Controlling the Band Structure of 2D Semiconductors by their Dielectric Environment

非技术描述：原子级薄二维（2D）材料领域始于十年前石墨烯的发现。这种只有一个或几个原子厚的材料的范围现在已经扩展到包括2D半导体，金属和绝缘体。这些材料的电学、光学和机械特性的独特组合开辟了许多新的科学前沿，并导致了各种纳米器件的发展-从晶体管到发光二极管和传感器。对该领域的进一步发展至关重要的是操纵电子在这些材料中的行为方式的能力，特别是通过控制它们的可用能级。材料的电子性质通常通过改变材料本身来改变，例如通过改变材料的化学成分。该项目探索了一种控制2D材料中电子行为的新方法。不是改变材料本身，而是通过改变周围介质来修改2D材料中的能级。对于只有一个或几个原子厚的2D材料，外部环境实际上改变了层内电子之间的作用力，从而改变了电子的可用能级。该研究考察了均匀和纳米结构的环境，作为开发一类新的纳米级电子和光学器件的构建模块的一种手段。参与该项目的研究生和本科生实习生学习准备，表征和分析2D材料和设备。重要的专业发展活动包括经常在小组内和斯坦福大学的部门论坛上介绍自己的研究和其他技术主题。技术说明：原子级薄的二维材料领域的发展的一个关键因素是在纳米尺度上控制它们的带隙。这对于探索材料的基本物理性质和实现许多技术应用至关重要，从调节光发射的光子能量到通过横向结控制输运特性。然而，传统的方法，如合金化，很难在纳米尺度上实现。该项目利用非局部介电屏蔽在原子级薄材料的限制下的显著影响，在2D半导体中外部调谐多体库仑相互作用。这允许简单地通过定制其介电环境来调谐2D材料的带结构和带隙。该研究涉及三个不同的组成部分：（1）具有不同带隙和环境屏蔽的2D异质结构的系统组装;（2）突然不连续处的带隙的真实空间轮廓和0 D和1D结构的局部化研究;以及（3）确定介电工程结中的导带和价带偏移。这些研究的表征工具包括光学光谱、电子能量损失光谱、阴极射线发光和扫描隧道光谱。

Non-technical description: The field of atomically thin two-dimensional (2D) materials started ten years ago with the discovery of graphene. The range of such materials that are only one or a few atoms thick has now expanded to encompass 2D semiconductors, metals, and insulators. The unique combination of electrical, optical, and mechanical properties of these materials has opened up many new scientific frontiers and led to the development of a variety of nanoscale devices - from transistors to light emitting diodes and sensors. Critical to the further development of the field is the ability to manipulate the way electrons behave in these materials, particularly by controlling their available energy levels. The electronic properties of a material are normally altered by modifying the material itself, for example, by changing the material's chemical composition. This project explores a new approach to controlling the behavior of electrons in 2D materials. Rather than changing the material itself, the energy levels in the 2D material are modified by changing the surrounding media. For 2D materials that are only one or a few atoms thick, the external environment actually modifies the forces acting between electrons within the layer and, hence, changes the available energy levels for electrons. The research examines both uniform and nanostructured environments as a means of developing building blocks for a new class of nanoscale electronic and optical devices. Graduate students and undergraduate interns involved in the project learn to prepare, characterize, and analyze 2D materials and devices. Important professional development activities include frequent presentations of their own research and other technical topics within the group and in departmental forums at Stanford University. Technical description: A critical factor for the development of the field of atomically thin two-dimensional 2D materials is the control of their band gap on the nanoscale. This is essential to explore the fundamental physical properties of the materials and to realize many technological applications, ranging from tuning the photon energy of light emission to the control of the transport characteristics by lateral junctions. Traditional approaches, like alloying, are, however, difficult to implement on the nanoscale. This project takes advantage of the pronounced influence of non-local dielectric screening in the limit of atomically thin materials to tune externally many-body Coulomb interactions in 2D semiconductors. This allows tuning of the band structure and band gap of the 2D material simply by tailoring its dielectric environment. The research involves three distinct components: (1) Systematic assembly of 2D heterostructures with varying band gaps and environmental screening; (2) real-space profiling of the band gap at abrupt discontinuities and study of localization for 0D and 1D structures; and (3) determination of conduction and valence band offsets across dielectrically engineered junctions. Characterization tools for these investigations include optical spectroscopy, electron energy-loss spectroscopy and cathodoluminescence, and scanning tunneling spectroscopy.