGR of one-dimensional gate doped semiconductors is accompanied by a stepwise increase of the carrier density by Δn=32meff b/(πℏ)2once the electrochemical potential reaches the valence or conduction band offset with b = (0.15 ± 0.05) eV nm (meff - effective carrier mass). Moreover, we show that the width of the spectroelectrochemical window of the first subband exciton of (1.55±0.05) eV corresponds to the fundamental band gap of the undoped (6,5) SWNTs in our samples and not to the renormalized band gap of the doped system. We also compare spectral changes in gate doped with those of chemically doped SWNTs. Femtosecond time-resolved pump-probe spectroscopy of chemically doped SWNTs provides evidence of exciton-trion coupling. These observations as well as a previously unidentified absorption band emerging at high doping levels in the Pauli-blocked region of the single-particle Hartree band structure, provide clear evidence for strong electronic correlations in the optical spectra of SWNTs.}, author = {{Tobias Hertel} and {Holger Hartleb} and {Florian Spaeth} and {Klaus H Eckstein} and {Melanie M Achsnich} and {Friedrich Schoeppler}}, journal = {229th ECS Meeting (May 29 - June 2, 2016)}, keywords = {swnt}, title = {(Invited) Evidence for Strong Electronic Correlations and Band-Gap Renormalization in Doped Single-Wall Carbon Nanotubes}, year = 2016 }
Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks. Bottacchi, Francesca; Bottacchi, Stefano; Späth, Florian; Namal, Imge; Hertel, Tobias; Anthopoulos, Thomas D. In Small (Weinheim an der Bergstrasse, Germany), 12(31), pp. 4211–4221. 2016.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
The current percolation in polymer-sorted semiconducting (7,5) single-walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field-effect measurements, atomic force microscopy (AFM), and conductive AFM (C-AFM) techniques. From AFM measurements, the nanotube length in the as-processed (7,5) SWNTs network is found to range from $$\approx$$100 to $$\approx$$1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage $$\approx$$58%. Analysis of the field-effect charge transport measurements in the SWNT network using a 2D homogeneous random-network stick-percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C-AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as-spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field-effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.

@article{bottacchi2016nanoscale, abstract = {The current percolation in polymer-sorted semiconducting (7,5) single-walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field-effect measurements, atomic force microscopy (AFM), and conductive AFM (C-AFM) techniques. From AFM measurements, the nanotube length in the as-processed (7,5) SWNTs network is found to range from $\approx$100 to $\approx$1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage $\approx$58{\%}. Analysis of the field-effect charge transport measurements in the SWNT network using a 2D homogeneous random-network stick-percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C-AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as-spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field-effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.}, author = {Bottacchi, Francesca and Bottacchi, Stefano and Sp{\"a}th, Florian and Namal, Imge and Hertel, Tobias and Anthopoulos, Thomas D.}, journal = {Small (Weinheim an der Bergstrasse, Germany)}, keywords = {transistors}, number = 31, pages = {4211–4221}, title = {Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks}, volume = 12, year = 2016 }
Photoluminescence from Thin Carbon Nanotube Films. Namal, Imge; Schoeppler, Friedrich; Hertel, Tobias. In ECS Meeting Abstracts, MA2016-01(8), p. 661. 2016.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Their remarkable light absorption and energy transport characteristics make single-wall carbon nanotubes (SWNTs) intriguing candidates for photovoltaic applications. Here, we study the photophysical properties of polymer wrapped SWNT films with varying thickness, so as to help identify processes which may affect device performance and that may impose limitations on the use of SWNT thin films as active layers in photovoltaic devices. Non-radiative exciton decay is of particular interest in this context and we have studied its dependence on the thickness of monochiral SWNT films. The films were made of PFOBPy wrapped and highly (6,5) enriched SWNT suspensions using a modified version of the filtration wet transfer technique reported earlier.1 Non radiative decay was probed by means of photoluminescence (PL) spectroscopy and by comparison of thin film data with data obtained from bulk suspensions. For thicker films we observe a pronounced decrease of the PL quantum yield (QY) for emission from the first subband S1 exciton state. However, the total PL QY is much less strongly affected by interactions within the thin film network because of a considerable shift of PL intensity to lower lying states. Surprisingly, some of the lower energy emission resembles PL phonon-sidebands of the first subband exciton which was previously attributed to the coupling between K-point phonons and dark excitons by Murakami et. al.2 Wu, Z.. Science 305, Nr. 5688 (2004): 1273–76. Murakami, Y., Benjamin L., Said K., Nobutsugu M., Tatsuya O., Shigeo M. Phys. Rev. B 79, Nr. 19 (2009): 195407.

@article{namal2016photoluminescence, abstract = {Their remarkable light absorption and energy transport characteristics make single-wall carbon nanotubes (SWNTs) intriguing candidates for photovoltaic applications. Here, we study the photophysical properties of polymer wrapped SWNT films with varying thickness, so as to help identify processes which may affect device performance and that may impose limitations on the use of SWNT thin films as active layers in photovoltaic devices. Non-radiative exciton decay is of particular interest in this context and we have studied its dependence on the thickness of monochiral SWNT films. The films were made of PFOBPy wrapped and highly (6,5) enriched SWNT suspensions using a modified version of the filtration wet transfer technique reported earlier.1 Non radiative decay was probed by means of photoluminescence (PL) spectroscopy and by comparison of thin film data with data obtained from bulk suspensions. For thicker films we observe a pronounced decrease of the PL quantum yield (QY) for emission from the first subband S1 exciton state. However, the total PL QY is much less strongly affected by interactions within the thin film network because of a considerable shift of PL intensity to lower lying states. Surprisingly, some of the lower energy emission resembles PL phonon-sidebands of the first subband exciton which was previously attributed to the coupling between K-point phonons and dark excitons by Murakami et. al.2 Wu, Z.. Science 305, Nr. 5688 (2004): 1273–76. Murakami, Y., Benjamin L., Said K., Nobutsugu M., Tatsuya O., Shigeo M. Phys. Rev. B 79, Nr. 19 (2009): 195407.}, author = {{Imge Namal} and {Friedrich Schoeppler} and {Tobias Hertel}}, journal = {ECS Meeting Abstracts}, keywords = {swnt}, number = 8, pages = 661, title = {Photoluminescence from Thin Carbon Nanotube Films}, volume = {MA2016-01}, year = 2016 }
Trion Dynamics in Hole-Doped (6,5) Single-Wall Carbon Nanotube Films. Eckstein, Klaus; Ickert, Roman; Kunkel, Pascal; Schoeppler, Friedrich; Hertel, Tobias. In ECS Meeting Abstracts, MA2016-01(8), p. 680. 2016.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Doping of semiconductor nanostructures is essential for their application in electronic as well as photonic devices. Recent studies have focused on the assessment of electrochemical gate doping (1,2) or chemical doping (3) of semiconducting single-wall carbon nanotubes (SWNTs) by absorption and photoluminescence spectroscopy. Moderate doping levels on the order of 0.5 holescenterdotnm-1 were found to lead to a redistribution of oscillator strength from the first subband exciton (S1) to the trion (2). Here, we report on an investigation of trion dynamics by femtosecond pump-probe transient absorption experiments (VIS pump/NIR probe and NIR pump/NIR probe) in moderately doped (6,5)-SWNT films. SWNTs were charged by gate-doping or by interaction with redox-active gold chloride. We find that optical excitation of energetically lower lying trionic states leads to a delayed rise of the first subband exciton's S1 photobleach (PB) signature on the ps time-scale. However, the amplitude of the S1 PB signal is surprisingly high considering that the energy separation of trion and exciton exceeds thermal energies by nearly an order of magnitude, making uphill thermal population transfer highly unlikely. Extremely weak photoluminescence from the S1 exciton following trion excitation in hole-doped films indicates that energy transfer from the trion to the first subband exciton is indeed negligible (lesssim 10-3). The photobleach is thus essentially due to ground state depletion and reflects the ps-dynamics of the trion decay. Excitation energy and temperature dependent transient absorption experiments reveal that this process is thermally activated. (1) J. Park et al., J. Am. Chem. Soc., 134, 14461 (2012). (2) H. Hartleb, F. Späth and T. Hertel, ACS Nano, 9, 10461 (2015). (3) R. Matsunaga, K. Matsuda and Y. Kanemitsu, Phys. Rev. Lett., 106, 037404 (2011).

@article{eckstein2016trion, abstract = {Doping of semiconductor nanostructures is essential for their application in electronic as well as photonic devices. Recent studies have focused on the assessment of electrochemical gate doping (1,2) or chemical doping (3) of semiconducting single-wall carbon nanotubes (SWNTs) by absorption and photoluminescence spectroscopy. Moderate doping levels on the order of 0.5 holescenterdotnm-1 were found to lead to a redistribution of oscillator strength from the first subband exciton (S1) to the trion (2). Here, we report on an investigation of trion dynamics by femtosecond pump-probe transient absorption experiments (VIS pump/NIR probe and NIR pump/NIR probe) in moderately doped (6,5)-SWNT films. SWNTs were charged by gate-doping or by interaction with redox-active gold chloride. We find that optical excitation of energetically lower lying trionic states leads to a delayed rise of the first subband exciton's S1 photobleach (PB) signature on the ps time-scale. However, the amplitude of the S1 PB signal is surprisingly high considering that the energy separation of trion and exciton exceeds thermal energies by nearly an order of magnitude, making uphill thermal population transfer highly unlikely. Extremely weak photoluminescence from the S1 exciton following trion excitation in hole-doped films indicates that energy transfer from the trion to the first subband exciton is indeed negligible (lesssim 10-3). The photobleach is thus essentially due to ground state depletion and reflects the ps-dynamics of the trion decay. Excitation energy and temperature dependent transient absorption experiments reveal that this process is thermally activated. (1) J. Park et al., J. Am. Chem. Soc., 134, 14461 (2012). (2) H. Hartleb, F. Späth and T. Hertel, ACS Nano, 9, 10461 (2015). (3) R. Matsunaga, K. Matsuda and Y. Kanemitsu, Phys. Rev. Lett., 106, 037404 (2011).}, author = {{Klaus Eckstein} and {Roman Ickert} and {Pascal Kunkel} and {Friedrich Schoeppler} and {Tobias Hertel}}, journal = {ECS Meeting Abstracts}, keywords = {tron}, number = 8, pages = 680, title = {Trion Dynamics in Hole-Doped (6,5) Single-Wall Carbon Nanotube Films}, volume = {MA2016-01}, year = 2016 }

2015[ to top ]

Ultrafast Charge Photogeneration and Dynamics in Semiconducting Carbon Nanotubes. Soavi, Giancarlo; Scotognella, Francesco; Viola, Daniele; Hefner, Timo; Hertel, Tobias; Lanzani, Guglielmo; Cerullo, Giulio. In Ultrafast Phenomena XIX, K. Yamanouchi, S. Cundiff, R. de Vivie-Riedle, M. Kuwata-Gonokami, L. DiMauro (eds.), pp. 360–362. Springer International Publishing, Cham, 2015.
- [ Abstract ]
- [ BibTeX ]
We show that charge-carriers are instantaneously photogenerated in semiconducting carbon nanotubes by identifying their spectral signature in transient absorption. We exploit carbon nanotubes as ideal systems for the study of charge-carriers dynamics in one dimension.

@inproceedings{soavi2015ultrafast, abstract = {We show that charge-carriers are instantaneously photogenerated in semiconducting carbon nanotubes by identifying their spectral signature in transient absorption. We exploit carbon nanotubes as ideal systems for the study of charge-carriers dynamics in one dimension.}, address = {Cham}, author = {Soavi, Giancarlo and Scotognella, Francesco and Viola, Daniele and Hefner, Timo and Hertel, Tobias and Lanzani, Guglielmo and Cerullo, Giulio}, booktitle = {Ultrafast Phenomena XIX}, editor = {Yamanouchi, Kaoru and Cundiff, Steven and de Vivie-Riedle, Regina and Kuwata-Gonokami, Makoto and DiMauro, Louis}, keywords = {swnt}, pages = {360–362}, publisher = {Springer International Publishing}, title = {Ultrafast Charge Photogeneration and Dynamics in Semiconducting Carbon Nanotubes}, year = 2015 }
Below-gap excitation of semiconducting single-wall carbon nanotubes. Soavi, G.; Grupp, A.; Budweg, A.; Scotognella, F.; Hefner, T.; Hertel, T.; Lanzani, G.; Leitenstorfer, A.; Cerullo, G.; Brida, D. In Nanoscale, 7(43), pp. 18337–18342. The Royal Society of Chemistry, 2015.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We investigate the optoelectronic properties of the semiconducting (6,5) species of single-walled carbon nanotubes by measuring ultrafast transient transmission changes with 20 fs time resolution. We demonstrate that photons with energy below the lowest exciton resonance efficiently lead to linear excitation of electronic states. This finding challenges the established picture of a vanishing optical absorption below the fundamental excitonic resonance. Our result points towards below-gap electronic states as an intrinsic property of semiconducting nanotubes.

@article{soavi2015belowgap, abstract = {We investigate the optoelectronic properties of the semiconducting (6{,}5) species of single-walled carbon nanotubes by measuring ultrafast transient transmission changes with 20 fs time resolution. We demonstrate that photons with energy below the lowest exciton resonance efficiently lead to linear excitation of electronic states. This finding challenges the established picture of a vanishing optical absorption below the fundamental excitonic resonance. Our result points towards below-gap electronic states as an intrinsic property of semiconducting nanotubes.}, author = {Soavi, G. and Grupp, A. and Budweg, A. and Scotognella, F. and Hefner, T. and Hertel, T. and Lanzani, G. and Leitenstorfer, A. and Cerullo, G. and Brida, D.}, journal = {Nanoscale}, keywords = {swnt}, number = 43, pages = {18337-18342}, publisher = {The Royal Society of Chemistry}, title = {Below-gap excitation of semiconducting single-wall carbon nanotubes}, volume = 7, year = 2015 }
High energetic excitons in carbon nanotubes directly probe charge-carriers. Soavi, Giancarlo; Scotognella, Francesco; Viola, Daniele; Hefner, Timo; Hertel, Tobias; Cerullo, Giulio; Lanzani, Guglielmo. In Scientific Reports, 5(1), p. 9681-. 2015.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S33 transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S11. The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D.

@article{soavi2015energetic, abstract = {Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S33 transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S11. The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D.}, author = {Soavi, Giancarlo and Scotognella, Francesco and Viola, Daniele and Hefner, Timo and Hertel, Tobias and Cerullo, Giulio and Lanzani, Guglielmo}, journal = {Scientific Reports}, keywords = {swnt}, number = 1, pages = {9681–}, title = {High energetic excitons in carbon nanotubes directly probe charge-carriers}, volume = 5, year = 2015 }
A comparison of Raman and photoluminescence spectra for the assessment of single-wall carbon nanotube sample quality. Kastner, Matthias; Stahl, Sabine; Vollert, Ivonne; Loi, Christian; Rühl, Nicolas; Hertel, Tobias; Schöppler, Friedrich. In Chemical Physics Letters, 635, pp. 245–249. 2015.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We compare mass-yields, Raman band intensity ratios and photoluminescence quantum yields (PLQY) of (6,5) single-wall carbon nanotubes (SWNT), produced by alcohol catalytic chemical vapor deposition at reaction temperatures ranging from 600°C to 850°C. The highest PLQYs for suspensions of unprocessed raw material and for density gradient ultracentrifuged (DGU) samples are obtained if SWNTs were grown at 850°C. Accordingly, the Raman defect band intensity ratios of DGU samples decrease toward the highest CVD growth temperatures. In contrast, PLQYs of non-DGU processed, polydisperse raw material suspensions, do not correlate with commonly used Raman D/G defect band intensity ratios.

@article{KASTNER2015245, abstract = {We compare mass-yields, Raman band intensity ratios and photoluminescence quantum yields (PLQY) of (6,5) single-wall carbon nanotubes (SWNT), produced by alcohol catalytic chemical vapor deposition at reaction temperatures ranging from 600°C to 850°C. The highest PLQYs for suspensions of unprocessed raw material and for density gradient ultracentrifuged (DGU) samples are obtained if SWNTs were grown at 850°C. Accordingly, the Raman defect band intensity ratios of DGU samples decrease toward the highest CVD growth temperatures. In contrast, PLQYs of non-DGU processed, polydisperse raw material suspensions, do not correlate with commonly used Raman D/G defect band intensity ratios.}, author = {Kastner, Matthias and Stahl, Sabine and Vollert, Ivonne and Loi, Christian and Rühl, Nicolas and Hertel, Tobias and Schöppler, Friedrich}, journal = {Chemical Physics Letters}, keywords = {swnt}, pages = {245 - 249}, title = {A comparison of Raman and photoluminescence spectra for the assessment of single-wall carbon nanotube sample quality}, volume = 635, year = 2015 }
Evidence for Strong Electronic Correlations in the Spectra of Gate-Doped Single-Wall Carbon Nanotubes. Hartleb, Holger; Späth, Florian; Hertel, Tobias. In ACS Nano, 9(10), pp. 10461–10470. 2015.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We have investigated the photophysical properties of electrochemically gate-doped semiconducting single-wall carbon nanotubes (s-SWNTs). A comparison of photoluminescence (PL) and simultaneously recorded absorption spectra reveals that free-carrier densities correlate well with the first sub-band exciton or trion oscillator strengths but not with PL intensities. We thus used a global analysis of the first sub-band exciton absorption for a detailed investigation of gate-doping, here of the (6,5) SWNT valence band. Our data are consistent with a doping-induced valence band shift according to Δϵv = n × b, where n is the free-carrier density, ϵv is the valence band edge, and b = 0.15 ± 0.05 eV·nm. We also predict such band gap renormalization of one-dimensional gate-doped semiconductors to be accompanied by a stepwise increase of the carrier density by Δn = (32meffb)/(πℏ)2 (meff is effective carrier mass). Moreover, we show that the width of the spectroelectrochemical window of the first sub-band exciton of 1.55 ± 0.05 eV corresponds to the fundamental band gap of the undoped (6,5) SWNTs in our samples and not to the renormalized band gap of the doped system. These observations as well as a previously unidentified absorption band emerging at high doping levels in the Pauli-blocked region of the single-particle Hartree band structure provide clear evidence for strong electronic correlations in the optical spectra of SWNTs.

@article{doi:10.1021/acsnano.5b04707, abstract = {We have investigated the photophysical properties of electrochemically gate-doped semiconducting single-wall carbon nanotubes (s-SWNTs). A comparison of photoluminescence (PL) and simultaneously recorded absorption spectra reveals that free-carrier densities correlate well with the first sub-band exciton or trion oscillator strengths but not with PL intensities. We thus used a global analysis of the first sub-band exciton absorption for a detailed investigation of gate-doping, here of the (6,5) SWNT valence band. Our data are consistent with a doping-induced valence band shift according to Δϵv = n × b, where n is the free-carrier density, ϵv is the valence band edge, and b = 0.15 ± 0.05 eV·nm. We also predict such band gap renormalization of one-dimensional gate-doped semiconductors to be accompanied by a stepwise increase of the carrier density by Δn = (32meffb)/(πℏ)2 (meff is effective carrier mass). Moreover, we show that the width of the spectroelectrochemical window of the first sub-band exciton of 1.55 ± 0.05 eV corresponds to the fundamental band gap of the undoped (6,5) SWNTs in our samples and not to the renormalized band gap of the doped system. These observations as well as a previously unidentified absorption band emerging at high doping levels in the Pauli-blocked region of the single-particle Hartree band structure provide clear evidence for strong electronic correlations in the optical spectra of SWNTs.}, author = {Hartleb, Holger and Späth, Florian and Hertel, Tobias}, journal = {ACS Nano}, keywords = {myown}, note = {PMID: 26381021}, number = 10, pages = {10461-10470}, title = {Evidence for Strong Electronic Correlations in the Spectra of Gate-Doped Single-Wall Carbon Nanotubes}, volume = 9, year = 2015 }
Polymer-sorted (6,5) single-walled carbon nanotubes for solution-processed low-voltage flexible microelectronics. Bottacchi, Francesca; Petti, Luisa; Späth, Florian; Namal, Imge; Tröster, Gerhard; Hertel, Tobias; Anthopoulos, Thomas D. In Applied Physics Letters, 106(19), p. 193302. 2015.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We report on low operating voltage transistors based on polymer-sorted semiconducting (6,5) single-walled carbon nanotube (SWNT) networks processed from solution at room temperature. The (6,5) SWNT...

@article{bottacchi2015polymersorted, abstract = {We report on low operating voltage transistors based on polymer-sorted semiconducting (6,5) single-walled carbon nanotube (SWNT) networks processed from solution at room temperature. The (6,5) SWNT...}, author = {Bottacchi, Francesca and Petti, Luisa and Sp{\"a}th, Florian and Namal, Imge and Tr{\"o}ster, Gerhard and Hertel, Tobias and Anthopoulos, Thomas D.}, journal = {Applied Physics Letters}, keywords = {transistors}, number = 19, pages = 193302, title = {Polymer-sorted (6,5) single-walled carbon nanotubes for solution-processed low-voltage flexible microelectronics}, volume = 106, year = 2015 }
Ultrafast Spectral Exciton Diffusion in Single-Wall Carbon Nanotubes Studied by Time-Resolved Hole Burning. Schilling, Daniel; Mann, Christoph; Kunkel, Pascal; Schöppler, Friedrich; Hertel, Tobias. In The Journal of Physical Chemistry C, 119(42), pp. 24116–24123. 2015.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We have studied ultrafast spectral diffusion (SD) within exciton bands of semiconducting single-wall carbon nanotubes (s-SWNTs) using one- and two-dimensional, near-infrared transient hole burning spectroscopy and time-resolved fluorescence spectroscopy at temperatures between 15 and 293 K. We find that inhomogeneous spectral broadening of 60 meV for s-SWNTs embedded in gelatin exceeds the homogeneous line width of 3.3 meV by over an order of magnitude. The experiments show that ultrafast spectral diffusion of excitons in gel-immobilized s-SWNTs on the 250 fs time scale can be attributed to axial intratube exciton diffusion. Comparison with kinetic Monte Carlo simulations suggests that the length-scale characteristic of the granularity of the axial potential energy landscape is about 24 nm.

@article{doi:10.1021/acs.jpcc.5b06865, abstract = {We have studied ultrafast spectral diffusion (SD) within exciton bands of semiconducting single-wall carbon nanotubes (s-SWNTs) using one- and two-dimensional, near-infrared transient hole burning spectroscopy and time-resolved fluorescence spectroscopy at temperatures between 15 and 293 K. We find that inhomogeneous spectral broadening of 60 meV for s-SWNTs embedded in gelatin exceeds the homogeneous line width of 3.3 meV by over an order of magnitude. The experiments show that ultrafast spectral diffusion of excitons in gel-immobilized s-SWNTs on the 250 fs time scale can be attributed to axial intratube exciton diffusion. Comparison with kinetic Monte Carlo simulations suggests that the length-scale characteristic of the granularity of the axial potential energy landscape is about 24 nm.}, author = {Schilling, Daniel and Mann, Christoph and Kunkel, Pascal and Schöppler, Friedrich and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {myown}, number = 42, pages = {24116-24123}, title = {Ultrafast Spectral Exciton Diffusion in Single-Wall Carbon Nanotubes Studied by Time-Resolved Hole Burning}, volume = 119, year = 2015 }

2014[ to top ]

Excitation and recombination dynamics of vacancy-related spin centers in silicon carbide. Hain, T. C.; Fuchs, F; Soltamov, V. A.; Baranov, P. G.; Astakhov, G. V.; Hertel, T; Dyakonov, V. In Journal of Applied Physics, 115(13), p. 133508-. American Institute of Physics, 2014.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We generate silicon vacancy related defects in high-quality epitaxial silicon carbide layers by means of electron irradiation. By controlling the irradiation fluence, the defect concentration is va...

@article{hain2014excitation, abstract = {We generate silicon vacancy related defects in high-quality epitaxial silicon carbide layers by means of electron irradiation. By controlling the irradiation fluence, the defect concentration is va...}, author = {Hain, T. C. and Fuchs, F and Soltamov, V. A. and Baranov, P. G. and Astakhov, G. V. and Hertel, T and Dyakonov, V}, booktitle = {Journal of Applied Physics}, journal = {Journal of Applied Physics}, keywords = {myown}, month = {apr}, number = 13, pages = {133508–}, publisher = {American Institute of Physics}, title = {Excitation and recombination dynamics of vacancy-related spin centers in silicon carbide}, volume = 115, year = 2014 }
Dynamical Contact Line Pinning and Zipping during Carbon Nanotube Coffee Stain Formation. Li, Han; Hain, Tilman C.; Muzha, Andreas; Schöppler, Friedrich; Hertel, Tobias. In ACS Nano, 8(6), pp. 6417–6424. 2014.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Thin films of single-wall carbon nanotubes (SWNTs) can be deposited onto solid substrates by evaporation-induced self-assembly. However, for this process to become more accessible to thin-film-based device fabrication requires optimization and a better understanding of the parameters and mechanisms governing nanoparticle film growth. Here, we focus on the role of contact-line (CL) dynamics at the edge of a receding meniscus for the deposition of thin nanoparticle films from colloidal suspensions. We find that film deposition rates can be increased by up to 2 orders of magnitude over earlier reports if parameters such as SWNT concentration, surfactant concentration, and height of the capillary bridge from which particles are deposited are properly adjusted. Most importantly we have also discovered that CL dynamics leading to the formation of striped films (coffee stains) are best described by dynamical pinning and kink-induced zipping. The existence of critical SWNT and surfactant concentrations as well as their role in determining stripe characteristics can be well accounted for by the proposed dynamical pinning and zipping model.

@article{doi:10.1021/nn501957y, abstract = {Thin films of single-wall carbon nanotubes (SWNTs) can be deposited onto solid substrates by evaporation-induced self-assembly. However, for this process to become more accessible to thin-film-based device fabrication requires optimization and a better understanding of the parameters and mechanisms governing nanoparticle film growth. Here, we focus on the role of contact-line (CL) dynamics at the edge of a receding meniscus for the deposition of thin nanoparticle films from colloidal suspensions. We find that film deposition rates can be increased by up to 2 orders of magnitude over earlier reports if parameters such as SWNT concentration, surfactant concentration, and height of the capillary bridge from which particles are deposited are properly adjusted. Most importantly we have also discovered that CL dynamics leading to the formation of striped films (coffee stains) are best described by dynamical pinning and kink-induced zipping. The existence of critical SWNT and surfactant concentrations as well as their role in determining stripe characteristics can be well accounted for by the proposed dynamical pinning and zipping model.}, author = {Li, Han and Hain, Tilman C. and Muzha, Andreas and Schöppler, Friedrich and Hertel, Tobias}, journal = {ACS Nano}, keywords = {stain}, note = {PMID: 24827029}, number = 6, pages = {6417-6424}, title = {Dynamical Contact Line Pinning and Zipping during Carbon Nanotube Coffee Stain Formation}, volume = 8, year = 2014 }
Triplet–triplet exciton dynamics in single-walled carbon nanotubes. Stich, Dominik; Späth, Florian; Kraus, Hannes; Sperlich, Andreas; Dyakonov, Vladimir; Hertel, Tobias. In Nature Photonics, 8(2), pp. 139–144. 2014.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Semiconducting single-walled carbon nanotubes (SWNTs) are considered as building blocks for novel optoelectronic and photonic devices. Energy transport, dissipation and nonlinear optical properties of such devices depend critically on the dynamics of singlet and triplet excitons. However, little is known about triplet excitons in SWNTs despite their important role in photovoltaic, photoelectric and other applications. We present pump–probe and spin-sensitive photoluminescence studies of semiconducting SWNTs that allow the determination of the quantum yield of triplet formation (5 ± 2%), the triplet lifetime (30 ± 10 µs) and the triplet exciton size (0.65 nm). Triplet–triplet annihilation is also found to induce delayed fluorescence. The power-law decay of pump–probe and time-resolved photoluminescence signals is characteristic of diffusion-limited annihilation in one-dimensional systems and allows an estimation of the triplet diffusion constant of 0.1 cm2 s−1. This work suggests that exciton annihilation in SWNTs is reduced by one-dimensional confinement of diffusive exciton motion.

@article{stich2014triplettriplet, abstract = {Semiconducting single-walled carbon nanotubes (SWNTs) are considered as building blocks for novel optoelectronic and photonic devices. Energy transport, dissipation and nonlinear optical properties of such devices depend critically on the dynamics of singlet and triplet excitons. However, little is known about triplet excitons in SWNTs despite their important role in photovoltaic, photoelectric and other applications. We present pump–probe and spin-sensitive photoluminescence studies of semiconducting SWNTs that allow the determination of the quantum yield of triplet formation (5 ± 2%), the triplet lifetime (30 ± 10 µs) and the triplet exciton size (0.65 nm). Triplet–triplet annihilation is also found to induce delayed fluorescence. The power-law decay of pump–probe and time-resolved photoluminescence signals is characteristic of diffusion-limited annihilation in one-dimensional systems and allows an estimation of the triplet diffusion constant of 0.1 cm2 s−1. This work suggests that exciton annihilation in SWNTs is reduced by one-dimensional confinement of diffusive exciton motion.}, author = {Stich, Dominik and Späth, Florian and Kraus, Hannes and Sperlich, Andreas and Dyakonov, Vladimir and Hertel, Tobias}, journal = {Nature Photonics}, keywords = {swnt}, number = 2, pages = {139–144}, title = {Triplet–triplet exciton dynamics in single-walled carbon nanotubes}, volume = 8, year = 2014 }

2013[ to top ]

Fluorescence Spectroscopy of Gel-Immobilized Single-Wall Carbon Nanotubes with Microfluidic Control of the Surfactant Environment. Bergler, Felix F.; Schöppler, Friedrich; Brunecker, Frank K.; Hailman, Michael; Hertel, Tobias. In The Journal of Physical Chemistry C, 117(25), pp. 13318–13323. American Chemical Society (ACS), 2013.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We describe the design and use of a microfluidic fluorescence cell for nanoparticle spectroscopy. The cell allows for microfluidic control of solvent and surfactant environments to study interfacial processes of nanoscale systems. We present a spectroscopic investigation on the effect of surfactant type and concentrations on the first subband exciton transition of (6,5) single-wall carbon nanotubes (SWNTs). SWNT fluorescence here serves as a surface-sensitive probe of changes at the surfactant–nanotube interface. The experiments show that a displacement of H2O or of adsorbed DNA by sodium cholate from solution leads to a pronounced blue-shift of exciton emission features and a roughly 5-fold increase of photoluminescence (PL) intensities. This is due to a combination of physical and chemical interactions. The major contribution to changes in PL quantum yield and exciton peak position can be attributed to changes in dielectric screening and its effect on exciton oscillator strengths.

@article{Bergler_2013, abstract = {We describe the design and use of a microfluidic fluorescence cell for nanoparticle spectroscopy. The cell allows for microfluidic control of solvent and surfactant environments to study interfacial processes of nanoscale systems. We present a spectroscopic investigation on the effect of surfactant type and concentrations on the first subband exciton transition of (6,5) single-wall carbon nanotubes (SWNTs). SWNT fluorescence here serves as a surface-sensitive probe of changes at the surfactant–nanotube interface. The experiments show that a displacement of H2O or of adsorbed DNA by sodium cholate from solution leads to a pronounced blue-shift of exciton emission features and a roughly 5-fold increase of photoluminescence (PL) intensities. This is due to a combination of physical and chemical interactions. The major contribution to changes in PL quantum yield and exciton peak position can be attributed to changes in dielectric screening and its effect on exciton oscillator strengths.}, author = {Bergler, Felix F. and Schöppler, Friedrich and Brunecker, Frank K. and Hailman, Michael and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {swnt}, month = {jun}, number = 25, pages = {13318–13323}, publisher = {American Chemical Society ({ACS})}, title = {Fluorescence Spectroscopy of Gel-Immobilized Single-Wall Carbon Nanotubes with Microfluidic Control of the Surfactant Environment}, volume = 117, year = 2013 }
Interface-Phenomena of Carbon Nanotubes. Hertel, Tobias; Brunecker, Frank; Ruehl, Nicolas; Schilling, Daniel; Spaeth, Florian. In NACHRICHTEN AUS DER CHEMIE, 61(6), pp. 632–635. 2013.
- [ BibTeX ]
- [ DOI ]
@article{hertel2013interfacephenomena, author = {Hertel, Tobias and Brunecker, Frank and Ruehl, Nicolas and Schilling, Daniel and Spaeth, Florian}, journal = {{NACHRICHTEN AUS DER CHEMIE}}, keywords = {myown}, month = {{JUN}}, number = {{6}}, pages = {{632-635}}, title = {{Interface-Phenomena of Carbon Nanotubes}}, volume = {{61}}, year = {{2013}} }
Photophysics of carbon nanotubes and nanotube composites. Hertel, Tobias; Bondarev, Igor. In Chemical Physics, 413, pp. 1–2. 2013.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Carbon nanotubes (CNTs) can be thought of as graphene sheets rolled-up into cylinders of one to a few nanometers in diameter and up to hundreds of microns in length [1]. Extensive work carried out in recent years, in particular on single-wall carbon nanotubes (SWNTs), has revealed intriguing physical properties of these molecular scale wires. At present, CNTs have been successfully integrated into miniaturized electronic, electromechanical, and chemical devices, scanning probes, and into nanocomposite materials [1], [2]. Over the past few years, optical nanomaterials research has also uncovered intriguing optical attributes of their physical properties, lending themselves to a variety of device applications [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. At the same time, carbon nanotubes offer an ideal testing ground for the investigation of one-dimensional nanosystems at the crossroads of molecular chemistry and condensed matter physics.

@article{hertel2013photophysics, abstract = {Carbon nanotubes (CNTs) can be thought of as graphene sheets rolled-up into cylinders of one to a few nanometers in diameter and up to hundreds of microns in length [1]. Extensive work carried out in recent years, in particular on single-wall carbon nanotubes (SWNTs), has revealed intriguing physical properties of these molecular scale wires. At present, CNTs have been successfully integrated into miniaturized electronic, electromechanical, and chemical devices, scanning probes, and into nanocomposite materials [1], [2]. Over the past few years, optical nanomaterials research has also uncovered intriguing optical attributes of their physical properties, lending themselves to a variety of device applications [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. At the same time, carbon nanotubes offer an ideal testing ground for the investigation of one-dimensional nanosystems at the crossroads of molecular chemistry and condensed matter physics.}, author = {Hertel, Tobias and Bondarev, Igor}, journal = {Chemical Physics}, keywords = {swnt}, note = {Photophysics of carbon nanotubes and nanotube composites}, pages = {1 - 2}, title = {Photophysics of carbon nanotubes and nanotube composites}, volume = 413, year = 2013 }
Ultrafast Photoconductivity of Graphene Nanoribbons and Carbon Nanotubes. Jensen, Søren A.; Ulbricht, Ronald; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus; Hertel, Tobias; Turchinovich, Dmitry; Bonn, Mischa. In Nano Letters, 13(12), pp. 5925–5930. 2013.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We present a comparative study of the ultrafast photoconductivity in two different forms of one-dimensional (1D) quantum-confined graphene nanostructures: structurally well-defined semiconducting graphene nanoribbons (GNRs) fabricated by a “bottom-up” chemical synthesis approach and semiconducting carbon nanotubes (CNTs) with a similar bandgap energy. Transient photoconductivities of both materials were measured using time-resolved terahertz spectroscopy, allowing for contact-free measurements of complex-valued photoconductivity spectra with subpicosecond time-resolution. We show that, while the THz photoresponse seems very different for the two systems, a single model of free carriers experiencing backscattering when moving along the long axis of the CNTs or GNRs provides a quantitative description of both sets of results, revealing significantly longer carrier scattering times for CNTs (ca. 150 fs) than for GNRs (ca. 30 fs) and in turn higher carrier mobilities. This difference can be explained by differences in band structures and phonon scattering and the greater structural rigidity of CNTs as compared to GNRs, minimizing the influence of bending and/or torsional defects on the electron transport.

@article{jensen2013ultrafast, abstract = {We present a comparative study of the ultrafast photoconductivity in two different forms of one-dimensional (1D) quantum-confined graphene nanostructures: structurally well-defined semiconducting graphene nanoribbons (GNRs) fabricated by a “bottom-up” chemical synthesis approach and semiconducting carbon nanotubes (CNTs) with a similar bandgap energy. Transient photoconductivities of both materials were measured using time-resolved terahertz spectroscopy, allowing for contact-free measurements of complex-valued photoconductivity spectra with subpicosecond time-resolution. We show that, while the THz photoresponse seems very different for the two systems, a single model of free carriers experiencing backscattering when moving along the long axis of the CNTs or GNRs provides a quantitative description of both sets of results, revealing significantly longer carrier scattering times for CNTs (ca. 150 fs) than for GNRs (ca. 30 fs) and in turn higher carrier mobilities. This difference can be explained by differences in band structures and phonon scattering and the greater structural rigidity of CNTs as compared to GNRs, minimizing the influence of bending and/or torsional defects on the electron transport.}, author = {Jensen, Søren A. and Ulbricht, Ronald and Narita, Akimitsu and Feng, Xinliang and Müllen, Klaus and Hertel, Tobias and Turchinovich, Dmitry and Bonn, Mischa}, journal = {Nano Letters}, keywords = {swnt}, note = {PMID: 24093134}, number = 12, pages = {5925-5930}, title = {Ultrafast Photoconductivity of Graphene Nanoribbons and Carbon Nanotubes}, volume = 13, year = 2013 }
Photoluminescence microscopy and spectroscopy of individualized and aggregated single-wall carbon nanotubes. Schöppler, Friedrich; Rühl, Nicolas; Hertel, Tobias. In Chemical Physics, 413, pp. 112–115. 2013.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We have studied photophysical properties of individual single-wall carbon nanotubes (SWNTs) and SWNT aggregates in colloidal suspensions using fluorescence microscopy, -spectroscopy and time-correlated single photon counting. The results suggest that SWNT aggregates, formed by addition of a divalent salt to a monomer SWNT suspension, have an open structure with a density of ≈12SWNTsμm−3. We also find a sizable spectral red-shift of 16nm, line-broadening of 23nm and reduced PL lifetimes in coagulated structures, independent of aggregate size for the range of structures studied here. Such changes may be used as fingerprint of SWNT aggregate formation in similar environments.

@article{SCHOPPLER2013112, abstract = {We have studied photophysical properties of individual single-wall carbon nanotubes (SWNTs) and SWNT aggregates in colloidal suspensions using fluorescence microscopy, -spectroscopy and time-correlated single photon counting. The results suggest that SWNT aggregates, formed by addition of a divalent salt to a monomer SWNT suspension, have an open structure with a density of ≈12SWNTsμm−3. We also find a sizable spectral red-shift of 16nm, line-broadening of 23nm and reduced PL lifetimes in coagulated structures, independent of aggregate size for the range of structures studied here. Such changes may be used as fingerprint of SWNT aggregate formation in similar environments.}, author = {Schöppler, Friedrich and Rühl, Nicolas and Hertel, Tobias}, journal = {Chemical Physics}, keywords = {swnt}, note = {Photophysics of carbon nanotubes and nanotube composites}, pages = {112 - 115}, title = {Photoluminescence microscopy and spectroscopy of individualized and aggregated single-wall carbon nanotubes}, volume = 413, year = 2013 }
Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes. Soavi, G.; Scotognella, F.; Brida, D.; Hefner, T.; Späth, F.; Antognazza, M. R.; Hertel, T.; Lanzani, G.; Cerullo, G. In The Journal of Physical Chemistry C, 117(20), pp. 10849–10855. American Chemical Society (ACS), 2013.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We show that excitons are not the unique outcome of photoexcitation in single-walled carbon nanotubes (SWNTs). Our experiments of transient photoinduced absorption suggest that charge carriers are formed with quantum yield of a few percent and that such species strongly affect the long-lived transient spectrum. Photogenerated charge carriers induce strong local electric fields that shift by the Stark effect the second subband exciton absorption in SWNTs, resulting in a characteristic derivative shape of the transient absorption spectra.

@article{Soavi_2013, abstract = {We show that excitons are not the unique outcome of photoexcitation in single-walled carbon nanotubes (SWNTs). Our experiments of transient photoinduced absorption suggest that charge carriers are formed with quantum yield of a few percent and that such species strongly affect the long-lived transient spectrum. Photogenerated charge carriers induce strong local electric fields that shift by the Stark effect the second subband exciton absorption in SWNTs, resulting in a characteristic derivative shape of the transient absorption spectra.}, author = {Soavi, G. and Scotognella, F. and Brida, D. and Hefner, T. and Späth, F. and Antognazza, M. R. and Hertel, T. and Lanzani, G. and Cerullo, G.}, journal = {The Journal of Physical Chemistry C}, keywords = {swnt}, month = {may}, number = 20, pages = {10849–10855}, publisher = {American Chemical Society ({ACS})}, title = {Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes}, volume = 117, year = 2013 }

2012[ to top ]

Influence of DNA conformation on the dispersion of SWNTs: single-strand DNAvs. hairpin DNA. Hain, Tilman C.; Kröker, Kristin; Stich, Dominik G.; Hertel, Tobias. In Soft Matter, 8(10), pp. 2820–2823. The Royal Society of Chemistry, 2012.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Dispersion of CNTs with single-strand (ss) and hairpin (hp) DNA yields suspensions with varying degree of individualization and selectivity towards specific tube types. Time-resolved fluorescence spectroscopy with dye labelled oligomers is used to investigate structural properties of DNA–SWNT conjugates. hpDNA is found to bind with its loop sideways to the CNT surface.

@article{C2SM07148G, abstract = {Dispersion of CNTs with single-strand (ss) and hairpin (hp) DNA yields suspensions with varying degree of individualization and selectivity towards specific tube types. Time-resolved fluorescence spectroscopy with dye labelled oligomers is used to investigate structural properties of DNA–SWNT conjugates. hpDNA is found to bind with its loop sideways to the CNT surface.}, author = {Hain, Tilman C. and Kröker, Kristin and Stich, Dominik G. and Hertel, Tobias}, journal = {Soft Matter}, keywords = {swnt}, number = 10, pages = {2820-2823}, publisher = {The Royal Society of Chemistry}, title = {Influence of DNA conformation on the dispersion of SWNTs: single-strand DNAvs. hairpin DNA}, volume = 8, year = 2012 }
Density gradient ultracentrifugation and stability of SWNT–peptide conjugates. Hartleb, Holger; Kröker, Kristin; Hertel, Tobias. In Chemical Physics Letters, 535, pp. 131–135. 2012.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
Density gradient ultracentrifugation was used for preparation of biocompatible single-wall carbon nanotube (SWNT)–peptide conjugates. Aggregate size selected SWNT suspensions were analyzed by absorption and photoluminescence spectroscopy to determine stability and structure of conjugates. The results suggest that conjugates with densities over 1.19g cm-3 consist of SWNT aggregates. More buoyant fractions with ρ≈1.18g cm-3 contain mostly individual SWNTs. Dilution experiments indicate that equilibration between SWNT-bound and free peptide in solution is approached within a few minutes making excess peptide in solution necessary to prevent peptide–SWNT conjugate aggregation. This has significant implications for the use of SWNT–peptide conjugates in biochemical applications.

@article{HARTLEB2012131, abstract = {Density gradient ultracentrifugation was used for preparation of biocompatible single-wall carbon nanotube (SWNT)–peptide conjugates. Aggregate size selected SWNT suspensions were analyzed by absorption and photoluminescence spectroscopy to determine stability and structure of conjugates. The results suggest that conjugates with densities over 1.19g cm-3 consist of SWNT aggregates. More buoyant fractions with ρ≈1.18g cm-3 contain mostly individual SWNTs. Dilution experiments indicate that equilibration between SWNT-bound and free peptide in solution is approached within a few minutes making excess peptide in solution necessary to prevent peptide–SWNT conjugate aggregation. This has significant implications for the use of SWNT–peptide conjugates in biochemical applications.}, author = {Hartleb, Holger and Kröker, Kristin and Hertel, Tobias}, journal = {Chemical Physics Letters}, keywords = {ultracentrifugation}, pages = {131 - 135}, title = {Density gradient ultracentrifugation and stability of SWNT–peptide conjugates}, volume = 535, year = 2012 }

2011[ to top ]

Molar Extinction Coefficient of Single-Wall Carbon Nanotubes. Schöppler, Friedrich; Mann, Christoph; Hain, Tilman C.; Neubauer, Felix M.; Privitera, Giulia; Bonaccorso, Francesco; Chu, Daping; Ferrari, Andrea C.; Hertel, Tobias. In The Journal of Physical Chemistry C, 115(30), pp. 14682–14686. 2011.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
The molar extinction coefficient of single-wall carbon nanotubes (SWNTs) is determined using fluorescence tagging, as well as atomic force microscopy (AFM) imaging, which facilitate the correlation of nanotube concentrations with absorption spectra. Tagging of SWNTs is achieved using fluorescence-labeled single-strand DNA oligomers as the dispersion additive, while AFM imaging is used to determine the mass of SWNTs in the retentate of vacuum-filtered colloidal SWNT suspensions. The resulting absorption cross section for the first exciton transition of (6,5) nanotubes of 1.7 × 10⁻¹⁷ cm² per C-atom corresponds to an extinction coefficient of (4400 ± 1000) M⁻¹·cm⁻¹, which is equivalent to an oscillator strength of 0.010 per carbon atom.

@article{schoeppler2011, abstract = {The molar extinction coefficient of single-wall carbon nanotubes (SWNTs) is determined using fluorescence tagging, as well as atomic force microscopy (AFM) imaging, which facilitate the correlation of nanotube concentrations with absorption spectra. Tagging of SWNTs is achieved using fluorescence-labeled single-strand DNA oligomers as the dispersion additive, while AFM imaging is used to determine the mass of SWNTs in the retentate of vacuum-filtered colloidal SWNT suspensions. The resulting absorption cross section for the first exciton transition of (6,5) nanotubes of 1.7 × 10⁻¹⁷ cm² per C-atom corresponds to an extinction coefficient of (4400 ± 1000) M⁻¹·cm⁻¹, which is equivalent to an oscillator strength of 0.010 per carbon atom.}, author = {Schöppler, Friedrich and Mann, Christoph and Hain, Tilman C. and Neubauer, Felix M. and Privitera, Giulia and Bonaccorso, Francesco and Chu, Daping and Ferrari, Andrea C. and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {swnt}, number = 30, pages = {14682-14686}, title = {Molar Extinction Coefficient of Single-Wall Carbon Nanotubes}, volume = 115, year = 2011 }
Free-Carrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime. Crochet, Jared J.; Hoseinkhani, Sajjad; Lüer, Larry; Hertel, Tobias; Doorn, Stephen K.; Lanzani, Guglielmo. In Phys. Rev. Lett., 107(25), p. 257402. American Physical Society, 2011.
- [ Abstract ]
- [ BibTeX ]
- [ URL ]
- [ DOI ]
We present evidence for the generation of free carriers in aggregated single-wall carbon nanotubes by photoexcitation in the energetic range of the π→π﹡ transition associated with the M saddle point of the graphene lattice. The underlying broad absorption culminating at 4.3 eV can be fit well with a Fano line shape that describes strong coupling of a saddle-point exciton to an underlying free electron-hole pair continuum. Moreover, it is demonstrated that transitions in this energetic region autoionize into the continuum by detecting features unique to the presence of free charges in the transient transmission spectra of the continuum-embedded second sub-band exciton, S₂.

@article{PhysRevLett.107.257402, abstract = {We present evidence for the generation of free carriers in aggregated single-wall carbon nanotubes by photoexcitation in the energetic range of the π→π﹡ transition associated with the M saddle point of the graphene lattice. The underlying broad absorption culminating at 4.3 eV can be fit well with a Fano line shape that describes strong coupling of a saddle-point exciton to an underlying free electron-hole pair continuum. Moreover, it is demonstrated that transitions in this energetic region autoionize into the continuum by detecting features unique to the presence of free charges in the transient transmission spectra of the continuum-embedded second sub-band exciton, S₂.}, author = {Crochet, Jared J. and Hoseinkhani, Sajjad and L\"uer, Larry and Hertel, Tobias and Doorn, Stephen K. and Lanzani, Guglielmo}, journal = {Phys. Rev. Lett.}, keywords = {ultraviolet}, month = {dec}, number = 25, pages = 257402, publisher = {American Physical Society}, title = {Free-Carrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime}, volume = 107, year = 2011 }

2010[ to top ]

Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates. Lueer, Larry; Crochet, Jared; Hertel, Tobias; Cerullo, Giulio; Lanzani, Guglielmo. In ACS NANO, 4(7), pp. 4265–4273. 2010.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We study excitation energy transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (<10 fs). Excitation in the visible spectral range, where the second excitonic transitions occur, is followed by fast intratube relaxation and subsequent energy transfer, in particular from the (8,4) tube toward other tubes, the latter process occurring in less than 10 fs.

@article{lueer2010ultrafast, abstract = {{We study excitation energy transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (<10 fs). Excitation in the visible spectral range, where the second excitonic transitions occur, is followed by fast intratube relaxation and subsequent energy transfer, in particular from the (8,4) tube toward other tubes, the latter process occurring in less than 10 fs.}}, author = {Lueer, Larry and Crochet, Jared and Hertel, Tobias and Cerullo, Giulio and Lanzani, Guglielmo}, journal = {{ACS NANO}}, keywords = {myown}, month = {{JUL}}, number = {{7}}, pages = {{4265-4273}}, title = {{Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates}}, volume = {{4}}, year = {{2010}} }
CARBON NANOTUBES A brighter future. Hertel, Tobias. In NATURE PHOTONICS, 4(2), pp. 77–78. 2010.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The demonstration of an LED made from a single electrostatically doped carbon nanotube p–n junction with dramatically improved light-emission efficiency marks an important advance for carbon nanotube photonics.

@article{hertel2010carbon, abstract = {The demonstration of an LED made from a single electrostatically doped carbon nanotube p–n junction with dramatically improved light-emission efficiency marks an important advance for carbon nanotube photonics.}, author = {Hertel, Tobias}, journal = {{NATURE PHOTONICS}}, keywords = {myown}, month = {{FEB}}, number = {{2}}, pages = {{77-78}}, title = {{CARBON NANOTUBES A brighter future}}, volume = {{4}}, year = {{2010}} }
Diffusion Limited Photoluminescence Quantum Yields in 1-D Semiconductors: Single-Wall Carbon Nanotubes. Hertel, Tobias; Himmelein, Sabine; Ackermann, Thomas; Stich, Dominik; Crochet, Jared. In ACS NANO, 4(12), pp. 7161–7168. 2010.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Photoluminescence quantum yields and nonradiative decay of the excitonic S-1, state in length fractionated (6,5) single-wall carbon nanotubes (SWNTs) are studied by continuous wave and time-resolved fluorescence spectroscopy. The experimental data are modeled by diffusion limited contact quenching of excitons at stationary quenching sites including tube ends. A combined analysis of the time-resolved photoluminescence decay and the length dependence of photoluminescence quantum yields (PL QYs) from SWNTs in sodium cholate suspensions allows to determine the exciton diffusion coefficient D = 10.7 +/- 0.4 cm(2)s(-1) and lifetime T-pL for long tubes of 20 +/- 1 ps. PL quantum yields Phi(pL) are found to scale with the inverse diffusion coefficient and the square of the mean quenching site distance, here I-d = 120 +/- 25 nm. The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.

@article{hertel2010diffusion, abstract = {{Photoluminescence quantum yields and nonradiative decay of the excitonic S-1, state in length fractionated (6,5) single-wall carbon nanotubes (SWNTs) are studied by continuous wave and time-resolved fluorescence spectroscopy. The experimental data are modeled by diffusion limited contact quenching of excitons at stationary quenching sites including tube ends. A combined analysis of the time-resolved photoluminescence decay and the length dependence of photoluminescence quantum yields (PL QYs) from SWNTs in sodium cholate suspensions allows to determine the exciton diffusion coefficient D = 10.7 +/- 0.4 cm(2)s(-1) and lifetime T-pL for long tubes of 20 +/- 1 ps. PL quantum yields Phi(pL) are found to scale with the inverse diffusion coefficient and the square of the mean quenching site distance, here I-d = 120 +/- 25 nm. The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.}}, author = {Hertel, Tobias and Himmelein, Sabine and Ackermann, Thomas and Stich, Dominik and Crochet, Jared}, journal = {{ACS NANO}}, keywords = {myown}, month = {{DEC}}, number = {{12}}, pages = {{7161-7168}}, title = {{Diffusion Limited Photoluminescence Quantum Yields in 1-D Semiconductors: Single-Wall Carbon Nanotubes}}, volume = {{4}}, year = {{2010}} }
Long lived photo excitations in (6,5) carbon nanotubes. Sciascia, C.; Crochet, J.; Hertel, T.; Lanzani, G. In EUROPEAN PHYSICAL JOURNAL B, 75(2), pp. 115–120. 2010.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We study the photo-induced transmission change (Delta T/T) upon quasi-continuum laser excitation, for different modulation frequency and excitation intensity in films of (6, 5) enriched CoMoCat single wall nanotubes (SWNTs) embedded in a polymeric matrix. This technique probes long-lived excited species, with lifetime in the 1 ms-1 s domain. The observed line-shape of Delta T/T spectra that can be assigned to a superposition of ground state bleaching and absorption modulation. We speculate that photo-excitation creates charges which get trapped in the sample, acquiring ms lifetime. This causes bleaching of the ground state absorption due to state filling and possibly a tube diameter expansion, with consequent blue shift of the optical transition.

@article{sciascia2010lived, abstract = {{We study the photo-induced transmission change (Delta T/T) upon quasi-continuum laser excitation, for different modulation frequency and excitation intensity in films of (6, 5) enriched CoMoCat single wall nanotubes (SWNTs) embedded in a polymeric matrix. This technique probes long-lived excited species, with lifetime in the 1 ms-1 s domain. The observed line-shape of Delta T/T spectra that can be assigned to a superposition of ground state bleaching and absorption modulation. We speculate that photo-excitation creates charges which get trapped in the sample, acquiring ms lifetime. This causes bleaching of the ground state absorption due to state filling and possibly a tube diameter expansion, with consequent blue shift of the optical transition.}}, author = {Sciascia, C. and Crochet, J. and Hertel, T. and Lanzani, G.}, journal = {{EUROPEAN PHYSICAL JOURNAL B}}, keywords = {myown}, month = {{MAY}}, number = {{2}}, pages = {{115-120}}, title = {{Long lived photo excitations in (6,5) carbon nanotubes}}, volume = {{75}}, year = {{2010}} }

2009[ to top ]

Ultrafast dynamics in metallic and semiconducting carbon nanotubes. Luer, Larry; Lanzani, Guglielmo; Crochet, Jared; Hertel, Tobias; Holt, Josh; Vardeny, Z. Valy. In PHYSICAL REVIEW B, 80(20). 2009.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Transient polarized pump/probe transmission changes upon pulse photoexcitation at 1.6 eV were measured in chiral enriched carbon nanotubes ensembles of both metallic and semiconductor characters, in a very broad probe spectral range from mid-IR to visible, combining several ultrafast laser systems. We identify a photoinduced absorption band at similar to 0.7 eV as due to hot Dirac fermions in the metallic tubes and measure its dynamics. The hot carriers equilibrate within the electronic system in similar to 400 fs and with the lattice within similar to 2 ps; in agreement with hot Dirac fermions thermalization in graphene. We also assign excitonic transitions in the semiconducting tubes, mainly of (6,5) chirality; in particular, an intraexciton transition at similar to 0.4 eV represents a lower limit for the exciton binding energy.

@article{luer2009ultrafast, abstract = {{Transient polarized pump/probe transmission changes upon pulse photoexcitation at 1.6 eV were measured in chiral enriched carbon nanotubes ensembles of both metallic and semiconductor characters, in a very broad probe spectral range from mid-IR to visible, combining several ultrafast laser systems. We identify a photoinduced absorption band at similar to 0.7 eV as due to hot Dirac fermions in the metallic tubes and measure its dynamics. The hot carriers equilibrate within the electronic system in similar to 400 fs and with the lattice within similar to 2 ps; in agreement with hot Dirac fermions thermalization in graphene. We also assign excitonic transitions in the semiconducting tubes, mainly of (6,5) chirality; in particular, an intraexciton transition at similar to 0.4 eV represents a lower limit for the exciton binding energy.}}, author = {Luer, Larry and Lanzani, Guglielmo and Crochet, Jared and Hertel, Tobias and Holt, Josh and Vardeny, Z. Valy}, journal = {{PHYSICAL REVIEW B}}, keywords = {myown}, month = {{NOV}}, number = {{20}}, title = {{Ultrafast dynamics in metallic and semiconducting carbon nanotubes}}, volume = {{80}}, year = {{2009}} }
Coherent Phonon Dynamics in Semiconducting Carbon Nanotubes: A Quantitative Study of Electron-Phonon Coupling. Lueer, Larry; Gadermaier, Christoph; Crochet, Jared; Hertel, Tobias; Brida, Daniele; Lanzani, Guglielmo. In PHYSICAL REVIEW LETTERS, 102(12). 2009.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We excite and detect coherent phonons in semiconducting (6,5) carbon nanotubes via a sub-10-fs pump-probe technique. Simulation of the amplitude and phase profile via time-dependent wave packet theory yields excellent agreement with experimental results under the assumption of molecular excitonic states and allows determining the electron-phonon coupling strength for the two dominant vibrational modes.

@article{lueer2009coherent, abstract = {{We excite and detect coherent phonons in semiconducting (6,5) carbon nanotubes via a sub-10-fs pump-probe technique. Simulation of the amplitude and phase profile via time-dependent wave packet theory yields excellent agreement with experimental results under the assumption of molecular excitonic states and allows determining the electron-phonon coupling strength for the two dominant vibrational modes.}}, author = {Lueer, Larry and Gadermaier, Christoph and Crochet, Jared and Hertel, Tobias and Brida, Daniele and Lanzani, Guglielmo}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{MAR 27}}, number = {{12}}, title = {{Coherent Phonon Dynamics in Semiconducting Carbon Nanotubes: A Quantitative Study of Electron-Phonon Coupling}}, volume = {{102}}, year = {{2009}} }
Size and mobility of excitons in (6,5) carbon nanotubes. Lueer, Larry; Hoseinkhani, Sajjad; Polli, Dario; Crochet, Jared; Hertel, Tobias; Lanzani, Guglielmo. In NATURE PHYSICS, 5(1), pp. 54–58. 2009.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Knowledge of excited-state dynamics in carbon nanotubes is determinant for their prospective use in optoelectronic applications. It is known that primary photoexcitations are quasi-one-dimensional excitons, the electron-hole correlation length ('exciton size') of which corresponds to a finite volume in the phase space. This volume can be directly measured by nonlinear spectroscopy provided the time resolution is short enough for probing before population relaxation. Here, we report on the experimental determination of exciton size and mobility in (6, 5) carbon nanotubes. The samples are sodium cholate suspensions of nanotubes (produced by the CoMoCat method) obtained by density-gradient ultracentrifugation. By using sub-15 fs near-infrared pulses to measure the nascent bleach of the lowest exciton resonance, we estimate the exciton size to be 2.0 +/- 0.7 nm. Exciton-exciton annihilation in our samples is found to be rather inefficient so that many excitons can coexist on a single nanotube.

@article{lueer2009mobility, abstract = {{Knowledge of excited-state dynamics in carbon nanotubes is determinant for their prospective use in optoelectronic applications. It is known that primary photoexcitations are quasi-one-dimensional excitons, the electron-hole correlation length ('exciton size') of which corresponds to a finite volume in the phase space. This volume can be directly measured by nonlinear spectroscopy provided the time resolution is short enough for probing before population relaxation. Here, we report on the experimental determination of exciton size and mobility in (6, 5) carbon nanotubes. The samples are sodium cholate suspensions of nanotubes (produced by the CoMoCat method) obtained by density-gradient ultracentrifugation. By using sub-15 fs near-infrared pulses to measure the nascent bleach of the lowest exciton resonance, we estimate the exciton size to be 2.0 +/- 0.7 nm. Exciton-exciton annihilation in our samples is found to be rather inefficient so that many excitons can coexist on a single nanotube.}}, author = {Lueer, Larry and Hoseinkhani, Sajjad and Polli, Dario and Crochet, Jared and Hertel, Tobias and Lanzani, Guglielmo}, journal = {{NATURE PHYSICS}}, keywords = {myown}, month = {{JAN}}, number = {{1}}, pages = {{54-58}}, title = {{Size and mobility of excitons in (6,5) carbon nanotubes}}, volume = {{5}}, year = {{2009}} }

2008[ to top ]

Intersubband decay of 1-D exciton resonances in carbon nanotubes. Hertel, Tobias; Perebeinos, Vasili; Crochet, Jared; Arnold, Katharina; Kappes, Manfred; Avouris, Phaedon. In NANO LETTERS, 8(1), pp. 87–91. 2008.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We have studied intersubband decay of E-22 excitons in semiconducting carbon nanotubes experimentally and theoretically. Photoluminescence excitation line widths of semiconducting nanotubes with chiral indicess (n,m) can be mapped onto a connectivity grid with curves of constant (n - m) and (2n + m). Moreover, the global behavior of E-22 line widths is best characterized by a strong increase with energy irrespective of their (n-m)mod(3) = +/- 1 family affiliation. Solution of the Bethe-Salpeter equations shows that the E-22 line widths are dominated by phonon assisted coupling to higher momentum states of the E-11 and E-12 exciton bands. The calculations also suggest that the branching ratio for decay into exciton bands vs free carrier bands, respectively is about 10:1.

@article{hertel2008intersubband, abstract = {{We have studied intersubband decay of E-22 excitons in semiconducting carbon nanotubes experimentally and theoretically. Photoluminescence excitation line widths of semiconducting nanotubes with chiral indicess (n,m) can be mapped onto a connectivity grid with curves of constant (n - m) and (2n + m). Moreover, the global behavior of E-22 line widths is best characterized by a strong increase with energy irrespective of their (n-m)mod(3) = +/- 1 family affiliation. Solution of the Bethe-Salpeter equations shows that the E-22 line widths are dominated by phonon assisted coupling to higher momentum states of the E-11 and E-12 exciton bands. The calculations also suggest that the branching ratio for decay into exciton bands vs free carrier bands, respectively is about 10:1.}}, author = {Hertel, Tobias and Perebeinos, Vasili and Crochet, Jared and Arnold, Katharina and Kappes, Manfred and Avouris, Phaedon}, journal = {{NANO LETTERS}}, keywords = {myown}, month = {{JAN}}, number = {{1}}, pages = {{87-91}}, title = {{Intersubband decay of 1-D exciton resonances in carbon nanotubes}}, volume = {{8}}, year = {{2008}} }
Ultrafast Spectroscopy of carbon nanotubes. Ma, Ying-Zhong; Hertel, Tobias; Vardeny, Zeev Valy; Fleming, Graham R.; Valkunas, Leonas. In CARBON NA NOTUBES: ADVANCED TOPICS IN THE SYNTHESIS, STRUCTURE, PROPERTIES AND APPLICATIONS, Vol. 111, A. Jorio, G. Dresselhaus, M. Dresselhaus (eds.), pp. 321–352. 2008.
- [ Abstract ]
- [ BibTeX ]
Time-domain spectroscopic studies provide a unique perspective on the materials properties and the microscopic processes underlying them in carbon nanotubes. Ultrafast spectroscopy is used to study the dynamics and kinetics of scattering and relaxation processes from the femtosecond (1 fs equivalent to 10(-15) s) to the picosecond timescale. This provides crucial information on carrier and exciton dynamics that underpin a variety of potential applications of carbon nanotubes, from their use as current-carrying quantum wires, through light-emitting or detecting nanodevices, to their use in light-harvesting technologies and photovoltaics. Background information on the ultrafast spectroscopic techniques of greatest applicability to nanotubes is also provided.

@incollection{ma2008ultrafast, abstract = {{Time-domain spectroscopic studies provide a unique perspective on the materials properties and the microscopic processes underlying them in carbon nanotubes. Ultrafast spectroscopy is used to study the dynamics and kinetics of scattering and relaxation processes from the femtosecond (1 fs equivalent to 10(-15) s) to the picosecond timescale. This provides crucial information on carrier and exciton dynamics that underpin a variety of potential applications of carbon nanotubes, from their use as current-carrying quantum wires, through light-emitting or detecting nanodevices, to their use in light-harvesting technologies and photovoltaics. Background information on the ultrafast spectroscopic techniques of greatest applicability to nanotubes is also provided.}}, author = {Ma, Ying-Zhong and Hertel, Tobias and Vardeny, Zeev Valy and Fleming, Graham R. and Valkunas, Leonas}, booktitle = {{CARBON NA NOTUBES: ADVANCED TOPICS IN THE SYNTHESIS, STRUCTURE, PROPERTIES AND APPLICATIONS}}, editor = {{Jorio, A and Dresselhaus, G and Dresselhaus, MS}}, keywords = {myown}, pages = {{321-352}}, series = {{Topics in Applied Physics}}, title = {{Ultrafast Spectroscopy of carbon nanotubes}}, volume = {{111}}, year = {{2008}} }
Length-dependent optical effects in single walled carbon nanotubes. Rajan, Aruna; Strano, Michael S.; Heller, Daniel A.; Hertel, Tobias; Schulten, Klaus. In JOURNAL OF PHYSICAL CHEMISTRY B, 112(19), pp. 6211–6213. 2008.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Recently, Heller et al. reported length-dependent effects on the relative photoluminescence (PL) quantum yield of single walled carbon nanotubes (SWNTs) [Heller et al J. Am. Chem. Soc. 2004, 126, 14567-14573]. We propose a simple model involving thermal diffusion of excitons along the nanotube axis and quenching at the ends, to explain the observed trend in their data. By fitting to our model, we extract a diffusion coefficient of 6 cm(2)/s for excitons in SWNTs. Assuming a mono exponential decay of exciton PL, we also predict that effective length-dependent PL lifetimes for these excitons lie in the range of 1 - 27 ps. Experimental observations are shown to be consistent with stochastic rather than wavepacket-like exciton migration, which is in agreement with ultrafast excitonic dephasing. Edge effects seem to limit the use of short SWNTs in imaging and optical sensing applications.

@article{rajan2008lengthdependent, abstract = {{Recently, Heller et al. reported length-dependent effects on the relative photoluminescence (PL) quantum yield of single walled carbon nanotubes (SWNTs) {[}Heller et al J. Am. Chem. Soc. 2004, 126, 14567-14573]. We propose a simple model involving thermal diffusion of excitons along the nanotube axis and quenching at the ends, to explain the observed trend in their data. By fitting to our model, we extract a diffusion coefficient of 6 cm(2)/s for excitons in SWNTs. Assuming a mono exponential decay of exciton PL, we also predict that effective length-dependent PL lifetimes for these excitons lie in the range of 1 - 27 ps. Experimental observations are shown to be consistent with stochastic rather than wavepacket-like exciton migration, which is in agreement with ultrafast excitonic dephasing. Edge effects seem to limit the use of short SWNTs in imaging and optical sensing applications.}}, author = {Rajan, Aruna and Strano, Michael S. and Heller, Daniel A. and Hertel, Tobias and Schulten, Klaus}, journal = {{JOURNAL OF PHYSICAL CHEMISTRY B}}, keywords = {myown}, month = {{MAY 15}}, number = {{19}}, pages = {{6211-6213}}, title = {{Length-dependent optical effects in single walled carbon nanotubes}}, volume = {{112}}, year = {{2008}} }

2007[ to top ]

Exciton dynamics in carbon nanotubes and nanotube aggregates. Hertel, Tobias; Crochet, Jared; Zhu, Zipeng; Arnold, Michael; Hersam, Mark C. In ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 233. Amer Chem Soc (ACS), Cellulose & Renewable Mat Div, 2007.
- [ BibTeX ]
@article{hertel2007exciton, author = {Hertel, Tobias and Crochet, Jared and Zhu, Zipeng and Arnold, Michael and Hersam, Mark C.}, journal = {{ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY}}, keywords = {swnt}, month = {{MAR 25}}, note = {{233rd National Meeting of the Cellulose-and-Renewable-Materials-Division of the American-Chemical-Society (ACS), Chicago, IL, MAR 25-29, 2007}}, organization = {{Amer Chem Soc (ACS), Cellulose \& Renewable Mat Div}}, title = {{Exciton dynamics in carbon nanotubes and nanotube aggregates}}, volume = {{233}}, year = {{2007}} }
Pump-probe spectroscopy of exciton dynamics in (6,5) carbon nanotubes. Zhu, Zipeng; Crochet, Jared; Arnold, Michael S.; Hersam, Mark C.; Ulbricht, Hendrick; Resasco, Daniel; Hertel, Tobias. In JOURNAL OF PHYSICAL CHEMISTRY C, 111(10), pp. 3831–3835. 2007.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We investigate exciton dynamics in isopycnically enriched (6,5) nanotube-DNA suspensions using femtosecond time-resolved pump-probe spectroscopy. The ground state recovery is characterized by a t(-0.45) +/- (0.03) power law behavior, indicative of a one-dimensional diffusion-limited reaction that is tentatively attributed to subdiffusive trapping of dark excitons. Spectral transients of bright (,40 singlet excitons within the Ell and E-22 manifolds exhibit a photobleach (PB) and a photoabsorption (PA) signal of similar strength. The PA is blue-shifted with respect to the PB-signal by 7.5 meV and is attributed to a transition from the dark singlet exciton (0)A(0) + to a (0)A(0)(-) + (0)A(0)(+) state within the two exciton E-11 manifold.

@article{zhu2007pumpprobe, abstract = {{We investigate exciton dynamics in isopycnically enriched (6,5) nanotube-DNA suspensions using femtosecond time-resolved pump-probe spectroscopy. The ground state recovery is characterized by a t(-0.45) +/- (0.03) power law behavior, indicative of a one-dimensional diffusion-limited reaction that is tentatively attributed to subdiffusive trapping of dark excitons. Spectral transients of bright (,40 singlet excitons within the Ell and E-22 manifolds exhibit a photobleach (PB) and a photoabsorption (PA) signal of similar strength. The PA is blue-shifted with respect to the PB-signal by 7.5 meV and is attributed to a transition from the dark singlet exciton (0)A(0) + to a (0)A(0)(-) + (0)A(0)(+) state within the two exciton E-11 manifold.}}, author = {Zhu, Zipeng and Crochet, Jared and Arnold, Michael S. and Hersam, Mark C. and Ulbricht, Hendrick and Resasco, Daniel and Hertel, Tobias}, journal = {{JOURNAL OF PHYSICAL CHEMISTRY C}}, keywords = {myown}, month = {{MAR 15}}, number = {{10}}, pages = {{3831-3835}}, title = {{Pump-probe spectroscopy of exciton dynamics in (6,5) carbon nanotubes}}, volume = {{111}}, year = {{2007}} }
Optical properties of structurally sorted single-wall carbon nanotube ensembles. Crochet, J.; Clemens, M.; Hertel, T. In PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 244(11), pp. 3964–3968. Univ Wien; MEFS Co Ltd; Rice Univ, Dept Chem, Appl Nanofluorescence; BRUKER OPTIK GmbH; HORIBA Jobin Yvon GmbH; NANOCYL S A; WILEY-VCH Verlag GmbH & Co, 2007.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Using density gradient ultracentrifugation and isopycnic fractionation we structurally sort traditionally prepared single-wall carbon nanotube suspensions, and show that fractions with photoluminescence (PL) quantum yields of over 1% can be isolated. Moreover, we find that traditionally prepared suspensions appear to be composed of a significant fraction of small nanotube aggregates which reduce the quantum yield of the ensembles by a factor of 5 with respect to the most photoluminescent fractions. In order to better understand the nature of the nonradiative decay processes that lead to lower ensemble PL quantum yields, nanotube ropes of diameter enriched tubes are made and compared with the aforementioned ensemble measurements. Changes in the optical properties are discussed in terms of energy transfer to metallic tubes, carbonaceous particles, and smaller band gap tubes. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.

@article{crochet2007optical, abstract = {{Using density gradient ultracentrifugation and isopycnic fractionation we structurally sort traditionally prepared single-wall carbon nanotube suspensions, and show that fractions with photoluminescence (PL) quantum yields of over 1\% can be isolated. Moreover, we find that traditionally prepared suspensions appear to be composed of a significant fraction of small nanotube aggregates which reduce the quantum yield of the ensembles by a factor of 5 with respect to the most photoluminescent fractions. In order to better understand the nature of the nonradiative decay processes that lead to lower ensemble PL quantum yields, nanotube ropes of diameter enriched tubes are made and compared with the aforementioned ensemble measurements. Changes in the optical properties are discussed in terms of energy transfer to metallic tubes, carbonaceous particles, and smaller band gap tubes. (c) 2007 WILEY-VCH Verlag GmbH \& Co. KGaA. Weinheim.}}, author = {Crochet, J. and Clemens, M. and Hertel, T.}, journal = {{PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}}, keywords = {myown}, month = {{NOV}}, note = {{21st International Winterschool on Electronic Properties of Novel Materials, Kirchberg, AUSTRIA, MAR 10-17, 2007}}, number = {{11}}, organization = {{Univ Wien; MEFS Co Ltd; Rice Univ, Dept Chem, Appl Nanofluorescence; BRUKER OPTIK GmbH; HORIBA Jobin Yvon GmbH; NANOCYL S A; WILEY-VCH Verlag GmbH \& Co}}, pages = {{3964-3968}}, title = {{Optical properties of structurally sorted single-wall carbon nanotube ensembles}}, volume = {{244}}, year = {{2007}} }

2006[ to top ]

Ultrafast dynamics of delocalized and localized electrons in carbon nanotubes. Perfetti, L; Kampfrath, T; Schapper, F; Hagen, A; Hertel, T; Aguirre, CM; Desjardins, P; Martel, R; Frischkorn, C; Wolf, M. In PHYSICAL REVIEW LETTERS, 96(2). 2006.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We report on the dynamics of the dielectric function of single-wall carbon nanotubes in the 10-30 THz frequency range after ultrafast laser excitation. The absence of a distinct free-carrier response is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. We find a feature of enhanced transmission caused by the blocking of optical transitions in small-gap tubes. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed.

@article{perfetti2006ultrafast, abstract = {{We report on the dynamics of the dielectric function of single-wall carbon nanotubes in the 10-30 THz frequency range after ultrafast laser excitation. The absence of a distinct free-carrier response is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. We find a feature of enhanced transmission caused by the blocking of optical transitions in small-gap tubes. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed.}}, author = {Perfetti, L and Kampfrath, T and Schapper, F and Hagen, A and Hertel, T and Aguirre, CM and Desjardins, P and Martel, R and Frischkorn, C and Wolf, M}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{JAN 20}}, number = {{2}}, title = {{Ultrafast dynamics of delocalized and localized electrons in carbon nanotubes}}, volume = {{96}}, year = {{2006}} }
Thermal desorption of gases and solvents from graphite and carbon nanotube surfaces. Ulbricht, Hendrik; Zacharia, Renju; Cindir, Nesibe; Hertel, Tobias. In CARBON, 44(14), pp. 2931–2942. 2006.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The interaction of 23 gases and solvents with the basal plane of highly oriented pyrolytic graphite (HOPG) and with single-wall carbon nanotube (SWCNT) samples is studied using thermal desorption spectroscopy. Pre-exponential frequency factors used for analysis of desorption traces are obtained from vapor pressure data. Activation energies for desorption at monolayer coverage are determined using the Redhead peak-maximum method. Binding energies of non-polar adsorbates to the HOPG surface are found to scale with the adsorbate polarizability providing clear evidence for the van der Waals character of the interaction. Low coverage desorption temperatures on SWCNT samples are found to be 50-100% higher than on HOPG. Such increase has previously been attributed to physisorption in higher coordinated sites such as grooves on the external SWCNT rope surfaces. Polar adsorbates on the other hand typically desorb at much higher temperatures from SWCNT samples which is here tentatively attributed to stronger interaction with defect sites. (c) 2006 Elsevier Ltd. All rights reserved.

@article{ulbricht2006thermal, abstract = {{The interaction of 23 gases and solvents with the basal plane of highly oriented pyrolytic graphite (HOPG) and with single-wall carbon nanotube (SWCNT) samples is studied using thermal desorption spectroscopy. Pre-exponential frequency factors used for analysis of desorption traces are obtained from vapor pressure data. Activation energies for desorption at monolayer coverage are determined using the Redhead peak-maximum method. Binding energies of non-polar adsorbates to the HOPG surface are found to scale with the adsorbate polarizability providing clear evidence for the van der Waals character of the interaction. Low coverage desorption temperatures on SWCNT samples are found to be 50-100\% higher than on HOPG. Such increase has previously been attributed to physisorption in higher coordinated sites such as grooves on the external SWCNT rope surfaces. Polar adsorbates on the other hand typically desorb at much higher temperatures from SWCNT samples which is here tentatively attributed to stronger interaction with defect sites. (c) 2006 Elsevier Ltd. All rights reserved.}}, author = {Ulbricht, Hendrik and Zacharia, Renju and Cindir, Nesibe and Hertel, Tobias}, journal = {{CARBON}}, keywords = {myown}, month = {{NOV}}, number = {{14}}, pages = {{2931-2942}}, title = {{Thermal desorption of gases and solvents from graphite and carbon nanotube surfaces}}, volume = {{44}}, year = {{2006}} }
Exciton dynamics probed in carbon nanotube suspensions with narrow diameter distribution. Hertel, Tobias; Zhu, Zipeng; Crochet, Jared; McPheeters, Claiborne; Ulbricht, Hendrik; Resasco, Daniel. In PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243(13), pp. 3186–3191. 2006.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We report on a pump-probe study of CoMoCAT nanotube suspensions with narrow chirality distribution. Visible pump pulses and a white light continuum are used for resonant excitation of the strongest dipole allowed E-22 subband exciton in the semiconducting (6, 5) tube and for broadband probe of the resulting spectral transients between 1300 nm and 480 nm, respectively. Transient spectra show signatures of both photobleaching (PB) and photoabsorption (PA) with practically identical decay- but slightly different rise-times. The experiments reveal that apparent variations of decay rates at different wavelengths do not reflect dynamics of different relaxation processes but are a consequence of the superposition of PB and blue-shifted PA response. (c) 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim.

@article{hertel2006exciton, abstract = {{We report on a pump-probe study of CoMoCAT nanotube suspensions with narrow chirality distribution. Visible pump pulses and a white light continuum are used for resonant excitation of the strongest dipole allowed E-22 subband exciton in the semiconducting (6, 5) tube and for broadband probe of the resulting spectral transients between 1300 nm and 480 nm, respectively. Transient spectra show signatures of both photobleaching (PB) and photoabsorption (PA) with practically identical decay- but slightly different rise-times. The experiments reveal that apparent variations of decay rates at different wavelengths do not reflect dynamics of different relaxation processes but are a consequence of the superposition of PB and blue-shifted PA response. (c) 2006 WILEY-VCH Verlag GmbH \& Co KGaA, Weinheim.}}, author = {Hertel, Tobias and Zhu, Zipeng and Crochet, Jared and McPheeters, Claiborne and Ulbricht, Hendrik and Resasco, Daniel}, journal = {{PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}}, keywords = {myown}, month = {{NOV}}, note = {{20th International Winterschool/Euroconference on Electronic Properties of Novel Materials, Kirchberg, AUSTRIA, MAR 04-11, 2006}}, number = {{13}}, pages = {{3186-3191}}, title = {{Exciton dynamics probed in carbon nanotube suspensions with narrow diameter distribution}}, volume = {{243}}, year = {{2006}} }

2005[ to top ]

Exponential decay lifetimes of excitons in individual single-walled carbon nanotubes. Hagen, A; Steiner, M; Raschke, MB; Lienau, C; Hertel, T; Qian, HH; Meixner, AJ; Hartschuh, A. In PHYSICAL REVIEW LETTERS, 95(19). 2005.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode.

@article{hagen2005exponential, abstract = {{The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode.}}, author = {Hagen, A and Steiner, M and Raschke, MB and Lienau, C and Hertel, T and Qian, HH and Meixner, AJ and Hartschuh, A}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{NOV 4}}, number = {{19}}, title = {{Exponential decay lifetimes of excitons in individual single-walled carbon nanotubes}}, volume = {{95}}, year = {{2005}} }
Spectroscopy of single- and double-wall carbon nanotubes in different environments. Hertel, T; Hagen, A; Talalaev, V; Arnold, K; Hennrich, F; Kappes, M; Rosenthal, S; McBride, J; Ulbricht, H; Flahaut, E. In NANO LETTERS, 5(3), pp. 511–514. 2005.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.

@article{hertel2005spectroscopy, abstract = {{Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.}}, author = {Hertel, T and Hagen, A and Talalaev, V and Arnold, K and Hennrich, F and Kappes, M and Rosenthal, S and McBride, J and Ulbricht, H and Flahaut, E}, journal = {{NANO LETTERS}}, keywords = {myown}, month = {{MAR}}, number = {{3}}, pages = {{511-514}}, title = {{Spectroscopy of single- and double-wall carbon nanotubes in different environments}}, volume = {{5}}, year = {{2005}} }

2004[ to top ]

Electronic structure and dynamics of optically excited single-wall carbon nanotubes. Hagen, A; Moos, G; Talalaev, V; Hertel, T. In APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 78(8), pp. 1137–1145. 2004.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We have studied the electronic structure and charge-carrier dynamics of individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using optical and electron-spectroscopic techniques. The electronic structure of semiconducting SWNTs in the band-gap region is analyzed using near-infrared absorption spectroscopy. A semi-empirical expression for E-11(S) transition energies, based on tight-binding calculations is found to give striking agreement with experimental data. Time-resolved PL from dispersed SWNT-micelles shows a decay with a time constant of about 15 ps. Using time-resolved photoemission we also find that the electron-phonon (e-ph) coupling in metallic tubes is characterized by a very small e-ph mass-enhancement of 0.0004. Ultrafast electron-electron scattering of photo-excited carriers in nanotube ropes is finally found to lead to internal thermalization of the electronic system within about 200 fs.

@article{hagen2004electronic, abstract = {{We have studied the electronic structure and charge-carrier dynamics of individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using optical and electron-spectroscopic techniques. The electronic structure of semiconducting SWNTs in the band-gap region is analyzed using near-infrared absorption spectroscopy. A semi-empirical expression for E-11(S) transition energies, based on tight-binding calculations is found to give striking agreement with experimental data. Time-resolved PL from dispersed SWNT-micelles shows a decay with a time constant of about 15 ps. Using time-resolved photoemission we also find that the electron-phonon (e-ph) coupling in metallic tubes is characterized by a very small e-ph mass-enhancement of 0.0004. Ultrafast electron-electron scattering of photo-excited carriers in nanotube ropes is finally found to lead to internal thermalization of the electronic system within about 200 fs.}}, author = {Hagen, A and Moos, G and Talalaev, V and Hertel, T}, journal = {{APPLIED PHYSICS A-MATERIALS SCIENCE \& PROCESSING}}, keywords = {myown}, month = {{MAY}}, number = {{8}}, pages = {{1137-1145}}, title = {{Electronic structure and dynamics of optically excited single-wall carbon nanotubes}}, volume = {{78}}, year = {{2004}} }
Carbon nanotubes - Building blocks of microelectronics of tomorrow?. Hertel, T. In NACHRICHTEN AUS DER CHEMIE, 52(2), pp. 137–140. 2004.
- [ BibTeX ]
- [ DOI ]
@article{hertel2004carbon, author = {Hertel, T}, journal = {{NACHRICHTEN AUS DER CHEMIE}}, keywords = {myown}, month = {{FEB}}, number = {{2}}, pages = {{137-140}}, title = {{Carbon nanotubes - Building blocks of microelectronics of tomorrow?}}, volume = {{52}}, year = {{2004}} }
Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons. Zacharia, R; Ulbricht, H; Hertel, T. In PHYSICAL REVIEW B, 69(15). 2004.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We have studied the interaction of polyaromatic hydrocarbons (PAHs) with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene, and ovalene at submonolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV, and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of two-dimensional adsorbate islands up to the desorption temperature. Preexponential frequency factors are found to be in the range 10(14)-10(21) s(-1) as obtained from both Falconer-Madix (isothermal desorption) analysis and Antoine's fit to vapor pressure data. The resulting binding energy per carbon atom of the PAH is 52+/-5 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is 61+/-5 meV/atom, which is considerably larger than previously reported experimental values.

@article{zacharia2004interlayer, abstract = {{We have studied the interaction of polyaromatic hydrocarbons (PAHs) with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene, and ovalene at submonolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV, and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of two-dimensional adsorbate islands up to the desorption temperature. Preexponential frequency factors are found to be in the range 10(14)-10(21) s(-1) as obtained from both Falconer-Madix (isothermal desorption) analysis and Antoine's fit to vapor pressure data. The resulting binding energy per carbon atom of the PAH is 52+/-5 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is 61+/-5 meV/atom, which is considerably larger than previously reported experimental values.}}, author = {Zacharia, R and Ulbricht, H and Hertel, T}, journal = {{PHYSICAL REVIEW B}}, keywords = {myown}, month = {{APR}}, number = {{15}}, title = {{Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons}}, volume = {{69}}, year = {{2004}} }

2003[ to top ]

Interaction of C-60 with carbon nanotubes and graphite. Ulbricht, H; Moos, G; Hertel, T. In PHYSICAL REVIEW LETTERS, 90(9). 2003.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The interaction of C-60 with single-wall carbon nanotubes (SWNTs) and graphite is studied experimentally by thermal desorption spectroscopy and theoretically by molecular-mechanics and molecular-dynamics calculations. The van der Waals parameters and force field for C-60-graphene and C-60-SWNT interactions are derived from the low-coverage C-60 binding energy to the graphite surface. We use these to compare the efficiency of different mechanisms by which C-60 can be encapsulated into SWNTs.

@article{ulbricht2003interaction, abstract = {{The interaction of C-60 with single-wall carbon nanotubes (SWNTs) and graphite is studied experimentally by thermal desorption spectroscopy and theoretically by molecular-mechanics and molecular-dynamics calculations. The van der Waals parameters and force field for C-60-graphene and C-60-SWNT interactions are derived from the low-coverage C-60 binding energy to the graphite surface. We use these to compare the efficiency of different mechanisms by which C-60 can be encapsulated into SWNTs.}}, author = {Ulbricht, H and Moos, G and Hertel, T}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{MAR 7}}, number = {{9}}, title = {{Interaction of C-60 with carbon nanotubes and graphite}}, volume = {{90}}, year = {{2003}} }
Temperature dependence of electron-to-lattice energy transfer in single-wall carbon nanotube bundles. Moos, G; Fasel, R; Hertel, T. In JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 3(1-2), pp. 145–149. 2003.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The electron-phonon coupling strength in single-wall carbon nanotube (SWNT) bundles has been studied directly in the time domain by femtosecond time-resolved photoelectron spectroscopy. We have measured the dependence of H(T-theta,T-I), the rate of energy transfer between the electronic system and the lattice as a function of electron and lattice temperatures T-theta and T-I. The experiments are consistent with a T-5 dependence of H on the electron and lattice temperatures, respectively. The results can be related to the e-ph mass enhancement parameter lambda. The experimentally obtained value for lambda/Theta(D)(2), where Theta(D) is the Debye temperature, suggests that e-ph scattering times at the Fermi level of SWNT bundles can be exceptionally long, exceeding 1.5 ps at room temperature.

@article{moos2003temperature, abstract = {{The electron-phonon coupling strength in single-wall carbon nanotube (SWNT) bundles has been studied directly in the time domain by femtosecond time-resolved photoelectron spectroscopy. We have measured the dependence of H(T-theta,T-I), the rate of energy transfer between the electronic system and the lattice as a function of electron and lattice temperatures T-theta and T-I. The experiments are consistent with a T-5 dependence of H on the electron and lattice temperatures, respectively. The results can be related to the e-ph mass enhancement parameter lambda. The experimentally obtained value for lambda/Theta(D)(2), where Theta(D) is the Debye temperature, suggests that e-ph scattering times at the Fermi level of SWNT bundles can be exceptionally long, exceeding 1.5 ps at room temperature.}}, author = {Moos, G and Fasel, R and Hertel, T}, journal = {{JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY}}, keywords = {myown}, month = {{FEB-APR}}, number = {{1-2}}, pages = {{145-149}}, title = {{Temperature dependence of electron-to-lattice energy transfer in single-wall carbon nanotube bundles}}, volume = {{3}}, year = {{2003}} }
Quantitative analysis of optical spectra from individual single-wall carbon nanotubes. Hagen, A; Hertel, T. In NANO LETTERS, 3(3), pp. 383–388. 2003.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We discuss how tight-binding band-structure calculations with a chirality- and diameter-dependent nearest-neighbor hopping integral may be used to relate well resolved features in the UV-VIS-NIR spectra of individual single-wall carbon nanotubes (SWNTs) to electronic excitations in specific tube types. The assignment of (n,m) indices to interband transitions in specific tube types can support a quantitative analysis of absorption spectra which may eventually be used for rapid screening and optimization of sample composition during SWNT synthesis.

@article{hagen2003quantitative, abstract = {{We discuss how tight-binding band-structure calculations with a chirality- and diameter-dependent nearest-neighbor hopping integral may be used to relate well resolved features in the UV-VIS-NIR spectra of individual single-wall carbon nanotubes (SWNTs) to electronic excitations in specific tube types. The assignment of (n,m) indices to interband transitions in specific tube types can support a quantitative analysis of absorption spectra which may eventually be used for rapid screening and optimization of sample composition during SWNT synthesis.}}, author = {Hagen, A and Hertel, T}, journal = {{NANO LETTERS}}, keywords = {myown}, month = {{MAR}}, number = {{3}}, pages = {{383-388}}, title = {{Quantitative analysis of optical spectra from individual single-wall carbon nanotubes}}, volume = {{3}}, year = {{2003}} }
Dynamics of C-60 encapsulation into single-wall carbon nanotubes. Ulbricht, H; Hertel, T. In JOURNAL OF PHYSICAL CHEMISTRY B, 107(51), pp. 14185–14190. 2003.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The encapsulation of C-60 into single-wall carbon nanotubes (SWNTs) is studied by molecular mechanics and kinetic Monte Carlo trajectory calculations van der Waals pair-potentials-experimentally derived from the interaction of C-60 with graphite-are used to calculate three-dimensional C-60-SWNT potential energy surfaces. We discuss encapsulation via different reaction paths through open tube ends or sidewall defects and compare capture of ballistic C-60 from the gas phase with encapsulation of C-60 preadsorbed on the surface of SWNT ropes. Encapsulation of preadsorbed C-60 is predicted to be the more likely process.

@article{ulbricht2003dynamics, abstract = {{The encapsulation of C-60 into single-wall carbon nanotubes (SWNTs) is studied by molecular mechanics and kinetic Monte Carlo trajectory calculations van der Waals pair-potentials-experimentally derived from the interaction of C-60 with graphite-are used to calculate three-dimensional C-60-SWNT potential energy surfaces. We discuss encapsulation via different reaction paths through open tube ends or sidewall defects and compare capture of ballistic C-60 from the gas phase with encapsulation of C-60 preadsorbed on the surface of SWNT ropes. Encapsulation of preadsorbed C-60 is predicted to be the more likely process.}}, author = {Ulbricht, H and Hertel, T}, journal = {{JOURNAL OF PHYSICAL CHEMISTRY B}}, keywords = {myown}, month = {{DEC 25}}, number = {{51}}, pages = {{14185-14190}}, title = {{Dynamics of C-60 encapsulation into single-wall carbon nanotubes}}, volume = {{107}}, year = {{2003}} }

2002[ to top ]

Charge-carrier dynamics in single-wall carbon nanotube bundles: a time-domain study. Hertel, T; Fasel, R; Moos, G. In APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 75(4), pp. 449–465. 2002.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We present a real-time investigation of ultra-fast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow us to study the processes governing the sub-picosecond and the picosecond dynamics of non-equilibrium charge carriers. On the sub-picosecond time scale the dynamics are dominated by ultra-fast electron-electron scattering processes, which lead to internal thermalization of the laser-excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi level - the highest energy studied experimentally. The subsequent cooling of the laser-heated electron gas to the lattice temperature by electron-phonon interaction occurs on the picosecond time scale and allows us to determine the electron-phonon mass enhancement parameter lambda. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper.

@article{hertel2002chargecarrier, abstract = {{We present a real-time investigation of ultra-fast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow us to study the processes governing the sub-picosecond and the picosecond dynamics of non-equilibrium charge carriers. On the sub-picosecond time scale the dynamics are dominated by ultra-fast electron-electron scattering processes, which lead to internal thermalization of the laser-excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi level - the highest energy studied experimentally. The subsequent cooling of the laser-heated electron gas to the lattice temperature by electron-phonon interaction occurs on the picosecond time scale and allows us to determine the electron-phonon mass enhancement parameter lambda. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper.}}, author = {Hertel, T and Fasel, R and Moos, G}, journal = {{APPLIED PHYSICS A-MATERIALS SCIENCE \& PROCESSING}}, keywords = {myown}, month = {{OCT}}, number = {{4}}, pages = {{449-465}}, title = {{Charge-carrier dynamics in single-wall carbon nanotube bundles: a time-domain study}}, volume = {{75}}, year = {{2002}} }
Physisorption of molecular oxygen on single-wall carbon nanotube bundles and graphite. Ulbricht, H; Moos, G; Hertel, T. In PHYSICAL REVIEW B, 66(7). 2002.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We present a study on the kinetics of oxygen adsorption and desorption from single-wall carbon nanotube (SWNT) and highly oriented pyrolytic graphite (HOPG) samples. Thermal-desorption spectra for SWNT samples show a broad desorption feature peaked at 62 K, which is shifted to a significantly higher temperature than the low-coverage desorption feature on HOPG. The low-coverage O-2 binding energy on SWNT bundles (18.5 kJ/mol) is 55% higher than that for adsorption on HOPG (12.0 kJ/mol). In combination with molecular mechanics calculations we show that the observed binding energies for both systems can be attributed to van der Waals interactions, i.e., physisorption. The experiments provide no evidence for a more strongly bound chemisorbed species or for dissociative oxygen adsorption.

@article{ulbricht2002physisorption, abstract = {{We present a study on the kinetics of oxygen adsorption and desorption from single-wall carbon nanotube (SWNT) and highly oriented pyrolytic graphite (HOPG) samples. Thermal-desorption spectra for SWNT samples show a broad desorption feature peaked at 62 K, which is shifted to a significantly higher temperature than the low-coverage desorption feature on HOPG. The low-coverage O-2 binding energy on SWNT bundles (18.5 kJ/mol) is 55\% higher than that for adsorption on HOPG (12.0 kJ/mol). In combination with molecular mechanics calculations we show that the observed binding energies for both systems can be attributed to van der Waals interactions, i.e., physisorption. The experiments provide no evidence for a more strongly bound chemisorbed species or for dissociative oxygen adsorption.}}, author = {Ulbricht, H and Moos, G and Hertel, T}, journal = {{PHYSICAL REVIEW B}}, keywords = {myown}, month = {{AUG 15}}, number = {{7}}, title = {{Physisorption of molecular oxygen on single-wall carbon nanotube bundles and graphite}}, volume = {{66}}, year = {{2002}} }
Desorption kinetics and interaction of Xe with single-wall carbon nanotube bundles. Ulbricht, H; Kriebel, J; Moos, G; Hertel, T. In CHEMICAL PHYSICS LETTERS, 363(3-4), pp. 252–260. 2002.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We present a study on the kinetics of xenon desorption from single-wall carbon nanotube (SWNT) bundles using thermal desorption spectroscopy (TDS). Desorption features from SWNT samples are broadened and peaked at higher temperature if compared to graphite. This can be explained using a coupled desorption-diffusion (CDD) model, which yields the low-coverage binding energy for Xe adsorption on SWNT bundles, 27 kJ mol(-1). The latter is about 25% higher than the monolayer binding energy on graphite, 21.9 kJ mol(-1). Using molecular mechanics calculations we find that this increase is consistent with adsorption in highly coordinated groove-sites on the external bundle surface or in endohedral sites inside of SWNTs. (C) 2002 Elsevier Science B.V. All rights reserved.

@article{ulbricht2002desorption, abstract = {{We present a study on the kinetics of xenon desorption from single-wall carbon nanotube (SWNT) bundles using thermal desorption spectroscopy (TDS). Desorption features from SWNT samples are broadened and peaked at higher temperature if compared to graphite. This can be explained using a coupled desorption-diffusion (CDD) model, which yields the low-coverage binding energy for Xe adsorption on SWNT bundles, 27 kJ mol(-1). The latter is about 25\% higher than the monolayer binding energy on graphite, 21.9 kJ mol(-1). Using molecular mechanics calculations we find that this increase is consistent with adsorption in highly coordinated groove-sites on the external bundle surface or in endohedral sites inside of SWNTs. (C) 2002 Elsevier Science B.V. All rights reserved.}}, author = {Ulbricht, H and Kriebel, J and Moos, G and Hertel, T}, journal = {{CHEMICAL PHYSICS LETTERS}}, keywords = {myown}, month = {{SEP 9}}, number = {{3-4}}, pages = {{252-260}}, title = {{Desorption kinetics and interaction of Xe with single-wall carbon nanotube bundles}}, volume = {{363}}, year = {{2002}} }

2001[ to top ]

Anisotropy of quasiparticle lifetimes and the role of disorder in graphite from ultrafast time-resolved photoemission spectroscopy. Moos, G; Gahl, C; Fasel, R; Wolf, M; Hertel, T. In PHYSICAL REVIEW LETTERS, 87(26). 2001.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 and 1.5 eV-the vicinity of a saddle point in the graphite band structure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g., defects or phonons, increases electron energy relaxation rates.

@article{moos2001anisotropy, abstract = {{Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 and 1.5 eV-the vicinity of a saddle point in the graphite band structure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g., defects or phonons, increases electron energy relaxation rates.}}, author = {Moos, G and Gahl, C and Fasel, R and Wolf, M and Hertel, T}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{DEC 24}}, number = {{26}}, title = {{Anisotropy of quasiparticle lifetimes and the role of disorder in graphite from ultrafast time-resolved photoemission spectroscopy}}, volume = {{87}}, year = {{2001}} }

2000[ to top ]

Electron-phonon interaction in single-wall carbon nanotubes: A time-domain study. Hertel, T; Moos, G. In PHYSICAL REVIEW LETTERS, 84(21), pp. 5002–5005. 2000.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We investigate the electron-phonon (e-ph) interaction in single-wall carbon nanotube samples at room temperature using femtosecond time-resolved photoemission. By probing electrons from the vicinity of the Fermi level we are able to study the e-ph interaction in the metallic nanotube species only. The observed electron dynamics can be used to calculate e-ph scattering matrix elements for twt, likely scattering scenarios: forward scattering from twistons and backscattering by longitudinal acoustic phonons. The corresponding matrix elements reveal an intrinsically weak e-ph interaction approximately 50% smaller than predicted by tight-binding calculations.

@article{hertel2000electronphonon, abstract = {{We investigate the electron-phonon (e-ph) interaction in single-wall carbon nanotube samples at room temperature using femtosecond time-resolved photoemission. By probing electrons from the vicinity of the Fermi level we are able to study the e-ph interaction in the metallic nanotube species only. The observed electron dynamics can be used to calculate e-ph scattering matrix elements for twt, likely scattering scenarios: forward scattering from twistons and backscattering by longitudinal acoustic phonons. The corresponding matrix elements reveal an intrinsically weak e-ph interaction approximately 50\% smaller than predicted by tight-binding calculations.}}, author = {Hertel, T and Moos, G}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{MAY 22}}, number = {{21}}, pages = {{5002-5005}}, title = {{Electron-phonon interaction in single-wall carbon nanotubes: A time-domain study}}, volume = {{84}}, year = {{2000}} }
Influence of excited electron lifetimes on the electronic structure of carbon nanotubes. Hertel, T; Moos, G. In CHEMICAL PHYSICS LETTERS, 320(3-4), pp. 359–364. 2000.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We have studied the dynamics of electrons in single-wall carbon nanotube (bucky paper) samples using femtosecond time-resolved photoemission. The lifetime of electrons excited to the pi* bands is found to decrease continuously from 130 fs at 0.2 eV down to less than 20 fs at energies above 1.5 eV with respect to the Fermi level. This should lead to a significant lifetime-induced broadening of the characteristic van Hove singularities in the nanotube density of states. (C) 2000 Elsevier Science B.V. All rights reserved.

@article{hertel2000influence, abstract = {{We have studied the dynamics of electrons in single-wall carbon nanotube (bucky paper) samples using femtosecond time-resolved photoemission. The lifetime of electrons excited to the pi{*} bands is found to decrease continuously from 130 fs at 0.2 eV down to less than 20 fs at energies above 1.5 eV with respect to the Fermi level. This should lead to a significant lifetime-induced broadening of the characteristic van Hove singularities in the nanotube density of states. (C) 2000 Elsevier Science B.V. All rights reserved.}}, author = {Hertel, T and Moos, G}, journal = {{CHEMICAL PHYSICS LETTERS}}, keywords = {myown}, month = {{APR 7}}, number = {{3-4}}, pages = {{359-364}}, title = {{Influence of excited electron lifetimes on the electronic structure of carbon nanotubes}}, volume = {{320}}, year = {{2000}} }

1999[ to top ]

Carbon nanotubes: nanomechanics, manipulation, and electronic devices. Avouris, P; Hertel, T; Martel, R; Schmidt, T; Shea, HR; Walkup, RE. In APPLIED SURFACE SCIENCE, 141(3-4), pp. 201–209. 1999.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Carbon nanotubes are novel materials with unique electrical and mechanical properties. Here we present results on their atomic structure and mechanical properties in the adsorbed state, on ways to manipulate individual nanotubes, on their electrical properties and, finally, on the fabrication and characteristics of nanotube-based electron devices. Specifically, atomic force microscopy (AFM) and molecular mechanics simulations are used to investigate the effects of van der Waals interactions on the atomic structure of adsorbed nanotubes. Both radial and axial structural deformations are identified and the interaction energy itself is obtained from the observed deformations. The conditions under which the structure of a nanotube will adjust to the topography of the substrate are defined. We show that the strong substrate-nanotube interaction allows the manipulation of both the position and shape of individual nanotubes at inert surfaces using the AFM. AFM manipulation is then utilized to position individual nanotubes on electrical pads so that their electrical characteristics can be evaluated. We demonstrate the operation of a field-effect transistor based on a single semiconducting nanotube and of a single-electron transistor using a nanotube bundle as Coulomb island. Finally, conducting nanotubes are employed as tips for AFM lithography. (C) 1999 Elsevier Science B.V. All rights reserved.

@article{avouris1999carbon, abstract = {{Carbon nanotubes are novel materials with unique electrical and mechanical properties. Here we present results on their atomic structure and mechanical properties in the adsorbed state, on ways to manipulate individual nanotubes, on their electrical properties and, finally, on the fabrication and characteristics of nanotube-based electron devices. Specifically, atomic force microscopy (AFM) and molecular mechanics simulations are used to investigate the effects of van der Waals interactions on the atomic structure of adsorbed nanotubes. Both radial and axial structural deformations are identified and the interaction energy itself is obtained from the observed deformations. The conditions under which the structure of a nanotube will adjust to the topography of the substrate are defined. We show that the strong substrate-nanotube interaction allows the manipulation of both the position and shape of individual nanotubes at inert surfaces using the AFM. AFM manipulation is then utilized to position individual nanotubes on electrical pads so that their electrical characteristics can be evaluated. We demonstrate the operation of a field-effect transistor based on a single semiconducting nanotube and of a single-electron transistor using a nanotube bundle as Coulomb island. Finally, conducting nanotubes are employed as tips for AFM lithography. (C) 1999 Elsevier Science B.V. All rights reserved.}}, author = {Avouris, P and Hertel, T and Martel, R and Schmidt, T and Shea, HR and Walkup, RE}, journal = {{APPLIED SURFACE SCIENCE}}, keywords = {myown}, month = {{MAR}}, note = {{International Symposium on Nano-Scale Modification of Surfaces, CTR JAPANESE AART \& TECHNOL MANGGHA, KRAKOW, POLAND, MAY 28-30, 1998}}, number = {{3-4}}, pages = {{201-209}}, title = {{Carbon nanotubes: nanomechanics, manipulation, and electronic devices}}, volume = {{141}}, year = {{1999}} }
Manipulation of carbon nanotubes and properties of nanotube field-effect transistors and rings. Shea, HR; Martel, R; Hertel, T; Schmidt, T; Avouris, P. In MICROELECTRONIC ENGINEERING, 46(1-4), pp. 101–104. 1999.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Using the tip of an atomic force microscope, we have manipulated individual carbon nanotubes on a patterned substrate, and have fabricated model nanodevices, including a room temperature field-effect transistor with a channel only 1.6 nm wide, as well as single-electron transistors. We have also developed a technique to produce rings of single-wall nanotubes with a very high yield.

@article{shea1999manipulation, abstract = {{Using the tip of an atomic force microscope, we have manipulated individual carbon nanotubes on a patterned substrate, and have fabricated model nanodevices, including a room temperature field-effect transistor with a channel only 1.6 nm wide, as well as single-electron transistors. We have also developed a technique to produce rings of single-wall nanotubes with a very high yield.}}, author = {Shea, HR and Martel, R and Hertel, T and Schmidt, T and Avouris, P}, journal = {{MICROELECTRONIC ENGINEERING}}, keywords = {myown}, month = {{MAY}}, note = {{International Conference on Micro- and Nanofabrication, LEUVEN, BELGIUM, SEP 22-24, 1998}}, number = {{1-4}}, pages = {{101-104}}, title = {{Manipulation of carbon nanotubes and properties of nanotube field-effect transistors and rings}}, volume = {{46}}, year = {{1999}} }

1998[ to top ]

Manipulation of individual carbon nanotubes and their interaction with surfaces. Hertel, T; Martel, R; Avouris, P. In JOURNAL OF PHYSICAL CHEMISTRY B, 102(6), pp. 910–915. 1998.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We demonstrate that the tip of an atomic force microscope (AFM) can be used to control the shape and position of individual multiwalled carbon nanotubes dispersed on a surface. Specifically we can bend, straighten, translate. rotate, and-under certain conditions-cut nanotubes. Such manipulations are feasible due to the interaction between nanotubes and the substrate, which can stabilize highly strained nanotube configurations. Direct evidence for this interaction is provided by the study of elastic distortions of tubes interacting with other tubes and the substrate, From the observed deformations of nanotubes with 100 Angstrom diameter. For example, we obtain a binding energy of 0.8 +/- 0.3 eV/Angstrom. This interaction forces nanotubes to conform to the structure of the substrate, and the resulting distortions should induce corresponding changes in their electronic structure and electrical transport properties.

@article{hertel1998manipulation, abstract = {{We demonstrate that the tip of an atomic force microscope (AFM) can be used to control the shape and position of individual multiwalled carbon nanotubes dispersed on a surface. Specifically we can bend, straighten, translate. rotate, and-under certain conditions-cut nanotubes. Such manipulations are feasible due to the interaction between nanotubes and the substrate, which can stabilize highly strained nanotube configurations. Direct evidence for this interaction is provided by the study of elastic distortions of tubes interacting with other tubes and the substrate, From the observed deformations of nanotubes with 100 Angstrom diameter. For example, we obtain a binding energy of 0.8 +/- 0.3 eV/Angstrom. This interaction forces nanotubes to conform to the structure of the substrate, and the resulting distortions should induce corresponding changes in their electronic structure and electrical transport properties.}}, author = {Hertel, T and Martel, R and Avouris, P}, journal = {{JOURNAL OF PHYSICAL CHEMISTRY B}}, keywords = {myown}, month = {{FEB 5}}, number = {{6}}, pages = {{910-915}}, title = {{Manipulation of individual carbon nanotubes and their interaction with surfaces}}, volume = {{102}}, year = {{1998}} }
AFM-tip-induced and current-induced local oxidation of silicon and metals. Avouris, P; Martel, R; Hertel, T; Sandstrom, R. In APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 66(2, S), pp. S659-S667. Free & Hanseat City Hamburg; German Min Educ Sci Res & Technol; German Sci Fdn; Commiss European Communities; Oxford Instruments GmbH; Omicron Vakuumphys GmbH; Topometrix GmbH; Digital Instruments, 1998.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Here we discuss two different processes that can be used to locally oxidize silicon or metals and are promising for the fabrication of model nanoelectronic devices. The first involves oxidation induced by a negatively biased conducting atomic force microscope (AFM) tip. We examine the kinetics and mechanism of this process and how factors such as the strength of the electric field, thickness of the oxide, and ambient humidity affect its fate and resolution. Weak ionic currents are detected, pointing to the electrochemical character of the process. Very fast initial oxidation rates are found to slow down dramatically as a result of the build up of stress and the reduction of the electric field strength. The lateral resolution is found to be largely determined by the defocusing of the electric field by a water him, surrounding the tip, whose extent is a function of ambient humidity. The second approach involves local oxidation induced by high current densities generated by forming constrictions in the current-carrying sample. This novel local oxidation process can be used to generate thin oxide tunneling barriers of 10-50 nm.

@article{avouris1998afmtipinduced, abstract = {{Here we discuss two different processes that can be used to locally oxidize silicon or metals and are promising for the fabrication of model nanoelectronic devices. The first involves oxidation induced by a negatively biased conducting atomic force microscope (AFM) tip. We examine the kinetics and mechanism of this process and how factors such as the strength of the electric field, thickness of the oxide, and ambient humidity affect its fate and resolution. Weak ionic currents are detected, pointing to the electrochemical character of the process. Very fast initial oxidation rates are found to slow down dramatically as a result of the build up of stress and the reduction of the electric field strength. The lateral resolution is found to be largely determined by the defocusing of the electric field by a water him, surrounding the tip, whose extent is a function of ambient humidity. The second approach involves local oxidation induced by high current densities generated by forming constrictions in the current-carrying sample. This novel local oxidation process can be used to generate thin oxide tunneling barriers of 10-50 nm.}}, author = {Avouris, P and Martel, R and Hertel, T and Sandstrom, R}, journal = {{APPLIED PHYSICS A-MATERIALS SCIENCE \& PROCESSING}}, keywords = {myown}, month = {{MAR}}, note = {{9th International Conference on Scanning Tunneling Microscopy/Spectroscopy and Related Techniques, HAMBURG, GERMANY, JUL 20-25, 1997}}, number = {{2, S}}, organization = {{Free \& Hanseat City Hamburg; German Min Educ Sci Res \& Technol; German Sci Fdn; Commiss European Communities; Oxford Instruments GmbH; Omicron Vakuumphys GmbH; Topometrix GmbH; Digital Instruments}}, pages = {{S659-S667}}, title = {{AFM-tip-induced and current-induced local oxidation of silicon and metals}}, volume = {{66}}, year = {{1998}} }
Deformation of carbon nanotubes by surface van der Waals forces. Hertel, T; Walkup, RE; Avouris, P. In PHYSICAL REVIEW B, 58(20), pp. 13870–13873. 1998.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The strength and effect of surface van der Waals forces on the shape of multiwalled and single-walled carbon nanotubes is investigated using atomic-force microscopy, continuum mechanics, and molecular-mechanics simulations. Our calculations show that depending on the tube diameter and number of shells, the van der Waals interaction between nanotubes and a substrate results in high binding energies, which has also been determined experimentally. Nanotubes on a substrate may consequently experience radial and axial deformations, which significantly modify the idealized geometry of free nanotubes. These findings have implications for electronic transport and the tribological properties of adsorbed nanotubes. [S0163-1829(98)05744-0].

@article{hertel1998deformation, abstract = {{The strength and effect of surface van der Waals forces on the shape of multiwalled and single-walled carbon nanotubes is investigated using atomic-force microscopy, continuum mechanics, and molecular-mechanics simulations. Our calculations show that depending on the tube diameter and number of shells, the van der Waals interaction between nanotubes and a substrate results in high binding energies, which has also been determined experimentally. Nanotubes on a substrate may consequently experience radial and axial deformations, which significantly modify the idealized geometry of free nanotubes. These findings have implications for electronic transport and the tribological properties of adsorbed nanotubes. {[}S0163-1829(98)05744-0].}}, author = {Hertel, T and Walkup, RE and Avouris, P}, journal = {{PHYSICAL REVIEW B}}, keywords = {myown}, month = {{NOV 15}}, number = {{20}}, pages = {{13870-13873}}, title = {{Deformation of carbon nanotubes by surface van der Waals forces}}, volume = {{58}}, year = {{1998}} }
Single- and multi-wall carbon nanotube field-effect transistors. Martel, R; Schmidt, T; Shea, HR; Hertel, T; Avouris, P. In APPLIED PHYSICS LETTERS, 73(17), pp. 2447–2449. 1998.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
We fabricated field-effect transistors based on individual single-and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations-in our case a collapsed tube-can make them operate as field-effect transistors. (C) 1998 American Institute of Physics. [S0003-6951(98)00143-0].

@article{martel1998single, abstract = {{We fabricated field-effect transistors based on individual single-and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations-in our case a collapsed tube-can make them operate as field-effect transistors. (C) 1998 American Institute of Physics. {[}S0003-6951(98)00143-0].}}, author = {Martel, R and Schmidt, T and Shea, HR and Hertel, T and Avouris, P}, journal = {{APPLIED PHYSICS LETTERS}}, keywords = {myown}, month = {{OCT 26}}, number = {{17}}, pages = {{2447-2449}}, title = {{Single- and multi-wall carbon nanotube field-effect transistors}}, volume = {{73}}, year = {{1998}} }

1997[ to top ]

Femtosecond time-resolved photoemission of electron dynamics in surface Rydberg states. Hertel, T; Knoesel, E; Hotzel, A; Wolf, M; Ertl, G. In JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A, 15(3, 2), pp. 1503–1509. Amer Vacuum Soc, 1997.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Femtosecond time-resolved photoelectron spectroscopy provides a unique tool to study the dynamics of optically excited electrons at surfaces directly in the time domain. We present a new model for two-photon photoelectron spectroscopy from surface and image potential (or Rydberg) states which is based on density matrix theory. The formalism accounts for the influence of both energy and phase relaxation on experimental spectra and thus permits the study of the nature of inelastic and elastic scattering processes at surfaces in more detail. The analysis of experimental data employing the proposed model reveals a new mechanism for optical excitation of electrons to normally unoccupied states at surfaces which is feasible due to the influence of electronic dephasing. We discuss the nature of different relaxation channels with respect to our studies of image state dynamics on the bare and Xe or Kr covered Cu(lll) surfaces. (C) 1997 American Vacuum Society.

@article{hertel1997femtosecond, abstract = {{Femtosecond time-resolved photoelectron spectroscopy provides a unique tool to study the dynamics of optically excited electrons at surfaces directly in the time domain. We present a new model for two-photon photoelectron spectroscopy from surface and image potential (or Rydberg) states which is based on density matrix theory. The formalism accounts for the influence of both energy and phase relaxation on experimental spectra and thus permits the study of the nature of inelastic and elastic scattering processes at surfaces in more detail. The analysis of experimental data employing the proposed model reveals a new mechanism for optical excitation of electrons to normally unoccupied states at surfaces which is feasible due to the influence of electronic dephasing. We discuss the nature of different relaxation channels with respect to our studies of image state dynamics on the bare and Xe or Kr covered Cu(lll) surfaces. (C) 1997 American Vacuum Society.}}, author = {Hertel, T and Knoesel, E and Hotzel, A and Wolf, M and Ertl, G}, journal = {{JOURNAL OF VACUUM SCIENCE \& TECHNOLOGY A}}, keywords = {myown}, month = {{MAY-JUN}}, note = {{43rd American-Vacuum-Society Symposium, PHILADELPHIA, PA, OCT 14-18, 1996}}, number = {{3, 2}}, organization = {{Amer Vacuum Soc}}, pages = {{1503-1509}}, title = {{Femtosecond time-resolved photoemission of electron dynamics in surface Rydberg states}}, volume = {{15}}, year = {{1997}} }
Atomic force microscope tip-induced local oxidation of silicon: Kinetics, mechanism, and nanofabrication. Avouris, P; Hertel, T; Martel, R. In APPLIED PHYSICS LETTERS, 71(2), pp. 285–287. 1997.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Atomic force microscope induced local oxidation of silicon is a process with a strong potential for use in proximal probe nanofabrication. Here we examine its kinetics and mechanism and how such factors as the strength of the electric field, ambient humidity, and thickness of the oxide affect its rate and resolution. Detection of electrochemical currents proves the anodization character of the process. Initial very fast oxidation rates are shown to slow down dramatically as a result of a self-limiting behavior resulting from the build up of stress and a reduction of the electric field strength. The lateral resolution is determined by the defocusing of the electric field in a condensed water film whose extent is a function of ambient humidity. (C) 1997 American Institute of Physics.

@article{avouris1997atomic, abstract = {{Atomic force microscope induced local oxidation of silicon is a process with a strong potential for use in proximal probe nanofabrication. Here we examine its kinetics and mechanism and how such factors as the strength of the electric field, ambient humidity, and thickness of the oxide affect its rate and resolution. Detection of electrochemical currents proves the anodization character of the process. Initial very fast oxidation rates are shown to slow down dramatically as a result of a self-limiting behavior resulting from the build up of stress and a reduction of the electric field strength. The lateral resolution is determined by the defocusing of the electric field in a condensed water film whose extent is a function of ambient humidity. (C) 1997 American Institute of Physics.}}, author = {Avouris, P and Hertel, T and Martel, R}, journal = {{APPLIED PHYSICS LETTERS}}, keywords = {myown}, month = {{JUL 14}}, number = {{2}}, pages = {{285-287}}, title = {{Atomic force microscope tip-induced local oxidation of silicon: Kinetics, mechanism, and nanofabrication}}, volume = {{71}}, year = {{1997}} }

1996[ to top ]

Dynamics of photoexcited electrons in metals studied with time-resolved two-photon photoemission. Knoesel, E; Hotzel, A; Hertel, T; Wolf, M; Ertl, G. In SURFACE SCIENCE, 368, pp. 76–81. 1996.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Femtosecond time-resolved two-photon photoemission (2PPE) is used to investigate the hot electron dynamics of Cu(111), Ag(110) and Ta(poly). The experimentally derived relaxation rates of photoexcited electrons scale with the available phase space for scattering with electrons below the Fermi level and are compared with predictions from Fermi liquid theory. A simulation employing rate equations to account for hot electron lifetimes, transport effects and the creation of secondary electrons demonstrates that the relaxation dynamics is strongly influenced by secondary electron cascades. With Ag(110) a pronounced temperature dependence of the photoemission yield is observed, indicating the importance of electron-phonon scattering for momentum conservation in the 2PPE process.

@article{knoesel1996dynamics, abstract = {{Femtosecond time-resolved two-photon photoemission (2PPE) is used to investigate the hot electron dynamics of Cu(111), Ag(110) and Ta(poly). The experimentally derived relaxation rates of photoexcited electrons scale with the available phase space for scattering with electrons below the Fermi level and are compared with predictions from Fermi liquid theory. A simulation employing rate equations to account for hot electron lifetimes, transport effects and the creation of secondary electrons demonstrates that the relaxation dynamics is strongly influenced by secondary electron cascades. With Ag(110) a pronounced temperature dependence of the photoemission yield is observed, indicating the importance of electron-phonon scattering for momentum conservation in the 2PPE process.}}, author = {Knoesel, E and Hotzel, A and Hertel, T and Wolf, M and Ertl, G}, journal = {{SURFACE SCIENCE}}, keywords = {myown}, month = {{DEC 10}}, note = {{8th International Conference on Vibrations at Surfaces, UNIV BIRMINGHAM, BIRMINGHAM, ENGLAND, JUN 23-27, 1996}}, pages = {{76-81}}, title = {{Dynamics of photoexcited electrons in metals studied with time-resolved two-photon photoemission}}, volume = {{368}}, year = {{1996}} }
Ultrafast electron dynamics at Cu(111): Response of an electron gas to optical excitation. Hertel, T; Knoesel, E; Wolf, M; Ertl, G. In PHYSICAL REVIEW LETTERS, 76(3), pp. 535–538. 1996.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Time-resolved two-photon photoemission is used to directly investigate the electron dynamics at a Cu(111) surface with 60 fs laser pulses. We find that the time evolution of the photoexcited electron population in the first image state can be described only by solving the optical Bloch equations to properly account for coherence in the excitation process. Our experiments also provide evidence that the dynamics of photoexcited bulk electrons is strongly influenced by hot electron cascades and that the initial relaxation rates are in agreement with Fermi liquid theory.

@article{hertel1996ultrafast, abstract = {{Time-resolved two-photon photoemission is used to directly investigate the electron dynamics at a Cu(111) surface with 60 fs laser pulses. We find that the time evolution of the photoexcited electron population in the first image state can be described only by solving the optical Bloch equations to properly account for coherence in the excitation process. Our experiments also provide evidence that the dynamics of photoexcited bulk electrons is strongly influenced by hot electron cascades and that the initial relaxation rates are in agreement with Fermi liquid theory.}}, author = {Hertel, T and Knoesel, E and Wolf, M and Ertl, G}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{JAN 15}}, number = {{3}}, pages = {{535-538}}, title = {{Ultrafast electron dynamics at Cu(111): Response of an electron gas to optical excitation}}, volume = {{76}}, year = {{1996}} }
Ultrafast dynamics of electrons in image-potential states on clean and Xe-covered Cu(111). Wolf, M; Knoesel, E; Hertel, T. In PHYSICAL REVIEW B, 54(8), pp. R5295-R5298. 1996.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Lifetimes of electrons in the n=1 and n=2 image states on Cu(111) are studied with femtosecond time-resolved photoemission. Adsorption of one monolayer of Xe results in a pronounced increase of the image-state lifetime, which for the n=1 state changes from 18+/-5 fs at clean Cu(111) to 75+/-15 fs at the Xe-covered surface. The slower relaxation rate induced by the Xe layer is attributed to a reduced overlap of the image-state wave function with bulk states. A density-matrix calculation reveals the importance of dephasing in the excitation process.

@article{wolf1996ultrafast, abstract = {{Lifetimes of electrons in the n=1 and n=2 image states on Cu(111) are studied with femtosecond time-resolved photoemission. Adsorption of one monolayer of Xe results in a pronounced increase of the image-state lifetime, which for the n=1 state changes from 18+/-5 fs at clean Cu(111) to 75+/-15 fs at the Xe-covered surface. The slower relaxation rate induced by the Xe layer is attributed to a reduced overlap of the image-state wave function with bulk states. A density-matrix calculation reveals the importance of dephasing in the excitation process.}}, author = {Wolf, M and Knoesel, E and Hertel, T}, journal = {{PHYSICAL REVIEW B}}, keywords = {myown}, month = {{AUG 15}}, number = {{8}}, pages = {{R5295-R5298}}, title = {{Ultrafast dynamics of electrons in image-potential states on clean and Xe-covered Cu(111)}}, volume = {{54}}, year = {{1996}} }
Angular distributions in stimulated desorption: Photodesorption of ammonia from a Cu(111) surface. Hertel, T; Wolf, M; Ertl, G. In CHEMICAL PHYSICS LETTERS, 257(1-2), pp. 155–162. 1996.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The angular distributions of flux and mean translational energy in stimulated desorption contain valuable information about the influence of substrate corrugation on the desorption dynamics. We present a simple model which predicts angular distributions of desorbing neutrals if the particle momentum parallel to the surface is conserved during the desorption process. The model allows the calculation of the temperature dependence of angular distributions and the observed decrease in translational energy with increasing angle of desorption. For the photostimulated desorption of NH3 from Cu(111) at two sample temperatures our analysis shows that substrate corrugation has no significant influence on the angular flux distributions which are essentially broadened by thermal motions of the molecules within the surface plane, i.e. due to the frustrated translation.

@article{hertel1996angular, abstract = {{The angular distributions of flux and mean translational energy in stimulated desorption contain valuable information about the influence of substrate corrugation on the desorption dynamics. We present a simple model which predicts angular distributions of desorbing neutrals if the particle momentum parallel to the surface is conserved during the desorption process. The model allows the calculation of the temperature dependence of angular distributions and the observed decrease in translational energy with increasing angle of desorption. For the photostimulated desorption of NH3 from Cu(111) at two sample temperatures our analysis shows that substrate corrugation has no significant influence on the angular flux distributions which are essentially broadened by thermal motions of the molecules within the surface plane, i.e. due to the frustrated translation.}}, author = {Hertel, T and Wolf, M and Ertl, G}, journal = {{CHEMICAL PHYSICS LETTERS}}, keywords = {myown}, month = {{JUL 19}}, number = {{1-2}}, pages = {{155-162}}, title = {{Angular distributions in stimulated desorption: Photodesorption of ammonia from a Cu(111) surface}}, volume = {{257}}, year = {{1996}} }
Internal quantum state distributions of NH3 photodesorbed from Cu(111) at 6.4 eV. Nessler, W; Bornscheuer, KH; Hertel, T; Hasselbrink, E. In CHEMICAL PHYSICS, 205(1-2), pp. 205–219. 1996.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The UV photodesorption (6.4 eV) of ammonia from Cu(111) has been investigated using resonance enhanced multiphoton ionization through the (B) over tilde <-- (X) over tilde transition for state specific detection. We have measured the relative populations of both inversion doublet states of the first three vibrational levels in the nu(2), mode and the rotational excitation of photodesorbed NH3. The average energy content in the nu(2) mode corresponds to a temperature of T-nu 2 = 1000 K which is significantly larger than the translational energy released normal to the surface [E(trans)]/2k = 600 K. A marked underpopulation of the antisymmetric nu(2) levels has been observed. The rotations can be described by a Boltzmann distribution with a temperature close to the surface temperature indicating that the molecules do not experience significant torque during the desorption process. No pronounced correlation effects in the population of different degrees of freedom are found. These experimental results are consistent with the assumption that the energy required for desorption is transferred from the initial electronic excitation to the molecule-surface bond via the intramolecular coordinate nu(2).

@article{nessler1996internal, abstract = {{The UV photodesorption (6.4 eV) of ammonia from Cu(111) has been investigated using resonance enhanced multiphoton ionization through the (B) over tilde <– (X) over tilde transition for state specific detection. We have measured the relative populations of both inversion doublet states of the first three vibrational levels in the nu(2), mode and the rotational excitation of photodesorbed NH3. The average energy content in the nu(2) mode corresponds to a temperature of T-nu 2 = 1000 K which is significantly larger than the translational energy released normal to the surface {[}E(trans)]/2k = 600 K. A marked underpopulation of the antisymmetric nu(2) levels has been observed. The rotations can be described by a Boltzmann distribution with a temperature close to the surface temperature indicating that the molecules do not experience significant torque during the desorption process. No pronounced correlation effects in the population of different degrees of freedom are found. These experimental results are consistent with the assumption that the energy required for desorption is transferred from the initial electronic excitation to the molecule-surface bond via the intramolecular coordinate nu(2).}}, author = {Nessler, W and Bornscheuer, KH and Hertel, T and Hasselbrink, E}, journal = {{CHEMICAL PHYSICS}}, keywords = {myown}, month = {{APR 15}}, number = {{1-2}}, pages = {{205-219}}, title = {{Internal quantum state distributions of NH3 photodesorbed from Cu(111) at 6.4 eV}}, volume = {{205}}, year = {{1996}} }

1995[ to top ]

FEMTOSECOND DYNAMICS OF ELECTRONIC EXCITATIONS OF ADSORBATES STUDIED BY 2-PHOTON PHOTOEMISSION PULSE CORRELATION - CO/CU(111). KNOESEL, E; HERTEL, T; WOLF, M; ERTL, G. In CHEMICAL PHYSICS LETTERS, 240(5-6), pp. 409–416. 1995.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
An equal pulse correlation technique based on angle-resolved two-photon photoemission is employed to investigate the lifetime of electronic excitations of an adsorbate on a single crystal metal surface. Photoemission from an occupied surface state on Cu(111) by a non-resonant two-photon process via the sp-band gap is used to characterize directly at the surface the width and shape of 65 fs laser pulses at a photon energy of h nu = 3.4 eV. The 2PPE correlation technique allows the establishment of an upper limit of tau < 20 fs for the lifetime of the 2 pi* resonance of CO adsorbed on Cu(111), whereas from the spectral width of the 2 pi* level a lower limit of approximate to 1 fs is estimated.

@article{knoesel1995femtosecond, abstract = {{An equal pulse correlation technique based on angle-resolved two-photon photoemission is employed to investigate the lifetime of electronic excitations of an adsorbate on a single crystal metal surface. Photoemission from an occupied surface state on Cu(111) by a non-resonant two-photon process via the sp-band gap is used to characterize directly at the surface the width and shape of 65 fs laser pulses at a photon energy of h nu = 3.4 eV. The 2PPE correlation technique allows the establishment of an upper limit of tau < 20 fs for the lifetime of the 2 pi{*} resonance of CO adsorbed on Cu(111), whereas from the spectral width of the 2 pi{*} level a lower limit of approximate to 1 fs is estimated.}}, author = {KNOESEL, E and HERTEL, T and WOLF, M and ERTL, G}, journal = {{CHEMICAL PHYSICS LETTERS}}, keywords = {myown}, month = {{JUL 7}}, number = {{5-6}}, pages = {{409-416}}, title = {{FEMTOSECOND DYNAMICS OF ELECTRONIC EXCITATIONS OF ADSORBATES STUDIED BY 2-PHOTON PHOTOEMISSION PULSE CORRELATION - CO/CU(111)}}, volume = {{240}}, year = {{1995}} }
ISOTOPE EFFECTS IN UV-PHOTOSTIMULATED DESORPTION OF AMMONIA FROM CU(111). WOLF, M; HERTEL, T; ERTL, G. In NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 101(1-2), pp. 56–59. Int Union Vacuum Sci Tech & Applicat, 1995.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
For UV photoinduced desorption of ammonia and deuterated ammonia from Cu (111) at hv = 6.4 eV, strong isotope effects are found, which increase from sigma(NH3)/sigma(ND3) = 1.9 +/- 0.5 to 4.1 +/- 1.2 when the coverage is decreased from 0.14 to 0.02 monolayer (ML), respectively. The coverage dependence of the isotope effect is attributed to a variation of the desorption probability with adsorbate binding energy. The magnitude of the isotope effect suggests that nuclear motion along an intramolecular coordinate during a short-lived electronic excitation facilitates desorption. We propose that after deexcitation to the electronic ground state potential energy surface, efficient coupling of the highly vibrational excited inversion mode with the molecule-surface coordinate leads to desorption. Trajectory calculations performed on a two-dimensional model potential energy surface predict that desorption occurs rapidly within a few vibrational periods of the umbrella mode (T-vib similar to 35 fs) - with comparable energy release into the translational and vibrational degrees of freedom.

@article{wolf1995isotope, abstract = {{For UV photoinduced desorption of ammonia and deuterated ammonia from Cu (111) at hv = 6.4 eV, strong isotope effects are found, which increase from sigma(NH3)/sigma(ND3) = 1.9 +/- 0.5 to 4.1 +/- 1.2 when the coverage is decreased from 0.14 to 0.02 monolayer (ML), respectively. The coverage dependence of the isotope effect is attributed to a variation of the desorption probability with adsorbate binding energy. The magnitude of the isotope effect suggests that nuclear motion along an intramolecular coordinate during a short-lived electronic excitation facilitates desorption. We propose that after deexcitation to the electronic ground state potential energy surface, efficient coupling of the highly vibrational excited inversion mode with the molecule-surface coordinate leads to desorption. Trajectory calculations performed on a two-dimensional model potential energy surface predict that desorption occurs rapidly within a few vibrational periods of the umbrella mode (T-vib similar to 35 fs) - with comparable energy release into the translational and vibrational degrees of freedom.}}, author = {WOLF, M and HERTEL, T and ERTL, G}, journal = {{NUCLEAR INSTRUMENTS \& METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}}, keywords = {myown}, month = {{JUN}}, note = {{6th International Workshop on Desorption Induced by Electronic Transitions (DIET VI), JAGIELLONIAN UNIV CONF CTR, PRZEGORZALY, POLAND, SEP 26-29, 1994}}, number = {{1-2}}, organization = {{Int Union Vacuum Sci Tech \& Applicat}}, pages = {{56-59}}, title = {{ISOTOPE EFFECTS IN UV-PHOTOSTIMULATED DESORPTION OF AMMONIA FROM CU(111)}}, volume = {{101}}, year = {{1995}} }
STRUCTURAL ASPECTS OF CESIUM-OXYGEN PHASES ON RU(0001). BLUDAU, H; OVER, H; HERTEL, T; GIERER, M; ERTL, G. In SURFACE SCIENCE, 342(1-3), pp. 134–154. 1995.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The adsorption of oxygen on the Ru(0001) surface precovered with submonolayer concentrations of cesium was investigated by means of low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES). The strong interaction between Cs and O manifests itself in the appearance of a diversity of ordered structures as indicated in a complex Theta(Cs)-Theta(O), phase diagram. The main emphasis has been put on structural aspects of these Cs-O overlayers. Real-space models for several of them are suggested on the basis of fully dynamical LEED calculations, quasi-kinematic LEED simulations and by applying the concept of LEED fingerprinting. The models are discussed in terms of their resemblance to Cs oxide bulk structures. Striking similarities to the CsO2 crystal structure have indeed been found for the oxygen-rich Cs-O phases on the Ru surface, namely the (3 x 2 root 3)rect and the (root 7 x root 7)R19.1 degrees structures, which are observed al a stoichiometry of two oxygen atoms per Cs atom. The common structural element consists of Cs-O-O chains being either twisted or linear. The (root 7 x root 7)R19.1 degrees-Cs-O phase can be observed over a wide Cs coverage range with optimum order always accomplished at an atomic ratio Cs:O=1:2.

@article{bludau1995structural, abstract = {{The adsorption of oxygen on the Ru(0001) surface precovered with submonolayer concentrations of cesium was investigated by means of low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES). The strong interaction between Cs and O manifests itself in the appearance of a diversity of ordered structures as indicated in a complex Theta(Cs)-Theta(O), phase diagram. The main emphasis has been put on structural aspects of these Cs-O overlayers. Real-space models for several of them are suggested on the basis of fully dynamical LEED calculations, quasi-kinematic LEED simulations and by applying the concept of LEED fingerprinting. The models are discussed in terms of their resemblance to Cs oxide bulk structures. Striking similarities to the CsO2 crystal structure have indeed been found for the oxygen-rich Cs-O phases on the Ru surface, namely the (3 x 2 root 3)rect and the (root 7 x root 7)R19.1 degrees structures, which are observed al a stoichiometry of two oxygen atoms per Cs atom. The common structural element consists of Cs-O-O chains being either twisted or linear. The (root 7 x root 7)R19.1 degrees-Cs-O phase can be observed over a wide Cs coverage range with optimum order always accomplished at an atomic ratio Cs:O=1:2.}}, author = {BLUDAU, H and OVER, H and HERTEL, T and GIERER, M and ERTL, G}, journal = {{SURFACE SCIENCE}}, keywords = {myown}, month = {{NOV 20}}, number = {{1-3}}, pages = {{134-154}}, title = {{STRUCTURAL ASPECTS OF CESIUM-OXYGEN PHASES ON RU(0001)}}, volume = {{342}}, year = {{1995}} }
UV PHOTOSTIMULATED DESORPTION OF AMMONIA FROM CU(111). HERTEL, T; WOLF, M; ERTL, G. In JOURNAL OF CHEMICAL PHYSICS, 102(8), pp. 3414–3430. 1995.
- [ BibTeX ]
- [ DOI ]
@article{hertel1995photostimulated, author = {HERTEL, T and WOLF, M and ERTL, G}, journal = {{JOURNAL OF CHEMICAL PHYSICS}}, keywords = {myown}, month = {{FEB 22}}, number = {{8}}, pages = {{3414-3430}}, title = {{UV PHOTOSTIMULATED DESORPTION OF AMMONIA FROM CU(111)}}, volume = {{102}}, year = {{1995}} }
ELECTRONIC EXCITATIONS OF ADSORBATES ANALYZED WITH 2-PHOTON PHOTOEMISSION - CO/CU(111). HERTEL, T; KNOESEL, E; WOLF, M; ERTL, G. In NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 101(1-2), pp. 53–55. Int Union Vacuum Sci Tech & Applicat, 1995.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Photoinduced excitations of normally unoccupied electronic states of adsorbates are investigated with two-photon photoemission (2PPE). We report 2PPE spectra from an ordered (1.4 x 1.4)-CO layer on Cu(111) for excitation with 3.5 eV less than or equal to hv less than or equal to 4.5eV photon energy. The energetic position, E(kin), of a CO induced state is found to shift as Delta E(kin) = 1 x Delta hv. This provides evidence for emission from a normally unoccupied state, located (3.3 +/- 0.2) eV above the Fermi level. Based on the polarization dependence of the emission from this adsorbate-induced state it is identified as the 2 pi* affinity level of chemisorbed CO on Cu(111) which is temporarily populated in the 2PPE process.

@article{hertel1995electronic, abstract = {{Photoinduced excitations of normally unoccupied electronic states of adsorbates are investigated with two-photon photoemission (2PPE). We report 2PPE spectra from an ordered (1.4 x 1.4)-CO layer on Cu(111) for excitation with 3.5 eV less than or equal to hv less than or equal to 4.5eV photon energy. The energetic position, E(kin), of a CO induced state is found to shift as Delta E(kin) = 1 x Delta hv. This provides evidence for emission from a normally unoccupied state, located (3.3 +/- 0.2) eV above the Fermi level. Based on the polarization dependence of the emission from this adsorbate-induced state it is identified as the 2 pi{*} affinity level of chemisorbed CO on Cu(111) which is temporarily populated in the 2PPE process.}}, author = {HERTEL, T and KNOESEL, E and WOLF, M and ERTL, G}, journal = {{NUCLEAR INSTRUMENTS \& METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}}, keywords = {myown}, month = {{JUN}}, note = {{6th International Workshop on Desorption Induced by Electronic Transitions (DIET VI), JAGIELLONIAN UNIV CONF CTR, PRZEGORZALY, POLAND, SEP 26-29, 1994}}, number = {{1-2}}, organization = {{Int Union Vacuum Sci Tech \& Applicat}}, pages = {{53-55}}, title = {{ELECTRONIC EXCITATIONS OF ADSORBATES ANALYZED WITH 2-PHOTON PHOTOEMISSION - CO/CU(111)}}, volume = {{101}}, year = {{1995}} }

1994[ to top ]

EMISSION OF EXOELECTRONS DURING OXIDATION OF CS VIA THERMAL-ACTIVATION OF A METASTABLE O2- SURFACE SPECIES. GROBECKER, R; SHI, H; BLUDAU, H; HERTEL, T; GREBER, T; BOTTCHER, A; JACOBI, K; ERTL, G. In PHYSICAL REVIEW LETTERS, 72(4), pp. 578–581. 1994.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Exposure of Cs surfaces to O2 causes the emission of exoelectrons. With a Cs monolayer on Ru(0001) the maximum yield is observed with an already partly oxidized surface on which a metastable O2-species could be identified. Thermally activated transformation (with an activation energy of 0.8 eV) of this phase leads to dissociation accompanied by exoelectron emission via Auger deexcitation.

@article{grobecker1994emission, abstract = {{Exposure of Cs surfaces to O2 causes the emission of exoelectrons. With a Cs monolayer on Ru(0001) the maximum yield is observed with an already partly oxidized surface on which a metastable O2-species could be identified. Thermally activated transformation (with an activation energy of 0.8 eV) of this phase leads to dissociation accompanied by exoelectron emission via Auger deexcitation.}}, author = {GROBECKER, R and SHI, H and BLUDAU, H and HERTEL, T and GREBER, T and BOTTCHER, A and JACOBI, K and ERTL, G}, journal = {{PHYSICAL REVIEW LETTERS}}, keywords = {myown}, month = {{JAN 24}}, number = {{4}}, pages = {{578-581}}, title = {{EMISSION OF EXOELECTRONS DURING OXIDATION OF CS VIA THERMAL-ACTIVATION OF A METASTABLE O2- SURFACE SPECIES}}, volume = {{72}}, year = {{1994}} }
UNOCCUPIED ADSORBATE STATES OF CO/CU(111) ANALYZED WITH 2-PHOTON PHOTOEMISSION. HERTEL, T; KNOESEL, E; HASSELBRINK, E; WOLF, M; ERTL, G. In SURFACE SCIENCE, 317(3), pp. L1147-L1151. 1994.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The unoccupied electronic states of the CO-(1.4 x 1.4) overlayer on Cu(111) have been studied with two-photon photoemission (2PPE). For excitation with 3.5 less-than-or-equal-to hnu less-than-or-equal-to 4.5 eV the kinetic energy of photoelectrons emitted from a CO induced state is found to shift as DELTAE(kin) = 1 . DELTAhnu. This provides evidence for emission from a normally unoccupied state, located 3.35 +/- 0.1 eV above the Fermi level, which is transiently populated in the 2PPE process. Based on the polarization dependence of the adsorbate induced feature this state is identified as being derived from the 2pi* affinity level of CO adsorbed on Cu(111). No dispersion of the CO 2pi* level is observed along GAMMABAR KBAR.

@article{hertel1994unoccupied, abstract = {{The unoccupied electronic states of the CO-(1.4 x 1.4) overlayer on Cu(111) have been studied with two-photon photoemission (2PPE). For excitation with 3.5 less-than-or-equal-to hnu less-than-or-equal-to 4.5 eV the kinetic energy of photoelectrons emitted from a CO induced state is found to shift as DELTAE(kin) = 1 . DELTAhnu. This provides evidence for emission from a normally unoccupied state, located 3.35 +/- 0.1 eV above the Fermi level, which is transiently populated in the 2PPE process. Based on the polarization dependence of the adsorbate induced feature this state is identified as being derived from the 2pi{*} affinity level of CO adsorbed on Cu(111). No dispersion of the CO 2pi{*} level is observed along GAMMABAR KBAR.}}, author = {HERTEL, T and KNOESEL, E and HASSELBRINK, E and WOLF, M and ERTL, G}, journal = {{SURFACE SCIENCE}}, keywords = {myown}, month = {{OCT 1}}, number = {{3}}, pages = {{L1147-L1151}}, title = {{UNOCCUPIED ADSORBATE STATES OF CO/CU(111) ANALYZED WITH 2-PHOTON PHOTOEMISSION}}, volume = {{317}}, year = {{1994}} }
STRUCTURAL-ANALYSES OF ORDERED RUBIDIUM PHASES ON RU(0001) USING LOW-ENERGY-ELECTRON DIFFRACTION. HERTEL, T; OVER, H; BLUDAU, H; GIERER, M; ERTL, G. In PHYSICAL REVIEW B, 50(11), pp. 8126–8129. 1994.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The adsorption geometries of the p(2X2) and (root 3x root 3)R30 degrees Rb/Ru(0001) phases at coverages of 0.25 and 0.33, respectively, were investigated using low-energy electron diffraction. The structure analyses revealed that Rb resides in the threefold fee site in the p(2X2) phase while for the (root 3x root 3)R30 degrees phase the hcp site is favored. The effective radius of Rb does not change with the Rb coverage within the error bars of this analysis. The concept of `'split positions'' has been used in order to take the enhanced motion of the adsorbate atoms parallel to the surface properly into account.

@article{hertel1994structuralanalyses, abstract = {{The adsorption geometries of the p(2X2) and (root 3x root 3)R30 degrees Rb/Ru(0001) phases at coverages of 0.25 and 0.33, respectively, were investigated using low-energy electron diffraction. The structure analyses revealed that Rb resides in the threefold fee site in the p(2X2) phase while for the (root 3x root 3)R30 degrees phase the hcp site is favored. The effective radius of Rb does not change with the Rb coverage within the error bars of this analysis. The concept of `'split positions'' has been used in order to take the enhanced motion of the adsorbate atoms parallel to the surface properly into account.}}, author = {HERTEL, T and OVER, H and BLUDAU, H and GIERER, M and ERTL, G}, journal = {{PHYSICAL REVIEW B}}, keywords = {myown}, month = {{SEP 15}}, number = {{11}}, pages = {{8126-8129}}, title = {{STRUCTURAL-ANALYSES OF ORDERED RUBIDIUM PHASES ON RU(0001) USING LOW-ENERGY-ELECTRON DIFFRACTION}}, volume = {{50}}, year = {{1994}} }
NA ADSORPTION ON RU(0001) - A LOW-ENERGY ELECTRON-DIFFRACTION ANALYSIS OF 3 ORDERED PHASES. HERTEL, T; OVER, H; BLUDAU, H; GIERER, M; ERTL, G. In SURFACE SCIENCE, 301(1-3), pp. 1–10. 1994.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
Sodium adsorbed on the Ru(0001) surface below room temperature forms a number of ordered phases, commensurate as well as incommensurate. They are investigated by low-energy electron-diffraction (LEED) and work-function measurements (Delta phi). The structural analyses for the p(2X2)-, (root 3x root 3)R30 degrees- and (3x3)-phases at coverages of 1/4, 1/3 and 4/9, respectively, reveal coverage-dependent adsorption sites. As for K adsorbed on Ru(0001), Na atoms occupy threefold fee-sites in the p(2X2) phase while for the (root 3x root 3)R30 degrees phase the hcp site is favoured. A structural analysis of the (3x3) phase, including 4 adsorbate atoms within the unit cell, revealed that nearest-neighbour distances are maximized at the cost of occupying low symmetry sites. This conclusion is supported by comparison of relative theoretical and experimental adsorbate-induced spot intensities. For all three phases the Na hard-sphere radii are nearly the same and close to the covalent Pauling radius.

@article{hertel1994adsorption, abstract = {{Sodium adsorbed on the Ru(0001) surface below room temperature forms a number of ordered phases, commensurate as well as incommensurate. They are investigated by low-energy electron-diffraction (LEED) and work-function measurements (Delta phi). The structural analyses for the p(2X2)-, (root 3x root 3)R30 degrees- and (3x3)-phases at coverages of 1/4, 1/3 and 4/9, respectively, reveal coverage-dependent adsorption sites. As for K adsorbed on Ru(0001), Na atoms occupy threefold fee-sites in the p(2X2) phase while for the (root 3x root 3)R30 degrees phase the hcp site is favoured. A structural analysis of the (3x3) phase, including 4 adsorbate atoms within the unit cell, revealed that nearest-neighbour distances are maximized at the cost of occupying low symmetry sites. This conclusion is supported by comparison of relative theoretical and experimental adsorbate-induced spot intensities. For all three phases the Na hard-sphere radii are nearly the same and close to the covalent Pauling radius.}}, author = {HERTEL, T and OVER, H and BLUDAU, H and GIERER, M and ERTL, G}, journal = {{SURFACE SCIENCE}}, keywords = {myown}, month = {{JAN 10}}, number = {{1-3}}, pages = {{1-10}}, title = {{NA ADSORPTION ON RU(0001) - A LOW-ENERGY ELECTRON-DIFFRACTION ANALYSIS OF 3 ORDERED PHASES}}, volume = {{301}}, year = {{1994}} }

1993[ to top ]

EPITAXIAL-GROWTH OF MAGNESIUM ON RU(0001). OVER, H; HERTEL, T; BLUDAU, H; PFLANZ, S; ERTL, G. In PHYSICAL REVIEW B, 48(8), pp. 5572–5578. 1993.
- [ Abstract ]
- [ BibTeX ]
- [ DOI ]
The type of growth and the local geometry of magnesium adsorbed on Ru(0001) were investigated by low-energy electron diffraction (LEED), work-function measurements, and thermal-desorption spectroscopy. Despite the large initial dipole moment of 4.5 D the LEED pattern revealed a hexagonal superstructure even at low global Mg coverage (theta(Mg)) greater-than-or-equal-to 0.05) with local coverage theta(Mg)=0.65 indicating island growth. In contrast to most alkali-metal/metal systems, where liquidlike adlayers are formed for coverages lower than 0.25, the repulsive dipole-dipole interaction is overcompensated by the overlap of wave functions due to the existence of two valence electrons. During completion of the first monolayer (theta(Mg)=0.75) the hexagonal superstructure is slightly compressed along the high-symmetry directions of the Ru substrate. This compression is completed when the Mg-Mg distance reaches the corresponding bulk value. Further deposition of Mg leads to a multilayer growth (most likely layer by layer). For such an epitaxially grown, 9-ML thick Mg film on Ru(0001), a LEED structural analysis was performed which revealed Mg bulklike values for the interlayer spacings.

@article{over1993epitaxialgrowth, abstract = {{The type of growth and the local geometry of magnesium adsorbed on Ru(0001) were investigated by low-energy electron diffraction (LEED), work-function measurements, and thermal-desorption spectroscopy. Despite the large initial dipole moment of 4.5 D the LEED pattern revealed a hexagonal superstructure even at low global Mg coverage (theta(Mg)) greater-than-or-equal-to 0.05) with local coverage theta(Mg)=0.65 indicating island growth. In contrast to most alkali-metal/metal systems, where liquidlike adlayers are formed for coverages lower than 0.25, the repulsive dipole-dipole interaction is overcompensated by the overlap of wave functions due to the existence of two valence electrons. During completion of the first monolayer (theta(Mg)=0.75) the hexagonal superstructure is slightly compressed along the high-symmetry directions of the Ru substrate. This compression is completed when the Mg-Mg distance reaches the corresponding bulk value. Further deposition of Mg leads to a multilayer growth (most likely layer by layer). For such an epitaxially grown, 9-ML thick Mg film on Ru(0001), a LEED structural analysis was performed which revealed Mg bulklike values for the interlayer spacings.}}, author = {OVER, H and HERTEL, T and BLUDAU, H and PFLANZ, S and ERTL, G}, journal = {{PHYSICAL REVIEW B}}, keywords = {myown}, month = {{AUG 15}}, number = {{8}}, pages = {{5572-5578}}, title = {{EPITAXIAL-GROWTH OF MAGNESIUM ON RU(0001)}}, volume = {{48}}, year = {{1993}} }

1992[ to top ]

COVERAGE-DEPENDENT ADSORPTION SITES IN THE K/RU(0001) SYSTEM - A LOW-ENERGY ELECTRON-DIFFRACTION ANALYSIS. GIERER, M; BLUDAU, H; HERTEL, T; OVER, H; MORITZ, W; ERTL, G. In SURFACE SCIENCE, 279(1-2), pp. L170-L174. 1992.
- [ Abstract ]
- [ BibTeX ]
The two ordered phases p(2 x 2) at a coverage theta = 0.25 and (square-root 3 x square-root 3)R30-degrees at theta = 0.33 of potassium adsorbed on Ru(0001) were analyzed by use of low-energy electron-diffraction (LEED). In the (square-root 3 x square-root 3)R30-degrees phase, the K atoms occupy threefold hcp sites, while in the p(2 x 2) phase the fcc site is favoured. In both phases, the K hard-sphere radii are nearly the same and close to the covalent Pauling radius.

@article{gierer1992coveragedependent, abstract = {{The two ordered phases p(2 x 2) at a coverage theta = 0.25 and (square-root 3 x square-root 3)R30-degrees at theta = 0.33 of potassium adsorbed on Ru(0001) were analyzed by use of low-energy electron-diffraction (LEED). In the (square-root 3 x square-root 3)R30-degrees phase, the K atoms occupy threefold hcp sites, while in the p(2 x 2) phase the fcc site is favoured. In both phases, the K hard-sphere radii are nearly the same and close to the covalent Pauling radius.}}, author = {GIERER, M and BLUDAU, H and HERTEL, T and OVER, H and MORITZ, W and ERTL, G}, journal = {{SURFACE SCIENCE}}, keywords = {myown}, month = {{DEC 1}}, number = {{1-2}}, pages = {{L170-L174}}, title = {{COVERAGE-DEPENDENT ADSORPTION SITES IN THE K/RU(0001) SYSTEM - A LOW-ENERGY ELECTRON-DIFFRACTION ANALYSIS}}, volume = {{279}}, year = {{1992}} }

News

Chance für Studierende: Mit KI die Zukunft der Lehre mitgestalten!

Bist Du bereit, an der Schnittstelle von KI und Bildung zu arbeiten? Bewirb Dich als studentische Hilfskraft am Lehrstuhl für Physikalische Chemie II und revolutioniere mit uns die Hochschullehre!

more

Hubland Süd, Geb. C5

Publications

Chance für Studierende: Mit KI die Zukunft der Lehre mitgestalten!

Data privacy protection

Data privacy protection

Picture credits