【作者】 聂锦兰；

【导师】 祖小涛；

【作者基本信息】 电子科技大学 ， 光学， 2008， 博士

【摘要】 表面吸附是表面科学的一个重要部分，其应用领域包括表面处理、催化及表面防腐蚀等。金刚石由于其在力学、电学、热学及光学等方面的优异性能，在光电子及半导体工业等领域具有广泛的应用前景。本论文通过对碱金属(AM)在金刚石(C)表面的吸附研究，探讨了表面处理对金刚石表面结构及电学性能的影响。铀(U)作为核武器和核能工业的重要材料在国防及开发利用核能中发挥着重要的作用。由于铀的化学性质活泼，表面腐蚀问题在其应用中显得尤为严峻。研究环境气体在铀表面的吸附现象对于进一步探讨其表面腐蚀机理及发展表面防腐蚀技术有着重要的意义。本论文采用基于密度泛函理论的第一性原理方法研究了碱金属在C(100)表面及氢气、氧气在α-U(001)表面的吸附，主要结论如下：1．系统研究了碱金属Na，K及Rb吸附于金刚石(100)表面的结构及电子性质。研究发现碱金属的稳定吸附构型对原子半径没有依赖性。在覆盖度为0.5ML时，它们的最稳定吸附位置都为谷桥位(T3)。在覆盖度为1ML时，两个碱金属吸附原子分别占据鞍位和谷桥位(HH+T3)。表面功函的计算表明，碱金属的吸附使得金刚石的表面功函大幅度降低，而当覆盖度增加到1ML的时候，表面功函又有较明显的回升，这与实验上所观察到的现象是一致的。我们把这归结为较高覆盖度下的偶极-偶极去极化作用。表面功函的降低使得体系的真空能级进入了金刚石能隙之内，形成了负电子亲和势。通过计算差分电荷密度，我们观察到碱金属吸附所诱导的“净电荷”大部分聚积在二聚化的碳原子与碱金属的键轴方向上，并且偏向碳原子。因此，我们认为碱金属与碳原子之间的化学键是极性共价键。由于碱金属的吸附，金刚石(100)表面反键表面态的能量明显降低，并在覆盖度为1ML时，与成键表面态发生交迭，从而金刚石表面呈现金属导电特性。这些研究结果表明碱金属吸附可以改变金刚石表面的导电类型，同时极性共价作用诱导了大幅度的表面功函下降现象，使得金刚石表面具有负电子亲和势。2．系统研究了氢气及氧气在α-U(001)表面的吸附，解离及扩散特性。研究发现氢气在α-U(001)表面表现出弱分子吸附特征，这与最近的实验研究对铀表面氢分子前驱体的存在的预言相吻合。结构分析表明，氢分子倾向于平行吸附在衬底铀原子的正上方，分子轴向沿100晶向。氧分子则倾向于以解离的形式吸附于α-U(001)表面，其解离伴随着两个氧原子以几乎相同的吸附高度分别占据两个相邻的凹陷位置。解离过程的研究发现氢分子在(α-U(001)表面的解离势垒很低，而氧分子的解离过程中则不存在势垒。在分子吸附态中，我们发现氢分子与铀原子之间只存在范德华作用力，而氧分子由于更接近铀表面，表现出与铀原子共价成键的化学吸附特征。对于解离吸附，氢原子失去了部分电荷并出现较弱的H1s与U6d电子态的杂化现象，表明H-U键为离子性与共价性混合的化学键。O-U键的特征则表现出对吸附结构的依赖性：吸附于凹陷位置的氧原子具有强烈的离子键特征，而吸附于顶位时，我们观察到了O2p-U5f-U6d电子态的杂化现象，表明此时的氧原子具有部分的共价性。分波态密度的分析表明，U5f与U6d电子都参与了与氢原子及氧原子的成键作用，但U5f与吸附物电子态的杂化现象比U6d要弱。这些研究结果表明当氢气和氧气吸附于铀表面时易于发生解离，从而导致铀表面的化学腐蚀。氢和氧原子与铀之间的化学作用以离子键为主，同时存在较弱的共价键特征。更多还原

【Abstract】 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.更多还原