Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides

Sara Barja ,Jeffrey B Neaton ,Adam Schwartzberg ,Choongyu Hwang ,Hyejin Ryu ,Shaul Aloni ,Sebastian Wickenburg ,M Hashimoto ,Miguel M Ugeda ,Artem Pulkin ,Michael F Crommie ,D Frank Ogletree ,Bruno Schuler ,Diana Y Qiu ,Sivan Refaely-Abramson ,Christopher Chen ,Oleg V Yazyev ,Steven G Louie ,Christoph Kastl ,Alexander Weber‐Bargioni

doi:10.1038/s41467-019-11342-2

Abstract

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe2 and WS2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies.

Highlights

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies
Our analysis suggests that the commonly observed point defects in MoSe2 and WS2, growth ex situ by molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) and measured in ultra high vacuum (UHV) after gentle annealing, are O substitutions at Se and S sites, respectively
A number of transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) studies have investigated point defects on various TMD samples[9,18], and the majority of chalcogen-site point defects have been identified as vacancies

Summary

Introduction

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. We combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe2 and WS2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. The strong dependence of the tunneling conditions on the STM contrast of the atomic lattice and the unclear differentiation between chalcogen and metal sublattices in former STM studies has led to a non-consistent interpretation of the defect type across the current literature[13,14,16,17,20,21,22,27,28] Sulphur vacancies, and their corresponding IGS, have been held responsible for unexpected catalytic activity in hydrogen evolution reactions reported in MoS214,15,17. Our comprehensive joint experimental and theoretical study reveals substitutional oxygen as a prolific point defect in 2D-TMDs and provides critical insight for future defect engineering in these systems

Methods

Results

Conclusion