First-principles study on the defect properties of two-dimensional materials

Recently, Defects in two-dimensional(2D) materials are revealed to be crucial to apprehend the various properties of materials. For example, they play the role of color centers, motivating electron transitions between the defect states. In addition, they have a large potential to be single photon emitters, which could be employed as a fundamental component of qubits. To exploit defects as a platform for future technology, we need to first appreciate the electronic structures of defects. It is, however, difficult to identify atomic defects especially when they are composed of vacancies and/or atoms with similar atomic numbers to those of host atoms, e.g., carbon or oxygen in hexagonal boron nitride (hBN). We perform first-principles calculations based on the density functional theory (DFT) to investigate the structural and electronic properties of defects in hBN and WSe₂, two prototypical 2D systems. As a first step, we examine the structural stabilities of defects in hBN and WSe₂ by evaluating the formation energies. Then we scrutinize their structural and electronic structures, focusing on levels of the defect states to estimate transition energies between the mid-gap states within HSE06, which is well-known for its great accuracy for the optical bandgap. We further consider the interaction between the defects in the hBN and WSe₂ with a few combinatory defects in an hBN/WSe₂/hBN heterostructure, which could affect the local geometry of defects, electronic structures, and their magnetic properties.