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

这项拨款是提交给NSF-EC计算材料科学的材料世界网项目的提案的结果。智力优势：在光电应用的纳米结构制造中，基于异质外延气相生长技术的半导体材料自组装方法的发展取得了巨大进展。特别是，所谓的定向自组装方法最近已被证明是基于使用预图图化衬底或多层生长中的垂直排列，允许高度控制尺寸分布和复杂图图化阵列的生长。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。这样的发展为未来超高密度磁记录介质的发展提供了革命性的潜力。该项目的主要目标是通过开发一套用于定向自组装预测建模的工具，推动多尺度计算材料科学的发展。这些工具将用于建立重要的微观和连续级现象的基本理解控制纳米级自组装。虽然要开发的计算工具是通用的，并且将应用于所研究的系统之外，但我们的方法在应用于特定系统中的可行性将被证明用于已充分研究的磁性材料系统Fe/Mo和Fe/W。我们提出了一种综合的方法，通过加入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。这项工作将包括从头计算表面能、表面应力和表面扩散系数，以及在沉积和退火过程中纳米结构形态和点组成演变的统计力学、介观和连续体计算，以及由此产生的自组装。通过传递参数，每一项努力都将传递给另一项，因为较小尺度的信息将用于较大尺度的计算，使我们能够跨越大范围的长度和时间尺度。利用这种方法，我们可以解决诸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控制点的空间和尺寸分布的因素等问题。这些问题的答案不仅具有根本的意义，而且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，均匀化技术和渐近展开与最先进的计算方法，如多网格求解器，自适应和复合有限元以及并行动力学蒙特卡罗模拟相结合。此外，这些模型将与欧盟的一个实验伙伴合作，在模型系统上进行严格和密集的测试和验证。更广泛的影响：对整个社区的主要影响是双重的：(1)为美国和欧洲的天才高中生开设晶体和外延生长的新强化暑期课程；(2)每个pi都有管理和认真的努力，将具有不同背景的本科生和研究生包括在这个项目中。美国暑期学校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策划和教授。欧共体的对应机构也将在cesar提供类似的项目。一些pi有规划和执行这类项目的经验。为了确保招聘的多样性，其中一位pi （Thornton）将负责直接联系目标学校的高中教师和研究人员，以识别代表性不足群体中的人才。此外，pi还将参加当地高中一级的科学博览会并担任评委，并利用这些论坛作为拓展活动的场所。

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.