Theoretical Investigation of X-Ray Absorption near Edge Spectroscopy (XANES) Angular Dependence of Aligned Carbon Nanotubes Grown by DC HF CVD Process
Materials Sciences and Applications
Carbon nanotubes (CNTs) grown on plain substrates SiO 2 /Si(100) by a direct current and hot filaments catalytic chemical vapor deposition process have been studied by synchrotron X-ray absorption near edge spectroscopy (XANES) technique to theoretically investigate the angular-dependence 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 * resonance decreases. This
... e 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 circularly). At the linear polarized light, π * resonance is a preference as well as at right-circular polarized. 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 surface plane for perfectly aligned HOPG or CNTs. This explains the incomplete extinction of π * intensity. We noticed at parallel orientation a region where any π * and σ * transitions did not expect because of the lack of polarization light.