Water-Assisted Growth of Twisted 3R-Stacked MoSe 2 Spirals and Its Dramatically Enhanced Second Harmonic Generations.

Enhanced second-harmonic generation (SHG) responses are reported in monolayer transition metal dichalcogenides (e.g., MX 2 , M: Mo, W; X: S, Se) due to the broken symmetries. The 3R-like stacked MX 2 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 (MoSe 2 ) 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 MoSe 2 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 MoSe 2 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.