Electroluminescence of monolayer WS$_2$ in a scanning tunneling microscope: the effect of bias polarity on the spectral and angular distribution of the emitted light

TL;DR

This study demonstrates how bias polarity in a scanning tunneling microscope influences the spectral and angular characteristics of electroluminescence from monolayer WS₂, revealing different excitonic contributions.

Contribution

It provides new insights into bias-dependent excitonic electroluminescence in monolayer WS₂ using optical spectroscopy and microscopy.

Findings

Bias polarity controls emission spectral features.

Positive bias favors charged exciton emission.

Negative bias reveals mixed dipole transition contributions.

Abstract

Inelastic electron tunneling in a scanning tunneling microscope (STM) is used to generate excitons in monolayer tungsten disulfide (WS$_2$). Excitonic electroluminescence is measured both at positive and negative sample bias. Using optical spectroscopy and Fourier-space optical microscopy, we show that the bias polarity of the tunnel junction determines the spectral and angular distribution of the emitted light. At positive sample bias, only emission from excitonic species featuring an in-plane transition dipole moment is detected. Based on the spectral distribution of the emitted light, we infer that the dominant contribution is from charged excitons, i.e., trions. At negative sample bias, additional contributions from lower-energy excitonic species are evidenced in the emission spectra and the angular distribution of the emitted light reveals a mixed character of in-plane and out-of-plane transition dipole moments.