Kinetic Monte Carlo Simulation of Nanoalloy Crystal Growth

本研究项目的目标是发展真实、准确、高效的计算机模拟技术来模拟合金材料。该项目的重点是模拟纳米团簇的生长，特别是合金，其大小从几百到几百万个原子不等。人们对纳米团簇的兴趣是广泛的，这在很大程度上是由于它们的特性是可调的。团簇的形态、结构和稳定性以及热、电子和化学性质会随着大小、组成和原子顺序而发生巨大变化。通常，有一些“神奇的尺寸”表现出特别不寻常的结构和特性。在某些情况下，只要添加一个掺杂原子，就能制造出截然不同的材料。应用广泛，包括光学、催化、纳米电子学和生物医学。虽然在纳米团簇上有大量的工作，但在这个项目中开发的技术是新的，并且非常适合于模拟原子尺度结构的生长和进化。这些技术的成功开发将填补现有仿真方法的重大空白。这个项目的重点是动力学蒙特卡罗（KMC）模拟纳米团簇的生长。迄今为止，纳米团簇模拟的所有工作基本上都是基于全局优化（寻找基态）或平衡蒙特卡罗模拟。相比之下，KMC模拟是为跟踪非平衡过程的动力学而设计的，因此特别适合于模拟原子尺度结构的生长和进化。KMC方法分为两大类：点阵和非点阵，这两种方法都将在本文中使用。基于网格的模型要简单得多，速度也要快几个数量级，这使得它们可以在更大的系统上工作，模拟时间更长。这种类型的模拟将用于研究包含数百万原子的枝晶簇的生长。该项目的这一部分还包含一个重要的建模组件，其主要目的是推导出由各种生长模型的公式决定表面能和平衡晶体形状的一般结果。这些结果将用于确定参数值和模型配置，这些参数值和模型配置可能会在随后的生长模拟中产生有趣的枝晶形态。该项目的非晶格部分将集中在包含数百或数千个原子的较小簇的生长上。这里的主要重点是开发更快的方法。这些新方法的试验场将是模拟具有核壳结构的合金纳米颗粒的生长。这可以看作是异质外延薄膜的纳米粒子模拟物。

The goal of this research project is to develop realistic, accurate, and efficient computer-simulation techniques for modeling alloy materials. The project focuses on simulation of the growth of nanoclusters, especially alloys, ranging in size from a few hundred to a few million atoms. Interest in nanoclusters is broad, arising in large part due to the tunable nature of their properties. The morphology, structure, and stability, along with the thermal, electronic, and chemical properties of clusters can vary dramatically with size, composition, and atomic ordering. Often, there are "magic sizes" that exhibit particularly unusual structures and properties. In some cases, the addition of as little as a single dopant atom can create a dramatically different material. Applications are diverse, including optics, catalysis, nanoelectronics, and biomedicine. While there is a large body of work on nanoclusters, the techniques under development in this project are new and uniquely suited to simulating the growth and evolution of atomistic-scale structures. The successful development of these techniques will fill a significant gap in existing simulation methods. This project focuses on kinetic Monte Carlo (KMC) simulation of the growth of nanoclusters. Essentially all of the work on nanocluster simulation to date is based on either global optimization (the search for ground states) or equilibrium Monte Carlo simulations. In contrast, KMC simulation is designed for following the dynamics of nonequilibrium processes and is therefore uniquely suited to simulating the growth and evolution of atomistic-scale structures. KMC methods fall into two broad categories: lattice and off-lattice, both of which will be used in this work. Lattice-based models are much simpler and orders of magnitude faster, allowing them to work with much larger systems for longer simulation times. This type of simulation will be used to study the growth of dendritic clusters containing several million atoms. This portion of the project also contains a significant modeling component, the main thrust of which is to derive general results for the way surface energy and equilibrium crystal shapes are determined by the formulation of various growth models. These results will then be used to identify parameter values and model configurations that are likely to yield interesting dendrite morphologies during the subsequent simulation of the growth. The off-lattice portion of the project will be focused on the growth of smaller clusters, containing hundreds or perhaps thousands of atoms. The main focus here will be on developing faster methods. The testing ground for these new methods will be simulating the growth of alloy nanoparticles with core-shell structures. This can be viewed as the nanoparticle analog of heteroepitaxial thin films.