Anisotropic phonon responses of 2D NbOX₂ (X=Cl, Br, I) under uniaxial tensile strain

The van der Waals crystal NbOX₂ (X = Cl, Br, I) has recently attracted much attention due to its remarkable in-plane anisotropy and substantial second-order nonlinear optical response. Moreover, the importance of modulating its physical properties through strain has become increasingly prominent. Herein, we investigate the anisotropic phonon response of NbOX₂ along various crystallographic directions under uniaxial tensile strain by Raman spectroscopy. The results show that opposing frequency shifts manifest in the Raman modes when uniaxial tensile strain is applied either parallel or perpendicular to the polar axis, and the highest frequency peak, P5, shows significant blue or red shifts. Density functional theory calculation results are consistent with the observed Raman shifts under strain. The change in Nb-O bond length is the main reason for the significant blue shift of P5. The anisotropic and pronounced frequency shifts of NbOX₂ under strain can serve as sensitive indicators for strain modulation, offering insights for potential applications such as flexible optoelectronic devices and strain sensors.