Intrinsic ferroelectricity and large bulk photovoltaic effect in novel two-dimensional buckled honeycomb-like lattice of NbP: first-principles study

Using first-principles calculations, we predict that the two-dimensional (2D) monolayers of NbP with the buckled honeycomb-like and puckered tetragonal structure can be obtained from the (110) and (001) orientations, respectively, of its bulk crystal structure. The electronic properties of these monolayers are spectacularly different as tetragonal lattice is metallic whereas the honeycomb-like lattice (h-NbP) is a semiconductor and exhibits intrinsic ferroelectricity originating from a rare sd (2)-sp (2) hybridization. The shift current bulk photovoltaic effect (BPVE) is systematically investigated in the h-NbP monolayer (1.21 angstrom thickness) using the Wannier interpolation method. Strong absorption of visible light at similar to 2 eV and a large 3D shift current of similar to 180 mu A V-2 is obtained which is attributed to the partial delocalization of Bloch states due to sd (2)-sp (2) hybridization. We compare the shift current response of h-NbP monolayer with that of some previously reported bulk ferroelectrics and 2D monolayers, suggesting that h-NbP monolayer can yield a large shift current at an ultimate thickness and is a promising 2D material for the BPVE application under the visible light. Strain effect is also investigated, revealing that the h-NbP monolayer is dynamically stable up to a strain limit of +/- 3%, and the shift current increases by similar to 9% at a compressive strain of -3% as the Bloch states are more delocalized due to the strengthening of sd (2)-sp (2) hybridization. The results presented in this study can pave the paths to fabricate the 2D monolayered structures of NbP, and realize the BPVE based next-generation solar cells of h-NbP monolayer.