A technique for the sp2/sp3 characterisation of carbon materials

Abstract

A method is devised to quantify the sp3 fraction of an amorphous carbon sample. It exploits the theoretical separation of the π* and σ* components of the carbon 1s electron energy-loss spectra. We perform density functional theory calculations on graphite and a series of Monte Carlo generated amorphous carbon structures of varying densities. We decompose the theoretically calculated K-edge of graphite into the π* and the σ* components. The resulting π* spectrum is adopted and assumed to be transferable to other carbon systems pending an appropriate parametrization of the broadening. The method is applied to the generated carbon structures and shown to be stable over a wide range of the energy window used for spectral integration. The sp3 fractions obtained for the series of amorphous carbon structures are found to be in good agreement with those obtained using the π-orbital axis vector approximation. The X-ray photoelectron spectra of the generated structures are also calculated. The sp3 fractions derived from these spectra are found to correspond well with the coordination architectures but fail to reproduce the sp3 fraction obtained using the C 1s energy-loss technique for low densities. We have also applied the C 1s energy-loss method on the energy-loss near-edge spectra of a series of plasma deposited amorphous carbon samples and found good agreement with results obtained using a functional fitting approach. Using dielectric function calculations we confirm the Ep ∝ n0.5e quasifree electron density dependence of the π + σ plasmon energies and deduce the ratio of effective electron band mass to the free electron mass as m*/m = 0.87. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)