【作者】 吴萍；

【导师】 曹更玉； 黄敏；

【作者基本信息】 中国科学院研究生院（武汉物理与数学研究所） ， 原子与分子物理， 2014， 博士

【摘要】 近年来,因为在纳米电子装置、原子尺度的磁存储装置、化学催化以及传感器等方面的潜在应用,贵金属纳米结构得到了广泛关注。尤其是金属纳米团簇和金属薄膜奇异的物理和化学性质成为科学家研究的热点。在催化、电子工业领域,贵金属团簇的应用大多离不开衬底的负载。而所选衬底的表面结构、热稳定性以及化学性质对团簇的制备、催化活性和电磁学性质可能存在影响。本文基于第一性原理利用自旋极化的密度泛函理论,研究了钯金属团簇或薄膜在不同性质的衬底上(包括NiAl(110),ZnO{0001}以及单层MoS2)的结构、稳定性、生长方式以及相关电磁学性质。首先,研究了吸附在NiA(110)表面的小尺寸的钯团簇的原子结构和电学特性以及一维钯原子链的共振态。结算表明单个钯原子吸附在Ni-桥位和A1-桥位时能量相差很小,与STM实验结果一致。在较低覆盖度下团簇采取一维链式生长；随覆盖度增加,团簇倾向于二维结构的生长。化学环境不同对钯原子的电子态分布存在很大影响,例如,NiAl衬底电子态的调制和与周围钯原子的相互作用。由于钯团簇与衬底的电荷转移导致Pdn(n=1-5)/NiAl(110)体系呈现磁性特征。另外,吸附在NiAl(110)表面两种无限长的钯原子链存在两种构型,其共振态的产生原因是钯原子的5sp轨道与衬底电子态杂化和相邻钯的5sp轨道的交叠。其次,研究了钯在不同覆盖度下吸附在ZnO{0001}极性表面结构及电磁性质,分析了Pd/ZnO的界面性质。钯在ZnO{0001}表面的吸附随着覆盖度的增加,表面形成能越大,体系能量上越稳定。在1ML覆盖度时钯原子倾向于吸附在ZnO(0001)表面的fcc位置,而钯吸附在ZnO(000-1)表面的氧原子的顶位能量最低。Pd/ZnO(0001)体系在费米能级出现带隙态,钯的电子态具有很强的局域性；Pd/ZnO(000-1)体系在费米能级附近100%的自旋极化,认为是钯与衬底的氧原子的强烈杂化引起的。钯单层与ZnO衬底之间存在电荷转移,导致体系功函数变化。此外,钯单层与ZnO{0001}界面形成肖特基接触,在光电器件方面存在潜在应用。最后,研究了钯等贵金属团簇在MoS2单层上的吸附性质。小尺寸的Pdn(n=1-5)团簇在MoS2单层上采取三维生长模式,吸附物与衬底之间存在电荷转移,在吸附体系表面形成偶极矩。讨论了钯等贵金属的吸附效应对单层MoS2的电磁学性质的调制,为基于MoS2的微纳米电子器件以及纳米催化的应用研究提供理论指导。比较研究了钯和金单层沉积在MoS2(0001)表面的吸附性质以及电学性质,研究发现金单层与衬底之间电荷转移区域较大,金作为MoS2纳米电子装置的候选接触电极可能具有比较好的导电性。对本论文研究工作做了总结并对今后的研究工作进行展望。更多还原

【Abstract】 Metal nanostructures have attracted considerable attention in recent years due to the potential applications in aspects of nanoscale electronics and atomic-scale storage devices as well as chemical catalysts and sensors. Inparticular, understanding the electronic, physical and chemical properties of metal clusters and monolayer became an issue attracting wide attention of a large number of researchers. Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of Pd clusters and Pd monolayer adsorbed on the substrates with different properties (including NiAl(110), ZnO{0001}, MoS2monolayer) were carried out with in the framework of spin density functional theory (DFT).Fristly, theoretical calculations on the geometry, stability, electronic and magnetic properties of small palladium clusters Pdn(n=1-5) adsorbed on the NiAl(110) alloy surface has been carried out. In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom. Among the possible structures considered for Pdn (n=1-5) clusters adsorbed on NiAl(110) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd1to Pd3) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd4and Pd5. Furthermore, metal-substrate bonding prevails over metal-metal bonding for Pd cluster adsorbed on NiAl(110) surface. The density of states for Pd atoms of Pd/NiAl(110) system are strongly affected by their chemical environment, such as the modification of NiAl and the effect of surrounding Pd. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(110) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices. In addition, the infinite Pd chains supported by NiAl(110) exist two configurations, the interactions among the increasing number of adatom resonance states enentually give rise to a band of resonance state, wihch is caused by substrate-mediated coupling and the direct5sp orbital overlap of adjacent Pd.Secondly, we have systemically studied the structural and electronic properties of Pd adsorbed on ZnO{0001} polar surface with different coverage, and analyzed the effect of interface of Pd/ZnO system. When the coverage increased to1ML, Pd atoms prefer to adsorb on fcc sites of ZnO{0001} surface; while Pd located at the top of O. After Pd adsorbed on ZnO(0001) surface, new band states emerged near the Fermi energy, and the electron density of Pd remain localized. Pd/ZnO (000-1) system is magnetic, which is caused by the hybridization between Pd and the O atom of substrate. Upon adsorption of Pd, the work function of system has change, which is caused by the change transfer between Pd and ZnO. The Schottky barrier height is0.65eV and1.16eV for Pd/ZnO(0001) and Pd/ZnO(000-1), respectively.Thridly, we have investigated firstly the structural, electronic and magnetic properties of Pdn (n=1-5) clusters adsorbed on MoS2monolayer. The initial growth of Pd clusters adopt Volmer-Weber mode, the Pd-Pd bonding is stronger than adsorbate-substrate bonds with the increasing of Pd coverage.The charge transfers from Pd clusters to substrate and the catalytic activity of Pd clusters with positive charge may be affected. Then the adsorption effects of the noble metal atoms (Ni, Pd, Pt, Cu, Ag, and Au) on electronic and magnetic properties of MoS2monolayer are investigated. To be comparaed, we present total energy calculations of the adsorption of Pd and Au monolayer on MoS2(0001) surface. We find that Au is suitable to be choosed as contact electrode for MoS2-based devices becaused of the stronger charge transfer between the Au adlayer and MoS2substrate.At last, we summarize the whole work and make an outlook to the effect of surface defects and reconstruction on the growth of metal clusters.更多还原