Publications

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• Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects. Li, Min-Ken; Riaz, Adnan; Wederhake, Martina; Fink, Karin; Saha, Avishek; Dehm, Simone; He, Xiaowei; Schoeppler, Friedrich; Kappes, Manfred M.; Htoon, Han; Popov, Valentin N.; Doom, Stephen K.; Hertel, Tobias; Hennrich, Frank; Krupke, Ralph in ACS NANO (2022).
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  @article{WOS:000819656200001, author = {Li, Min-Ken and Riaz, Adnan and Wederhake, Martina and Fink, Karin and Saha, Avishek and Dehm, Simone and He, Xiaowei and Schoeppler, Friedrich and Kappes, Manfred M. and Htoon, Han and Popov, Valentin N. and Doom, Stephen K. and Hertel, Tobias and Hennrich, Frank and Krupke, Ralph}, journal = {ACS NANO}, keywords = {}, title = {Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects}, 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 (2022). 6(4) 2101010.
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  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 = {}, 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 (2022). ASAP.
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  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 = {}, note = {PMID: 35732039}, pages = {ASAP}, title = {Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects}, year = 2022 }

• 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).
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  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 = {}, 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 (2021).
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  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 = {}, month = {03}, publisher = {American Chemical Society ({ACS})}, title = {Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes}, year = 2021 }
• 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 (2021). 253 119567.
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  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 = {}, pages = 119567, title = {Coherent two-dimensional electronic spectroelectrochemistry}, volume = 253, year = 2021 }

• 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 (2020). 15(3) 164–166.
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  @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 = {}, number = 3, pages = {164–166}, title = {Banning carbon nanotubes would be scientifically unjustified and damaging to innovation}, volume = 15, year = 2020 }

• Optical Spectroscopy of Doped Carbon Nanotubes. Hertel, Tobias in Handbook of Carbon Nanomaterials (2019). (Vol. 10) 191–236.
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  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 = {}, 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 (2019). 123(49) 30001–30006.
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  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 = {}, number = 49, pages = {30001-30006}, title = {Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy}, volume = 123, year = 2019 }

• Direct Tracking of Ultrafast Carrier Motion Dynamics in Semiconducting Single-Wall Carbon Nanotubes. Jasinskas, Vidmantas; Oberndorfer, Florian; Pakstas, Vidas; Hertel, Tobias; Gulbinas, Vidmantas in JOURNAL OF PHYSICAL CHEMISTRY C (2018). 122(28) 16424–16430.
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  An Ultrafast all-optical electric field strength measurement technique based on the electric-field-induced second harmonic generation together with conventional transient photocurrent measurements has been applied for the direct tracking of carrier separation and motion dynamics in semiconducting single-wall carbon nanotubes (SWNTs). Thin films of (6,5)-SWNT-enriched samples were prepared on combs of interdigitated electrodes with different concentrations of fullerene derivative phenyl-C-61-butyric acid methyl ester (PCBM) serving as an electron acceptor. Neutral delocalized excitons photogenerated in the samples with high PCBM concentration were found to form localized charge-transfer (CT) states within less than 1 ps with the electron transferred to the PCBM and a hole remaining on the SWNT. The hole's drift along the length of individual nanotubes was found to take similar to 200-1000 ps and is most likely limited by dissociation of the CT states rather than by the hole's mobility. A fraction of about (30-50)% of generated charge carriers was found to recombine within 3.6 ns measurement interval.

  @article{jasinskas2018direct, abstract = {An Ultrafast all-optical electric field strength measurement technique based on the electric-field-induced second harmonic generation together with conventional transient photocurrent measurements has been applied for the direct tracking of carrier separation and motion dynamics in semiconducting single-wall carbon nanotubes (SWNTs). Thin films of (6,5)-SWNT-enriched samples were prepared on combs of interdigitated electrodes with different concentrations of fullerene derivative phenyl-C-61-butyric acid methyl ester (PCBM) serving as an electron acceptor. Neutral delocalized excitons photogenerated in the samples with high PCBM concentration were found to form localized charge-transfer (CT) states within less than 1 ps with the electron transferred to the PCBM and a hole remaining on the SWNT. The hole's drift along the length of individual nanotubes was found to take similar to 200-1000 ps and is most likely limited by dissociation of the CT states rather than by the hole's mobility. A fraction of about (30-50)% of generated charge carriers was found to recombine within 3.6 ns measurement interval.}, author = {Jasinskas, Vidmantas and Oberndorfer, Florian and Pakstas, Vidas and Hertel, Tobias and Gulbinas, Vidmantas}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, keywords = {}, number = 28, pages = {16424–16430}, publisher = {AMER CHEMICAL SOC}, title = {Direct Tracking of Ultrafast Carrier Motion Dynamics in Semiconducting Single-Wall Carbon Nanotubes}, volume = 122, year = 2018 }

• 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 (2017). 9(2017-07-26, 34) 12441–12448.
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  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 = {}, 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 }
• Carrier photogeneration, drift and recombination in a semiconducting carbon nanotube network. Eckstein, A.; Bertasius, V.; Jasinskas, V.; Namal, I.; Hertel, T.; Gulbinas, V. in NANOSCALE (2017). 9(34) 12441–12448.
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  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{eckstein2017carrier, 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 Bertasius, V. and Jasinskas, V. and Namal, I. and Hertel, T. and Gulbinas, V.}, journal = {NANOSCALE}, keywords = {}, number = 34, pages = {12441–12448}, publisher = {ROYAL SOC CHEMISTRY}, title = {Carrier photogeneration, drift and recombination in a semiconducting carbon nanotube network}, volume = 9, year = 2017 }
• Localized Charges Control Exciton Energetics and Energy Dissipation in Doped Carbon Nanotubes. Eckstein, Klaus H.; Hartleb, Holger; Achsnich, Melanie M.; Schoeppler, Friedrich; Hertel, Tobias in ACS NANO (2017). 11(10) 10401–10408.
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  Doping by chemical or physical means is key for the development of future semiconductor technologies. Ideally, charge carriers should be able to move freely in a homogeneous environment. Here, we report on evidence suggesting that excess carriers in electrochemically p-doped semiconducting single-wall carbon nanotubes (s-SWNTs) become localized, most likely due to poorly screened Coulomb interactions with counterions in the Helmholtz layer. A quantitative analysis of blue-shift, broadening, and asymmetry of the first exciton absorption band also reveals that doping leads to hard segmentation of s-SWNTs with intrinsic undoped segments being separated by randomly distributed charge puddles approximately 4 nm in width. Light absorption in these doped segments is associated with the formation of trions, spatially separated from neutral excitons. Acceleration of exciton decay in doped samples is governed by diffusive excitors transport to, and nonradiative decay at charge puddles within 3.2 ps in moderately doped s-SWNTs. The results suggest that conventional band-filling in s-SWNTs breaks down due to inhomogeneous electrochemical doping.

  @article{eckstein2017localized, abstract = {Doping by chemical or physical means is key for the development of future semiconductor technologies. Ideally, charge carriers should be able to move freely in a homogeneous environment. Here, we report on evidence suggesting that excess carriers in electrochemically p-doped semiconducting single-wall carbon nanotubes (s-SWNTs) become localized, most likely due to poorly screened Coulomb interactions with counterions in the Helmholtz layer. A quantitative analysis of blue-shift, broadening, and asymmetry of the first exciton absorption band also reveals that doping leads to hard segmentation of s-SWNTs with intrinsic undoped segments being separated by randomly distributed charge puddles approximately 4 nm in width. Light absorption in these doped segments is associated with the formation of trions, spatially separated from neutral excitons. Acceleration of exciton decay in doped samples is governed by diffusive excitors transport to, and nonradiative decay at charge puddles within 3.2 ps in moderately doped s-SWNTs. The results suggest that conventional band-filling in s-SWNTs breaks down due to inhomogeneous electrochemical doping.}, author = {Eckstein, Klaus H. and Hartleb, Holger and Achsnich, Melanie M. and Schoeppler, Friedrich and Hertel, Tobias}, journal = {ACS NANO}, keywords = {}, number = 10, pages = {10401–10408}, publisher = {AMER CHEMICAL SOC}, title = {Localized Charges Control Exciton Energetics and Energy Dissipation in Doped Carbon Nanotubes}, volume = 11, year = 2017 }
• Localized Charges Control Exciton Energetics and Energy Dissipation in Doped Carbon Nanotubes. Eckstein, Klaus H.; Hartleb, Holger; Achsnich, Melanie M.; Schöppler, Friedrich; Hertel, Tobias in ACS Nano (2017). 11(10) 10401–10408.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Eckstein_2017, author = {Eckstein, Klaus H. and Hartleb, Holger and Achsnich, Melanie M. and Schöppler, Friedrich and Hertel, Tobias}, journal = {{ACS} Nano}, keywords = {}, month = {09}, number = 10, pages = {10401–10408}, publisher = {American Chemical Society ({ACS})}, title = {Localized Charges Control Exciton Energetics and Energy Dissipation in Doped Carbon Nanotubes}, volume = 11, year = 2017 }

• 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) (2016). 12(31) 4211–4221.
  • [ 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 = {}, number = 31, pages = {4211–4221}, title = {Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks}, volume = 12, year = 2016 }
• Substrate-Mediated Cooperative Adsorption of Sodium Cholate on (6,5) Single-Wall Carbon Nanotubes. Bergler, Felix F.; Stahl, Sabine; Goy, Annika; Schoeppler, Friedrich; Hertel, Tobias in LANGMUIR (2016). 32(37) 9598–9603.
  • [ Abstract ]
  • [ BibTeX ]
  The interaction of sodium cholate (NaC) with (6,5) single-wall carbon nanotubes (SWNTs) is investigated using photoluminescence spectroscopy. Dilution of SWNT-NaC suspensions is accompanied by changes in the exciton PL quantum yield and peak emission energy. An abrupt change of the exciton emission peak energy at NaC concentrations between 10 and 14 mM indicates strongly cooperative formation of a micellar phase on (6,5) SWNT surfaces with a Hill coefficient of n(H) = 65 +/- 6. This is in contrast to the formation of free NaC micelles with aggregation numbers of only about 4 and suggests that the cooperativity of NaC micelle formation on nanotube surfaces is strongly substrate-enhanced. The temperature dependence of this previously unobserved transition is used for a determination of Delta(mic)(G) over bar (Theta)/(1 + beta) = -(11.4 +/- 0.2) kJ.mol(-1) which, for typical Na+ counterion binding with beta approximate to 0.2, yields a free SWNT-NaC micellization enthalpy Delta(mic)(G) over bar (Theta) of -13.7 kJ.mol(-1).

  @article{bergler2016substratemediated, abstract = {The interaction of sodium cholate (NaC) with (6,5) single-wall carbon nanotubes (SWNTs) is investigated using photoluminescence spectroscopy. Dilution of SWNT-NaC suspensions is accompanied by changes in the exciton PL quantum yield and peak emission energy. An abrupt change of the exciton emission peak energy at NaC concentrations between 10 and 14 mM indicates strongly cooperative formation of a micellar phase on (6,5) SWNT surfaces with a Hill coefficient of n(H) = 65 +/- 6. This is in contrast to the formation of free NaC micelles with aggregation numbers of only about 4 and suggests that the cooperativity of NaC micelle formation on nanotube surfaces is strongly substrate-enhanced. The temperature dependence of this previously unobserved transition is used for a determination of Delta(mic)(G) over bar (Theta)/(1 + beta) = -(11.4 +/- 0.2) kJ.mol(-1) which, for typical Na+ counterion binding with beta approximate to 0.2, yields a free SWNT-NaC micellization enthalpy Delta(mic)(G) over bar (Theta) of -13.7 kJ.mol(-1).}, author = {Bergler, Felix F. and Stahl, Sabine and Goy, Annika and Schoeppler, Friedrich and Hertel, Tobias}, journal = {LANGMUIR}, keywords = {}, number = 37, pages = {9598–9603}, publisher = {AMER CHEMICAL SOC}, title = {Substrate-Mediated Cooperative Adsorption of Sodium Cholate on (6,5) Single-Wall Carbon Nanotubes}, volume = 32, year = 2016 }
• Substrate-Mediated Cooperative Adsorption of Sodium Cholate on (6,5) Single-Wall Carbon Nanotubes. Bergler, Felix F.; Stahl, Sabine; Goy, Annika; Schöppler, Friedrich; Hertel, Tobias in Langmuir (2016). 32(37) 9598–9603.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Bergler_2016, author = {Bergler, Felix F. and Stahl, Sabine and Goy, Annika and Schöppler, Friedrich and Hertel, Tobias}, journal = {Langmuir}, keywords = {}, month = {09}, number = 37, pages = {9598–9603}, publisher = {American Chemical Society ({ACS})}, title = {Substrate-Mediated Cooperative Adsorption of Sodium Cholate on (6,5) Single-Wall Carbon Nanotubes}, volume = 32, year = 2016 }
• Chirality Specific Triplet Exciton Dynamics in Highly Enriched (6,5) and (7,5) Carbon Nanotube Networks. Abudulimu, Abasi; Spaeth, Florian; Namal, Imge; Hertel, Tobias; Lüer, Larry in The Journal of Physical Chemistry C (2016). 120(35) 19778–19784.
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  @article{Abudulimu_2016, author = {Abudulimu, Abasi and Spaeth, Florian and Namal, Imge and Hertel, Tobias and Lüer, Larry}, journal = {The Journal of Physical Chemistry C}, keywords = {}, month = {08}, number = 35, pages = {19778–19784}, publisher = {American Chemical Society ({ACS})}, title = {Chirality Specific Triplet Exciton Dynamics in Highly Enriched (6,5) and (7,5) Carbon Nanotube Networks}, volume = 120, year = 2016 }
• 13 nm Exciton Size in (6,5) Single-Wall Carbon Nanotubes. Mann, Christoph; Hertel, Tobias in JOURNAL OF PHYSICAL CHEMISTRY LETTERS (2016). 7(12) 2276–2280.
  • [ Abstract ]
  • [ BibTeX ]
  Electron-hole correlation lengths, also termed exciton size, for (6,5) single-wall carbon nanotubes (SWNTs) are determined using femtosecond time-resolved pump-probe spectroscopy. The phase space filling model is used to obtain the sizes of the first subband exciton in samples of isolated and of bundled SWNTs. The experiments indicate that the exciton size of (13 +/- 3) nm is a factor of 6 higher than previous experimental estimates and theoretical predictions for vacuum suspended SWNTs. This surprising result may be attributed at least in part to the effect of the dielectric environment on exciton sizes and supports recent theoretical findings predicting that screening in SWNTs may enhance rather than reduce electron-hole interactions for separations larger than the tube diameter. Thereby, the work also points to the unique nature of screening and electronic correlations in one-dimensional semiconductors.

  @article{mann2016exciton, abstract = {Electron-hole correlation lengths, also termed exciton size, for (6,5) single-wall carbon nanotubes (SWNTs) are determined using femtosecond time-resolved pump-probe spectroscopy. The phase space filling model is used to obtain the sizes of the first subband exciton in samples of isolated and of bundled SWNTs. The experiments indicate that the exciton size of (13 +/- 3) nm is a factor of 6 higher than previous experimental estimates and theoretical predictions for vacuum suspended SWNTs. This surprising result may be attributed at least in part to the effect of the dielectric environment on exciton sizes and supports recent theoretical findings predicting that screening in SWNTs may enhance rather than reduce electron-hole interactions for separations larger than the tube diameter. Thereby, the work also points to the unique nature of screening and electronic correlations in one-dimensional semiconductors.}, author = {Mann, Christoph and Hertel, Tobias}, journal = {JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, keywords = {}, number = 12, pages = {2276–2280}, publisher = {AMER CHEMICAL SOC}, title = {13 nm Exciton Size in (6,5) Single-Wall Carbon Nanotubes}, volume = 7, year = 2016 }
• Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks. Bottacchi, Francesca; Bottacchi, Stefano; Spaeth, Florian; Namal, Imge; Hertel, Tobias; Anthopoulos, Thomas D. in SMALL (2016). 12(31) 4211–4221.
  • [ Abstract ]
  • [ BibTeX ]
  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 approximate to 100 to approximate to 1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage approximate to 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 approximate to 100 to approximate to 1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage approximate to 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 Spaeth, Florian and Namal, Imge and Hertel, Tobias and Anthopoulos, Thomas D.}, journal = {SMALL}, keywords = {}, number = 31, pages = {4211–4221}, publisher = {WILEY-V C H VERLAG GMBH}, title = {Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks}, volume = 12, year = 2016 }
• Excitation quenching in polyfluorene polymers bound to (6,5) single-wall carbon nanotubes. Eckstein, Angela; Karpicz, Renata; Augulis, Ramunas; Redeckas, Kipras; Vengris, Mikas; Namal, Imge; Hertel, Tobias; Gulbinas, Vidmantas in CHEMICAL PHYSICS (2016). 467 1–5.
  • [ Abstract ]
  • [ BibTeX ]
  Fluorescence quenching of polyfluorene-based polymer (PFO-BPy) attached to single-wall carbon nanotubes (SWNT) has been investigated by means of fluorescence and transient absorption spectroscopy. Fluorescence of SWNT/PFO-BPy complex suspension in chlorobenzene decays nonexponentialy on a time scale from several to several hundreds of picoseconds, and experiences slower depolarization if compared with free polymer in solution. We attribute the slowly quenched and more polarized fluorescence to the polymer tail segments that are not in direct contact with the SWNT. Polymer fluorescence in SWNT/PFOBPy solid films is quenched about 10 times faster than in suspensions. The initial excited state quenching in films occurs with a 2.3 ps time constant, which we attribute to the polymer chains intimately attached to the SWNT. Slow changes of the spectroscopic properties of the SWNT/PFO-BPy suspensions with time revealed that their aging is associated with the desorption of polymers from SWNT, increasing their tendency to aggregate.

  @article{eckstein2016excitation, abstract = {Fluorescence quenching of polyfluorene-based polymer (PFO-BPy) attached to single-wall carbon nanotubes (SWNT) has been investigated by means of fluorescence and transient absorption spectroscopy. Fluorescence of SWNT/PFO-BPy complex suspension in chlorobenzene decays nonexponentialy on a time scale from several to several hundreds of picoseconds, and experiences slower depolarization if compared with free polymer in solution. We attribute the slowly quenched and more polarized fluorescence to the polymer tail segments that are not in direct contact with the SWNT. Polymer fluorescence in SWNT/PFOBPy solid films is quenched about 10 times faster than in suspensions. The initial excited state quenching in films occurs with a 2.3 ps time constant, which we attribute to the polymer chains intimately attached to the SWNT. Slow changes of the spectroscopic properties of the SWNT/PFO-BPy suspensions with time revealed that their aging is associated with the desorption of polymers from SWNT, increasing their tendency to aggregate.}, author = {Eckstein, Angela and Karpicz, Renata and Augulis, Ramunas and Redeckas, Kipras and Vengris, Mikas and Namal, Imge and Hertel, Tobias and Gulbinas, Vidmantas}, journal = {CHEMICAL PHYSICS}, keywords = {}, pages = {1–5}, publisher = {ELSEVIER SCIENCE BV}, title = {Excitation quenching in polyfluorene polymers bound to (6,5) single-wall carbon nanotubes}, volume = 467, year = 2016 }

• 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 (2015). 5
  • [ Abstract ]
  • [ BibTeX ]
  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 S-33 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 S-33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S-11. 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 S-33 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 S-33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S-11. 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 = {}, publisher = {NATURE PUBLISHING GROUP}, title = {High energetic excitons in carbon nanotubes directly probe charge-carriers}, volume = 5, year = 2015 }
• Polymer-sorted (6,5) single-walled carbon nanotubes for solution-processed low-voltage flexible microelectronics. Bottacchi, Francesca; Petti, Luisa; Spaeth, Florian; Namal, Imge; Troester, Gerhard; Hertel, Tobias; Anthopoulos, Thomas D. in APPLIED PHYSICS LETTERS (2015). 106(19)
  • [ Abstract ]
  • [ BibTeX ]
  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) SWNTs were separated from the as-received carbon nanotubes mixture using a polyfluorene-based derivative as the sorting and dispersing polymer agent. As-prepared devices exhibit primarily p-type behavior with channel current on/off ratio >10(3) and hole mobility approximate to 2 cm(2) V-1 s(-1). These transistor characteristics enable realization of low-voltage unipolar inverters with wide noise margins and high signal gain (>5). Polymer/(6,5) SWNT transistors were also fabricated on free-standing polyimide foils. The devices exhibit even higher hole mobility (approximate to 8 cm(2) V-1 s(-1)) and on/off ratios (>10(4)) while remaining fully functional when bent to a radius of 4 mm.

  @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) SWNTs were separated from the as-received carbon nanotubes mixture using a polyfluorene-based derivative as the sorting and dispersing polymer agent. As-prepared devices exhibit primarily p-type behavior with channel current on/off ratio >10(3) and hole mobility approximate to 2 cm(2) V-1 s(-1). These transistor characteristics enable realization of low-voltage unipolar inverters with wide noise margins and high signal gain (>5). Polymer/(6,5) SWNT transistors were also fabricated on free-standing polyimide foils. The devices exhibit even higher hole mobility (approximate to 8 cm(2) V-1 s(-1)) and on/off ratios (>10(4)) while remaining fully functional when bent to a radius of 4 mm.}, author = {Bottacchi, Francesca and Petti, Luisa and Spaeth, Florian and Namal, Imge and Troester, Gerhard and Hertel, Tobias and Anthopoulos, Thomas D.}, journal = {APPLIED PHYSICS LETTERS}, keywords = {}, number = 19, publisher = {AMER INST PHYSICS}, title = {Polymer-sorted (6,5) single-walled carbon nanotubes for solution-processed low-voltage flexible microelectronics}, volume = 106, 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 (2015). 9(10) 10461–10470.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Hartleb_2015, author = {Hartleb, Holger and Späth, Florian and Hertel, Tobias}, journal = {{ACS} Nano}, keywords = {}, month = {09}, number = 10, pages = {10461–10470}, publisher = {American Chemical Society ({ACS})}, title = {Evidence for Strong Electronic Correlations in the Spectra of Gate-Doped Single-Wall Carbon Nanotubes}, volume = 9, 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 (2015). 635 245–249.
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  @article{Kastner_2015, 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 = {}, month = {08}, pages = {245–249}, publisher = {Elsevier {BV}}, title = {A comparison of Raman and photoluminescence spectra for the assessment of single-wall carbon nanotube sample quality}, volume = 635, 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 (2015). 106(19) 193302.
  • [ 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 = {}, number = 19, pages = 193302, title = {Polymer-sorted (6,5) single-walled carbon nanotubes for solution-processed low-voltage flexible microelectronics}, volume = 106, 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 (2015). 7(43) 18337–18342.
  • [ Abstract ]
  • [ BibTeX ]
  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 = {}, number = 43, pages = {18337–18342}, publisher = {ROYAL SOC CHEMISTRY}, title = {Below-gap excitation of semiconducting single-wall carbon nanotubes}, volume = 7, year = 2015 }
• Evidence for Strong Electronic Correlations in the Spectra of Gate-Doped Single-Wall Carbon Nanotubes. Hartleb, Holger; Spaeth, Florian; Hertel, Tobias in ACS NANO (2015). 9(10) 10461–10470.
  • [ Abstract ]
  • [ BibTeX ]
  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 Delta epsilon(v) = n x b, where n is the free-carrier density, epsilon(v) 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 Delta n = (32m(eff)b)/(pi h)(2) (m(eff) 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{hartleb2015evidence, 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 Delta epsilon(v) = n x b, where n is the free-carrier density, epsilon(v) 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 Delta n = (32m(eff)b)/(pi h)(2) (m(eff) 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 Spaeth, Florian and Hertel, Tobias}, journal = {ACS NANO}, keywords = {}, number = 10, pages = {10461–10470}, publisher = {AMER CHEMICAL SOC}, title = {Evidence for Strong Electronic Correlations in the Spectra of Gate-Doped Single-Wall Carbon Nanotubes}, volume = 9, 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 (2015). 119(42) 24116–24123.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Schilling_2015, author = {Schilling, Daniel and Mann, Christoph and Kunkel, Pascal and Schöppler, Friedrich and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {}, month = 10, number = 42, pages = {24116–24123}, publisher = {American Chemical Society ({ACS})}, title = {Ultrafast Spectral Exciton Diffusion in Single-Wall Carbon Nanotubes Studied by Time-Resolved Hole Burning}, volume = 119, 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; Ruehl, Nicolas; Hertel, Tobias; Schoeppler, Friedrich in CHEMICAL PHYSICS LETTERS (2015). 635 245–249.
  • [ Abstract ]
  • [ BibTeX ]
  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 degrees C to 850 degrees 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 degrees 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{kastner2015comparison, 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 degrees C to 850 degrees 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 degrees 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 Ruehl, Nicolas and Hertel, Tobias and Schoeppler, Friedrich}, journal = {CHEMICAL PHYSICS LETTERS}, keywords = {}, pages = {245–249}, publisher = {ELSEVIER SCIENCE BV}, title = {A comparison of Raman and photoluminescence spectra for the assessment of single-wall carbon nanotube sample quality}, volume = 635, year = 2015 }
• Ultrafast Spectral Exciton Diffusion in Single-Wall Carbon Nanotubes Studied by Time-Resolved Hole Burning. Schilling, Daniel; Mann, Christoph; Kunkel, Pascal; Schoeppler, Friedrich; Hertel, Tobias in JOURNAL OF PHYSICAL CHEMISTRY C (2015). 119(42) 24116–24123.
  • [ Abstract ]
  • [ BibTeX ]
  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{schilling2015ultrafast, 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 Schoeppler, Friedrich and Hertel, Tobias}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, keywords = {}, number = 42, pages = {24116–24123}, publisher = {AMER CHEMICAL SOC}, title = {Ultrafast Spectral Exciton Diffusion in Single-Wall Carbon Nanotubes Studied by Time-Resolved Hole Burning}, volume = 119, 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 (2015). 106(19) 193302.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Bottacchi_2015, author = {Bottacchi, Francesca and Petti, Luisa and Späth, Florian and Namal, Imge and Tröster, Gerhard and Hertel, Tobias and Anthopoulos, Thomas D.}, journal = {Applied Physics Letters}, keywords = {}, month = {05}, number = 19, pages = 193302, publisher = {{AIP} Publishing}, title = {Polymer-sorted (6,5) single-walled carbon nanotubes for solution-processed low-voltage flexible microelectronics}, volume = 106, year = 2015 }

• 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 (2014). 8(6) 6417–6424.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Li_2014, author = {Li, Han and Hain, Tilman C. and Muzha, Andreas and Schöppler, Friedrich and Hertel, Tobias}, journal = {{ACS} Nano}, keywords = {}, month = {05}, number = 6, pages = {6417–6424}, publisher = {American Chemical Society ({ACS})}, 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; Spaeth, Florian; Kraus, Hannes; Sperlich, Andreas; Dyakonov, Vladimir; Hertel, Tobias in NATURE PHOTONICS (2014). 8(2) 139–144.
  • [ Abstract ]
  • [ BibTeX ]
  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 mu s) 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 cm(2) 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 mu s) 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 cm(2) s(-1). This work suggests that exciton annihilation in SWNTs is reduced by one-dimensional confinement of diffusive exciton motion.}, author = {Stich, Dominik and Spaeth, Florian and Kraus, Hannes and Sperlich, Andreas and Dyakonov, Vladimir and Hertel, Tobias}, journal = {NATURE PHOTONICS}, keywords = {}, number = 2, pages = {139–144}, publisher = {NATURE PUBLISHING GROUP}, title = {Triplet-triplet exciton dynamics in single-walled carbon nanotubes}, volume = 8, year = 2014 }
• 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 (2014). 115(13)
  • [ Abstract ]
  • [ BibTeX ]
  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 varied over several orders of magnitude. We establish the excitation profile for optical pumping of these defects and evaluate the optimum excitation wavelength of 770 nm. We also measure the photoluminescence dynamics at room temperature and find a monoexponential decay with a characteristic lifetime of 6.1 ns. The integrated photoluminescence intensity depends linear on the excitation power density up to 20 kW/cm(2), indicating a relatively small absorption cross section of these defects.

  @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 varied over several orders of magnitude. We establish the excitation profile for optical pumping of these defects and evaluate the optimum excitation wavelength of 770 nm. We also measure the photoluminescence dynamics at room temperature and find a monoexponential decay with a characteristic lifetime of 6.1 ns. The integrated photoluminescence intensity depends linear on the excitation power density up to 20 kW/cm(2), indicating a relatively small absorption cross section of these defects.}, 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.}, journal = {JOURNAL OF APPLIED PHYSICS}, keywords = {}, number = 13, publisher = {AMER INST 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; Schoeppler, Friedrich; Hertel, Tobias in ACS NANO (2014). 8(6) 6417–6424.
  • [ Abstract ]
  • [ BibTeX ]
  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{li2014dynamical, 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 Schoeppler, Friedrich and Hertel, Tobias}, journal = {ACS NANO}, keywords = {}, number = 6, pages = {6417–6424}, publisher = {AMER CHEMICAL SOC}, 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 (2014). 8(2) 139–144.
  • [ 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 = {}, number = 2, pages = {139–144}, title = {Triplet–triplet exciton dynamics in single-walled carbon nanotubes}, volume = 8, year = 2014 }

• Interface-Phenomena of Carbon Nanotubes. Hertel, Tobias; Brunecker, Frank; Ruehl, Nicolas; Schilling, Daniel; Spaeth, Florian (2013). 632–635.
  • [ BibTeX ]
  @editorialmaterial{hertel2013interfacephenomena, author = {Hertel, Tobias and Brunecker, Frank and Ruehl, Nicolas and Schilling, Daniel and Spaeth, Florian}, journal = {NACHRICHTEN AUS DER CHEMIE}, keywords = {}, number = 6, pages = {632–635}, publisher = {GESELLSCHAFT DEUTSCHER CHEMIKER E V}, title = {Interface-Phenomena of Carbon Nanotubes}, volume = 61, year = 2013 }
• 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 (2013). 117(25) 13318–13323.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Bergler_2013, 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 = {}, month = {06}, 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 }
• Photophysics of carbon nanotubes and nanotube composites. Hertel, Tobias; Bondarev, Igor (2013). 1–2.
  • [ BibTeX ]
  @editorialmaterial{hertel2013photophysics, author = {Hertel, Tobias and Bondarev, Igor}, journal = {CHEMICAL PHYSICS}, keywords = {}, pages = {1–2}, publisher = {ELSEVIER SCIENCE BV}, title = {Photophysics of carbon nanotubes and nanotube composites}, volume = 413, year = 2013 }
• Grenzflächenphänomene an Kohlenstoffnanoröhren. Hertel, Tobias; Brunecker, Frank; Rühl, Nicolas; Schilling, Daniel; Späth, Florian in Nachrichten aus der Chemie (2013). 61(6) 632–635.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Hertel_2013, author = {Hertel, Tobias and Brunecker, Frank and Rühl, Nicolas and Schilling, Daniel and Späth, Florian}, journal = {Nachrichten aus der Chemie}, keywords = {}, month = {06}, number = 6, pages = {632–635}, publisher = {Wiley}, title = {Grenzflächenphänomene an Kohlenstoffnanoröhren}, volume = 61, year = 2013 }
• Photoluminescence microscopy and spectroscopy of individualized and aggregated single-wall carbon nanotubes. Schöppler, Friedrich; Rühl, Nicolas; Hertel, Tobias in Chemical Physics (2013). 413 112–115.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Sch_ppler_2013, author = {Schöppler, Friedrich and Rühl, Nicolas and Hertel, Tobias}, journal = {Chemical Physics}, keywords = {}, month = {02}, pages = {112–115}, publisher = {Elsevier {BV}}, 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.; Spaeth, F.; Antognazza, M. R.; Hertel, T.; Lanzani, G.; Cerullo, G. in JOURNAL OF PHYSICAL CHEMISTRY C (2013). 117(20) 10849–10855.
  • [ Abstract ]
  • [ BibTeX ]
  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{soavi2013ultrafast, 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 Spaeth, F. and Antognazza, M. R. and Hertel, T. and Lanzani, G. and Cerullo, G.}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, keywords = {}, number = 20, pages = {10849–10855}, publisher = {AMER CHEMICAL SOC}, title = {Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes}, volume = 117, 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 (2013). 117(20) 10849–10855.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Soavi_2013, 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 = {}, month = {05}, number = 20, pages = {10849–10855}, publisher = {American Chemical Society ({ACS})}, title = {Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes}, volume = 117, year = 2013 }
• Photoluminescence microscopy and spectroscopy of individualized and aggregated single-wall carbon nanotubes. Schoeppler, Friedrich; Ruehl, Nicolas; Hertel, Tobias in CHEMICAL PHYSICS (2013). 413 112–115.
  • [ Abstract ]
  • [ BibTeX ]
  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 approximate to 12 SWNTs mu m (3). We also find a sizable spectral red-shift of 16 nm, line-broadening of 23 nm 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{schoeppler2013photoluminescence, 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 approximate to 12 SWNTs mu m (3). We also find a sizable spectral red-shift of 16 nm, line-broadening of 23 nm 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 = {Schoeppler, Friedrich and Ruehl, Nicolas and Hertel, Tobias}, journal = {CHEMICAL PHYSICS}, keywords = {}, pages = {112–115}, publisher = {ELSEVIER SCIENCE BV}, title = {Photoluminescence microscopy and spectroscopy of individualized and aggregated single-wall carbon nanotubes}, volume = 413, year = 2013 }
• Fluorescence Spectroscopy of Gel-Immobilized Single-Wall Carbon Nanotubes with Microfluidic Control of the Surfactant Environment. Bergler, Felix F.; Schoeppler, Friedrich; Brunecker, Frank K.; Hailman, Michael; Hertel, Tobias in JOURNAL OF PHYSICAL CHEMISTRY C (2013). 117(25) 13318–13323.
  • [ Abstract ]
  • [ BibTeX ]
  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 surfactantnanotube 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{bergler2013fluorescence, 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 surfactantnanotube 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 Schoeppler, Friedrich and Brunecker, Frank K. and Hailman, Michael and Hertel, Tobias}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, keywords = {}, number = 25, pages = {13318–13323}, publisher = {AMER CHEMICAL SOC}, title = {Fluorescence Spectroscopy of Gel-Immobilized Single-Wall Carbon Nanotubes with Microfluidic Control of the Surfactant Environment}, volume = 117, year = 2013 }

• Influence of DNA conformation on the dispersion of SWNTs: single-strand DNA vs. hairpin DNA. Hain, Tilman C.; Kroeker, Kristin; Stich, Dominik G.; Hertel, Tobias in SOFT MATTER (2012). 8(10) 2820–2823.
  • [ Abstract ]
  • [ BibTeX ]
  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{hain2012influence, 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 Kroeker, Kristin and Stich, Dominik G. and Hertel, Tobias}, journal = {SOFT MATTER}, keywords = {}, number = 10, pages = {2820–2823}, publisher = {ROYAL SOC CHEMISTRY}, title = {Influence of DNA conformation on the dispersion of SWNTs: single-strand DNA vs. hairpin DNA}, volume = 8, year = 2012 }
• Influence of DNA conformation on the dispersion of SWNTs: single-strand DNA vs. hairpin DNA. Hain, Tilman C.; Kröker, Kristin; Stich, Dominik G.; Hertel, Tobias in Soft Matter (2012). 8(10) 2820.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Hain_2012, author = {Hain, Tilman C. and Kröker, Kristin and Stich, Dominik G. and Hertel, Tobias}, journal = {Soft Matter}, keywords = {}, number = 10, pages = 2820, publisher = {Royal Society of Chemistry ({RSC})}, title = {Influence of {DNA} conformation on the dispersion of {SWNTs}: single-strand {DNA} vs. 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 (2012). 535 131–135.
  • [ 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 = {}, pages = {131 - 135}, title = {Density gradient ultracentrifugation and stability of SWNT–peptide conjugates}, volume = 535, year = 2012 }

• 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 (2011). 115(30) 14682–14686.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Sch_ppler_2011, 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 = {}, month = {07}, number = 30, pages = {14682–14686}, publisher = {American Chemical Society ({ACS})}, title = {Molar Extinction Coefficient of Single-Wall Carbon Nanotubes}, volume = 115, year = 2011 }
• Molar Extinction Coefficient of Single-Wall Carbon Nanotubes. Schoeppler, Friedrich; Mann, Christoph; Hain, Tilman C.; Neubauer, Felix M.; Privitera, Giulia; Bonaccorso, Francesco; Chu, Daping; Ferrari, Andrea C.; Hertel, Tobias in JOURNAL OF PHYSICAL CHEMISTRY C (2011). 115(30) 14682–14686.
  • [ Abstract ]
  • [ BibTeX ]
  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 x 10(-17) cm(2) 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{schoeppler2011molar, 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 x 10(-17) cm(2) 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 = {Schoeppler, 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 = {JOURNAL OF PHYSICAL CHEMISTRY C}, keywords = {}, number = 30, pages = {14682–14686}, publisher = {AMER CHEMICAL SOC}, 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. (2011). 107(25) 257402.
  • [ 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 = {}, month = 12, 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 }
• Free-Carrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime. Crochet, Jared J.; Hoseinkhani, Sajjad; Lueer, Larry; Hertel, Tobias; Doorn, Stephen K.; Lanzani, Guglielmo in PHYSICAL REVIEW LETTERS (2011). 107(25)
  • [ Abstract ]
  • [ BibTeX ]
  We present evidence for the generation of free carriers in aggregated single-wall carbon nanotubes by photoexcitation in the energetic range of the pi ->pi* 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, S(2).

  @article{crochet2011freecarrier, abstract = {We present evidence for the generation of free carriers in aggregated single-wall carbon nanotubes by photoexcitation in the energetic range of the pi ->pi* 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, S(2).}, author = {Crochet, Jared J. and Hoseinkhani, Sajjad and Lueer, Larry and Hertel, Tobias and Doorn, Stephen K. and Lanzani, Guglielmo}, journal = {PHYSICAL REVIEW LETTERS}, keywords = {}, number = 25, publisher = {AMER PHYSICAL SOC}, title = {Free-Carrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime}, volume = 107, 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 Physical Review Letters (2011). 107(25)
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Crochet_2011, author = {Crochet, Jared J. and Hoseinkhani, Sajjad and Lüer, Larry and Hertel, Tobias and Doorn, Stephen K. and Lanzani, Guglielmo}, journal = {Physical Review Letters}, keywords = {}, month = 12, number = 25, publisher = {American Physical Society ({APS})}, title = {Free-Carrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime}, volume = 107, year = 2011 }

• Long lived photo excitations in (6,5) carbon nanotubes. Sciascia, C.; Crochet, J.; Hertel, T.; Lanzani, G. in EUROPEAN PHYSICAL JOURNAL B (2010). 75(2) 115–120.
  • [ Abstract ]
  • [ BibTeX ]
  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 = {}, number = 2, pages = {115–120}, publisher = {SPRINGER}, title = {Long lived photo excitations in (6,5) carbon nanotubes}, volume = 75, 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 (2010). 4(12) 7161–7168.
  • [ Abstract ]
  • [ BibTeX ]
  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 = {}, number = 12, pages = {7161–7168}, publisher = {AMER CHEMICAL SOC}, title = {Diffusion Limited Photoluminescence Quantum Yields in 1-D Semiconductors: Single-Wall Carbon Nanotubes}, volume = 4, year = 2010 }
• Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates. Lueer, Larry; Crochet, Jared; Hertel, Tobias; Cerullo, Giulio; Lanzani, Guglielmo in ACS NANO (2010). 4(7) 4265–4273.
  • [ Abstract ]
  • [ BibTeX ]
  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 = {}, number = 7, pages = {4265–4273}, publisher = {AMER CHEMICAL SOC}, title = {Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates}, volume = 4, year = 2010 }
• A brighter future. Hertel, Tobias in Nature Photonics (2010). 4(2) 77–78.
  • [ BibTeX ]
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  @article{Hertel_2010, author = {Hertel, Tobias}, journal = {Nature Photonics}, keywords = {}, month = {02}, number = 2, pages = {77–78}, publisher = {Springer Nature}, title = {A brighter future}, volume = 4, year = 2010 }
• Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates. Lüer, Larry; Crochet, Jared; Hertel, Tobias; Cerullo, Giulio; Lanzani, Guglielmo in ACS Nano (2010). 4(7) 4265–4273.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{L_er_2010, author = {Lüer, Larry and Crochet, Jared and Hertel, Tobias and Cerullo, Giulio and Lanzani, Guglielmo}, journal = {{ACS} Nano}, keywords = {}, month = {06}, number = 7, pages = {4265–4273}, publisher = {American Chemical Society ({ACS})}, title = {Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates}, volume = 4, year = 2010 }
• CARBON NANOTUBES A brighter future. Hertel, Tobias (2010). 77–78.
  • [ BibTeX ]
  @news{hertel2010carbon, author = {Hertel, Tobias}, journal = {NATURE PHOTONICS}, keywords = {}, number = 2, pages = {77–78}, publisher = {NATURE PUBLISHING GROUP}, title = {CARBON NANOTUBES A brighter future}, volume = 4, year = 2010 }

• 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 (2009). 102(12)
  • [ Abstract ]
  • [ BibTeX ]
  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 = {}, number = 12, publisher = {AMER PHYSICAL SOC}, title = {Coherent Phonon Dynamics in Semiconducting Carbon Nanotubes: A Quantitative Study of Electron-Phonon Coupling}, volume = 102, year = 2009 }
• 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 (2009). 80(20)
  • [ Abstract ]
  • [ BibTeX ]
  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 = {}, number = 20, publisher = {AMER PHYSICAL SOC}, title = {Ultrafast dynamics in metallic and semiconducting carbon nanotubes}, volume = 80, year = 2009 }

• Size and mobility of excitons in (6, 5) carbon~nanotubes. Lüer, Larry; Hoseinkhani, Sajjad; Polli, Dario; Crochet, Jared; Hertel, Tobias; Lanzani, Guglielmo in Nature Physics (2008). 5(1) 54–58.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{L_er_2008, author = {Lüer, Larry and Hoseinkhani, Sajjad and Polli, Dario and Crochet, Jared and Hertel, Tobias and Lanzani, Guglielmo}, journal = {Nature Physics}, keywords = {}, month = 11, number = 1, pages = {54–58}, publisher = {Springer Nature}, title = {Size and mobility of excitons in (6, 5) carbon nanotubes}, volume = 5, year = 2008 }
• 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 (2008). 8(1) 87–91.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Hertel_2008, author = {Hertel, Tobias and Perebeinos, Vasili and Crochet, Jared and Arnold, Katharina and Kappes, Manfred and Avouris, Phaedon}, journal = {Nano Letters}, keywords = {}, month = {01}, number = 1, pages = {87–91}, publisher = {American Chemical Society ({ACS})}, title = {Intersubband Decay of 1-D Exciton Resonances in Carbon Nanotubes}, volume = 8, year = 2008 }

• Quantum Yield Heterogeneities of Aqueous Single-Wall Carbon Nanotube Suspensions. Crochet, Jared; Clemens, Michael; Hertel, Tobias in Journal of the American Chemical Society (2007). 129(26) 8058–8059.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Crochet_2007, author = {Crochet, Jared and Clemens, Michael and Hertel, Tobias}, journal = {Journal of the American Chemical Society}, keywords = {}, month = {07}, number = 26, pages = {8058–8059}, publisher = {American Chemical Society ({ACS})}, title = {Quantum Yield Heterogeneities of Aqueous Single-Wall Carbon Nanotube Suspensions}, volume = 129, year = 2007 }
• Pump-Probe Spectroscopy of Exciton Dynamics in (6,5) Carbon Nanotubes. Zhu, Zipeng; Crochet, Jared; Arnold, Michael S.; Hersam, Mark C.; Ulbricht, Hendrik; Resasco, Daniel; Hertel, Tobias in The Journal of Physical Chemistry C (2007). 111(10) 3831–3835.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Zhu_2007, author = {Zhu, Zipeng and Crochet, Jared and Arnold, Michael S. and Hersam, Mark C. and Ulbricht, Hendrik and Resasco, Daniel and Hertel, Tobias}, journal = {The Journal of Physical Chemistry C}, keywords = {}, month = {03}, number = 10, pages = {3831–3835}, publisher = {American Chemical Society ({ACS})}, 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) (2007). 244(11) 3964–3968.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Crochet_2007, author = {Crochet, J. and Clemens, M. and Hertel, T.}, journal = {physica status solidi (b)}, keywords = {}, month = 11, number = 11, pages = {3964–3968}, publisher = {Wiley}, title = {Optical properties of structurally sorted single-wall carbon nanotube ensembles}, volume = 244, year = 2007 }
• Ultrafast Spectroscopy of Carbon Nanotubes. Ma, Ying-Zhong; Hertel, Tobias; Vardeny, Zeev Valy; Fleming, Graham R.; Valkunas, Leonas in Topics in Applied Physics (2007). 321–353.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @incollection{Ma_2007, author = {Ma, Ying-Zhong and Hertel, Tobias and Vardeny, Zeev Valy and Fleming, Graham R. and Valkunas, Leonas}, booktitle = {Topics in Applied Physics}, keywords = {}, pages = {321–353}, publisher = {Springer Berlin Heidelberg}, title = {Ultrafast Spectroscopy of Carbon Nanotubes}, year = 2007 }

• Thermal desorption of gases and solvents from graphite and carbon nanotube surfaces. Ulbricht, Hendrik; Zacharia, Renju; Cindir, Nesibe; Hertel, Tobias in Carbon (2006). 44(14) 2931–2942.
  • [ BibTeX ]
  • [ URL ]
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  @article{Ulbricht_2006, author = {Ulbricht, Hendrik and Zacharia, Renju and Cindir, Nesibe and Hertel, Tobias}, journal = {Carbon}, keywords = {}, month = 11, number = 14, pages = {2931–2942}, publisher = {Elsevier {BV}}, title = {Thermal desorption of gases and solvents from graphite and carbon nanotube surfaces}, volume = 44, year = 2006 }
• 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 (2006). 96(2)
  • [ 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 = {}, month = {{JAN 20}}, number = {{2}}, title = {{Ultrafast dynamics of delocalized and localized electrons in carbon nanotubes}}, volume = {{96}}, 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) (2006). 243(13) 3186–3191.
  • [ BibTeX ]
  • [ URL ]
  • [ DOI ]
  @article{Hertel_2006, author = {Hertel, Tobias and Zhu, Zipeng and Crochet, Jared and McPheeters, Claiborne and Ulbricht, Hendrik and Resasco, Daniel}, journal = {physica status solidi (b)}, keywords = {}, month = 11, number = 13, pages = {3186–3191}, publisher = {Wiley}, title = {Exciton dynamics probed in carbon nanotube suspensions with narrow diameter distribution}, volume = 243, year = 2006 }

• Exponential Decay Lifetimes of Excitons in Individual Single-Walled Carbon Nanotubes. Hagen, Axel; Steiner, Mathias; Raschke, Markus B.; Lienau, Christoph; Hertel, Tobias; Qian, Huihong; Meixner, Alfred J.; Hartschuh, Achim in Physical Review Letters (2005). 95(19)
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  @article{Hagen_2005, author = {Hagen, Axel and Steiner, Mathias and Raschke, Markus B. and Lienau, Christoph and Hertel, Tobias and Qian, Huihong and Meixner, Alfred J. and Hartschuh, Achim}, journal = {Physical Review Letters}, keywords = {}, month = 10, number = 19, publisher = {American Physical Society ({APS})}, title = {Exponential Decay Lifetimes of Excitons in Individual Single-Walled Carbon Nanotubes}, volume = 95, year = 2005 }

• Carbon nanotubes - Building blocks of microelectronics of tomorrow?. Hertel, T in NACHRICHTEN AUS DER CHEMIE (2004). 52(2) 137–140.
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  @article{hertel2004carbon, author = {Hertel, T}, journal = {{NACHRICHTEN AUS DER CHEMIE}}, keywords = {}, 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, Renju; Ulbricht, Hendrik; Hertel, Tobias in Physical Review B (2004). 69(15)
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  @article{Zacharia_2004, author = {Zacharia, Renju and Ulbricht, Hendrik and Hertel, Tobias}, journal = {Physical Review B}, keywords = {}, month = {04}, number = 15, publisher = {American Physical Society ({APS})}, title = {Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons}, volume = 69, year = 2004 }
• Electronic structure and dynamics of optically excited single-wall carbon nanotubes. Hagen, A.; Moos, G.; Talalaev, V.; Hertel, T. in Applied Physics A (2004). 78(8) 1137–1145.
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  @article{Hagen_2004, author = {Hagen, A. and Moos, G. and Talalaev, V. and Hertel, T.}, journal = {Applied Physics A}, keywords = {}, month = {05}, number = 8, pages = {1137–1145}, publisher = {Springer Nature}, title = {Electronic structure and dynamics of optically excited single-wall carbon nanotubes}, volume = 78, year = 2004 }

• Dynamics of C60Encapsulation into Single-Wall Carbon Nanotubes. Ulbricht, Hendrik; Hertel, Tobias in The Journal of Physical Chemistry B (2003). 107(51) 14185–14190.
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  @article{Ulbricht_2003, author = {Ulbricht, Hendrik and Hertel, Tobias}, journal = {The Journal of Physical Chemistry B}, keywords = {}, month = 12, number = 51, pages = {14185–14190}, publisher = {American Chemical Society ({ACS})}, title = {Dynamics of C60Encapsulation into Single-Wall Carbon Nanotubes}, volume = 107, year = 2003 }
• Quantitative Analysis of Optical Spectra from Individual Single-Wall Carbon Nanotubes. Hagen, Axel; Hertel, Tobias in Nano Letters (2003). 3(3) 383–388.
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  @article{Hagen_2003, author = {Hagen, Axel and Hertel, Tobias}, journal = {Nano Letters}, keywords = {}, month = {03}, number = 3, pages = {383–388}, publisher = {American Chemical Society ({ACS})}, title = {Quantitative Analysis of Optical Spectra from Individual Single-Wall Carbon Nanotubes}, volume = 3, year = 2003 }

• 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 (2002). 75(4) 449–465.
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  @article{Hertel_2002, author = {Hertel, T. and Fasel, R. and Moos, G.}, journal = {Applied Physics A: Materials Science {\&} Processing}, keywords = {}, month = 10, number = 4, pages = {449–465}, publisher = {Springer Nature}, 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, Hendrik; Moos, Gunnar; Hertel, Tobias in Physical Review B (2002). 66(7)
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  @article{Ulbricht_2002, author = {Ulbricht, Hendrik and Moos, Gunnar and Hertel, Tobias}, journal = {Physical Review B}, keywords = {}, month = {08}, number = 7, publisher = {American Physical Society ({APS})}, title = {Physisorption of molecular oxygen on single-wall carbon nanotube bundles and graphite}, volume = 66, year = 2002 }

• Adsorption and desorption of weakly bonded adsorbates from single-wall carbon nanotube bundles. Hertel, Tobias in AIP Conference Proceedings (2001).
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  @inproceedings{Hertel_2001, author = {Hertel, Tobias}, booktitle = {{AIP} Conference Proceedings}, keywords = {}, publisher = {{AIP}}, title = {Adsorption and desorption of weakly bonded adsorbates from single-wall carbon nanotube bundles}, year = 2001 }
• 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 (2001). 87(26)
  • [ Abstract ]
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  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 = {}, 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}} }

• Influence of excited electron lifetimes on the electronic structure of carbon nanotubes. Hertel, T; Moos, G in CHEMICAL PHYSICS LETTERS (2000). 320(3-4) 359–364.
  • [ Abstract ]
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  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 = {}, 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}} }

• Carbon nanotubes: nanomechanics, manipulation, and electronic devices. Avouris, P; Hertel, T; Martel, R; Schmidt, T; Shea, HR; Walkup, RE in APPLIED SURFACE SCIENCE (1999). 141(3-4) 201–209.
  • [ Abstract ]
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  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 = {}, 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 (1999). 46(1-4) 101–104.
  • [ Abstract ]
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  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 = {}, 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}} }

• 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 (1998). 66(2, S) S659-S667.
  • [ Abstract ]
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  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 = {}, 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}} }
• Single- and multi-wall carbon nanotube field-effect transistors. Martel, R; Schmidt, T; Shea, HR; Hertel, T; Avouris, P in APPLIED PHYSICS LETTERS (1998). 73(17) 2447–2449.
  • [ Abstract ]
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  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 = {}, month = {{OCT 26}}, number = {{17}}, pages = {{2447-2449}}, title = {{Single- and multi-wall carbon nanotube field-effect transistors}}, volume = {{73}}, year = {{1998}} }
• Manipulation of individual carbon nanotubes and their interaction with surfaces. Hertel, T; Martel, R; Avouris, P in JOURNAL OF PHYSICAL CHEMISTRY B (1998). 102(6) 910–915.
  • [ Abstract ]
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  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 = {}, month = {{FEB 5}}, number = {{6}}, pages = {{910-915}}, title = {{Manipulation of individual carbon nanotubes and their interaction with surfaces}}, volume = {{102}}, year = {{1998}} }
• Deformation of carbon nanotubes by surface van der Waals forces. Hertel, T; Walkup, RE; Avouris, P in PHYSICAL REVIEW B (1998). 58(20) 13870–13873.
  • [ Abstract ]
  • [ BibTeX ]
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  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 = {}, month = {{NOV 15}}, number = {{20}}, pages = {{13870-13873}}, title = {{Deformation of carbon nanotubes by surface van der Waals forces}}, volume = {{58}}, year = {{1998}} }

• Atomic force microscope tip-induced local oxidation of silicon: Kinetics, mechanism, and nanofabrication. Avouris, P; Hertel, T; Martel, R in APPLIED PHYSICS LETTERS (1997). 71(2) 285–287.
  • [ Abstract ]
  • [ BibTeX ]
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  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 = {}, 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}} }
• 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 (1997). 15(3, 2) 1503–1509.
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  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 = {}, 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}} }

• 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 (1996). 205(1-2) 205–219.
  • [ Abstract ]
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  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 = {}, 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}} }
• 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 (1996). 54(8) R5295-R5298.
  • [ Abstract ]
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  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 = {}, 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}} }

• 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 (1995). 101(1-2) 53–55.
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  • [ 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 = {}, 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}} }
• UV PHOTOSTIMULATED DESORPTION OF AMMONIA FROM CU(111). HERTEL, T; WOLF, M; ERTL, G in JOURNAL OF CHEMICAL PHYSICS (1995). 102(8) 3414–3430.
  • [ BibTeX ]
  • [ DOI ]
  @article{hertel1995photostimulated, author = {HERTEL, T and WOLF, M and ERTL, G}, journal = {{JOURNAL OF CHEMICAL PHYSICS}}, keywords = {}, month = {{FEB 22}}, number = {{8}}, pages = {{3414-3430}}, title = {{UV PHOTOSTIMULATED DESORPTION OF AMMONIA FROM CU(111)}}, volume = {{102}}, 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 (1995). 101(1-2) 56–59.
  • [ 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 = {}, 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}} }

• 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 (1994). 72(4) 578–581.
  • [ 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 = {}, 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}} }
• 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 (1994). 50(11) 8126–8129.
  • [ 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 = {}, 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}} }
• 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 (1994). 317(3) L1147-L1151.
  • [ 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 = {}, 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}} }

• EPITAXIAL-GROWTH OF MAGNESIUM ON RU(0001). OVER, H; HERTEL, T; BLUDAU, H; PFLANZ, S; ERTL, G in PHYSICAL REVIEW B (1993). 48(8) 5572–5578.
  • [ 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 = {}, month = {{AUG 15}}, number = {{8}}, pages = {{5572-5578}}, title = {{EPITAXIAL-GROWTH OF MAGNESIUM ON RU(0001)}}, volume = {{48}}, year = {{1993}} }

• 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 (1992). 279(1-2) L170-L174.
  • [ 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 = {}, 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}} }