OP: Momentum Conservation in Optoelectronic Processes at 2D Van der Waals Semiconductor Heterojunctions

非技术描述：过渡金属二硫化物（TMD）单层是最薄的半导体材料，厚度为一到三个原子（即纳米的一小部分），通常被称为二维半导体。这些材料可以作为未来电子和光电子应用的材料平台，因为传统的半导体技术正在达到尺寸极限。该项目的主要目标是了解电荷如何在二维半导体之间的界面上移动，这是许多电子和光电子设备操作的核心过程。该研究将先进的生长和加工技术与各种光谱表征方法相结合。此外，该项目还为学生提供培训机会，范围从K-12、社区学院、本科生到研究生。技术描述：二维半导体界面上的光学和光电子过程必须同时满足能量和动量守恒；后者包括自旋和晶体动量。过渡金属二硫族化合物（TMD）单层的六角形结构导致动量空间K和-K有六个谷，具有相反的自旋轨道分裂。单层中的K或-K谷通常与另一层中的K或-K谷不对齐。因此，电荷在界面上的转移伴随着平行动量的变化。然而，人们对动量守恒的机制知之甚少，这在很大程度上是由于迄今为止应用于tmd的实验技术中缺乏动量分辨率。本研究通过实验直接测量电子在时域中的能量和动量来解决这个问题，因为它在TMD单层的K（或-K）谷中被激发，作为自由电子转移到第二层单层或形成层间激子，或与界面上的空穴重新结合。这是由最先进的实验技术，时间分辨双光子光发射光谱与近红外到可见的TMD单层或异质结的激发和极紫外激光电离激发的电子。电离后的电子在能量空间和动量空间都能以飞秒时间分辨率被探测到。这种直接的实验方法促进了对TMD异质结层间激子的理解，并指导了基于二维半导体的未来光电技术的发展。

Nontechnical Description: Transition metal dichalcogenide (TMD) monolayers are the thinnest semiconductor materials, with thickness on the order of one to three atoms (i.e., a fraction of a nanometer) and are often called two-dimensional semiconductors. These materials may serve as a material platform for future electronics and optoelectronics applications as the conventional semiconductor technologies are reaching dimensional limits. The main goal of this project is to understand how charges move across the interface between two-dimensional semiconductors, a process central to the operation of many electronic and optoelectronic devices. The research combines advanced growth and processing technologies with various spectroscopic characterization methods. In addition, the project offers training opportunities for students, ranging from K-12, community college, and undergraduates to graduate students. Technical Description: Optical and optoelectronic processes at two-dimensional semiconductor interfaces must satisfy the conservation of both energy and momentum; the later includes both spin and crystal momentum. The hexagonal structure of a transition metal dichalcogenide (TMD) monolayer leads to six valleys in momentum space, K and -K, with opposite spin-orbital splitting. The K or -K valleys in one monolayer are usually not aligned with those of the other. Thus, charge transfer across the interface is accompanied by change in parallel momentum. However, little is known about the mechanism for momentum conservation, due in a large part to the lack of momentum resolution in experimental techniques applied to date to the TMDs. This research experimentally tackles this problem by directly measuring the energy and momentum of the electron in the time domain, as it is excited in the K (or -K) valley of a TMD monolayer, transferred to the second monolayer as a free electron or to form an inter-layer exciton, and or recombine with the hole across the interface. This is enabled by a state-of-the-art experimental techniques, time-resolved two-photon photoemission spectroscopy with near-IR to visible excitation of the TMD monolayers or heterojunctions and an extreme ultraviolet laser to ionize the excited electron. The ionized electron is detected in both energy and momentum spaces with femtosecond time resolution. Such a direct experimental approach advances the understanding of interlayer excitons at TMD heterojunctions and guides the development of future optoelectronic technologies based on two-dimensional semiconductors.