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：过渡金属二北元化物（TMD）单层是最薄的半导体材料，其厚度在一到三个原子的级数（即纳米表的一部分），通常称为二维半导体。随着常规半导体技术达到尺寸限制，这些材料可以用作将来电子产品和光电应用应用的材料平台。该项目的主要目标是了解二维半导体之间的电荷如何在界面上移动，这是许多电子和光电设备运行的核心过程。该研究将先进的生长和加工技术与各种光谱表征方法结合在一起。此外，该项目为学生提供了培训机会，从K-12，社区学院和本科生到研究生。技术描述：二维半导体接口处的光学和光电过程必须满足能量和动量的保护；后来包括自旋和晶体动量。过渡金属二甲基化金（TMD）单层的六边形结构在动量空间，K和-K中导致六个山谷，具有相反的自旋轨道分裂。一个单层中的K或-K山谷通常与另一个单层不符。因此，跨界面的电荷转移伴随着平行动量的变化。然而，对于动量保护的机制知之甚少，这在很大程度上是由于迄今为止应用于TMD的实验技术缺乏动量分辨率。这项研究通过直接测量时间域中电子的能量和动量来解决这个问题，因为它在TMD单层的K（或-K）山谷中被激发，将其转移到第二个单层作为自由电子或形成层间单层的激子，或者与孔中的孔相位。这是通过最先进的实验技术来实现的，具有时间分辨的两光子光发射光谱，近红外，可见刺激TMD单层或异质结，以及极端的紫外线激光，以使激发电子电离。以飞秒时间分辨率在能量和动量空间中都检测到电离电子。这种直接的实验方法在TMD异质界面上对层间激子的理解发展，并指导基于二维半导体的未来光电技术的开发。