ARTICLE pubs.acs.org/JPCC
Structure and Electronic and Transport Properties of Transition Metal Intercalated Graphene and Graphene-Hexagonal-Boron-Nitride Bilayer Jing Zhou,†,‡ Lu Wang,‡ Rui Qin,† Jiaxin Zheng,† Wai Ning Mei,*,‡ P. A. Dowben,§ Shigeru Nagase,|| Zhengxiang Gao,† and Jing Lu*,† †
State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P. R. China Department of Physics, University of Nebraska at Omaha, Omaha, Nebraska 68182-0266, United States § Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, Theodore Jorgensen Hall, 855 North 16th Street, University of Nebraska—Lincoln, P.O. Box 880299, Lincoln, Nebraska 68588-0299, United States Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan
)
‡
ABSTRACT: Structural, electronic, and magnetic properties of the Fe-, Co-, Ni-, and V-intercalated graphene bilayer sandwich (denoted by C2|M|C2, M = Fe, Co, Ni, and V) and graphene on hexagonal boron nitride (h-BN) bilayer sandwich (denoted by C2|M|BN, M = Fe, Co, Ni, and V) are studied by using density functional theory method. We find that both the graphene bilayer and graphene-h-BN bilayer in all the C2|M|C2 and C2| M|BN sandwiches favor AB stacking over AA stacking mode. The Fe, Co, and Ni atoms prefer to be located over the center of CC bonds whereas V atoms prefer to be located above the C atoms on graphene, and they all prefer to be located above the N atoms on h-BN sheet, regardless of the stacking mode. The C2|Fe| C2, C2|Co|C2, C2|Fe|BN, and C2|Co|BN sandwiches of AB stacking are all ferromagnetic metals with the spin polarization of 86%, 67%, 65%, and 46% at the Fermi level, respectively. By contrast, both C2|Ni|C2 and C2|Ni|BN sandwiches of AB stacking are nonmagnetic semiconductors with bandgaps of 0.64 and 0.23 eV, respectively, which provide a novel strategy of opening a bandgap of graphene. From the quantum transport calculation, we obtain a giant room-temperature magnetoresistance of ∼200% in the spin valve device based on AB stacking C2|Fe|C2 sandwich.
1. INTRODUCTION Graphene has attracted great recent interest due to its peculiar electronic properties. The long spin relaxation time and length of graphene make it a suitable material in spintronics devices.14 The popular spintronics devices include spin-filter and spinvalve. Normally, materials with a high degree of spin polarization at Fermi level (Ef) lead to high spin filter efficiency and high magnetoresistance. The graphene based spin valve has low magnetoresistance because there is no spin polarization in pure graphene. Introducing spin-polarized transition metal (TM) into graphene layers seems to be a feasible method of introducing high spin-polarization to graphene devices. Theoretically, it3 has been reported that there exists high spin-polarization at Ef in the Fe, Co, and Ni adatoms and dimers adsorbed single-layer graphene.5 There has also been a theoretical report on the spin-polarization found at Ef in Mn-substituted bilayer graphene.6 On the other hand, graphene has very high carrier mobility, 15 000 cm2/V s for SiO2-substrate-supported sample and 200 000 cm2/V s for suspended sample.1,2,7,8 Consequently, opening and tuning the graphene bandgap without reducing the carrier mobility will expand the application of graphene devices, for example, the graphene field-effect transistor. Several approaches have been made on single-layer or bilayer graphene r 2011 American Chemical Society
for bandgap engineering both experimentally and theoretically, including applying a perpendicular electric field on bilayer graphene,911 donor or acceptor doping on one side of bilayer graphene,1214 donor and acceptor codoping on both sides of bilayer graphene,12 hydrogenation,1518 functionalization of graphene,19 and fabricating graphene nanomesh,20 etc. The former three approaches open the band gap of bilayer graphene via breaking the inversion symmetry of bilayer graphene. Graphite intercalation compounds (GICs) are complex materials with formula XCy where element X layer is intercalated between the graphite layers. The limit of GICs is single-atomlayer intercalated bilayer graphene sandwich structure. Several fundamental questions concerning the transition metal (TM) intercalated bilayer graphene sandwich arise: (1) What are the favorable structures of the graphene sandwich? (2) How are the magnetic moments of the intercalated TM atoms coupled? (3) Can the intercalated TM metal atoms offer strong spin polarization at the Fermi level? (4) Can the bandgap be opened in TM intercalated graphene bilayer sandwich? Received: September 30, 2011 Revised: November 14, 2011 Published: November 15, 2011 25273
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Table 1. Binding Energy per TM Atom, Energy Difference between FM and AFM States per TM Atom, Localized Magnetic Moment (M) per TM Atom, the Nearest TMCarbon Distance (dTM‑C), Graphene Layer Separation (dlayer), and Mulliken Charge (Q) per TM Atom of TM Metal Atom Graphene Sandwich Calculated by Using LDA Method Eb (eV)
stacking C2|Fe|C2 C2|Co|C2
ΔE (eV)
d (Å)
dTM‑C (Å)
dlayer (Å)
Q (e)
M (μB)
AA
AFM
1.17
0.03
2.48
2.05
3.85
0.72
0.28
AB
FM
1.18
0.14
2.48
2.05
3.93
0.72
0.31
AA
FM
1.04
0.00
2.49
2.04
3.87
0.69
0.14
1.09
0.00
2.49
2.04
3.91
0.70
0.14
AB C2|Ni|C2
AA AB
NM
0.84 0.86
2.49 2.50
1.99 2.04
3.83 3.91
0.66 0.65
0.00 0.00
C2|V|C2
AA
NM
1.34
2.51
2.12
4.06
0.76
0.02
1.37
2.51
2.10
4.21
0.79
0.01
AB C2|Fe|BN
AB
FM
1.43
0.01
2.49
1.98
3.94
0.66
0.33
0.00
C2|Co|BN
AB
FM
1.22
2.51
1.99
3.93
0.63
0.21
C2|Ni|BN
AB
NM
0.91
2.52
1.97
3.94
0.58
0.00
C2|V|BN
AB
NM
1.55
2.53
2.09
4.18
0.72
0.04
In a recent experiment, graphene on hexagonal boron nitride (h-BN) has been reported. It is also interesting to check the TM intercalated graphene-h-BN sandwich structure. In this article, we investigated the structural, electronic and transport properties of the TM intercalated graphene bilayer sandwich (denoted by C2|M|C2, M = Fe, Co, Ni, and V) and graphene-h-BN bilayer sandwich (denoted by C2|M|BN, M = Fe, Co, Ni, and V) by using density functional theory (DFT) method and quantum transport calculation. We locate the favorable position of the TM atoms between bilayer graphene, or between single-layer graphene and hexagonal boron nitride sheet. The structural, electronic, and magnetic properties depend strongly on the species of intercalated TM metals.
2. COMPUTATIONAL DETAILS The interatomic distance of the bulk crystal Fe (body-centered cubic), Co (hexagonal close-packed), Ni (face-centered cubic), and V (body-centered cubic) of the (111), (001), (111), and (111) surface, are 2.48, 2.51, 2.49, and 2.62 Å, respectively. All of these transition metal lattice constants are comparable with the graphene period of 2.46 Å. Consequently, we intercalate single hexagonal transitional metal sheet between bilayer graphene, and also graphene-h-BN. Three configurations are considered. The metal atoms are (1) above the carbon atoms, (2) above the midpoint of a CC bond, and (3) above the center of a hexagon on graphene. We consider both the AB and AA stacking of the bilayer graphene and construct supercells consisting of four carbon atoms and one TM atom. We also construct supercells for graphene-h-BN sandwich of two carbon atoms, one boron atom, one nitrogen atom, and one TM atom. Vacuum space of 12 Å normal to the sandwich plane is used to avoid interaction between sandwiches. Spin-polarized calculations are performed by using the ultrasoft pseudopotential plane-wave method implemented in the CASTEP package.21 We carried out tests in the generalized gradient approximation (GGA) of PerdewWang 1991 (PW91) for the exchange-correlation energy and find the length of CC bond generated by the local density approximation (LDA) is closer to the experiment result (1.42 Å). Therefore, all the calculations are performed within LDA unless otherwise noted. Geometry optimization is performed for both the atomic positions and the lattice
Figure 1. Top and side views of the optimized structures of the (a) Fe-, Co-, Ni-, and (b) V-intercalated bilayer graphene sandwich. Atoms represented as follows: gray ball, C; light blue ball, Fe, Co, or Ni; dark pink ball, V.
constant with a plane wave cutoff energy of 300 eV and 7 7 1 k points until the maximum atomic force is less than 0.01 eV/Å. The single point energy calculation is performed with a larger cutoff energy of 400 eV and 12 12 1 k points. The optimized interlayer distance of bilayer graphene is 3.32 Å, which is in good agreement with the experimental value of 3.34 Å of graphite.22 A two-probe model is constructed to study the transport properties. The transport properties are computed by using the DFT coupled with the NEGF formalism implemented in the ATK2008.10 code.23,24 The norm-conserving pseudopotentials of the TroullierMartins type25 is used. Single-ζ basis set is used, the mesh cutoff is chosen as 150 Ry, and the electron temperature is set to 300 K. The structures of the scattering region are optimized until the maximum atomic forces are less than 0.03 eV/Å. The spin-resolved current Iσ under a bias voltage Vbias is calculated with the LandauerB€uttiker formula26 Iσ ðVbias Þ ¼
e h
Z fTσ ðE, Vbias Þ½fL ðE, Vbias Þ fR ðE, Vbias ÞgdE
ð1Þ
where Tσ(E, Vbias) is the spin-resolved transmission probability, fL/R(E, Vbias) is the FermiDirac distribution function for the 25274
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Figure 2. Top and side views of the optimized structures of the (a) Fe-, Co-, Ni-, and (b) V-intercalated graphene and hexagonal boron nitride sheet sandwich. Atoms are represented as follows: gray ball, C; light blue ball, Fe, Co, or Ni; dark pink ball, V; light pink ball, B; dark blue ball, N.
left (L)/right (R) electrode, and σ is a spin index. The bias voltage effect on the device is taken into account self-consistently
3. RESULTS AND DISCUSSION In Table 1, we summarize the essential results such as the binding energy per TM atom (Eb), energy difference between the FM and AFM states per TM atom (ΔE), lattice constant (a), the nearest TM-carbon distance (dTM‑C), graphene layer separation (dlayer), Mulliken charge (Q), and localized magnetic moment (M) per TM atom of the TM intercalated graphene sandwich. 3.1. TM Sheet Intercalated Bilayer Graphene Sandwich. The optimized configurations of the Fe-, Co-, Ni-, and V-intercalated bilayer graphene sandwich are shown in Figure 1a,b. In both the AA and AB stacking graphene sandwich, Fe, Co, and Ni atoms all prefer to be sited above the midpoint of a CC bond as shown in Figure 1a; however, V atoms prefer to be sited above C atoms as shown in Figure 1b. The AB stacking sandwiches are more stable than that of the AA stacking by 0.03, 0.06, 0.03, and 0.05 eV per TM atom for Fe-, Co-, Ni-, and V-intercalated bilayer graphene sandwich, respectively. The TM interatomic distance is 2.482.51 Å. The nearest TMC distance is 1.992.12 Å, typical of TMC covalent bond length. The distance between two graphene layers is 3.834.21 Å, significantly larger than that of the pure bilayer graphene of 3.43 Å. The binding energy of the TM-sheet-intercalated graphene sandwich is defined as
Eb ¼ ðEtot ETM EG Þ=nTM
ð2Þ
where Etot represents the total energies of the TM-graphene composites. ETM and EG are the total energy of TM atom sheet and pure bilayer graphene, respectively, and nTM the number of TM atoms per supercell. Thus, Eb can be used to check the stability of the system studied. From Table 1, the formations of TM intercalated bilayer graphene sandwich are all exothermic with binding energies of 0.84 to 1.36 eV per TM atom, respectively. The stability increases in the order of C2|Ni|C2 < C2|Co|C2 < C2|Fe|C2 < C2|V|C2. Then, we study the magnetism of the graphene sandwiches. Ferromagnetically (FM) coupled, antiferromagnetically (AF) coupled, and nonmagnetic (NM) TM sheets are considered
Figure 3. Ferromagnetic and antiferromagnetic configurations of the (a) Fe-, Co-, Ni-, and (b) V-intercalated AB-stacking bilayer graphene sandwich, (c) Fe-, Co-, Ni-, and (d) V-intercalated graphene and hexagonal boron nitride sheet sandwich. Green arrows show the relative directions of magnetic moments. Atoms are represented as follows: gray ball, C; light blue ball, Fe, Co, or Ni; dark pink ball, V; light pink ball, B; dark blue ball, N.
Figure 4. Spin density of the (a) Fe-, (b) Co-intercalated AB-stacking bilayer graphene sandwich and (c) Fe-, (d) Co-intercalated graphene and hexagonal boron nitride sheet sandwich (isovalue: 0.15 au). Atoms are represented as follows: gray ball, C; light blue ball, Fe, Co; light pink ball, B; dark blue ball, N.
for all the graphene sandwiches, as shown in Figure 3a,b. The ABstacking TM-intercalated graphene sandwiches of C2|Fe|C2 and C2|Co|C2 are both FM coupled in the ground state, and the AFM coupling is 0.14 and 0.00 eV higher in energy, respectively. The FM-AFM energy difference of the AB stacking C2|Fe|C2 is larger than the room temperature value of 0.026 eV and suggests that the FM state can be stabilized at room temperature. The magnetic moment of the intercalated Fe and Co atoms is 0.31 and 0.14 μB per TM atom, respectively. Figure 4 shows the spin density of the FM C2|Fe|C2 and C2|Co|C2 sandwich, and the induced magnetic moments are mainly localized on the Fe/Co 25275
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Figure 5. Band structures and DOS of the ferromagnetic (a) Fe-, (b) Co-, and (d) V-, and nonmagnetic (c) Ni-intercalated AB-stacking bilayer graphene sandwich. Blue indicates majority spin; red indicates minority spin. K is the Dirac point of graphene. The Fermi level and the valence band maximum are set to zero.
atoms. On the other hand, the AB stacking C2|Ni|C2 and C2|V| C2 are both NM. It is noteworthy that the local magnetic moment in graphene cannot be effectively described by a single-impurity unit cell and is often spuriously suppressed when a periodic boundary condition (PBC) is used because of the enhanced interaction in with the reduced metallic system.27,28 However, in our model, the TM atoms are strongly interacting, forming a closely packed monolayer, and more than one TM atoms are included in our supercell. Therefore, the obtained magnetic moments for the TM atoms are reliable in our model. The DFT-GGA calculations are known to underestimate the on-site correlation effects between 3d electrons of TM atoms, which are important in predicting the magnetic interaction. Hence, we have also performed calculations within the LDA + U scheme by using the ultrasoft pseudopotential plane wave method implemented in CASTEP.21 The recommended Hubbard parameter U is 2.5 eV for Fe, Co, Ni, and V atoms. The inclusion of the Hubbard U term causes the binding energy to slightly decrease by 00.1 eV and the energy difference between the FM and AFM states to slightly increase by 00.01 eV, thus stabilizing the FM states. The localized magnetic moment per TM atom is nearly intact. We show the ground-state band structures and corresponding DOSs of the TM-sheet-intercalated bilayer graphene sandwich in Figure 5ad, respectively. The Fe-, Co-, and V-intercalated sandwiches are all metallic, and moreover, the spin is highly polarized at Ef in both C2|Fe|C2 and C2|Co|C2 sandwiches. The
spin polarization at Ef, P(Ef), is defined as PðEf Þ ¼
DðEf , maj Þ DðEf , min Þ DðEf , maj Þ þ DðEf , min Þ
ð3Þ
where D(Ef, maj) and D(Ef, min) represent the values of DOS of majority and minority spin at Ef, respectively. The P(Ef) of the C2|Fe|C2 and C2|Co|C2 sandwiches is 86% and 67%, respectively. In Figure 6a,b, we show the atom-resolved partial density of states (PDOS) of the C2|Fe|C2 and C2|Co|C2 sandwiches. We find that the DOS of both majority and minority spin at Ef are primarily contributed from the Fe 3d and Co 3d orbitals, respectively, which is quite close to the situation in TM atom filled SWCNTs. The high spin polarization at Ef makes the C2| Fe|C2 and C2|Co|C2 sandwiches suitable for spintronic device application. The C2|Ni|C2 sandwich is semiconducting with a direct band gap of 0.7 eV, as shown in Figure 5c. We calculated the effective mass of the C2|Ni|C2 sandwich at the K point. Both the hole mass and the electron mass at the K point are ∼me, which are much higher than that of pure bilayer graphene of 0.03me (where me is the free electron mass). As a result, we cannot expect high carrier mobility in the C2|Ni|C2 bilayer graphene sandwich. From the simple relation between carrier mobility u and effective mass m, u = eτ/m, where τ is the scattering time, we estimate that the carrier mobility in the SiO2-substrate-supported C2|Ni|C2 25276
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sandwich is about 500 cm2/V s, assuming τ is the same between bilayer graphene and the C2|Ni|C2 sandwich. The band dispersions of C2|Fe|C2, C2|Co|C2, and C2|Ni|C2 sandwiches are similar, and there is a band gap in the shown energy (4 to 4 eV). The electronic structure difference between C2|Fe|C2, C2|Co|C2, and C2|Ni|C2 sandwiches can be understood in terms of an upshift of Ef with the increasing valence electron number. Both the Fe- and Co-intercalated sandwiches are metallic, but the energy difference from Ef to the lower edge of the band gap is smaller in C2|Co|C2 than in C2|Fe|C2. In the case of C2|Ni|C2, the electron filling reach the lower edge of the band gap, leading to a semiconductor. 3.2. TM Sheet Intercalated Single-Layer Graphene and Hexagonal Boron Nitride Sheet Sandwich. The optimized
configurations of the Fe-, Co-, Ni-, and V-intercalated grapheneh-BN bilayer sandwich are shown in Figure 2a,b, respectively. Similarly, Fe, Co, and Ni atoms all prefer to be located above the midpoint of a CC bond, and V atoms prefer to be located above C atoms. On the other hand, all the TM atoms prefer to be located above N atoms. The TM interatomic distance is 2.492.53 Å, slightly larger than that in bilayer graphene sandwich by 0.010.02 Å. The nearest TMC distance is 1.972.09 Å, smaller than that in bilayer graphene sandwich by 0.010.07 Å. This length is typical of covalent TMC bond. Simultaneously, the distance between TM and B/N atoms is also typical of covalent bond length. The distance between graphene and BN sheet layer is 3.934.18 Å, larger than that of the pure BN-graphene of 3.43 Å.
Figure 6. Atom- and spin-resolved partial DOS of the ferromagnetic (a) Fe- and (b) Co-intercalated AB-stacking bilayer graphene sandwich. The Fermi level is set to zero.
Figure 8. Atom- and spin-resolved partial DOS of the ferromagnetic (a) Fe- and (b) Co-intercalated graphene and BN sheet sandwich. The Fermi level is set to zero.
Figure 7. Band structure and DOS of the ferromagnetic (a) Fe-, (b) Co-, and (d) V-, and nonmagnetic (c) Ni-intercalated graphene and hexagonal boron nitride sheet sandwich. Blue indicates majority spin; red indicates minority spin. K is the Dirac point of graphene. The Fermi level and the valence band maximum are set to zero. 25277
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Figure 9. Fe-intercalated bilayer graphene sandwich based spin valve device: (a) schematic model. The spin polarization directions of the two ferromagnetic electrodes can be controlled by the magnetic field. P and AP denote the configurations, in which the spin polarization directions of the two electrodes are parallel and antiparallel, respectively. (b) Transmission spectrum of the P and AP configurations with Vbias = 0.002 V. (c) Transmission eigenstates for the P and AP configurations at Ef and the Γ point. The isovalue is 0.2 au. (d, e) Bias dependence of the current in the P and AP configurations (d) and the magnetoresistances (e). Gray ball indicates C; red ball indicates Fe.
The binding energy of the TM sheet intercalated graphene sandwich is defined as Eb ¼ ðEtot ETM EG EBN Þ=nTM
ð4Þ
where Etot represents the total energy of the TMgraphene composites. ETM, EG, and EBN are the total energy of TM atom sheet, graphene, and BN sheet, respectively. nTM is the number of TM atoms per supercell. The formations of TM metal sheet intercalated graphene and BN sheet sandwich are all exothermic with binding energies of 0.91 to 1.55 eV per TM atom,
respectively. The graphene-h-BN bilayer sandwiches are more stable than the corresponding graphene bilayer sandwiches by 0.050.25 eV, and the stabilities are following the same increasing order of C2|Ni|BN < C2|Co|BN < C2|Fe|BN < C2|V|BN as bilayer graphene sandwich. Then we study the magnetism of the functionalized graphene sandwiches. FM, AF, and NM TM sheets are considered for all the graphene-h-BN sandwiches, as shown in Figure 3c,d. The AB-stacking TM-intercalated graphene sandwich of C2|Fe|BN and C2|Co|BN are both FM coupled in ground states, and the 25278
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AFM coupling is 0.01 and 0.00 eV higher in energy, respectively. The FM-AFM energy differences of the AB stacking C2|Fe|BN and C2|Co|BN are both smaller than the room temperature of 0.026 eV, which suggests that the FM state cannot be stabilized at room temperature. The magnetic moment of the intercalated Fe and Co atoms is 0.33 and 0.21 μB per TM atom, respectively, larger than that in the corresponding bilayer graphene sandwich. On the other hand, the AB stacking C2|Ni|BN and C2|V|BN sandwiches are both NM, as C2|Ni|C2 and C2|V|C2 bilayer graphene sandwiches are. Figures 7 and 8 address DOS and PDOS. We show the ground-state band structures and DOSs of the Fe, Co, Ni, and V sheet intercalated graphene-h-BN sandwich in Figure 7ad, respectively. Both the C2|Fe|BN and C2|Co|BN sandwiches are metallic, with spin polarization at Ef, and the calculated P(Ef) is 65% and 46%, respectively, which is smaller than the corresponding one in bilayer graphene sandwich. In Figure 8a,b, we show the atomresolved partial density of states (PDOS) of the C2|Fe|BN and C2| Co|BN sandwiches and find that the DOSs of both majority and minority spin at Ef are also primarily contributed from the Fe and Co 3d orbitals, respectively. The C2|V|BN sandwich is a nonmagnetic metal. The C2NiBN sandwich is a nonmagnetic semiconductor with a direct band gap of ∼0.2 eV, much smaller than that in the C2|Ni|C2 sandwich of ∼0.7 eV. We calculated the effective mass at the K point. Both the hole mass and the electron mass are ∼0.2me, smaller than that of the C2|Ni|C2 sandwich. A higher carrier mobility of 2500 cm2/V s may be achieved in the C2|Ni|BN sandwich rather than the C2|Ni|C2 bilayer graphene sandwich. 3.3. Transport Properties of TM Sheet Intercalated Bilayer Graphene Sandwich. Since the spin polarization of FM coupled C2|Fe|C2 sandwich is quite high, we built a spin valve device based on AB-stacking Fe-intercalated bilayer graphene sandwich. The schematic model is shown in Figure 9a. The spin polarization directions of the two ferromagnetic electrodes can be controlled by the magnetic field. P and AP denote two configurations, in which the spin polarization directions of the two electrodes are parallel and antiparallel, respectively. The transmission spectra of the P and AP configurations with Vbias = 0.002 V are shown in Figure 9b. The transmission coefficients of the P configuration around Ef are much larger than those of the AP configuration and furthermore differ significantly between the majority and minority spins. In the P configuration, the transmission coefficients of the minority spin are about 2 times the majority spin at Ef, whereas in the AP configuration, the transmissions of the two spins are generally the same. The difference between P and AP configurations is reflected in the transmission eigenchannel at Ef and the Γ point displayed in Figure 9c. The transmission eigenvalue of the P configuration is 0.99, which means the scattering is weak, and actually most of the incoming wave is able to reach to the other lead. On the contrary, the transmission eigenvalue of the AP configuration is 0.32, and the corresponding incoming wave function is apparently scattered. The larger transmission coefficients within the bias window in the P configuration as compared to those in the AP configuration lead to the current in the P configuration (IP) that is larger than that in the AP configuration (IAP) at Vbias = 0.002 V. Figure 9d shows the bias dependence of IP and IAP in 0.0020.2 V. IP is always larger than IAP in the checked bias range. The magnetoresistance (MR) is defined as MR ¼
IP IAP IAP
ð5Þ
The calculated bias dependence of MR at room temperature is shown in Figure 9e. The MR has a very high value of 195% at a small bias of 0.002 V and then generally decreases with the increasing bias. The maximum MR value in experiment at room temperature is a few hundred percent.2931 Therefore, the calculated maximum MR in the C2|Fe|C2 sandwich based spinvalve is comparable with the available maximum experimental MR. On the other hand, the MR of 195% is also comparable with the calculated MR of 110% in Ni-graphene system.32 However, the calculated MR of C2|Fe|C2 sandwich is not as high as those (million to billion) for a graphene nanoribbon spin valve.33,34 The origin of the strong magnetism of Fe- and Co-intercalated graphene (or graphene/BN) bilayer can be understood in terms of Stoner criterion. For example, the calculated density of states at Ef, N(Ef), of C2|Fe|C2 sandwich is 1.2 states/Fe atom in the nonmagnetic state. Taking the recommended the on-site Coulomb repulsion U = 2.5 eV,21 we have N(Ef)U/2 = 1.5 > 1. According to Stoner criterion, spontaneous magnetization will take place in C2|Fe|C2 sandwich. Bulk ferric chloride (FeCl3) GIC exhibits a stable ferromagnetic transition at temperature of T = 8.5 K.35 Very recently, FeCl3 monolayer has been intercalated between bilayer graphene, forming a minimal GIC.36 A slight upturn in resistance related to magnetic transition is observed. This success is expected to promote the synthesis of TM-intercalated graphene bilayer sandwich and graphene-h-BN bilayer sandwich.
4. CONCLUSIONS In this work, we investigate the geometrical structures and electronic and transport properties of the Fe-, Co-, Ni-, and V-intercalated graphene bilayer sandwich and graphene-h-BN bilayer sandwich by using a density functional theory method. We find the favorable configurations of TM atom on graphene/ h-BN sheet and also the preferred stacking mode. The C2|Fe|C2, C2|Co|C2, C2|Fe|BN, and C2|Co|BN sandwiches of AB stacking are all ferromagnetic metals with high spin polarization at Ef. By contrast, both C2|Ni|C2 and C2|Ni|BN sandwiches of AB stacking are nonmagnetic semiconductors with bandgaps, which provide a novel strategy of opening a bandgap of graphene. From the quantum transport calculation, we obtain a giant roomtemperature magnetoresistance of ∼200% in the AB stacking C2| Fe|C2 sandwich based spin valve device. There may be potential application of the transition metal intercalated graphene sandwich in spintronics devices. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (J.L.);
[email protected] (W.N.M.).
’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197–200. (3) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191–1196. (4) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. 25279
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