First-Principles Studies Of Surface Adsorption And Growth

Adsorption on surfaces is an important topic to study. First principles calculation is a powerful tool to reveal atomic details of surface processes. In this thesis, mainly using density function theory (DFT), the following systems are studied:growth mechanism of the two ZnO polar surfaces, effects of carbide formation to graphene growth, intermediate species in graphene growth on Cu(110) surface, VOPC adsorption on graphene.In chapter1, we first give a brief introduction of DFT, including the exchange-correlation functional and several models to describe van der Waals (VDW) interaction. Then, transition state theory and numcerical methods to locate transition states are introduced. At last, background of scanning tunneling microscopy (STM) is introduced, including the methods of STM simulation.In chapter2, growth mechanism of ZnO (000±1) polar surfaces are studied. Through comparison of the structures and formation energy of ZnxOy (x+y=1-6) clusters adsorbed on these two surfaces, some interesting results are obtained. On the (0001)-Zn surface, O atoms adsorb on hollow sites at the initial stage of growth. Then Zn atoms come in, and the stable structure becomes bulk-like for some specific clusters. On the (0001)-O surface, Zn cluster adsorption leads to stable cage structures formed by pulling substrate O out. In clusters with both Zn and O, O atoms avoid directly bonding with the surface, and no energetically favorable bulk-like structure is found. On the basis of the prediction of these surface adsorption behaviors, experimentally observed growth rate and surface roughness differences on these two polar surfaces can be understood.In chapter3, effects of steps to ZnO growth are studied. At first the stability of various steps are systematically studied. Via formation energy comparison, several stable steps on the two polar surfaces are identified. Then, diffusion of Zn/O adatom on armchair step and zigzag step are studied. Diffusion barriers on (0001)-Zn terminated steps are very small. Influence of step to growth is thus small. For (0001)-O terminated steps, diffusion over step is much more difficult than on terrace. The difference of ES barriers should have significant effects on the morphology of grown surfaces.In chapter4, effects of carbide formation in graphene growth are studied. Besides carbon solubility, carbide formation possibility is another important factor to differentiate various substrate materials in graphene growth. A recent experiment indicates that the formation of transition metal carbides (TMCs) can suppress carbon precipitation. In this study, Mo2C, a representative of TMCs, is used to study the effects of carbide formation in graphene growth from first principles. Carbon diffusion in Mo2C bulk turns out to be very difficult and it becomes much easier on the Mo2C (001) surface. Therefore, a simultaneous carbon segregation/precipitation suppression and graphene growth is possible. A direction depended diffusion behavior is observed on the (101) surface, which is thus less favorable for graphene growth compared to the (001) surface.In chapter5, intermediate states of graphene growth on Cu (110) surface is studied. Through the comparison of structures, formation energy and STM simulations of various C1Hx and C2HX clusters on Cu (110) surface, the possibility of clusters without H can be completely excluded. Compared with experiment, we find in initial stage the cluster array observed in STM images is most possibly to be C2H5.In chapter6, possibility of graphene to be used as a substrate for molecular device is discussed. STM experiment found a single dipole molecule, vanadyl phthalocyanine (VOPC), has bistable configurations in self-assembled closely packed monolayer on graphite by controlling the polarity of the pulse voltage applied to the tip. The structures and density of electronic states of the bistable states observed in STM and STS are studied by first-principle calculation. The results demonstrate that the structure with lower energy is O-up VOPC and another polarity reversal structure is O-down VOPC.