Selective Growth and Robust Valley Polarization of Bilayer 3R-MoS2

Abstract

It turns out that as-grown bilayers are predominantly 3R-type, not more common 2H-type, as verified by microscopic and spectroscopic characterizations. As expected, 3R bilayer showed a significantly higher valley polarization compared with centrosymmetric 2H bilayer, which undergoes efficient interlayer scattering across contrasting valleys because of their vertical alignment of the K and K^' points in momentum space. Interestingly, 3R bilayer showed even higher valley polarization compared with the monolayer counterpart. Moreover, 3R bilayer reasonably maintained its valley efficiency over a very wide range of excitation power density from ~0.16 kW/cm2 to ~0.16 MW/cm2 at both low and room temperatures. These observations are rather surprising, because valley dephasing could be more efficient in the bilayer via both interlayer and intralayer scatterings, whereas only intralayer scattering is allowed in monolayer. The improved valley polarization of 3R bilayer can be attributed to its indirect-gap nature, where valley-polarized excitons can relax into the valley-insensitive band edge, which otherwise scatter into the contrasting valley to effectively cancel out the initial valley polarization. Our results provide a facile route for the growth of 3R-MoS2 bilayers that could be utilized as a platform for advancing valleytronics