Strain direction dependent optical bandgap modulation and negative Poisson's ratio in layered violet phosphorus

Two-dimensional layered materials, especially structures with a negative Poisson's ratio (NPR), provide an ideal platform for engineering optical properties through strain control because of their extremely high mechanical elasticity and sensitive dependence of material properties on mechanical strain. Violet phosphorus (VP) exhibits intrinsic NPR and anisotropic optical properties arising from its unique crystalline structure, which is suitable for strain monitoring applications via photoluminescence (PL) spectroscopy. In this paper, a combined experimental and theoretical effort is made to investigate the effects of mechanical strain along different crystallographic orientations on various spectral features of VP PL. It is found that VP exhibits anomalous strain-dependent bandgap increase, showing anisotropic modulation rates of +16.60 meV/% (a-axis) and +10.87 meV/% (b-axis). The abnormal phenomenon could be correlated with electronic band structure based on first-principles calculations. The bandgap increase results from reduced conduction band wavefunctions with the increase of strain. More importantly, the NPR of VP under strain increases its average interlayer spacing, where reduced interlayer interactions drive the monotonic optical bandgap modulation. These findings provide essential insights into the strain-induced optical tunability of VP nanosheets, paving the way for advanced photonic and optoelectronic applications.