Two-Dimensional CuO Inside the Supportive Bilayer Graphene Matrix

Article Cite This: J. Phys. Chem. C 2019, 123, 17459−17465

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Two-Dimensional CuO Inside the Supportive Bilayer Graphene Matrix D. G. Kvashnin,*,†,‡ A. G. Kvashnin,§ E. Kano,*,∥,⊥ A. Hashimoto,∥,⊥ M. Takeguchi,∥ H. Naramoto,# S. Sakai,∥,# and P. B. Sorokin†,‡,#,∇ †

National University of Science and Technology MISiS, 4 Leninskiy Prospekt, Moscow 119049, Russian Federation Emanuel Institute of Biochemical Physics of RAS, 4 Kosygin Street, Moscow 119334, Russian Federation § Skolkovo Innovation Center, Skolkovo Institute of Science and Technology, 3 Nobel Street, Moscow 121205, Russian Federation ∥ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan ⊥ Graduate School of Pure and Applied Science, University of Tsukuba, 1-2-1 Sengen, Tsukuba 305-0047, Japan # National Institutes for Quantum and Radiological Science and Technology QST, 1233 Watanuki, Takasaki 370-1292, Japan ∇ Technological Institute for Superhard and Novel Carbon Materials, 7a Centralnaya Street, Troitsk, Moscow 108840, Russian Federation

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‡

S Supporting Information *

ABSTRACT: New two-dimensional (2D) copper monoxide located at the pore in a bilayer graphene matrix is investigated in both theoretical and experimental ways. Strict analysis of the lattice stability with respect to the magnetic state reveals that the CuO monolayer displays an antiferromagnetic rectangular structure as a favorable state. It is shown that 2D CuO can be confined in a bilayered graphene pore terminated with Cu atoms forming spin-polarized states on the graphene/CuO interface. It is found that the co-presence of Cu and O atoms around the bilayered graphene edge plays an essential role in the 2D CuO formation process.

S

structure18 may complicate the formation of monolayers inside the graphene pore. In contrast to the defects in graphene monolayers, pores in bilayer graphene (BG) have a stable geometry of the edges and no healing was observed.19−21 Indeed, the free edges of the neighbored graphene layers tend to connect with each other to minimize the edge energy as was shown in ref 22. Such a process was shown to be energetically favorable and the connection of the edges proceeds without any energy barrier accompanying the formation of stable and chemically inert boundaries. Here, we present a comprehensive investigation of a twodimensional copper oxide monolayer inside a bilayered graphene pore (BGP) by employing experimental and theoretical approaches. The atomic structures of both freestanding CuO and implemented CuO in BG were studied and discussed in comparison with the experimental data. The density functional theory methods were used to calculate the thermal stability and electronic and magnetic properties. The binding energy for each possible type of interface was calculated. We also performed the investigation of the growth

uccessful synthesis and investigation of graphene established a wide family of two-dimensional (2D) materials containing compounds of different compositions and atomic structures with various sorts of physical and chemical properties. Such a family may even consist of 2D films without layered bulk counterparts, which allows one to suggest the expandability of the family on new 2D materials with unusual atomic structures. For example, 2D metal oxide and carbide films can form monolayer structure like graphene and h-BN but also exhibit an ordered magnetic structure and specific rectangular lattice.1−4 Recently, much effort has been made to synthesize 2D metal oxide films, which were successfully cleaved from the surface of bulk layered oxides.5−8 Theoretical and experimental studies on Fe,9 FeO,3 CuO,2 and ZnO10 monolayers suspended in a graphene pore suggest the possibility of formation of monolayered lateral heterostructures made of 2D metal oxide and graphene matrix. Herein, copper oxide is especially important due to possible high-temperature superconductivity11−14 and promising applications in catalytic substrates,15 solar cells,16 thin-film transistors,17 and others. To design new 2D heterostructures with tunable electronic and magnetic properties, it is highly desirable to produce pores in graphene in a controlled manner. However, the trend of graphene defects to heal with restoring a perfect graphitic © 2019 American Chemical Society

Received: June 5, 2019 Published: June 14, 2019 17459

DOI: 10.1021/acs.jpcc.9b05353 J. Phys. Chem. C 2019, 123, 17459−17465

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The Journal of Physical Chemistry C process of 2D CuO in BGP using the evolutionary algorithm USPEX.

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METHODS Calculations of the atomic structure, stability, and spindependent electronic properties were carried out using the DFT + U method23 describing transition-metal compounds within general gradient approximation24 and using the augmented plane-wave method25 implemented in VASP software.26−28 First, we defined the parameters U−J for the correct description of electronic properties, in particular, the value of the band gap of bulk (three-dimensional) CuO by taking into account the intra-atomic Coulombic and exchange interactions. The U−J parameter was found to be 8.5 eV to obtain the band gap of about 1.5 eV.14 Due to the large size of the considered supercells (more than 250 atoms), the energy cutoff was set at 250 eV. Atomic structure relaxation was carried out until the interatomic forces became less than 0.001 eV/Å. To avoid spurious interactions caused by periodic images of the considered supercells, the lattice vector in the nonperiodic direction was set at ∼15 Å. Behaviors of Cu and CuO clusters in BG nanopores under constant temperature were described by means of the molecular dynamics (MD) method using the Nosé−Hoover thermostat.29,30 The total simulation time varied from 0.15 to 0.35 ns with the time step of 0.1 fs. The simulation of the formation of the CuO layer between the two edges of BG was carried out using the firstprinciples fixed-composition evolutionary algorithm as implemented in the USPEX code adapted for the surfaces.31−34 Search for the stable crystal structure of a free-standing twodimensional CuO monolayer was performed using the USPEX code as well. In both cases, the evolutionary searches were combined with structure relaxations using density functional theory (DFT)35,36 also within the generalized gradient approximation24 (spin-polarized generalized gradient approximation in the case of free-standing CuO) and the projector augmented plane-wave method 25 implemented in the VASP26−28 package. The plane-wave energy cutoff of 500 eV and the k-mesh of 0.05 × 2π/Å−1 resolution ensure an excellent convergence of total energies. For the isolated slabs, the monopole, dipole, and quadrupole corrections were taken into account using a method discussed in refs 37 and 38. During the structure search, the first generation (80 structures for prediction of the interface and 120 structures for the isolated monolayer) was produced randomly; the successive generations were obtained by applying 40% heredity, 10% soft mutation, 20% transmutation operations and 30% of the generation was produced using symmetry random generators. Each of the considered supercells contained a vacuum layer of 20 Å. The unusual two-dimensional nature of CuO is different from its bulk counterpart,14 requiring detailed investigation of its atomic structure. We performed an evolutionary search for a stable free-standing CuO monolayer using the USPEX code.31,32,39 The predicted lowest-energy structures were then relaxed with ferromagnetic and antiferromagnetic spin ordering to define the favorable magnetic state. In Figure 1, the atomic structures of several predicted CuO monolayers are shown. It was found that all predicted monolayers are magnetic with preliminary antiferromagnetic spin ordering, except for the fourth predicted structure with ferromagnetic ordering (Figure 1). The experimentally observed1 CuO layer with a rectangular lattice displays the lowest energy. The lattice

Figure 1. Energy difference between predicted 2D CuO structures and corresponding bulk antiferromagnetic counterpart. Yellow, blue, and red bars correspond to antiferromagnetic, ferromagnetic, and nonmagnetic states of 2D CuO, respectively. The corresponding atomic structures with an energetically favorable spin configuration are presented. Cu and O atoms are shown in brown and red colors, respectively.

parameters of the predicted structure with a rectangular lattice are a = 5.468 Å and b = 5.457 Å. The CuO structure containing 8-membered rings is less energetically favorable and may be considered as a defective orthorhombic CuO observed experimentally.1 The graphene-like structure with a hexagonal symmetry (h-CuO) has a higher energy, and the sublattice atoms in the unit cell tend to leave the plane, forming a silicene-like corrugated structure. It is worth noting that hCuO is energetically unfavorable without the corrugation even compared to the latter structure. The atomic structure with the highest energy among considered ones belongs to the orthorhombic CuO containing Cu−O−Cu chains with a ferromagnetic order different from the other predicted CuO structures. We used a partially bilayered graphene common to both the experiments in ref 1 and this report. The graphene sheet was synthesized by chemical vapor deposition under the condition determined from ref 40 to make AB-stacked BG. The number of layers of graphene in each area was confirmed by dark-field transmission electron microscopy (TEM) and high-resolution TEM techniques (see pp 29−32 in ref 41). In ref 1, we confirmed the structure and composition of a 2D CuO monolayer at the atomic level by annular dark-field scanning TEM (ADF-STEM) combined with electron energy-loss spectroscopy (EELS). We used TEM-EELS for this report instead, and the result suggested that the sample was composed of C, Cu, and O, although the TEM-EELS spectra are not only from 2D CuO in graphene but also from a wide area of the sample. Figure 2a shows the experimentally observed TEM image of 2D CuO in a graphene pore. Figure 2b−d shows the selected magnified and filtered TEM images obtained from the same area in a TEM movie. Before taking the movie, we irradiated a strong electron beam in a wide area to make a pore (BGP) in a bilayered graphene area. The microscope was operated at 80 kV at room temperature. Graphene is much stable when heated at around 500 °C, while it can be easily etched away at room temperature under electron beam irradiation because of oxygen-based contaminants remaining on the graphene surface.42 The TEM movie was then acquired at a speed of 1 frame/s. The area of bilayered graphene was partially etched away, and the 2D CuO surrounded by not only BG but also 17460

DOI: 10.1021/acs.jpcc.9b05353 J. Phys. Chem. C 2019, 123, 17459−17465

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tends to buckle, indicating its instability as a two-dimensional free-standing substance. Previous experimental and theoretical works1−3,44 showed that transition metals can form two-dimensional stable structures with rectangular lattice only by combining with oxygen or carbon. According to the reported data,1 we suggest that the observed structure is a 2D copper oxide. Taking into account the recent studies of the 2D Fe,9 ZnO,10 and FeO3 monolayers inside graphene nanopores, we simulated the simple models of CuO monolayers within the BGP with the AA and AB graphene stacking (see Figures 3 and S3 in the

Figure 2. Experimental TEM image of 2D CuO inside BGP. The image was taken under over-focus so that the metal atoms appeared bright. (b−d) Filtered TEM images of the 2D CuO region. When Cu atoms at the edge migrated (from (b) to (c)), the 2D CuO changed its orientation (from (c) to (d)) while preserving the rectangular lattice.

monolayer graphene was generated (see Figure S1 in the Supporting Information). In the images of Figure 2b−d, one can clearly see the rectangular arrangement of atoms, with the same feature as observed in the previous report.1 The 2D CuO in ref 1 was on monolayer graphene, while the 2D CuO in the present images is arranged in the graphene pore with a diameter of ∼2 nm. The atoms especially at the edge of the graphene pore are unstable, and they may migrate away every few seconds under electron beam irradiation,43 which leads to the orientation change of the 2D CuO as seen in Figure 2b−d. Interestingly, even after such a reorientation, CuO maintained its 2D rectangular lattice structure in the graphene pore. We obtained a value of the Cu−Cu distance of 2.78 ± 0.03 Å in this report (using the graphene lattice constant as a reference), similar to that of 2.83 ± 0.06 Å in ref 1. According to the TEM data, we can conclude that a part of the edges is BG, which motivates us to investigate the effect of a BG matrix on the structure and stability of the copper-based monolayers formed therein. To provide a theoretical foundation for the presence of a Cu-based monolayer in BGP, we first studied the stability of the orthorhombic monolayer consisting of only Cu atoms inside the pore with the AA and AB graphene stacking (see Figures 2a and S2 in the Supporting Information, respectively). The Cu monolayer was placed in BGP in a rectangular shape. This simple model provides an opportunity to study the features of the interface between the Cu monolayer and zigzag/armchair edges of BG. The geometry relaxation simulation clearly shows that the Cu monolayer with a rectangular lattice tends to transform to the hexagonal one. This represents the fact that even under the confined condition, the pure metallic structure favors the “hexagonal close packing” in the two-dimensional state regardless of the graphene stacking. The obtained results show the possibility of stabilization of the hexagonal Cu monolayer in BGP. In contrast to this, our calculation in ref 1 showed that a small two-dimensional copper cluster placed on the graphene surface

Figure 3. Simulation of atomic structure of the pure Cu monolayer inside BGP with AA stacking of graphene layers before and after geometry relaxation. (b) MD simulations of the CuO monolayer inside the BGP at T = 600 K for 0 and 3 ps time steps. Copper, oxygen, and carbon atoms are depicted by brown, red, and green colors, respectively.

Supporting Information, respectively). We found that the CuO monolayer undergoes small changes in the atomic structure in the confined state for both stacking cases. The relaxed atomic structures of the CuO monolayers are shown in Figures 3b and S3a in the Supporting Information for the AA and AB stacking BGs, respectively. Ab initio molecular dynamic (MD) simulation at high temperature (600 K) for 3 ps showed the stability of the 2D CuO in BGP (see Figures 3b and S3b). During the MD simulation, the rectangular lattice of CuO remains stable and the interface between the CuO monolayer and BGP does not change. It was found that difference in the stacking of BG does not impact significantly the interface between the BGP edges and CuO monolayer. Since the graphene stacking merely affects the stability of the 2D CuO in BGP, only AA stacking was studied further. The interface between copper oxide and graphene is of particular interest from both fundamental and practical points of view because it has a crucial impact on the properties of the confined CuO monolayer. Due to the rectangular symmetry of the 2D CuO, its edges can be named in the same manner as in ref 45. The edges oriented parallel to the lattice vectors and containing both Cu and O atoms were defined as a linear (LN) type. The edges including only one atomic type could be named as zigzag (ZZ), according to its atomic arrangement (Figure 4a). It should be noted that the latter edge could be terminated by copper and oxygen, but in the presented work, only the termination with metallic atoms was studied. In the case of BG, two types of edges: zigzag and armchair (AC), were considered. Due to the mismatch and special mutual 17461

DOI: 10.1021/acs.jpcc.9b05353 J. Phys. Chem. C 2019, 123, 17459−17465

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Figure 4. Schematic representation of 2D CuO with denotations of the edges. (b) Atomic structures and the corresponding names of four considered CuO/BG interfaces.

We studied how the CuO/BG interface can influence the spin-dependent electronic properties of the 2D CuO. The calculations were performed for the most stable CuO/BG interfaces (ZZ−ZZ and AC−ZZ) as shown in Figure 5. A detailed analysis through the comparison between the spinresolved densities of states (DOS) of the CuO/BG interface and of its individual components (Sections (i)−(iv) in Figure 5) reveals that the formation of the CuO/BG interfaces changes the electronic properties of BG significantly. Visualization of the electron density distribution (top panels of Figure 5) suggests that in both the energetically preferable interfaces (ZZ−ZZ and AC−ZZ), the electron density in CuO is mostly redistributed at the region close to the interface. The individual BG component displays a DOS that is symmetric with respect to the majority and minority spin states (Figure 5, Section (i)), while the connection with the CuO component (Figure 5, Section (ii)) leads to the appearance of spin density on the interface carbon and copper atoms. Spin polarization (SP) of the interface atoms derived from the interfacial DOS (Figure 5, Sections (iii)−(iv)) varies depending on the type of the CuO/BG interface. It was found that the ZZ−ZZ interface displays a small SP in carbon and copper atoms at the Fermi energy (about 1%, see Sections (v) and (vi) in Figure 5a), while the less energetically preferable AC−ZZ interface displays a higher SP value (about 4 and 8%) at the Fermi energy for carbon and copper atoms, respectively (Sections (v) and (vi) in Figure 5b). On the other hand, the interface effect on the spin properties of the CuO region has the opposite character. The isolated 2D CuO region displays a pronounced spin asymmetry originating from the presence of the chemically active edges terminated by Cu (see Figure 5, Section (ii)). After the formation of the CuO/BG interfaces, the spin asymmetry on copper atoms drastically decreases through the spin redistribution between CuO and the edges of BG (see Figure 5, Section (iv)). The similar effect was observed in both interfaces considered. The above results allow us to discuss the prospects of forming the interfaces of this kind for potential use in spintronics. The formation process of the CuO monolayer in BGP was investigated in a step-by-step manner using the evolutionary algorithm USPEX.31−34 The supercell comprising two bilayered graphene edges containing 64 carbon atoms was used as a substrate. Structural evolution in the region between BG edges was observed by depositing a certain amount of Cu and O atoms on the substrate. The simulations were performed under three different conditions of the Cu:O ratios of 10:8, 12:12 and 14:12. The obtained results are shown in Figure 6a−c for Cu:O = 10:8 (a), 12:12 (b) and 14:12 (c).

orientation between graphene and the CuO monolayer, several possible types of interfaces can be considered: ZZ−LN, ZZ− ZZ, AC−LN, and AC−ZZ (the first and second parts label the type of the BG and CuO edge, respectively) (Figure 4b). The binding energy of the interface was calculated using the equation Eb =

ECuO/BG − E BG − ECuO (1)

2L

where ECuO/BG, EBG, and ECuO are the total energy of the interface and the individual components (BG and CuO) and L is the length of the interface within the unit cell, which was multiplied by 2 due to the presence of the two interfaces in the considered CuO/BG supercell (Figure 4b). The calculated values of the binding energies are presented in Table 1. Table 1. Calculated Values of Binding Energies of CuO/BG Interfaces interface configuration

ZZ−LN

AC−LN

ZZ−ZZ

AC−ZZ

binding energy (eV/Å)

1.04

−0.078

−2.63

−0.52

Termination of the bilayered graphene edges by chemically active copper atoms leads to the energy preference of the interfaces with the AC−ZZ and ZZ−ZZ configurations; however, the relative stability of the considered interfaces mainly comes from their geometry. The most energetically favorable ZZ−ZZ interface (−2.63 eV/Å) represents the almost perfect connection of two undistorted layers of BG and CuO by Cu termination. The Cu atoms on the edges make a strong connection with the ZZ edge of BG due to the small lattice mismatch (less than 1%) between the corresponding CuO and the graphene superlattices. In the case of the AC−ZZ interface, the presence of a Cu termination also leads to the formation of a low-energy interface between CuO and the AC edges of BG. However, the presence of the mixed edges (ZZ− LN and AC−LN) in BGP does not allow the formation of a smooth interface between the bilayered graphene edge and CuO like in the case of the AC−ZZ interface. Cu and O atoms on the edge tend to move in the bridge position between two graphene layers, which causes a local corrugation of the CuO lattice near the interface (see Figure 4b). Copper and oxygen atoms move out from the plane of the CuO monolayer toward different graphene layers. In both the ZZ and AC edges of BG, oxygen atoms prevent the formation of the C−C bonds between two graphene layers in the bilayered edge, which leads to the increase of the whole interface energy. 17462

DOI: 10.1021/acs.jpcc.9b05353 J. Phys. Chem. C 2019, 123, 17459−17465

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Figure 5. Electron density distribution after the formation of ZZ−ZZ (a) and AC−ZZ (b) interfaces. DOS of individual BG and CuO edges partially resolved for C (i) and Cu (ii) atoms, respectively. DOS of the CuO/BG interface partially resolved for C (iii) and Cu (iv) atoms. Energy dependence of spin polarization of C (v) and Cu (vi) atoms on the CuO/BG interface. The majority and minority spin states are shown as red and blue lines, respectively.

Figure 6. Atomic structure of the CuO monolayer formed between the bilayered graphene edges predicted by evolutionary algorithm USPEX. The Cu:O ratio was chosen as (a) 10:8, (b) 12:12 and (c) 14:12 to consider the structural evolution depending on the deposited amounts of copper and oxygen atoms. Carbon atoms are shown in brown color, copper atoms in blue, and oxygen atoms in red.

During the evolutionary search in the first generation, the Cu and O atoms were deposited randomly between the edges of the BG components. The evolution algorithm was used to perform a global optimization to find a stable structure of the CuO/BG interface. The distance between the graphene edges, which is equivalent to the pore size, was chosen to be 11.5 Å. The result of the simulations with deficiency of O atoms (Cu:O = 10:8) is shown in Figure 6a. A small amount of Cu and O atoms initiates the early step of the interface growing process, which begins at the bilayered graphene edges and then two Cu−O parts move toward each other (Figure 6a). Increasing the number of deposited atoms (Cu:O = 12:12) leads to the connection of the two Cu−O parts on each of the bilayered graphene edges (Figure 6b). Further increase of the number of deposited atoms (Cu:O = 14:12) induces the interface formation with Cu2O2 square units (highlighted by

blue) as a dominant structural motif (Figure 6c). Due to the lattice mismatch between BG and the orthorhombic CuO monolayer, the formation of a uniform interface is not possible and the O−O bond with the length of 1.49 Å is formed as seen in Figure 6b,c. The distance between the neighboring Cu atoms at the boundary of the two CuO monolayers is ∼2.3 Å. Such an interface contains equal amounts of O−O and Cu−Cu bonds along the boundary. The obtained results claim the possibility of the formation of an atomically thick rectangular CuO monolayer as well as the possible appearance of a mirror symmetry one-dimensional topological defect therein. In the present work, the atomic structure and electronic properties of a novel CuO monolayer were investigated in both theoretical and experimental ways. The systematic study of atomic geometry unambiguously revealed that the 2D CuO in the ground state displays a rectangular atomic structure and 17463

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antiferromagnetic spin ordering. Experimental observation of the 2D CuO in the bilayered graphene pore motivated us to study such heterostructure in detail. The analysis of the formation process of the 2D CuO in BGP using the evolutionary algorithm allowed us to reveal the structural features of the interface-growing process, whereas stability analysis allowed us conclude that even a small nanometer-sized 2D CuO patch in BGP displays stability at high temperature of up to 600 K. The investigation of spin-dependent electronic properties of the 2D CuO/bilayered graphene interface revealed noticeable spin polarization in the vicinity of the Fermi energy. Our findings testify that the bilayered graphene pore can be considered as a promising template for the synthesis of new 2D materials with specific electronic properties therein.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b05353. TEM images of CuO and graphene; simulation of atomic structure of the pure Cu monolayer; and results of geometry relaxation of the CuO monolayer (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.G.K.). *E-mail: [email protected] (E.K.). ORCID

D. G. Kvashnin: 0000-0003-3320-6657 A. G. Kvashnin: 0000-0002-0718-6691 S. Sakai: 0000-0002-0367-9106 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Shiro Entani (QST), Dr. Songtian Li (QST), and the members of QST for fruitful discussion. TEM observation was supported by the “Nanotechnology Platform project” sponsored by MEXT, Japan. This work was partially supported by JSPS KAKENHI, Japan (Grant No. 15J04118 for E.K., 25390035 for A.H., and 16H03875 for S.S). A.H. acknowledges the Global Research Center for Environment and Energy based on Nanomaterials Science, MEXT, Japan. S.S. acknowledges the QST Advanced Study Laboratory (Res. Gr. For Advanced Quantum Functional Materials). The features of stability and atomic structures 2D Cu and CuO layer was supported by the Russian Science Foundation (Project identifier: 17-72-20223). The analysis of bilayered graphene atomic structure was supported by RFBR 17-02-01095. The study of interface effects was supported by Increase Competitiveness Program of NUST “MISiS” No. K2-2019-016 and Grant of President of Russian Federation for government support of young DSc. (MD-1046.2019.2). We are grateful to the supercomputer cluster provided by the Materials Modelling and Development Laboratory at NUST “MISIS” (supported via the grant from the Ministry of Education and Science of the Russian Federation No. 14.Y26.31.0005), and to the Joint Supercomputer Center of the Russian Academy of Sciences and the Arkuda supercomputer of Skolkovo Foundation. 17464

DOI: 10.1021/acs.jpcc.9b05353 J. Phys. Chem. C 2019, 123, 17459−17465

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DOI: 10.1021/acs.jpcc.9b05353 J. Phys. Chem. C 2019, 123, 17459−17465