Defects, band bending and ionization rings in MoS2

TL;DR

This study uses STM and STS to investigate sulphur vacancies in bulk MoS2, revealing tip-induced band bending effects that influence defect spectra interpretation and clarify previous discrepancies.

Contribution

It demonstrates the importance of considering tip-induced band bending when analyzing defect states in MoS2 using STM/STS.

Findings

Spectroscopic features disperse with tip position and bias.

Tip-induced band bending causes ionization of sulphur vacancies.

Careful interpretation of defect spectra is necessary.

Abstract

Chalcogen vacancies in transition metal dichalcogenides are widely acknowledged as both donor dopants and as a source of disorder. The electronic structure of sulphur vacancies in MoS2 however is still controversial, with discrepancies in the literature pertaining to the origin of the in-gap features observed via scanning tunneling spectroscopy (STS) on single sulphur vacancies. Here we use a combination of scanning tunnelling microscopy (STM) and STS to study embedded sulphur vacancies in bulk MoS2 crystals. We observe spectroscopic features dispersing in real space and in energy, which we interpret as tip position- and bias-dependent ionization of the sulphur vacancy donor due to tip induced band bending (TIBB). The observations indicate that care must be taken in interpreting defect spectra as reflecting in-gap density of states, and may explain discrepancies in the literature.