Core-level Soft X-ray Spectroscopy of PbO-Based Photoconductors and Nanosheets of MoO3

Abstract

The successful incorporation into electronic devices of any novel material, whether nano-scaled or in bulk form, requires an understanding of the relationship between the structural and electronic properties of these materials. Soft X-ray core-level spectroscopy involves the creation of core-holes, which result in the transition of the valence electrons. These transitions provide invaluable information about the electronic and structural properties of a material. Ab initio calculations based on density functional theory allow the experimental observations to be interpreted, which makes it possible to link the electronic properties to the underlying atomic arrangements. We used these experimental and theoretical methods to study two different classes of metal oxides.

The conventional imaging material, amorphous selenium, used in direct conversion digital flat-panel X-ray detectors, is an adequate photoconductor only for low-energy applications such as mammography. Research into alternative materials with sufficiently high quantum yields, even at photon energies above 60 keV, is required to overcome the shortcomings of amorphous selenium. Owing to their high electron density, PbO-based photoconductors are promising alternatives. This thesis presents an investigation of the electronic and structural properties and the relationship between them for various differently prepared PbO films.

MoO3 belongs to a class of materials known as transition metal oxides. Owing to their layered structure, it is possible to isolate a single or a few layers of the material to produce two-dimensional nanosheets. Because of their reduced dimensionality, these materials have several potential applications, including in electronic logic devices, photovoltaic devices, photodetectors, and biosensors, and they could be used for biomedical diagnosis and therapy. These materials also exhibit a higher theoretical specific capacitance, making them excellent materials to use in supercapacitors. Due to enhanced many-body effects caused by reduced dimensionality, we showed that the two-dimensional MoO3 nanosheets experience a variety of X-ray induced excitations, including dd transitions, electron-hole bound states (excitons), and spin-flip excitations (magnons).