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.

Abstract2D semiconducting transition metal dichalcogenides (TMDCs), most with a formula of MX2 (M=Mo, W; X=S, Se, etc.), have emerged as promising channel materials for next‐generation integrated circuits, considering their dangling‐bond‐free surfaces, moderate bandgaps, relatively high carrier mobilities, etc. Wafer‐scale preparation of 2D MX2 films holds fundamental significance for realizing their applications. Chemical vapor deposition (CVD) is recognized as the most promising method for preparing electronic‐grade 2D MX2 films. This review hereby summarizes the recent progress in CVD syntheses of wafer‐scale 2D MX2 films and their applications in logic operations, data storage, and image capturing/processing related fields. The first part focuses on the wafer‐scale syntheses of 2D MX2 films through designing homogeneous metal precursor supply routes (e.g., precoating soluble precursor, feeding gaseous precursor, designing independent multisource supply or face‐to‐face metal precursor supply routes). The second part highlights the epitaxial growth of monolayer MX2 single crystals on single‐crystal Au substrates and well‐designed sapphire substrates. The third part introduces various functional device/circuit related applications of CVD‐derived 2D MX2 wafers. Finally, challenges and prospects are discussed from the viewpoints of the controlled synthesis, reliable characterization, and damage‐free transfer of 2D MX2, as well as the fabrication and integration of high‐performance devices.

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