Water‐Assisted Growth of Twisted 3R‐Stacked MoSe₂ Spirals and Its Dramatically Enhanced Second Harmonic Generations

Abstract

Enhanced second‐harmonic generation (SHG) responses are reported in monolayer transition metal dichalcogenides (e.g., MX2, M: Mo, W; X: S, Se) due to the broken symmetries. The 3R‐like stacked MX2 spiral structures possessing the similar broken inversion symmetry should present dramatically enhanced SHG responses, thus providing great flexibility in designing miniaturized on‐chip nonlinear optical devices. To achieve this, the first direct synthesis of twisted 3R‐stacked chiral molybdenum diselenide (MoSe2) spiral structures with specific screw dislocations (SD) arms is reported, via designing a water‐assisted chemical vapor transport (CVT) approach. The study also clarifies the formation mechanism of the MoSe2 spiral structures, by precisely regulating the precursor supply accompanying with multiscale characterizations. Significantly, an up to three orders of magnitude enhancement of the SHG responses in twisted 3R stacked MoSe2 spirals is demonstrated, which is proposed to arise from the synergistic effects of broken inversion symmetry, strong light–matter interaction, and band nesting effects. Briefly, the work provides an efficient synthetic route for achieving the 3R‐stacked TMDCs spirals, which can serve as perfect platforms for promoting their applications in on‐chip nonlinear optical devices.

Nonlinear Optical Response Modulated by Lattice Reconstruction in Small‐Angle Twisted Bilayer MoS2

“…The components of the SHG spectra are intricately linked to these multiple leaps; for example, certain optical leaps in some components of the C exciton evoke a stronger nonlinear optical response than others. [56,57] These transitions occur at distinct k points encircling the Г point. These k points bear significance as they embody regions where the self-selected orbital splitting within the valence band is minimal, and the spin-orbit splitting dwindles to zero as the k point converges with the Г point.…”

Toward Edge Engineering of Two-Dimensional Layered Transition-Metal Dichalcogenides by Chemical Vapor Deposition

“…The 3D edges of the TMD layers primarily originate from their corresponding 3D spirals and 1D nanotubes. The controllable growth of 3D spiral domains is achieved via screw-dislocation-driven (SDD) growth mechanism, which has been successfully employed in synthesizing various layered spiral materials using different fabrication techniques. ,, For example, MoS 2 , , WS 2 , WSe 2 , MoTe 2 , graphene, BN, and SnS have been synthesized via CVD. Additionally, aqueous solution synthesis has also been utilized to fabricate spiral conjugated block copolymers, CoAl layered double hydroxide, and Ni­(OH) 2 .…”