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

Noncentrosymmetric transition-metal dichalcogenides, particularly their 3 R polymorphs, provide a robust setting for valleytronics. Here, we report on the selective growth of monolayers and bilayers of MoS 2, which were acquired from two closely but differently oriented substrates in a chemical vapor deposition reactor. It turns out that as-grown bilayers are predominantly 3 R -type, not more common 2 H -type, as verified by microscopic and spectroscopic characterization. As expected, the 3 R bilayer showed a significantly higher valley polarization compared with the centrosymmetric 2 H bilayer, which undergoes efficient interlayer scattering across contrasting valleys because of their vertical alignment of the K and K ′ points in momentum space. Interestingly, the 3 R bilayer showed even higher valley polarization compared with the monolayer counterpart. Moreover, the 3 R bilayer reasonably maintained its valley efficiency over a very wide range of excitation power density from ∼0.16 kW/cm 2 to ∼0.16 MW/cm 2 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 the monolayer. The improved valley polarization of the 3 R 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 3 R -MoS 2 bilayers that could be utilized as a platform for advancing valleytronics.