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.
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.
