The development of future valley based electronics or valleytronics requires a high degree of valley polarization (VP) in large area monolayer (1L)-MoS2. Though it is possible to synthesize 1L-MoS2 films with large area coverage, VP property of as-grown films is found to be very poor. Here, we investigate the role of physisorbed air molecules and strain on the luminescence and the VP characteristics of large area monolayer MoS2 grown on various substrates by a microcavity based chemical vapor deposition (CVD) technique. The study shows that the removal of adsorbates from sulfur vacancy ( VS) sites not only suppresses the broad sub-bandgap luminescence feature that typically dominates low temperature photoluminescence (PL) spectra of these films but also significantly enhances VP. Post-growth transfer of the 1L-MoS2 film from sapphire to a SiO2/Si substrate by a polystyrene assisted process is found to be highly effective in improving the polarization characteristic (∼80%) of K-valleys through relaxation of the biaxial tensile strain and the removal of physisorbed air molecules from the VS sites. The process is also found to provide long lasting protection for MoS2 films from air. The finding, thus, creates much needed opportunity to use CVD grown large area 1L-MoS2 for realization of valleytronics of the future.

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Using temperature dependent photoluminescence (PL), polarization resolved PL and Raman spectroscopy, we investigate the effect of in situ vacuum annealing as well as the relaxation of strain on the luminescence and the valley polarization properties of large area strictly monolayer (1L)-MoS 2 , grown on sapphire and SiO 2 /Si substrates by a microcavity based chemical vapor deposition (CVD) technique. The study shows that the strain as well as the physisorption of air molecules at the sulfur vacancy (V S ) sites play key roles in governing the optical quality of CVD grown 1L-MoS 2 . Removal of air molecules from the V S sites enhances the relative strength of the A-exciton/trion transition as compared to the broad luminescence (BL) band arising from those defects at low temperatures. It has also been found that such removal helps in improving the valley polarization property of the film. Relaxation of biaxial tensile strain, which has been achieved by post growth transferring of 1L-MoS 2 film from the sapphire to a SiO 2 /Si substrate by a polystyrene assisted transfer process, is also found to be helpful to get back the high polarization character (∼80%) of the valleys. The study further shows that the transfer process not only facilitates the removal of physisorbed air molecules from the V S sites but also puts in place a long lasting capping layer on MoS 2 that shields the film from reacting with air and hence enhances the relative yield of A-exciton/trion transition by suppressing the BL transition. The study thus creates an opportunity to use CVD grown large area 1L-MoS 2 for the development of optoelectronic as well as valleytronic devices for practical applications for the future.