Theoretical Investigation of X-Ray Absorption near Edge Spectroscopy (XANES) Angular Dependence of Aligned Carbon Nanotubes Grown by DC HF CVD Process

Abstract

Carbon nanotubes (CNTs) grown on plain substrates SiO2/Si(100) by a direct current and hot fila- ments catalytic chemical vapor deposition process have been studied by synchrotron X-ray ab- sorption near edge spectroscopy (XANES) technique to theoretically investigate the angular-de- pendence of carbone (C) K-edge π* and σ* transitions. Experimental XANES spectra show that π* resonance increases with the incidence angle from normal to grazing incidence angle while σ * re- sonance decreases. This has been explained by the sine-square and cosine-square dependencies of π * and σ * intensities, respectively. These results were confirmed by theoretical XANES curves of highly oriented pyrolytic graphite (HOPG) and CNTs plotted versus incidence angle. It has been shown that π* and σ* transitions strongly depend on the nature of polarized light (linearly or cir- cularly). At the linear polarized light, π* resonance is a preference as well as at right-circular pola- rized. At the left-circular polarized light, σ * resonance is a preference. The π * intensities are high at parallel orientation and the σ* intensities are low at normal orientation. The smallest π* intensity is noticed at normal orientation, where the π * orbitals are supposed to be lying parallel to the sur- face plane for perfectly aligned HOPG or CNTs. This explains the incomplete extinction of π * inten- sity. We noticed at parallel orientation a region where any π* and σ* transitions did not expect be- cause of the lack of polarization light. * Corresponding author.