CAREER: Many-electron interactions and excited-state properties in two-dimensional van der waals interfaces

Washington University, Saint Louis MO

Investigators

Abstract

NON-TECHNICAL SUMMARY Interfaces in solids, such as p-n junctions and heterojunctions in diodes and transistors, are excellent platforms for fundamental scientific research, and they are the foundation of the microelectronics industry. In these structures, a very large number of electrons interact with each other and shape the conductance, thermal activity, magnetism, optical response, and other fundamental properties of the materials. This CAREER award supports theoretical and computational research and education, in which the PI will focus on many-electron interactions in a new type of structure that can be viewed as "sandwiches" of two-dimensional layers of different materials. The layers are held together by weak forces called van der Waals (vdW) forces. Materials with interfaces in which vdW forces dominate combine the properties of the different layers in exotic ways to produce novel effects, such as unusual light-matter interactions and charge transport. Thus, such materials are useful for studying new physical phenomena and for designing small electronic devices of targeted functionalities. The PI will use parameter-free quantum mechanical simulations and simpler models based on these computations to explore how electronic interactions at vdW interfaces yield unique electrical transport and optical properties. This research can pave the way towards manipulating electronic, thermal, and optical properties of devices, such as diodes, transistors and low-cost photovoltaic solar cells. The educational component of this activity includes student mentoring, course development, seminars and workshops, and diverse collaborations involving students. In particular, the PI will focus on various K-12 activities. Collaborations with neighboring and international institutes will provide opportunities for training under-represented minority students in science, technology, engineering, and mathematics fields. TECHNICAL SUMMARY Graphene and related two-dimensional structures have attracted a great deal of scientific and technological interest over the last decade. Towards this end, few-layer heterostructures and superlattices held together by interlayer van der Waals (vdW) interactions have recently been fabricated. Creating few-layer composites of different materials allows one to achieve a wide range of new material characteristics - the individual materials' properties do not simply superpose. Accordingly, it is very likely that vdW interfaces will be central to the future of scientific and industrial research based on two-dimensional materials. This CAREER award supports theoretical and computational research and education focused on many-electron interactions and excited-state properties of these interfaces. The research plan includes three facets: 1. At the single-particle excitation level, the PI and his team will compute the quasiparticle band offsets and charge transfer at vdW junctions, which dictate their electrical and transport properties. The team will further develop practical methods for engineering these characteristic properties. Ultimately, the aim is to design a new type of low-power-dissipation diode that is based on quantum tunneling across vdW junctions. 2. At the two-particle excitation level, the project focuses on optical excitations: excitons (electron-hole pairs). The electron-hole binding energy spectra of unique interlayer and intralayer excitons will be computed for vdW heterostructures, which is ultimately expected to lead to a parameter-free exciton model. The team will also focus on physical and chemical approaches for manipulating the energy spectra, optical activities, and lifetimes of these excitons in few-layer and superlattice structures. These ideas will be applied toward achieving exciton Bose-Einstein condensation and designing photovoltaic devices. 3. At the higher-order excitations level, the project will focus on three-body interactions (trions) and four-body interactions (biexcitons). By modeling the screened Coulomb interactions in vdW interfaces based on first principles simulations, the PI will develop efficient means for obtaining the energy spectra and optical activities of trions and biexcitons. These models are particularly useful for interpreting experimental results and predicting new optical devices without requiring costly first-principles simulations. The educational component of this activity includes student mentoring, course development, seminars and workshops, and diverse collaborations involving students. In particular, the PI will focus on various K-12 activities. Collaborations with neighboring and international institutes will provide opportunities for training under-represented minority students in science, technology, engineering, and mathematics fields.