Tailoring the Properties of Heterostructures of Monolayers: Epitaxial Growth and Doping

非技术描述：半导体材料的异质结构，提供了增强的电学和光学特性，远远超过了每个单独的组成材料，已经成为现代电信，节能显示和能量收集技术的关键实现元素。该项目探索了一种新的方法来合成由二维材料片制成的异质结构，其中片内的原子形成强键，但层之间的相互作用非常弱。原子分辨率成像和计算是为了理解这种特征的各向异性键如何促进类似乐高积木的单原子层的生长和组装，从而允许设计和合成具有定制特性和功能的新材料，超越目前存在于自然界的材料的限制。该项目为研究生和本科生提供材料合成和原子尺度表征以及电子结构计算领域的跨学科培训。电子结构代码的开放源代码分发继续为科学界提供我们团队代码开发工作的好处。一项正在进行的教师研究经验外展计划将二维材料的前沿研究带给高中生，以激发他们对科学和工程的兴趣。技术描述：本项目旨在从原子尺度上理解1)二维过渡金属二硫族化合物在范德瓦尔斯外延生长过程中固有的晶界形成，以及2)其异质结构的掺杂和带隙工程。利用集成分子束外延、扫描隧道显微镜（STM）和原子力显微镜（AFM）的能力，通过原位隧道光谱和力偏压光谱分别确定了材料的固有特性，如横向结的带偏移和功函数。通过非原位拉曼光谱和光致发光进一步获得层厚度、掺杂和带隙。通过这些空间平均光谱测量获得的信息与原子尺度的结构信息相关，如垂直结的排列、界面混合、局部应变和由原位原子分辨率STM/AFM成像以及非原位透射电子显微镜获得的无序。密度功能理论计算与感兴趣的实验系统紧密耦合，解决与结构特性（例如，修饰vdW生长的能量学以及缺陷和杂质的影响）和电子特性（界面状态，带偏移，晶界上的电荷效应和功函数）相关的问题。这种集成的方法可以在原子尺度上控制单层异质结构的生长、带隙工程和表征，所有必要的步骤都可以定制其电子和光学特性。

Nontechnical Description: Heterostructures of semiconductor materials, which provide enhanced electrical and optical characteristics well beyond that of each individual constituent material, have been the key enabling element in modern telecommunication, energy-efficient displays, and energy harvesting technologies. This project explores a new approach to synthesize heterostructures made of sheets of two-dimensional materials, in which atoms within a sheet form strong bonds but interactions between the layers are very weak. Atomic-resolution imaging and calculations are carried out to understand how this characteristic anisotropic bonding facilitates the growth and assembling of the single atomic layers similar to LEGO blocks, thus allowing the design and synthesis of new materials with tailored properties and functionalities beyond the limits of materials that currently exist in nature. This project provides graduate and undergraduate students interdisciplinary training in areas of materials synthesis and characterization at the atomic scale, as well as electronic structure calculations. The open source distribution of electronic structure codes continues to provide the scientific community the benefits of our team's code development efforts. An ongoing Research Experience for Teachers outreach program brings cutting-edge research on two-dimensional materials to high-school students to inspire their interest in science and engineering.Technical Description: This project aims to gain an atomic scale understanding of 1) the inherent grain boundary formation during the van der Waals (vdW) epitaxial growth of two-dimensional transition-metal dichalcogenides, and 2) the doping and bandgap engineering of their heterostructures. Leveraging the capability of integrating molecular beam epitaxy, scanning tunneling microscopy (STM) and atomic force microscopy (AFM), the intrinsic materials properties such as band offsets across lateral junctions and work functions are determined by in-situ tunneling spectroscopy and force-bias spectroscopy, respectively. Layer thickness, doping, and band gaps are further accessed by ex-situ Raman spectroscopy and photoluminescence. The information obtained by these spatially averaged spectroscopic measurements is correlated with atomic scale structural information such as the alignment of vertical junctions, interface intermixing, local strain, and disorder obtained by in-situ atomic resolution STM/AFM imaging, as well as ex-situ transmission electron microscopy. Density functional theory calculations are tightly coupled to the experimental systems of interest, addressing issues related to structural properties (e.g., energetics of the modified vdW growth and the impact of defects and impurities) and electronic properties (interface states, band offsets, charging effects at grain boundaries, and work functions). This integrated approach enables the controlled growth, bandgap engineering, and characterization of heterostructures of monolayers at the atomic scale, all necessary steps towards tailoring their electronic and optical properties.