Single Layer Bismuth Iodide: Computational Exploration of Structural, Electrical, Mechanical and Optical Properties

Abstract

Layered graphitic materials exhibit new intriguing electronic structure and the search for new types of two-dimensional (2D) monolayer is of importance for the fabrication of next generation miniature electronic and optoelectronic devices. By means of density functional theory (DFT) computations, we investigated in detail the structural, electronic, mechanical and optical properties of the single-layer bismuth iodide (BiI3) nanosheet. Monolayer BiI3 is dynamically stable as confirmed by the computed phonon spectrum. The cleavage energy (Ecl) and interlayer coupling strength of bulk BiI3 are comparable to the experimental values of graphite, which indicates that the exfoliation of BiI3 is highly feasible. The obtained stress-strain curve shows that the BiI3 nanosheet is a brittle material with a breaking strain of 13%. The BiI3 monolayer has an indirect band gap of 1.57 eV with spin orbit coupling (SOC), indicating its potential application for solar cells. Furthermore, the band gap of BiI3 monolayer can be modulated by biaxial strain. Most interestingly, interfacing electrically active graphene with monolayer BiI3 nanosheet leads to enhanced light absorption compared to that in pure monolayer BiI3 nanosheet, highlighting its great potential applications in photonics and photovoltaic solar cells.

Figure 1

(a) Side view of BiI₃ bulk crystal; (b) top and (c) side views of BiI₃ nanosheet; red and green ball represent the iodine and bismuth atoms, respectively; (d) top and (e) side views of Iso-surface (0.045 ev/au³) for electronic density of monolayer BiI₃.

Figure 2

(a) Phonon dispersion of BiI₃ monolayer; (b) cleavage energy in J/m² (blue line) and its derivative σ in GPa (red line) as a function of the separation distance d for a fracture in BiI₃ monolayer. Inset: Separating a monolayer from its neighbouring tri-layer.

Figure 3

Band structure and total DOS of BiI₃ monolayer (a) without SOC; (b) with SOC. Inset is 2D Brillouin zone.

Figure 4

(a) Stress-stain curve under biaxial strain. Inset: top view of monolayer BiI₃ and the directions of strain; (b) The band gap at different strain without SOC (red dashed line) and with SOC (blue dashed line); insert: band gap as a function of strain for mono-, bi- and tri- layer BiI₃.

Figure 5

(a) The imaginary part of dielectric function ε₂(ω) of BiI₃ monolayer without SOC (red line) and with SOC (blue line); (b) ε₂(ω) of BiI₃ monolayer (red line) and the graphene/BiI₃ composite (blue line).