Kinetic Monte Carlo Simulation of Nanoalloy Crystal Growth

本研究项目的目标是开发逼真的，准确的，高效的计算机模拟技术建模合金材料。该项目的重点是模拟纳米团簇的生长，特别是合金，尺寸从几百到几百万个原子不等。人们对纳米团簇的兴趣很广泛，这在很大程度上是由于其性质的可调性质。团簇的形态、结构和稳定性沿着热、电子和化学性质可以随着尺寸、组成和原子有序而显著变化。通常，有一些“神奇的尺寸”表现出特别不寻常的结构和性质。在某些情况下，添加少至单个掺杂剂原子可以产生显著不同的材料。应用范围广泛，包括光学、催化、纳米电子学和生物医学。虽然有大量关于纳米团簇的工作，但该项目正在开发的技术是新的，并且独特地适合于模拟原子尺度结构的生长和演化。这些技术的成功开发将填补现有模拟方法的一个重大空白。本项目的重点是纳米团簇生长的动力学蒙特卡罗（KMC）模拟。基本上所有的工作纳米团簇模拟到目前为止是基于全局优化（寻找基态）或平衡蒙特卡罗模拟。相比之下，KMC模拟是为了遵循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演化。KMC方法分为两大类：晶格和非晶格，这两种方法都将在这项工作中使用。基于格子的模型更简单，速度也快了几个数量级，使它们能够在更长的模拟时间内与更大的系统一起工作。这种类型的模拟将用于研究含有数百万个原子的树枝状团簇的生长。该项目的这一部分还包含一个重要的建模组件，其主要目的是通过制定各种生长模型来确定表面能和平衡晶体形状的方式。然后，这些结果将被用来确定参数值和模型配置，可能会产生有趣的枝晶形态在随后的生长模拟。该项目的非晶格部分将专注于包含数百或数千个原子的较小原子团的生长。这里的主要重点将是开发更快的方法。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。这可以被视为异质外延薄膜的纳米颗粒类似物。

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.