Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X29: Focus Session: Carbon Nanotubes and Related Materials XV: Electronic Structure and Optical Properties |
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Sponsoring Units: DMP Chair: Eva Andrei, Rutgers University Room: Morial Convention Center 221 |
Friday, March 14, 2008 8:00AM - 8:12AM |
X29.00001: Saturation of Photoluminescence from Carbon Nanotubes at High Laser Intensities: Exciton-Exciton Annihilation near the Mott Density Yoichi Murakami, Junichiro Kono We have carried out a nonlinear photoluminescence excitation (PLE) spectroscopy study of carbon nanotube ensembles using intense, femtosecond, and wavelength-tunable optical pulses. For all PL features we examined, their intensities were seen to saturate at high laser fluence, irrespective of whether the excitation was resonant or non-resonant with the $E_{22}$ transition. As the fluence was increased from the linear regime to the saturation regime, excitation resonances at $E_{22}$ energies gradually broadened and eventually became completely flat at the highest fluence, indicating that the PL intensity became independent of the excitation wavelength. However, the energies and lineshape of PL \textit{emission peaks} did not show any changes throughout the entire range of fluence used. Through absorption spectroscopy at high laser intensities, we also demonstrated that $E_{22}$ \textit{absorption peaks} do not show any shift or broadening even at high laser fluence, indicating that state-filling or scattering is not the cause of the observed ``flattening'' of the excitation spectra. We developed a model to explain these observations by carefully taking into account the spatial overlap of excitons when the average inter-exciton distance approaches the Bohr radius in the exciton-exciton annihilation process. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X29.00002: Curvature Effects on the E$_{33}$ and E$_{44}$ Exciton Transitions in Semiconducting Single-Walled Carbon Nanotubes Stephen Doorn, Erik Haroz, Sergei Bachilo, Bruce Weisman We discuss recent measurements of the E$_{33}$ and E$_{44}$ transitions of small diameter (0.7 to 1.2 nm) single-walled carbon nanotubes using deep blue (415 to 465 nm) resonance Raman spectroscopy and photoluminescence excitation spectroscopy in the UV and blue regions (280 to 480 nm). Individual radial breathing mode features, as well as Raman and photoluminescence excitation maxima, are assigned to specific nanotube chiralities. Transition-dependent trends in RBM intensities are discussed. We present a scaling law analysis of transition energies and show that energies for nanotubes with diameter less than 0.9 nm are not explained by previous scaling law descriptions for larger diameter nanotubes. This new behavior at small diameters is interpreted in terms of both a crossing-over of the E$_{33}$ and E$_{44}$ trend lines for a given 2n+m branch, and a chirality dependence in the many-body exciton effects that becomes significant at high curvatures. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X29.00003: Exciton binding energies in metallic single-walled carbon nanotubes are comparable to those in semiconducting ones Zhendong Wang, Sumit Mazumdar Excitons in metallic single-walled carbon nanotubes (M-SWCNTs) have attracted both theoretical \footnote{J. Deslippe {\it et al.}, Nano. Lett. \textbf{7},1626-1630 (2007).} and experimental \footnote{F. Wang {\it et al.}, unpublished} attention recently. It has been claimed that exciton binding energies in M-SWCNTs are an order of magnitude smaller than those in semiconducting single-walled carbon nanotubes (S-SWCNTs). We have investigated M-SWCNTs within a $\pi$-electron Hamiltonian that has previously reproduced quantitatively the absolute energies as well as the binding energies of both longitudinal \footnote{Z. Wang, H. Zhao and S. Mazumdar, Phys. Rev. B {\bf 74}, 195406 (2006).} and transverse \footnote{Z. Wang, H. Zhao and S. Mazumdar, Phys. Rev. B {\bf 76}, 115431 (2007).} excitons in S-SWCNTs. We are able once again to reproduce quantitatively the available absolute exciton energies and the optical absorption spectra of M-SWCNTs with diameters 0.9 - 1.4 nm. While we need a dielectric constant larger than in the S-SWCNTs, our calculated exciton binding energies in this diameter range are 0.2 - 0.3 eV, only slightly smaller than those in S-SWCNTs with similar diameters. \footnote{Supported by NSF-DMR-0705163.} [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X29.00004: First-principles calculation of carrier dynamics in CNTs Invited Speaker: |
Friday, March 14, 2008 9:12AM - 9:24AM |
X29.00005: Experimental evidence for the spin triplet exciton in single wall carbon nanotubes A. Mohite, B. Alphenaar, T. Santos, J. Moodera Optical transitions in single wall carbon nanotubes (SWNTs) are dominated by the formation of excitonic states. Most excitonic states predicted for carbon nanotubes are not observable under normal conditions, including the so-called dark excitonic states and the spin triplet states. Here, we describe a photocurrent measurement technique that allows for the first observation of the spin triplet state. A thin (3 nm) semi-transparent film of Europium Sulphide (EuS) is used as a top contact to CVD grown nanotubes. EuS is a spin filtering tunnel barrier. At low temperatures, the conduction band in the EuS splits, so that the spin-up level lies 0.36 eV below the spin-down level. We monitor the nanotube photocurrent at 4.2K for magnetic fields between 0 and 50 mT. With increasing magnetic field, a low energy photocurrent peak splits off from the singlet exciton peak. No splitting is observed in the free carrier peak, but its position shifts by approximately 7 meV. From this we estimate that the EuS layer produces an effective magnetic field of 7 T. This is too small to produce observable Zeeman splitting, or for the dark exciton to be observed. We instead suggest that the EuS provides spin orbit coupling to mix the singlet and triplet states, making the triplet state optically active. Supported by ONR (No. N00014-06-1-0228) and NSF. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X29.00006: Diffusion limited optical effects in Single Walled Carbon Nanotubes (SWNTs) Aruna Rajan, Michael Strano, Tobias Hertel, Klaus Schulten Recently, it was observed that SWNTs smaller than a few hundred~nm have very low quantum yields. We propose that this is due to thermal diffusion of excitons along the nanotube axis and quenching at the ends. By fitting spectroscopic data to a one-dimensional diffusion model, we extract a diffusion coefficient of $6~$cm$^{2}/$sec for excitons in~($7$,$5$) SWNTs. Assuming a mono-exponential decay of exciton photoluminescence, we also predict that the effective length-dependent photoluminescence lifetimes for these excitons lie in the range of $1$~ps to $27$~ps. Experimental observations are shown to be consistent with stochastic rather than wave packet-like exciton migration, which is in agreement with ultrafast excitonic dephasing. Edge effects seem to limit the use of short SWNTs as optical sensors. Calculation of the form of the diffusion coefficient assuming exciton-phonon coupling as the underlying mechanism will be discussed. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X29.00007: $K$-momentum dark exciton energy in carbon nanotubes O. N. Torrens, J. M. Kikkawa, M. Zheng Phonon sideband optical spectroscopy determines the energy of the dark $K$-momentum exciton for (6,5) carbon nanotubes (CNTs). One-phonon sidebands appear in absorption and emission, split by two zone-boundary ($K$-point) phonons. Their average energy locates the $E_{11} \quad K$-momentum exciton 36 meV above the $E_{11}$ bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the $K$-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of CNTs. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X29.00008: Low Magnetic Field Effects in Single-Walled Carbon Nanotubes D. E. Milkie, O. N. Torrens, J. M. Kikkawa We observe room temperature, sub-Tesla magnetic field effects on the photoluminescence of aqueous suspensions of single-walled carbon nanotubes and on the electrical transport of carbon nanotube composites. The nearly identical field scale found in both cases suggests a common origin for the phenomena. In aqueous suspensions, weak magnetic fields decrease the nanotube photoluminescence intensity by $\sim $0.1{\%} - 1{\%}, an effect which saturates by $\sim $1 Tesla. We explore this magnetic darkening as a function of surfactant, magnetic field and direction, temperature, and nanotube chirality. For carbon nanotube epoxy composites and aerogels, low magnetic fields produce a similar decrease in the device resistivity, which is found to be temperature dependent. We discuss these new magnetic field effects in the context of excitonic magneto physics and magnetic field effects observed in other organic semiconductor systems. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X29.00009: Dark excitons in single-walled carbon nanotubes investigated by electroabsorption spectroscopy Hideo Kishida, Y. Nagasawa, S. Imamura, A. Nakamura We report electroabsorption (EA) spectra in micelle-wrapped single walled carbon nanotubes (SWNT). By applying the high electric field (up to 85kV/cm), the absorption spectra of semiconducting SWNTs show field-induced change in the region of $E_{11}$ and $E_{22}$ transitions. The EA spectra are essentially reproduced by the second derivative curves of the absorption spectra. Such spectral features indicate that the bright (one-photon allowed) exciton and dark (two-photon allowed) exciton for each chiral index are nearly degenerate. The closer scrutiny of the EA spectra reveals that the dark excitons for several chiral indices are located on the higher energy side of the bright states. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X29.00010: High-Field Magneto-Photoluminescence Spectroscopy of Highly-Aligned Carbon Nanotubes J. Shaver, J. Kono, S. A. Crooker, J. A. Fagan, E. K. Hobbie We have investigated excitons in semiconducting single-walled carbon nanotubes (SWNTs) through low-temperature magneto-photoluminescence (PL) of highly-aligned SWNT films in magnetic fields ($\textbf{\textit{B}}$) up to 55~T. The magnetic field was generated using the 60~T long pulse magnet powered by a 1.4 GVA motor-generator at the National High Magnetic Field Lab in Los Alamos, NM. Polyacrylic acid films containing DNA suspended CoMoCAT SWNTs were stretch-aligned, and the alignment factor was analyzed by polarized Raman spectroscopy$^1$. Utilizing two well-defined measurement geometries, SWNTs~$\parallel$~$\textbf{\textit{B}}$ and SWNTs~$\perp$~$\textbf{\textit{B}}$, we provide unambiguous evidence that the PL from excitons in SWNTs is sensitive only to the $\textbf{\textit{B}}$-component parallel to the tube axis. We developed a theoretical model of one-dimensional magneto-excitons, based on recently-proposed exchange-split bright and dark exciton bands with Aharonov-Bohm-phase-dependent energies, masses, and oscillator strengths, which successfully reproduces our observations$^2$. \newline 1. Fagan \textit{et al.} Phys. Rev. Lett. \textbf{98}, 147402 (2007) \newline 2. Shaver \textit{et al.} Nano Lett. \textbf{7}, 1851 (2007) [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X29.00011: Time-Domain Ab Initio Studies of Excitation Dynamics in Carbon Nanotubes Bradley Habenicht, Oleg Prezhdo We have developed state-of-the-art non-adiabatic molecular dynamics techniques and implemented them within time-dependent density functional theory in order to model the ultrafast excitation processes in CNTs at the atomistic level and in real time. Our ab initio studies of CNTs directly mimic the experimental data and reveal many intriguing features of the excitation dynamics, including non-radiative fluorescence quenching, fast intrinsic intraband relaxation, phonon-induced component of fluorescence linewidths, the importance of defects, the dependence of the relaxation rate on the excitation energy and intensity, and a detailed understanding of the role of active phonon modes. \textbf{1}. C. F. Craig, W. R. Duncan, O. V. Prezhdo ``Trajectory surface hopping in the time-dependent Kohn-Sham theory for electron-nuclear dynamics'', \textit{Phys. Rev. Lett}.,\textbf{ 95} 163001 (2005) \textbf{2}. B. F. Habenicht, C. F. Craig, O. V. Prezhdo, ``Electron and hole relaxation dynamics in a semiconducting carbon nanotube'', \textit{Phys. Rev. Lett.} \textbf{96} 187401 (2006) \textbf{3}. B. F. Habenicht, H. Kamisaka, K. Yamashita and O. V. Prezhdo, ``Ultrafast vibrationally-induced dephasing of electronic excitations in semiconducting carbon nanotubes, \textit{Nano Lett.,} submitted \textbf{4}. B. F. Habenicht, C. F. Craig, O. V. Prezhdo, ``Quenching of fluorescence in a semiconducting carbon nanotube: time-domain ab initio study'', \textit{Phys. Rev. Lett.}, submitted. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X29.00012: Free Carrier Auger Relaxation of Excitons in Carbon Nanotubes Jesse Kinder, Eugene Mele We study a relaxation pathway in doped carbon nanotubes in which optically pumped excitonic states interact with free electrons or holes through an Auger process. The exciton recombines nonradiatively, transferring its energy and momentum to a free carrier. The calculated decay rate depends on temperature, exciton density, and the amount of doping. For optically excited bright excitons, conservation of energy and momentum forbid this decay below a critical doping density. For the heavier dark excitons, recombination is allowed for any nonzero doping density. By studying the phase space for this Auger process and exciton-exciton annihilation, we find that the free carrier interaction can dominate the relaxation rate at low exciton densities. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X29.00013: Magnetic Brightening of Dark Excitons in Individual Single-Walled Carbon Nanotubes Ajit Srivastava, Junichiro Kono, Han Htoon, Victor I. Klimov We have performed micro-photoluminescence (PL) studies on \textit{individual} single-walled carbon nanotubes (SWNTs) at varying temperatures ($T$ = 4 K -- 100 K) in magnetic fields ($B)$ up to 5 T, which provide direct evidence for the existence of dark excitons in SWNTs. Only when the $B$ was parallel to the tube axis, we observed the appearance of a \textit{secondary peak at a lower energy} with respect to the main emission peak. The secondary peak increased in intensity with increasing $B$ at the expense of the main peak. At the lowest $T$, a complete reversal of emission intensity from the main peak to the side peak was seen as the $B$ was increased. However, the main peak was recovered as the $T$ was increased at a fixed $B$. These behaviors can be explained by assigning the main and secondary peaks to the lowest-energy bright and dark singlet exciton states, respectively. The absence of these behaviors in $B$ perpendicular to the tube axis convincingly suggests that brightening is induced by the Aharonov-Bohm phase. The zero-field dark-bright splitting is found to be $\sim $1-2 meV, which is lower than most theoretical predictions. [Preview Abstract] |
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