Point and complex defects in monolayer Pd Se 2 : Evolution of electronic structure and emergence of magnetism

Layered transition-metal dichalcogenides (TMDs) constitute an emerging class of materials that provide researchers a platform to realize fundamental studies and to design promising optoelectronic devices. While defects are an almost unavoidable part of TMDs, they bring additional interesting properties absent in defect-free layers. Moreover, the controlled introduction of defects in TMDs makes it possible to tailor the electromagnetic properties of the materials. Here we report defect-induced properties of single-layer $\mathrm{Pd}{\mathrm{Se}}_{2}$ and demonstrate the emergence of magnetism at the nanoscale. Our first-principle calculations indicate that Se vacancies create in-gap defect states, which can be responsible for trapping of carriers. The complex square ${V}_{\mathrm{Pd}+4\mathrm{Se}}$ vacancy induces spin-polarized states with a total local magnetic moment of $2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per defect, making it possible to introduce magnetization into $\mathrm{Pd}{\mathrm{Se}}_{2}$ and therefore expand the family of two-dimensional (2D) magnets. The defect formation energies are much lower compared to many other TMD materials that can explain the presence of a large number of Se defects after mechanical exfoliation of $\mathrm{Pd}{\mathrm{Se}}_{2}$ layers, while the central location of the Pd atoms preserves them from exfoliation-induced defect formation. The negatively charged vacancies are prone to form and in many cases demonstrate spin-polarized states. The small diffusion barrier of ${\mathrm{V}}_{\mathrm{Se}}$ in 2D $\mathrm{Pd}{\mathrm{Se}}_{2}$ leads to an easy room-temperature migration, while ${\mathrm{V}}_{\mathrm{Pd}}$ demonstrates a high diffusion barrier preventing its spontaneous migration. As a guide for experimentalists, we simulate and characterize scanning tunneling microscope images in valence and conduction states and estimate the electron-beam energy for a controllable production of various defect patterns. These intriguing findings make $\mathrm{Pd}{\mathrm{Se}}_{2}$ an ideal platform to study defect-induced phenomena.