Propriedades das espumas grafíticas e dos nanotubos de carbono Fo

Abstract

In this work we applied experimental techniques, such as Raman spcetroscopy and electron microscopy, and group theoretical analysis to study the structural, electronic and vibrational properties of graphitic materials. A Raman spectroscopy study of graphitic foams was performed for probing the spatial and laser excitation energy dependence of the double resonance Raman peaks. We have observed changes in the D band relative intensity, and on the relative contribution from turbostratic (2D) graphite and from highly aligned (3D) graphite to the G0 band. The D band integrated intensity was found to decrease linearly with increasing laser energy (EL), in contrast with previous experiments on nanographite twhich showed a E −4 L dependence. The calculation of the skewness (which is a measure of the asymmetry of a distribution) of the G0 band profile was found to be a good qualitative measure of the relative density of 2D and 3D graphite in a given region of the sample. The direct comparison between spatial distribution of the D band relative intensity and the skewness of the G0 band, suggests a correlation between the presence of defects and the high density of 2D graphite. We also reviewed the basic properties of carbon nanotubes from the standpoint of group theory. The zone folding scheme is explained in the light of the helical symmetry of the nanotubes. The group theory for chiral and achiral nanotubes is presented in detail, and the irreducible representations of the factor groups of the wavevector k are obtained. The quantum numbers which label those representations are discussed in terms of the linear and quasi-angular momenta. Finally, we extend the results of group theory to shed light on the electronic and excitonic properties of carbon nanotubes. Selection rules for the optical absorption are discussed for the case where the electron–electron interaction is strong and the exciton binding energies cannot be neglected. Trasmission Electron Microscopy (TEM) and electron microdiffraction experiments were performed on carbon nanotubes in small bundles grown directly on the TEM grid. A precise (n, m) identification was obtained in the case of a two-nanotube rope. This result shows that the microdifraction technique, which can be performed on any simple TEM apparatus, can be used to provide structural determination of the carbon nanotubes in the sample. Finally, spectroscopic studies were performed in acid treated carbon nanotubes. Initially, Single-wall carbon nanotubes (SWNTs) prepared by the arc discharge method were oxidized using nitric acid. The samples were analyzed by using Raman scattering and Fourier transformed infrared spectroscopy (FTIR). The FTIR results indicate the presence of −COOH acid groups in the treated samples. The up shifts observed in the radial breathing mode frequencies suggest that SWNTs behave as donors after the acid treatment, with charge transfer occurring from the nanotubes to the −COOH groups. Ab initio calculations of SWNTs interacting with −COOH acid groups support the charge transfer process from the nanotubes to the carboxyl groups. To better understand the charge transfer effects on carbon nanotubes, Raman spectroscopy experiments were also performed on a mixture of Single-wall and Double-wall carbon nanotubes for different relative concentrations. In this experiment, two sets of samples were analyzed, one which was exposed to H2SO4 for 5 s and one which is pristine. The H2SO4 is known to act as an acceptor for the electrons of graphitic materials. The effect of the hole doping on the vibrational and electronic properties of the double and single-wall carbon nanotubes is probed using Resonant Raman scattering with different excitation energies probing different nanotubes. The doping effects on the inner and outer walls of double-wall nanotubes is also discussed.