Collaborative Research: Remote epitaxy on van der Waals materials: unveiling adatom interaction, growing single-crystal membranes, and producing unconventional heterostructures

第1部分：非技术描述科学技术随着新材料平台的发现而发展，如碳纳米管或合成聚合物。研究小组并没有追求一种全新的材料，而是瞄准了一种新的扭曲，使现有的材料变得更平、更薄、更轻。早在2017年，首席研究员就发明了远程外延技术，该技术允许在石墨烯涂层模板上生长纳米级材料，之后可以剥离。这些轻薄的独立材料可以成为轻质、柔性设备的基石，具有前所未有的性能。到目前为止，对远程外延的研究大多局限于对半导体中常用的氮化镓和砷化镓等材料的经验观察。为了利用远程外延作为各种材料系统之间生长和异质集成的理想平台，研究小组旨在从基础层面研究远程外延的机制。通过探索各种二维（2D）和三维（3D）材料的机制，研究小组希望创造更广泛的可用于远程外延的材料系统。此外，能够将不同的材料系统结合在一起可以发现新的功能，从而使物理科学受益，通过改进当前的设备制造技术及其性能，为工程科学和工业带来进步。研究小组计划重点回答三个主要问题，以进一步了解远程外延：(1)附着原子与底层2D/3D衬底相互作用的性质，(2)2D材料和界面对远程外延的影响，以及(3)涉及附着原子/原子核迁移和2D表面缺陷形成的动态过程。为了回答这些问题，研究小组将首先揭示远程外延是否真的在“远程”意义上发生，这是先于任何其他问题的最基本的难题。这个问题很难回答，因为人们很容易只观察到外延的结果，而不是附着原子的成核。这个问题是通过有意地对2D层进行图案化和使用具有不同生长特性的各种材料作为epilayer来解决的。其次，通过研究二维中间层的类型、结晶度和厚度如何改变附着原子和底物的远程相互作用，探讨了二维层的影响。为了显示二维层的内在作用，尝试在具有不同离子性的各种衬底上直接生长二维层。第三，通过了解成核机制、缺陷分析和剥离剥离，研究了二维层上附着原子的热力学和动力学、它们的合并成核以及核-核相互作用。基于这些结果，研究小组计划在工程成核条件下，通过远程外延在直接生长的2D层上生长高质量的超薄膜。由于远程外延允许混合维度异质结构，因此进一步研究3D/各种2D/3D夹层结构，以研究电子态与磁性能之间的新物理耦合。对于科学界来说，这些结果有望利用远程外延来研究传统方法无法实现的多功能和耦合材料平台。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL DESCRIPTIONScience and technology have developed alongside with discovery of new materials platform, such as carbon nanotubes, or synthetic polymers. Instead of chasing after a completely new materials, the research team targets a novel twist, making currently existing materials to become flatter, thinner, and lighter. Back in 2017, the principal investigator invented remote epitaxy, which allows growing nanoscale materials on graphene-coated templates to be exfoliated afterwards. These thin freestanding materials can become building blocks for lightweight, flexible devices having unprecedented performance. Until now, studies on remote epitaxy have been mostly limited to empirical observations on materials such as gallium nitride and gallium arsenide typically used in semiconductors. To utilize remote epitaxy as an ideal platform for growth and hetero-integration between various materials systems, the research team aims to study the mechanism of remote epitaxy at a fundamental level. By exploring the mechanism on various two-dimensional (2D) and three-dimensional (3D) materials, the research team expects to create a wider range of materials systems available for remote epitaxy. Also, being able to combine different materials systems together can benefit the physical sciences by discovering new functionalities, bringing advancements in engineering sciences and industry by improving current device fabrication techniques and their performances. PART 2: TECHNICAL DESCRIPTIONThe research team plans to focus on answering three major questions to understand remote epitaxy further: (1) the nature of the adatoms interaction with the underlying 2D/3D substrates, (2) the impacts of 2D materials and interfaces on remote epitaxy, and (3) the dynamic processes involved in adatom/nuclei migration and defect formation on 2D surfaces. To answer these questions, the research team will first reveal whether the remote epitaxy truly occurs in a ‘remote’ sense, which is the most fundamental conundrum that precedes any other questions. This has been difficult to answer because one can easily observe only the results of epitaxy, not the nucleation of adatoms. This issue is tackled by intentionally patterning the 2D layer and employing various materials with different growth properties as the epilayers. Second, impact of 2D layers is explored by studying how the type, crystallinity, and thickness of 2D interlayers alter the remote interaction of adatoms and substrates. In order to show the intrinsic role of 2D layers, direct growth of 2D layers is attempted on various substrates having different ionicity. Third, thermodynamics and kinetics of adatoms, their mergence to form nuclei, and nucleus-nucleus interaction on 2D layers are studied by understanding the nucleation mechanism, performing defect analysis, and exfoliating epilayers. Based on these results, the research team plans to demonstrate high-quality ultrathin films grown by remote epitaxy on directly-grown 2D layers with engineered nucleation conditions. As remote epitaxy allows mixed dimensional heterostructures, further study is done on 3D/various 2D/3D sandwiched structures to investigate new physical couplings between electronic states and magnetic properties. To the scientific community, these results are expected to utilize remote epitaxy for studying multifunctional and coupled material platforms that were not achievable by conventional methods.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.