Graphene-silicene bilayer: A nanocapacitor with permanent dipole and piezoelectricity effect

Using density functional theory, we study the electronic properties of a graphene-silicene bilayer (GSB). A single layer of silicene binds to the graphene layer with adhesion energy of about 25 meV/atom. This adhesion energy between the two layers follows accurately the well-known $-1/z^2$ dispersion energy as found between two infinite parallel plates. In small flakes of GSB with hydrogenated edges, negative charge is transferred from the graphene layer to the silicene layer, producing a permanent and a switchable polar bilayer, while in an infinite GSB, the negative charge is transferred from the silicene layer to the graphene layer. The graphene-silicene bilayer is a good candidate for a nanocapacitor with piezoelectric capabilities. We found that the permanent dipole of the bilayer can be tuned by an external perpendicular electric field.

Figure 1

(a), (b) Top and side view of the optimized graphene-silicene bilayer (the white atoms at the edges refer to the H atoms). (c), (d) Side and top view of the portion inside the red rectangle of panel (a). The thick solid black lines in panel (d) indicate the moiré pattern and the red or blue circles are guides to the eye in order to see more clearly the stacked structure. The large (small) dots refer to Si (C) atoms.

Figure 2

(a) Side and the top view of the unit cell used for the periodic boundary calculations. (b) The corresponding band structure. The valance and conductance bands are shown by blue and red curves, respectively.

Figure 3

Frontier molecular orbitals in GSB. (a) HOMO, (b) LUMO, and (c) dipole moment in the optimized flake of GSB.

Figure 4

The variation of the binding energy of the graphene-silicene bilayer with interlayer distance. Solid line is the best fit to $A/z^9-B/z^2$.

Figure 5

The schematic model for a flake of GSB in the presence of an upward external electric field.

Figure 6

The variation of (a) electric dipole moment, (b) transferred charge, and (c) interlayer distance with applied electric field perpendicular to the GSB layers (small flake).

Figure 7

(a) The variation of the induced dipole with the interlayer distance between the graphene and silicene layers. (b) The nonlinear variation of the force versus the dipole moment. The inset shows the variation between force and distance.