Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A27: Focus Session: Carbon Nanotubes: Optical Properties I |
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Sponsoring Units: DMP Chair: Marcus Freitag, IBM Room: LACC 501C |
Monday, March 21, 2005 8:00AM - 8:36AM |
A27.00001: Optical Spectroscopy of Individual Single-Walled Carbon Nanotubes by Rayleigh Scattering Invited Speaker: Optical spectroscopy of \textit{individual} nanostructures has greatly enhanced our understanding of nanoscale physics. For single-wall carbon nanotubes (SWNTs), there is a particularly strong motivation for such techniques, since the properties of SWNTs vary enormously with their precise physical structure. To date, both fluorescence and Raman scattering have shown the sensitivity to probe individual SWNTs. While fluorescence is an excellent experimental method, it is limited to semiconducting nanotubes displaying reasonable fluorescence efficiency. Raman scattering provides complementary information, but is weak and requires the identification of an electronic resonance to observe a signal. In this paper, we describe a new spectroscopic approach for investigating individual SWNTs and other nanostructures.$^{1}$ The method is based on Rayleigh scattering. The approach has the advantage of relying on the ubiquitous linear polarizability of the material, a response present for fluorescing and non-fluorescing species alike and displaying resonances at the transition energies of the system. This method has yielded high-quality spectra over the visible and near-IR spectral range from both individual semiconducting and metallic SWNTs. A key element in the experiment is use of supercontinuum radiation as the light source. This source, produced by passing femtosecond laser pulses through a microstructured fiber, provides radiation with the broad spectrum of a light bulb, but with the brightness of a laser. The experiment also employs SWNTs suspended across slit structures and viewed in a dark-field configuration to eliminate background scattering. Rayleigh scattering spectra of electronic transitions in semiconducting and metallic nanotubes will be presented, as will be results on the polarization dependence of the transitions. The method will be shown to be appropriate for the characterization of different spatial segments of a given SWNT and for the examination of tube-tube interactions in small bundles of SWNTs. This work is supported by the NSF NSEC at Columbia University, NYSTAR, and the DOE-BES. It was performed in collaboration with Feng Wang, Matthew Y. Sfeir, Limin Huang, Chia-Chin Chuang, James C. Hone, Stephen P. O'Brien,$^{ }$and Louis E. Brus. $^{1 }$M. Y. Sfeir, F. Wang, L. Huang, et al., Science \textbf{306}, 1540 (2004). [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A27.00002: Temperature Dependence of Tunable Raman and Fluorescence Spectroscopy of Individual Suspended Carbon Nanotubes Stephen Cronin, Yan Yin, Andrew Walsh, Alexander Stolyarov, Anna Swan, M. Selim Unlu, Bennett Goldberg, Michael Tinkham The temperature dependence of tunable Raman spectra are measured on single-wall carbon nanotubes grown by chemical vapor deposition (CVD) over trenches etched in quartz substrates. The suspended portion of the nanotubes exhibit enormous enhancement of the Raman and fluorescence signals relative to the unsuspended portion. Raman and fluorescence spectra are taken on the same individual nanotubes using a tunable Ti:Sapphire laser over the range 750nm-830nm. Both the Raman mode frequencies and the subband transition energies (E$_{ii}$) are observed to shift with temperature. By preparing nanotubes suspended in free space over a trench we eliminate the effects of interaction with the substrate or surfactant molecules, which, as will be shown, can be quite significant. The temperature dependence of the linewidths and resonance windows are also discussed. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A27.00003: Auger Recombination of Excitons in Single-Walled Carbon Nanotubes Feng Wang, Gordana Dukovic, Mark Hybertsen, Louis Brus, Tony Heinz Single-walled carbon nanotubes (SWNTs) as prototypical 1- dimensional systems exhibit enhanced carrier-carrier interactions. As a consequence, one would expect semiconducting SWNTs containing multiple electron-hole pairs to display rapid Auger recombination. We have investigated this issue experimentally by examining the efficiency and temporal evolution of the fluorescence emission from SWNTs after excitation by a femtosecond laser pulse$^{1}$. The behavior as a function of the pump excitation fluence, which controls the initial electron-hole density, reveals the presence of Auger recombination through a decrease in fluorescence efficiency and the emergence of a rapid decay channel when multiple electron-hole pairs are present in a SWNT.$^{2}$ Similar fluence-dependent effects have also recently been reported by Ma et al$^{3}$. Quantitative analysis yields an Auger recombination rate of $\sim $1/ps$^{ }$for just 2 electron-hole pairs in a 400 nm long SWNT. This rapid Auger rate limits the sustainable electron-hole density that can be achieved within a single nanotube. We compare our experimental finding with a theoretical estimate of the Auger rate in SWNTs based on a point-contact interaction model. $^{1 }$F. Wang, et al., Phys. Rev. Lett. \textbf{92}, 177401 (2004). $^{2 }$F. Wang, et al., Phys. Rev. B, in press. $^{3 }$Y. Z. Ma, et al., J. Chem. Phys. \textbf{120}, 3368 (2004). [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A27.00004: Observation of Carbon Nanotube Optical Bistability D. Milkie, C. Staii, S. Paulson, E. Hindman, A.T. Johnson, J.M. Kikkawa We present scanning confocal microscopy studies of suspended carbon nanotubes grown by chemically-assisted vapor deposition across micron-sized apertures. Maps of the photoluminescence emission taken at the E11 peak show unusual `holes' and `rings' with subwavelength spatial features. These features result from abrupt $\sim $20 meV blue shifts in the emission energy due to small changes in the excitation position. Polarization and intensity dependent studies show that this switching behavior depends on the intensity of light absorbed into the nanotube, and additional spatial structure is seen by varying the excitation wavelength. Our findings suggest that the phenomenon represents a true bistability of the E11 transition, and perhaps a many-body effect, as no intermediate emission wavelengths are observed. This work was supported by DARPA/ONR N00015-01-1-0831, NSF DMR 00-79909, SENS, NSF IGERT DGE-0221664 and in part (SP and ATJ) by the Commonwealth of Pennsylvania's Ben Franklin Technology Development Authority through the Nanotechnology Institute. Authors SP and ATJ acknowledge financial support through the Nanotechnology Institute of the Commonwealth of Pennsylvania. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A27.00005: Stability of Excitons in Carbon Nanotubes under High Laser Excitations G.N. Ostojic, S. Zaric, J. Kono, V.C. Moore, R.H. Hauge, R.E. Smalley The behavior of excitons and free carriers strongly depends on the carrier density. In 3-D and 2-D solids, increasing the number of carriers reduces the exciton binding energy via screening, and when the average separation reaches the Bohr radius, a transition from an excitonic insulating state to a conducting electron-hole ($e$-$h$) plasma occurs (i.e., the Mott transition). However, the unique nature of 1-D Coulomb interaction may alter this scenario. We have used nondegenerate pump-probe spectroscopy with a widely tunable pump and a white light continuum probe to monitor the behavior of excitons in single-walled carbon nanotubes for different carrier densities. In addition to already known band filling effects, broadening of absorption peaks is identified from complex, spectrally dependent pump-probe signals including both photo-induced absorption and bleaching. From the excitation conditions, we estimate that the average 1-D density of the photo-excited $e$-$h$ pairs is in the order of the Mott density. However, throughout the observed time range, covering both high and low density regimes, the positions of E$_{11}$ absorption peaks are unchanged. This stability of excitons is similar to excitons in GaAs quantum wires and thus indicates a unique property of 1-D solids. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A27.00006: NIR-luminescence mapping and Raman spectroscopy of single-walled carbon-13 nanotubes Yuhei Miyauchi, Shohei Chiashi, Shigeo Maruyama Photoluminescence and Raman scatterings of single-walled carbon nanotubes (SWNTs) synthesized from isotopically-modified ethanol were studied. Using Alcohol catalytic CVD (ACCVD) technique optimized for the efficient production of SWNTs from very small amount of ethanol, SWNTs consisting of carbon-13 isotope (SW$^{13}$CNTs) were synthesized in addition to normal SWNTs consisting of mainly $^{12}$C. The vibrational features of SW$^{13}$CNTs were compared with those of normal SWNTs through NIR-luminescence mapping and Raman spectroscopy. There was almost no change in Raman spectra shape of SW$^{13}$CNTs except for the Raman shift frequency down-shifted as much as square-root of mass ratio 12/13. In addition to Raman spectroscopy, we have mapped the NIR-luminescence of D$_{2}$O-surfactant dispersions of both SW$^{13}$CNTs and SW$^{12}$CNT. By comparing the two maps, luminescence peaks corresponding to electronic transitions with vibrational excitation were identified. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A27.00007: Low Temperature Photoluminescence of Surfactant-suspended SWNT’s Flavio Plentz, Henrique Ribeiro, Marcos Pimenta, Ado Jorio, Michael Strano Photoluminescence (PL) has been a powerful method for the investigation of bulk semiconductors and novel artificial low dimensional semiconductor heterostructures such as quantum wells, quantum wires and quantum dots. Recently bright PL from semiconductor, surfactant-suspended, isolated single walled carbon nanotubes (SWNT's) and from isolated SWNT's grown between silicon oxide pillars have been observed. Low temperature PL measurements in pillar suspended SWNT's have also been reported. In this latter study small blue shifts of the PL peak energies and PL excitation energies, and previously unreported PL peaks have been found. In this work we present results on the low temperature photoluminescence of surfactant-suspended SWNT's. We have found that, upon freezing of the SDS-suspended SWNT's solution, the SWNT's remains strongly luminescent. We perform PL and photoluminescence excitation (PLE) measurements in temperatures ranging from 3.5 to 210K, for excitation energies between 0.8 and 1.75eV and compare the results with the PL and PLE maps obtained for the same suspension at 300K. We found different behavior for the energy shifts for distinct nanotubes families. We observe a blue shift in E11 transition for mod(2n+m,3)=1 nanotubes and a red shift is observed for mod(2n+m,3)=2 nanotubes. We discuss our results in terms of temperature and strain effects in the electronic structure. The brasilian authors aknowledge CNPq, FINEP and FAPEMIG. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:24AM |
A27.00008: Optical Characterization and Applications of Single Walled Carbon Nanotubes Invited Speaker: Recent advances in the dispersion and separation of single walled carbon nanotubes have led to new methods of optical characterization and some novel applications. We find that Raman spectroscopy can be used to probe the aggregation state of single-walled carbon nanotubes in solution or as solids with a range of varying morphologies. Carbon nanotubes experience an orthogonal electronic dispersion when in electrical contact that broadens (from 40 meV to roughly 80 meV) and shifts the interband transition to lower energy (by 60 meV). We show that the magnitude of this shift is dependent on the extent of bundle organization and the inter-nanotube contact area. In the Raman spectrum, aggregation shifts the effective excitation profile and causes peaks to increase or decrease, depending on where the transition lies, relative to the excitation wavelength. The findings are particularly relevant for evaluating nanotube separation processes, where relative peak changes in the Raman spectrum can be confused for selective enrichment. We have also used gel electrophoresis and column chromatography conducted on individually dispersed, ultrasonicated single-walled carbon nanotubes to yield simultaneous separation by tube length and diameter. Electroelution after electrophoresis is shown to produce highly resolved fractions of nanotubes with average lengths between 92 and 435 nm. Separation by diameter is concomitant with length fractionation, and nanotubes that have been cut shortest also possess the greatest relative enrichments of large-diameter species. The relative quantum yield decreases nonlinearly as the nanotube length becomes shorter. These findings enable new applications of nanotubes as sensors and biomarkers. Particularly, molecular detection using near infrared (n-IR) light between 0.9 and 1.3 eV has important biomedical applications because of greater tissue penetration and reduced auto-fluorescent background in thick tissue or whole blood media. Carbon nanotubes have a tunable n-IR emission that responds to changes in the local dielectric function but remains stable to permanent photobleaching. We report the synthesis and successful testing of solution phase, near-infrared sensors, with $\beta$-D-glucose sensing as a model system, using single walled carbon nanotubes that modulate their emission in response to the adsorption of specific biomolecules. New types of non-covalent functionalization using electron withdrawing molecules are shown to provide sites for transferring electrons in and out of the nanotube. We also show two distinct mechanisms of signal transduction -- fluorescence quenching and charge transfer. The results demonstrate new opportunities for nanoparticle optical sensors that operate in strongly absorbing media of relevance to medicine or biology. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A27.00009: Optical Properties of Carbon Nanotubes in Semiconductor/NT Hybrids O. N. Torrens, D. E. Milkie, M. F. Islam, A. G. Yodh, J. M. Kikkawa, A. Khandekar, T. F. Kuech We report on etch processing and optical measurements of hybrid GaAs-Nanotube materials. Samples are created by OMVPE growth over GaAs vicinal substrates coated with single-wall laser-oven carbon nanotubes (SWNTs). We use scanning confocal Raman and photoluminesence microscopy to study the incorporation of SWNTs into the GaAs matrix. We further present procedures for patterning these samples into islands separated by V-grooves and interconnected by suspended SWNTs. Strategies for performing time-resolved studies of spin coherent transport across SWNTs will be discussed. Preliminary results of two-color, time-resolved Faraday rotation in unpatterned hybrid structures reveal unconventional phase shifts in coherent dynamics of spins in the GaAs matrix. JMK acknowledges support from DARPA/ONR N00015-01-1-0831, TFK acknowledges support from ARO, and DEM acknowledges support from NSF IGERT DGE-0221664. [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A27.00010: Polarization dependence of the optical absorption of single-walled carbon nanotubes Yoichi Murakami, Erik Einarsson, Tadao Edamura, Shigeo Maruyama Anisotropic optical absorption properties of single-walled carbon nanotubes (SWNTs) are determined from the measurements of a recently developed vertically aligned SWNT film grown on an optically polished quartz substrate. In addition to the inter-subband absorption below 3 eV, we present the remarkable polarization dependences of absorption peaks at 4.5 eV and 5.25 eV. Origins of these absorption peaks are clarified and their important relevance to the optical properties of graphite is revealed. A method of determining a nematic order parameter of the vertically aligned SWNT film by separating the collinear absorption peak at 4.5 eV from other transition dipoles is introduced. Subsequently, the intrinsic optical absorption cross-sections of the SWNTs for 0.5 - 6 eV are determined for both parallel and cross-polarized light. It is shown that the tail of the non-collinear absorption peak at 5.25 eV contributes appreciably to the absorption of cross-polarized light even in the inter-subband transition region below 3 eV. [Preview Abstract] |
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