Sub-diffraction-resolved spatial distribution of emitting excitons in STM-induced luminescence of 2D semiconductors via Richardson-Lucy deconvolution

TL;DR

This paper presents a method to resolve the nanoscale spatial distribution of exciton emission in 2D semiconductors using Richardson-Lucy deconvolution of STM-induced luminescence images, revealing new insights into exciton transport.

Contribution

It introduces a novel application of Richardson-Lucy deconvolution to surpass diffraction limits in STM-induced luminescence imaging of 2D materials.

Findings

Resolved sub-diffraction emission patterns in WSe2 and WS2 monolayers.

Discovered dependence of emission distribution on tunneling current.

Identified hot spots of emission far from excitation source.

Abstract

Using scanning tunneling microscopy-induced luminescence (STML), the optical properties of two-dimensional (2D) semiconductors may be investigated at the nanoscale. This is possible because the tunneling current under the tip is an extremely localized electrical excitation source. However, in most STML applications, the spatial distribution of the emission relative to the excitation point is unresolved. Yet this distribution contains key information about how the interaction of excitons with injected charge carriers affects the luminescence of these materials, and about exciton transport. Resolving this spatial distribution at the nanoscale is relevant both for a fundamental understanding of exciton physics and for device applications; yet it remains a significant challenge. In this work, we resolve the spatial distribution of the emission beyond the diffraction limit of light by deconvolving real-space optical microscopy images of the STML using an iterative algorithm, i.e., Richardson-Lucy (RL) deconvolution. To showcase this technique, we apply it to the STML of monolayer tungsten diselenide ($\mathrm{WSe_2}$) and tungsten disulfide ($\mathrm{WS_2}$). Thus, we highlight hitherto ignored or misunderstood aspects of STML on 2D semiconductors related to exciton and charge carrier transport, namely the dependence of the spatial distribution of emission on the tunnel current setpoint and the origin of the emission from hot spots located micrometers from the excitation source.