Sanibel Symposium

Two-dimensional (2D) semiconductor materials, such as transition metal dichalcogenides, monochalcogenides, and phosphorene have attracted extensive research interests for potential applications in optoelectronics, spintronics, photovoltaics, and catalysis. To harness the potential of 2D semiconductors for electronic devices requires better control and understanding of impurities, defects, and dopants and how they control the carrier concentration, character, and mobility in 2D materials. Accurate determination of their formation energies and charge transition levels enable us to predict the effect of the defect on the electronic properties and is crucial for the design of novel 2D optoelectronic and spintronic devices. Unfortunately, experimental data of defect concentrations are scarce due to the difficulty of measuring defects in low-dimensional systems and establishing and maintaining thermodynamic equilibrium in these systems.

Density-functional theory (DFT) provides a tool that can predict these quantities accurately. However, modeling charged defects in single-layer materials with common plane-wave DFT approaches poses additional challenges which lead to the divergence of the energy with vacuum spacing. Recently, Freysoldt and Neugebauer developed a correction scheme that employs a generalized dipole approach and restores the appropriate electrostatic boundary conditions for charged 2D materials. We apply this correction scheme to compute the formation energies and charge transition levels associated with Sulphur vacancies in MoS2. We also compare the electronic structures of the defect in different charge states to gain insights into the effect of defects on bonding and magnetism. We investigate the convergence of these defect properties with respect to vacuum spacing, in-plane supercell dimensions, i.e., defect concentration, and different levels of theory.