Surface analysis of low dimensional materials : revealing their electronic properties by advanced spectroscopy

Abstract: Low-dimensional materials (0D, 1D, 2D) have been widely used to develop modern miniaturized (micro- and nano-) technology. The use of these materials come from their extraordinary optical, electrical, thermal, and mechanical properties, which are very different from the bulk crystal. To understand low-dimensional materials there is a large interest in studying the surface states of such materials, because the topmost few atomic layers possess an atomic arrangement and electronic structure different from the crystal bulk and hence responsible for many of the novel properties. In surface science, the techniques typically probe the topmost 1-10 nm of surfaces exposed to vacuum. X-ray photoemission spectroscopy (XPS) is the most common surface technique used because of its relatively easy handling and good ability to reveal important information on the surface oxidation states. XPS involves radiation of light that penetrates a sample up to 10 nm depth. Ultraviolet photoemission spectroscopy (UPS) is another surface-sensitive technique, with a slightly lower probing depth, on average about 2.5 nm. For the research in this thesis, a vacuum system has been constructed that contains surface analytical equipment for UPS, Angle Resolved Photospectroscopy and Low-electron energy diffraction. Normally, XPS and UPS are used as individual techniques as they both determine different properties of the material. However, hereby for many applications both are used in conjunction because they complement each other and provide a comprehensive analysis of the samples structure and electronic properties.The aim of this thesis is to present surface analytical measurements carried on low-dimensional materials. Among the materials studied is Graphene, used to as a proof-of-principle experiment in the vacuum system constructed as it has been extensively studied and information can be easily found. The second material was Cu2O thin films that shows different chemical and electronic properties depending on the oxidation level. The third material is nanoporous GaN that exhibit V-pits that modify the properties of the material depending on the hole sizes. The fourth material are trigonal Te nanowires, 1D nanostructures that has a narrow direct bandgap, a high-hole mobility, and a high current density. Finally, an application of the constructed setup is show characterizing the electronic properties of NiFeMo solids.