2D boron dichalcogenides from the substitution of Mo with ionic B2 pair in MoX2 (X = S, Se and Te): high stability, large excitonic effect and high charge carrier mobility

Abstract

Two-dimensional (2D) transition metal dichalcogenides are regarded as promising candidates for nanoelectronic devices, due to their novel electronic properties. Motivated by the similarity of valence electrons between Mo and B₂ pairs, we design a new type of 2D MoS₂-like material, i.e. boron dichalcogenides, through the global minimization search and density functional theory methods. Free standing trigonal and hexagonal phase boron dichalcogenide (B₂X₂, X = S, Se and Te) monolayers are predicted to be highly stable. The T-B₂X₂ and H-B₂X₂ monolayers are all semiconducting with indirect bandgaps ranging from 2.14 eV to 4.01 eV and large excitonic effects. Particularly, H-B₂X₂ exhibits high carrier mobility of up to 6.23 × 10⁵ cm² (V⁻¹ s⁻¹), which can be comparable to that of graphene. Therefore, 2D boron dichalcogenides have great potential for applications in high-performance flexible field-effect transistors and light emitters. Furthermore, the optical spectrum of these monolayers reveals that the absorption is in the ultraviolet region, suggesting future applications in ultraviolet optoelectronic devices.