Enhancement of valley polarization in CVD grown monolayer MoS<inf>2</inf>films

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.