Curvature-Induced Giant Second-Harmonic Generation in WS₂ Nanoscroll on a Metallic Film.

Abstract

Curvature in two-dimensional (2D) materials presents an effective approach for modulating their electronic structures and nonlinear optical properties; however, its influence on transition metal dichalcogenides (TMDs) remains insufficiently understood. In this study, we introduce a straightforward method to convert monolayer WS₂ into quasi-one-dimensional nanoscrolls via solvent-assisted rolling on metallic substrates, yielding a curved multilayer architecture characterized by pronounced strain gradients and symmetry breaking. The resulting WS₂ nanoscrolls demonstrate an enhancement in second-harmonic generation (SHG) exceeding 4 orders of magnitude (up to 1.4 × 10⁴) compared to monolayers, accompanied by significant amplification of Raman scattering signals. We attribute this substantial nonlinear optical response to the combined effects of curvature-induced inversion symmetry breaking, coherent interlayer coupling, and strain-mediated modulation of the nonlinear susceptibility. In addition, the metallic substrate significantly modifies the local electromagnetic environment through mirror-induced field enhancement and increased radiative decay channels, leading to enhanced local fields and improved emission efficiency, which further contribute to the observed signal amplification. Polarization-resolved SHG measurements reveal pronounced anisotropy consistent with strain-induced redistribution of nonlinear tensor components, while Raman and photoluminescence analyses confirm the coexistence of tensile and compressive strain, leading to phonon softening and bandgap narrowing. These findings highlight the important role of curvature in manipulating light-matter interactions in 2D materials and propose a generalizable strategy for engineering enhanced nonlinear responses in van der Waals systems beyond conventional planar configurations.