A new technique for the sp$^2$/sp$^3$ characterisation of carbon materials

Abstract

We present a technique to determine the sp$^3$/sp$^2$ ratio of carbon materials which is based on the electron energy-loss spectroscopy and which uses the theoretical spectrum of graphite obtained from ab initio electronic structure calculations. The method relies on the separation of the $π^*$ and $σ^*$ components of the carbon K-edge of graphite. This $π^*$ spectrum is adopted and assumed to be transferable to other carbon systems given an appropriate parametrisation of the broadening. The method is applied on a series of Monte-Carlo generated amorphous carbon structures and is shown to be stable over a wide range of the energy windows for which spectral integration is performed. The results are found to be in good agreement with the sp$^3$ fraction obtained from a microscopic scheme which uses the $π-$orbital axis vector (POAV1) analysis. The technique was also applied on a series of experimental spectra of amorphous carbon and was found to be in good agreement with the results obtained from a functional fitting approach.