UC Berkeley

Point defects dictate the performance of many materials in electronic and optoelectronic applications. As scientists and engineers continue to push the limits of nanoscale electronic materials, the role of point defects and the importance of understanding them grows ever larger. Emerging materials for electronic and optoelectronic applications include monolayer (ML) transition metal dichalcogenides (TMDs), whose point defect properties are an active area of research from both applied and theoretical perspective. In recent years, automated calculations based on density functional theory have become an increasingly popular and valuable tool for understanding atomic scale properties and informing materials design; how- ever, several barriers exist in application of such techniques to point defect properties. These barriers include predictive limitations of semi-local density functional theory, which is known to suffer from inaccurate prediction of point defect formation energies and charge transition levels, and the computational cost of more accurate methods such as hybrid density func- tional theory. In order to accelerate the design of novel electronic materials, a thorough understanding of point defects as guided by accurate, efficient first principles calculations is required. In this dissertation, first principles calculations based on hybrid density functional theory are applied to the study of point defects in MoX2 (X=S, Se, Te), an emerging family of two dimensional materials attracting much interest for electronic and optoelectronic ap- plications. It begins with an introduction to the growing importance of density functional theory in materials design, highlighting the value of automated workflows and the need for their extension to point defect properties. The basics of density functional theory are then briefly reviewed. Subsequently, our newly implemented workflow for hybrid density func- tional theory calculations is described. We introduce the software used in the remainder of the dissertation, including benchmarking to support the validity of subsequent case studies. Finally, the workflow is applied to two relevant case studies. First, in a study of the intrinsic defects in MoX2 monolayer systems, we identify the most prominent point defects in these systems, showing that MoS2 and MoSe2 are Frenkel forming materials while MoTe2 is a Schottky former. We also show that controlling growth conditions can reduce overall de- fect concentrations, especially in MoS2, while noting that post-growth treatments are likely necessary achieving sufficiently low concentrations in practical devices. We conclude with a study of substitutional dopants in MoX2 monolayer systems. By investigating several candi- date dopants, including the effect of substrate screening and variations in chemical potential, we identify As, Cl, Nb, and Re as promising candidates for doping MoX2 monolayers, while noting limitations of calculations and highlighting the need for further research of substrate effects.