OP: Interlayer Excitons in Double Layer Black Phosphorus

University Of Arkansas, Fayetteville AR

Investigators

Abstract

Nontechnical description: When a semiconductor absorbs light, an electron can be promoted to a higher energy level and leave behind a hole. The attraction between the negatively charged electron and the positively charged hole can cause the two particles to stick together. This bound electron-hole pair is called an exciton. In a new class of atomically thin semiconductors, excitons are extraordinarily stable, even at room temperature. The research team aims to understand, using both experiment and theory, the behavior of excitons in black phosphorus, and to find out whether it will be possible to construct potentially transformative optoelectronic devices using excitons in this class of materials. In particular, double layer black phosphorus (two atomically thin layers separated by an insulating spacer) provides a compelling combination of properties for the exploration of excitons, including longer exciton lifetimes and vertical orientation of the positive and negative charges making up the exciton. Both of these properties, combined with the relatively high electrical conductivity of black phosphorus, will promote both the fundamental study and device applications of excitons in this material. This research activity provides mentoring and training of a widely inclusive group of high school, undergraduate, and graduate students. Outreach activities focus on encouraging the enrollment of underrepresented groups at the University of Arkansas. Technical description: Absent screening by a 3D bulk, 2D materials generate strong Coulomb interactions and therefore host excitons at high temperatures. This project aims to characterize and understand the basic properties of excitons in pristine, encapsulated, few-layer black phosphorus. Given sufficient carrier mobility and exciton lifetimes, it is possible to create excitonic devices that transport excitons from one part of a circuit to another. This movement of excitons could be used to encode and transmit information, or it could be used to modulate optical signals in a nanoscale structure. In particular, double layer black phosphorus (two atomically thin layers separated by a dielectric spacer) provides a compelling combination of properties for the exploration of excitons, including enhanced lifetimes, relatively high and anisotropic mobility, and vertically-aligned dipoles that promote control of exciton motion by non-uniform electric fields. The research team measures and analyzes exciton energies, linewidths, positions, and lifetimes as a function of black phosphorus thickness, temperature, and charge density (both electrons and holes) to provide baseline material properties of pristine black phosphorus. This information provides input to effective theoretical models, which guides the creation and understanding of excitonic devices in which excitons are controlled by gates voltages.