First-principles Study Of Surface Adsorption On Diamond And Uranium

Adsorption is an important part of surface science, application areas of whichinvolve surface treatment technique and surface corrosion prevention et al. Owning toit's outstanding performance in mechanics, electricity, thermal and optics, diamondsurface has wide application prospect in photoelectronic and semiconductor industry.The current work has studied the adsorption of Alkali metal (AM) on diamond surfaceand focused on the adsorption induced influences on the diamond (C) surface structuraland electronic properties. As a kind of important material used in nuclear weapon andnuclear energy industry, Uranium (U) plays an important role in the national defenseand the developing of nuclear energy. We have studied the adsorption of hydrogen andoxygen on Uranium surface which has important significance in further investigatingthe mechanism of surface corrosion and improving the corrosion prevention technology.First principle calculations based on density functional theory have been performed tostudy the adsorption of AM on C(100) surface and hydrogen/oxygen on U(001) surface.The main results are as follows:1. A systematic study has been carried out to investigate the structural andelectronic properties of the adsorption systems of Na, K and Rb on C(100) surface. It isfound that the stable adsorption site for AM is independent to its atomic radius. At thecoverage of 0.5 ML, all considered AMs favorite the valley-bridge (T3) site. As thecoverage increases to 1 ML, one of the adsorbates still prefers T3 site with another oneoccupying the pedestal (HH) site. The calculation of surface work function has shownthat the adsorption of AM lead to the dramatic decrease of surface work function. As thecoverage increases to 1 ML, the surface work function shows increment relative to thatatΘ=0.5 ML. This is in consistent with what has been experimentally observed and maybe ascribed to the dipole-dipole depolarization effect at higher coverages. The decreaseof surface work function has pulled down the vacuum level below the conduction bandminimum which indicates a negative electron affinity. The charge difference densityanalysis has shown that the AM induced "net charge" mainly locates along the bondaxis between the dimerized carbon atom and AM. The deviation of the "net charge"from AM atoms indicates polarized covalent AM-C bond. The analysis of the projected density of states has given supports to the donation of AMs states and C2p states to theAM-C bond. We found that the AM adsorption has lowered the energy of anti-bondingsurface states which show overlapping with bonding surface states for adsorption atΘ=1ML, thus resulting in a metallic diamond surface. In summary, AM adsorption canchange the conductivity type of C(100) surface and induce negative electron affinity dueto the polarized covalent bonding nature of AM-C which has lead to dramatic decreaseof surface work function.2. We have systematically studied the adsorption, dissociation and diffusion ofhydrogen and oxygen onα-U(001) surface. Weak molecular adsorption has been foundfor hydrogen, agreeing well with a recent experimental work which suggests theexistence of the hydrogen molecular precursor on uranium surface. The structuralanalysis showed that hydrogen prefers to adsorb above a substrate atom with H-H axisalong 100 direction. Oxygen was found to favor dissociated adsorption onα-U(001)surface. The dissociation of oxygen is followed by the occupancy of the twoneighboring hollow sites by the two oxygen atoms. The dissociation investigation hasrevealed a low dissociation barrier for hydrogen and no barrier for oxygen. For themolecular adsorption structures, we found van der walls type interaction betweenhydrogen and uranium atom. For adsorbed oxygen molecular which has lower heightfrom the Uranium surface, we found covalent bonding of O-U. For the dissociatedadsorption, hydrogen atom loses some charge with weak hybridization found betweenHis and U6d states which indicates mixing of ionic and covalent characters for H-Ubond. The bonding nature of O-U bond shows dependence on the adsorption site. Strongionic character has been observed for oxygen adsorbed on hollow site while some covalent characterarises for the top site adsorption with the strong hybridization of O2p-U5f-U6d states. Theprojected density of states analysis has shown that both U5f and U6d states are involvedin the bonding with hydrogen and oxygen. The hybridization of U5f with adsorbatesstates is weaker than that of U6d states. These results suggest that hydrogen and oxygenmolecules are easily dissociated when approach Uranium surface which is consequentlyfollowed by chemical corrosion. The chemical interaction between hydrogen/oxygenand Uranium are mainly ionic bonding with mixing of weak covalent bonding.