RUI: Structure and Kinetics of Epitaxially-Grown Surface Alloys

这是一个本科院校研究奖（RUI），涉及本科生参与拟议的研究。利用动力学蒙特卡罗模拟，仔细分析实验研究和最先进的扩散势垒总能量计算相结合，首席研究员（PI）最近证明了通过异质外延沉积生长的表面合金的结构和发展可以完全解释。这样的系统，就像他们在这里所做的那样，涉及到不止一种原子种类，并将沉积的种类混合在几层基质中，很难准确地建模，因此取得的成功是值得注意的。将这项工作扩展到其他双金属系统也有很强的技术动机，因为其中一些系统与表面磁性和催化具有重要的相关性。用于在原子水平上处理这类系统的表面动力学的模型是基于控制局部原子排列的能量势垒的简单的、依赖于配位的、参数化的近似。包括直接移动和交换（协调的，两个原子）移动。根据实验信息，结合VASP协议和微推弹性带法计算的总能量，推导出必要的参数。通过动力学蒙特卡罗模拟（BLK算法）确定了基于这些移动和相关障碍的表面合金体系的动力学发展和演变，该算法也忠实地处理了沉积过程。这种方法已经成功地完全解释了Rh/Ag（001）体系中有趣的覆盖相关结构，关于这项工作的第一份报告最近发表在《表面科学》杂志上。在目前的研究中，这种处理和模型将与德国乌尔姆的R. Jurgen实验小组合作，扩展到其他制度和系统，该小组在双金属生长系统研究方面具有广泛的实验能力，并且是Rh/Ag（001）系统的大部分STM数据的来源。第一个扩展将是在较高温度下的Rh/Ag（001）体系，然后是不同基底对称的其他软金属基体表面合金。之后，我们将把该方法扩展到具有工业意义的较硬衬底情况。这样的系统应该类似地进化，但需要更高的温度来实现必要的移动性。在对模型进行这些扩展时，我们还将测试对活化能势垒的配位依赖性模型进行扩展的必要性。该小组还对金属平台上的附着原子和空缺进行了详细的分析。我们已经证明，低覆盖下的访问概率函数可以通过简单的松弛方法计算得到，并将继续将这种方法应用于由移动吸附原子或空位催化的各种表面过程。%%%这是本科院校研究奖（RUI），涉及本科生进行拟议的研究。利用动力学蒙特卡罗模拟、对实验研究的仔细分析和最先进的扩散屏障总能量计算相结合，首席研究员（PI）最近证明了通过沉积生长的表面合金的结构和发展可以完全解释。在目前的研究中，该处理和模型将与德国乌尔姆的R. Jurgen实验小组合作，扩展到其他制度和系统，该小组在双金属生长系统研究方面具有广泛的实验能力，并且是先前研究的许多数据的来源

This is an award for Research at Undergraduate Institutions (RUI) that involves undergraduate students in the proposed research. Using a combination of kinetic Monte Carlo simulation, careful analysis of experimental studies and state-of-the-art total energy calculations for diffusion barriers, the principal investigator (PI) has recently demonstrated that the structure and development of surface alloys grown via heteroepitaxial deposition can be fully accounted for. Such systems, involving as they do here more than one atomic species and mixing of the deposited species several layers into the substrate, are challenging to model accurately and so the success achieved is noteworthy. There are also strong technological motivations for extending this work to other bi-metallic systems since several of them have important relevance for surface magnetism and catalysis. The model developed for treating the surface kinetics of such systems at the atomic level is based on a simple, coordination dependent, parametrized approximation for the energy barriers governing local atomic arrangements. Both direct moves and exchange (coordinated, two-atom) moves are included. The necessary parameters are deduced from experimental information combined with total energy calculations using the VASP protocol and the nudged elastic band method of mapping out diffusion pathways and barriers. The kinetic development and evolution of the surface alloy system based on these moves and the associated barriers are determined via a kinetic Monte Carlo simulation (BLK algorithm), which also treats the deposition process faithfully. This approach has succeeded in fully accounting for the interesting coverage dependent structures seen for the Rh/Ag(001) system, and a first report on this work has recently appeared in the journal Surface Science.In the present research this treatment and model will be extended to other regimes and systems in collaboration with the experimental group of R. Jurgen in Ulm, Germany, which has extensive experimental capability for study of bi-metallic growth systems and was the source of much of the STM data for the Rh/Ag(001) system. The first extension will be to the Rh/Ag(001) system at higher temperature and then to other soft-metal substrate surface alloys of differing substrate symmetry. After that we will extend the method to harder substrate cases of industrial significance. Such systems should evolve similarly, but require higher temperatures to achieve the necessary mobility. In making these extensions to the model we will also test for the necessity of extensions to our model for the coordination dependence of the activation energy barriers.The group also has underway a detailed analysis of adatoms and vacancies on metallic terraces. We have shown that the visit probability function at low coverage can be obtained via simple relaxation method calculations and will continue work to apply this approach to various surface processes catalyzed by mobile adatoms or vacancies. %%%This is an award for Research at Undergraduate Institutions (RUI) that involves undergraduate students in the proposed research. Using a combination of kinetic Monte Carlo simulation, careful analysis of experimental studies and state-of-the-art total energy calculations for diffusion barriers, the principal investigator (PI) has recently demonstrated that the structure and development of surface alloys grown via deposition can be fully accounted for. In the present research this treatment and model will be extended to other regimes and systems in collaboration with the experimental group of R. Jurgen in Ulm, Germany, which has extensive experimental capability for study of bi-metallic growth systems and was the source of much of the data for the previous research.***