Suppression of Defects and Deep Levels Using Isoelectronic Tungsten Substitution in Monolayer MoSe₂

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1603850 (1 of 10) atomically thin systems.[5,6] The variety of TMD monolayers synthesized to date provide a wide range of direct bandgaps suitable for various optoelectronic devices with different functionalities. The greatly enhanced Coulomb interactions in monolayer TMDs provide ideal opportunities for exploring many-body phenomena such as carrier multiplication that results in tightly bound trions.[7–9] Moreover, circular dichroism arising from the direct bandgap at the K and K′ points of the Brillouin zone opens up ways to manipulate electrons in the valley for spintronics.[10–12] However, despite the direct bandgap, the PL efficiency (or quantum yield) of monolayer TMDs is still poor. This is mainly attributed to defectrelated nonradiative recombination and, at higher excitation powers, biexcitonic recombination.[13–15] Compared with monolayers mechanically exfoliated from bulk crystals, those grown directly through chemical vapor deposition (CVD) generally show a higher density of defects (mainly chalcogen vacancies) in the lattice. Since CVD has the greatest potential to synthesize wafer scale two-dimensional (2D) layers for industrial device fabrication, suppressing the formation or detrimental effects of defect states in the CVDgrown TMD monolayers to increase the PL efficiency is very important for realizing potential optoelectronic applications for 2D TMDs. Suppression of Defects and Deep Levels Using Isoelectronic Tungsten Substitution in Monolayer MoSe2