New two-dimensional anisotropic monolayers of phosphide binary compounds with wide fundamental bandgaps

The newly discovered 2D GaP and InP layers have unique anisotropic structural, electronic, and mechanical properties. Their crystalline structures show orthorhombic lattices symmetry and high buckling of 2.14 Å-2.46 Å. The speeds of sound in their phono dispersions along the Γ-Y direction are higher than those along the Γ-X direction, reflecting anisotropy in their elastic constants which were further confirmed from their strong directional dependence of Young’s moduli. They have wide fundamental bandgaps (2.89 eV for GaP and 2.59 eV for InP, respectively), which were found to be tunable under the strain either along armchair or zigzag directions. A direct-indirect bandgap transition was found under certain strains along zigzag or biaxial orientations, reflecting their promising applications for the strain-induced bandgap engineering in nanoelectronics and photovoltaics.