Pulsed cathodoluminescence of WS2 nanocrystals at various electron excitation energy densities: Defect induced sub-band gap emission

Abstract

Pulsed cathodoluminescence spectra and luminescence decay kinetics of WS2 nanocrystals were studied under a high energy electron pulse excitation. Increasing electron energy density inputs to the WS2 nanocrystals lead to enhancement of the electron-hole number and their lifetimes. The maximum luminescence intensity is reached for the highest electron energy dose. The electrons, interacting with WS2 nanocrystals, form defect vacancies, wherein excited electron-holes create bound excitons which further recombine as a distinct sub-band gap emission. The lifetime of bound excitons does not depend on increasing electron doses due to the limitation of the enhancing number of bound excitons by increasing density of defects, which are radiative recombination active centers. The number of bound excitons, i.e. luminescence intensity, is proportional to the density of defects, which can be tuned by electron doses. Micro-photoluminescence measurements of the WS2 monolayer showed formation of excitons and trions.