Structure of amorphous carbon from near-edge and extended x-ray absorption spectroscopy

Mechanically hard (ha-C) and soft (sa-C) amorphous carbon films of 2.9 and 2.20.3em0exg0.2em0excm^-3 approximate densities were prepared by filtered cathodic arc deposition and analyzed by near--edge x-ray absorption spectroscopy (NEXAFS) and extended x-ray absorption spectroscopy (EXAFS) to determine their structure. The analysis observed an insignificant level of π bond conjugation in both kind of films. EXAFS distinguished two types of atomic environments in them: one semiordered with well defined bond lengths, and the other with so strong bond disorder that its contribution to EXAFS was undetectable. The proportion of atoms in the semiordered atomic environments was of less than 40% in both films. Their bond lengths were similar to those of diamond in the ha-C films and to graphite in the sa-C. NEXAFS spectra analysis was based on the linear relation between σ* energy and bond length. It served to quantify the proportion of sp³ bonded atoms in a-C, to deduce the average bond length of the atoms undetected by EXAFS, and to determine the level of π bond conjugation in the films. The sp³ concentration estimated with the proposed method was of 44% in the ha-C films and 10% in the sa-C films. These values were consistent with the EXAFS results, but disagreed with those obtained based on the traditional π*∕σ* intensity ratio method which overestimated sp³ concentrations. Annealing of the ha-C films up to its almost complete graphitization caused a gradual reduction in bond length of the semiordered environments with no differentiation between two phases, diamondlike and graphitelike, at any temperature. This result support models that explain sp³ bond promotion in a-C as caused by the high compressive stress attained by a strongly disordered sp² dense structure during film deposition.