Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.

Oxygen substitution in sputter-deposited MoS2 films studied by extended X-ray absorption fine structure, X-ray photoelectron spectroscopy and X-ray diffraction

Abstract: The morphology and crystallite structure/orientation of sputter-deposited MoS 2 solid lubricant films have been shown to be strongly affected by oxygen-containing species in the sputtering chamber. We have studied the effect of oxygen incorporation within these films using extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Films produced at The Aerospace Corporation, the National Centre of Tribology (U.K.), Hohman Plating and Manufacturing, and NASA Lewis Research Center were studied to determine common characteristics of different MoS 2 films produced by sputter deposition. In agreement with previous results, XPS of the films indicated a bulk anion:cation ratio ( i.e. (S+O):Mo) of about 2, and XRD revealed that the films consisted completely of MoS 2 -like phases that were compressed in the ( h k 0) directions as compared with pure MoS 2 . This “edge plane compression” appeared to correlate with oxygen content. EXAFS confirmed the absence of MoO 2 in the films and indicated that the films actually consist of two similar MoS 2 -like phases. Correlation of the EXAFS results with those obtained from XPS and XRD indicated that the two phases are MoS 2 and an MoS 2- x O x substitutional solution. The solid solution may be described as MoS 2 with oxygen atoms that have substituted for sulfur atoms in the MoS 2 crystal lattice. Relative peak height changes in the EXAFS radial distribution curves indicated that increasing amounts of oxygen cause a reduction in the size of crystallites in the films. The presence of the MoS 2- x O x phase may explain the superior tribological performance of MoS 2 films in some applications, which has been shown to correlate with oxygen impurities in the films.

Comparison of extended x-ray absorption fine structure and Scherrer analysis of x-ray diffraction as methods for determining mean sizes of polydisperse nanoparticles

Abstract: Curve fitting of extended x-ray absorption fine structure (EXAFS) spectra, transmission electron microscopy (TEM) imaging, and Scherrer analysis of x-ray diffraction (XRD) are compared as methods for determining the mean crystallite size in polydisperse samples of platinum nanoparticles. By applying the techniques to mixtures of pure samples, it is found that EXAFS correctly determines the relative mean sizes of these polydisperse samples, while XRD tends to be weighted more toward the largest crystallites in the sample. Results for TEM are not clear cut, due to polycrystallinity and aggregation, but are consistent with the other results.

Synchrotron-radiation studies of the transition of hydrogenated amorphous carbon to graphitic carbon

Abstract: Using the synchrotron-radiation facility at Brookhaven National Laboratory, we have studied the electronic structure of thin hydrogenated amorphous carbon films produced by dc glow discharges of hydrocarbons. Valence-band photoemission measurements on these films yielded information on the occupied valence-band density of states, while near-edge absorption at the carbon K edge elucidated features of the unoccupied conduction band. The as-deposited, unannealed films are essentially amorphous, showing no extended x-ray absorption fine-structure (EXAFS) features. Valence-band photoemission results show that these films are insulating, suggesting tetrahedral (diamondlike) coordination. The near-edge-absorption results, however, show some unsaturated carbon-carbon bonds as evidenced by a 1s..--> pi..( feature. Upon annealing, the films become more graphitic as evidenced by both the photoemission and near-edge-absorption measurements showing fine structure in agreement with that observed for crystalline or microcrystalline graphite. They also become conductive. Annealing is accompanied by evolution of hydrogen from the films, and this appears correlated with the development of certain features of the band structure as the film orders. The annealed films also show the beginnings of features in the EXAFS region.