The study of physical and chemical properties of twodimensional (2D) semiconductors has attracted remarkable scientific and technological interests. [1][2][3][4][5] Among which, transition metal dichalcogenides (TMDCs) with over 40 types of metal and chalcogen combinations, are one of the most focused families. [6] Transition metal dichalcogenides MX 2 (M = Mo, W; X = S, Se, Te) are layered semiconductors superimposed by weak van der Waals forces (vdW) with layerdependent tunability of optical bandgap from visible to near-infrared (IR) spectrum. [7,8] The strong light-matter interaction makes them prominent for photoelectric and solar materials. [9][10][11] MoS 2 and WS 2 are the two main representatives of TMDCs. Compared with MoS 2 , WS 2 is possibly more attractive due to its 20 times higher of photoluminescent efficiency, better thermal stability, larger valence band splitting, and theoretically reduced effective carrier masses. [12][13][14] However, far from the theoretical predictions, monolayer WS 2 devices show very limited carrier mobilities, photoresponse speeds, and on-off ratios. [15,16] For instance, the measured mobilities and photoresponse time of monolayer WS 2 -based field-effect transistors (FETs) are typically in the range of 1-50 cm 2 V À1 s À1 and 5-2000 ms, respectively, [17][18][19][20][21] which are far less than the predicted mobility of >500 cm 2 V À1 s À1 and photoresponse speed of <100 ns, respectively. [6,13,[22][23][24] The lowered performance may be marginal because of the deficiency of surface passivation, the main reason is commonly attributed to the defects produced in the synthesis, since trapping states at the electronic band edges in 2D WS 2 are significantly more active and especially fatal to its photoelectronic performances compared with their bulk counterparts. [25] The reported photoluminescent quantum yields of CVD-grown monolayer WS 2 are only 0.01%-6% of the mechanically exfoliated monolayer WS 2 because of the high density of vacancy defects. These defects inevitably come from the missing atoms in CVD synthesizing process due to the second law of thermodynamics. [26] However, CVD-synthesized WS 2 usually shows much larger in-plane sizes for applications compared with mechanically exfoliated samples. [27][28][29] Therefore, it is important to understand the defect effects on the electronic structures of 2D materials.Sulfur atomic vacancy defect has long been regarded as the main source of defect for its highest density in CVD-grown WS 2 . [30] Many methods have been proposed to reduce the density of sulfur defects, including high-temperature treatments, chemical atmosphere reductions, and irradiations by ultraviolet or electron beams. [31,32] Such methods are effective for reducing