First-principles study of metal adatom adsorption on graphene

The adsorption of 12 different metal adatoms on graphene is studied using first-principles density-functional theory with the generalized gradient approximation. The adsorption energy, geometry, density of states (DOS), dipole moment, and work function of each adatom-graphene system are calculated. For the adatoms studied from groups I–III of the Periodic Table, the results are consistent with ionic bonding, and the adsorption is characterized by minimal change in the graphene electronic states and large charge transfer. For transition, noble, and group IV metals, the calculations are consistent with covalent bonding, and the adsorption is characterized by strong hybridization between adatom and graphene electronic states. For ionically bonded adatoms, the charge transfer is calculated quantitatively using two methods, one based on the DOS and the other based on the real-space-charge density. A variation in dipole moments and work-function shifts across the different adatoms is observed. In particular, the work-function shift shows a general correlation with the induced interfacial dipole of the adatom-graphene system and the ionization potential of the isolated atom.

Figure 1

(Color online) Adatom on the hollow site in a periodic $4\times 4$ arrangement on a graphene surface.

Figure 2

(Color online) The three adsorption sites considered: hollow $\left(H\right)$, bridge $\left(B\right)$, and top $\left(T\right)$.

Figure 3

(Color online) Spin-up (top) and spin-down (bottom) total DOS for K on the $H$ site of graphene and for isolated graphene. The energy is relative to $E_F$ of the K-graphene system. The curves are aligned at the Dirac point.

Figure 4

(Color online) Spin-up (top) and spin-down (bottom) total DOS, PDOS on the graphene states, and PDOS on the $\text{K}s$ states for K on the $H$ site of graphene. The energy is relative to $E_F$.

Figure 5

(Color online) Spin-up total DOS, PDOS on the graphene states, and PDOS on the $\text{Al}s$ and $p$ states for Al on the $H$ site of graphene. Spin-up and spin-down states are degenerate. The energy is relative to $E_F$.

Figure 6

(Color online) Spin-up (top) and spin-down (bottom) total DOS, PDOS on the graphene states, and PDOS on the $\text{Ti}s$, $p$, and $d$ states for Ti on the $H$ site of graphene. The energy is relative to $E_F$.

Figure 7

(Color online) Spin-up (top) and spin-down (bottom) total DOS, PDOS on the graphene states, and PDOS on the $\text{Fe}s$, $p$, and $d$ states for Fe on the $H$ site of graphene. The energy is relative to $E_F$.

Figure 8

(Color online) Spin up total DOS, PDOS on the graphene states, and PDOS on the $\text{Pd}s$, $p$, and $d$ states for Pd on the $B$ site of graphene. Spin-up and spin-down states are degenerate. The energy is relative to $E_F$.

Figure 9

(Color online) The $x\text{−}y$ planar-averaged electron charge-density difference $\left(\Delta {\rho }_{\text{av}}\right)$ for K on graphene $H$ site as a function of position in the $z$ direction. Two vertical lines indicate the position of the graphene sheet $\left(0\text{Å}\right)$ and the position of the adatom $\left(1.64\text{Å}\right)$. The arrow at $1.64\text{Å}$ indicates $R_{\text{cut}}$ used in calculating $\Delta q_{\rho }$.

Figure 10

(Color online) Plot of the work-function shift $\left(\Delta \Phi \right)$ relative to isolated graphene versus dipole moment $\left(p\right)$ for the $4\times 4$ adatom coverage. The red squares are elements from groups I–III; other elements are marked by green circles. The dotted blue line is given by Eq. (7).

Figure 11

(Color online) Plot of calculated work-function shift $\left(\Delta \Phi \right)$ for the $4\times 4$ coverage versus IP from experiment (Ref. 46). The dotted line is given by Eq. (8).