Kinetics of Thin Film Growth on van der Waals Surfaces

非技术描述：亚微米厚度的涂层通常用于各种各样的应用，包括但不限于：汽车，生物医学，催化，国防，能源，电子和光电子工业。这些涂层中所用材料的性能和使用寿命通常由材料的结晶度决定。传统上，改善和/或控制涂层的结晶度需要在高温下进行处理，使用高能光束，和/或长时间延长退火。原子薄片材料，如石墨和六方氮化硼（hBN，也称为白石墨），被称为范德华（vdW）层，可以帮助合成结晶度更好的材料。这个项目的目标是了解导致提高vdW层结晶度的机制。通过该项目培养了一名博士生，并将研究成果教授给工程专业的本科生，并通过会议报告和技术出版物进行传播。通过在技术会议上组织重点研讨会，首席研究员打算提高对有家庭的研究生面临的挑战的认识（并找到可行的解决方案）。技术描述：这个项目的目的是在原子尺度上理解控制vdW表面固体薄膜生长的因素。研究小组获得的初步结果表明，溅射沉积在hBN层上的薄膜比同外延生长的薄膜具有更好的结晶性。为了理解这一有趣的结果，我们假设vdW表面上的吸附原子迁移率高于传统3D固体，并且衬底、vdW层和沉积物之间存在明确的晶体取向关系。该项目使用原位变温扫描隧道显微镜实验来验证这一假设，并测试这些观察结果的普遍性。通过对薄膜生长的时间分辨测量，定量确定了vdw层对表面扩散率和形态演化动力学的作用。该项目为合成各种应用的高质量晶体薄膜提供了潜在的技术重要场所的新见解。这种方法可能是提高高难熔性化合物和热不稳定材料结晶度的最有效方法，对于这些材料，温度升高对材料的微观结构和组成几乎没有影响或有害影响。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。

Non-technical description: Coatings with sub-micrometer thicknesses are commonly used in a wide variety of applications including but not limited to: automotive, biomedical, catalysis, defense, energy, electronic, and optoelectronic industries. Properties and life-time performance of materials used in these coatings are often dictated by the crystallinity of the materials. Traditionally, improving and/or controlling the crystallinity in coatings require processing at high-temperatures, use of energetic beams, and/or extended annealing for long durations. Atomically-thin sheets of materials such as graphite and hexagonal boron nitride (hBN, also known as white graphite), referred to as van der Waals (vdW) layers, can aid the synthesis of materials with better crystallinity. The goal of this project is to understand the mechanisms leading to enhancing crystallinity on vdW layers. One doctoral student is trained through this project and the research results are taught to undergraduate students in engineering and disseminated through conference presentations and technical publications. By organizing focused workshops at technical conferences, the principal investigator intends to promote awareness of (and find viable solutions to) the challenges faced by graduate students with families.Technical description:The aim of this project is to develop an atomic-scale understanding of the factors controlling the growth of thin solid films on vdW surfaces. Preliminary results obtained by the team suggest that thin films sputter-deposited on hBN layers exhibit better crystallinity than homoepitaxially grown thin films. To understand this intriguing result, it is hypothesized that adatom mobility on vdW surfaces is higher than that on conventional 3D solids and that there exist well-defined crystallographic orientation relations between the substrate, vdW layer, and the deposit. The project uses in situ variable-temperature scanning tunneling microscopy experiments to validate this hypothesis and to test the generality of these observations. Through time-resolved measurements of the thin film growth, the role of the vdW-layer on surface diffusivities and morphological evolution kinetics are quantitatively determined. This project is providing new insights into a potentially technologically important venue for synthesizing high-quality crystalline thin films for a variety of applications. This approach may be the most effective way to improve crystallinity of highly refractory compounds and thermally unstable materials for which increasing temperature has little or deleterious effect on the microstructure and composition of the material.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.