Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S29: Focus Session: Carbon Nanotubes and Related Materials XI: Optical Spectroscopy |
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Sponsoring Units: DMP Chair: James Hone, Columbia University Room: Morial Convention Center 221 |
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S29.00001: \textit{In situ} Raman Spectroscopy of Suspended Carbon Nanotubes under High Voltage Bias Invited Speaker: We report recent results of Raman spectroscopy taken of individual suspended single-walled carbon nanotubes exhibiting negative differential conductance (NDC) \textit{in situ} under high voltage biases [1]. The transverse and longitudinal optical phonon modes ($G_{+}$ and $G_{-}$ band) are found to respond differently to the applied voltage bias. We observe preferential downshifting of only one optical phonon mode while the other remains largely unchanged, indicating a non-equilibrium phonon population caused by the preferential electron-phonon coupling of only one optical phonon polarization. This preferential coupling is caused by the differences between the two Kohn anomalies in the TO and LO phonon branches [2]. Surprisingly, in most metallic nanotubes, the narrow $G_{+}$ band (TO band) is more strongly heated by electron-phonon scattering at high biases. The non-equilibrium phonon populations produced under high biases are corroborated by anti-Stokes Raman spectroscopy. We correlate the optically measured phonon population to the electrically measured resistivity using a Landauer model to determine key scattering parameters. The electron-phonon scattering mechanism revealed by these measurements and this analysis show the importance of electron-phonon scattering by phonon absorption from the large non-equilibrium phonon population in explaining the observed negative differential conductance [3]. \newline \newline [1] A. W. Bushmaker, V. V. Deshpande , M. W. Bockrath, S. B. Cronin, Direct observation of mode selective electron-phonon coupling in suspended carbon nanotubes, \textit{Nano Lett.}, in press (2007). \newline [2] S. Piscanec, M. Lazzeri, J. Robertson, A. C. Ferrari, F. Mauri, \textit{Phys. Rev. B} \textbf{75}, 35427 (2007). \newline [3] E. Pop, D. Mann, J Cao, Q. Wang, K. Goodson, H. Dai, \textit{Phys. Rev. Lett.} \textbf{95}, 155505 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S29.00002: Measurement of the absorption cross section of individual single walled carbon nanotubes St\'ephane Berciaud, Laurent Cognet, R. Bruce Weisman, Brahim Lounis We combined time--resolved and cw luminescence studies on highly luminescent individual (6,5) single walled carbon nanotubes in aqueous environments to yield the first experimental determination of the absorption cross section of individual nanotubes. Luminescence decays systematically exhibited a bi-exponential behavior with a short component (mean 45ps) accounting for most of the integrated signal, followed by a weak tail decaying on a 250 ps timescale. We obtained a mean statistical value of 1.0 10$^{-17}$ cm$^{2}$ per carbon atom for nanotubes resonantly excited at their second order optical transition, a value independently obtained by photothermal absorption measurements also performed on individual (6,5) nanotubes. Precise knowledge of the absorption cross section of individual nanotubes is essential for the determination of nanotube quantum yield as well as in quantitative studies of multi-excitonic processes. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S29.00003: Length-dependent optical properties of single-walled carbon nanotubes J. R. Simpson, J. A. Fagan, B. J. Bauer, E. K. Hobbie, A. R. Hight Walker Length proves to be an important physical characteristic of single-walled carbon nanotubes (SWCNTs) strongly affecting optical absorption, photoluminescence, and resonance Raman spectroscopies.\footnote{J. A. Fagan \textit{et al.}, J. Am. Chem. Soc. \textbf{129}, 10607 (2007).} Our samples include HiPco, CoMoCat, and arc-discharge SWCNTs dispersed in aqueous solutions by wrapping with either DNA or surfactant and exhibiting an exceptionally low degree of SWCNT bundling/clustering. Size-exclusion chromatography or density sorting ultracentrifugation collect length fractions ranging in size from approximately 50 nm to 1000 nm. The optical spectral weight specific to the SWCNT electronic states and photoluminescence peak emission intensity, compared to their underlying backgrounds, scale approximately linearly with length. All observed Raman vibrational modes exhibit a monotonic increase of scattering intensity with nanotube length. Resonance Raman of the radial breathing mode reveals a blueshift of the excitation energy for shorter nanotubes. Localization of bound excitons along the nanotube may explain the observed length-dependent optical properties. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S29.00004: Reflectance spectra of individual single walled carbon nanotubes Hualing Zeng We report back scattering spectroscopic measurements on individual single walled carbon nanotubes (SWNTs). The reflectance spectra show geometry-dependent resonant peaks corresponding to optical transitions between Van Hove singularities in SWNTs' joint density of states. All nanotubes display certain colours as their reflectance spectra demonstrate strong energy dependence. This approach was proved to be an effective tool to probe geometric structures and optical properties of individual SWNTs. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S29.00005: Inverted spectra of SWCNT films John Lehman, Katherine Hurst, Lara Roberson, Kathryn Nield, John Hamlin Diffuse Reflectance for purified single wall carbon nanotube (SWCNT) films and its relation to absorptance in the wavelength range 0.6 $\mu $m to 2 $\mu $m are inverted when compared to absorptivity data in the literature. This surprising behavior has been corroborated by diffuse reflectance measurements and shows that the reflectance is a substantial part of the unique optical behavior. Typically, the absorptance is fairly assumed to be complementary to the transmittance, while the reflectivity is insignificant. Only in certain instances (see for example, Barnes, et. al[1], Wang, et. al[2]), is the small reflectance explicitly accounted for. In the present work, we present diffuse reflectance and specular absorptance at normal incidence of SWCNT films. \newline [1] T. M. Barnes, J. van de Lagemaat, D. Levi,1 G. Rumbles, T. J. Coutts, C. L. Weeks, D. A. Britz, I. Levitsky, J. Peltola, P. Glatkowski, Phys. Rev. B \textbf{75}, 235410 (2007). \newline [2] F. Wang, M. Y. Sfeir, L. Huang, X. M. H. Huang, Wu, J. Kim, J. Hone, S. O'Brien, L. E. Brus, T. F. Heinz, PRL \textbf{96}, 167401 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S29.00006: Broadband Rayleigh Scattering and Photoconductivity Spectra of Individual Semiconducting Single-Walled Carbon Nanotubes Matthew Sfeir, Sami Rosenblatt, Yang Wu, Hugen Yan, Christophe Voisin, Bhupesh Chandra, Robert Caldwell, Yuyao Shan, James Hone, Tony F. Heinz, James A. Misewich Combining a Fourier-transform measurement of photoconductivity with Rayleigh spectroscopy, we have identified the four lowest-lying optical transitions from specific, individual single-walled carbon nanotubes. In these investigations we made use of the previously reported transfer technique [1] to obtain samples with optimized arrangements both for Rayleigh (freely suspended) and photoconductivity (transistor geometry) spectroscopy. The combination of these two optical characterization techniques yields high-resolution spectra of the electronic transitions of individual nanotubes over a spectral range extending from 0.3 -- 2.7 eV. We will discuss the details of the spectra that we have obtained for individual single-walled nanotubes of defined chiral index, including the observation of asymmetric lineshapes for the lowest-lying optical transition. [1] X. M. H. Huang, et al., Nano Lett$.$ \textbf{5}, 1515 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S29.00007: Optical phonon and hot carrier lifetimes in single-walled carbon nanotubes by time-resolved anti-Stokes Raman scattering Kwangu Kang, David Cahill, Taner Ozel, Moonsub Shim The lifetimes of optical phonon and photoexcited carriers in both semiconducting and metallic single-walled carbon nanotubes are determined by time-resolved Raman scattering using a subpicosecond pump-probe method. Non-equilibrium populations of electronic and phonon excitations are observed by incoherent anti-Stokes Raman scattering from a broad continuum and the G mode, respectively. HiPco nanotubes with E$_{22}$ transitions and arc-discharge nanotubes with E$_{11}$ transitions dominate the spectra because of their resonance with the photon energy. To separate Raman scattering created by the probe beam from scattering created by the pump beam, we have developed a two- color pump-probe technique based on the broad bandwidth of the Ti:sapphire laser oscillator and narrow bandpass optical filters. For semiconducting tubes, the optical phonon lifetimes decrease from 1.2 ps to 0.9 ps with increasing laser fluence. The optical phonon lifetimes of metallic tubes, on the other hand, increase from 0.6 ps to 1.1 ps. The hot carrier lifetime is approximately 0.3 ps. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S29.00008: Raman Spectroscopy of isolated Double Wall Carbon Nanotubes (DWNT) Federico Villalpando-Paez, Alfonso Reina Cecco, Daisuke Shimamoto, Antonio G. Souza Filho, Hyungbin Son, Yoong A. Kim, Endo Morinobu , Mauricio Terrones, Mildred Dresselhaus We have developed a method to perform Raman spectroscopy on isolated double wall carbon nanotubes (DWNT). By identifying isolated DWNTs and obtaining their Raman spectra using different laser lines, we are able to find DWNTs whose inner and outer walls are in resonance with the same laser line or with more than one laser lines ranging from 514nm to 785nm. The inner and outer walls of a DWNT can be metallic (M) or semiconducting (S) and each of the four possible configurations (M/M, M/S, S/S, S/M) has different electronic properties. The obtained Raman spectra show simplified radial breathing mode (RBM), G and G' line shapes that allow us to study the inter layer interactions and make comparisons to previous experiments on DWNT bundles and double layer graphene. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S29.00009: Theory of coherent phonon spectroscopy in carbon nanotubes G. D. Sanders, C. J. Stanton, Y. S. Lim, K. J. Yee, J. H. Kim, E. H. Haroz, L. G. Booshehri, J. Kono We develop a theory for the generation and detection of coherent phonons in single wall carbon nanotubes. Coherent phonons are generated in the nanotube by ultrafast laser pulses via the deformation potential electron-phonon interaction with the photogenerated carriers. The electronic states are treated in a tight binding formalism which gives a description of the states over the nanotube Brillouin zone while the nanotube phonon modes are treated in a valence force field model that includes bond-stretching, in-plane and out-of-plane bond-bending, and bond-twisting interactions. Equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations. In coherent phonon spectroscopy the coherent phonons are detected by ultrafast pump probe differential transmission measurements. We find that for uniform illumination with a 5 fs pump pulse only the q = 0 radial breathing mode and a high frequency G mode are strongly excited. We will discuss excitation strengths for different coherent phonon modes and compare to recent experiments. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S29.00010: Stability of carbon nanotubes to laser irradiation probed by Raman spectroscopy. Alexander Soldatov, David Olevik, Manuel Dossot, Edward McRae CNTs in a bundled state suffer from overheating effects - exposure to laser irradiation leads to a reversible shift of the RBM resonance window at a moderate laser fluence [1] or even to damaging of certain nanotube types at higher fluencies. Here we report on our systematic study of the influence of laser irradiation on the Raman spectra of HiPCO-produced single wall CNTs. Specifically, we have examined Raman response of bundled CNTs to: i) laser power density; ii) exposure time and iii) photon energy (1.96 and 2.33 eV). Our results show that irreversible destruction of CNTs in the bundles takes place at even a moderate laser power density ($\sim $500 W/cm$^{2})$. Notably, the tubes with smaller diameters are influenced first and the rate of CNT damage increases with photon energy. Finally, we determined that the threshold for the RBM spectrum profile to change at $\sim $200 W/cm$^{2}$, which is apparently below the laser fluencies used typically in Raman experiments on CNT bundles. Based on these results we developed a regime of Raman data collection which was recently used to identify functionalization of different types of CNTs [2] from their RBM Raman spectra. [1] C. Fantini, et al. Phys. Rev. Lett., \textbf{93}, 147406 (2004). [2] J. Liu, et al. Carbon, \textbf{45}, 885, (2007). [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S29.00011: Surface Enhanced Raman Spectroscopy (SERS) and Scanning Electron Microscopy of Individual SERS Hot Spots Rajay Kumar, Stephen Cronin We measure Raman spectroscopy and scanning electron microscopy before and after depositing silver nanoparticles on carbon nanotubes. Individual SERS ``hot spots'' are identified with respect to a lithographically defined grid using micro-Raman spectroscopy. Carbon nanotubes' extremely large aspect ratio enables subsequent imaging of the nanoparticle geometry together with the SERS active molecule. The SERS enhancement factor is determined by comparing the Raman intensity of an individual nanotube before and after nanoparticle deposition. The data, published in R. Kumar et al., Appl. Phys. Lett., 91 (2007), reports SERS enhancement factors up to 134,000 and nanoparticle heating exceeding 600C, as evidenced by the local burnout of nanotubes in SERS hot spot regions. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S29.00012: Electrical Transport Studies of (n,n) Armchair Carbon Nanotubes Robert Caldwell, Bhupesh Chandra, Christophe Voisin, Tony F. Heinz, James Hone By using Rayleigh scattering spectroscopy, a simple mechanical transfer process, and standard E-beam lithography fabrication of metallic leads, we can probe the electrical properties of individual single-walled carbon nanotubes of known chiral indices (n,m) on the substrate of our choosing. Using these techniques, we have discovered that (n,n) `armchair' nanotubes consistently deviate from the predicted metallic behavior, specifically showing a gap in current -- gate voltage curves. We present detailed studies of the transport behavior of these devices, including conductivity as a function of bias, length, and temperature. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S29.00013: Supercurrent in Single Wall Carbon Nanotube Josephson Junctions Gang Liu, Yong Zhang, Chunning Lau We investigate transport in highly transparent single-wall carbon nanotube Josephson Junctions. Gate tunable supercurrent, multiple Andreev reflections and hysteresis current-voltage characteristics are observed, corresponding to on- and off-resonance transmission of charges via the nanotube's quantized energy levels. In the talk we will discuss the dependence of supercurrent on temperature, source-drain separation and gate voltage, and compare with various theoretical models. [Preview Abstract] |
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