Uniaxial strain-induced giant Raman enhancement in few-layer γ-InSe

Abstract

InSe is a promising two-dimensional van der Waals semiconductor, exhibiting exceptional plasticity in its bulk single-crystalline form. This notable mechanical behavior originates from interlayer gliding and cross-layer dislocation slip, facilitated by the synergistic interplay between long-range In–Se Coulomb interactions spanning the van der Waals gap and soft intralayer In–Se bonding. To investigate anisotropic responses under strain, few-layer γ-InSe flakes on gold-coated transparent flexible PET substrates were subjected to ∼1.74% uniaxial tensile strain via substrate stretching, applied independently along both the armchair and zigzag crystallographic directions of γ-InSe. All Raman spectra of strained few-layer γ-InSe exhibit red shifts due to In–Se bond elongation, with distinct strain sensitivities observed across phonon modes [A1(Γ13) mode shows a shift rate of −0.6 cm−1/%, while the A1(Γ11), A1(Γ12), and E(Γ33) modes exhibit lower sensitivity at around −0.2 cm−1/%]. Under uniaxial strain, the absolute intensity of the dominant Raman peaks decreases significantly, whereas the relative intensity ratio of the emerging peaks at 147 and 155 cm−1 to the strongest peak (115 cm–1) shows a marked increase. Furthermore, angle-resolved polarized Raman spectroscopy (ARPRS) reveals a characteristic four-lobed pattern in the angular intensity distribution of γ-InSe, consistent with in-plane phonon anisotropy under C3v symmetry. This characteristic enables determination of the in-plane crystalline orientation through combined ARPRS measurements and group theory calculations. Our results demonstrate the suitability of the flexible substrate for such strain processes and establish Raman spectroscopy as an ultrasensitive method for strain determination in γ-InSe.