Remarkable anisotropic phonon response in uniaxially strained few-layer black phosphorus

Black phosphorus (BP) is a good candidate for studying strain effects on twodimensional (2D) materials beyond graphene and transition-metal dichalcogenides. This is because of its particular ability to sustain high strain and remarkably anisotropic mechanical properties resulting from its unique puckered structure. We here investigate the dependence of lattice vibrational frequencies on crystallographic orientations in uniaxially strained few-layer BP by in-situ strained Raman spectroscopy. The out-of-plane A_g¹ mode is sensitive to uniaxial strain along the near-armchair direction whereas the in-plane B_2g and A_g² modes are sensitive to strain in the near-zigzag direction. For uniaxial strains applied away from these directions, all three phonon modes are linearly redshifted. Our experimental observation is explained by the anisotropic influence of uniaxial tensile strain on structural properties of BP using density functional theory. This study demonstrates the possibility of selective tuning of in-plane and out-of-plane phonon modes in BP by uniaxial strain and makes strain engineering a promising avenue for extensively modulating the optical and mechanical properties of 2D materials.