Stone-Wales defects in silicene: Formation, stability, and reactivity of defect sites

During the synthesis of ultrathin materials with hexagonal lattice structure Stone-Wales (SW) type of defects are quite likely to be formed and the existence of such topological defects in the graphenelike structures results in dramatic changes of their electronic and mechanical properties. Here we investigate the formation and reactivity of such SW defects in silicene. We report the energy barrier for the formation of SW defects in freestanding ($\sim$2.4 eV) and Ag(111)-supported ($\sim$2.8 eV) silicene and found it to be significantly lower than in graphene ($\sim$9.2 eV). Moreover, the buckled nature of silicene provides a large energy barrier for the healing of the SW defect and therefore defective silicene is stable even at high temperatures. Silicene with SW defects is semiconducting with a direct band gap of 0.02 eV and this value depends on the concentration of defects. Furthermore, nitrogen substitution in SW-defected silicene shows that the defect lattice sites are the least preferable substitution locations for the N atoms. Our findings show the easy formation of SW defects in silicene and also provide a guideline for band gap engineering in silicene-based materials through such defects.

Figure 1

The top and side view of (a) perfect and (b) SW-defected silicene. (c) Energetics of formation of a SW defect in silicene via the bond rotation (highlighted). The black (open squares) curve is obtained by relaxing the first-nearest hexagons to the silicene dimer in freestanding silicene layer. Red (open circles) and blue (open triangles) curves are obtained by relaxing up to the second-nearest hexagons to the silicon dimer in the freestanding and supported silicene [on a Ag(111) surface], respectively. (d) The actual bond rotation steps (0${}^{\circ }$, 45${}^{\circ }$, and 90${}^{\circ }$) in silicene supported on the Ag(111) surface. The uppermost and lower-lying Ag atoms are shown by gray (light) and black (dark).

Figure 2

Phonon dispersion of pure (dashed line) and SW-defected silicene (filled curve). Atomic motions for two phonon modes, which have imaginary eigenfrequencies, are shown on the right. Only the motions of atoms in defect core and neighboring atoms are shown for clarity. Blue and red arrows represent displacements of Si atoms along $-z$ and $+z$ directions, respectively.

Figure 3

The electronic band structure for a 6$\times$6 supercell of (a) perfect and (b) SW-defected silicene. (c) Band decomposed charge densities of the conduction band around the Fermi level at $\Gamma$, $K$, and $M$ symmetry points (the isosurface is set to $0.57\times {10}^{-3}$ e/Å${}^3$)

Figure 4

(a) The electronic band structure of N-doped silicene, (b) top and side view of the structure. (c) 3D and contour plot of difference charge density (${\rho }_{\uparrow }-{\rho }_{\downarrow }$). The slicing plane is marked by dot-dashed line in (b).

Figure 5

The substitutional sites of the SW defect in silicene with associated letter, and the side view of the same structure.