Metal Surface Of The Semiconductor And The First Principle Of The Semi-metal Films Studied

The dissertation is devoted to the study of physical and chemical properties of novel materials from first principles. With the progress in density functional theory (DFT) and its numerical methods, DFT based first-principles calculation has become a routine method for condensed matter theory, quantum chemistry and material science. In this dissertation, we study a variety of materials, from conventional semiconductor, organic semiconductor to semimetal, adsorbed on transitional metal surfaces. The concerned properties include geometrical struc-tures and electronic structures. In the first chapter, we introduce the basic con-cept of DFT and review its recent progress. Finding good approximation for exchange-correlation functional is one of the main targets in DFT. With the de-velopment of modern functional, DFT leads to more and more accurate results. In addition, extension of DFT to the time dependent region and combination of DFT with dynamic mean field theory (DMFT) are also active topics now. All these progresses lead DFT applicable to a broad range of problems. This trend is illustrated by some examples at the end of this chapter.In chapter 2, we focus on the numerical methods of DFT. As a first step, we need to discrete the analytical formulation. Based on different discretization schemes, a variety of numerical methods are introduced into DFT, including modern wavelet method. Meanwhile, the linear scaling algorithms of DFT are getting mature, which makes the application of DFT to large systems such as big biological molecules. With so many methods, it is difficult to choose a best one, each method favors a certain kind of problems. Although it is important to develop methods and program, external packages are still often used. At the end of this chapter, we briefly introduce some widely used simulation packages.Starting from chapter 3, we begin to focus on properties of real materials from first principles. Germanium, as a typical semiconductor, shows very inter-esting behaviors on closed packed substrate of transition metal. Ruthenium is a very important transition metal in the catalysis industry, and the close packed Ru(0001) surface have some special electronic structure. Ge growth on Ru(00 01) can be considered as a model system for well understanding growth behavior of semiconductors on close packed surface of transition metals. Recently scan-ning tunneling microscopy (STM) investigations of germanium growth on the Ru(0001) surface was reported, and the results showed a ((21)^1/2×(21)^1/2)R10.9°superstructure in the sub-monolayer regime, we report on coverage-dependent adsorption energy of germanium on the Ru(0001) surface by using the state-of-the art DFT calculation method, and with a relatively stable model structure of Ru(0001)-(21)^1/2×(21)^1/2-3Ge. The scanning tunneling microscopy (STM) images and the scanning tunneling spectra (STS) images are well simulated, consisting well with the STM measurements on the ((21)^1/2×(21)^1/2)R10.9°superstructure of Ge/Ru(0001) system in the monolayer regime.In chapter 4, we report our theoretical investigations of tetracene adsorption on the transition metal Ru(1010) surface by using ab initio DFT calculations. Several adsorption geometries of flat-lying tetracene molecule with its long axis parallel and perpendicular to the [1210] direction of Ru(1010) substrate were investigated in details. The most stable adsorption structures are determined to be for the molecule adsorbed between the top and the second Ru atomic rows, with its long molecular axis along the [1210] direction and an adsorption energy of 4.23 eV, and for tetracene adsorbed with its long molecular axis perpendicular to [1210] with a slightly smaller adsorption energy of 4.19 eV. The adsorption energy difference of about 0.04 eV between these two adsorption configurations consists well with the statistic orientational distributions observed in the STM measurements), and the both adsorption configurations match very well with the STM experiments for tetracene overlayer on Ru(1010). By comparing densities of states before and after tetracene adsorption, we conclude that tetracene ad-sorption on the Ru(1010) surface is determined by the coupling of the Ru d-band and the filled p-orbitals of tetracene.The above two chapters concern semiconductor only. In chapter 5, we study a kind of novel 2-D semimetal material:graphene. graphene has very unusual electronic properties. The elementary excitations are linearly dispersing Dirac fermions with vanishing electronic density of states, whose properties can be con- trolled by application of external electric and magnetic field, or by altering sample geometry and topology. Because of its linear (photon-like) energy-momentum dispersion relationship, The electron mobility in graphene is very high. With such features, graphene is promising in many applications, such as bipolar de-vices which comprises junctions between hole-like and electron-like regions, and p-n junctions which can be configured by gate-voltage within a single atomic layer. Graphene based devices can be expected to have much more advantages than silicon based devices. We have carried out a first principles investigation on graphene grown on Cu(111) surface with a h-BN buffer layer. Electrons transfer from the copper substrate to graphene through the BN buffer layer is predicted which results in a n-doped graphene in the absense of a gate voltage. A gap opening at the Dirac point just below the Fermi-level is observed. The size of the gap is comparable to that of a typical narrow gap semiconductor. The Fermi-level can be easily shifted inside this gap to make graphene a semiconductor. Considering that the gap size of graphene on h-BN or clean Cu(111) is only a few meV, it would be possible to engineering the gap of graphene through the tuning the interaction between the buffer layer and the metal surface by choosing different metal surface or buffer layer.