Electronic and Magnetic Properties of Ti and Fe on Graphene

Departamento de Fısica, UniVersidade Federal do Ceará, Caixa Postal 6030, ... UniVersidade de Sa˜o Paulo, Caixa Postal 66318, 05315-970, Sa˜o Paul...
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J. Phys. Chem. C 2008, 112, 9163–9167

9163

Electronic and Magnetic Properties of Ti and Fe on Graphene Ivana Zanella Departamento de Fı´sica, UniVersidade Federal do Ceara´, Caixa Postal 6030, Campus do Pici, 60455-900, Fortaleza, CE, Brazil

Solange B. Fagan* Centro de Cieˆncias Naturais e Tecnolo´gicas, Centro UniVersita´rio Franciscano, 97010-032, Santa Maria, RS, Brazil

R. Mota Departamento de Fı´sica, UniVersidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil, and International Center for Condensed Matter Physics, UniVersidade de Brası´lia, 70919-970, Brası´lia, DF, Brazil

A. Fazzio Instituto de Fı´sica, UniVersidade de Sa˜o Paulo, 05315-970, Sa˜o Paulo, SP, Brazil Instituto de Fı´sica, UniVersidade de Sa˜o Paulo, Caixa Postal 66318, 05315-970, Sa˜o Paulo, SP, Brazil ReceiVed: December 12, 2007; ReVised Manuscript ReceiVed: April 8, 2008

The structural, electronic and magnetic properties of Fe and Ti atomic wires and the complete covering when adsorbed on graphene are presented through ab initio calculations based on density functional theory. The most stable configurations are investigated for Fe and Ti in different concentrations adsorbed on the graphene surface, and the corresponding binding energies are calculated. The results show a tendency of the Ti atoms to cover uniformly the graphene surface, whereas the Fe atoms form clusters. The adsorption of the transition metal on the graphene surface changes significantly the electronic density of states near the graphene Fermi region. In all arrangements studied, a charge transfer is observed from the adsorbed species to the graphene surface due to the high hybridizations between the systems. Introduction Materials based on graphene have demonstrated at present a wide range of potential applications. Buckyballs, nanotubes, and others structures can all be viewed as derivatives of graphene. Graphene itself was experimentally discovered a few years ago by micromechanical cleavage of graphite.1 Since its beginning, graphene has been understood as a promising candidate for several technological applications due to its properties such as scalability, chemical stability, and ballistic transport at room temperature, among others.2,3 Moreover, there is an extraordinary interest in the magnetic order in low-dimensional systems that can be used in recording media, magnetic inks, and spintronic devices. For such intentions, it is interesting to understand the behavior of transition metals (TM) as, for example, Fe and Ti structures adsorbed on the graphene surface. It is also relevant to analyze these TM atoms adsorbed on materials with a weak concentration of free electrons due to the possible increasing of the electrical and thermal conduction. The formation of the free electron gas is dependent on the neighbor’s adsorbed metals on the surface. Recently, several works report the interaction of 3d-transition metals on carbon nanostructures.4–8 Kru¨ger et al.6 consider the magnetic behavior of TM monolayers on a graphite surface using the tight-binding linear muffin-tin orbital method, without geometry optimizations. Previous works for TM atoms adsorbed * Corresponding author. E-mail: [email protected]; solange.fagan@ gmail.com.

on carbon nanostructures show that the geometry optimization is essential to describe correctly the corresponding results.9–11 Rahman12 presents an experimental result for metal atoms intercalated in graphite sheets observing changes on the electrical properties of graphene layers resulting in strongly magnetic graphite. Nishioka and Goldman13 studied a magnetoresistance in a spin valve arrangement composed by multilayer graphene with Co contacts. The covering of carbon nanostructures with a range of transition metal atoms has been experimentally verified to depend on the specific adsorbed element, generating alternatively wires or clusters.14 Previous experimental14 and theoretical9 results show that Ti atoms tend to cover homogeneously the carbon nanotubes surface. A significant charge transfer is observed from Ti atoms to carbon nanotubes as well as a resulting magnetization in the involved systems that is a consequence of the high hybridizations between the Ti and C atoms.9 In contrast, the Fe atoms adsorbed on the carbon nanotube surface tend to agglomerate preserving the high spin polarizations of the Fe structures.10,11,14 Therefore, the research on the metal atoms adsorbed on graphene layers is a very promising field for several electronic and magnetic applications.3,13,15–17 Then, the behavior of the Fe and Ti atoms on the graphene surface can be used to extrapolate an understanding of the others 3d-transition metal atoms on this carbon nanostructure. In this paper, the electronic and magnetic properties for Fe and Ti in different concentrations adsorbed on the graphene

10.1021/jp711691r CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

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surface are determined. The results are compared with the previous one for the corresponding Fe and Ti structures interacting with single-walled carbon nanotubes (SWCNT).9–11 Methodology The electronic and structural properties of Fe and Ti in different concentrations adsorbed on the graphene surface were analyzed through total energy ab initio calculations based on the density functional theory.18,19 The SIESTA (Spanish Initiative for the Electronic Simulations of Thousand of Atoms) code that performs fully self-consistent simulations by solving the spin-polarized Kohn-Sham equations using numerical atomic orbitals as basis sets is used.20 In all calculations, the double-ζ basis set plus polarization function was used. Due to the large overlap between the semicore and the valence states, the 3s and 3p electrons of Ti were explicitly included in the calculation.21 The exchange-correlation potential was treated through a generalized gradient approximation for the exchange-correlation term, as proposed by Perdew et al.22 The interactions between ionic cores and valence electrons are described by norm conserving Troullier-Martins pseudopotentials.23 A cutoff of 200 Ry for the grid integration was utilized to represent the charge density. The structural optimizations were performed using the conjugate gradient algorithm until all the residual forces for all atoms were smaller than 0.03 eV/Å. Because the Fermi surface of graphene is a single point, Monkhorst-Pack grids24 of 5 × 5 × 1 k points were used for atomic relaxations in supercells that are demonstrated in recent calculations25 to reproduce the experimental results. In this work, the considered TM concentrations on the graphene surface are (i) 1 TM per 32 C atoms; (ii) 4 or 8 TM per 32 C; and (iii) 16 TM per 32 C, labeled as TM atomic, wires and decorating arrangements, respectively. The binding energies for all the studied configurations of Fe and Ti adsorbed on graphene are calculated through the equation

EB) -[ET(graphene + nX) -ET(graphene) - nET(X)] (1) where ET(graphene + nX) is the total energy for the system with graphene plus Fe or Ti atoms, where n is the number of Fe or Ti atoms involved in the system, ET(graphene) is the total energy for the graphene surface and ET(X) is the total energy of an isolated Fe or Ti atom. Results and Discussion Several configurations for atoms and wires of Fe adsorbed on graphene surfaces are studied. The following sections will present the structural, electronic, and magnetic results for these systems. I. Fe atoms in Graphene. Figure 1 shows the possible sites for the atomic adsorption of Fe and Ti on the graphene carbon sheet. The Fe or Ti atoms were positioned on the graphene at the top of the C atom, labeled TOP position, at the middle of the C-C bond, labeled bridge BC position, and at the hexagonal center site, labeled HC site, as indicated in Figure 1. Table 1 presents the closest distances between the TM metals and the C atoms of the graphene surface, the binding energies (calculated using eq 1), the spin polarization of the resulting systems and the charge transfers for the Fe or Ti atoms. Among all studied configurations, the HC are the most stable sites for the Fe and also for the Ti atoms on the graphene surface, with a binding energy of 1.10 and 3.26 eV, respectively. This high interaction can be understood in terms of the modifications in the effective

Figure 1. Schematic view for the adsorption sites for Fe or Ti atomic on the graphene surface.

TABLE 1: Distances between TM Atoms and C Atoms of the Graphene Surface (D(TM-C)), Binding Energies, Spin Polarizations, Charge Transfers, and Effective Valence Configurations for the Fe or Ti Atoms when Adsorbed on the Graphene Surfacea TM atomic configuration Fe-HC Fe-TOP Fe-BC Ti-HC Ti-TOP Ti-BC-unstable

spin charge D(TM-C) EB polarization transfer effective TM (Å) (eV) (µB) (e-) configuration 2.13 2.18 2.24 2.26 2.12

1.10 0.60 0.58 3.26 2.55

2.00 4.00 4.00 2.71 2.88

+0.14 +0.03 +0.05 +1.08 +0.92

4sp0.86 3d7.0 4sp1.37 3d6.6 4sp1.35 3d6.6 4s0.12 3d2.8 4s0.48 3d2.6

a The plus sign in the charge transfer indicates that the corresponding TM atom loses electron.

valence configurations of the Fe or Ti atoms. For the atomic Fe on the TOP and BC positions, the binding energies are almost half the ones from HC. For the Ti case, the BC configuration is unstable and the atom moves to the TOP or HC sites. Comparing with previous results,9–11 the Fe and Ti atoms also stay on the HC configuration on the SWCNT adsorption. It is observed that the Fe and Ti on this configuration present binding energies just about 1.40 and 3.30 eV for Fe and Ti, respectively, that are slightly higher than the graphene adsorption. These values can be explained in terms of the curvature of the SWCNTs outer surfaces. The results for the spin polarization, electronic charge transfer, and effective Fe-valence configuration can clarify the behavior of the HC compared with the BC and TOP positions. The relatively high value of the binding energy for the HC configuration is an indication of the high hybridization of the Fe-graphene valence states, generating a low spin configuration compared with the atomic Fe or with the BC and TOP sites. We have also studied the HC configuration with a spin polarization of 4.00 µB, but the HC arrangement with low spin (2.00 µB) is 0.4 eV more stable than with the high one (4.00 µB). For the atomic Ti adsorbed on the graphene surface, a high charge transfer, about one electron, from the Ti to the graphene surface is observed. The same behavior is observed for the atomic Ti on the SWCNT surface.9 Analyzing the spin polarization, the Ti adsorbed on graphene presents a value (around 2.7 µB) higher than adsorbed on the SWCNT (around 1.8 µB). Figure 2 presents the electronic total densities of states (DOS) for the pristine graphene (Figure 2a) and Fe and Ti atomic adsorbed on the graphene for the HC configurations, Figures 2b,c, respec-

Properties of Ti and Fe on Graphene

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Figure 3. Relaxed structures for Fe and Ti wires adsorbed on graphene: (a) HC-HC, (b) BC-BC, and (c) ZIG configurations.

TABLE 2: Distances TM-TM, Binding Energies, and Total Spin Polarization per TM Wires Adsorbed on the Graphene Surface in Different Configurations

Figure 2. DOS along the high symmetry graphene points for the majority (black lines with positive values) and minority (red lines with negative values) spin configurations for (a) graphene, (b) Fe-HC, and (c) Ti-HC configurations. Plots for the electronic charge densities for the polarized charge (majority minus minority electronic charge) for the (d) Fe and (e) Ti atoms on the HC configuration adsorbed on the graphene surface in different views. The green and blue contours correspond respectively to the majority and minority spin polarization.

tively. When the Fe atom is adsorbed on the graphene sheet, the DOS alters, and localized levels are introduced at the valence and at the conduction bands in the majority spin configuration, for all the arrangements. For the minority spin configuration, the electronic levels near to the top of the valence band suffer several modifications due to the high hybridization on the Fe-graphene levels and with one localized Fe-3d level. This high hybridization is also observed on the Fe atom adsorbed on the SWCNT surface.10,11 For the Ti adsorbed on the graphene surface DOS, localized levels are observed on the majority and minority spins configurations, which are basically Ti-3d levels. The graphene levels are hybridized with 4s and 3d Ti levels altering the valence and conduction bands of the original graphene, resulting in a high charge transfer between the Ti and the C atoms of the graphene surface near the adsorption site, as can be observed in the Table 1. The Figure 2d,e presents the plot for the difference between the majority and minority electronic charge densities for the Fe and Ti adsorbed on the graphene surface. For the Fe and Ti systems, a high spin polarization is observed on the TM atoms and the C neighbors.

TM wire configuration

distance TM-TM (Å)

EB per TM atom (eV)

spin polarization per TM atom (µB)

Fe-HC-HC Fe-BC-BC Fe-ZIG Ti-HC-HC Ti-BC-BC Ti-ZIG

2.47 2.3/2.6 2.43/2.46 2.46 2.56 2.43

1.99 2.14 2.75 4.05 3.94 4.96

3.25 3.25 2.88 2.80 2.00 0.00

II. Fe and Ti Wires on Graphene. Several configurations of Fe and Ti wires interacting with the graphene surface are considered. The most stable structures are (i) HC-HC wires, where the TM atoms are approaching to the center of the graphene hexagon, (ii) BC-BC wires, where the TM atoms are above the C-C bonds of the graphene surface, and (iii) ZIG wires, where the adsorbed atoms are above the C graphene atoms “forming” an angle of 60 ° between TM-TM atoms. The resulting configurations for HC-HC, BC-BC, and ZIG wires are shown in panels a-c of Figures 3, respectively. On the HC-HC configurations, the Fe or Ti atoms are approaching the center of a C hexagon with Fe-Fe and Ti-Ti distances of 2.47 and 2.46 Å, respectively. The behavior for the Fe or Ti wire on the BC-BC configurations is very similar to the HC-HC one, with Fe-Fe or Ti-Ti distances of 2.30/ 2.60 and 2.56 Å, respectively, where the Fe or Ti atoms directly approach above the C-C bonds. The ZIG Fe and Ti wire configurations present the adsorbed atoms distributed directly above the C graphene atoms “forming” an angle of 60° with Fe-Fe or Ti-Ti with interatomic distances of 2.43/2.46 and 2.43 Å, respectively. In Table 2, the minimum TM-C distances, binding energies (EB/atom), and the spin polarization per TM atom are presented for TM wires interacting with the graphene surface. The most stable obtained structure is with the TM wire in the ZIG configuration, for both Fe and Ti cases. The ZIG Fe-wire is 0.75 and 0.61 eV/atom more stable than HC-HC and BC-BC Fe wires, respectively. In general, the Fe wires adsorbed on the graphene surface present higher binding energies compared with the atomic Fe adsorbed on the graphene surface. This behavior is very similar to the one observed for Fe, atoms and wires, adsorbed on SWCNT.10,11 For the Ti wire adsorbed on the graphene surface, the most stable configuration is also the ZIG arrangement (Figure 3c). In terms of binding energies, the ZIG wire is 0.91 and 1.02 eV/atom more stable than HC-HC and BC-BC Ti wires, respectively. This value is in the same order as observed for the Ti wire on the SWCNT surface (4.0 eV for the ZIG configuration with high spin).9

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Figure 5. Different views for the configurations for (a) Fe and (b) Ti decorating the graphene surface

Figure 4. DOS along the high symmetry graphene points for the majority (black lines with positive values) and minority (red lines with negative values) spin configurations for the (a) pristine graphene, (b) Fe wire, and (c) Ti wire in ZIG configuration adsorbed on the graphene. In panel d, plots for the electronic charge densities for the polarized charge (majority minus minority electronic charge) for the Fe wire in the ZIG configuration adsorbed on the graphene surface in different views. The green and blue contours correspond respectively for the majority and minority spin polarizations.

It is also interesting to notice that the most stable configuration for the Fe wire is observed for high spin configuration (around 2.8 µB). The electronic spin polarization at the Fe atoms and the C atoms near the adsorption can be observed in Figure 4d. In contrast, for the Ti-wire configuration, the most stable system presents a low spin configuration in the ZIG positions (0.0 µB). In the Figure 4a-c, the DOS for Fe and Ti wires on the graphene surface in the ZIG configurations are shown. It is observed an increase in the DOS at the valence and conduction bands due to the high hybridization levels between TM and C atoms. III. Fe and Ti Decorating Graphene. Fe and Ti decorating the graphene surface are also analyzed, as shown in Figure 5. The TM atoms are originally placed on the most stable configuration of the atomic structure, the HC site for both Fe and Ti. For the Fe atom, it is interesting to observe that the average Fe-Fe distance is 2.4 Å, similar to the obtained value for the Fe-Fe distance in Fe wires on graphene. For the Fe-C neighbor atoms, the bond length is around 3.98 Å, and that is very large compared with the Fe-C distance (2.13 Å on the HC atomic configuration), showing that the Fe atoms are displacing out of the graphene surface. The binding energy per Fe atom is 3.65 eV. This value is very high compared with the Fe in the atomic (1.10 eV) and in the ZIG wire (2.75 eV) configurations. Observing the bond length values and the binding energies, it can be concluded that the Fe atoms tend to form clusters and aggregate with each other going away from the graphene surface. Another interesting point to observe is the spin polarization per Fe atom on the decorating configuration on the graphene sheet that is 2.78 µB per Fe atom. This value

Figure 6. Binding energy per atom versus TM concentration on the graphene surface.

is higher than the Fe in atomic adsorption (2.0 µB) and similar with the Fe ZIG wire spin configuration (2.88 µB). For the Ti decorated graphene, the Ti-Ti distance stays around 2.47 Å and the Ti-C 2.68 Å. The binding energy per Ti atom in this configuration is around 4.61 eV, presenting no spin polarization in this arrangement. Figure 6 presents the relation of the binding energies per TM atom with different concentrations on the graphene surface. It is observed that the stability of the Fe atoms enhances when the concentration increases on the graphene surface, indicating an agglomeration of the Fe structures, also observed in the corresponding bond lengths results. For the Ti structures, it is observed that, in terms of the binding energies (Figure 6), the system achieves a large stabilization in the atomic, wire, or decorating arrangements, with the Ti structures preferring to cover homogeneously the graphene surface. Conclusions The structural and electronic properties of Fe and Ti adsorbed on the graphene surface in atomic, wires, and decorating

Properties of Ti and Fe on Graphene configurations are discussed through first principles calculations. The hexagonal center sites are demonstrated, for both Fe and Ti atoms, to be the most stable configurations on the graphene surface with binding energies around 1.10 and 3.26 eV, respectively. For the Fe or Ti wires the most stable configuration is, for both, that with the Fe or Ti atoms forming the ZIG wires with 2.75 and 4.96 eV for the corresponding binding energies, respectively. The configuration with all of the hexagons of the graphene covered with a TM atom is also analyzed. Comparing the binding energies for different concentrations of TM atoms decorating the graphene surface, they are compatible with the tendency of Ti atoms to uniformly distribute on the graphene surface, in contrast with the propensity of Fe atoms to clusterization. These results are similar to the previously obtained with SWCNTs.9–11 High densities of electronic states are obtained on the graphene Fermi regions for all the TM atoms adsorbed configurations. In the atomic and wires arrangements for Fe cases, the resulting most stable systems present relevant spin polarizations as a consequence to the Fe-graphene hybridizations. From our results, the role played by TM atoms adsorbed on graphene, from structural and magnetic point of view, these systems are very promising materials to be used for potential applications in magnetic sensors or electronic devices. Acknowledgment. The authors acknowledge CENAPADSP for computer time and financial support from Brazilian agencies CNPq, FAPERGS (Grant 07/01129), and FUNCAP (Grant 350220/2006-9). S.B.F. acknowledges L’oreal/Paris for the Grant for the Brazilian Woman in Science/2006. R.M. thanks to the Centro Internacional de Fı´sica da Mate´ria Condensada, UnB, Brası´lia-DF, Brazil.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9167 References and Notes (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonoset, S. V. Science 2004, 306–666. (2) Stankovich, S.; Dikin, D. A.; Dommett, G.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E.; et al. Nature 2006, 442, 282. (3) Geim, A. K.; Novoselov, K. S. Nature Mat 2007, 6, 183. (4) Sun, Q.; Wang, Q.; Jena, P.; Kawazoe, Y. J. Am. Chem. Soc. 2005, 127, 14582. (5) Krasnov, P. O.; Ding, F.; Singh, A. K.; Yakobson, B. I. J. Phys. Chem. C 2007, 111, 17977. (6) Kru¨ger, P.; Rakotomahevitra, A.; Parlebas, J. C.; Demangeat, C. Phys. ReV. B 1998, 57, 5276. (7) Uchoa, B.; Lin, C.-Yi; Castro Neto, A. H. Phys ReV B 2008, 77, 035420. (8) Chen, L.; Wu, R.; Kioussis, N.; Blanco, J. R. J. Appl. Phys. 1997, 81, 4161. (9) Fagan, S. B.; Fazzio, A.; Mota, R. Nanotechnology 2006, 17, 1154. (10) Fagan, S. B.; Mota, R.; Silva, A. J. R.; Fazzio, A. Phys. ReV. B 2003, 67, 205414. (11) Fagan, S. B.; Mota, R.; Silva, A. J. R.; Fazzio, A. Microelectronics J. 2003, 34, 481. (12) Rahman, F. Appl. Phys. A: Mater. Sci. Process. 2007, 86, 221. (13) Nishioka, M.; Goldman, A. M. Appl. Phys. Lett. 2007, 90, 252505. (14) Zhang, Y.; Franklin, N. W.; Chen, R. J.; Dai, H. Chem. Phys.Lett 2000, 331, 35. (15) Zhang, Y.; Tan, Y. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (16) Novoselov, K. S.; McCann, E.; Morozov, S. V.; Falko, V. I.; Katsnelson, M. I.; Zeitler, U.; et al. Nat. Phys 2006, 2, 177. (17) Uchoa, B.; Castro Neto, A. H. Phys. ReV. Lett. 2007, 98, 146801. (18) Hohenberg, P.; Kohn, W. Phys. ReV. 1964, 136, B864. (19) Kohn, W.; Sham, J. L. Phys. ReV. 1965, 140, 1433. (20) Artacho, E.; Sanchez-Portal, D.; Ordejon, P.; Garcia, A.; Soler, J. M. Phys. Stat. Sol.(b) 1999, 215, 809. (21) Junquera, J.; Zimmer, M.; Ordejo´n, P.; Ghosez, P. Phys. ReV. 2003, B67, 155327. (22) Perdew, J. P.; Burke, E.; Ernzenhof, M. Phys. ReV. Lett. 1996, 77, 3865. (23) Troullier, N.; Martins, J. L. Phys. ReV. B 1991, 43, 1993. (24) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (25) Valencia, F.; Romero, A. H.; Ancilotto, F.; Silvestrelli, P. L. J. Phys. Chem. B 2006, 110, 14832.

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