Chemical Functionalization of Silicene: Spontaneous Structural Transition and Exotic Electronic Properties

The use of newly discovered silicene for various optoelectronic applications depends largely on the possibility of controlling its electronic properties by chemical functionalization. To investigate this possibility, we systemically study the structural and electronic properties of chemically functionalized silicene by employing first-principles calculations combined with the cluster expansion approach. Interestingly, we find that chemically functionalized epitaxial silicene is generally accompanied by a spontaneous structural transition, which originates from the preference of $s{p}^3$ hybridization of silicon. To realized continuously tunable band gaps, chemical functionalization of freestanding silicene at $\sim 900\mathrm{K}$ is proposed. Finally, we predict that metastable silicene can also be used as an important host material to produce novel functional materials via substitutional doping. For example, the discovered ordered ${\mathrm{Si}}_8{\mathrm{P}}_4$ could be a strong candidate for thin-film solar cell absorbers beyond bulk Si.

Figure 1

(a) The calculated formation energies $E_f$ of epitaxial ${\mathrm{SiF}}_x$ along with the corresponding CE fits as a function of $x$. The $E_f$ of $\sim 6000$ symmetry-inequivalent structures calculated from CE are also plotted here. Inset: The top and side views of the structures of silicene and ${\mathrm{SiF}}_1$. (b) is the same as (a) but for epitaxial ${\mathrm{SiO}}_x$ and the $\sim 10000$ symmetry-inequivalent structures calculated from CE are shown. The top and side views of the structure of ${\mathrm{SiO}}_{0.5}$ are shown as an inset in Fig. 1b. The top and side views of the four discovered intermediate ground state structures for (c) ${\mathrm{Si}}_{12}{\mathrm{F}}_4$, (d) ${\mathrm{Si}}_6{\mathrm{F}}_3$ (e) ${\mathrm{Si}}_{12}{\mathrm{F}}_{11}$, and (f) ${\mathrm{Si}}_{14}{\mathrm{O}}_4$. In these structures, the blue, green, and red atoms represent the Si, F, and O, respectively. The three-bonded Si atoms are highlighted by orange color and the unit cells are marked by a dashed line.

Figure 2

The electronic band structures for these discovered ground states of epitaxial ${\mathrm{SiF}}_x$ and ${\mathrm{SiO}}_x$: (a) ${\mathrm{Si}}_{12}{\mathrm{F}}_4$, (b) ${\mathrm{Si}}_6{\mathrm{F}}_3$, (c) ${\mathrm{Si}}_{12}{\mathrm{F}}_{11}$, and (d) ${\mathrm{Si}}_{14}{\mathrm{O}}_4$. The positions of CBM and VBM are highlighted by red dots. The Fermi level is set to zero.

Figure 3

The MC-calculated phase diagrams of double-sided (a) ${\mathrm{SiF}}_x$, (b) ${\mathrm{SiH}}_x$, and (c) ${\mathrm{SiO}}_x$. The area of phase separation in these phase diagrams is also highlighted by shaded colors. The side views of double-sided ${\mathrm{SiF}}_1$, ${\mathrm{SiH}}_1$, and ${\mathrm{SiO}}_1$ are shown as insets in (a)–(c). The yellow, green, white, and red atoms represent Si, F, H, and O, respectively. (d) The calculated band gaps of double-sided ${\mathrm{SiF}}_x$ and ${\mathrm{SiH}}_x$ alloys as a function of F or H concentration.

Figure 4

(a) The calculated formation energies $E_f$ of ${\mathrm{Si}}_{1-x}{\mathrm{P}}_x$ along with the corresponding CE fits as a function of P concentration $x$. The $E_f$ of $\sim 17000$ symmetry-inequivalent structures calculated from CE are also plotted here. (b) The top and side view of the discovered ground state ${\mathrm{Si}}_8{\mathrm{P}}_4$. The blue and pink atoms represent Si and P, respectively. (c) The calculated band structure of ${\mathrm{Si}}_8{\mathrm{P}}_4$. (d) The calculated optical absorption (imaginary part of dielectric function) of ${\mathrm{Si}}_8{\mathrm{P}}_4$. The result of diamond Si is also plotted for comparison.