Scanning Tunneling Microscope Characterization of Chalcogen Site Defects in Flux Grown WSe₂

Within the family of two dimensional (2D) materials, transition metal dichalcogenides (TMDs) have gained much research interest due to their potential for filling the semiconductor role in 2D devices. Defects play a critical role in modifying semiconductor electronic properties, both detrimental by limiting device performance, and beneficial as a tool to engineer the electronic properties. The extreme 2D nature of TMDs leads to a strong sensitivity to defects from confinement and reduced screening.^[1] A number of different atomic defects have already been experimentally observed in TMDs, but connecting each defect species with their impact on the electronic properties is critical. Recent studies have shown that defect concentrations below 10¹¹ cm^-2 are achievable for TMD single crystals synthesized by a self-flux growth method,^[2] but the identity of these defects still remains unclear. Of the possible defects in TMDs, chalcogen vacancies are shown to be the most energetically favorable with the lowest formation energy,^[3] but previous studies have shown that oxygen passivated vacancies are the most abundant defect in CVD grown samples. Here we report the characterization of chalcogen site defects in flux grown WSe₂ using scanning tunneling microscopy and spectroscopy (STM/S). We study the electronic and structural properties of the defects in both bulk and monolayer as-grown WSe₂. We further induce vacancies by thermal annealing in UHV, and oxygen passivate them to show how the defects influence the WSe₂ electronic properties. Our study sheds light on the how one of the most abundant defects in TMDs, chalcogen site defects, can be used to engineer the electronic properties of TMDs.<br/> <br/>[1] Q. Liang, et al., <i>ACS Nano</i>, <b>15</b>, (2021) .<br/> <br/>[2] D. Edelburg, et al., <i>Nano. Lett.</i>, <b>19</b>, (2019).<br/> <br/>[3] H.-P. Komsa, et al., <i>PRB</i>, <b>88</b>, (2013).<br/> <br/>[4] S. Barja et. al., <i>Nat. Comm.</i>, <b>10</b>, (2019).