Density-functional Theory Calculations And STM Image Simulations Of Surface Adsorption On Transition Metals

First-principles calculations have made significant contributions to our understanding of condensed-matter systems and solid-state properties. We can do profound investigation to the physical nature about the properties of solid and surface, and predict many material qualities and their tendency of variations from microscopic view by calculations. With the rapid development of theories and methods, first principle methods are greatly improved in applicability and accuracy. But the first principles investigations for the complicated solids and surface systems, especially for the first-row elements or systems with d or f electrons, are still in initial stage or have not obtain ideal result. Recently, more and more interests have been attracted for study those systems. The main contents being studied and important results in this research presented as following:1.. First, the surface atomic geometry, structural relaxations, and electronic states of clean Cu(110) surface and oxygen-adsorbed Cu(110) (2×1)-O surface have been studied by using ab initio total energy calculations. The density functional calculations have been carried out for three possible adsorbed positions of oxygen and the most favorable one has been determined by total energy comparison. It is reveled that the added-row reconstruction is the most stable one with maximum adsorption energy in Cu(110) (2×1)-O surface and the adsorbed 0 atom is beyond the outmost surface Cu layer slightly. The adsorbates lie approximately 0.016nm above the outermost Cu layer and the hybridized band derived from Cu 3d-O 2p hybridization locates in the range of-5.5 --6.0 eV below EF. The adsorption energy of oxygen in this configuration is measured to be-1.94 eV with respect to oxygen molecule. The work functions of clean Cu(110) and oxygen-adsorbed Cu(110) (2×1) surface are calculated to be 4.51 eV and 4.68 eV, respectively. The surface electronic structures show that the cohesive effect between adsorbates and the substrate Cu atoms is essentially due to the Cu 3d-O 2p interaction.2. Then, we have studied the influence of hydrogen contamination on the atomic geometry of Ti(0001) surface by using the density-functional theory and the projector-augmented wave method. Based on the optimized structural parameters of hcp Ti from the PAW total energy calculation, the surface relaxations, surface energy and work function of clean Ti(0001) surface were calculated in the same way. The adsorption geometries and total energies of several coverages of hydrogen on Ti(0001) surface including p(1×1),p(1×2),((?))R30°, and p(2×2), were studied for the hcp and fcc site absorptions combined with the both sites occupation in p(1×1) structure. These results suggest that the Ti(0001) p(lxl)/H geometry has the largest energy gain among each conformation, so under the condition of low coverage and low H2 pressure, the most possible conformation is p(1×1)-H adsorption. The shrink of Ti(0001) surface with H contamination was-3.7% from available experiments and this work yields-2.85% for hcp and-4.31% for fcc adsorption geometries, respectively. It is deduced that the most possible adsorption configuration for a hydrogen contaminated Ti(0001) surface is a mixture of hcp and fcc adsorptions. For a clean Ti(0001) surface the surface contraction is calculated to be near-7.0% while the experimental measurement predicted-4.9%. This observation implies that even for a "clean" Ti(0001) surface there is still about 13.6% surface area covered with hydrogen adsorption. These results reflect that the hydrogen contamination could affect the Ti(0001) surface structure dramatically. Furthermore the present study could yield a conclusion naturally that the shrink of the Ti(0001) surface will be reduced with the increase of H atom adsorption below 1.0 ML.3. We also have investigated the influence of oxygen adsorption on the surface geometry and electronic properties of Ag(100) surface by using the density-functional theory calculations. The total energy calculations based on projector-augmented wave (PAW) method have been preformed to describe the adsorption geometry at several coverages of oxygen adsorption including p(1×1), c(2×2), and ((?))R45°, and a series of essential physical properties at these coverages on Ag(100) surface such as the surface relaxation, adsorption energy, work function and so on. The results presented in this work show that for an Ag(100) ((?))R45°-2O geometry, the most stable atomic structure is a type of missing-row reconstruction. Eventually, this structural change causes the various displacements of surface atoms which have been calculated in this work. The calculations on the local density of states reveal that in the Ag(100) ((?))R45°-2O geometry the cohesive effect between the adsorbed oxygen atoms and the substrate Ag layer is essentially due to the sufficient Ag4d-O2p orbital hybridization. Finally we have.simulated the STM images for several bias-voltages and tip heights, providing experiments abundant data and theoretical supports. 4. Our next work is that total energy calculations on the atomic geometry and adsorption properties of Ni(100)/H2O surface are performed by using the state-of-art density-functional theory in the implementation of the projector-augmented wave (PAW) method. The adsorptions of molecular water have been investigated in the surface periodicities of p(3×3) combined with 4 different sites. It is concluded that water adsorbed on the Ni(100) surface prefers the T1 site with its axis tilted away from the surface normal. In considering the small adsorption energy-33 meV, it is easily to conclude that adsorption behind in the Ni(100)/H2O surface has the type of physical adsorption.5. In chapter 5, the STM images and surface relaxations of reconstructed W(100) c(2×2) surface have been investigated by using the density-functional theory calculations. The distorted displacementδof tungsten atom along [110] presents as 0.27 A combined with a distorted energy of 80.6 meV/atom, and the relaxation of W(100) c(2×2) surface is calculated to be-7.6% for△d12/d0, and+0.8% for△d12/d0 respectively while the surface work functionΦis 4.55 eV. The calculated STM images of the W(100) c(2×2) surface display unusual features as follows:The protrusions in STM image along [110] axis lie in the middle of zig-zag chain of tungsten atoms while the dark regions in STM images correspond to the valley between neighbor zig-zag chains due to surface reconstruction. The typical corrugation of STM tip has been calculated between 0.08-0.13 A for the negative bias voltages while in the positive bias voltage regions the corrugation varies from 0.19 A to 0.24 A.6. At the end, we have investigated the STM images and surface relaxations of oxygen adsorption on Mo(110) p(2×2) surface by using the density-functional theory calculations. The relaxation of oxygen adsorption on Mo(110)p(2×2) surface is calculated to be-2.10% for△d12/d0 which is much smaller than that of clean one. The surface work functionΦof clean Mo(110) surface is 4.65 eV while the surface energyσrepresents 0.019eV/atom with the distance of Mo-O dMo-O 1.20 A and the adsorption energy Eads-4.52eV. The calculated STM images of the Mo(110)p(2×2) surface on different bias voltages display unusual features as follows:The protrusions in STM image along [111] axis is the oxygen atoms while the valley in STM images correspond to the region between neighbor Mo atoms. The typical corrugation of STM tip has been calculated to show that:higher the STM tip is, sharper the corrugation will be.