Soft X-ray Spectroscopy of Metal Nitrides and Oxides: Uncovering Structure-property Relationships in Phosphors for pc-LEDs

This thesis engages a problem that is seemingly a little incongruous: to develop an under- standing of, and link together, the structure-property relationships of LED-phosphors and frontier polymorphs of vanadium oxides using the same experimental and theoretical frame- work. Soft x-ray spectroscopy and density functional theory calculations are used as the probe of, and interpretive tool for, the electronic structures of these materials. By comparing and contrasting the observed electronic structures to those of other, similar systems, changes in electronic structure are linked to changes in crystal structure and atomic composition. From this vantage point the LED-phosphors, which are essentially metal nitrides, and vana- dium oxides are not as disparate as they would seem at first glance. Their properties lie on a continuum, allowing them to be readily understood within a single framework. At the same time, there are significant insights to be gained about each group, by studying the other. In both groups the interplay between localization and delocalization of metal d-states is seen to be key to their functional properties, both as a result of and in conjunction with, the influence of their ligands. The effects of the ligands are seen to stem directly from their local charge densities, as well as their separation from and arrangement around the metal sites. In addition the interaction of non-metal sites is seen to be critical in many cases. Structure-property relationships are outlined for this suite of materials. Critical material properties, such as the band gaps, and the location of metal states in the band gaps, are determined experimentally. Several analysis techniques are developed that prove critical for the analyses of these compounds. Included in this is a thorough analysis of x-ray self- absorption in vanadium oxides, which is too often ignored. Ultimately a coherent framework for understanding the properties of these compounds is developed and it is hoped that this will serve as the basis of further refinement of their useful properties