RUI: Structure and Kinetics of Epitaxially-Grown Surface Alloys

这是一个本科院校研究奖(RUI)，邀请本科生参与拟议的研究。通过结合动力学蒙特卡罗模拟、实验研究的仔细分析和最新的扩散势垒总能量计算，首席研究员(PI)最近证明，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。这类系统涉及一个以上的原子物种，并将沉积的物种混合到衬底中几层，对准确建模具有挑战性，因此所取得的成功是值得注意的。也有强烈的技术动机将这项工作扩展到其他双金属系统，因为它们中的几个与表面磁性和催化具有重要的相关性。为在原子水平上处理这类体系的表面动力学而开发的模型是基于控制局域原子排列的能垒的简单的、配位相关的、参数化的近似。包括直接移动和交换(协调的、双原子的)移动。根据实验信息，结合VASP协议的总能量计算和绘制扩散路径和势垒的微缩弹性带方法，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势垒的表面合金系统的动力学发展和演化，该模拟也忠实地处理了沉积过程。这个方法已经成功地完全解释了Rh/Ag(001)系统有趣的覆盖相关结构，最近关于这项工作的第一篇报告发表在《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的实验小组合作扩展到其他制度和系统，该小组具有广泛的实验能力来研究双金属生长系统，并且是Rh/Ag(001)系统的大部分STM数据的来源。第一个扩展将在较高温度下扩展到Rh/Ag(001)系统，然后扩展到不同衬底对称性的其他软金属衬底表面合金。之后，我们将把该方法推广到具有工业意义的硬质衬底情况。这类系统的进化应该类似，但需要更高的温度才能实现必要的机动性。在对模型进行这些扩展时，我们还将测试对我们的模型进行扩展的必要性，以确定活化能势垒的配位依赖性。该小组还正在对金属平台上的吸附原子和空位进行详细的分析。我们已经证明，低覆盖率下的访问概率函数可以通过简单的松弛方法计算得到，并将继续将该方法应用于移动原子或空位催化的各种表面过程。%这是一个本科院校研究奖(RUI)，邀请本科生参与拟议的研究。通过结合动力学蒙特卡罗模拟、实验研究的仔细分析和最新的扩散势垒总能量计算，首席研究员(PI)最近证明，通过沉积生长的表面合金的结构和发展可以完全解释。在本研究中，这一处理和模型将与德国乌尔姆的R.Jurgen的实验小组合作，扩展到其他制度和系统，该小组在研究双金属生长系统方面具有广泛的实验能力，是以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.***