Final-state effects in X-ray spectra from transition metal oxides and silicates

Abstract

Due to their chemical selectivity and the large amount of information that can be gained about the charge and coordination number (CN) of an element, X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) are routinely used to study metal centres in a variety of synthetic (e.g., alloys, ceramics, films) and natural (e.g., plants, soils) matrices. However, many competing effects influence spectral energies, and the ability to separate these effects is difficult. In particular, the effect of CN and the next-nearest neighbour (NNN) on XPS binding energies (BE) of inorganic solids have not been well-studied. In this work, the constituent elements of several industrially-relevant materials have been substituted and the resulting shifts in XPS and XANES spectral energies have been investigated, leading to a better understanding of the different effects that can cause these shifts. With increasing Zn content in SrFe(1-x)Zn(x)O(3-δ) (0 ≤ x ≤ 1), an oxygen-deficient perovskite-type structure, examination of Fe K- and Zn K-edge XANES spectra shows that greater oxygen deficiency (δ) lowers the transition-metal CN. Substitution of Fe by Zn results in shifts in the metal 2p XPS BEs that are much greater than the shifts observed in the corresponding L2,3-edge XANES absorption energies. As the number of electron-rich O2- anions surrounding the metal centres decreases, there is less electron density to screen the core-hole generated by XANES or XPS processes. Consequently, the poorly-screened core-hole exerts a stronger influence on the system, whose electrons relax to a greater extent. Further, O is electronegative compared to other atoms in the structure, and its tendency to tightly bind electrons restricts the ability of electrons from the material to relax around a core-hole on a metal centre. As the CN decreases, the magnitude of final-state relaxation around the core-hole increases, lowering the final-state energy and the observed BE. When the same core-electron is excited, this effect is more pronounced in XPS than in XANES, where the excited electron partially screens the core-hole. Investigations of (TiO2)x(SiO2)1-x (0 ≤ x ≤ 1), an amorphous metal-silicate, showed that the use of both hard (Ti K-edge) and soft (Ti L2,3-edge) X-rays provides a useful way to monitor changes in the bulk and surface, respectively. The bulk and surface regimes are critical for the applications of the amorphous transition-metal silicates, which are now being used as high-κ dielectric materials for use in semiconductors. Comparison of Ti K- and L2,3-edge spectra revealed that Ti atoms at the surface have a higher average CN than in the bulk, likely due to the presence of surface hydroxide and water groups that can coordinate to the Ti centres. The O K-edge, Ti L2,3-edge, and Si L2,3-edge XANES absorption energies showed little to no change with Ti content, while the O 1s, Ti 2p, and Si 2p XPS BEs were found to decrease significantly with increasing Ti content. As Ti replaces electronegative Si atoms, electrons in the material become less tightly bound and can relax to a greater extent around a core-hole. The larger degree of relaxation screens the core-hole more effectively in the final-state, lowering the final-state energy and all core-line BEs in these materials. Investigations of amorphous quaternary [(ZrO2)x(TiO2)y(SiO2)1-x-y (x + y = 0.20, 0.30)] and related ternary [(ZrO2)x(SiO2)1-x (0 ≤ x ≤ 1)] silicates found similar results. Namely, final-state relaxation increases with the amount of incorporated metal-oxide. The increase in final-state relaxation with total metal content has been confirmed empirically through analysis of the Auger parameter, which also increases with total metal content. These studies provide more examples that help us improve our understanding of the many influences that makes analysis of XPS spectra complicated, and highlight large changes in BE (>1 eV) that can occur without any changes in ground-state energies (e.g., oxidation state).