Theoretical investigations of novel Janus Pb2SSe monolayer as a potential multifunctional material for piezoelectric, photovoltaic, and thermoelectric applications

Abstract

Two-dimensional Janus nanomaterials, due to their unique electronic, optical, and piezoelectric characteristics resulting from the antisymmetric structures, exhibit great prospects in multifunctional energy application to alleviate the energy crisis. Monolayer Janus Pb₂SSe, with a black phosphorus-like structure and an indirect band gap of 1.59 eV as well as high carrier mobility (526–2105 cm² V⁻¹ s⁻¹), displays outstanding potentials in the energy conversion between nanomechanical energy, solar energy, waste heat, and electricity, which has been comprehensively studied utilizing DFT-based simulations. The research results reveal that monolayer Pb₂SSe not only possesses giant in-plane piezoelectricity of d₁₁ = 75.1 pm V⁻¹ but also superhigh out-of-plane piezoelectric coefficients (d₃₁ = 0.5 pm V⁻¹ and d₃₃ = 15.7 pm V⁻¹). Meanwhile, by constructing Pb₂SSe bilayers, the out-of-plane piezoelectric coefficients can be significantly enhanced (d₃₁ = 19.2 pm V⁻¹ and d₃₃ = 194.7 pm V⁻¹). In addition, owing to the small conduction band offset, suitable donor band gap and excellent light absorption capability in the Pb₂SSe/SnSe (Pb₂SSe/GeSe) heterostructure, the power conversion efficiencies were calculated to be up to 20.02% (Pb₂SSe/SnSe) and 19.28% (Pb₂SSe/GeSe), making it a promising candidate for solar energy collection. Furthermore, from the thermoelectric electron and phonon transport calculations, it can be found that the Pb₂SSe monolayer is an n-type thermoelectric material with ultrahigh ZT = 2.19 (1.52) at room temperature, which can be traced back to its ultralow κ_L = 0.78 (0.99) W m⁻¹ K⁻¹, and superhigh PF = 10.18 (8.25) mW m⁻¹ K⁻² along the x(y) direction at the optimal doping concentration at 300 K. The abovementioned versatile characteristics in the Janus Pb₂SSe monolayer, along with its comprehensive stabilities (energy, dynamic, thermal, and mechanical stabilities), highlight its potential in clean energy harvesting.