Defects in WS2 monolayer calculated with a nonlocal functional: any difference from GGA?

Abstract

Density functional theory (DFT) with generalised gradient approximation (GGA) functionals is commonly used to predict defect properties in 2D transition metal dichalcogenides (TMDs). Since GGA functionals often underestimate band gaps of semiconductors and incorrectly describe the character of electron localisation in defects and their level positions within the band gap, it is important to assess the accuracy of these predictions. To this end, we used the non-local density functional Perdew—Burke—Ernzerhof (PBE)0-TC-LRC to calculate the properties of a wide range of intrinsic defects in monolayer WS2. The properties, such as geometry, in-gap states, charge transition levels, electronic structure and the electron/hole localisation of the lowest formation energy defects are discussed in detail. They are broadly similar to those predicted by the GGA PBE functional, but exhibit numerous quantitative differences caused by the degree of electron and hole localisation in charged states. For some anti-site defects, more significant differences are seen, with both changes in defect geometries (differences of up to 0.5 Å) as well as defect level positions within the band gap of WS2. This work provides an insight into the performance of functionals chosen for future DFT calculations of TMDs with respect to the desired defect properties.