A Hybrid-DFT Study of Intrinsic Point Defects in $MX_2$ ($M$=Mo, W; $X$=S, Se) Monolayers

TL;DR

This study uses hybrid DFT to analyze the structural and electronic properties of point defects in monolayer $MX_2$ materials, revealing defect stability influenced by temperature and charge state, with implications for material engineering.

Contribution

It provides a comprehensive hybrid DFT analysis of point defects in $MX_2$ monolayers, including charged defects, vibrational effects, and defect imaging, which advances understanding of defect behavior in these materials.

Findings

Charged vacancy defects can be negatively charged.

Finite-temperature vibrations significantly affect defect stability.

Vibrational modes of defects can be visualized via Raman scattering.

Abstract

Defects can strongly influence the electronic, optical and mechanical properties of 2D materials, making defect stability under different thermodynamic conditions crucial for material-property engineering. In this paper, we present an account of the structural and electronic characteristics of point defects in monolayer transition metal dichalcogenides $MX_2$ with $M$=Mo/W and $X$= S/Se, calculated with density-functional theory using the hybrid HSE06 exchange correlation functional including many-body dispersion corrections. For the simulation of charged defects, we employ a charge compensation scheme based on the virtual crystal approximation (VCA). We relate the stability and the electronic structure of charged vacancy defects in monolayer MoS$_2$ to an explicit calculation of the S monovacancy in MoS$_2$ supported on Au(111), and find convincing indication that the defect is negatively charged. Moreover, we show that the finite-temperature vibrational contributions to the free energy of defect formation can change the stability transition between adatoms and monovacancies by 300--400 K. Finally, we probe defect vibrational properties by calculating a tip-enhanced Raman scattering image of a vibrational mode of a MoS$_2$ cluster with and without an S monovacancy.