S. G. Chou

Family behavior of the optical transition energies in single-wall carbon nanotubes of smaller diameters

Using the extended tight-binding model that allows bond lengths and angles to vary, the optical transition energies Eii in single-wall carbon nanotubes are calculated as a function of inverse tube diameter. After geometrical structure optimization, the 2n+m=constant family behavior observed in photoluminescence (PL) experiments is obtained, and detailed agreement between the calculations and PL experiments is achieved after including many-body corrections.

Potential dependent surface Raman spectroscopy of single wall carbon nanotube films on platinum electrodes

The analysis of the resonance Raman spectra of single-walled carbon nanotubes in an electrochemically controlled aqueous H2SO4 environment using different laser excitation energies shows major reversible and irreversible differences in the main vibrational features regarding their intensities, lineshapes, and frequencies for different applied potentials. These differences arise from the electrochemically induced changes in the occupation of electronic states for metallic and semiconducting nanotubes.

Quantitative evaluation of the octadecylamine-assisted bulk separation of semiconducting and metallic single-wall carbon nanotubes by resonance Raman spectroscopy

The selective stabilization of octadecylamine (ODA) on semiconducting (S) single-wall carbon nanotubes (SWNTs) has been reported to provide a means for the bulk separation of S from metallic (M) SWNTs. Utilizing resonance Raman spectroscopy and, in particular, the relative changes in the integrated intensities of the radial-breathing mode region, a generic method has been developed to provide quantitative evaluation of the separation efficiency between M and S SWNTs along with diameter separation. The ODA-assisted separation is shown to provide S enrichment by a factor of 5 for SWNTs prepared by high pressure CO decomposition and greater S enrichment for SWNTs with diameters below 1nm.

Electrochemical gating of individual single-wall carbon nanotubes observed by electron transport measurements and resonant Raman spectroscopy

Metal electrodes patterned lithographically on top of individual single-wall carbon nanotubes are used to gate the nanotubes with respect to a reference electrode in an electrolyte drop. The gating is found to have a dramatic effect on both the Raman spectra and electron transport of the nanotubes. Current through metallic nanotubes is found to increase sharply with electrochemical gate voltage, indicating that the Fermi energy reaches valence and conduction band van Hove singularities. Using resonant confocal micro-Raman spectroscopy, we observe a 9 cm−1 upshift of the tangential mode vibrational frequency, as well as a 90% decrease in intensity, by applying 1 V between an individual nanotube and a silver reference electrode in a dilute H2SO4 solution. The mechanisms for the shifts of the Raman mode frequencies are discussed on the basis of changes in the lattice constant of heavily charged nanotubes.

Length characterization of DNA-wrapped carbon nanotubes using Raman spectroscopy

A systematic resonance Raman study has been carried out on DNA-wrapped single walled carbon nanotubes (SWCNTs) of three different average lengths ⟨Ltube⟩ using seven different values of laser excitation energy Elaser. The dependence of the intensity ratio of the D-band and G-band Raman features (ID∕IG) on ⟨Ltube⟩ indicates that nanotube length can be used as an important structural parameter for Raman characterization. By systematically varying Elaser, the ratio ID∕IG is found to be much stronger for metallic than for semiconducting SWCNTs but appears to have the same functional dependence on Elaser and ⟨Ltube⟩ or crystallite size as does nanographite.

Environment effects on the Raman spectra of individual single-wall carbon nanotubes: Suspended and grown on polycrystalline silicon

An enhanced Raman signal is observed from individual suspended single-wall carbon nanotubes (SWNTs) and from isolated SWNTs grown on an n-doped polycrystalline silicon film used in standard silicon processing. The radial breathing modes of the Raman spectra taken from suspended SWNTs exhibit narrow linewidths, which indicate a relatively unperturbed environment for suspended SWNTs. Clear Raman signals from intermediate frequency modes in the frequency range from 520to1200cm−1 are presented, which might allow a detailed study of the phonon band structure of individual SWNTs.

Trigonal Anisotropy in Graphite and Carbon Nanotubes

We discuss here how the trigonal warping effect of the electronic structure is relevant to optical processes in graphite and carbon nanotubes. The electron-photon, electron-phonon, and elastic scattering matrix elements have a common factor of the coefficients of Bloch wave funtions of the A and B atoms in the graphite unit cell. Because of the three fold symmetry around the Fermi energy point (the K or K′ point), the matrix elements show a trigonal anisotropy which can be observed in both resonance Raman and photoluminescence spectroscopy. This anisotropy is essential for understanding the chirality dependence of the Raman intensity and the optical response of single wall carbon nanotubes.

Spectroscopy of small diameter single‐wall carbon nanotubes

Resonance Raman spectroscopy of small diameter (below 1.2 nm) single‐wall carbon nanotubes (SWCNT) is presented. The diameter and chirality dependent many‐body corrections to the tight binding based Kataura plot are discussed. The radial breathing modes also show small chirality dependence, giving evidence for the deviations of the small nanotube diameters from the ideal folded graphene structure. The use of spectroscopy for the characterization of small environmental effects and the (n,m) population on HiPco and CoMoCAT SWNT samples is pointed out. The richness of the intermediate frequency modes is highlighted.

Carbon nanotube photo-physics

A review is given of how resonance Raman spectroscopy (RRS) and photoluminescence (PL) can be used to reveal unique information about nanostructures, 1nm in diameter, thus providing new techniques for probing the electronic and vibrational properties of nanostructures with particular regard to nanosensing applications. Special attention is given to recent advances made in this field and to perspectives about future research directions.