Controlling exciton dynamics at interfaces using moiré potentials

NON-TECHNICAL SUMMARY:

Two-dimensional (2D) layered crystals, such as transition metal dichalcogenides, have received much attention recently. Notably, their properties can be tailored by stacking different crystals together without the requirement of lattice matching found in traditional inorganic semiconductors. One promising way to control material properties is to vary the relative orientation and the lattice size of the two atomically thin crystals to create a nanoscale moiré pattern.

This project will combine 2D-layered crystals with molecular crystals to produce a wide range of moiré patterns. The research team will then investigate the dependence of optical and electronic properties on the different types of moiré patterns. The fundamental knowledge gained by this project allows researchers to create tailor-made interfaces useful in devices such as solar cells, LEDs, and photodetectors, as well as quantum emitters.

The project will train undergraduate and graduate students on cutting-edge research capabilities in nanoscale material design, fabrication, and characterization in a collaborative environment. There will be a particular emphasis on recruiting students from underrepresented groups. Outreach activities to the public include a summer camp for K-12 students, outreach seminars, and science outreach websites.

TECHNICAL SUMMARY:

A wide range of moiré patterns can be created by interfacing 2D transition metal dichalcogenide crystals with organic molecular crystals through weak van der Waals forces. The moiré pattern produces a nanoscale-periodic potential, which in turn provides a unique way to control the electron dynamics at the interface. The research team combines theoretical and experimental efforts to understand how the moiré potential affects the properties of the interlayer exciton (IX) formed at the interface.

The goal is to search for interfaces with two distinct classes of moiré potential that favor either photo-to-electrical conversion or light emission, and to demonstrate the moiré potential as an effective way to control the competition between the free carrier generation and the radiative recombination of the IX. A combination of time-resolved and/or angle-resolved photoemission spectroscopy, and spatiotemporal optical pump-probe spectroscopy are used to probe the IX dynamics and the band structure at the interface.

The experimentally measured IX dynamics are correlated with band alignments, interface interactions, and the moiré potential determined by theoretical methods based on density functional theory, including meta-GGAs for rapid screening and hybrid functionals for more accurate calculations. van der Waals interactions are explicitly included. Wannier functions are used to simulate moiré potentials.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.