We study the effects of Rhenium (Re) deposited on epitaxial monolayer graphene grown on SiC(0001) and after subsequent annealing at different temperatures, by performing high resolution photoelectron spectroscopy (PES) and angle resolved photoelectron spectroscopy (ARPES). The graphene-Re system is found to be thermally stable. While no intercalation or chemical reaction of the Re is detected after deposition and subsequent annealing up to 1200 degrees C, a gradual decrease in the binding energy of the Re 4f doublet is observed. We propose that a larger mobility of the Re atoms with increasing annealing temperature and hopping of Re atoms between different defective sites on the graphene sample could induce this decrease of Re 4f binding energy. This is corroborated by first principles density functional theory (DFT) calculations of the Re core-level binding energy shift. No change in the doping or splitting of the initial monolayer graphene electronic band structure is observed after Re deposition and annealing up to 1200 degrees C, only a broadening of the bands. (C) 2017 Elsevier B.V. All rights reserved.

Nanoclusters have gained a huge interest due to their unique properties. They represent an intermediate state between an atom and a solid, which manifests itself in their atomic configurations and electronic structure. The applications of nanoclusters require detailed understanding of their properties and strongly depend on the ability to control their synthesis process. Significant effort has been invested in modelling of nanoclusters properties. However, the complexity of these systems is such that many aspects of their growth process and properties are yet to be understood.

My thesis focuses on describing structural and electronic properties of nanoclusters. In particular, the model for nanoparticles growth in plasma condition is developed and applied, allowing to describe the influence of the plasma conditions on the evaporation, growth and morphological transformation processes. The mechanism driving the morphology transition from icosahedral to decahedral phase is suggested based on force-fields models. Spectroscopic methods allow for precise characterization of nanoclusters and constitute an important tool for analysis of their electronic structure of valence band as well as core-states. The special attention in the thesis is paid to the core-states of nanoclusters and influences that affect them. In particular, the effects of local coordination, interatomic distances and confinement effects are investigated in metal nanoclusters by density functional theory methods. These effects and their contribution to spectroscopic features of nanoclusters in X-ray photoemission are modelled. The relation between the reactivity of nanoclusters and their spectroscopic features calculated in different approximations are revealed and explained. Ceria is a very important system for many applications due to the ability of cerium atoms to change their oxidation state depending on the environment. The shift of the oxidation state and its effects on the core-states is examined with X-ray absorption measurements and modelling allowing to build a rigid foundation for interpretation of the measured spectra and characterization of electronic structure of ceria nanoparticles.  

A pressure-induced interaction between core electrons, the core-level crossing (CLC) transition, has been observed in hcp Os at P approximate to 400 GPa [L. Dubrovinsky et al., Nature (London) 525, 226 (2015)]. By carrying out a systematic theoretical study for all metals of the 5d series (Hf, Ta, W, Re, Os, Ir, Pt, Au) we have found that the CLC transition is a general effect for this series of metals. While in Pt it occurs at approximate to 1500 GPa, at a pressure substantially higher than in Os, in Ir it occurs already at 80 GPa. Moreover, we predict that in Re the CLC transition may take place already at ambient pressure. We explain the effect of the CLC and analyze the shift of the transition pressure across the series within the Thomas-Fermi model. In particular, we show that the effect has many common features with the atomic collapse in rare-earth elements.

The morphology transition from the thermodynamically favorable to the unfavorable phase during growth of freestanding copper nanoclusters is studied by molecular dynamics simulations. We give a detailed description of the kinetics and thermodynamics of the process. A universal mechanism of a solid-solid transition, from icosahedral to decahedral morphology in the nanoclusters, is proposed. We show that a formation of distorted NC during the growth process with islands of incoming atoms localized in certain parts of the grown particle may shift the energy balance between Ih and Dh phases in favor of the latter leading to the morphology transition deep within the thermodynamic stability field of the former. The role of diffusion in the morphology transition is revealed. In particular, it is shown that fast diffusion should suppress the morphology transition and favor homogeneous growth of the nanoclusters.

A recently developed method of nanoclusters growth in a pulsed plasma is studied by means of molecular dynamics. A model that allows one to consider high-energy charged particles in classical molecular dynamics is suggested, and applied for studies of single impact events in nanoclusters growth. In particular, we provide a comparative analysis of the well-studied inert gas aggregation method and the growth from ions in a plasma. The importance to consider of the angular distribution of incoming ions in the simulations of the nanocluster growth is underlined. A detailed study of the energy transfer from the incoming ions to a nanocluster, as well as the diffusion of incoming ions on the cluster surface, is carried out. Our results are important for understanding and control of the nanocluster growth process.