Tuning miscibility and morphology at the nanoscale: First principles investigations

Abstract

This thesis is divided into five chapters. (A)The first chapter in the thesis gives a brief introduction and motivation for studying low dimensional systeM.S.. We have discussed about how the properties change very drastically when going from bulk to a lower dimensional scale. The importance of theoretical calculations and how it can help in the study of such materials are discussed. Computational science can help suggest new materials with desired properties, and can also help understand and explain experimental results by analysing the factors involved separately. The theoretical tools used for the calculations in the work reported in this thesis are briefly mentioned. (B)The second chapter of the thesis gives a brief description on density functional theory (DFT). An introduction to the many-body problem and the approximations involved are discussed. The techniques used for implementing the density functional theory like plane-wave basis set, k-point sampling and smearing are explained. The theory of calculating forces on the atoM.S. by the Hellmann-Feynman theorem is discussed. The framework of spinpolarised DFT used for the calculation of magnetic properties of systeM.S. is also briefly described. vii (C) The third chapter is dedicated to the study of “Surface alloys on a W(110) substrate”. It deals with two-dimensional systeM.S. of surface alloys, obtained by mixing two different metals over the surface of another metal. Metals that do not form alloys in the bulk phase may allow an atomic level mixing at the surface. Surface alloys become even more interesting when at least one of the constituents is magnetic, as both lower coordination at the surface and the change in effective coordination number due to alloying can have significant impact on the magnetic properties of the system. We chose eight different metals, three magnetic and five non-magnetic metals, belonging to a range of sizes and studied the surface alloy formation over the W(110). We studied the properties of the single-component monolayers of these metals over W(110). We calculated preferred surface sizes of the atoM.S. on the W(110) surface using stress calculations. We also investigated the stability and the magnetic property of the alloys formed (four configurations of alloys considered for every pair of magnetic and nonmagnetic metals considered by us). We have tried to analyse the stability of these surface alloys by separating the elastic and chemical contributions to the energy. (D) The fourth chapter is devoted to the study of “Controlling morphology of Au clusters by substrate doping”. We have carried out first principles investigations on the stable geometry and morphology of a 20-atom Au cluster. Au20 clusters are found to be catalytically active and this property is especially desirable for oxidation reactions as Au clusters were found to adsorb oxygen molecules strongly and cause an activation of the O-O bond. The Au clusters were found to be more catalytically active when they were deposited on defect-rich MgO (F-center defects) than on MgO without any defects. This is due to the formation of slightly negatively charged Au cluster on defect-rich MgO. It was futher calculated and experimentally proven that Au clusters on Mo-supported thin film of MgO preferred a two dimensional planar geometry than the stable tetrahedral geometry of the free cluster. Here, the planar geometry was found to be more negatively charged than the tetrahedral geometry and hence, the catalytic activity of the planar cluster was predicted to be higher. In this project, we have developed a strategy to alter the tetrahedral geometry of the free cluster to the catalytically more active planar geometry by depositing the cluster over Al-doped MgO. (E) In the fifth chapter the salient features of the findings of the thesis are summarised and some outlook for future projects are discussed.