Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q24: Focus Session: Optical Response of Nanotubes |
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Sponsoring Units: DMP Chair: Achim Hartschuh, University of Munich Room: 326 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q24.00001: Optical near-field investigations of individual single-walled carbon nanotubes Invited Speaker: Optical excitation of semiconducting nanotubes creates excitons that determine nearly all light-based applications. Near-field photoluminescence (PL) and Raman imaging with a spatial resolution better than 15 nm was used to probe the spectroscopic properties of excitonic states along single nanotubes on substrates [1,2]. The PL intensity was found to decrease towards the nanotube ends on a length scale of few 10 nm probably caused by exciton transport to localized end states followed by efficient non-radiative recombination. DNA-wrapping of nanotubes results in pronounced emission energy variations on a length scale of few 10 nm indicating the potential of the material for nanoscale sensing applications [3]. Inter-nanotube energy transfer was studied for different pairs of semiconducting nanotubes forming bundles and crossings [4]. Efficient transfer is found to be limited to a few nanometres because of competing fast non-radiative relaxation and can be explained in terms of electromagnetic near-field coupling. We also report on our recent experimental results on time-resolved near-field PL measurements, electrically gated nanotubes and the PL of nanotubes on metal surfaces. \\[4pt] References: \\[0pt] [1] A. Hartschuh, Angew. Chem. Int. Ed. 47, 8178 (2008). \\[0pt] [2] I. O. Maciel et. al, Nature Mat. 7, 878 (2008). \\[0pt] [3] H. Qian et. al, Nano Lett. 8, 2706 (2008). \\[0pt] [4] H. Qian et. al, Nano Lett. 8, 1363 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q24.00002: Electroluminescence Spectral Shape in Carbon Nanotube Field Effect Transistors under High Bias Conditions Megumi Kinoshita, Vasili Perebeinos, Mathias Steiner, Phaedon Avouris In carbon nanotube field effect transistors, electroluminescence excited by intra-nanotube impact excitation at high source-drain bias reveals strongly broadened electronic transitions (FWHM $\sim$150 to $\sim$300 meV for the lowest energy peak observed) in the E$_{11}$ to E$_{22}$ energy range. Through the bias and polarization dependence of the spectra, we investigate the production mechanism of these states and consider possible causes for their bias-dependent broad lineshapes, including exciton-exciton annihilation, and high electron and phonon temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q24.00003: Electroluminescence from Carbon Nanotube Network Transistors Elyse Adam, Carla Aguirre, Matthieu Paillet, Benoit Cardin St-Antoine, Francois Meunier, Patrick Desjardins, David Menard, Richard Martel A spectroscopic study of the electroluminescence properties of individual carbon nanotube (CNFET) and carbon nanotube network (NNFET) field effect transistors has been performed. As expected, the measurements on metallic and semiconducting CNFET showed that only semiconducting carbon nanotubes produce electroluminescent signals. The narrow emission line widths of CNFET ($\sim $80 meV) compared to that of NNFETs ($\sim $180 meV) indicates that the light emission from carbon nanotube networks involves many carbon nanotubes. Moreover, electroluminescence spectra from NNFETs made from three different sources of carbon nanotubes (laser ablation, CoMoCAT and DWNT) have shown major differences which, based on comparisons with their corresponding absorption spectra, indicate that only larger diameter carbon nanotubes contribute to light emission. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q24.00004: Confocal imaging and excitation spectra of photoluminescence from carbon nanotubes suspended over trenches of various widths S. Moritsubo, T. Murai, T. Shimada, Y. Murakami, S. Maruyama, Y. K. Kato Carbon nanotubes (CNTs) have novel optical properties, such as strongly bound excitons and sensitivity to surrounding environments, because of their unique structure. In particular, it is well known that CNTs lying on substrates show photoluminescence (PL) quenching caused by very effective substrate-induced nonradiative decay of photoexcited excitons. In this study, we formed trenches with various widths on SiO$_{2}$/Si substrates in order to prepare suspended CNTs. Using ethanol as carbon source, chemical vapor deposition was used for the synthesis of CNTs. PL images were collected by a home-built laser-scanning confocal microscope system utilizing a fast steering mirror. In addition, PL excitation spectra were taken using a wavelength tunable Ti:sapphire laser. We analyzed these PL images and excitation spectra in order to clarify the interaction between excitons and the substrate. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q24.00005: Near infrared photoresponse study of large area multi-walled carbon nanotube film with different electrode spacing Biddut Sarker, M. Arif, Paul Stokes, Alamgir Kabir, Saiful I. Khondaker Photoconductivity of carbon nanotube have generated considerable debate in terms of whether the photoresponse is (i) due to photon induced charge carrier (excitonic), (ii) due to heating of the CNT network (bolometric), or (iii) caused by photodesorption of oxygen molecules at the surface of the CNT. In addition, the role of the metal electrode -- CNT contact's effect on the photoresponse has also been debated. In this talk, we will present near -infrared photoresponse study of multi-walled carbon nanotube (MWCNT) film with different electrode spacings. We found that there is a large enhancement of photocurrent upon laser illumination and the photocurrent strongly depends on the position of the laser spot with maximum response occurring at the metal -- film interface. We also show that the photoresponse is rather slow ($\sim $1s) and increases with increasing electrode spacing. We will discuss the origin of the position dependent photocurrent and slow time response. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q24.00006: Transparency and conductivity of carbon nanotube networks Jan Obrzut The conductivity of films made of single wall carbon nanotubes longer than 200 nm closely follows the percolation theory for two-dimensional (2D) networks . The scaling universal exponents describing the ``percolation'' transition from an insulating to conducting state with increasing concentration are consistent with 2D percolation model. A sheet of tubes about 820 nm long becomes conducting at an amazingly low concentration of about 18x10$^{-9}$ g/cm$^{2}$. In comparison, batches of short nanotubes or mixed-length batches form more 3D networks that conduct noticeably worse. Furthermore, the conductivity percolation threshold (x$_{c}$) varies with the aspect ratio Length (L) as, x$_{c} \sim$1/L, a result that is also in accordance with the percolation theory. We also show that contrary to current predictions, these sheets do not have optical properties similar to thin metallic films. Our results indicate that the correlation between the optical properties and the electrical conductivity of these sheets is again better predicted by the general percolation theory. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q24.00007: Carrier renormalization effects on the optical response of doped semiconducting single-walled-carbon nanotubes Sheng Ju, Cheol-Hwan Park, Steven Louie It is known that many-electron effects dramatically change the optical properties of single-walled carbon nanotubes (SWCNTs). Recently, researchers have succeeded in tuning the Fermi energy of an individual SWCNT by applying a gate voltage or by introducing adsorbate dopants. Therefore, the optical response of doped SWCNTs is not only interesting from a pure scientific point of view but also important for the application of these systems. We present here first-principles calculations, based on the GW-Bethe Salpeter equation (GW-BSE) approach, of the quasiparticle (single-particle excitation) spectrum and the optical (electron-hole excitation) spectrum of doped SWCNTs. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q24.00008: Optical properties of doped semiconducting single-walled carbon nanotubes. Catalin D. Spataru, Francois Leonard We studied how the optical response of semiconducting single-walled carbon nanotubes changes upon doping. We performed ab initio calculations of the optical absorption spectrum of the p-doped (10,0) SWCNT, employing a many-electron Green's function approach that determines both the quasiparticle and electron-hole excitations from first principles. We found that the absorption spectrum changes dramatically upon doping, due to both quasiparticle and excitonic effects. In the independent quasiparticle picture, the electron-hole continua are strongly red-shifted with respect to the undoped case due to the metallic character acquired by the electronic screening upon doping. However, the main optical features in the absorption spectrum, including both quasiparticle and electron-hole interaction effects, are only slightly shifted, but qualitatively very different, with respect to the undoped case. Small doping levels (where the Fermi level lies below the valence band maximum by an energy much smaller than the binding energy of excitons in the undoped SWCNT) are sufficient to bleach band-gap absorption. In addition, while resonant excitons associated with the second electron-hole continuum can still exist in the doped SWCNT, their binding energy is much reduced, to a level typical of metallic SWCNTs of similar diameter. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q24.00009: Origins of optical absorption components of metallic and semiconducting single-wall carbon nanotubes in ultra-violet region Kazuhiro Yanagi, Takeshi Saito, Yasumitsu Miyata, Hiromichi Kataura There are large absorption components in the optical absorption spectra of single-wall carbon nanotubes (SWCNTs) in the ultraviolet (UV) region ($\sim $5 eV). Clarification of the origins of the UV absorption is important, since the tails of the UV components influence the transparency of nanotubes and impede their uses for transparent conducting films. However, the origins have not been correctly understood yet. Such UV absorption components are assumed to be caused by $\pi $-plasmons, however, recently contributions from $\pi -\pi $* transition at the M point were also suggested. To understand the origins of UV component, here we clarified how the electronic structure (metallic or semiconducting) and the diameters of SWCNTs influence the UV optical absorption features. We clearly identified two components in UV region, and revealed dependence of the components on their diameters. Remarkably, dependence of the peak-energies of one component on diameters could not be explained by plasmon model, implying the presence of different origins than plasmons in the UV absorption components. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q24.00010: Environmental Change in UV Absorption by Single-Walled Carbon Nanotubes Yoichi Murakami, Shigeo Maruyama We investigated the UV absorption of single-walled carbon nanotubes (SWNTs) in the 4 - 6 eV range, which has been customarily referred to as the ``pi-plasmon'' as a whole. The optical absorption spectra of aligned SWNTs were measured in different dielectric environments. The experimental results unambiguously show that, for the two different UV absorption components existing in the 4 - 6 eV range, only the feature at 5.0 - 5.3 eV exhibits remarkable spectral changes, while the other feature at $\sim $ 4.5 eV remains unchanged. We attribute the former (5.0 - 5.3 eV) to a dipolar surface plasmon in the radial direction of SWNTs. On the other hand, the experimental result raises a fundamental question as to whether it is appropriate to classify the UV feature at $\sim $ 4.5 eV as a pi-plasmon. We will discuss its relation with the UV absorption at $\sim $ 4.5 eV in graphite/graphene that has long been recognized as the interband transition at M point of the Brillouin Zone. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q24.00011: Electron-electron interaction effects on the photophysics of metallic single-walled carbon nanotubes Sumit Mazumdar, Demetra Psiachos, Zhendong Wang, Roberto Badilla Within a molecular Hamiltonian appropriate for correlated $\pi$-electron systems, we show that optical excitations polarized parallel to the nanotube axes in the so-called metallic single-walled carbon nanotubes are to excitons. Our calculated absolute exciton energies in twelve different metallic single-walled carbon nanotubes, with diameters in the range 0.8 - 1.4 nm, are in nearly quantitative agreement with experimental results. We have also calculated the absorption spectrum for the (21,21) single-walled carbon nanotube in the E$_{22}$ region. Our calculated spectrum gives an excellent fit to the experimental absorption spectrum. In all cases our calculated exciton binding energies are only slightly smaller than those of semiconducting nanotubes with comparable diameters. As in the semiconducting nanotubes we predict in the metallic nanotubes a two-photon exciton above the lowest longitudinally polarized exciton that can be detected by ultrafast pump-probe spectroscopy. We also predict a subgap absorption polarized perpendicular to the nanotube axes below the lowest longitudinal exciton, blueshifted from the exact midgap by electron-electron interactions. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q24.00012: Fano Resonance in Single-Walled Cabron Nanotube Devices Gang Liu, Yong Zhang, Chunning Lau We have observed Fano resonance in a short carbon nanotube device. The device's transport spectroscopy exhibits inverse Coulomb blockade structures superimposed on Fabry Perot resonance patterns, indicating the quantum interference between a well-coupled channel and a poorly-coupled channel. Our results have implication on the detection of charges' phase and phase coherence in an electronic interferometer. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q24.00013: Optical Absorption Spectra of Charge Doped Single-Walled Carbon Nanotubes from First-principles Calculations Guangfu Luo, Jing Lu, Wai-Ning Mei, Lu Wang, Lin Lai, Jing Zhou, Rui Qin, Hong Li, Zhengxiang Gao The optical absorption spectrum of single-walled carbon nanotubes (SWCNTs) under charge doping is often interpreted within the graphene zone-folding and rigid-band model. Based on the periodic boundary model together with a uniform background countercharge, our density functional theory calculations, however, show that the spectrum response deviates from the expectations of such model. Specifically, the SWCNT band structures can differ qualitatively from the zone-folding ones, and with the increasing doping level, the absorption peaks will blench in a non-sequential energy order. The on-tube bands in SWCNTs sometimes change obviously even under low charge doping level, and accordingly cause spectra peaks to shift, split, and merge. At the end of this paper, we discuss briefly the applicability of the present results at the GW-BSE theory level and in other SWCNT-like systems. [Preview Abstract] |
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