Thermally Induced Defects on WSe2

The 2D nature of transition metal dichalcogenides (TMDs) makes their electronic and optical performance highly susceptible to the presence of defects. At elevated temperatures, which can be reached during growth or in operation, additional defects can be introduced and lead to further material degradation. Therefore, by studying the impact of temperature on 2D-T MDs the formation of defects and their respective degradation pathways can be established. The electronic and geometric structure and density of thermally induced defects on 2D tungsten diselenide (WSe2) layers were examined using scanning tunneling microscopy/spectroscopy (STM/STS). WSe2 layers were grown on highly ordered pyrolytic graphite (HOPG), via molecular beam epitaxy (MBE) and annealed at 600 degrees C, which caused a 7-fold increase in overall defect density. A layer-dependent trend emerged whereby the defect density on the first layer was greater than the second, suggesting that the TMD-graphite and TMD-TMD van der Waals interactions influence the formation energy of thermally growth defects. The defect inventory included single-point vacancies and a collection of larger defects with complex geometric and electronic signatures. These defects were classified by matching their unique electronic structures with their respective topographical presentation via spatially resolved STS maps. Defect states at the conduction and valence band edges introduced n- or p-type character and generally lowered the local band gap around each defect site. A unique defect structure displayed an increased band gap, likely as a consequence of local delamination of the TMD due to subsurface Se-cluster formation. Density functional theory (DFT) was used to examine select defects and supported the interpretation of the STM/STS work with density of states (DOS) and local-integrated DOS calculations. The assessment of the geometric and electronic signatures and details of the local doping profile around all defect sites deepened our understanding of the thermal stability of WSe2.