High Luminescence Efficiency in MoS₂ Grown\nby Chemical Vapor Deposition

Abstract

One\nof the major challenges facing the rapidly growing field of\ntwo-dimensional (2D) transition metal dichalcogenides (TMDCs) is the\ndevelopment of growth techniques to enable large-area synthesis of\nhigh-quality materials. Chemical vapor deposition (CVD) is one of\nthe leading techniques for the synthesis of TMDCs; however, the quality\nof the material produced is limited by defects formed during the growth\nprocess. A very useful nondestructive technique that can be utilized\nto probe defects in semiconductors is the room-temperature photoluminescence\n(PL) quantum yield (QY). It was recently demonstrated that a PL QY\nnear 100% can be obtained in MoS₂ and WS₂ monolayers\nprepared by micromechanical exfoliation by treating samples with an\norganic superacid: bis­(trifluoromethane)­sulfonimide (TFSI).\nHere we have performed a thorough exploration of this chemical treatment\non CVD-grown MoS₂ samples. We find that the as-grown monolayers\nmust be transferred to a secondary substrate, which releases strain,\nto obtain high QY by TFSI treatment. Furthermore, we find that the\nsulfur precursor temperature during synthesis of the MoS₂ plays a critical role in the effectiveness of the treatment. By\nsatisfying the aforementioned conditions we show that the PL QY of\nCVD-grown monolayers can be improved from ∼0.1% in the as-grown\ncase to ∼30% after treatment, with enhancement factors ranging\nfrom 100 to 1500× depending on the initial monolayer quality.\nWe also found that after TFSI treatment the PL emission from MoS₂ films was visible by eye despite the low absorption (5–10%).\nThe discovery of an effective passivation strategy will speed the\ndevelopment of scalable high-performance optoelectronic and electronic\ndevices based on MoS₂.