Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H18: Focus Session: Carbon Nanotubes: Electronic and Optical Properties II |
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Sponsoring Units: DMP Chair: Vasili Perebeinos, IBM Room: Baltimore Convention Center 315 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H18.00001: Two-photon photoluminescence and exciton binding energies in single-walled carbon nanotubes Invited Speaker: The optical properties of carbon nanotubes are thought to be dominated by excitonic effects. Exciton binding energies between 100 meV and 1 eV have been predicted, depending on diameter, chiral angle, and surrounding medium. Experimental proof for these large binding energies, however, has been missing so far. Here we present direct experimental evidence that the elementary optical excitations of carbon nanotubes are strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole pairs, stable even at room temperature. By comparing one-photon and two-photon luminescence excitation spectra, we probe excitonic states with distinctly different wavefunction symmetry [1]. The energy splitting between one-photon active and two-photon active states is a distinct fingerprint of excitonic interactions in carbon nanotubes and binding energies between 300 and 500 meV are derived. Our results are strongly supported by \textit{ab initio} calculations of two-photon absorption spectra [2]. We discover that for all the tubes studied, excitonic effects are very strong and are crucial for the one-photon as well as the two-photon spectra in both the peak shapes and positions. The implications of these results for the fluorescence yield and lifetime of single-walled carbon nanotubes [3] will be discussed. [1] J. Maultzsch et al., Phys. Rev. B 72, Rxxx (2005); cond-mat/0505150. [2] E. Chang, G. Bussi, A. Ruini, and E. Molinari, Phys. Rev. Lett. 92, 196401 (2004). [3] A. Hagen et al., Phys. Rev. Lett. 95, 197401 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:27PM |
H18.00002: Ab initio study of the optical properties of carbon nanotubes Invited Speaker: We present an ab initio study of the optical properties of carbon nanotubes. We use state-of-the-art electronic structure methods based on many-body perturbation theory to compute the optical absorption and resonance Raman spectra of large tubes which have up to 200 atoms [1,2]. Our symmetry-based method makes the study of large tubes feasible within the many- body framework and also allows us to understand the symmetry properties of the excitons and selection rules. We include a study of the so-called dark excitons which are crucial for understanding luminescence efficiency in carbon nanotubes. The mechanism that explains the dark-bright splitting can be understood within our symmetry-based approach. Finally, we present an analysis of the two-photon spectra for several carbon nanotubes, a theoretical analysis which, in conjunction with combined one- and two-photon experiments, allows one to measure the binding energy of excitons. We find in all cases that the excitonic binding energy is large, ranging from 0.5 to 0.9 eV depending on the diameter of the tube, and that the excitonic wavefunction is Wannier-like and extended over many atoms. Our studies for the one- and two-photon absorption and resonance Raman spectra have been fruitful for understanding the corresponding experiments. In particular, our theoretical results are in good agreement with one- and two-photon absorption experiments [3-5]. The results for resonance Raman show that such a spectroscopic technique is a good alternative to optical absorption since it allows for the selection of tubes of a given diameter while probing the same excited states. 1. E.K. Chang, G. Bussi, A. Ruini, and E. Molinari, Phys. Rev. Lett. 92, 196401 (2004). 2. E.K. Chang, G. Bussi, A. Ruini, and E. Molinari, Phys. Rev. B 72, 195423 (2005). 3. M. Y. Sfeir et al., Science 306, 1540 (2004). 4. J. Maultzsch et al., to be published in Phys. Rev. B, see also cond-mat/0505150. 5. Z. M. Li et. al., Phys. Rev. Lett. 87, 127401 (2001). [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H18.00003: Structural Dependence of Excitonic Optical Transitions and Band-Gap Energies in Carbon Nanotubes Gordana Dukovic, Feng Wang, Daohua Song, Matthew Sfeir, Tony Heinz, Louis Brus The optical transitions in semiconducting carbon nanotubes have recently been ascribed to excitons. We have used two-photon excitation spectroscopy to measure the energies of exciton states of different symmetries by taking advantage of the different selection rules for two-photon absorption and one-photon fluorescence in this quasi-one-dimensional molecule. The measured exciton binding energies are on the order of hundreds of meV. Here we report the investigation of structural dependence of the exciton energies for different nanotube species, ranging in diameter from 0.76 to 1.2 nm. The exciton binding energies vary inversely with nanotube diameter, ranging from 420 to 270 meV in this sample. This result is in agreement with theoretical predictions. In addition, we have found that the measured band-gap energies are significantly blue-shifted from those predicted by tight-binding calculations, which may have implications for the transport properties of semiconducting carbon nanotubes. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H18.00004: Ultrafast decay of $E_{22}$ subband excitons in semiconducting SWNTs Tobias Hertel, Jared J. Crochet, Zipeng Zhu We use CW and time-resolved spectroscopy for an investigation of intra- and interband relaxation of excited states in micelle suspended SWNTs. We observe that the dynamics of $E_{22}$ subband exciton decay in SWNTs is surprisingly similar to the dynamics of photoexcited carriers in bulk graphite. Moreover we find a clear correlation of the $E_{22}$ exciton decay rate with the excited state energy. We compare these results with earlier theoretical studies of quasi-particle decay in graphite and discuss the role of electron-electron and electron-phonon coupling for the dynamics of higher excited states. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H18.00005: Real-time {\em ab initio} simulations of excited carrier dynamics in carbon nanotubes. Yoshiyuki Miyamoto, Angel Rubio Secades, David Tomanek Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a $(3,3)$ carbon nanotube at different temperatures. Following an $h\nu=6.8$~eV photo-excitation, the carrier decay is initially dominated by efficient energy transfer to the electronic degrees of freedom. About 200~fs after the photoexcitation in a nanotube initially at room temperature, the electron-hole gap is reduced to nearly half its initial value and the decay mechanism becomes dominated by coupling to phonons. We show that the onset point and damping rate within the phonon regime change with the initial ion velocities, which is a manifestation of a temperature dependent electron-phonon coupling. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H18.00006: Pump-probe excitation spectroscopy of chirality enriched SWNT suspensions Zipeng Zhu, Michael Arnold, Jared J. Crochet, Mark Hersam, Clay McPheeters, Daniel E. Resasco, Hendrik Ulbricht, Tobias Hertel We study the transient optical response of chirality enriched SWNT samples by conventional pump-probe and pump-probe excitation spectroscopy using visible pump and white-light continuum probe pulses. Comocat starting material is suspended using either SDBS or DNA as surfactants. In addition, DNA suspended samples have been isopycnically fractionated. Transient spectra of these sample are significantly less congested than multi-disperse suspensions, which greatly simplifies the interpretation of photo-bleaching and photo-absorption transients. The newly developed technique of pump-probe excitation spectroscopy (PPES) also allows to study previously inaccessible aspects of the dynamics of the second subband $E_{22}$ exciton decay. Experiments indicate that $E_{22}\rightarrow E_{11}$ interband relaxation rates are higher than the decay of $G$-mode phonon sidebands seen not only in PPES but also in photoluminescence excitation spectra. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H18.00007: Intrinsic and extrinsic effects in the temperature dependent photoluminescence of semiconducting carbon nanotubes Denis Karaiskaj, Chaiwat Engtrakul, Timothy McDonald, Michael J. Heben, Angelo Mascarenhas The temperature dependence of the band gap of semiconducting carbon nanotubes was measured for ten different nanotube species. The unprecedented effectiveness in avoiding the effect of external strain, or any other effects originating from the surrounding environment, lead to an accurate measurement of the band gap temperature dependence, giving fundamental insight into the nanotube electron-phonon interaction. Small but reproducible energy shifts of the emission lines with temperature were observed, showing a moderate chirality dependence, well in agreement with recent theoretical calculations [1]. In addition to the energy shift, a substantial narrowing of the emission lines was also observed. The removal of the temperature shift of the band gap, allows the precise measurement of the effect of external strain on carbon nanotubes in different environments. \newline \newline [1] Rodrigo B. Capaz, Catalin D. Spataru, Paul Tangney, Marvin L. Cohen, and Steven G. Louie, Phys. Rev. Lett. \textbf{94}, 036801 (20050). [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H18.00008: Strong anisotropy in the THz absorption spectra of stretch-aligned single walled carbon nanotubes Y. Iwasa, N. Akima, H. Matsui, N. Toyota, S. Brown, A. M. Barbour, J. Cao, J. L. Musfeldt, M. Shiraishi, H. Shimoda, O. Zhou Polarized THz spectroscopy is crucial for understanding the low-energy electronic structure and carrier dynamics in single walled carbon nanotubes (SWNTs), as well as for exploring polarization-sensitive THz applications. We prepared stretch-aligned SWNT/polymer composites, and measured the polarized absorption spectra from the THz through the visible region. The low-frequency electronic excitations are predominantly polarized parallel to the tube direction. The peak centered near 100 cm$^{-1}$ is discussed in terms of a curvature-induced gap and a plasmon resonance due to a finite size/wavelength effects in the SWNTs. The broad middle infrared structure that is observed in unoriented films with spaghetti-like morphology disappears in stretch-aligned samples, suggesting that this middle infrared feature may be due to in-gap states in the semiconducting tubes caused by the highly disordered morphology of the unoriented films. Hole doping effects were also investigated, and conversion of semiconducting tubes to more conducting ones is demonstrated. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H18.00009: Ultrafast dynamics of the mid-infrared response of carbon nanotubes Christian Frischkorn, Tobias Kampfrath, Luca Perfetti, Martin Wolf We report on time-resolved measurements of low-energy excitations in carbon nanotubes and compare these with results obtained for graphite. The system’s mid-infrared response has been obtained from time-resolved THz spectroscopy data in the 10 - 30 THz spectral range. We find essentially two processes governing an electronic current dynamics in carbon nanotubes. First, strongly bound excitons are the main photoproduct in large-band gap tubes and thus prevent a typical free-carrier response, while in small-gap and metallic tubes carrier localization due to defects is observed as manifested in a substantial dichroism. In these measurements, the reduced polarizability perpendicular to the tube axis is exploited. In the case of graphite, our results show that strongly coupled optical phonons in the graphite layer dominate the ultrafast energy and transport relaxation dynamics after optical excitation [1]. These phonon modes heat up on a femtosecond time scale and cool down with a time constant of several picoseconds. Moreover, the observed pronounced increase in the Drude relaxation rate significantly originates from these few active lattice vibrations. - [1] PRL 95, 187403 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H18.00010: Photoluminescence excitation spectroscopy of individual single walled carbon nanotubes Jacques Lefebvre, Jeffery Bond, Yoshikazu Homma, Paul Finnie Photoluminescence excitation spectroscopy is an essential tool for understanding the optical properties of single walled carbon nanotubes. In many cases, such measurements are performed on large ensembles and, aside from the main spectral features, reliable assignment is most often difficult. In order to address the existence of additional emission and absorption levels/bands in PLE spectra, this work has focused on the study of individual and small ensembles of single walled carbon nanotubes. Such carbon nanotubes are grown by chemical vapor deposition on pre-patterned substrates allowing individual nanotubes to be suspended in free space. The measurements are performed in air, at room temperature, without post growth processing of the nanotubes. [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H18.00011: Optical Spectroscopy of individual Single-Walled Carbon Nanotubes of Defined Chiral Structure Matthew Sfeir, Tobias Beetz, Feng Wang, Limin Huang, X.M. Henry Huang, James Hone, Stephen O'Brien, James Misewich, Tony Heinz, YiMei Zhu, Louis Brus Single-walled carbon nanotubes (SWNTs) constitute of a family of nanostructures with over one hundred members. Each specific structure has distinct electronic and optical properties, varying from semiconducting to metallic. Here we study the correlation between electronic structure and physical structure in SWNTs by combing high-sensitivity Rayleigh spectroscopy and electron diffraction techniques. Simultaneous application of these experimental techniques on individual SWNTs allows for measurement of electronic transitions in tubes of fully and independently determined structure$.$ In this manner, we directly verify the ``family behavior'' that has guided previous spectroscopic assignment of optical transitions in semiconducting SWNTs. We also demonstrate the previously unobserved, but theoretically predicted splitting of optical transitions in metallic SWNTs. [Preview Abstract] |
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