Electronic and magnetic properties of $3d$ transition-metal atom adsorbed graphene and graphene nanoribbons

In this paper, we theoretically studied the electronic and magnetic properties of graphene and graphene nanoribbons functionalized by $3d$ transition-metal (TM) atoms. The binding energies and electronic and magnetic properties were investigated for the cases where TM atoms adsorbed to a single side and double sides of graphene. We found that $3d$ TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 and 1.95 eV depending on their species and coverage density. Upon TM atom adsorption, graphene becomes a magnetic metal. TM atoms can also be adsorbed on graphene nanoribbons with armchair edge shapes (AGNR’s). Binding of TM atoms to the edge hexagons of AGNR yields the minimum energy state for all TM atom species examined in this work and in all ribbon widths under consideration. Depending on the ribbon width and adsorbed TM atom species, AGNR, which is a nonmagnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or antiferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNR’s half-metallic with a 100% spin polarization at the Fermi level. Present results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of $3d$ TM atoms.

Figure 1

(Color online) (a) A $\left(2\times 2\right)$ cell of graphene and the six possible adsorption sites for the TM atoms. B1, H1, and T1 are the single-sided adsorption sites, whereas B2, H2, and T2 are the possible additional sites for the double-sided adsorption. (b) A $\left(4\times 4\right)$ cell of graphene and the four sites we consider for the adsorption of the second TM atom from below when the first TM atom is adsorbed on H1 from above. Calculations have been performed by using supercell geometry and hence the above adsorption geometries have been periodically repeated in two dimensions.

Figure 2

(Color online) Analysis of the energetics of Ti, Co, and Fe moving from H1 to T1 and from H1 to B1. The transient paths are given in the inset. Total energy of the unit cell is plotted for each path $\left(H1\to T1,H1\to B1\right)$, which is divided into sections with equal lengths.

Figure 3

(Color online) (a) Band structure of the bare graphene calculated for the $\left(2\times 2\right)$ cell. (b) Band structure for one Co adsorbed on each $\left(2\times 2\right)$ cell of graphene. Blue (dark) and orange (light) curves indicate the majority and minority spin bands, respectively. The zero of the energy is set to the Fermi energy.

Figure 4

(Color online) Spin resolved charge accumulation (i.e., $\Delta {\rho }_{\uparrow (\downarrow )}>0$) obtained from the charge density difference calculation for one Ti atom adsorbed on each $\left(4\times 4\right)$ cell of graphene (see the text). Blue (dark) and yellow (light) regions indicate the isosurfaces of majority and minority spin states, respectively.

Figure 5

(Color online) (a)–(f) possible hollow sites for adsorption on AGNR’s with $N_a=4$, 5, 6, 7, 8, and 9. For all $N_a$, H1 is the edge hollow site. H0 appears for $N_a=5$, 7, and 9, which indicates that the middle hollow site fulfills the reflection symmetry. H2 and H3 are the remaining sites if they are different from the previous ones, with H2 being closer to H1. The unit cells are indicated by dashed lines.

Figure 6

(Color online) Transition state analysis of Ti adsorbed on 7-AGNR between H0 and H1 sites above the bridge site. (a) Top view of three adsorption sites of Ti on 7-AGNR from H0 to H1, i.e., H0, bridge, and H1 sites are shown. (b) Side view for these three adsorption sites. Adsorption to the C-C bridge gives the farthest position to the AGNR plane. (c) Total energy per unit cell for Ti adsorption on the path from H0 to H1 (see the text).

Figure 7

(Color online) (a) Band structures of 5-AGNR and $\theta =2$ coverages of 5-AGNR (b) with Fe and (c) with Ti. Fermi energy is set to zero. In (b) and (c), blue (dark) curves are the bands with majority spin, and orange (light) curves are the bands with minority spin. Fe adsorption opens a gap of 0.10 eV for the minority spin, while the majority spin is metallic. Adsorption of Ti makes the minority spin metallic, while the majority spin has an energy gap of 0.16 eV at the Fermi energy.