Defects engineering in 2D materials through non-equilibrium synthesis and processing

Defects in atomically thin 2D materials not only influence their existing properties, but also could induce the emerging new functionality. However, the lack of control over forming specific defects in 2D materials makes them hard for their real applications in microelectronics and quantum information science. To understand the evolution of defect in 2D crystals during synthesis and processing and their emergent functionalities, we have recently developed in situ diagnostics both at the microscale using optical imaging and laser spectroscopy, and at the atomic scale using electron microscopy and spectroscopy. Here I will present our recent progress on how to develop non-equilibrium CVD approaches to enable the defect control during synthesis, including the selective formation of antisite defects of 2D materials, and embedded twin boundaries in bilayer crystals. By using a dilute W-Au alloy as the substrate to maintain W-poor (S-rich) growth conditions that were predicted to reduce the formation energy of antisite defects, the S_W and S2_W antisite defects were selectively during the growth of WS₂ monolayers. The observed localized defect states induced by those antisite defects in the bandgap of monolayer WS2 could be used for long-lifetime quantum emitters. In addition, by using the two-step CVD method, the twin grain boundaries in the bottom layer are preferred formed through the top-layer confined epitaxial growth of the oriented domains on the substrate. The Mo isotope labeling Mo, and subsequent laser thinning combined with Raman spectroscopy, TOF-SIMS, and Z-STEM imaging were used to reveal the stack sequence and growth mechanisms. Therefore, the nonequilibrium synthesis is an effective approach to engineer defects and tailor the electronic and optical properties of a wide variety of atomically thin 2D materials for designed functionalities, such as local magnetism, long-lifetime quantum emission and novel electronics.