Biomedical subjects

A G Souza Filho

Publications and source records attributed to A G Souza Filho.

8 recordsLinked to original sources

PubMed2005

Strain-induced interference effects on the resonance Raman cross section of carbon nanotubes.

In this Letter, we report the effects of strain on the electronic properties of single-wall carbon nanotubes. When we normalize the electronic transition energies to the corresponding values obtained for unstrained tubes, we obtain that, regardless of the tube diameter, all the data collapse onto universal curves following an n - m = constant family pattern. In the case of metallic tubes, quantum interference effects on the Raman cross section are predicted for strained tubes when the energies of the lower and the upper components have nearly the same values. Experimental evidence for the strain-induced Raman cross section changes is observed in single nanotube spectroscopy.

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PubMed2004

One-dimensional character of combination modes in the resonance Raman scattering of carbon nanotubes.

Resonance Raman spectroscopy with an energy tunable system is used to analyze the 600-1100 cm(-1) spectral region in single-wall carbon nanotubes. Sharp peaks are associated with the combination of zone folded optic and acoustic branches from 2D graphite. These combination modes exhibit a peculiar dependence on the excitation laser energy that is explained on the basis of a highly selective resonance process that considers phonons and electrons in low dimensional materials.

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PubMed2003

Resonance Raman spectra of carbon nanotubes by cross-polarized light.

Resonance Raman studies on single wall carbon nanotubes (SWNTs) show that resonance with cross polarized light, i.e., with the E(mu,mu+/-1) van Hove singularities in the joint density of states needs to be taken into account when analyzing the Raman and optical absorption spectra from isolated SWNTs. This study is performed by analyzing the polarization, laser energy, and diameter dependence of two Raman features, the tangential modes (G band) and a second-order mode (G' band), at the isolated SWNT level.

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PubMed2003

Phonon trigonal warping effect in graphite and carbon nanotubes.

The one-dimensional structure of carbon nanotubes leads to quantum confinement of the wave vectors for the electronic states, thus making the double resonance Raman process selective, not only of the magnitude, but also of the direction of the phonon wave vectors. This additional selectivity allows us to reconstruct the phonon dispersion relations of 2D graphite, by probing individual single wall carbon nanotubes of different chiralities by resonance Raman spectroscopy, and using different laser excitation energies. In particular, we are able to measure the anisotropy, or the trigonal warping effect, in the phonon dispersion relations around the hexagonal corner of the Brillouin zone of graphite.

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PubMed2003

The concept of cutting lines in carbon nanotube science.

A review is presented of one-dimensional cutting lines that are utilized to obtain the physical properties of carbon nanotubes from the corresponding properties of graphite by the zone-folding scheme. Quantization effects in general low-dimensional systems are briefly discussed, followed by a more detailed consideration of one-dimensional single-wall carbon nanotubes. The geometrical structure of the nanotube is described, from which quantum confined states are constructed. These allowed states in the momentum space of graphite are known as cutting lines. Different representations of the cutting lines in momentum space are introduced. Electronic and phonon dispersion relations for nanotubes are derived by using cutting lines and the zone-folding scheme. The relation between cutting lines and singularities in the electronic density of states is considered. The selection rules for carbon nanotubes are shown to be directly connected with the cutting lines. Different experimental techniques are considered that confirm the validity of cutting lines and the zone-folding approach.

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PubMed2002

Single nanotube Raman spectroscopy.

A review is presented on the observation of the resonant Raman spectra from one isolated single wall carbon nanotube, focusing on the important structural information that is provided by single nanotube spectroscopy including the (n, m) determination of the individual tubes. The special sensitivity of the radial breathing mode to the (n, m) determination is emphasized, and the corroboration of this (n, m) assignment by diameter- and chirality-dependent phenomena in other Raman modes, such as the G-band, D-band, and G'-band features is also discussed. The significance of single nanotube spectroscopy for future nanotube research in general is briefly reviewed.

Journal Article↗

PubMed2001

Probing phonon dispersion relations of graphite by double resonance Raman scattering.

The phonon dispersion relations of graphite can be probed over a wide range of the Brillouin zone by double resonance Raman spectroscopy. The double resonance Raman process provides us with new assignments for the dispersive and nondispersive features observed in the Raman spectra of disordered graphite and carbon nanotubes, some features having been incorrectly assigned previously, or not assigned at all.

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PubMedDate not supplied

Science and applications of single-nanotube Raman spectroscopy.

A review is presented of the resonance Raman spectra from individual isolated single-wall carbon nanotubes (SWNTs). A brief summary is given of how the measurements are made. Why the resonance Raman effect allows single-carbon nanotube spectra to be observed easily and under normal operating conditions is summarized. The important structural information that is provided by single-nanotube spectroscopy using one laser line is discussed, and what else can be learned from tunable laser experiments is reviewed. Particular attention is given to the determination of the nanotube diameter and of the energy of its van Hove singularities Eii. Applications of single-nanotube spectroscopy are emphasized, such as measurements of isolated SWNTs connected with circuit-based samples and of isolated SWNTs mounted on an atomic force microscope tip. A critical assessment of the opportunities and limitations of the resonance Raman method for structural (n, m) identification is presented. The trigonal warping effect, which is central to the (n, m) identification in resonance Raman spectroscopy, is discussed in simple terms, and the importance of this effect in nanotube science and applications is reviewed.

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