Probing Charge, Spin and Thermoelectric Transport in Atomically Thin Materials

Non-technical abstract: The silicon transistor is the fundamental building block of modern electronics, the continued shrinking of which propelled the exponential growth of computing power over the past several decades. This trend cannot continue as the size of a transistor approaches that of an atom. New electronics beyond silicon calls for new operational principles that are drastically different from that of a conventional transistor, which controls the current flow by controlling the charge of the carriers. Equally important to today's society is the development of energy-harvesting materials and devices that could convert heat to electricity efficiently. Atomically thin layered materials, which consist of layers of atoms strongly bonded within each layer but weakly bonded between layers, offer excellent opportunities to tackle both challenges. The first objective of this project is to understand how a quantum mechanical property of an electron called 'spin' propagates in atomically thin materials and to develop a new type of valve that controls an electric current flow by controlling the spin of the carriers. The second objective of this project is to understand how electric current transports and dissipates in atomically thin materials and how to engineer the surface chemistry of the materials to make them efficient heat-to-electricity converters. Knowledge gained in this research is expected to have significant impact on the development of next-generation nanoelectronics and energy-harvesting devices. The research activities train students of all levels with necessary skills to advance nanoscience and nanotechnology and promote the participation of under-represented groups.

Technical abstract: This project seeks to significantly advance the fundamental understandings of charge, spin and thermoelectric transport in atomically thin transition metal dichalcogenides (TMDs) and explore their unique application potentials. One distinguishing property of TMDs that may lead to low-power electronic applications is the interlocking of the spin, valley and layer degrees of freedom in these materials. The first thrust of the project aims to systematically study spin and valley relaxation pathways in few-layer TMDs using magneto-transport measurements. The knowledge acquired is used to design and implement a novel spin-valley-layer valve in bilayer TMDs, leveraging the extensive device fabrication expertise of the PI?' lab. Measurements seek to understand its operation principles and evaluate its performances. The second thrust of the project focuses on understanding and controlling the charge and thermoelectric transport in TMD materials towards thermoelectric applications. One activity of this thrust aims to establish a much-needed quantitative understanding of the electron-phonon interactions in TMD materials by studying the temperature-dependent sheet resistance of the materials in the high-carrier density regime. A second activity exploits their band structures and surface nature to engineer desired thermoelectric responses. Experiments seek to enhance the thermopower of TMD materials using surface covalent functionalization. Measurements are supported by computations. Research carried out in this project is expected to produce timely and critical knowledge to stimulate and underpin the development of potential electronic, spintronic and thermoelectric applications of TMD materials. The research activities equip students with necessary STEM skills while summer camp activities promote science leadership and aim to broaden the reach of science to under-represented groups.