Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V3: Spectroscopy of Carbon Nanotubes |
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Sponsoring Units: DCMP Chair: Tobias Hertel, Vanderbilt University Room: LACC 515B |
Thursday, March 24, 2005 11:15AM - 11:51AM |
V3.00001: Mobile Ambipolar Domain in Carbon-nanotube Infrared Emitters Invited Speaker: Carbon nanotube field-effect transistors emit infrared light when biased in the ambipolar regime where electrons are injected at one contact and holes at the other. Not only is the radiation confined to the nanometer-scale width of the carbon nanotube, but also along the axis of the tube, due to an electron-hole recombination length on the micron scale. The light spot can be electronically positioned anywhere along the length of the intrinsic carbon nanotube by means of a single back gate. This device constitutes a novel spatially controllable light-source and allows unprecedented insight into electronic transport in nanotube field-effect transistors. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:27PM |
V3.00002: Luminescence from suspended carbon nanotubes Invited Speaker: Single walled carbon nanotubes are produced in a wide range of species with different diameter and chirality. Since two of three nanotube species are predicted to be semiconductors with a direct bandgap, efficient photoluminescence is to be expected for this materials system. Recent experimental breakthroughs have shown that proper isolation of the nanotube from its environment is crucial, and suspension of nanotubes on substrates patterned with pillars is proving to be a method of choice. This presentation will review two years of research on the luminescence properties of suspended nanotubes, from ensemble measurements to individual nanotubes. Some properties are truly remarkable and can be identified as consequences of the stiffness of the carbon-carbon bond, the one-dimensional nature of SWNTs, and their maximally exposed surface area. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 1:03PM |
V3.00003: Low Temperature, Photoluminescence and Photoluminescence Excitation Studies of Individual Carbon Nanotubes Invited Speaker: Semiconducting, single-walled carbon nanotubes are nanoscale, near-infrared light emitters that have great potential for a wide variety of optoelectronic applications. A detail understanding of fundamental photophysics of nanotubes is essential to fully exploit this potential. To address the nature of fundamental photoexcitations in nanotubes, we perform for the first time low-temperature, photoluminescence (PL) and PL excitation (PLE) studies at the single-nanotube level. In our PL spectra, we observed two types of features. Some of the nanotubes show sharp (sub-meV to a few meV linewidths), symmetric spectral lines that can be attributed to on- dimensional (1D) excitons. On the other hand, we also detect broad (> 6meV linewidths), asymmetric peaks that show strong thermal broadening on their high-energy sides. The spectral shape as well as the unusual temperature dependence of these peaks can be explained in terms of the Fermi-edge-singularity effect that arises from many-body interactions of photoexcited carriers with pre-existing population of carriers introduced into nanotubes at the preparation stage (unintentional doping). Our PLE spectra, in addition to features due to a direct excitation to the second electronic transition, exhibit a number of strong phonon-assisted transitions involving the excitation of one or more phonon modes together with the first electronic state. Surprisingly, the phonon replicas are as intense as the zero-phonon transition associated with the second electronic state. In contrast to a small width of emission lines, most of the PLE features are characterized by tens of meV linewidths indicating significant lifetime broadening induced by inelastic electron-phonon scattering. All of these observations suggest that strong electron-phonon coupling gives rise to a significantly more complex structure of nanotube absorption spectra than it is assumed in a simple picture of optical transitions dominated by singularities in the 1D energy spectrum. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:39PM |
V3.00004: Intrinsic and Extrinsic Magnetic Anisotropies of Single-Wall Carbon Nanotubes Invited Speaker: Interpretation of bulk magnetization measurements of single-wall carbon nanotubes (SWNTs) is often complicated by the presence of ferromagnetic (FM) catalyst impurities. I will discuss how magnetic alignment of SWNTs in suspension can be used to detect FM impurities attached to nanotubes. A combination of Raman scattering and polarized absorbance is first used to determine the bare optical polarized absorbance cross-sections for light parallel and perpendicular to the nanotube axis. Next, these spectra serve as a benchmark for performing quantitative and high-resolution studies of SWNT alignment in suspensions. These studies reveal that even after chemical purification FM moments significantly enhance SWNT alignment and have an easy axis along the SWNT axis. Lowering the FM impurity content using magnetic gradient fractionation produces a concomitant reduction in the number of SWNTs whose alignment is dominated by FM anisotropy. These studies permit an estimate of the tethered FM moment size for both laser-oven and HiPCO SWNTs, and give an accurate measure of SWNT diamagnetic anisotropy. Studies of DNA-wrapped SWNTs available from DuPont show essentially no FM impurity moment. Since the latter samples can be selectively enriched with single SWNT species, we are able to compare diamagnetic responses for different wrapping vectors (m,n). Work done in collaboration with M.F. Islam, D. E. Milkie, O.N. Torrens, C. L. Kane and A. G. Yodh at PENN and M. Zheng, G.B. Onoa, T. Gierke at DuPont CR{\&}D. Support NSF through DMR-0203378, DMR-079909 and DGE-0221664, NASA through NAG8-2172, DARPA/ONR through N00014-01-1-0831, and SENS. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 2:15PM |
V3.00005: Modulation of the energy gap in carbon nanotubes threaded by magnetic field Invited Speaker: Carbon nanotubes are molecules that have an atomic lattice equivalent to the hexagonal lattice of a single layer of graphite, seamlessly rolled into a cylinder. Their electronic properties are determined by the chirality or wrapping angle. Depending on the chirality, the molecule may have a gap in the electronic spectrum and behave as a semiconductor, or it may have zero gap and exhibit properties of a one-dimensional metal. Although, practically, it is impossible to change the chirality of a given molecule, it is possible to achieve an equivalent effect by applying a strong magnetic field along the axis of the nanotube. This behavior arises from Aharonov-Bohm coupling of the magnetic vector-potential, which is determined by the magnetic flux threading the nanotube, to the orbital motion of the electrons. In particular, the energy gap is predicted to oscillate periodically with magnetic flux, with a period of h/e. This effect, known as Ajiki-Ando (AA) splitting [1], offers the possibility of interconversion of metallic tubes into semiconducting ones and vice-versa, via a magnetic field. Our recent experiments[2] provide an experimental evidence for the AA energy-gap modulation. These measurements were performed on single electron tunneling (SET) transistors based on multiwall carbon nanotubes, in the quantum dot regime. Multiwall nanotubes are unique molecules in that they allow an application of a full magnetic flux quantum, due to their large diameter. The SET transistors used in this study showed the usual pattern of Coulomb diamonds and signatures of resonant tunneling and Zeeman splitting. Therefore the observed pattern of Coulomb peaks and their displacements with the magnetic field could be interpreted as a pattern of single-electron energy levels and was used to study their response to the magnetic flux. Spectroscopic measurements at higher bias showed an energy gap, which was induced and modulated by the magnetic flux. The period of the observed modulation was h/e, as expected for the AA splitting, while the modulation amplitude was lower than expected. [1] H. Ajiki, T. Ando, J. Phys. Soc. Jpn., Vol.62, p.1255 (1993). [2] U.C. Coskun, T.-C. Wei, S. Vishveshwara, P. M. Goldbart, and A. Bezryadin, Science, Vol.304 p.1132 (2004). [Preview Abstract] |
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