Determining the sp2/sp3 bonding concentrations of carbon films using X-ray absorption spectroscopy

Abstract

The sp2 bonding concentrations of nitrogen-doped amorphous carbon samples and nanodiamond films were determined from their soft X-ray absorption spectra. The amorphous carbon (a-C) samples were deposited under atmospheres of varying nitrogen concentrations onto polytetrafluoroethylene (PTFE) polymer and silicon substrates. The nanodiamond films were synthesized on silicon substrates in a CH4/H2 gas mixture by microwave plasma chemical vapour deposition. The sp2 bonding concentrations in the a-C films (deposited on PTFE substrates) increase from 74% to 93% as nitrogen doping increases, with a step-like increase in sp2 fraction when nitrogen concentrations in the films exceed 27%. The a-C films on silicon substrates display the same trend of increasing sp2 concentration as a function of greater nitrogen concentration. Nanodiamond deposition conditions, such as bias voltage and methane concentration, affect the purity of the film. Our analysis reveals sp2 bonding concentrations in these samples ranging from a few percent to 11%.PACS Nos.: 78.70.Dm, 61.10.Ht, 61.46.+w, 81.05.Uw