Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A28: Focus Session: Optical Properties of Nanostructures I: Carbon Nanotubes |
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Sponsoring Units: DMP Chair: Riichiro Saito, Tohoku University Room: Morial Convention Center 220 |
Monday, March 10, 2008 8:00AM - 8:36AM |
A28.00001: Optical Spectroscopy of Individual Single-Walled Carbon Nanotubes Invited Speaker: Single-walled carbon nanotubes (SWCNTs) constitute a fascinating class of 1-dimensional materials. While the electrical, thermal, and mechanical properties of SWCNTs have been studied in depth for some time, only recently have their optical properties emerged as a major topic of research. This interest reflects several factors: The importance of nanotube optical properties for analysis of their structure, quality, and growth; the potential of nanotubes for diverse optoelectronic applications; and the unique physics of the excited states and dynamical processes in these 1-D structures. In this paper we will present an overview of our current understanding of excited electronic states in nanotubes and of the methods and applications of single-molecule optical spectroscopy of carbon nanotubes. We will present recent results on optical spectroscopy of individual carbon nanotubes as perturbed by their external environment, including by the application of strain and deposition of adsorbates. These studies were carried out in collaboration with the research groups of Profs. Louis Brus, Jim Hone, and Stephen O'Brien at Columbia and Prof. Feng Wang at UC Berkeley. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A28.00002: Photophysics of Nanostructures: Tubes, Sheets, and Ribbons Invited Speaker: The restricted geometry of nanostructures often gives rise to novel, unexpected properties and phenomena. In particular, symmetry and many-electron effects can play a vital role in determining the behaviors of these systems. In this talk, I discuss some recent work on using first-principles theory and computation (employing the GW-BSE approach) to understand and predict the electronic structure and optical response of carbon nanostructures including nanotubes, graphene and graphene nanoribbons. We show that, owing to reduced dimensionality, many-electron interaction (self-energy and excitonic) effects change qualitatively the nature of the photo-excited states in both semiconducting and metallic carbon nanotubes. Exciton states with extraordinarily large binding energies and an unusual spectrum (arising from an ``anti-screening'' phenomenon) are predicted for the semiconducting tubes. Moreover, unlike bulk systems, we discover that excitons also exist in the metallic carbon nanotubes. Similar studies show that excitonic effects are equally dominant in the optical spectra of the graphene nanoribbons, although the characters of the exciton states are quite different. Other quasi-1D systems, such as the boron-nitride nanotubes and semiconductor nanowires, exhibit similar behavior. We discuss the physics behind these phenomena and present comparison of our theoretical predictions to recent measurements. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A28.00003: Effective One-Dimensional Electron-Hole Interaction in Single-Walled Carbon Nanotubes Jack Deslippe, Mario Dipoppa, David Prendergast, Rodrigo Capaz, Steven Louie Using the results of ab initio GW-Bethe-Salpeter-Equation (GW-BSE) calculations on the excitonic effects in single-walled carbon nanotubes (SWCNTs), we derive a 1D quantum model for the electron-hole interaction in both semiconducting and metallic SWCNTs. The model includes the important effects of spatial dependent screening and reproduces the exciton binding energies and envelope wave functions of the complete GW-BSE solution of the electron-hole excited states. The inclusion of the spatial dependence in the dielectric function is essential to capture the positioning of the higher exciton states in the spectrum whose calculated energies differ dramatically from those obtained using previous models based on constant dielectric screening. The present effective interaction can be used to calculate the binding energies of exciton states in a range of SWCNTs, which would be impractical by ab initio study. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A28.00004: Environmental effect for exciton transition energy of single carbon nanotubes Riichiro Saito, Kentaro Sato, Park JinSung, Yuhei Miyauchi, Shigeo Maruyama, Mildred Dresselhaus, Gene Dresselhaus The exiton transition energies of single wall carbon nanotubes which are observed in resonance Raman, photo-absorption, and photoluminescense spectroscopies, depend on the surrouding materials (environmental effect). The environmental effect can be explained by screeining of the excitonic states by the dielectric materials. We calculate the transition energies for many different (n,m) carbon nanotubes up to 4eV and to 3nm in diameter. The calculated results are compare with many experimental data with different conditions for samples. The energy shift for the exciton transition energies can be explained by a fitting parameter of static dielectric constants of surrouding materials. However we will show that the effective dielectric constant has a unique, type, metallicity, diameter, and energy dependence of the dielectric constants in order to reproduce the exciton energies for the wide range of diameter and excitation energies. By analyzing the data, we will give a simple formula for the dielectric constants for carbon nanotubes themselves and the surrounding materials as a function of chirality and diameter of single wall carbon nanotubes. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A28.00005: Surface Exciton-Plasmons in Carbon Nanotubes Igor Bondarev, Kevin Tatur, Lilia Woods We study theoretically the interactions of excitonic states with surface electromagnetic modes of a single-walled carbon nanotube. We use our previously developed Green's function formalism to quantize an electromagnetic field in the presence of quasi-1D absorbing bodies [1]. We show that these interactions result in the exciton-plasmon coupling that is significant in its strength due to the presence of weakly-dispersive low-energy ($\sim $0.5-2eV) interband surface plasmon modes [2] and large exciton excitation energies $\sim $1eV in small-diameter nanotubes [3]. We estimate the exciton-plasmon Rabi splitting to be $\sim $0.01-0.1eV which is close to that observed in organic semiconductors [4] and much larger than that reported for hybrid semiconductor-metal nanoparticle molecules [5]. We calculate the exciton absorption lineshape and demonstrate a clear line splitting effect as the exciton energy is tuned to the closest interband surface plasmon resonance. \newline [1] I.V.Bondarev and Ph.Lambin, Phys. Rev. B72, 035451 (2005). [2] T.Pichler, et al., Phys. Rev. Lett. 80, 4729 (1998). [3] D.Spataru, et al., Phys. Rev. Lett. 95, 247402 (2005). [4] J.Belessa, et al., Phys. Rev. Lett. 93, 036404 (2004). [5] W.Zhang, A.O.Govorov, G.W.Bryant, Phys. Rev. Lett. 97, 146804 (2006). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A28.00006: Cross-polarized optical absorption of single-walled carbon nanotubes probed by photoluminescence excitation spectroscopy, UV-Vis-IR and polarized Raman Scatterings Invited Speaker: Because of the depolarization effect, or so-called antenna effect, optical absorption of single-walled carbon nanotubes (SWNTs) is weak when excited by light polarized perpendicular to the nanotube axis. However, in photoluminescence (PL) excitation spectra of isolated SWNTs, PL peaks due to cross-polarized excitation can be clearly identified. By decomposing the cross-polarized component, the optical transition energy of E12 or E21 can be? measured, and the smaller exciton binding energy for perpendicular excitations is concluded [1]. Cross-polarized absorption is dominant in the absorption of a vertically aligned film of SWNTs [2] when excited from the top of the film. In our previous study, a pi-plasmon absorption at 5.25 eV was revealed in contrast to 4.5 eV for parallel excitation [3]. Resonant Raman scattering from such a film is also influenced by the cross-polarized excitation [4]. Even though a Kataura plot for the E33 and E44 range has been proposed by using such a vertically aligned film [5], polarized Raman scattering spectra reveal more complicated features in the system because of the small bundle size, typically 5-8 nanotubes [6]. \newline References: \newline [1] Y. Miyauchi, M. Oba, S. Maruyama, Phys. Rev. B 74 (2006) 205440. \newline [2] Y. Murakami, S. Chiashi, Y. Miyauchi, M. Hu, M. Ogura, T. Okubo, S. Maruyama, Chem. Phys. Lett. 385 (2004) 298. \newline [3] Y. Murakami, E. Einarsson, T. Edamura, S. Maruyama, Phys. Rev. Lett. 94 (2005) 087402. \newline [4] Y. Murakami, S. Chiashi, E. Einarsson, S. Maruyama, Phys. Rev. B 71 (2005) 085403. \newline [5] P. T. Araujo, S. K. Doorn, S. Kilina, S. Tretiak, E. Einarsson, S. Maruyama, H. Chacham, M. A. Pimenta, A. Jorio, Phys. Rev. Lett. 88 (2007) 067401. \newline [6] E. Einarsson, H. Shiozawa, C. Kramberger, M. H. Ruemmeli, A. Gruneis, T. Pichler, S. Maruyama, J. Phys. Chem. C (2007) published on Web. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A28.00007: Carbon nanotube excited states: the role of the environment Invited Speaker: The nature of the excited states and the radiative and non-radiative decay of isolated CNTs will be briefly reviewed. I will then focus on the role of the environment, external fields and photon confinement on these properties. I will discuss the use of photovoltage microscopy as a means of imaging environment-induced potential fluctuations in CNTs, presence and size of Schottky barriers at contacts and the extent of band-bending. Scanning resonant micro-Raman scattering will be used to evaluate the CNT excited state shifts induced by the substrate, and, also the phonon frequency and linewidth changes due to the fields of trapped substrate charges, and charge-transfer effects. Variations in local charge density will be determined by making use of the dependence of electron-phonon coupling on the local charge density. The non-radiative decay of free and localized excitons will then be examined and it will be shown that a phonon-assisted electronic decay mechanism made possible by interaction with the environment can dominate the lifetime of excited CNTs. Finally, results on the modification of the radiative properties of CNTs by changing the photon field mode density for CNTs enclosed in micro-cavities will be presented. [Preview Abstract] |
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