Accurate quantum mechanical modeling of defects in two-dimensional and three-dimensional materials

Layered materials beyond graphite such as hexagonal boron nitride (hBN) and transition
metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS2) are presently under intense study, and most applications require knowledge about their defects.
In this work, first, density functional theory (DFT) has been employed to study the defects
properties in hBN. According to earlier studies, the screened hybrid functional of Heyd,
Scuseria, and Ernzerhof (HSE), with parameters tuned to reproduce the relative position
of the band edges and to satisfy the generalized Koopmans’ theorem (gKT) , is capable of providing defect properties very accurately in traditional bulk semiconductors. This success is concluded to be connected to the proper description of electronic screening. I investigate whether such a functional can be optimized for layer compund such as hBN. I find that while the optimization is possible for bulk hBN, the optimized parameters can only be met approximately for the atomatically thin monolayer (ML) hBN. So, generally, the quantitative accuracy of defect calculations in 2D layers is limited.
Using an optimized hybrid functional in bulk hBN, which reproduces the gap and satisfies
the generalized Koopmans condition, an Ni configuration is found to be lower in energy than the ones reported so far. The (0/-) charge transition level is also much deeper, so Ni acts as a very efficient compensating center in n-type samples. Its calculated photoluminescence (PL) at 3.0 eV agrees well with the position of an N-sensitive band measured at 3.1 eV. Hyperfine interactions have been also performed to investigate the three boron electron (TBC) electron paramagnetic resonance (EPR) center in bulk hBN. I find that the carbon substitutional on the nitrogen site (CN) is the source of carbon-associated TBC in hBN in agreement with a previous study. I also show that the nitrogen vacancy (VN) cannot be the origin of the electron-associated produced TBC in hBN and in thermal equilibrium it cannot exist in ntype samples.
Similarly, defects in van der Waals layered MoS2, play an important role. I present
our first principle investigation of a water splitting for a defective monolayer molybdenum
disulfide (ML MoS2). Defects such as molybdenum vacancy (VMo) and a vacancy complex of Mo and three S (VMoS3 ) in two configurations are considered. The complex defect VMoS3, where a pair of sulfur vacancies is placed on the top plane and the third vacancy on the bottom plane, is found to be sufficiently active to split the water molecule into OH and H. The water dissociation not occur for the complex defect VMoS3 if all sulfur vacancies are located at the surface. The presented results are critical for designing chemical sensing and hydrogen generation devices from MoS2.