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Tuning the Dirac Cone of Bilayer and Bulk Structure Graphene by Intercalating First Row Transition Metals Using First Principles Calculations Srimanta Pakhira, and Jose L. Mendoza-Cortes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11761 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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The Journal of Physical Chemistry

Tuning the Dirac Cone of Bilayer and Bulk Structure Graphene by Intercalating First Row Transition Metals using First Principles Calculations Srimanta Pakhira†,‡,¶,§,k and Jose L. Mendoza-Cortes∗,†,‡,¶,§ Condensed Matter Theory - National High Magnetic Field Laboratory (NHMFL), Florida State University, Tallahassee, FL, 32310, USA., Department of Chemical & Biomedical Engineering, Florida A&M University - Florida State University, Joint College of Engineering, Tallahassee, FL, 32310, USA., Department of Scientific Computing, Florida State University, Tallahassee, FL, 32304, USA., and Materials Science and Engineering, High Performance Materials Institute (HPMI), Florida State University, Tallahassee, FL, 32310, USA. E-mail: [email protected] Phone: +1-850-410-6298. Fax: +1-850-410-6150

∗

To whom correspondence should be addressed Condensed Matter Theory - National High Magnetic Field Laboratory (NHMFL), Florida State University, Tallahassee, FL, 32310, USA. ‡ Department of Chemical & Biomedical Engineering, Florida A&M University - Florida State University, Joint College of Engineering, Tallahassee, FL, 32310, USA. ¶ Department of Scientific Computing, Florida State University, Tallahassee, FL, 32304, USA. § Materials Science and Engineering, High Performance Materials Institute (HPMI), Florida State University, Tallahassee, FL, 32310, USA. k Current Address: Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Simrol, Indore-453552, MP, India. †

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Abstract Modern nanoscience has focused on two-dimensional (2D) layer structure materials which have garnered tremendous attention due to their unique physical, chemical and electronic properties since the discovery of graphene in 2004. Recent advancement in graphene nanotechnology opens a new avenue of creating 2D bilayer graphene (BLG) intercalates. Using first-principles dispersion-corrected DFT techniques, we have designed 20 new materials in-silico by intercalating first row transition metals (TMs) with BLG, i.e. 10 layered structure and 10 bulk crystal structures of TM intercalated in BLG. We investigated the equilibrium structure and electronic properties of layered and bulk structure BLG intercalated with first row TMs (Sc-Zn). The present DFT-D calculations show that the 2pz sub-shells of C atoms in graphene and the 3dyz sub-shells of the TM atoms provide the electron density near the Fermi energy level controlling the material properties of the BLG-intercalated materials. This article highlights how the Dirac point moves in both the BLG and bulk-BLG given a different TM intercalated materials. The implications of controllable electronic structure and properties of intercalated BLG-TM materials for future device applications are discussed. This work opens up new avenues for the efficient production of two-dimensional and threedimensional carbon-based intercalated materials with promising future applications in nanomaterial science.

Introduction In honeycomb lattices like graphene, the existence of the Dirac point results from the planar trigonal connectivity of the sites and its sublattice symmetry. 1 Graphene is a wonderful material with many superlatives to its name. Monolayer graphene (MLG) is an infinite one atom thick honeycomb membrane of carbons in which each atom connects to three surrounding other carbons by sp2 hybridized bonds, and it is often treated theoretically as a free-standing two-dimensional (2D) sheet. It has attracted great attention in nanomaterial

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science and nanotechnology since its first experimental fabrication in 2004, 2 due to its exceptional electronic properties linked to the Dirac physics of its low energy quasi-particles. 3,4 The presence of a Dirac Cone in graphene, which represents linear energy dispersion at the Fermi level, gives graphene massless fermions leading to various quantum Hall effects, ultra high carrier mobility, and many other novel phenomena and properties. MLG has been found to have excellent electrical, electronic and 2D layer material properties. Hindrances to using graphene in many applications, such as electronics have been due to the lack of a band gap. In modern digital electronic and nanoelectronic devices, MLG has the well known zero band gap issue 5 which makes a high on-off ratio difficult; deeming it unsuitable for transistors, which are the foundation of all modern electronic and digital devices. Aside from the peculiar massless Dirac fermions that emerge in MLG, stacking graphene sheets on top of the other may result in very different energy-momentum relations, 6 expanding the electronic versatility of this 2D carbon system. In the simplest architecture, two graphene layers can be arranged in an AA configuration; 4 the physical properties of such bilayer graphene (BLG) are correlated with the stacking order and relative twist angle, 7,8 with each type possessing a unique π-electron landscape. 9 The AA-stacked BLG unit cell is constructed in such a way to have one atom in one layer exactly above one atom of the second layer of graphene. 4 Similar to MLG; BLG is an atomically thin 2D layer material that has generated extensive research effort in modern nanoscience in this decade. Unlike MLG which has an unusual Dirac Cone band structure at low energies, 4,10 BLG has a parabolic band with zero band gap. A recent experiment showed that BLG presents an interesting case in terms of its electronic properties at low energy 11 leading to an unconventional quantum Hall effect which arises from the chiral nature of the charge carriers in BLG. 1,10 Its band structure around the K and K’ points displays hyperbolic bands touching at the neutrality point if trigonal warping terms are neglected. 12 More interestingly, the low-energy electronic structure and high density of states (DOSs) can potentially lead to be an intriguing material in that its electronic structure can be altered by intercalating metal atoms between two

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graphene layers. BLG can be both the Dirac and the Schrödinger systems, depending on its stacking structure. 4 When BLG is in the Bernal AB-stacked form, in which the upper graphene is shifted in-plane by (~a +~b)/3 with respect to the lower one, it forms a 2D Schrödinger system having a parabolic band dispersion (where ~a and ~b are the unit vectors of the graphene sheet). 4,13 On the other hand, when it is in the AA-stacked form, in which the lattice of two layers is stacked without lateral shift, it forms a 2D Dirac system having a linear band dispersion. 14 This difference between the AB- and AA-stacked BLGs causes some difference in their physical and electronic properties. Specially, the Berry Phase is different between the two systems; that of Dirac system is π, while that of Schrödinger system is 2π. A “Berry Phase” is a phase angle (i.e., running between 0 and 2π) that describes the global phase evolution of a complex vector as it is carried around a path in its vector space. To fabricate practical devices based on graphene and BLG, it is essential to modify and control the electronic properties and parameters such as the band structures, DOSs, sign and concentration of carriers as well as the band gap at the Dirac point. Despite the unique intrinsic properties of MLG and BLG layer materials, various modification methods have been applied to these materials that yield even more exciting outcomes. In fact, several attempts have been made to achieve material engineering by (i) introducing an external electric field 15,16 ; (ii) depositing atoms or molecules on a graphene sheet 17 ; (iii) alloying and hybridization 18 ; and (iv) intercalation 4,19–21 to mention a few. Each of these categories provide unique perspectives and advantages in studying both the fundamental science as well as applications in 2D materials. Among a variety of graphene-based materials, graphene intercalation materials are formed by insertion of molecular or atomic layers with various chemical species between graphite layers. 4,19–24 In graphite, however, intercalation of guest atoms and molecules into graphite layers is known to considerably modify the electronic structure, leading to unique physical properties and technological applications such as superconductivity and rechargeable batteries. 21,23,25 Intercalated graphene materials with a

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periodic arrangement of an alternating sequence of intercalant and graphene layers exhibit a variety of exotic electronic properties ranging from superconductivity to magnetism. 19,20,26 Recent advancement in graphene nanotechnology opens a new avenue of creating few-layer graphene intercalates. Combining layers of 2D materials in particular sequences to form multilayer structures provides an avenue for the manipulation of mechanical and electronic properties and for creating heterostructure devices. 3,4,22,23,27 As stated earlier, among all the approaches, the intercalation method is one of the best methods to control the material properties of graphene, BLG and graphite. 4,19,24,28,29 Alkalimetal intercalated graphite and graphene have been intensively studied for decades, where alkali metal atoms are found to form ordered structures at the hollow sites of hexagonal carbon rings. To the best of our knowledge, the intercalation of graphene or graphite was studied mainly by using alkali atoms or ions (Li, Na and K) which has many important applications in modern science 19–21 such as Li-ion or Na-ion battery 28,30 However, intercalation of transition metal (TM) atoms into 2D layered structure and 3D bulk structure materials (such as bilayer graphene or graphite) has not been studied systematically and extensively yet. The few reported studies have shown that they provide rich electrical, material and chemical properties that are distinctly different from those of pristine materials. 4 In particular, intercalation into bulk BLG has attracted special attention, since graphite intercalation compounds show various fascinating physical properties such as superconductivity and magnetism. 19,20,26 Bui et al. 31 carried out a theoretical investigation on 2D BLG-Cr nanostructure material and they found that the 2pz orbitals of graphene layers are aligned antiferromagnetically with respect to the Cr layer, but they did not study the electronic and material properties nor the details of the crystal structure. Schwingenschlögl and co-workers computationally investigated the electronic properties and magnetic behavior of a 3D graphitic network in ABA and AAA stacking with intercalated transition metal atoms (Mn, Fe, Co, Ni, and Cu) and they proposed that these spin-polarized TM-intercalated materials can be utilized in spintronic and nanoelectronic applications. 32 Xu and co-workers 33 theoretically

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investigated various ordered structures of 3D TM-intercalated BLG (where TM atoms are Sc and Ti) with biaxial strain and they computed the electronic and magnetic properties of these TM-intercalated BLG materials. Their results indicated that the strong interaction between TM atoms and graphene comes mainly from the hybridization between p orbitals from C and d orbitals from TM. BLG with different TM/carbon hexagons ratios (where TM = Ti, Cr, Mn, Fe) and insertion patterns were computationally studied by Zhang et al., 34 and they showed a way to control magnetic and electronic properties of BLG. Recently, we investigated how the electronic properties of 2D BLG can be tuned by intercalation of Vanadium (V), Niobium (Nb), Tantalum (Ta) atoms and interestingly we found that the 2D layer structure BLG-V showed a Dirac Cone in its band structure. 4 We have also discussed why the Dirac Cone appears in the intercalated BLG materials, 4 considering only V-B TM atoms (such as Vanadium, Niobium and Tantalum) in the periodic table. Thus, the electronic structure and properties change as a function of the number of graphene layers as well as by intercalation of foreign atoms, i.e. TM atoms. Even though experimental and theoretical efforts on the TM intercalated BLGs are still limited; deep understanding toward modulating electronic properties on these materials is still lacking, which could be controlled by metal species or insertion of TM atoms and intercalation patterns. Inspired by TM functionalized single layer graphene, intercalating TM atoms into BLG are expected to enhance the stability by the d-orbitals of the TM and p-orbitals from graphene, and provide an alternate way to tune and control the electronic and material properties of BLG. In the present article, we have focused on the intercalation effects of all the first row transition metals on both the BLG and bulk-BLG showing the band structure and density of states (DOSs) near the Fermi energy level (EF ) as well as principles that control the electronic properties. In order to design these novel devices (both layer and bulk structures BLG) it is critical to understand the structural and electronic properties of these intercalation compounds, specially inter-layer coupling of the bilayer graphene and transition metals in 2D (BLG-TM)

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and 3D (bulk-BLG-TM) structures. In this work, we use a periodic dispersion-corrected unrestricted hybrid DFT i.e. DFT-D 35–40 method to show how the structure and material properties of both the layer and bulk structures of BLG have been changed by intercalating first row transition metals (TMs). We studied the band structures and density of states (DOSs) of first row transition metals (Sc-Zn) intercalated in both the BLG and bulk-BLG. The present DFT-D study found a Dirac Cone at the K-point in the BLG-V among all the BLG-TM 2D layer materials while the other BLG-TM materials conserve the Dirac Point in their band structures. This study also found the Dirac point is close to the Fermi energy (EF ) level in Mn and Cu intercalated bulk-BLG. Thus, we have further discussed how the Dirac Point is controlled and moved due to the TM intercalation in both BLG and bulkBLG materials. This study may allow bilayer graphene, new avenues to develop 2D Dirac materials and modern electronic devices following recent trends in science and technology. In this paper, we perform first-principles exhaustive investigation of first row TM atoms intercalation in BLG and bulk-BLG. We computationally designed a total of twenty TM intercalated materials: (i) ten TM-intercalated-BLG 2D layer materials (in short BLG-TM) and (ii) ten TM intercalated bulk structure BLG materials (in short bulk-BLG-TM). We have systematically studied the material properties of the aforementioned intercalated BLG materials in this study including equilibrium structure, stability and electronic properties. We observed in our computation that the Dirac point is at the Fermi energy level (EF ) of the 2D layer structure BLG-V (where V has the electronic configuration [Ar]3d3 4s2 ) formed the Dirac Cone at the K-point. However, when the BLG was intercalated by Ti ([Ar]3d2 4s2 ), where Ti has less d-orbital electrons compared to V, the Dirac point at the K-point moved down from the EF about 2 eV (see Scheme 1a). Similarly, the Dirac point at K moved below the EF in the 2D layer BLG-Fe where Fe ([Ar]3d6 4s2 ) has more d-orbital electrons compared to V as shown on the right side in Scheme 1a. Thus, these calculations reveal that the Dirac Cone and Dirac point are controlled by the d-orbital electrons of the TMs which are interacting with the p-orbital electrons of graphene in both the BLG-TM and bulk-BLG-TM

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Energy (eV) w.r.t. vacuum

a) 2D Bilayer Graphene TM Intercalated Compounds 4

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V=[Ar]3d34s2

Ti=[Ar]3d24s2 Less Electrons

For comparison

-3

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K

Fe=[Ar]3d64s2 More Electrons

b) Bulk Structure BLG TM Intercalated Compounds Energy(eV)

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Cr=[Ar]3d44s2 Less Electrons

M

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Mn=[Ar]3d 4s

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2

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M

Fe=[Ar]3d64s2 More Electrons

Scheme 1: Band structures of (a) 2D-layered materials: BLG-V; BLG-Ti and BLG-Fe and (b) 3D-bulk materials: bulk-BLG-Mn, bulk-BLG-Cr and bulk-BLG-Fe. In these structures, the Dirac point is moved due to the different TM atoms i.e. changing the number of d electrons in the system. In some cases, the Dirac point can touch the Fermi Energy level resulting into a Dirac Cone. 8


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materials. The Dirac features still exist in the TM intercalated bulk structure BLG material (bulk-BLG-TM); see Scheme 1b. For the bulk structure, the present study found that the Dirac point is close to the (EF ) in the BLG-Mn and BLG-Cr as shown in Scheme 1b. In the bulk structure intercalated BLG structure i.e. bulk-BLG-TM, the Dirac point moved up and down depending on the TM atoms in BLG intercalated materials. Thus, in this study we present the factors that controls the Dirac point in BLG and bulk analog of BLG. The unit cell of the BLG-TM and bulk-BLG-TM are depicted in Scheme 2.

Scheme 2: Unit cells of the 2D-material: BLG-TM (a) side view, (b) top view; and 3Dmaterial: bulk-BLG-TM (c) side view and (d) top view are shown here. The unit cells are highlighted by a parallelogram.

Methods and Computational Details First-principles calculations based on hybrid density functional theory (DFT) were used to perform all the periodic boundary computations as implemented in the ab initio CRYSTAL14 suite code, which makes use of localized Gaussian basis sets. 41,42 This approach differs from plane-wave codes (e.g. VASP, Quantum Espresso etc.) however both reaching similar results. For hybrid density functionals the localized Gaussian basis set codes are more naturally 9



suited for solving the Hartree-Fock part of the solution. The equilibrium geometries of BLG-TM and bulk-BLG-TM (TMs: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) were obtained by the dispersion-corrected hybrid unrestricted DFT method, i.e. UB3LYP-D2, or DFT-D in short. 35–39,43,44 The semi-empirical Grimme’s “-D2” dispersion corrections were added in the present calculations in order to incorporate van der Waals dispersion effects on the system. 38 This level of theory has shown to give correct electronic properties of 2D/3D materials. 4,45–47 The DFT-D methods (i.e. here UB3LYP-D2) has shown to give quite accurate thermochemistry for both covalently bonded systems and systems dominated by dispersion forces. 4,38,39,43,45–47 Here, we have employed the most popular and extensively used hybrid density functional with dispersion, UB3LYP-D2, because the parameters are available and it is the best compromise of accuracy and speed; and predicts geometry and electronic structure well. In the present computation, triple-zeta valence with polarization quality (TZVP) Gaussian basis sets were used for the C and all TM atoms. 48 20 materials with BLG-TM as building blocks were designed including 10 BLG-TM and 10 bulk-BLG-TM materials: (a) BLG-Sc, (b) BLG-Ti, (c) BLG-V, (d) BLG-Cr, (e) BLG-Mn, (f) BLG-Fe, (g) BLG-Co, (h) BLG-Ni, (i) BLG-Cu and (j) BLG-Zn. Each one has been prepared by adding one TM atom in one unit cell between two graphene layers to construct the BLG-TM and bulk BLG-TM structures as shown in Figure 1 and Figure 2, respectively. The threshold used for evaluating the convergence of the energy, forces, and electron density was 10−7 a.u. for each parameter. The BLG-TM unit cell constructed this way contains one configuration, known as AA, where one atom is exactly above another atom of the other layer of graphene. In both the BLG and bulk-BLG, the TM atoms are inserted into the space between two graphene layers and form intercalated structures, which corresponds to TM/C8 (graphene where the concentration of metal and C atoms is C:TM = 8:1 in brief TM/C8 ) and results in the maximum capacity of bulk BLG during TM atoms intercalation. In both cases, one TM atom was intercalated per unit cell of BLG. Integration inside of the first Brillouin zone was sampled on a 15 x 15 x 1 k-mesh grids

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b)

a)

BLG-Sc

c)

BLG-Ti

d)

BLG-Cr

BLG-V

e)

f)

BLG-Mn

g)

BLG-Fe

h)

BLG-Co

i)

BLG-Ni

j)

BLG-Cu

BLG-Zn

Figure 1: Equilibrium structures of BLG-TM; a) BLG-Sc, b) BLG-Ti, c) BLG-V, d) BLGCr, e) BLG-Mn, f) BLG-Fe, g) BLG-Co, h) BLG-Ni, i) BLG-Cu and j) BLG-Zn. Notice how this structure is infinite periodic in two directions (2D material) and only two layers of graphene are used in the calculation.

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for 2D layered materials and on 20 x 20 x 20 k-mesh grids for the bulk structures, with a resolution of around 2π x 1/60 Å−1 for both the optimization and material properties calculations (band structures and DOSs). We have plotted the bands along a high symmetric k-direction, M − K − Γ − M for the bilayer materials and M − Γ − K − M − L − H − A for the bulk-BLG-TM materials in the first Brillouin zone. Electrostatic potential calculations have been included in the present computation for slabs/surfaces i.e. the energy is reported with respect to the vacuum for the bilayer materials. The band structures and DOSs of the BLG-TM and bulk-BLG-TM materials are shown in Figure 3 and Figure 4, respectively. The contributions of the sub-shells of the C and TM atoms (such as px , py , pz , dyz etc.) in the total DOSs have been also computed for both the BLG-TM bilayer and bulk-BLG-TM materials. The conventional unit cells of the BLG-TM materials have a 2D characteristic in the x and y directions. The z direction, on the other hand, is treated within a vacuum by employing a ∼ 500 Å length for the z axis to accommodate the vacuum environment. The mathematical expression was used to calculate the binding energy of the pristine BLG and BLG-TM, ∆Eb , are given below:

∆Eb = E BLG−T M − E BLG − ET M

(1)

where E BLG−T M is the energy of the BLG-TM, E BLG is the energy of the pristine BLG and ET M is the energy of TM atom.

Results and Discussion The optimized structures of BLG-TM and bulk-BLG-TM materials are shown in Figure 1 and 2, respectively, and the average C-TM and C-C bond distances as well as the intercalation distances between two layers (d) are reported in Table 1. The electronic properties derived from the band diagram and the density of states are also shown in Table 1. The equilibrium 12


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Bulk-BLG-Sc

Bulk-BLG-Ti

Bulk-BLG-V

Bulk-BLG-Cr

Bulk-BLG-Mn

Bulk-BLG-Fe

Bulk-BLG-Co

Bulk-BLG-Ni

Bulk-BLG-Cu

Bulk-BLG-Zn

Figure 2: Equilibrium structure of bulk-BLG-TM intercalated materials; a) Bulk-BLG-Sc, b) Bulk-BLG-Ti, c) Bulk-BLG-V, d) Bulk-BLG-Cr, e) Bulk-BLG-Mn, f) Bulk-BLG-Fe, g) Bulk-BLG-Co, h) Bulk-BLG-Ni, i) Bulk-BLG-Cu and j) Bulk-BLG-Zn. Notice how this structure is infinite periodic in 3 directions (3D material) and only 4 layers are shown. 13



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M

-7

DOS (arb. units)

4

4

3

3

2

2

1

30 24 18 12 6 300 24 18 12 6 0

-4

2

40 30 20 10 0

-4

EF

3

2

40 30 20 10 0

-3

3

40 30 20 10 0

-2

-3

40 30 20 10 0

-2

30 24 18 12 6 300 24 18 12 6

0 -1

Total

40 30 20 10 0

Energy (eV) w.r.to vac Energy (eV) w.r.to vac

c)

0 -1

Sc s p d f

s p d

C

Zn s p d f

s p d

Total

1

0

0

-1

-1

-2

-2

-3

-3

-4

-4

-5

-5

-6

-6

-7

-7

-8

EF

-8

-9

-9

-10

-10

M

K

BLG-Zn

Γ

M

30 24 18 12 6 300 24 18 12 6 0

b) C

40 30 20 10 0

1

40 30 20 10 0

2

1


a) 2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

DOS (arb. units)

Figure 3: Band structure and DOSs of the BLG-TM materials; a) BLG-Sc, b) BLG-Ti, c) BLG-V, d) BLG-Cr, e) BLG-Mn, f) BLG-Fe, g) BLG-Co, h) BLG-Ni, i) BLG-Cu and j) BLG-Zn. The contribution of the individuals s and p orbitals of the C atoms, and s, p and d orbitals of the TM atoms were shown along with total DOSs. 14 ACS Paragon Plus Environment

5

4

4 p d

3

3

3

3

-4

-1

1 0 -1

EF

-2

L

H

A

DOS (arb. units)

5 4

3

3

Energy (eV)

4

1 0 -1 -2

1 0 -1 -2

-5

5

2

C

s p d

Mn

s p d f

Total

2 1 0

EF

-1 -2

f)

3

2

2

Energy (eV)

3

0 -1 -2

C

s p d

Co

s p d f

Total

1 0 -1

EF

-2

h)

Energy (eV)

4

4

1

0

40 30 20 10 100 0 75 50 25 0 12 0 80 40

DOS (arb. units)

Γ

K M BLG-Cr

L

H

-5

A

0 -1 -2

-5

A

-4

M

DOS (arb. units)

2

-5

H

-3

1

-4

L

-2

2

-3

K M BLG-Mn

EF

3

-4

Γ

0 -1

3

-3

M

1

4

-4

-3 -4

M

Γ

K M BLG-Fe

L

H

-5

A

DOS (arb. units)

6

6

5

5

4

4

3 2 1 0 -1

1 0 -1

-3

-3

-5

-4

2

2

1 0 -1 -2 -3

C

s p d

Cu

s p d f

Total

1 0 EF

-1 -2 -3

j)

-1 -2 -3 -4

800 60 40 20 100 0 75 50 25 0

40

-5

DOS (arb. units)

30 20 10

1

-7

A

4

0

-6

H

-4

A

1

-6

L

H

2

-5

K M BLG-Cu

L

2

-5

Γ

K M BLG-Ni

3

-4

M

Γ

3

-4

-6

M

4

Energy (eV)

3

Energy (eV)

4

3

Energy (eV)

4

DOS (arb. units)

EF

DOS (arb. units) C

Zn

s p d

Total

s p d f

0 -1 -2 -3

EF

-4 -5 -6

M

Γ

K M L BLG-Zn

H

A

-7

30 20 10

30 20 10 0 60 40 20 0 60 45 30 15 0

-4

A

Total

s p d f

s p d

2

-5

H

Ni

C

3

-4

L

EF

-2

-2

M BLG-Co

Total

s p d f

0

-2

K

Fe

s p d

-1

-3

Γ

C

1

-3

M

Total

s p d f

2

4

-3

-5

45 30 15

3

2

-4

Energy (eV)

K M BLG-V

4

3

-3

0 75 50 25 120 0 80 40 0

Γ

s d

4

-4

Cr

C

5 p

-3 30 20 10

M

DOS (arb. units)

6

5

0

0

2

6

30

3

d)

-4

A

0

1

Total

H

0 60

2

s p d f

L

0 30 20 10

4

3

V

s p d

K M BLG-Ti

0 40 30 20 10 0

C

Γ

10

5

M

Energy (eV)

5

DOS (arb. units)

30 20 10

A

Energy (eV)

H

30 20 10

L

Energy (eV)

K M BLG-Sc

Energy (eV)

Γ

0

-5

0 90 60 30

-3

-5

M

EF

0 90 60 30

-2

-3

0 60 40 20 0 60 40 20 0

-2

Energy (eV)

Energy (eV) Energy (eV)

0 -1

-4

-4

Energy (eV)

-1

-3

-3

i)

s p d f

1

-4

-2

g)

EF

s -2 p d

0

2

-3

4

e)

-1

1

0 80 60 40 20 0 80 60 40 20 0

-2

0

2

Total

s p d f

0 90 60 30

-1

1

s

0 30 20 10

0

2

Ti

C

0

1

Total

Energy (eV)

2

Sc

C

Energy (eV)

5

4

Energy (eV)

5

4

20

c)

b)

5

30 20 10

Energy (eV)

a)

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

Page 15 of 34

DOS (arb. units)

Figure 4: Band structure and DOSs of bulk-BLG-TM materials; a) Bulk-BLG-Sc, b) BulkBLG-Ti, c) Bulk-BLG-V, d) Bulk-BLG-Cr, e) Bulk-BLG-Mn, f) Bulk-BLG-Fe, g) BulkBLG-Co, h) Bulk-BLG-Ni, i) Bulk-BLG-Cu and j) Bulk-BLG-Zn. The contribution of the individuals s and p orbitals of the C atoms, and s, p and d orbitals of the TM atoms were shown along with total DOSs. 15



lattice constants of the optimized structures of both the BLG-TM and bulk-BLG-TM materials are reported in the Supporting Information. The present DFT calculation found that the equilibrium average C-TM, C-C bond distances and intercalation distances (d) between two graphene layers were changed in both the 2D layer and bulk structure BLG-TM materials as depicted in Table 1. The average C-C bonds in graphene sheet in both the BLG-TM and bulk-BLG-TM materials have a resulting length ∼1.435 Å, which is longer that the C-C bond in an isolated monolayer graphene or bilayer graphene given our recent DFT calculations; 1.416 Å and 1.421 Å, respectively. The C-C and C-TM equilibrium bond distances and d of the other BLG-TM layer materials are in good agreement with the previous computational results. 31,33,34 The intercalation distances (d) of both the BLG-TM and bulk-BLG-TM are plotted in Figure 5 and the trend is similar for the bilayer and bulk analogs. 6

Intercalation distance (d) in Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

2D BLG−TM Bulk BLG−TM

5.5

5 4.5

4

3.5

3

Sc

Ti

V

Cr

Mn Fe

Co

Ni

Cu

Zn

Figure 5: Equilibrium intercalation distance (d in Å) of both the BLG-TM and bulk-BLGTM materials. To estimate the likelihood for the synthesis of these materials, we have calculated the binding energy (∆Eb ) of both systems; BLG-TM and bulk-BLG-TM. The binding energy was calculated using the mathematical expression in Eq. 1, and reported in Table 2. The present DFT study found that all the binding energies are negative indicating that the BLG16


Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TM are more stable than BLG and TM alone. The present DFT-D method showed that the 2D layer structure BLG-Zn has negative binding energy indicating a stable material where as the bulk-BLG-Zn is not a stable compound (i.e. the bulk structure material) as ∆Eb is positive, which is about 0.17 eV as shown in Table 2. Our computational study reveals that both the layer and bulk BLG-Ti materials have the most favorable binding energy among all materials studied here indicating it is the most stable compound, while the second most stable compound is BLG-Sc; see Table 2. The binding energy for both the BLG-TM and bulk-BLG-TM have been plotted and shown in Figure 6. This plot shows how the ∆Eb varies with the TM intercalated BLG materials and BLG-Ti is the most favorable material as it shows the lowest binding energy among all other materials studied here. Both the BLGMn and bulk-BLG-Mn have higher ∆Eb as depicted in Figure 6. This plot also shows how the ∆Eb varies with the TM intercalated BLG materials, with the most stable compounds showing the more negative energies. Table 1: Equilibrium average C-TM and C-C bond distances, intercalation distances (d) and electronic state for the BLG-TM and bulk-BLG-TM materials for the different first row transition metals (TM). The C-TM, C-C bond distances and d were expressed in Å. The Mulliken spin population analysis on the TM atoms was also shown as ‘Spin’. BLG-TM TM C-TM

C-C

d

Sc Ti V Cr Mn Fe Co Ni Cu Zn

1.431 1.438 1.439 1.436 1.434 1.432 1.431 1.430 1.429 1.416

3.760 3.444 3.441 3.363 3.640 3.664 3.461 3.547 3.519 5.785

2.378 2.253 2.243 2.219 2.326 2.335 2.252 2.294 2.274 3.222

Spin

Bulk-BLG-TM State

0.004 Metal 1.087 Metal 2.336 Metal 2.811 Metal 4.034 Metal 2.740 Metal 1.671 Semi-Metal 0.619 Semi-Metal 0.002 Metal 0.000 Metal

C-TM

C-C

d

Spin

State

2.393 2.257 2.259 2.267 2.309 2.247 2.221 2.233 2.238 3.182

1.439 1.442 1.449 1.438 1.438 1.436 1.441 1.430 1.432 1.413

3.800 3.425 3.450 3.471 3.584 3.423 3.368 3.408 3.413 5.701

0.002 1.101 2.664 3.704 4.143 2.861 1.574 0.304 0.000 0.000

Metal Metal Metal Metal Metal Metal Metal Metal Metal Metal

The pore surface area (SA ) of the BLG-TM and pore surface area (SA ), pore volume (VP ) and density (ρ) of the bulk-BLG-TM were estimated and reported in Table 2. Interestingly, 17



Table 2: The binding energy ∆Eb of both the BLG-TM and bulk-BLG-TM. Surface Area (SA ), Pore Volume (Vp ), and Density (ρ) were estimated. The units of the ∆Eb , SA , Vp and ρ were expressed in eV, m2 gm−1 , cm3 gm−1 and gm cm−3 . SA and Vp were estimated from rolling a H2 molecule with an initial solvent diameter of 2.80 Å, over the surface. BLG-TM

Bulk-BLG-TM

TM

∆Eb (eV)

EF (eV)

Dirac Point


-7.67 -13.6 -5.97 -6.30 -0.44 -4.64 -2.73 -3.34 -4.70 -1.89

-2.487 -2.526 -2.49 -2.067 -2.084 -2.262 -2.085 -2.047 -2.049 -2.113

K K K K K K K K K K

Binding energy ∆Eb (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

SA TM (m2 /g) 1101 1085 1069 1064 1048 1045 1032 1031 1011 1007

1 0 −1 −2 −3 −4 −5 −6 −7 −8 −9 −10 −11 −12 −13 −14

Sc


∆Eb (eV)

EF (eV)

Dirac SA VP ρ Point (m2 /g) (cm3 /g) (g/cm3 )

-7.44 -13.48 -6.41 -6.17 -0.15 -4.13 -2.68 -3.44 -4.74 +0.17

-0.987 0.06 -1.007 -0.266 -0.493 -0.934 -1.302 -1.118 -0.543 -3.391

K, K, K, K, K, K, K, K, K, K,

H H H H H H H H H H

620 116 124 168 358 104 88 97 166 1203

0.198 0.184 0.182 0.181 0.183 0.175 0.170 0.172 0.168 0.217

5.05 5.43 5.48 5.13 5.46 5.70 5.87 5.81 5.93 4.60

2D BLG−TM Bulk BLG−TM

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Figure 6: Binding energy (∆Eb in eV) vs. both the BLG-TM and bulk-BLG-TM materials. our present DFT-D study found that BLG-TM have large surface area compared to bulkBLG-TM, and plotted in Figure 7. This calculation indicated that the pore surface area of the BLG-TM was decreased when the larger TM atom was intercalated in BLG. It means that BLG-Sc has the largest pore surface area and BLG-Zn has the lowest pore surface area; see Table 2 and Figure 7. We also observed that the pore volume of the bulk-BLG-TM 18


Page 19 of 34

1200

Pore Surface Area SA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

800 2D BLG−TM

600 Bulk BLG−TM

400

200

0

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Figure 7: Pore surface area (SA in m2 gm−1 ) for both the BLG-TM and bulk-BLG-TM materials. materials decreases as the size of the TM atoms increases. Obtaining the equilibrium structures of both the BLG-TM and bulk-BLG-TM materials, we investigated how the electronic properties of the materials were changed by intercalating the first row transition metals. It has been shown previously that the 2D AA-stacked BLG is a pure semiconductor with a small band gap ∼ 0.25 eV and it has Dirac features. In the present study, we found that the material properties were drastically changed by intercalating first row TMs in the pristine AA-stacked BLG. For example, the band structure calculation showed that BLG-Sc and bulk-BLG-Sc behave as a metal (see Figure 3a and 4a) with large electron density around the Fermi Energy level (EF ). This is due to the contribution of the p-orbital electrons of the C atoms and the d-orbital electron of the Sc atom. The electronic properties of the BLG-Ti and bulk-BLG-Ti structure materials were reported in Figure 3b and 4b, respectively. The present computation found that the intercalation distance d and C-Ti bond distances were decreased whereas C-C bond distances increased in both the BLG-Ti and bulk-BLG-Ti materials compared to BLG-Sc (see Table 1) analog. Interestingly, we found a band crossing point i.e. Dirac point that still exists in both the BLG-Ti materials, but the valance band (VB) and conduction band (CB) are over19



lapped on each other making them conductor. A large electron density was found around the EF due to VB and CB overlapping around the Fermi level as depicted in the “Total” DOSs. A detailed discussion of the contribution of individual sub-shells electron density of the C and TM atoms on the total density of states will be explained later in this article. By intercalating a Vanadium (V) atom in BLG; the BLG-V and bulk-BLG-V materials were prepared. The present calculations found that the intercalation distance d almost remains the same in the bilayer structure while d increasing by 0.025 Å in the bulk structure relative to BLG-Ti. The most interesting results were found in BLG-V as depicted in Figure 3c as the addition of a single V atom intercalated in AA-stacked BLG (i.e. BLG-1V) yields a Dirac material. The calculations reveals that BLG-V has a Dirac Cone in its band structure, and other prominent graphene features. The DOSs around the Fermi energy level indicate metallic behavior. A detailed discussion about the reasons of the Dirac Cone in BLG-V was reported recently. 4 The electronic properties of the bulk-BLG-V were shown in Figure 4c. The Dirac Cone did not appear in the band structure of the bulk-BLG-V material, but the Dirac features and Dirac points remained at the K- and H-points while valence and conduction bands are overlapped making it conducting as shown in Figure 4c. Interestingly, we found that the CB and VB touch each other just above the EF at the H-point (i.e. Dirac Point) making bulk-BLG-V a good conductor. A large electron density appears around the Fermi level which is coming mainly from the p-orbital electrons of C atoms in graphene and d orbital electrons of V, which provide the electron density around the EF as depicted in the right hand side of Figure 4c. The structures and electronic properties were changed due to the addition of Cr and Mn atoms in BLG and bulk-BLG. Both the bond distances (C-Cr and C-C) and intercalation distance d were decreased in the BLG-Cr in comparison with the BLG-V material. But for the case of bulk-BLG-Cr; the C-Cr and d distances were increased whereas C-C bond distances were decreased by 0.011 Å compared to bulk-BLG-V. The C-Mn and d distances were increased in both the BLG-Mn and bulk-BLG-Mn compared to BLG-Cr and bulk-BLG-

20


Page 20 of 34

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr; see Table 1. The Dirac Cone was moved down towards the VB after adding Cr and Mn in the BLG materials. In both the BLG-Cr and BLG-Mn, the VB and CB are overlapped below the Fermi energy level, making them conductors. Thus, the total DOSs calculations found a large electron density around the EF level, and the electron density is coming mainly from the p-orbitals of carbon of the graphene with a small contribution from the d-orbital electrons as depicted in Figure 3d and Figure 3e, respectively. The electronic properties of bulk-BLG-Cr and bulk-BLG-Mn are reported in Figure 4d-e. The valence and conduction bands of the BLG-Cr overlapped and the band crossing or Dirac point was pushed down towards the valence bands and it appeared around 1 eV below the EF . The large electron density that has appeared around the EF suggests, it is a conductor. The present DFT calculation found that another band crossing point, i.e. Dirac point, at the K-point which was close to the EF in the bulk-BLG-Mn as shown in Figure 4e. Interestingly, it was observed that the d-orbital electrons of Mn are not providing any significant electron density in the total DOSs as shown in Figure 4e where as p-orbital electrons of the C atoms are significantly contributing the electron density on the total DOSs. Thus, the DOSs calculation indicates that graphene features are dominant in bulk-BLG-Mn. The discussions about the geometrical structures and electronic properties of the BLGTM and bulk-BLG-TM ( where TM = Fe, Co, Ni, Cu and Zn) materials are reported in detail in the Supporting Information. The band structures, DOSs and individual components of the s-, p-orbiatls electron density of the C atoms and the s-, p-, d-orbitals electron density of the TM atoms (here Fe, Co, Ni, Cu and Zn) of the aforementioned materials are depicted in Figure 3f-j and Figure 4f-j, respectively. The most important findings about the material or electronic properties of the BLG-Fe, BLG-Co and bulk-BLG-Fe and bulk-BLG-Co materials are discussed here. The electronic properties of the Fe and Co atoms intercalated in BLG and bulk-BLG are shown in Figure 3f and in Figure 3g, respectively. Both of the band structures and DOSs of BLG were changed due to the intercalation of the Fe and Co atoms, and the band structure calculations showed that the Dirac point (i.e. band crossing) is

21



pushed down slightly from the EF by about 0.2 eV and 0.5 eV in the BLG-Fe and BLG-Co materials resulting in a conductor as depicted in Figure 3f and in Figure 3g, respectively. Interestingly, the present computations found that the BLG-Co shows semi-metallic behavior like graphene, although the VB and CB are overlapped onto each other, but the total DOSs calculations show that the electron density is close to zero at the EF (see Figure 3g) like monolayer graphene, 4 on the other hand BLG-Fe shows metallic behavior. The DOSs show that the total DOSs follows the p-orbital electron density character of C atoms in graphene and d-orbitals electron has negligible contribution indicating that BLG-Co is a metal with relatively low electron density around the Fermi energy level as depicted in Figure 3g. The calculations found that VB and CB touch each other at the EF yielding a Dirac point at the H-point in bulk-BLG-Co, and at the same time VB and CB touch each other at the K-point around 1.3 eV above the EF . This result means that the graphene features are dominant around the H-point in bulk-BLG-Co. Thus we can predict that bulk-BLG-Co is a Dirac material whereas the 2D layer structure is a metal with zero band gap. The Dirac points at the K-point and H-point are pushed down in the BLG and bulk-BLG materials when they were intercalated by Ni and Cu atoms; see Figure 3h-i and 4h-i. Our present computation reveals that the total DOSs of the BLG-Ni is about zero at the EF although the VB and CB are overlapped below the Fermi energy level, whereas BLG-Cu has large electron density around the EF . In the case of the layer structure; BLG-Cu, a large electron density appears in the total DOSs around the Fermi level resulting in a conductor due to the overlapping of VB and CB below the EF at the K-point as depicted in Figure 3i. The band structure calculations showed that for bulk-BLG-Cu, the Dirac point appeared around the K-point; see 4i. Electron density, which follows the p-orbital character of C atoms of the graphene, occurs around the EF making it as a Dirac material. Additionally for BLG-Zn, the Dirac point at the K-point has been pushed down below the EF level and the total DOSs showed large electron distribution around the EF due to overlap of valence and conduction bands; see Figure 3j. Similarly, the VB and CB of the bulk-BLG-Zn overlap

22


Page 22 of 34

Page 23 of 34

above the EF level resulting in a large electron density around the EF . The present DFT calculation reveals that the total density of states of both the materials (bilayer and bulk structure) follow the major contribution by the p-orbitals, which also causes the large electron density around the Fermi level making them a conductor.

1

-1

0

-2

-1 EF

-5

-5

-4

-6

-6

-5

3

Cp

Fe d d z2 dxz dyz dx2-y2 dxy

px py pz

2

g)5 4 3

Cp

2

1

h)5 4 3

0

0

-1

-1 EF

-2

-3

-3

-4

-4

-5

-5

-6

-6

0

0

-1

-1

-1

-2

EF -2

-7

BLG-Mn

Ni d d z2 dxz dyz dx2-y2 dxy

px py pz

i) 4 3 2

Cp

Cu d dz2 dxz dyz dx2-y2 dxy

px py pz

j)

3 2 0

0

-1

-2

EF

-4

-3

-5 -6

-5

-5

-6

-6

-6

-9

-6

-7

-7

-7

-10

-7

40 30 20 10 0

BLG-Cu

0

40 30 20 10

-8

40 30 20 10 0

40 30 20 10 0

40 30 20 10 0

0

40 30 20 10

40 30 20 10 0

BLG-Ni

0

-5

40 30 20 10

-5

0

-4

40 30 20 10

-4

BLG-Co

EF

-3

-4

BLG-Fe

Zn d dz2 dxz dyz dx2-y2 dxy

px py pz

-2

-3

-4

Cp

1

1

-1 EF

4

-3

-3

EF

BLG-Cr

1

0

EF

Cp

2

1

-2

Mn d d z2 dxz dyz dx2-y2 dxy

px py pz

2 1

-7

Cp

DOS (arb. units)

Co d dz2 dxz dyz dx2-y2 dxy

px py pz

-6

3

1

BLG-V

BLG-Ti

e)4

0

4

EF

40 30 20 10 0

40 30 20 10

40 30 20 10

40 30 20 10 0

-7

BLG-Sc

0

-3

2

Cr d dz2 dxz dyz dx2-y2 dxy

px py pz

-2

-2

-4

3

Cp

0

0

-3

-4

2

d)4

40 30 20 10

-3

px py pz

40 30 20 10

EF

3

Vd dz2 dxz dyz dx2-y2 dxy

Cp

40 30 20 10 0

-2

c) 4

0

1

-1

-7

f)

Ti d dz2 d xz dyz dx2-y2 dxy

px py pz

40 30 20 10

2

Cp

40 30 20 10 0

Energy (eV)

0

b)3

40 30 20 10 0

Sc d d z2 d xz dyz dx2-y2 dxy

px py pz

0

1

Cp

40 30 20 10

2

0

a)

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BLG-Zn

DOS (arb. units) Figure 8: Sub-shells DOSs (i.e. the density of states of the px , py and pz of the p-orbital of C atoms, and the dz2 , dxz , dyz , dx2 −y2 , and dxy of the d-orbital of TM atoms) of the BLGTM materials; a) BLG-Sc, b) BLG-Ti, c) BLG-V, d) BLG-Cr, e) BLG-Mn, f) BLG-Fe, g) BLG-Co, h) BLG-Ni, i) BLG-Cu and j) BLG-Zn. We have further investigated in detail the contribution of the individual sub-shells electrons of the p-orbitals of C atoms and d-orbitals of TM atoms in the total DOSs of all 23



the BLG-TM and bulk-BLG-TM studied here. The individual components of the sub-shells electron density (i.e. px , py , pz , dz2 , dxy , dyz , dxz , and dx2 −y2 ) of all the BLG-TM and bulkBLG-TM are shown in Figure 8 and Figure 9, respectively. The present study reveals that the pz sub-shell of carbon is the main contributing components, which provides the large electron density in the total DOSs of all the bilayer and bulk structure BLG materials. This calculation shows that the total DOSs follow the same trace of the pz sub-shell of carbon atoms in graphene. Among all the d-sub-shells of TMs, in most of the cases, dxy and dyz provide the electron density around the EF level from the transition metals side in the total DOSs for BLG-TM and bulk-BLG-TM as shown in Figure 8 and Figure 9, respectively. Of course there are some exceptions, for example: bulk structure of BLG-Sc showed that dz2 is another contributing component in the total DOSs along with dxy and dyz as shown in Figure 9a. The present calculations reveal that the TM atoms are making 2D layer and bulk structure BLG conductors or metals except for BLG-Co and BLG-Ni, where both the 2D layered and the 3D bulk materials show metallic behavior although they have less electron density around the Fermi energy level. The Dirac point and Dirac Cone move depending on the type of the transition metals intercalation for both the intercalated BLG-TM and bulk-BLG-TM materials. This study found that the Dirac point is pushed down below the EF level in the 2D intercalated BLG except for BLG-V, which showed the Dirac Cone in its band structure. The reasons for the Dirac Cone were discussed in detail in a previous study. 4 Recently, Banerjee and his co-workers 23 experimentally showed that graphene intercalated compounds (i.e. doped multilayer graphene nanoribbons) paves the way for graphene as the next generation interconnect materials for a variety of semiconductor technologies and applications. They also computed the band structure calculations of the FeCl3 intercalated ML-GNR material where the Dirac point moved above the Fermi energy level around 0.68 eV, which is useful in interconnect technologies for integrated circuits (IC). 23 In general, Cu-based interconnects employed in a wide range of integrated circuit products are fast

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b) 5

4

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2

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1

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dyz dx2-y2 dxy

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0

0 30 20 10

30 20 10

4 3 2

p Shellsd C p Shells pCShellsFed C pCShells px dz2 py dxz pz

EF

dx2-y2 dxy

-5

C p Shells

-4

h)6

3 2

4

0

0 -1

EF

C p Shells px py pz

dz2 dxz

dyz dx2-y2

-2

-4

-4 -5

-5

2

-1

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-1

EF -3

BLG-Cr

C p Shells px py pz

1

1

BLG-Mn

j) 4

Cu d Shells dz2 dxz

3

dyz dx2-y2 dxy

2

EF

-1 -2 -3

-4

-3

-5

-6

-5

-5

-4

-6

-7

DOS (arb. units)

EF

BLG-Cu

40 30 20 10 0 40 30 20 10 0

-4

40 30 20 10 0 40 30 20 10 0

20 15 10 5 0 40 30 20 10 0

-5

-4

0

-4

10

-2

20 15 10 5 0 30 20

-3

40 30 20 10 100 0 75 50 25 0

-3

BLG-Ni

Zn d Shells

0

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BLG-Co

C p Shells px py pz

1

-2

BLG-Fe

EF

-1

-3

3

dz2

dxz dyz

0 EF

-3

i) 4

Mn d Shells

dx2-y2 dxy

2

-2

0

EF

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-1

2

dz2 dxz

dyz dx2-y2 dxy

Ni d Shells

dxy

3 px py pz

dyz dx2-y2

C p Shells px py pz

1

DOS (arb. units)

Co d Shells

4

dxy

BLG-V

5

1

-1

0

-3

1

EF

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-4

dz2 dxz

2 1

-3

px py pz

3

dxy

-1

BLG-Ti

dyz

4

1

-2

g) 4

5

2

BLG-Sc

f)

5 d)6 C pCShells Cu d Shells e) p ShellsCr

dyz dx2-y2

3

-2

dz2

P Shells Shells Shells C VVDDShells CP p V D Shellsd px dz2 py dxz pz

75 50 25

px py pz

-4 -5

4

0

-3

c)5

EF -1

-1 -2

V D Shells C pCShells p V ShellsTi D Shellsd px dz2 py dxz pz

75 50 25 0 75 50 25 0

Sc d Shells

0 75 50 25 0

C p Shells

0 75 50 25

5

30 20 10

Energy (eV)

a)

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30 20 10 0 40 30 20 10 0

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BLG-Zn

Figure 9: Sub-shells DOSs (i.e. the density of states of the px , py and pz of the p-orbital of the C atoms, and the dz2 , dxz , dyz , dx2 −y2 , and dxy of the d-orbital of the TM atoms) of bulkBLG-TM materials; ; a) bulk-BLG-Sc, b) bulk-BLG-Ti, c) bulk-BLG-V, d) bulk-BLG-Cr, e) bulk-BLG-Mn, f) bulk-BLG-Fe, g) bulk-BLG-Co, h) bulk-BLG-Ni, i) bulk-BLG-Cu and j) bulk-BLG-Zn.

25



approaching a dead-end due to their increasing resistivity and diminishing current carrying capacity with scaling, which severely degrades both performance and reliability. Here, we found several materials with Dirac points, which might be useful for the next-generation IC and interconnect technology. The interesting thing is that the Dirac point appears in Cr, Mn, Fe, Co, Ni, Cu and Zn intercalated bulk structure BLG at the K-point, which have a similar type of band structure to the FeCl3 intercalated ML-GNR material. The Dirac point is close to the Fermi level of both the Cu and Mn intercalated bulk structure BLG i.e. bulk-BLGCu and bulk-BLG-Mn. Thus, these results indicate that these BLG-TM and bulk-TM-BLG materials might be also useful for the next-generation IC and interconnect technology and they can potentially remove the Cu-based interconnect technology. We showed in this study that these materials are experimentally realizable given their thermodynamic stability, thus holding the promise for a next generation of materials for interconnects.

Conclusion In summary, first row transition metals (Sc to Zn) intercalated 2D layer as well as bulk structure bilayer graphene have been investigated using first-principles dispersion-corrected hybrid density functional theory calculations. This amounts to the prediction of 20 materials. The equilibrium geometry were calculated for both the 2D-layered and 3D-bulk TM intercalated materials, BLG-TM and bulk-BLG-TM, respectively. Their electronic properties (both the band structures and DOSs) were computed by the aforementioned first principles method. The individual components of the sub-shells of the p-orbitals (i.e. px , py and pz ) of C atoms and the d-orbitals (i.e. dz2 , dxz , dyz , dx2 −y2 , and dxy ) of the TM atoms, which are taking part in the total electron density in DOSs have also reported along with the DOSs calculations. Among all the 2D layer nanostructure materials, the present study found only the BLG-V material has a Dirac Cone in its band structure, while the others BLG-TMs still contains the Dirac point. Intercalation of first row transition metal atom in bilayer graphene

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opens new possibilities to engineer electronic properties by moving the Dirac point by controlling the TM intercalate. Similarly, transition metal atom intercalation in bulk structure BLG opens up the possibility of a general understanding of the intercalation behavior of BLG, modifying the two-dimensional properties and the next generation interconnect material for a variety of carbon-based semiconductor technologies, integrated circuits and other applications. This study will enhance the recent advancement in graphene nanotechnology, semiconductor technology, interconnect technology and opens a new avenue of creating both 2D layer as well as bulk structure bilayer graphene intercalates. Our results show that the interaction of TM atoms and bilayer graphene are thermodynamically stable and the Dirac point can be controlled by a judicious choice of the TM to make Dirac materials. Thus, first row transition metal intercalation in both 2D layer and 3D bulk bilayer graphene could also offer an interesting research approach for the creation of heterostructures made of different TM layers and different mixtures of concentrations of TM, which might give rise to even more exotic properties with applications that require intrinsec control of the electronic properties in materials sciences and nanotechnology.

Acknowledgement This work was supported by start up funds from Florida State University (FSU). J.L.MC. gratefully acknowledges the support from the Energy and Materials Initiative at FSU. The authors thank the High Performance Computer cluster at the Research Computing Center in FSU, for providing computational resources and support. S.P. is grateful to Mr. Kevin P. Lucht from FSU for helpful discussions and guidance with computational resources. S.P. also thanks DST-SERB, Govt. of India for providing his highly prestigious Ramanujan Faculty Fellowship. The authors thanks to the reviewers for their suggestions and comments.

27



Supporting Information Available The supporting Information is available free of charge on the ACS Publications website. Section 1: Detail Discussions of the Structure and Electronic Properties of the BLG-TM and bulk-BLG-TM (where TM: Fe-Zn). Section 2: Optimized Structures (.cif format) of the 2D layer structure BLG-TM materials. Section 3: Optimized Structures (.cif format) of the 3D bulk crystal structure BLG-TM materials (i.e. TM intercalated BLG). This material is available free of charge via the Internet at http://pubs.acs.org/.

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TOC Graphic

3 2

-3

EF

-4 -5 -6 -7 -8 -9 10

M

K

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4 3 2 1

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La

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Bu

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Tuning the Dirac Cone of Bilayer and Bulk ... - ACS Publications

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