Article pubs.acs.org/JPCC
Two-Dimensional MnO2/Graphene Interface: Half-Metallicity and Quantum Anomalous Hall State Li-Yong Gan,† Qingyun Zhang,‡ Chun-Sheng Guo,*,† Udo Schwingenschlögl,*,‡ and Yong Zhao†,§ †
Key Laboratory of Advanced Technology of Materials (Ministry of Education), Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu, Sichuan 610031, China ‡ PSE, Division, KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia § School of Materials Science and Engineering, University of New South Wales, Sydney, 2052 New South Wales Australia
ABSTRACT: We explore the electronic properties of the MnO2/graphene interface by first-principles calculations, showing that MnO2 becomes half-metallic. MnO2 in the MnO2/graphene/MnO2 system provides time-reversal and inversion symmetry breaking. Spin splitting by proximity occurs at the Dirac points and a topologically nontrivial band gap is opened, enabling a quantum anomalous Hall state. The half-metallicity, spin splitting, and size of the band gap depend on the interfacial interaction, which can be tuned by strain engineering. metallic substrates would lead to short circuits,34−37 attention has been drawn to the coupling of graphene to magnetic insulators;38−43 however, 2D materials are rarely considered for exploiting the proximity effect due to a limited availability of monolayer systems with intrinsic magnetism,44 although this approach could facilitate all-2D graphene-based spintronics. The magnetic material itself also can be significantly affected by the contact to graphene as its electronic structure may be modified, potentially yielding novel functionalities.22 Monolayer manganese dioxide (MnO2) is predicted to be a ferromagnetic semiconductor with high Curie temperature.44 Recently, it has been shown experimentally that a low Mn vacancy concentration induces half-metallicity,45 reflecting a high susceptibility to external stimuli. Thus, monolayer MnO2 is an ideal material to explore the interaction with graphene. Although the MnO2/graphene hybrid system has been realized experimentally and subsequently has drawn extensive attention in electrochemical applications,46−50 an analysis of its electronic structure from a theoretical point of view is still lacking. For this reason we present in the following detailed results from firstprinciples calculations, which provide important insights into
1. INTRODUCTION Exfoliation of graphene has triggered the exploration of the class of 2D materials.1 Many new 2D systems have been fabricated, such as boron nitride, transition-metal dichalchogenides, and early transition-metal carbides/carbonitrides.2−7 These materials exhibit physical and chemical properties distinctly different from their bulk counterparts. In particular, they potentially provide flexibility, low weight, and dramatically reduced device sizes, thus being considered as key to future applications in nanoelectronics, optoelectronics, energy conversion, and energy storage.8−14 With the research on basic properties of 2D materials probably having passed its zenith, the focus nowadays is shifting to applications.15 It has been demonstrated that the integration of 2D materials into vertical van der Waals heterostructures is a powerful strategy to leverage and combine the features of different 2D components.13−21 Such hybrid systems preserve well individual intrinsic material properties due to the absence of strong chemical bonding, while interfacial charge transfer and coupling have the potential to yield novel functionalities.22−26 Introduction of spin polarization is a prerequisite for realizing graphene-based spintronics, a field of intense present interest.27−29 The deposition of adatoms has been considered30,31 but is probably not a feasible strategy because often cluster formation cannot be avoided.32,33 Because magnetic © 2015 American Chemical Society
Received: August 25, 2015 Revised: October 4, 2015 Published: October 7, 2015 2119
DOI: 10.1021/acs.jpcc.5b08272 J. Phys. Chem. C 2016, 120, 2119−2125
Article
The Journal of Physical Chemistry C
Figure 1. Top views of the (a) Osecond-centered, (b) Ofirst-centered, and (c) Mn-centered configurations of the MnO2/graphene hybrid system. The gray, blue, large red, and small red balls represent C, Mn, Ofirst, and Osecond atoms, respectively. (d) Binding energy per C atom as a function of the interface distance. Equilibrium distances are indicated by stars. (e) Spin-polarized band structure of the Mn-centered configuration at the equilibrium position (red: spin majority, blue: spin minority).
upper O layer, and Mn layer on top of the hollow site of graphene. The average value of the MnO2 and graphene lattice constants is adopted and kept fixed in the MnO2/graphene and graphene/MnO2/graphene systems. The interfacial binding energy is defined as EB = EHS − EM − EG, where EHS, EM, and EG are the total energies of the hybrid system, individual MnO2 monolayer, and graphene, respectively. The binding energy (per C atom) as a function of the interface distance is given in Figure 1d. Clearly, the formation of the MnO2/graphene interface is exothermic. The three energy profiles almost overlap, revealing 3.03 Å equilibrium interlayer distance and 90 meV binding energy. This indicates a weak coupling that is not sensitive to the stacking pattern. In Figure 1e we show the spinresolved band structure of the Mn-centered configuration, which is slightly energetically favorable, at the equilibrium distance. We observe that the electronic properties of the components are well-preserved, while remarkable new features emerge: The conduction band of MnO2 crosses the Fermi energy (EF), giving rise to half-metallicity, and spin polarization arises in graphene by proximity coupling to MnO2. The Heyd− Scuseria−Ernzerhof exchange correlation functional58 yields the same half-metallic feature for MnO2, with the spin majority conduction band crossing EF. In the following, we aim at exploring the influence of external factors on the material properties and thus investigate graphene/MnO2/graphene and MnO2/graphene/MnO2 hybrid systems. We first focus on the graphene/MnO2/graphene system. Contact to graphene results in occupation of states at the conduction band minimum (CBM) of MnO2, transforming it into a half-metal. To unravel this transition, we visualize in Figure 2a the charge redistribution at the interfaces by subtracting the charge of the hybrid system from that obtained for the isolated components. Charge shifts mainly from the
the material properties. In particular, we demonstrate that the interfacial coupling induces half-metallicity in MnO2 and that the magnetic proximity enables a quantum anomalous Hall state in graphene.
2. COMPUTATIONAL METHODS First-principles calculations are performed using the Vienna AbInitio Simulation Package with the spin-polarized Perdew− Burke−Ernzerhof exchange correlation functional. To model strong electronic correlations in the localized Mn 3d orbitals, we include an on-site Coulomb interaction with effective parameter U − J = 3.9 eV.51,52 Although this approach is limited with respect to the anisotropic Coulomb interaction,53,54 our results on the magnetism of monolayer MnO2 agree well with those based on the Heyd−Scuseria−Ernzerhof exchange correlation functional.44 A cutoff energy of 500 eV and a 30 × 30 × 1 k-mesh are used. The geometry is optimized until all residual forces are
