First–principles study of the structural and electronic properties of graphene absorbed on MnO(111)

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Accepted Manuscript

First–principles study of the structural and electronic properties of graphene absorbed on MnO(111) surfaces

Victor V. Ilyasov, B.Ch. Meshi, I. Popova, Igor V. Ershov, Nguyen N. Hieu, Chuong V. Nguyen

PII: S2210-271X(16)30431-5

DOI: http://dx.doi.org/10.1016/j.comptc.2016.10.017

Reference: COMPTC 2285

To appear in: Computational & Theoretical Chemistry

Please cite this article as: V.V. Ilyasov, B.Ch. Meshi, I. Popova, I.V. Ershov, N.N. Hieu, C.V. Nguyen, First–

principles study of the structural and electronic properties of graphene absorbed on MnO(111) surfaces, Computational & Theoretical Chemistry (2016), doi: http://dx.doi.org/10.1016/j.comptc.2016.10.017

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First–principles study of the structural and electronic properties of graphene absorbed on MnO(111)

surfaces

Victor V. Ilyasov

^a

, B. Ch. Meshi

^a

, I. Popova

^a

, Igor V. Ershov

^a

, Nguyen N. Hieu

^b

, Chuong V. Nguyen

^b,c,

∗

aDon State Technical University, Rostov on Don, Russia

bInstitute of Research and Development, Duy Tan University, Da Nang, Vietnam

cDepartment of Material Science and Engineering, Faculty of Mechanical Engineering, Le Quy Don Technical University, Ha Noi, Vietnam

Abstract

In this work, adsorption of graphene on polar MnO(111) surface with and without hydrogen coverage was investigated by density functional theory. Local atomic reconstructions of the graphene/H:MnO(111) interface and their thermodynamic and electronic properties were analyzed for different adsorption models. Bond length and adsorption energy were found for different reconstructions of surface atomic structure in the graphene/H:MnO(111) systems. Effect of graphene adsorption on the electronic spectrum of the graphene/H:MnO(111) interface was also studied. The effective charge of carbon atoms and nearest-neighbor atoms were determined for the considered adsorption models. Our calculations show that the charge transfer from carbon atom to nearest-neighbor atoms is due to reconstruction of the local atomic and electronic structures, correlating with the interface hydrogenation concentration. At the interface hydrogen concentration ofΘ = 1.0 ML (monolayer), the

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p–njunction was observed in the graphene and a new state,n–type semiconductor, is qualitatively emerged.

Key words: graphene; interface; electronic properties; adsorption; semiconductor.

1 Introduction

Graphene is one of the most important two-dimensional (2D) carbon nanos- tructures for production of spintronic devices due to its remarkable physical properties [1–3]. Graphene on various surfaces has been investigated by both theoretical and experimental studies [4–8]. Features of the atomic and band structure of the interface between graphene and MnO(001) surface for ferromagnetic and antifer- romagnetic ordering have been studied by density functional theory (DFT) [9, 10].

Based on DFT calculations of the structural energy in the graphene/MnO(001) systems, their stability was evaluated and the chemical bond energy was determined [9, 10]. Spontaneous spin polarization in the 3d bands of manganese and the 2pbands of the oxygen and carbon atoms in the SLG/MnO(001) system has been investigated [10]. By using DFT+U approximation, Ershov et al show that the MnO(111) polar surface (terminated by oxygen atoms) has a semi-metallic band structure [9]. This character is useful application of these surfaces in spintronic devices for realization of spin-dependent transport of electrons.

The electronic properties of graphene are very sensitive to substrate [6], including changes in the condition of the substrate surface. Graphene can be bonded to a polar surfaces (111) of α–quartz [11]. It is known that graphene is adsorbed on the oxygen-terminated polar surfaces (111) and the charge transfer between graphene and oxygen has been observed [12]. There are no earlier studies of the

∗ Corresponding author.

Email address:chuongnguyen11@gmail.com(Chuong V. Nguyen).

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chemosorption processes in the graphene/MnO(111) systems. We believe that the changes in the structural and electronic properties of graphene as a result of interaction with the dielectric substrate deserve consideration in detail.

This work contains our first-principles analyses of interaction of graphene with different interfacial surface configurations, including polar surfaces (111) with a single (double, triple) unsaturated bond for the oxygen surface atoms, and with a fully hydrogenated O–polar surface of the substrate. We used DFT to study the adsorption of graphene, the structural and electronic properties in the graphene/H:MnO(111)

systems. Possible modulation of band gap and interface surface properties of graphene/H:MnO(111) systems are also investigated.

2 Model and computational method

The theoretical model of the graphene/H:MnO(111) system is built on the triperiodic slab scheme. We used a supercell of 58 atoms terminated by a monolayer of oxygen atoms and containing unit cells(2×2)of MnO in the (111) plane.

This surface is formed by cleaving of the MnO face-centered cubic lattice along the body diagonal, and is characterized by intercalation of the atomic layers of oxygen with the layers of manganese. The neighboring layers of manganese have the oppo- site direction of the spin magnetic moments, which is characteristics of the antifer- romagnetic structure. Previous theoretical study shows that this surface is stable at high oxygen pressure values, and is one of the possible thermodynamically stable forms of existence of MnO polar surfaces [13]. Fig. 1(a) shows a fragment of the slab simulating the graphene/H:MnO(111) interface. The possible positions of the atomic hydrogen on the polar surface (111) are shown in Figs. 1(b,c,d). Graphene is centered on the substrate’s oxygen atom, which is in line with the bonding position over the oxygen atom. This configuration requires the minimum energy in compar-

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ison with the other bonding positions. We have considered four different configurations for the hydrogen atom in the interface between graphene and MnO(111) slab: four (or three) hydrogen atoms are bonded with the oxygen atoms in the upper layer (in the case of full coverage, the fourth hydrogen atom is covered by the carbon atom number 10); two hydrogen atoms are bonded with the nearest-neighbor oxygen atoms; one hydrogen atom is bonded with the oxygen atom in the upper layer. For comparison, we have considered the graphene/MnO(111) system with an unhydrogenated interface.

Fig. 1. (Color online) Model of orientation of the carbon and hydrogen atoms in relative to the substrate. Slab fragment simulating the interface with different views: (a,c) graphene/H:MnO(111) and (b,d) graphene/MnO(111)

The H:MnO(111) slab consists of 11 nonequivalent planes in the [111] direc-

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tion: four manganese planes, five oxygen planes, and two hydrogen planes. The selected vacuum gap width was 12 Å, which made it possible to exclude all interaction caused by slab translations in the [111] direction. In this work, we performed the self-consistent calculations of the total energy based on DFT with the pseudopotential approximation (Quantum-Espresso code) [14]. For the exchange- correlation energy, we used the functional form of Perdew-Burke-Ernzerhof (PBE) model within the generalized gradient approximation (GGA) [15,16]. For the plane waves used in the expansion of the pseudo-wavefunctions, the cutoff energy was 952 eV. For calculation of all surfaces, we used the k–points generation scheme by the Monkhorst–Pack method with an 2D(9×9×1)mesh. The achieved total cell energy convergence was at least 10⁻⁶ Ry/cell. Ultrasoft pseudopotentials in the Vanderbilt parametrization scheme were used to describe the interaction between the valence electrons with the core. The pseudopotentials were built with the use of the more reliable Troullier-Martins scheme [17]. The following electronic configurations were used for the corresponding atoms: Mn – [Ar]3s²3p⁶d⁵, O – [He]2s²2p⁴, C – [He]2s²2p². The [Ar] and [Ne] states are core states.

It is well known that the local and semi-local approximations for the exchange- correlation energy (LDA, GGA, including PBE) within the framework of the DFT cannot be used for correct description of the dispersion interaction in the layered structures [18, 19]. Since the dispersion interaction is due to solely non-local dy- namic correlation effects, it makes no sense to use the hybrid exchange-correlation functionals for DFT calculations, because they consider only the non-local (Hartree- Fock) exchange. In this work, we allowed for van der Waals interaction in our system within the framework of the DFT by using a semi-empirical potential of the C6R⁻⁶ type introduced into the total energy functional (DFT-D2) in accordance with [18]:E_DFT–D =E_DFT+E_disp. For calculations by the DFT-D2 method, we used the well-known PBE exchange-correlation functional adjusted for dispersion (PBE-

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D2). To achieve a more correct description of the3dstates of the Mn metal in the oxide structure, we entered the Hubbard correctionU = 6eV as in Ref. [13]. The effect of the lower surface of the slab on the band structure was reduced to the minimum through passivation by hydrogen atom, i.e. we considered a H:MnO(111):H type of substrate.

The oxygen adsorption energy in the graphene/H:MnO(111) system was determined on the basis of the expression [20]:

E_ads= (E_slab+SLG−E_slap−E_SLG)/N, (1)

whereE_slab+SLGis the total energy of the supercell with interface,E_slap is the total energy of the substrate unit cell, E_SLG is the energy of graphene, and N is the number of carbon atoms in the calculated graphene cell.

Based on the Löwdin population analysis [21], we determined the effective charges on the carbon atom and on the nearest-neighbor surface atoms of oxygen and manganese for five carbon adsorption models of the graphene/H:MnO(111).

3 Results and discussion

3.1 Atomic structure of 2D graphene/H:MnO(111) system

At first, we studied the features of the atomic structure of the interface between graphene and the H:MnO(111) hydrogenated substrate terminated by oxygen. For this purpose, relaxation was performed for two upper double atomic layers (Mn, O) in the slab of manganese monoxide and graphene monolayer for different hydrogen concentrations of the interface oxygen. Initially, graphene was located at the distance of 2 Å from the H:MnO(111):H surface. The lower two double layers (Mn,O) and one bottom layer of hydrogen in the 2D graphene/H:MnO(111) system were "frozen". Relaxation was continued until the aggregate of all forces

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Table 1

C–O, C–H, C–C, C–Mn, and Mn–O bond lengths with different hydrogen concentrations (hydrogenation) of the interface in the graphene/H:MnO(111) and graphene/MnO(111) systems after relaxation. The data for the atoms involved in chemical bonds with graphene is putted in parentheses as shown in Fig. 1(b). All bond lengths are in Å.

Bond type Hydrogen coverage of the interfaceΘ

0 0.25 0.50 0.75 1.0

C–O 2.69 (1.44) 3.41 3.25 3.41 3.40 (3.33) 3.36 [22]; 3.40 [23]

C–H - 2.48 2.34 2.47 2.47 (2.37)

C–C 1.37 1.37 1.37 1.37 1.37

C–Mn 3.69 4.52 4.60 4.74 4.55

Mn–O 2.11 2.16 2.1 2.17 2.20

acting on the system became less than 0.001 eV/Å. The atomic structure of the four-layer slab containing graphene for the four different configurations of the graphene/H:MnO(111) system after relaxation is shown in Fig. 1(a). The equilibrium lattice constants were found, and the atomic positions of the carbon atoms in graphene and the atoms in the upper two double layers of manganese monoxide were determined. We also determined the bond lengths between the carbon atoms in graphene, and the distances between the interface atom pairs, as well as the length of the Mn–O bond in the upper layers of the manganese monoxide slab for the five different configurations of the graphene/H:MnO(111) and graphene/MnO(111) systems after relaxation. The result is listed in Tab. 1.

Our DFT calculations of the interplanar distances in the graphene/H:MnO(111) system show that in the direction of h111i there emerges a distance between the layers of manganese and oxygen atoms, and this distance depends on the hydrogen concentration of the interface polar surface (111). The calculated results are listed

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Table 2

Vertical distances (in Å) between the hydrogen atom and the oxygen in the first layera, the oxygen first layer and the manganese layerb, the manganese layer and the oxygen in the third layerc, the oxygen in the lower layer and the passivation hydrogend, the energyEads

of graphene adsorption on the unhydrogenated MnO(111) surface and on the hydrogenated H:MnO(111) surface.

Interface hydrogen concentrationΘ a b c d E_ads, meV/atom Method

0 - 1.00 1.51 0.96 86 PBE-D2

- 0.91 1.40 0.96 34 PBE

0.25 0.96 1.19 1.48 0.96 59 PBE-D2

0.50 0.96 1.21 1.43 0.96 56 PBE-D2

0.75 0.96 1.20 1.46 0.96 53 PBE-D2

1.0 0.96 1.24 1.45 0.96 51 PBE-D2

in Tab. 2. In particular, this distance isb = 1.19Å andb = 1.00Å corresponding to atΘ = 0.25ML (monolayer) andΘ = 0, respectively. The distance between the manganese second layer and the oxygen third layer wasc= 1.48Å andc= 1.50Å for the hydrogen concentration ofΘ = 0.25ML andΘ = 0, respectively. In this part, we focus on the changes in these distances for different reconstructions of the atomic surface structure of the graphene/H:MnO(111) interface at different hydrogen concentrations. At the bottom of the slab, the O-H bond length has been taken to be a similar the bond in the interface (see Tab. 2). These bond lengths (see Tab. 1) and the a, b, c,and ddistances (see Tab. 2) for different configurations of the atomic surface in the graphene/H:MnO(111) interface partially characterize the local atomic structure. AtΘ = 1.0ML, the distance increases tob = 1.24Å, which reflects the general tendency in cases of interface hydrogenation.

From Tabs. 1 and 2, we can see that the restructuring of the local atomic structure conditioned by the bonding position of the hydrogen atoms on the polar surface of the graphene/H:MnO(111) interface. For instance, in the upper double atomic

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layer (Mn,O) the length of the Mn–O bond at Θ = 1.0ML has grown by 1.8 % in comparison toΘ = 0.25ML. At the same time, at Θ = 1.0 ML there is a reduction of the average lengths of the C–O and C–H bonds by 0.5 % to 3.4 % (for the C₁₀ atom) in comparison to the case of Θ = 0.25 ML. In the process of relaxation, e.g. forΘ = 1.0ML, the graphene layer, initially located at the distance of 2 Å from the H:MnO(111) surface, moved away a distance of 2.37 Å from the hydrogen monolayer. As a result of relaxation, the average bond length between the atoms of carbon and oxygen was more than d_C–O = 3.36Å. This value is typical of the van der Waals distance between oxygen atoms with two unpaired electrons andsp²–hybridized carbon [22]. The hydroxyl groups (OH) were oriented perpen- dicularly to the interface surface, as shown in Fig. 1(a). The O–H bond lengths wered_O–H = 0.96Å for all considered configurations. For the considered pair of oxygen and hydrogen atoms the electronegativity difference (ED) was 1.24 X [24], while the d_O–H bond length was comparable with the H–O hydrogen bond length in water (0.96 Å [25]). The above findings correlate with the physical concept of the hydrogen bond [26] and make it possible to assign the O–H bond simulated in this work to the hydrogen type. It is wll known that [25, 26], the hydrogen bond energy is approximately 10 times higher than the van der Waals interaction energy, but weaker, by an order of magnitude, than the covalent bond, which is of prime importance for our further analysis.

The results of the DFT calculations also showed that the lengths of the σ–

bonds in graphene are well-described by means of the PBE-D2 functional. For the graphene/H:MnO(111) heterostructure, the average length of the C–C bond in graphene was 1.37 Å, which points to its deformation in the considered supercell with substrate. The C–C bond length was 3.5 % for the deformation case and it did not significantly affect on the band spectrum of graphene in the low-energy region.

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3.2 Adsorption energy of graphene on H:MnO(111) surface

Adsorption of graphene on the H:MnO(111) hydrogenated surface leads to change in the interplanar distances between the upper layers of the manganese and oxygen atoms (see Tab. 2). For the considered five configurations differing in the hydrogen concentration of the oxygen interface layer in the graphene/H:MnO(111) system, we obtained the the energy of adsorption of graphene on non-hydrogenated and hydrogenated substrates. The results are listed in Tab. 2. From Tab. 2, we can see that the hydrogen passivation of one surface atom reduces the adsorption energy by ∆E_ads = 27 meV/atom. The nature of this phenomenon has not been discussed in previous studies. When the hydrogen concentration of the oxygen interface layer increases toΘ = 1.0ML, the adsorption energy decreases by 1.7 times. The 40.7 % decrease of the adsorption energy after hydrogenation of the graphene/H:MnO(111) interface is connected, in our opinion, with reduced interaction between graphene and the substrate. Note that in this system, hydrogenation of the oxygen interface layer is the controlling mechanism in the occurring chemosorption processes.

Our calcualtions for the graphene adsorption energy (0.086 eV/atom) in the unhydrogenated graphene/MnO(111) system agree with the previous estimation (0.11 eV/atom [7]) and (0.040 eV/atom [19]) for the SLG/Al2O3(0001) system terminated by oxygen and aluminum, respectively.

Our additional DFT calculations show that in the absence of hydrogenation (passivation) the interface between graphene and the MnO(111) substrate undergoes a geometrical reconstruction as shown in Fig. 1(b). In this case, the atomic structure of graphene changed in comparison with the structure of graphene adsorbed on the hydrogenated H:MnO(111) substrate. In the graphene/MnO(111) system, the carbon atom number 10 [see Figs. 1(b,d)] positioned over oxygen (on

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top of O) approaching the surface oxygen atom to the distance of 1.44 Å. It is well known that the C–O bond length in ethylene oxides and ethers is from 1.42 to 1.49 Å [27, 28]. Therefore, it can be assumed that the C–O bond is formed in the carbon over oxygen positions. An indirect evidence of the existence of such bond is the adsorption energy of the carbon atom in the position over oxygen be- ingE_ads = −0.086 eV/atom. However, such carbon atoms make only 25 percent in the calculated graphene cell. The other carbon atoms remain at the distance of 2.69 Å from the oxygen interface layer, which is 1.6 times more than the maximum recorded C–O bond length (1.622 Å [29]) in 1,4,7-tri-tern-butyloxatriquinane. In the previous study [7], Entani et al have been experimentally found that the vertical distance (2.6 Å) between graphene and the α–Al₂O₃(0001) surface is terminated by oxygen. The adsorption energy in the SLG/α–Al₂O₃(0001) interface was from 0.11 to 0.13 eV/atom [7], which is close to our results (0.086 eV/atom) for the graphene/MnO(111) system without hydrogenation of the interface. In the physical view, Entani and co-worker believed that the interaction between theπ–electrons in graphene and the unsaturatedp_z–electrons in the oxygen upper layer is of the electrostatic type, not of the van der Waals type [7]. This study consider the distribution of the effective charges on the interface atoms, as described below. Another matter of interest is the role and the effect of the substrate’s near-surface metal type on the nature of the interfacial interaction. The change in distance between the layer of manganese atoms and graphene (from 3.69 Å to 4.74 Å) plays lesser role than the oxygen polar surface (111) in the interface. However, we will next demonstrate the effect of near-surface manganese layer on the interaction between graphene and the substrate. Note that in the SLG/α–Al₂O₃(0001) interface, the oxygen layer did not manifest any special properties in comparison with the substrate metal surfaces [7, 30].

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Table 3

Effective charges on the interface atoms for different hydrogen concentrations Θ of the oxygen polar layer in the graphene/H:MnO(111) system. The data for the atoms involved in chemical bonds with graphene is putted in parentheses.

Hydrogen concentrationΘ Effective charge on the atom,e

Mn O H C

0 0.93 (0.90) −0.42 (−0.20) - 0.03 (0.32)

0.25 0.89 −0.61 −0.06 0.032 0.50 0.89 −0.59 −0.03 0.020 0.75 0.91 −0.56 −0.01 0.010 1.0 0.93 −0.55 −0.04 0.019

To gain a better understanding of the chemosorption processes, we study the distribution of the effective charges on the atoms of the graphene/H:MnO(111) interface for different hydrogenation rates. In Tab. 3, we show our DFT calculations of the effective charges on the carbon atoms and on the nearest-neighbor surface atoms of hydrogen, oxygen and manganese for the considered configurations of the graphene/H:MnO(111) systems.

As shown in Tab. 3, general tendency for charge transfer from the carbon atoms to the interface (hydrogen and oxygen) atoms is similar as reported in Refs. [12, 23]. This general pattern is not true only for the obtained values of the effective charges on the atoms of manganese, but also on oxygen and carbon atoms in the unhydrogenated interfacial surface. In the graphene/MnO(111) system, the significant charge transfer from the oxygen atoms to the carbon atoms is partially re- plenished by charge transfer from the manganese atoms to the oxygen interface atoms. Based on our DFT calculations, it can be asserted that the neighbor carbon and oxygen atoms have effective charges very low (−0.20e) for the oxygen and very high(0.32e) for the carbon. In ten carbon atoms forming two hexagons

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in Fig. 1(d), only three atoms have positive effective charge with the total value ofQ_ef = 0.38e, while seven atoms have a negative charge with the total value of Q_ef =−0.18e. Considering that the charge may be redistributed among the carbon

atoms, it can be assumed that the graphene sheet is charged positively (+0.20e).

The four oxygen atoms in the upper layer have the total charge ofQ_ef = −1.47e.

A simple calculation of the potential energy of electrostatic interaction between graphene and the substrate yields an estimate of 2 eV. This value reflects the interaction of the whole graphene sheet with the substrate, therefore one carbon atom should account for about 0.14 eV/atom. Based on the above simple estimation, we can be surmised that electrostatic interaction explains the graphene chemosorption mechanism (0.086 eV/atom) in our contemplated system.

Note that for the hydrogen concentration of Θ = 0.75 ML, the effective charges on the carbon atoms were reduced twice, while on the hydrogen atoms they were increased thrice in the totally hydrogenated interface (Θ = 1.0ML). Specific features of the atomic configuration may be responsible for this effect. Note that in an atomic configuration withΘ = 0.75 ML, the distance between the oxygen interface layer and the underlying manganese layer is reduced by 3% (see Tab. 2) in comparison to the case of totally hydrogenated interface (Θ = 1.0ML). In this case, the recorded difference between the effective charges is accounted for inten- sity of the transfer mechanisms. In configurations with the hydrogen concentration of 0, 0.25 ML, 0.5 ML, 0.75 ML, and 1.0 ML, charge transfer is observed from the manganese atoms to the oxygen atoms. Generally, charge transfer determines the mechanisms of the process of graphene chemosorption on the polar surface (111) and it is conditioned by the significant differences in the Pauling’s electronegativity of the manganese atoms (1.55X), oxygen (3.44X), carbon (2.55X), and hydrogen (2.20X) [24] However, it is well known that, under the conditions of weak interaction, the governing mechanism for interface charge transfer is that of physical

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absorption [7]. The charge transfer process will continue until an equilibrium of the chemical potentials of graphene is established and the H:MnO(111) substrate is reached after their joining.

From configurations of the atomic surfaces in the graphene/H:MnO(111) interface, we can see that hydrogenation of the interface leads to a significant restructuring of the local atomic structure. The hydrogenation of the oxygen interface layer partially reduces the interaction between graphene and the H:MnO(111) substrate (see Tab. 2). The disturbance in the local atomic structure affects to the electronic energy spectrum of the surface atoms of carbon, oxygen, manganese and hydrogen in the considered configurations of the graphene/H:MnO(111) systems.

3.3 The electronic structure of systems SLG/H:MnO(111)

In order to deeply understand the physical nature of the interaction between graphene and hydrogenated H:MnO(111) surface, we consider the band structure of five different configurations of the graphene/H:MnO(111) and graphene/MnO(111) systems after relaxation. Fig. 2 show the band structure and the density of states (DOS) of the graphene/H:MnO(111) system at hydrogen concentration of 0, 0.25 ML, and 0.50 ML. The the band structure and DOS of this system at hydrogen concentration of 0.75 ML, and 1.0 ML are shown in Fig. 3. Both spin up and spin down are taken into account.

First, we study the effect of the hydrogen concentration of the oxygen upper layer in the MnO(111) substrate on the band structure of substrate. We start with the MnO(111):H substrate, where only the lower layer of oxygen was passivated by hydrogen. It is known [31] that the electronic properties of the transition metal oxides are fully determined by their band structure near the Fermi level. Therefore, this work focus on band structures of the substrate and the graphene/H:MnO(111)

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PDOS (arb. units) C-s,p,

PDOS (arb. units) Mn-s,p,d

PDOS (arb. units) O-s,p

DOS (arb. units Total

Γ M Κ Γ Γ M Κ Γ

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

MnO SGL 0

MnO SGL 0.25

MnO SGL 0.5 (a)

(b)

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spin up spin down

Fig. 2. (Color online) DFT calculation of the band structure, the partial DOS for the atoms of carbon, manganese and oxygen, the total DOS for the graphene/H:MnO(111) systems:

(a) without hydrogenation, (b)Θ = 0.25ML, (c) andΘ = 0.50. Both spins are taken into account. The Fermi level equals to zero.

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system near the Fermi level. Next, we consider the interaction between graphene and MnO(111) substrate without hydrogenation. As shown in Fig. 2(a), band structure of the adsorbed graphene for both spins undergoes significant changes relative to the electronic structure of the graphene without a substrate (see Fig. 4).

We know that the non-hydrogenated substrate contributes to opening of the band gap between the bonding and the antibondingπ-bands of graphene, about 0.85 eV, in the electronic subsystems for both spins. Additionally, in the spin-down electronic subsystem, a narrower forbidden gapE_g = 0.20eV is formed at the Fermi level. The linear dispersion law at the tops of the π-bands (at the Dirac point)

PDOS (arb. units) C-s,p

DOS (arb. u Total

PDOS (arb. units) C-s,p

DOS (arb. u Total

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

(a)

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

(b)

MnO SLG 0.75 MC

MnO SLG 1.0 MC

spin up spin down

Fig. 3. (Color online) DFT calculations of the band structure, the partial DOS for the atoms of carbon, manganese and oxygen. The total DOS of the graphene/H:MnO(111) systems for the hydrogen coverage of 0.75 ML (a) and 1.0 ML (b). Both spins are taken into account.

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changes to the parabolic relation, which testifies to emergence of the finite effective masses of the carriers. Besides, these bands tend approaching to the Fermi level. This points to the fact that despite of the weak bond between graphene and the substrate (E_ads = 0.086 eV/atom), this interaction governs the charge transfer in the interface of the considered system. This leads to the charge transfer in the interface shifting the Fermi level to the Dirac cone top, and it is controlled by the substrate’s work function channel [4, 30]. The charge transfer in the interface shifts the Fermi level relative to the Dirac cone top, and is controlled by the substrate’s work function channel. The Fermi level shift in graphene is determined as ∆E_F = E_F − E_D , where E_D is the middle of the forbidden gap (or the top of the Dirac cone) of adsorbed graphene on the surface of MnO(111) [30].

We can see that the adsorption of graphene on the unhydrogenated substrate in the graphene/MnO(111) system is accompanied by a downward shift of the Fermi level. Besides, the doped graphene (+0.20e) has the band structure of a p-type semiconductor. This phenomenon may be connected with the difference in the work functions between graphene and the MnO(111) surface, as observed in the graphene/metal interface [4, 30]. Therefore we studied, based on the DFT calculations, the effect of hydrogenation on the work function of adsorbed graphene and of the O-polar interfacial surface. Our DFT calculations of the work function are presented in Tab. 4. Our calculations shown that, the work function of freestanding graphene is 4.50 eV, which is in good agreement with previous theoretical study of 4.26 eV [8] and experimental value of 4.49 eV [32]. As shown in Tab. 4, in the case of the adsorption of graphene on the unhydrogenated MnO(111) surface terminated by oxygen,∆E_F is equal to 1.33 eV and this is a maximum value. In this case, the difference in work function between MnO(111) and graphene without substrate is (W −W_G) = 3.2 eV. Based on the general principles of physics, one can expect the charge transfer from graphene to the substrate. This process is known as p-

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doping of graphene. When the hydrogenation mechanism is applied, the value of the downward shift of the Fermi level decreases practically linearly. In particular, for the graphene/H(0.5):MnO(111) configuration, where the hydrogen coverage is Θ = 0.5ML, adsorption of graphene on the substrate causes the downward shift of the Fermi level ofE_g = 0.65eV. This system also shows the charge transfer of 0.02efrom graphene to the hydrogen atoms (see Tab. 3). The band gap in this case isEg = 13meV, and the band structure can be regarded as ap-type semiconductor [see Fig. 2(c)]. WhenΘ = 0.75ML, the downward shift of the Fermi level is just

∆E_F = 0.02eV. The difference in work function is(W −W_G) = −0.21eV. The band gap isE_g = 11 meV and the charge transfer from graphene to the hydrogen atoms is0.010e(see Tab. 3). Thus, we can see that when the hydrogen concentration increases, the charge transfer from graphene to the substrate decreases. The band structure of the graphene/H(0.75):MnO(111) system remains that of ap–type semiconductor (see Fig. 3).

When the hydrogen concentration increases toΘ = 1.0ML, the Fermi level shifts upward by the value of ∆E_F = −0.09eV. In this case, according to [30], the system corresponds to then–type semiconductor, subject to presence of a forbidden band. The dependence of band gap on the hydrogen concentration is also shown in Tab. 4. The band gap is inversely proportional to the hydrogen concentration. Besides, the work function of the graphene adsorbed on the hydrogenated H(1.0):MnO(111) surface is two times higher than one of the substrate. The re- verse is true for the unhydrogenated MnO(111) surface, where the graphene work function is 1.7 times less than that of the substrate. It should be emphasized that hydrogenation of the interface leads to a significant change in the electronic properties of the interface, in particular, reduction of the work function for the adsorbed graphene and for the manganese monoxide surface.

It is important that the process of interface hydrogenation to the hydrogen con-

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Table 4

Band gapE_g, graphene work functionW_G, substrate work functionW_S, Fermi level shift

∆E_F, local magnetic moment MM on the interface atoms for different hydrogen concentrations of the oxygen polar layer in the graphene/H:MnO(111) system. The data for the atoms involved in chemical bonds with graphene is putted in parentheses.

Hydrogen coverage of the interfaceΘ E_g, eV W_G, eV W_S, eV ∆E_F, eV

0 0.84 (0.20) 4.50 7.72 1.33

4.58 [7] 8.9 [29] 1.3 [7]

0.25 0.005 4.47 6.45 0.98

0.50 0.013 4.41 5.34 0.65

0.75 0.011 3.93 4.29 0.02

1.0 0.001 3.93 1.38 −0.09

centration ofΘ = 1.0ML makes it possible to obtain a qualitatively new state, that isn–type semiconductor. The latter makes it possible to developn-type graphene field-effect transistors, which is of significant interest for nanoelectronics. The hydrogen concentration of the oxygen interface layer in the graphene/H:MnO(111) system can be used as a possible controlling mechanism of thep−njunction. In particular, the previous work have been shown that we can control the electronic properties of graphene by means of chemical modification of the substrate surface, preliminary to graphene deposition [22]. Besides, the graphene adsorption energy is 0.210 eV/cell, which is typical character of the processes of physical adsorption [22].

Band structures of the H:MnO(111) ultrathin layer for the different hydrogen concentrations are shown in Fig. 4. We can see that, H:MnO(111) ultrathin layer with spin-up electronic subsystem is a direct bandgap semiconductor with the band gap about 2 eV [Fig. 4(a)]. At the same time, the electronic band structure with the spin-down subsystem has the metal-type conductivity with two localized bands

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MnO 111 MnO 111 (Θ = 0.25)

SLG

spin up spin down spin up spin down

spin up spin down

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

−5

−3

−1 1 3 5

0

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

Ε − ΕF, eV

−5

−3

−1 1 3 5

Ε − ΕF, eV 0

(a) (b)

(c) (d)

MnO 111 (Θ = 0.75) spin up spin down

Fig. 4. (Color online) DFT calculation of the band structure for both spins in the H:MnO(111) systems: without hydrogenation (a), hydrogen coverage of 0.25 ML (b) and 0.75 ML (c), and graphene without substrate (d).

intersecting at the Fermi level. Apparently the nature of these states is related to the disturbance in coordination of the oxygen surface layer atoms, and breaking of the Mn–O bonds. The latter leads to formation of unsaturated bonds of the oxygen surface layer atoms.

For the hydrogen concentration ofΘ = 0.25ML [Fig. 4(b)], the spin-down electronic subsystem demonstrates two unsaturated bonds of the oxygen atoms on the upper surface of the substrate, which represent two localized bands in the Fermi level region. Note that they are energy-degenerate around the Γ point in the first Brillouin zone. One of the bands is largely populated, while the other band is only

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partially populated around the K point. It is known [23] that each oxygen surface atom has one orbital with a single isolated electron. Since we used, in the upper layer of the supercell, 4 oxygen atoms (non-interaction each other). Because of the great distance, they will form 4 separate bands. Two of these are localized in the Fermi level region, while two other lie within the energy range from 0.6 eV to 1.0 eV. When one of the oxygen atoms is passivated by hydrogen, then there are three localized states insignificantly shifted into the forbidden gap [Fig. 4(b)]. The latter is true for the spin-down electronic subsystem, while the spin-up subsystem is practically unchanged. Note that the high density of theE(k)dispersion curves in Fig. 4(b) is due to the use of 58 basis atoms in the calculated supercell.

When three oxygen atoms are passivated by hydrogen, then in the spin-down electronic subsystem there are two localized states [Fig. 4(c)]. The bands are shifted to the valence band top. In this situation, only one band is partially populated around the M point of the Brillouin zone. In the case of total hydrogenation of the interface, when four oxygen atoms are passivated by hydrogen atoms, the spin- down electronic subsystem shows a significant shift of the localized states in the energy range from 0.5 to 0.8 eV. This analysis leads to the conclusion that the hydrogen concentration ofΘ = 1.0ML for the MnO(111) substrate causes a significant restructuring of the electronic structure around the Fermi level. The specific features of formation of the energy bands near the Fermi level are well-illustrated by the partial DOS patterns of the atoms of carbon, manganese and oxygen, and by the total patterns (Figs. 2 and 3).

4 Conclusion

In conclusion, using the first-principle calculations based on density functional theory, we have studied the local atomic structure, the thermodynamic and elec-

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tronic properties of the graphene/H:MnO(111) system for different reconstructions of the interface conditioned by interaction with graphene and hydrogenation of the interface. We have considered five reconstructions of the atomic surface of manganese monoxide terminated by oxygen, conditioned by the interface hydrogenation. This work has shown that hydrogenation of the interface leads to a significant change in its electronic properties. In particular, a significant reduction is observed in the work function for the adsorbed graphene and the hydrogenated surface of manganese monoxide, which controls the mechanisms of charge transfer in the interface. The charge transfer results inp-doping of graphene and emergence of the semimetal-semiconductor junction. In the case of the interface hydrogenation of Θ = 1.0ML, thep–njunction is formed in graphene, and a qualitatively new state emerges. That is then-type semiconductor. The latter opens up the possibility of development of graphene field-effect transistors of then-type, which is of consid- erable interest for nanoelectronics.

Our DFT study of the interaction between graphene and a dielectric surface shows that the use of the PBE-D2 approximation for description of the exchange- correlation energy was justified and yielded comparable values for the energy of graphene adsorption onto dielectric and semiconducting substrates [5, 33, 34]. This study also shows that the electronic spectrum in the graphene/H:MnO(111) system can be modulated by hydrogenation of the oxygen interface layer, which is suggestive of a high potential for future application of this system in spintronics.

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HIGHLIGHTS

1. DFT study of adsorption of graphene on MnO (111) surface with and without hydrogen coverage

2. Effect of graphene adsorption on electronic properties of graphene/H:MnO(111) with different reconstructions was studied

3. Electronic spectrum of graphene/H:MnO(111) system can be modulated by hydrogenation of the oxygen interface layer

4. The p–n junction was observed in the graphene and a new state, n–type semiconductor, is qualitatively emerged

Highlights