Electrical control of the chemical bonding of fluorine on graphene

We study the electronic structure of diluted F atoms chemisorbed on graphene using density functional theory calculations. We show that the nature of the chemical bonding of a F atom adsorbed on top of a C atom in graphene strongly depends on carrier doping. In neutral samples the F impurities induce a ${\mathit{sp}}^3$-like bonding of the C atom below, generating a local distortion of the hexagonal lattice. As the graphene is electron-doped, the C atom retracts back to the graphene plane and for high doping (${10}^{14}$ cm${}^{-2}$) its electronic structure corresponds to a nearly pure ${\mathit{sp}}^2$ configuration. We interpret this ${\mathit{sp}}^3$-${\mathit{sp}}^2$ doping-induced crossover in terms of a simple tight-binding model and discuss the physical consequences of this change.

Figure 1

(a) Unit cell used in our DFT calculations (60 C atoms). (b) schematic lateral view of the defect showing the local puckering of the graphene sheet caused by the adatom. Partial density of states (PDOS) for the undoped case projected on the F atom (c), the ${\mathrm{C}}_0$ atom (d), a ${\mathrm{C}}_0$-nearest neighbor C atom, ${\mathrm{C}}_n$ (e), and a “bulk” ${\mathrm{C}}_{\text{b}}$ atom (f).

Figure 2

PDOS for electron-doped graphene with one (a),(b) and two (c),(d) additional electrons per DFT unit cell. Labels are the same as in Fig. 1. Notice the reduction of the hybridization between the F atom and the graphene sheet as the doping increases.

Figure 3

Partial density of states projected onto the $p_z$ atomic orbitals for the F, ${\mathrm{C}}_0$, and ${\mathrm{C}}_n$ atoms for undoped graphene (top panels) and for the electron doped ($+2$) case (bottom panels). The thick lines correspond to the DFT results while the thin lines are the results of the tight-binding model with the parameters discussed in the text. The unit cell contains $60$ C atoms in both calculations.

Figure 4

Occupation of the F, ${\mathrm{C}}_0$, and ${\mathrm{C}}_n$ atoms for a simplified tight-binding model in the ${\mathit{sp}}^3$ and ${\mathit{sp}}^2$ configurations, as a function of the carrier density. Note that the charge of the F atom is higher in the ${\mathit{sp}}^2$ configuration.