A DFT study of bandgap tuning in chloro-fluoro silicene

Abstract

The structural, electronic and optical properties of silicene and its derivatives are investigated in the present work by employing density functional theory (DFT). The Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) is used as the exchange-correlation potential. Our results provide helpful insight for tailoring the band gap of silicene via functionalization of chlorine and fluorine. First, relaxation of all the materials is performed to obtain the appropriate structural parameters. Cl-Si showed the highest lattice parameter 4.31 Å value, while it also possesses the highest buckling of 0.73 Å among all the derivatives of silicene. We also study the electronic charge density, charge difference density and electrostatic potential, to check the bonding characteristics and charge transfer between Si-halides. The electronic properties, band structures and density of states (DOS) of all the materials are calculated using the PBE-GGA as well as the modified Becke-Johnson (mBJ) on PBE-GGA. Pristine silicene is found to have a negligibly small band gap but with the adsorption of chlorine and fluorine atoms, its band gap can be opened. The band gap of Cl-Si and F-Si is calculated to be 1.7 eV and 0.6 eV, respectively, while Cl-F-Si has a band gap of 1.1 eV. Moreover, the optical properties of silicene and its derivatives are explored, which includes dielectric constants ε₁ and ε₂, refractive indices n, extinction coefficients k, optical conductivity σ and absorption coefficients I. The calculated binding energies and phonon band structures confirm the stability of Cl-Si, Cl-F-Si, and F-Si. We also calculated the photocatalytic properties which show silicine has a good response to reduction, and the other materials to oxidation. A comparison of our current work to recent work in which graphene was functionalized with halides, is also presented and we observe that silicene is a much better alternative for graphene in terms of semiconductors and photovoltaics applications.

Fig. 1. Structure of Si, Cl–Si, F–Si and Cl–F–Si.

Fig. 2. Charge density of (a) Cl–Si, (b) Cl–F–Si and (c) F–Si.

Fig. 3. Isosurface plots of charge density difference of (a) Cl–Si, (b) Cl–F–Si and (c) F–Si.

Fig. 4. Electrostatic potential and work function of (a) Si, (b) Cl–Si, (c) Cl–F–Si and (d) F–Si.

Fig. 5. Phonon dispersion of (a) Si, (b) Cl–Si, (c) Cl–F–Si and (d) F–Si.

Fig. 6. Band structures of (a) Si, (b) Cl–Si, (c) F–Si and (d) Cl–F–Si.

Fig. 7. TDOS of (a) Si, (b) Cl–Si, (c) F–Si and (d) Cl–F–Si.

Fig. 8. PDOS of (a) Si, (b) Cl–Si, (c) F–Si and (d) Cl–F–Si.

Fig. 9. Optical properties of Si, (a) ε₁ and ε₂, (b) n and k, (c) σ and (d) I.

Fig. 10. Optical properties of Cl–Si, F–Si and Cl–F–Si, (a) ε₁, (b) ε₂ and (c) I.

Fig. 11. Photocatalytic properties of silicene and its derivatives.