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

Abstract

Noncentrosymmetric transition-metal dichalcogenides, particularly their 3R polymorphs, provide a robust setting for valleytronics. Here, we report on the selective growth of monolayers and bilayers of MoS2, 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 3R-type, not more common 2H-type, as verified by microscopic and spectroscopic characterization. As expected, the 3R bilayer showed a significantly higher valley polarization compared with the 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, the 3R bilayer showed even higher valley polarization compared with the monolayer counterpart. Moreover, the 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 the monolayer. The improved valley polarization of the 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.

The Intrinsic Thermodynamic Difficulty and a Step‐Guided Mechanism for the Epitaxial Growth of Uniform Multilayer MoS2 with Controllable Thickness

“…[26] Presently, the growth of multilayer MoS 2 films has commonly been achieved through the layer-by-layer growth mode via chemical vapor deposition (CVD) methods. [16,27,28] However, the multilayer MoS 2 films prepared by this route show poor uniformity on the thickness and domain size. This is attributed to that the nucleation of top-layer has to be occurred on the surface of previously grown bottom MoS 2 films, making it difficult to control the thickness and domain size of grown MoS 2 domain.…”

Recent Progress in Phase Regulation, Functionalization, and Biosensing Applications of Polyphase MoS2

“…Reproduced with permission. [ 24 ] Copyright 2021, American Chemical Society. C) Bandgaps of 1T, 1T′, and 1T″′.…”

“…Unlike 2H‐MoS 2 , the optical absorption energy difference ( E B − E A ) in 3R‐MoS 2 does not change with layer numbers (Figure 3E,F), because the interlayer waves are decoupled. [ 24 ]…”

“…Therefore, the change in the position of peak versus layer numbers was not obvious, and only a slight blueshift occurred as the thickness decreased (Figure 3H). [ 24 ] This property makes SEMI‐MoS 2 an important material in the field of photodetectors.…”

“…Unlike 2H-MoS 2 , the optical absorption energy difference (E B − E A ) in 3R-MoS 2 does not change with layer numbers (Figure 3E,F), because the interlayer waves are decoupled. [24] When bulk MoS 2 is peeled off to monolayer or few layers, a strong photoluminescence (PL) phenomenon emerges, indicating the transition of indirect bandgap to direct in d-electron system. [39] According to relevant research results, the fluorescence quantum production efficiency of monolayer 2H-MoS 2 is four orders of magnitude higher than that of its bulk.…”