Carbon Nanotubes: Optical Properties


M. A. Pimenta Departmento de Fisica, Universidade Federal de Minas Gerais

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The interaction between light and solids gives rise to many optical properties in solid-state systems that can be observed by a variety of experimental techniques such as photo absorption and emission, infrared absorption, Raman scattering, photoelectron and Auger spectroscopy, and ultraviolet and X-ray photoelectron spectroscopy. Especially in one-dimensional (1-D) solids, such as carbon nanotubes, much useful information on the optical properties can be obtained by exploiting low-dimensional effects, such as the direction of the polarization of the light relative to the nanotube axis, the resonance between the laser excitation energy and a singularity in the joint density of states, and the variety of crystal structures of nanotubes stemming from their chirality. Thus, by rotating the orientation angle of the nanotube relative to the incident and scattered light polarization directions, by changing the laser excitation energies, or by selecting different geometries for the nanotubes, we can systematically investigate the optical properties for many different types of carbon nanotubes in a consistent way. Especially fruitful has been the study of the resonance Raman spectra from only one nanotube by using micro-Raman measurements of a nanotube on a Si/SiO2 substrate. Because of the large amount of detailed information that can be obtained from this technique, micro-Raman spectroscopy is considered to be a standard nanotechnique for probing the optical properties of a nanotube in a quick, nondestructive way at room temperature and atmospheric pressure.

Since the reported observation of carbon nanotubes in 1991, high-resolution transmission electron microscopy (HRTEM) and scanning probe microscopy [SPM, such as scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), Kelvin force microscopy (KFM), magnetic force microscopy (MFM), and so on] have provided definitive tools for observing tiny samples on nanometer length scales. However, for single-wall carbon nanotubes (SWNTs), the high quality required of the measurement instruments and of the personnel to observe structure at the atomic level restricts the number of instruments and researchers who can make such measurements.

The finding of so many new phenomena and concepts in the nanotube field in the past decade reminds us of the good old times of the adventurers on sailing ships who ventured out to find the new world. In the next decade, nanotube research will enter a new period in which nanotube samples are easily obtained, in which many nanotube-based industrial technologies and applications will be developed, and in which fundamental physics and chemistry discoveries will continue to be made, but more and more the research will be stimulated by the needs of the applications and by the availability of new measurement tools. In this sense, a standard technique to characterize nanotubes is highly desirable. Here it is proposed that resonance Raman spectroscopy should fill this role.