NSF-EC Cooperative Activity in Computational Materials Research: Bridging Atomistic to Continuum - Multiscale Investigation of Self-Assembling Magnetic Dots During Epitaxial Growth

这笔赠款是提交给NSF-EC计算材料科学材料世界网络计划的一项建议的结果。智力优势：在制造用于光电子应用的纳米结构方面，基于异质外延气相生长技术的半导体材料自组装方法的开发取得了巨大进展。具体地说，最近已经证明了所谓的定向自组装方法，该方法基于在多层生长中使用预图案化衬底或垂直对准，从而允许对复杂图案化阵列的尺寸分布和生长进行高度控制。虽然半导体系统已经取得了巨大的进步，但金属纳米结构的自组装还没有达到同样的程度。这些发展为未来超高密度磁记录媒体的发展带来了革命性的变化。该项目的主要目标是通过开发一套用于定向自组装的预测建模的工具来推动多尺度计算材料科学的最新发展。这些工具将被用来建立对支配纳米级自组装的重要微观和连续统级现象的基本理解。尽管将要开发的计算工具是通用的，并将在所研究的系统之外具有应用，但我们将以研究良好的磁性材料系统Fe/Mo和Fe/W为例，证明我们的方法在具体系统中应用的可行性。我们提出了一种综合的方法，通过联合来自欧洲共同体和美国的计算专家的力量，跨越原子尺度到连续尺度。这项工作将涉及表面能、表面应力和表面扩散系数的从头计算，以及统计力学、介观和连续计算在沉积和退火过程中纳米结构形态和组成的演变以及由此产生的自组装。通过参数传递，每种努力都将相互补充，因为较小规模的信息将被用于较大规模的计算，使我们能够在广泛的长度和时间尺度之间架起桥梁。使用这种方法，我们可以解决一些问题，如动力学和热力学效应之间的相互作用如何导致纳米结构的形成，以及是什么控制了点的空间和尺寸分布。对这些问题的回答不仅具有根本的意义，而且还使我们能够深入了解产生大规模自组织磁点阵列的过程。为了完成这些特定的任务，我们将现代计算工具(如相场模型、齐次化技术和渐近展开)与最先进的计算方法(如多重网格求解器、自适应和组合有限元以及并行动力学蒙特卡罗模拟)相结合。此外，与欧盟的一个实验伙伴合作，这些模型将接受对模型系统的尖锐和密集的测试和验证。广泛的影响：对整个社区的主要影响是两方面的：(1)为美国和欧洲的天才高中生开设关于晶体和外延生长的新的强化暑期课程；(2)每个PI都管理和认真地努力将不同背景的本科生和研究生包括在这个项目中。美国暑期班将与加州大学欧文分校的加州州立数学与科学暑期学校(COSMOS)合作提供，并将由美国所有的PI计划和授课。欧共体的同行也将在Ceasar提供类似的项目。一些个人投资经理有规划和执行这类项目的经验。为了确保招聘的多样性，一名个人投资主任(桑顿)将负责与高中教师和研究人员联系，以便直接在目标学校寻找代表性不足群体中的人才。此外，私人投资机构将出席和评判高中一级的地方科学博览会，并利用这些论坛作为外展活动的场所。

This grant is a result of a proposal submitted to the Materials World Net program on NSF-EC Computational Materials Science.Intellectual Merits: In the fabrication of nanostructures for optoelectronic applications, tremendous advances have been realized in the development of self- assembly methods for semiconductor materials based on heteroepitaxial vapor-phase growth techniques. In particular, so-called directed self-assembly methods have been demonstrated recently based on the use of pre-patterned substrates or vertical alignment in growth of multilayers, allowing a high degree of control over size distributions and growth of complex patterned arrays. While tremendous progress has been made for semiconductor systems, self- assembly of metallic nanostructures has not been advanced to the same degree. Such developments offer the potential to revolutionize the development of future media for ultra-high-density magnetic recording. The primary goal of the project is to advance the state of the art in multiscale computational materials science through development of a suite of tools for predictive modeling of directed self- assembly. These tools will be utilized to build fundamental understanding of important microscopic and continuum-level phenomena governing nanoscale self-assembly. Although the computational tools to be developed are general and will have application beyond the systems studied, the feasibility of our approach in applications to specific systems will be demonstrated for the well-studied magnetic material systems Fe/Mo and Fe/W. We propose an integrated approach by joining forces of computational experts from the European Community and the United States, spanning the atomistic to the continuum scales. The work will involve ab initio calculations of surface energies, surface stress, and surface diffusion coefficients, as well as statistical mechanics, mesoscopic and continuum calculations of the evolution of nanostructural morphology and composition of dots during both deposition and annealing, and the resulting self-assembly. By parameter passing, each effort will feed into the other, as the information at the smaller scale will be employed in the larger scale calculations, enabling us to bridge a wide range of length and time scales. Using this approach, we can address questions such as how the interplay between kinetic and thermodynamic effects lead to nanostructural formation and what controls the spatial and size distributions of the dots. Answers to these questions are not only of fundamental interest but also allow us to provide insights into processes that produce large-scale self-organized arrays of magnetic dots. To fulfill these specific tasks, we combine modern computational tools, like phase-field models, homogenization techniques and asymptotic expansions with state- of-the-art computational methods, such as multigrid solvers, adaptive and composite finite elements and parallel kinetic Monte Carlo simulations. Furthermore, in collaboration with an experimental partner in EU, the models will be subjected to sharp and intensive tests and validation on the model systems.Broader Impacts: The major impacts on the community at large are two-fold: (1) New intensive summer courses on crystal and epitaxial growth for gifted high school students in both US and Europe, and (2) managed and conscientious effort by every one of the PIs to include undergraduate and graduate students with diverse backgrounds in this project. The US summer school will be offered in cooperation with the California State Summer School for Mathematics and Science (COSMOS) at the University of California, Irvine and will be planned and taught by all of the PIs in the US. The EC counterpart will also offer a similar program at CEASAR. Several of the PIs have experience with planning and executing such programs. To ensure diversity in recruitment, one of the PIs (Thornton) will be in charge of contacting high school teachers and researchersat targeted schools directly to identify talents in under-represented groups. In addition, PIs will attend and judge local science fairs at the high school level and utilize such forums as a ground for outreach activities.