Exploring the valleytronic, optical, and piezoelectric properties of Janus MoBXY2 (X = N, P; Y = S, Se, Te) monolayers for multifunctional applications

Abstract

Inspired by recent studies on MoS₂ and MoSi₂N₄, we propose and investigate Janus MoBXY₂ (X = N, P; Y = S, Se, Te) monolayers, which exhibit robust dynamic and thermal stabilities. Our findings reveal that all these monolayers are semiconductors, with MoBNS₂ and MoBPTe₂ exhibiting direct band gaps at the K/K′ points, resulting in degenerate valleys and significant valley spin splitting (VSS) in the valence band. Notably, Berry curvatures at K and K′ points, with opposite signs, suggest potential for inducing the valley Hall effect (VHE). Furthermore, MoBNS₂ and MoBPTe₂ demonstrate pronounced optical absorption in the visible light region and high carrier mobility, especially MoBNS₂ with a hole mobility reaching up to 2.0 × 10³ cm² V⁻¹ s⁻¹ along the zig-zag direction. Attributed to their Janus structure, these monolayers exhibit strong in-plane and out-of-plane piezoelectric responses, with d₁₁ values ranging from 1.268 to 4.754 pm V⁻¹ and d₃₁ values ranging from 0.053 to 0.137 pm V⁻¹. Investigation of strain effects highlights possibilities for indirect-to-direct band gap transitions and manipulation of band gaps and VSS. This study not only enriches the understanding of MoBXY₂ monolayer properties but also suggests their promising applications in valleytronics, photovoltaics, and piezoelectric devices.