Strain-induced anomalous Raman shifting in ferroelectric NbOI2

NbOI2 exhibits a notable in-plane anisotropic ferroelectric property and a pronounced second-order nonlinear optical response, which has great potential applications in nanoelectronics. However, the collective behavior of lattice dynamics, which represents the atomic origin of emerging novel phenomena, has not been systematically explored, especially in the context of external strain. In this study, we investigate the phonon responses of NbOI₂ under both uniaxial and biaxial tensile strains, applied along two distinct crystallographic orientations. The characteristic P5(LO) Raman mode, which involves in-plane vibrations primarily of Nb-O atoms, demonstrates opposite frequency shifts, with anomalously blue- or redshifts observed when uniaxial tensile strain is applied parallel or perpendicular to the polar axis, respectively. Biaxial strain consistently induced a blueshift in the Raman mode, distinct from that of many conventional two-dimensional materials. These experimental observations are supported by density-functional theory calculations, which reveal the strain-induced deviations in the Nb atomic position-induced pronounced blueshift. This study highlights the distinctive strain sensitivity of NbOI2, offering a useful feedback mechanism for strain manipulation, and providing insights into the potential of NbOI₂ for applications in flexible optoelectronics and strain sensors.