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

OP：二维范德华半导体异质结光电过程中的动量守恒

• 批准号: 1608437
• 负责人: Xiaoyang Zhu
• 金额: $ 50万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-01 至 2019-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1608437&HistoricalAwards=false
• 关键词: OP; Momentum; Conservation; Optoelectronic; Processes

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.

非技术描述：过渡金属二硫属化物（TMD）单层是最薄的半导体材料，其厚度大约为一到三个原子（即，纳米的几分之一）并且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限制。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，从K-12，社区学院和本科生到研究生。技术说明：二维半导体界面的光学和光电过程必须满足能量和动量守恒，后者包括自旋和晶体动量。过渡金属二硫属化物（TMD）单层的六边形结构导致动量空间中的六个谷，K和-K，具有相反的自旋轨道分裂。一个单层中的K或-K谷通常不与另一个单层中的K或-K谷对齐。因此，穿过界面的电荷转移伴随着平行动量的变化。然而，鲜为人知的是动量守恒的机制，在很大程度上是由于缺乏动量分辨率的实验技术应用到日期的TMD。这项研究实验解决了这个问题，通过直接测量的能量和动量的电子在时域中，因为它是激发在一个TMD单层的K（或-K）谷，转移到第二个单层作为自由电子或形成层间激子，和/或重组与孔跨界面。这是一个国家的最先进的实验技术，时间分辨的双光子光电子能谱与近红外可见光激发的TMD单层或异质结和极紫外激光激发的电子。电离的电子被检测到的能量和动量空间与飞秒时间分辨率。这种直接的实验方法推进了对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。