Magnetism in Oxygen-Functionalized Hexagonal Boron Nitride

Dec 15, 2014 - Wide-bandgap hexagonal boron nitride (h-BN) nanosheets, white graphene, can be uniformly grown on conducting Cu foils with high quality...
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Magnetism in Oxygen-Functionalized Hexagonal Boron Nitride Nanosheet on Copper Substrate Yufeng Guo* and Wanlin Guo State Key Laboratory of Mechanics and Control of Mechanical Structures and MOE Key Laboratory for Intelligent Nano Materials and Devices, Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ABSTRACT: Wide-bandgap hexagonal boron nitride (h-BN) nanosheets, white graphene, can be uniformly grown on conducting Cu foils with high quality. Here, we show that h-BN monolayers on Cu substrates exhibit ferromagnetic, antiferromagnetic, or ferrimagnetic properties with ozone (O3) molecules or oxygen−hydrogen (O−H2) groups bonded on its B atoms, depending on the adsorption density and configuration of these functional groups. The magnetisms in the O3- and O−H2-functionalized h-BN sheets originate from unpaired electrons in the ozone molecule and electrons transferred from the H2 molecule to the O atom, respectively. The Cu substrate plays an essential role in stabilizing the chemisorption of O3 or O−H2 on the h-BN sheets. Magnetic composite structures composed of Cu, h-BN, and oxygen provide a feasible route to facilitate the design of two-dimensional h-BN-based spin devices.

of the h-BN nanosheets.30−32 Oxygen and its allotropes are very chemically reactive and suitable for surface modification and engineering. Even though h-BN nanosheets are resistant to oxidation by molecular oxygen,8,9,33 the interactions of oxygen allotropes such as triatomic oxygen (ozone, O3) and atomic oxygen (O) with h-BN nanosheets on growth substrates and the subsequent influence on their magnetic properties have not been sufficiently investigated. In this study, we show by spin-polarized first-principles calculations that O3-functionalized an h-BN monolayer on a Cu substrate can exhibit ferromagnetism (FM), antiferromagnetism (AFM), or ferrimagnetism (FIM) by control of the O3 adsorption density and configuration. The magnetism of the O3-functionalized h-BN sheet is attributed to the bonding of one O atom of the O3 molecule with a B atom leading to unpaired electrons on the other two O atoms. The O−H2functionalized h-BN monolayer on Cu is magnetic with halfmetallic properties when the O−H2 group adsorbs on the B atom. Without the Cu substrate, the bonding of O3 or O−H2 with the B atoms is unstable, and the whole system becomes nonmagnetic.

1. INTRODUCTION Dielectrics are crucial components in microelectronic devices including field-effect transistors, digital circuits, and interconnect wiring. Shrinkage of device dimensions and circuit elements results in improvements in performance, and requires the thickness of dielectric layers to be scaled down as well.1,2 The hexagonal boron nitride (h-BN) nanosheet is a graphenelike two-dimensional (2D) material with the same honeycomb basal plane but alternating B and N atoms substituting for carbon that exhibits excellent mechanical properties3−5 and high chemical stability.6−9 The h-BN nanosheet is a nonmagnetic insulator with a large band gap and has dielectric properties very similar to those of SiO2 materials,10 which makes it ideally suitable for use as an ultrathin dielectric separation layer or flat dielectric substrate in graphene electronic devices.11−13 Recently, high-quality and uniform hBN nanosheets over large areas were successfully synthesized and achieved on Cu foils by chemical vapor deposition methods.14−17 In addition to its use as a dielectric layer, to impart more functionality to BN sheets, on growth substrate without an extra transfer process is attractive and promising for practical applications of this type of 2D material. Magnetism in 2D materials is an exciting research field in both fundamental physics and applied nanotechnology. For hBN nanosheets, theoretical works predict that magnetism can be substantially achieved by structural defects,18,19 impurities,20,21 and chemical decoration and doping.22−25 Viable surface functionalizations of boron nitride nanosheets through covalent or noncovalent bonding of atoms, molecules, and functional groups have been confirmed by experiments,26−29 in which fluorination induces magnetism in the h-BN nanosheets.26 On the other hand, supporting substrates could lead to a significant change in the physical and chemical properties © 2014 American Chemical Society

2. MODEL AND METHOD We chose a rhombus unit cell with a lattice length of 1.0224 nm where a 4 × 4 h-BN monolayer (32 atoms) is placed on a 4 × 4 Cu(111) substrate. The Cu(111) substrate (80 atoms) is composed of five layers, with the atoms in the bottom layer fixed during structural relaxation and a vacuum region larger than 2.5 nm in the direction perpendicular to the h-BN and substrate planes. According to experimental results, the lattice Received: December 9, 2014 Published: December 15, 2014 873

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The Journal of Physical Chemistry C parameter for h-BN is 0.2524 nm.34 To match the Cu substrate, the lattice of the h-BN nanosheets is stretched by 1.3%. All computations were performed within the framework of density functional theory (DFT) as implemented in the VASP code by using the projector augmented wave method with the Perdew− Burke−Ernzerhof (PBE) exchange-correlation functional.35−37 The influence of van der Waals interactions was considered by using a modified version of vdW-DF, referred to as “optB86bvdW”, in which the revPBE exchange functional of the original vdW-DF method of Dion et al. is replaced with the optB86b exchange functional to yield more accurate equilibrium interatomic distances and energies for a wide range of systems.38,39 The whole system was relaxed by using a conjugate-gradient algorithm until the force on each atom was less than 0.1 eV/nm. After structural relaxation, computations with an energy cutoff of 500 eV and special k points sampled on a 6 × 6 × 1 Monkhorst−Pack mesh40 were employed to calculate the exact energy. Our calculations showed that the 1.3% stretching strain slightly influences the band structure and energy gap of the h-BN layer.

2. RESULTS AND DISCUSSION An O3 molecule was placed on different surface locations of a Cu-supported h-BN monolayer, and then those systems were relaxed into equilibrium states. We found that the most stable adsorption state with the lowest total energy is O 3 chemisorption on the h-BN sheet, as shown in Figure 1a, where the O3 molecule bonds with the B atom. The length of the O−B bond is 0.146 nm, with the nearby O−O bond stretched from the original 0.129 to 0.167 nm and the top O− O bond at 0.127 nm. The cohesive energy of the ozone molecule on the BN surface is 4.92 eV, which was calculated as Ec = Etot − Ebn/sub − Eo, where Etot is the total energy, Ebn/sub is the energy of the Cu-supported h-BN sheet, and Eo is the energy of a single O3 molecule. Subsequently, we studied the magnetic properties and compared the spin-unpolarized and spin-polarized situations (α-spin and β-spin or spin-up and spin-down were resolved) of O3-functionalized h-BN/Cu. The total energy of the spin-polarized system was found to be 220 meV per unit cell lower than that of the spin-unpolarized system, and magnetism was found to be favored as the ground state for this Cu−BN−O3 system. Figure 1b shows the corresponding charge density difference, Δρspin = ρspin‑up − ρspin‑down, between the charge densities of the spin-up and spindown configurations for the system in the magnetic state. The splitting of the spin-up and spin-down densities occurs upon O atom bonding with the B atom, and positive Δρspin values dominate over the other two O atoms. To better understand the role of the O3 molecule in the magnetic and electronic properties of this composite structure, the local density of states (LDOS) for the spin-up and spin-down configurations for the h-BN monolayer and O3 are shown in Figure 1c. For the BN sheet, the spin-polarized LDOS are completely symmetrical, and there is no magnetism. In contrast, the spin-polarized LDOS for the ozone molecule are asymmetrical. Both the spin density difference and asymmetrical LDOS demonstrate that the magnetism of the whole system originates from the O3 molecule and is mainly attributable to the top two O atoms. The corresponding magnetic moments of the two O atoms are 0.29 and 0.37 μB. It is known that the oxygen molecule is paramagnetic owing to unpaired electrons in the p orbital but that the ozone molecule is nonmagnetic. When the O3 molecule is bonded to the BN sheet, the O−B bond (0.146

Figure 1. (a) Optimized structure of Cu-supported h-BN monolayer with an O3 molecule absorbed on its surface in the unit cell (dashed lines). Red, green, blue, and yellow dots represent oxygen, boron, nitrogen, and copper atoms, respectively. (b) Contour plot of the corresponding charge density difference Δρspin (in units of 0.01 e/Å3) between the spin-up and spin-down charge densities. (For clarity, the Cu substrate is not shown.) White represents positive values of Δρspin, and purple represents negative values of Δρspin. (c) Spin-resolved LDOS (in units of states/atom) of the h-BN monolayer and O3 molecule. Here, spin-up is denoted by a positive value, and spin-down is denoted by a negative value. (d) Spin-resolved band structures of O3-functionalized h-BN/Cu contributed by only B, N, and O atoms. Larger circles indicate larger spectral weights and contributions from the B, N, and O atoms. The Fermi level was set to zero.

nm) is shorter than the adjacent O−O bond (0.167 nm). Therefore, the bottom O atom has a stronger interaction with the B atom than with the nearby O atom. This makes the top two O atoms of the ozone molecule separate from the bottom O atom and behave like an O2 molecule, which consequently creates some unpaired electrons again and leads to a magnetic ground state. Without O3 adsorption, the h-BN monolayer is physisorbed on the Cu substrate and retains its insulating properties. Moreover, the O3-adsorbed h-BN sheet on the Cu substrate becomes a magnetic semiconductor, as shown by the band structures in Figure 1d. For a B atom in the h-BN lattice, there are six B atoms in the first, second, and third adjacent sites. The corresponding B−B distances are 0.256, 0.442, and 0.512 nm, respectively. To study the effects of increasing the O3 molecule density on the magnetic properties, we placed another O3 molecule on the first, second, and third adjacent B atoms. The relaxed structures of those cases are shown in Figure 2a−c. The cohesive energies between the O3 molecules and underlying h-BN/Cu are −9.36, −9.89, and −9.19 eV, respectively. Two O3 molecules are attracted and close to each other when they are on the first adjacent B atoms. Our DFT calculations showed that antiferromagnetism is favored as the ground state for this 2O3-functionalized h-BN/Cu system (our calculations showed that this system cannot converge to a ferromagnetic state) with an energy 58.5 meV lower than that of nonmagnetic state, as shown in Figure 2a. For the other two cases, the structures of 874

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Figure 2. Optimized O3-functionalized h-BN/Cu structures with two O3 molecules absorbed on B atoms at the (a) first, (b) second, and (c) third adjacent sites and three O3 molecules at the (d) first and (e) third adjacent sites. The corresponding charge density differences Δρspin (in units of 0.01 e/Å3) for the FM, AFM, and FIM states and the energy differences between magnetic and nonmagnetic states are also shown. Here, the energy of the nonmagnetic state was set to zero. The color designations are the same as in Figure 1.

the adsorbed O3 molecules are slightly influenced because of a sufficiently large distance, and the ferromagnetic states are their ground states, as shown in Figure 2b,c. However, the energy differences between the FM and AFM systems are only −1.7 and −3.0 meV, respectively. Then, we considered more O3 molecules adsorbed on the B atoms. With three O3 molecules bonded at the first adjacent sites, the ferrimagnetic state became the ground state, with a total magnetic moment of 1.0 μB and an energy 370.2 meV lower than that of the ferromagnetic state (Figure 2d). For three O3 molecules on the third adjacent sites, the ferromagnetic state was still the ground state (Figure 2e). No antiferromagnetic state was found for odd numbers of O3 molecules. The O3-functionalized h-BN/Cu system will preserve ferromagnetism once the adsorbed O3 molecules are on the second adjacent sites or farther apart. It can be seen from Figure 2d that the energy difference between the FIM and FM states is 3 orders of magnitude higher than that of the other cases between the FM and AFM or FIM states. This means that a more stable magnetic state can be achieved by control of the O3 density and configuration. The top two O atoms of the O3 molecule mainly contribute to the magnetism in O3-functionalized h-BN/Cu. Each O3 molecule in Figure 2a,b has one nearest O3 molecule, whereas the O3 molecule in Figure 2c has two. For O3 molecules bonded to the third adjacent B atoms (Figure 2c,e), the whole system exhibits a better uniformity and symmetry of O3 distribution. Therefore, we considered the change in the total magnetic moment of the ground state of O3 at the third adjacent sites. The ground states for these O3functionalized h-BN/Cu structures including two to four O3 molecules in the unit cell were all found to be ferromagnetic. As shown in Figure 3a, the total magnetic moment per O3 molecule increased from 0.85 to 1.0 μB with increasing number of O3 molecules. However, the energy difference between the FM and AFM states was −3 meV for two O3 molecules at the third adjacent sites. For three O3 molecules, the energy difference between the FM and FIM states was −5.7 meV. For four O3 molecules, the energy differences between the FM and FIM states and between the FM and AFM states were −1.4 and −1.9 meV, respectively. These small energy differences between

Figure 3. (a) Total magnetic moment of O3-functionalized h-BN/Cu as a function of the number of O3 molecules, which are located at the third adjacent sites. The inset shows the charge density difference Δρspin for four O3 molecules in the unit cell with the ferromagnetic state. (b−d) Spin-resolved LDOS (in units of states/atom) of two O3 molecules of the 2O3-functionalized h-BN/Cu at the (b) first adjacent sites with the AFM state and at the (c,d) third adjacent sites with the (c) AFM and (d) FM states.

the FM and AFM or FM and FIM states indicate that these systems are probably paramagnetic at room temperature. Moreover, the 4O3-adsorbed h-BN sheet on the Cu substrate (inset in Figure 3a) is still a ferromagnetic semiconductor. To understand the mechanism of the magnetic ordering difference, Figure 3b−d presents the spin-resolved LDOS of two O3 molecules bonding on the first and third adjacent sites. At the first adjacent site with the AFM state, because of strong interactions between the two O3 molecules, the LDOS of the spin-up and spin-down states of the first O3 molecule are different from that of the second O3 molecule, as shown in Figure 3b. The opposite spin arrangement on the two O3 molecules is more favored when they are on the first adjacent sites. In contrast, the LDOS of the first O3 molecules are approximately antisymmetric (the value of the LDOS of the spin-up or spin-down configurations of the first O3 molecule approximately equals that of spin-down or spin-up configuration of the second O3 molecule) to that of the second O3 molecule for the case at the third adjacent sites with the AFM state (Figure 3c), whereas they are completely the same for the FM state (Figure 3d). Moreover, our calculations show that the spin-resolved LDOS of two O3 molecules on second adjacent sites are similar to those on third adjacent sites. These results 875

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polarized LDOS of the h-BN sheet and O in Figure 4c. Moreover, the peaks crossing the Fermi level of the spin-up LDOS and the spin-polarized band structures (Figure 4d) demonstrate that the O−H2-functionalized h-BN sheet exhibits half-metallic properties. We calculated the charge density difference as Δρ = ρtot − ρsub − ρa, where ρtot is the total charge density, ρsub is the charge density of the Cu-supported hBN sheet with an O atom, and ρa is the charge density of a hydrogen molecule. Charge transfer from the H2 molecule brings additional and unpaired electrons in the O atom, as shown in Figure 5, and accordingly causes magnetism in the whole system.

indicate that the O3 molecule is slightly influenced by another O3 molecule when they are separated by a sufficiently large distance and the two O3 molecules are prone to retain the same spin arrangement, but such a slight interaction or coupling between them leads to a slight difference in the energies of the FM and AFM states. Furthermore, we also considered the adsorption of an O2 molecule and an O atom on a Cu-supported h-BN monolayer. The O2 molecule physisorbs on the BN sheet and cannot form a stable composite structure with the Cu−BN system, but the O atom favors vertical bonding with the B atom of the h-BN sheet.31 As a result, using the same procedure, we studied the magnetic properties of an oxygen−hydrogen- (O−H2-) functionalized h-BN sheet on a Cu substrate where an O atom vertically bonds with the B atom. According to our calculations, O-functionalized h-BN/Cu is nonmagnetic. However, the O−H2-functionalized h-BN sheet becomes magnetic when a H2 molecule is adsorbed on the O site, as shown in Figure 4. The distance between the O atom and H2

Figure 5. Contour plots of the 2D projection of the change in charge density Δρ (in units of e/Å3) for O−H2-functionalized h-BN/Cu. Red, green, blue, yellow, and gray dots represent oxygen, boron, nitrogen, copper, and hydrogen atoms, respectively.

Referring to the effects of an increasing O−H2 density, further calculations showed that the FM state is favored as the ground state of 2O−2H2-functionalized h-BN/Cu if the two O atoms are located on third adjacent B atoms, as shown by Figure 6, but the total magnetic moment (0.17 μB) and energy

Figure 6. Optimized structures of 2O−2H2-functionalized h-BN/Cu with two O atoms located at the third adjacent sites and corresponding charge density differences Δρspin for the FM and AFM states, as well as energy differences. Red, green, blue, yellow, and gray dots represent oxygen, boron, nitrogen, copper, and hydrogen atoms, respectively.

Figure 4. (a) Optimized structure of O−H2-functionalized h-BN/Cu with O on the B atom. (b) Contour plots of the corresponding charge density difference Δρspin (in units of 0.01 e/Å3). (c) Spin-polarized LDOS (in units of states/atom) of the h-BN monolayer and the O atom. (d) Spin-resolved band structures of O−H2-functionalized hBN/Cu contributed by only B, N, and O atoms. Larger circles indicate larger spectral weights and contributions from the B, N, and O atoms. The Fermi level was set to zero.

difference between the magnetic and nonmagnetic states decrease significantly because of stronger interactions between the neighboring N atoms and Cu substrate. For one O−H2 group, the distance between the Cu substrate and the N atom neighboring the O atom is 0.3 nm. If two O−H2 groups are located on third adjacent B atoms, the distance between the neighboring N and Cu atoms decreases to 0.22 nm, and the FM state is favored as the ground state, but the total magnetic moment and energy difference between the magnetic and nonmagnetic states decrease significantly as the increasing interaction between the neighboring N atoms and Cu substrate. When two O atoms bond on second adjacent sites, the distance between the neighboring N and Cu atoms is 0.21 nm, and the 2O−2H2-functionalized h-BN/Cu system is nonmagnetic because of stronger interlayer interactions. The bonding of O

molecule is 0.242 nm, and the cohesive energy between the H2 and the Cu−BN−O system is only −0.07 eV, so the adsorption of the H2 molecule occurs by physisorption. The energy difference between the magnetic and nonmagnetic states of O− H2-functionalized h-BN/Cu is −48 meV, 1 order of magnitude lower that of O3-functionalized h-BN/Cu, and the total magnetic moment is only 0.6 μB, indicating a weaker magnetism in O−H2-functionalized h-BN/Cu. As shown in Figure 4b, the charge density difference, Δρspin, distributes not only on the O atom but also on the neighboring N atoms. This means that both the O and h-BN sheet contribute to the magnetism, which is further confirmed by the asymmetric spin876

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The Journal of Physical Chemistry C atoms on first adjacent B atoms is unstable. The O atoms will detach from the h-BN sheet and form an oxygen molecule. The Cu substrate actually plays an essential role in stabilizing the adsorption and bonding of O3 and O−H2 on the h-BN sheet by transferring charges to the O−B bond (Figure 7).

(11072109, 11472131); the Jiangsu NSF (BK20131356); the Fundamental Research Funds for the Central Universities of China (No. NE2012005); and the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and Astronautics) (Grants 0413G01, MCMS-0414G01, MCMS-0412G01), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and sponsored by Qing Lan Project. We thank Prof. Zhuhua Zhang for his helpful discussion.

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Figure 7. Contour plots of the 2D projection of the change in charge density Δρ (Δρ = ρtot − ρsub − ρa, where ρtot is the total charge density, ρsub is the charge density of the Cu-supported h-BN sheet without O atoms, and ρa is the charge density of an ozone molecule) (in units of e/Å3) for O3-functionalized h-BN/Cu. Red, green, blue, and yellow dots represent oxygen, boron, nitrogen, and copper atoms, respectively.

Without the Cu substrate, the O3 molecule would detach from the h-BN sheet with the breaking of the O−B bond, and the chemisorption of O3 would become physisorption. The vertical O−B bond in the Cu−BN−O system is also not stable after the Cu substrate is removed, and the O atom will bond with the adjacent N atom, forming a BNO triangle ring.31 In addition to being used as a growth substrate or catalyst, the Cu substrate could be an important component in the construction of h-BNbased composite structures and functional devices.

4. CONCLUSIONS In summary, our DFT calculations reveal abundant magnetic properties in O3- and O−H2-functionalized h-BN/Cu systems. The FM, AFM, and FIM characteristics of O3-functionalized hBN/Cu systems and their magnetic stability are dependent on the O3 density and configuration. The magnetism is attributed to unpaired electrons in the top two O atoms of the O3 molecule. In contrast, the O−H2-functionalized h-BN sheet becomes magnetic after adsorption of a H2 molecule on the O site, and its surface magnetism is induced by the transfer of additional electrons from the H2 molecule to the O atom. The role of the Cu substrate is to stabilize the chemisorption of O3 and O−H2 on the h-BN sheets. These results provide new insights into the direct utilization of growth substrates, instead of transferring h-BN onto other substrates, and surface functionalization to build h-BN-based magnetic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-25-84890513. Fax: 8625-84895827. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for New Century Excellent Talents in University (NCET-13-0855); the 973 Program (2013CB932604, 2012CB933403); the NSF 877

DOI: 10.1021/jp5122799 J. Phys. Chem. C 2015, 119, 873−878

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DOI: 10.1021/jp5122799 J. Phys. Chem. C 2015, 119, 873−878