Monolayer and bilayer BP as efficient optoelectronic materials in visible and ultraviolet regions

• Phonon dispersion of bilayer BP is similar to monolayer but with out-of-plane mode splitting. • Phonon band gap of monolayer and bilayer BP decreases with increasing tensile strain. • Monolayer and bilayer BP show dynamical stability under tensile strain. • Bilayer BP shows lower absorption edge than monolayer. • Compressive strain increases absorption intensity and width in monolayer and bilayer BP. Hexagonal boron phosphide (BP) shows significant promise for optoelectronic applications due to its high carrier mobility and moderate band gap. Strain inevitably occurs during the synthesis of two-dimensional (2D) nanomaterials. In this work, a detailed study on the strain-dependent phonon dispersion and optical properties of monolayer and bilayer BP was performed. The calculated phonon dispersion curves demonstrate that both monolayer and bilayer BP systems remain dynamically stable under tensile strain. By increasing tensile strain, the LO and TO modes soften and the phonon band gap between optical and acoustic modes becomes narrower. The LO and TO modes display expanded phonon spectra under large tensile strains in contrast to the ZA mode. The absorption edge of bilayer BP is located around 0.6 eV, lower in energy with respect to the monolayer BP at ∼ 0.8 eV. The results indicate that the optical absorption may be enhanced by applying the compressive strain. With their broad absorption range and strain-tunable absorption strength and phononic properties, monolayer and bilayer BP are highly promising for next-generation nanodevices.