Ab Initio Study of Structural, Electronic, and Magnetic Properties of

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Ab Initio Study of Structural, Electronic and Magnetic Properties of Transition Metal Atoms Intercalated AA-stacked Bilayer Graphene Xiuyun Zhang, Xinli Zhao, and Yongjun Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07488 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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The Journal of Physical Chemistry

Ab Initio Study of Structural, Electronic and Magnetic Properties of Transition Metal Atoms Intercalated AA-stacked Bilayer Graphene

Xiuyun Zhang*, Xinli Zhao Yongjun Liu Department of Physics, Yangzhou University, Yangzhou, 225002, China

Abstract The structural, electronic and magnetic properties of transition metal atoms intercalated bilayer graphene, [GTMG]x:ys, (x, y is integer, TM=Ti, Cr, Mn, Fe) with different TM/carbon hexagons ratios and insertion patterns, are systematically studied by density functional theory calculations. All the studied systems are thermodynamically stable and competitive ionic-covalent bonding characters are dominated in the TM-graphene interaction. Most studied systems are ferromagnetic, particularly, [GCrG]1:18, [GCrG]1:9 and [GFeG]1:6(1) and [GTMG]1:6(2) (TM=Cr, Mn, Fe) exhibit large magnetic moment of 4.43µΒ, 5.60µΒ, 7.02µΒ,10.85 µΒ, 9.04 µΒ and 5.19 µΒ per unit cell, respectively. In contrast, [GCrG]1:8 and [GFeG]1:8 are ferrimagnetic while eight other [GTMG]x:ys are nonmagnetic. Moreover, five intercalation nanostructures of [GTMG]1:18 (TM=Ti, Mn), [GTMG]1:9 (TM=Ti, Mn) and [GTiG]1:6 are semiconductors with the gaps of 0.141eV/0.824eV, 0.413eV/0.668eV, and 0.087eV, respectively. Comparison on different isomers with same TM/carbon hexagons ratios showed that the electronic and magnetic properties of these [GTMG]x:ys are largely dependent on the TM atoms arrangement. For thus, an effective way to control the electronic and magnetic properties of graphene based nanostructures is proposed. 1. Introduction Graphene, the single-layer hexagonal carbon network of carbon atoms with sp2 hybridization, has attracted great attention due to its exceptional physical and chemical properties.1-3 However, two crucial issues regarding the potential application in the fields of electronic and spintronics devices, a band gap with high on/off ratio and magnetization, have incited the study interests of theoretical and experimental scientists. In the past decades, great efforts have been made on the gap opening as well as the introduction of magnetism of

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graphene, such as, quantum confinement by fabricating graphene into a narrow ribbon,4-8 chemical doping or physisorption of various asdorbates,9-11 applying uniaxial tensile strain,12 adsorbing graphene onto substrates,13,14 et al. Among the mechanisms, the adsorption of metal adatoms has been proved an effective way to tailor the electronic and magnetic properties of graphene.15-28 Using density functional theory, Zenella, et al. observed significant charges transfer in the Ti/Fe adsorbed graphene.15 Similarly, Liu et al. examined ionic interaction inside the alkali- atoms/graphene, mixed covalent and ionic interaction for group-III/graphene, and covalent interaction for 3d TM atoms/graphene.16 Interestingly, Hu et al. observed a broad range of gaps over 0.2eV by covering the 5d TM adatoms on graphene.17 In addition, Krasheninnikov et al.18 and Johll, et al.19 investigated interesting magnetic behaviors of the TMs embedded defective graphene. And very recently, Qu et al. found that the adsorption of 5d TM atoms give rise to magnetism on graphene.20 In addition, Kealy et al. found that the cap of organic ligands on TM adsorbed graphene produce different electronic and magnetic properties.21 By comparing the electronic and magnetic properties of metallocene-decorated graphene, Li et al. found that cobaltocene displays an obvious electron transfer to graphene.22 Kang et al. found that the stability of ferrocene/graphene system is largely sensitive to their arrangements.23 Plachinda et al.24 and Avdoshenk et al.25 found that functionalizing graphene with half-sandwich metal-arene molecules can tailor the properties of graphene alternately. Compared with single layer graphene, the properties of bi- or few-layer graphene (BLG or FLG) are more flexible due to the breaking of the symmetry between the A and B sub-lattices.29 Mainly two types of interlayer stackings are identified for BLG: i) AA-stacking with every carbon atoms from the first (1st) layer locating just above the ones of the second (2ed) layer, and ii) AB-stacking with half of the carbon atoms from 1st layer having AA-stacking mode while the other half of the carbon atoms sitting above the hollow sites of the 2ed layer. Experimentally, most formed BLGs display AB-stacking, while few products with AA stacking were found.30,31 Inspired by the TM functionalized single layer graphene, intercalating TM atoms into BLG are expected, on one hand, to enhance the stability by the d-π coordination interaction between TM and graphene, on the other hand, to be regarded as an alternate way to tune the electronic and magnetic properties of BLG. Experimentally, Maassen et al. found that the interaction of graphene and TM layer induces


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large spin injection values and open the energy gaps of graphene to tens of meV at K point.32 Theoretically, Pinto et al. found that the Au intercalated graphene leads to n-doping of graphene and induce different properties from single layer graphene.26 Besides, Bui et al. studied the different intercalation nanostructures with Cr atoms intercalated on two graphene layers, and investigated intercalation ratio dependent magnetic properties.33 Even though, experimental and theoretical efforts on the TM intercalated BLGs are still limited, and deep understanding towards the properties modulation by metal species, insertion concentrations and intercalation patterns, et al. is still lacking. In this study, we systematically study the structural, electronic and magnetic properties of TM intercalated AA-stacked BLGs, [GTMG]x:ys (TM=Ti, Cr, Mn, Fe), with different TM/hexagons ratios (RTM-Hex) and TM arrangements. Our results show that the interaction of TM atoms and bilayer graphene are so strong and interesting electronic and magnetic behaviors are identified.

2. Computational Methodology and Models All calculations are performed within the framework of spin-polarized DFT as implemented

in

the

Vienna

Ab

initio

Simulation

Package

(VASP).34,35

The

exchange–correlation potentials are treated by the generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerholf (PBE).36 The interaction between valence electrons and ion cores is described by the projected augmented wave (PAW) method.37,38 Furthermore, the DFT-D2 method taking into account of van der Waals (VDW) interaction was adopted.39 Initially, we tested the geometry optimization of single layer graphene and got the lattice constant of 2.47Å, which agrees well with previous results.40,41 As shown in Figure 1, five BLG slabs with nine different TM/carbon hexagons ratios are considered: (i) the 3×3 slabs with RTM-Hex of 1:18 (b) and 1:9 (d); (ii) the 3×4 slabs with the RTM-Hex of 1:24 (a), 1:12 (c), 1:8 (e) and 1:6 (f); (iii) the 1×2 slab with the RTM-Hex of 1:4 (g); (iv) the 1×1 slab with RTM-Hex of 1:2 (h). Accordingly, the Monkhorst-Pack grid of 15×15×1 to 25×25×1 are used for the geometry optimization, and much more dense k-point grids of 25×25 ×1 to 45×45 ×1 are set for electronic properties exploration. To eliminate unnecessary interaction between molecules in neighboring unit cells, the size of the unit cell along the z axis was set as large as 15 Å. During the ion relaxation, the gaussian smearing of the electron occupations was



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applied (selected using the flag ISMEAR = 0) with a broadening of 0.05 eV. The energy cutoff for the plane wavefunction is 400 eV and the force acting on each atom is less than 0.01 eV/Å. The ions in the periodic unit are allowed to fully relax.

Figure 1. Scheme of [GTMG]x:ys with different RTM-Hexs, x:y= 1:24,1:18, 1:12, 1:9, 1:8, 1:6. The parentheses are used to distinguish the three isomers of [GTMG]1:6s. (a-j) Top view and (k-t) side view.

3. Results and Discussion 3.1 Geometries of [GTMG]x:y intercalation nanostructures The optimized structures of studied [GTMG]x:ys are shown in Figure S1-S5 in supporting information (SI). Similar with initial structures, the TMs in the optimized [GTMG]x:ys keep certain separation and no aggregations occur. In order to verify the possibility of TM clustering, we calculate the migration barriers for TM atoms sandwiched by two-side graphene (see Figure S6 in SI). Different from the TM adsorbed single layer graphene,16 the migration barriers of TM atoms though the double layer graphene are rather high (>1.2eV), therefore, it is difficult for them to gather. In the studied [GTMG]x:ys, most TM atoms are energetically favored the hollow sites of BLGs. Exceptions are found for [GMnG]1:2 and [GFeG]1:2, which the TM atoms are energetically preferred for the atop sites. The C-C bond lengths of graphene are in the range of 1.390Ǻ ~1.451Å and the TM-C bond lengths (the


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carbon atoms of the hexagonal rings closest to the TM atom) are around 1.942 Å ~2.853Å. Moreover, a close to “V” shape change tendency is found for the TM layer-graphene layer distances (DTM-G, see Figure 2(a) and Table 1, 2). For example, the shortest DTM-G is 1.829 Å for [GTiG]1:12, 1.669Å for [GCrG]1:8, 1.642 Å for [GMnG]1:6 and 1.7 Å for [GFeG]1:3, respectively. Besides, obvious TM dependent DTM-Gs are found for these [GTMG]x:ys, e. g. [GTiG]x:ys have the longest DTM-Gs except [GTiG]1:2 and [GTiG]1:18, followed by [GFeG]x:ys, [GCrG]x:ys and [GMnG]x:ys, of which, [GCrG]x:ys and [GMnG]x:ys have close DTM-Gs at same RTM-Hex. In previous study, Bui et al. have observed that DCr-Gs at RTM-Hex of 1:2 and 1:6 is 1.945 Å and 1.639Å,30 our results of 2.005 Å and 1.674 Å are about 3% and 2.1% longer, respectively.

Table 1. The systems (sys), intercalated TM atoms (TM), the C-C bond lengths of graphene (LC-C), the TM-C distance with the carbon atoms in the hexagonal rings nearest to TM atoms (DTM-C), the distance of TM layer to graphene layer (DTM-G), the binding energies (Eb) per TM atom, the magnetic moments (MMs) and the electronic states of these [GTMG]x:y (EG) systems, x:y=1:24, 1:18, 1:12, 1:9, 1:8.

Sys [GTMG]1:24

[GTMG]1:18

[GTMG]1:12

[GTMG]1:9

[GTMG]1:8

TM

LC-C (Å)

Ti Cr Mn Fe Ti Cr Mn Fe Ti Cr Mn Fe Ti Cr Mn Fe Ti Cr Mn Fe

1.418-1.43 1.413-1.43 1.417-1.43 1.419-1.43 1.418-1.43 1.422-1.43 1.420-1.43 1.420-1.43 1.413-1.43 1.404-1.44 1.411-1.44 1.412-1.43 1.411-1.43 1.413-4.43 1.413-1.44 1.414-1.43 1.413-1.44 1.401-1.44 1.406-1.44 1.408-1.43

DTM-G(Å ) 1.921 1.715 1.694 1.781 1.899 2.046 1.778 1.734 1.829 1.673 1.654 1.737 1.867 1.804 1.662 1.722 1.845 1.669 1.648 1.722

DTM-C(Å) 2.394-2.40 2.236 2.214-2.22 2.280-2.29 2.379-2.38 2.301-2.73 2.284 2.249-2.25 2.310-2.41 2.171-2.21 2.138-2.24 2.241-2.27 2.328-2.42 2.297-2.30 2.148-2.20 2.229-2.24 2.287-2.54 2.164-2.22 2.113-2.22 2.201-2.26

Eb(eV ) 3.76 1.13 1.07 2.02 3.83 1.03 0.41 1.76 3.82 1.32 1.28 2.18 2.90 0.15 0.40 1.28 4.07 1.33 1.36 2.14


MM(µ µΒ) ) 2.44 0.21 0 0.32 2.34 4.43 0.34 0.04 2.16 0.64 0 3.46 2.55 5.60 0 3.70 0.49 0.22 0 3.08

EG Ti-1.21G+0.61 Cr-0.98G+0.45 Mn-0.86G+0.43 Fe-0.77G+0.39 Ti-1.22G+0.611 Cr-1.02G+0.51 Mn-0.75G+0.38 Fe-0.76G+0.38 Ti-2.52G+1.26 Cr-1.97G+0.99 Mn-1.73G+0.87 Fe-1.53G+0.77 Ti-2.50G+1.25 Cr-2.19G+1.09 Mn-1.73G+0.87 Fe-1.54G+0.77 Ti-3.48G+1.74 Cr-2.93G+1.46 Mn-2.60G+1.30 Fe-2.30G+1.15


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Figure 2. (a) The distances of TM layer to graphene layer, DTM-G, (b) the binding energies per TM atom, Eb, and (c) the charges transferred to one graphene layer, ∆e, of [GTMG]x:ys per TM, TM=Ti, Cr, Mn, Fe, (d-g) top and side view of charge density differences of [GTMG]1:24, TM=Ti, Cr, Mn, Fe.

3.2 Stabilities of [GTMG]x:ys intercalation structures To explore the chemical stabilities of these [GTMG]x:y intercalation structures, we calculate the binding energies defined as: Eb=-{E([GTMG]x:y]-2E[G]-nE[TM]}/n

(1)

Which the E[·] respects the total energy of [GTMG]x:ys, one-layer graphene and TM atoms, respectively. n is the number of TM atoms per unit cell. As shown in Figure 2(b), the binding energies of all the studied [GTMG]x:ys are positive, indicating that the bonding of TM atoms and graphene are energetically stable. The largest binding energy is found for [GTiG]1:8 , ~4.07eV, and the lowest one is for [GCrG]1:9, ~0.15eV. Similar with trend of TM-G distances, the binding energies of [GTMG]x:ys vary with the intercalation concentration of TM atoms. For example, all the [GTMG]x:ys have the least binding energies at x:y=1:9, correspondence

to

the

largest

one

for

[GFeG]x:ys/[GMnG]x:ys

at

x:y=1:2

and

[GTiG]x:ys/[GCrG]x:ys at x:y=1:6. In addition, a distinct element dependent stability is observed, such as, the largest binding energies are found for [GTiG]x:ys, ~2.9eV-4.3eV,


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followed by [GFeG]x:ys of 1.65eV-3.15eV, and the least ones are found for [GCrG]x:ys and [GMnG]x:ys, both have very close binding energies, around 0.15eV-2.24eV. The distinct energy profile can be well understood from the competitive covalent-ionic bonding nature between TM atoms and graphene. As shown in Figure 2(d-e) and Figure S7-S10 in SI, significant electrons transfer occurs between TM atoms and two-side graphene layer, of which, the TM atoms are electron donors and the graphene acts as electron acceptors. The transferred electrons in [GTMG]x:ys are summerrized in Figure 2c and Table 1, 2. It is found that very close charges are transferred to graphene for the [GTMG]x:ys with small RTM-Hexs, e.g. x:y=1:24, 1:18, 1:12 and 1:9, respectively. While it is reduced gradually with the increase of intercalation concentration and the transferred electrons are cut down by about 1/3 for [GTMG]1:2s. The inconsistent variation of binding energies with respect to electrons transferred stems from the increase of covalent TM/G interaction (see Figure S7-S10 in SI), which the electron density between TM-G increase with the RTM-Hexs. On the other hand, TM dependent

electrons

transferred

are

also

found

for

these

[GTMG]x:ys,

e.g.

∆e([GTiG]x:ys)>∆e([GCrG]x:ys)>∆e([GMnG]x:ys)>∆e([GFeG]x:ys). Moreover, although the electrons transferred to graphene in [GCrG]x:ys and [GMnG]x:ys are a bit larger than those of [GFeG]x:ys, the stable electron configuration of half occupied 3d orbitals in Cr(3d54s1) and Mn(3d54s2), make the d-π interaction less stable.

Table 2. The systems (sys), intercalated TM atoms (TM), the C-C bond lengths of

graphene(LC-C), the TM-C distance with the carbon atoms in the benzeniod hexagonal rings nearest to TM atoms (DTM-C), the distance of TM layer to graphene layer (DTM-G), the binding energies (Eb) per TM atom, the magnetic moments (MMs) and the electronic states of these [GTMG]x:y (EG) systems, x:y=1:6, 1:4, 1:2, respectively.

Sys

TM

LC-C (Å)

DTM-G(Å)

DTM-C(Å)

Eb(eV)

MM(µ µΒ)

EG

[GTMG]1:6

Ti

1.412-1.451

1.895

2.238-2.853

4.30

0

Ti-4.43G+2.23

Cr

1.390-1.445

1.674

2.168-2.291

1.38

2.15

Cr-3.83G+1.92

Mn

1.397-1.448

1.642

2.066-2.388

1.34

0

Mn-3.35G+1.68

Fe

1.399-1.438

1.715

2.136-2.256

2.25

7.02

Fe-3.0G+1.50



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[GTMG]1:4

[GTMG]1:2

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Ti

1.424-1.428

1.914

2.369-2.405

3.74

0

Ti-1.04G+0.52

Cr

1.425-1.434

1.755

2.227-2.293

0.91

1.12

Cr-0.87G+0.43

Mn

1.425-1.435

1.727

2.209-2.267

0.81

1.08

Mn-0.74G+0.37

Fe

1.427-1.431

1.835

2.322-2.350

1.73

2.06

Fe-0.65G+0.32

Ti

1.426

1.996

2.453-2.454

3.27

0

Ti-0.75G+0.37

Cr

1.426

2.005

2.461

0.90

0.51

Cr-0.55G+0.28

Mn

1.428

2.049

1.942

2.24

1.53

Mn-0.38G+0.19

Fe

1.426

2.069

2.069-3.495

3.15

2.81

Fe-0.36G+0.18

3.3 Magnetic properties of [GTMG]x:y intercalation structures Interestingly, a wide variety of magnetic properties are found for theses [GTMG]x:ys (see Figure 3 and Table 1, 2). For [GTiG]x:ys, high magnetic moments (>2.0µΒ) are found for those structures with low RTM-Hex, that is, 2.44µΒ, 2.34µΒ, 2.16µΒ and 2.55µΒ are found for [GTiG]x:ys at x:y =1:24, 1:18, 1:12, and 1:9, respectively. With the intercalation concentration increase, the magnetic moment decreases rapidly to 0.49µΒ for [GTiG]1:8 and completely disappears at x:y = 1:6, 1:4 and 1:2, respectively. In contrast, zero magnetic moments are found for [GMnG]x:ys with lower intercalation ratios( x:y= 1:24, 1:18, 1:12, 1:9 and 1:6), while for high intercalation of [GMnG]1:4 and [GMnG]1:2, the magnetic moments are increased to 1.08µΒ and 1.53µΒ, respectively Differently, an up-down trend with respect to RTM-Hex is found for the magnetic moments of [GCrG]x:ys, which the largest magnetic moment is 5.6µΒ at x:y=1:9 and the lowest one is 0.21µΒ at x:y=1:24. As for [GFeG]x:ys, close to zero magnetic moments are found for those with low intercalation ratios at x:y=1:24 and 1:18, then it increase with the intercalation density and reach the peak of 7.02µΒ at x:y=1:6. Theoretically, Johll, et al found that the magnetic moment of Fe adsorbed single-layer graphene with 4×4 unit cell is 2.0µΒ,19 while the small magnetic moment for [GFeG]1:24 (0.32µΒ) and [GFeG]1:18 (0.04µΒ) in this study show that the capture of another graphene layer affect the magnetic moment significantly. Furthermore, different magnetic orders are found for these [GTMG]x:ys (see Figure 4e-g and Figure S11-S15 in SI), e.g. ferromagnetic coupling is favored for TM atoms in most [GTMG]x:ys, paramagnetic orders are found for [GCrG]1:24, [GMnG]x:y (x:y=1:24, 1:12, 1:9, 1:8, 1:6), and [GTiG]x:y (x:y=1:6, 1:4, 1:2), antiferromagnetic (AFM) coupling is found for [GTMG]1:8s, TM=Ti, Cr and Fe, respectively.


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Figure 3 (a) Total magnetic moments of [GTMG]x:ys, TM=Ti, Cr, Mn, Fe; (b-d) the density of states and (e-g) spin density plots of [GFeG]1:24, [GFeG]1:12 and [GFeG]1:6, respectively.

Deep analysis show that the magnetic behaviors of these [GTMG]x:ys come mainly from the spin polarized d valence electrons states of TM atoms. In the symmetric hexagonal crystal field of graphene, the degenerated five degenerated d orbitals are split into single occupied dz2, two generated (dxy, dx2-y2) and (dxz, dyz) orbitals, which can be clearly seen in figure 3(b-d) and Figure S16-S20 in SI. On one hand, the dxy and dx2-y2 orbitals of TM atom hybrid with the σ orbital of graphene forming stable bonding orbitals, which situate below the fermi level and deep in energy. On the other hand, the dxz and dyz orbitals of TM atom hybrid with the π orbitals of graphene forming antibonding orbitals near to or above the Fermi level. Differently, the dz2 orbital of TM atom is localized in the plots of density of states. Taking [GFeG]x:ys as example, the small magnetic moment (0.32µΒ) of [GFeG]1:24 comes from the dyz orbital of Fe atom, and for [GFeG]1:12 (3.46µΒ) and [GFeG]1:6(7.02µΒ), their magnetic moment are mainly contributed by dxz/dyz orbitals of Fe atom. As for other [GTMG]x:ys, the magnetic moments can be clearly seen from the DOS plots in SI.

3.4 Electronic properties of [GTMG]x:ys intercalation structures



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Interestingly, rich electronic properties, such as, metallic, semiconducting (SC) and half metallic (HM) properties are found for these [GTMG]x:ys. As shown in Figure 4(a-d) and Figure S23 (e) in SI, the conducting band and valence bands at Dirac point for [GTMG]1:18, [GTMG]1:9 (TM=Ti, Mn) and [GTiG]1:6 are opened, namely, the gaps are opened to 0.141eV/0.824eV, 0.413eV/0.668eV and 0.087eV, respectively. Particularly, the fermi level locates between the conduction bands and valence bands of [GTiG]1:6, showing that it is perfect SC without adjusting the Fermi level. Furthermore, four [GTMG]x:ys are found to be quasi HMs with semiconducting behavior in one spin channel while being metallic in the opposite one (see Figure 4e-f). For example, the HM band gaps of [GCrG]1:24 [GMnG]1:4, [GCrG]1:9 and [GFeG]1:9 are 0.01eV, 0.336eV, 0.66eV and 0.123eV, respectively, showing that they are candidates of robust half metals with high HM stabilities. As for other [GTMG]x:ys, they are all metals.

Figure 4.Bandstructures of (a-d) [GTMG]1:18 and [GTMG]1:9, TM=Ti, Mn, the red dash dotted lines respect the bands of “BLG”; (e-f) bandstructures of [GCrG]1:24, [GMnG]1:4 and (g,h) [GTMG]1:9, TM=Cr, Fe, the black solid lines respect the spin up electrons, and the red solid lines are the spin down electrons. 3.5 Comparison of [GTMG]x:ys with same intercalation ratios Finally, to explore the influence of TM arrangement style to the stabilities, electronic and magnetic properties of [GTMG]x:ys, we compare three kinds of [GTMG]x:ys configurations at


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x:y =1:6, TM=Ti, Cr, Mn, Fe: (1) a 3×4 AA-stacked BLG supercell sandwiches four TM atoms on the hollow sites per unit cell (Figure 1f); (2) a 3×3 AA-stacked BLG supercell with three TM atoms intercalated per unit cell (Figure 1i); (3) a 3 × 3 AA-stacked BLG supercell encapsulates one TM atom per unit cell (Figure 1j). The optimized structures are shown in Figure S5 in SI and detailed geometric parameters are summarized in Table S1 in SI. It is showed that the stabilities of these [GTMG]1:6s vary with the TM elements as well as their arrangement,

that

is,

the

binding

energies

follow

the

Eb[GTMG]1:6(1)s>Eb[GTMG]1:6(2)s>Eb[GTMG]1:6(3)s

order

of or

Eb[GTiG]1:6s>Eb[GFeG]1:6s>Eb[GCr(Mn)G]1:6s (see Figure 5a). Except [GMnG]1:6(2) and [GMnG]1:6(3), the electrons transferred to graphene increase smoothly from [GTMG]1:6(1)s to [GTMG]1:6(3)s (Figure 5b). And the inconsistent relationship between the stabilities and the electrostatic interaction can also be clearly understood from the competition between ionic and covalent bonding nature in the [GTMG]1:6s, the covalent interaction decrease with the ionic interaction increase from [GTMG]1:6(1) to [GTMG]1:6(3) (see Figure 5j-p).

Figure 5. (a) The binding energies (Eb), (b) the charges (∆e) transferred to one graphene layer, (c) the magnetic moments of [GTMG]1:6s, TM=Ti, Cr, Mn, Fe, (d-i) the density of states and (j-p) the charge density difference plot of [GMnG]1:6s and [GFeG]1:6s, the insets show the spin density plots.



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On the other hand, different magnetic behaviors are found for the [GTMG]1:6s isomers. For example, ferromagnetic coupling is found for the TM atoms of [GTMG]1:6(2) and [GTMG]1:6(3), of which, the magnetic moments of [GTMG]1:6(2) are 2.77 µΒ, 10.85 µΒ, 9.04 µΒ and 5.19 µΒ for TM=Ti. Cr, Mn, Fe, respectively, about three times of [GTMG]1:6(3)s (see Figure 5c, S15 in SI). In contrast, distinct magnetic orders are found for [GTMG]1:6(1) (Figure S13 e-h). Ferromagnetic coupling is found for Fe atoms of [GFeG]1:6(1) (Figure S13h), which total magnetic moment is as large as 7.01µΒ. Different case is found for [GCrG]1:6(1) (Figure S13f), which ferromagnetic coupling is found for two Cr atoms spaced by two hexagons with the local magnetic moments of 1.106 µΒ and 1.107 µΒ, respectively. While zero residual magnetic moments are found for the other two Cr atoms. As a result, the magnetic moment of this system is 2.15µΒ. Besides, [GTiG]1:6(1) and [GMnG]1:6(1) are paramagnetic with zero magnetic moment (Figure S13e, g). Interestingly, the electronic properties of such [GTMG]1:6s are sensitive to the TM arrangements. For [GTMG]1:6(1) (see Figure S23 (e-h)), SC properties are found for TM=Ti, while metallic properties are found for TM=Cr, Mn and Fe. As for [GTMG]1:6(2) (see Figure 6a-d), HM or quasi HM are found for TM=Cr, Mn, Fe, with the HM gaps of 0.090eV, 0.525eV and 0385eV, respectively, while metallic property is found for [GTiG]1:6(2). And for [GTMG]1:6(3), SC property is found for TM=Ti with an indirect gap of 0.412eV, HM and quasi HM property is found for TM=Cr, Mn and Fe, with a HM gap around 0.125eV, 0.545eV and 0.348eV(see Figure 6e-h), respectively.


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Figure 6. Bandstructures of (a-d) [GTMG]1:6(2) and (e-h) [GTMG]1:6(3), TM=Ti, Cr, Mn, Fe. The black solid lines respect the spin up electrons, and the red solid lines are the spin down electrons.

IV. Conclusion Using density functional theory, we systematically studied the structural, electronic and magnetic properties of transition metal intercalated bilayer graphene, [GTMG]x:ys, (x:y=1:24, 1:18, 1:12, 1:9, 1:8, 1:6, 1:4, 1:2, TM=Ti, Cr, Mn, Fe, respectively) with different intercalation ratios and TM arrangement patterns. Except [GCrG]1:18, [GCrG]1:9 and [GMnG]1:9, all theother [GTMG]x:ys are thermodynamically stable with the binding energies are larger than 0.80eV per TM atom. Besides, both ionic and covalent interaction dominates in the binding of TM and graphene. Furthermore, the electronic and magnetic properties of these [GTMG]x:ys are investigated to strongly affected by the intercalation ratios and TM elements. Most studied systems are ferromagnetic, of which, [GFeG]1:6, [GCrG]1:9 and [GCrG]1:18 have large magnetic moment of 7.02µΒ, 5.6µΒ and 4.43µΒ, respectively, while fewer systems are nonmagnetic. Interestingly, five intercalation structures of [GTMG]1:18, [GTMG]1:9 (TM=Ti, Mn) and [GMnG]1:6, are SCs with the gaps of 0.141eV/0.824eV, 0.413eV/0.668eV, and 0.087eV, respectively. Even with same intercalation ratios, the



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electronic and magnetic properties of [GTMG]x:ys are largely defined by the TM patterns. Our results show that the [GTMG]x:ys are good candidates for electronic and spintronics based devices.

Corresponding Authors *Dr. X.Y. Zhang, e-mail: [email protected], telephone number, +86-0514-87975466

Supporting Information Available: Optimized structures, plots of charge density differences, spin densities, density of states and band structures of all the studied [GTMG]x:ys, TM=Ti, Cr, Mn, Fe, respectively; The systems (sys), intercalated TM atom (TM), the C-C bond lengths of graphene(LC-C), the TM-C distance with the carbon atoms in the benzeniod hexagonal rings nearest to TM atoms (DTM-C), the distance of TM layer to graphene layer (DTM-G), the binding energies (Eb) per TM atom and the electronic states(EG) and magnetic moments (MM) of three kinds of [GTMG]1:6 systems, TM=Ti, Cr, Mn, Fe, respectively. This material is available free of charge via the net at http://pubs.acs.org.

Acknowledgments This work is supported by the NSFC (11574262, 11104241). The authors thank the computational resources at the YZU.

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