Abstract
Two-dimensional (2D) crystals exhibit unique and exceptional properties and show promise for various applications. In this work, we systematically studied the structures of a 2D boronphosphide (BP) monolayer with different stoichiometric ratios (BPx, x = 1, 2, 3, 4, 5, 6 and 7) and observed that each compound had a stable 2D structure with metallic or semiconducting electronic properties. Surprisingly, for the BP5 compounds, we discovered a rare penta-graphene-like 2D structure with a tetragonal lattice. This monolayer was a semiconductor with a quasi-direct band gap of 2.68 eV. More importantly, investigation of the strain effect revealed that small uniaxial strain can trigger the band gap of the penta-BP5 monolayer to transition from a quasi-direct to direct band gap, whereas moderate biaxial strain can cause the penta-BP5 to transform from a semiconductor into a metal, indicating the great potential of this material for nanoelectronic device applications based on strain-engineering techniques. The wide and tuneable band gap of monolayer penta-BP5 makes it more advantageous for high-frequency-response optoelectronic materials than the currently popular 2D systems, such as transition metal dichalcogenides and black phosphorus. These unique structural and electronic properties of 2D BP sheets make them promising for many potential applications in future nanodevices.
Highlights
Two-dimensional (2D) materials exhibit fascinating electronic properties and show great potential for various applications, such as electronics, optoelectronics and solar cells
transition metal dichalcogenides (TMDCs) and black phosphorus have disadvantages; for example, their band gaps are mostly less than 2.0 eV, which results in their failure to respond to photons with wavelengths less than 620 nm, such as blue and ultraviolet (UV) light-emitting diodes (LEDs) and photodetectors[15]
We systematically studied the structures of 2D BP monolayers with different stoichiometries, including BP, BP2, BP3, BP4, BP5, BP6 and BP7, using particle swarm optimisation (PSO) combined with ab initio molecular dynamics (MD) calculations
Summary
Two-dimensional (2D) materials exhibit fascinating electronic properties and show great potential for various applications, such as electronics, optoelectronics and solar cells. The corresponding monolayer structures have been synthesised experimentally, and all of them exhibit excellent properties different from those of their bulk counterparts[6, 9,10,11,12]. In some semiconducting TMDCs, the bulk material usually has an indirect band gap, whereas the corresponding monolayer has a direct band gap. TMDCs and black phosphorus have disadvantages; for example, their band gaps are mostly less than 2.0 eV, which results in their failure to respond to photons with wavelengths less than 620 nm, such as blue and ultraviolet (UV) light-emitting diodes (LEDs) and photodetectors[15] Another approach to further open the band gap of a structure involves modifying its basic configuration because the properties are closely related to the structural configurations. The 2D materials with ratios of 1:3, 1:6, and 1:7 exhibited semiconducting properties with indirect band gaps of 0.8–2 eV, whereas the 2D materials with 1:2 and 1:4 ratios exhibited metallic properties
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