The structure of a-C: What NEXAFS and EXAFS see
Mechanically hard ha-C and soft sa-C amorphous carbon films of 2.9 and 2.2 g cm-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 pi 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 40percent 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 sigma* 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 bond conjugation in the films. The sp3 concentration estimated with the proposed method was of 44percent in the ha-C films and 10percent in the sa-C films. These values were consistent with the EXAFS results, but disagreed with those obtained based on the traditional pi*/sigma* 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 resultsupport 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.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Advanced Light Source Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 936737
- Report Number(s):
- LBNL-871E; TRN: US0805688
- Journal Information:
- Physical Review B, Vol. 76, Issue 094201; Related Information: Journal Publication Date: 2007
- Country of Publication:
- United States
- Language:
- English
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