| Effective modifiction of the electrical, magnetic, and optical properties of semiconductors is highly desired for modern electronics and is one of central topic in condensed matter physics. Metal oxides and metal chalcogenides possess very diverse and interesting structures and exhibit useful physical properties which have many applications in modern technologies. However, their intrinsic physical properties do not always fulfill the need for practical use. Besides, in order to realize certain functional process, materials have to match each other in some physical properties. For these reasons, their surface properties or band structures need to be modified.Benefiting from the development of related theory, numeral algorithm, and computation technique, first-principles calculation based on density functional theory is now a common method for condensed matter physics, quantum chemistry, and material science research. In this thesis, the surface work-function modification of indium tin oxide, the band structure modification of MoO3, and the electronic and magnetic properties controlling of transition metal dichalcogenides (TMDCs) are investigated using density functional theory calculations. Our main research conclusions are summarized as following:(1) The impact of halogenation, in particular Cl and F, on the work functions of indium tin oxide (ITO) surfaces was studied. We found that a strong surface dipole layer induced by the halogen, rather than a change in the electrochemical potential (that is, Fermi level) of the ITO, led to a dramatic increase in the work function. However, the work function for F-coated ITO was lower than that of Cl-coated ITO. This result contradicts the well-known fact that F is much more electronegative than Cl. Detailed computations reveal that both electronegativity and atomic size collectively contribute to the extraordinarily high work function of Cl-ITO.(2) The band structure of MoO3can be significantly altered by a distortion in the octahedral coordination structure. We discovered that, in addition to epitaxial type of structural strain, chemical force such as hydrogen inclusion can also cause extended lattice distortion. The lattice distortion in hydrogenated MoO3led to a significant reduction of the energy gap, overshadowing the Moss-Burstein effect of band filling. Charge doping simulations revealed that filling of conduction band drives the lattice distortion. This suggests that any charge transfer or n-type electron doping could lead to lattice distortion and consequentially a reduction in energy gap.(3) We propose that a half semiconducting state can exist in trigonal-prismatic transition metal dichalcogenide (TMDC) monolayers of dl configuration. In that state both electrons and holes are spin polarized and share the same spin channel. On the basis of hybrid density functional theory, we predict in particular that VS2monolayers are half semiconductors with a direct band gap. Moreover, we find that the conduction electron spin orientation of VS2switches under moderate strain. Our predictions thus open up intriguing possibilities for applications of VS2in spintronics and optoelectronics. Our analysis of trigonal-prismatic group-V MX2(M=V, Nb, Ta; X=S, Se, Te) monolayers reveals a broad diversity of electronic states that can be understood qualitatively in terms of localization of d electrons.According to these investigations, some central conclusions are reached:atomic scale parameters such as atomic radius, molecule bond length, and lattice distance are crucial for the properties of solid in terms of electrical dipole, orbital coupling, and electron localization. Bear in mind the important role of these atomic parameters could help better control of the physical properties of materials. |
