Half metal in two-dimensional hexagonal organometallic framework
© Hu et al.; licensee Springer. 2014
Received: 17 October 2014
Accepted: 4 December 2014
Published: 20 December 2014
Two-dimensional (2D) hexagonal organometallic framework (HOMF) made of triphenyl-metal molecules bridged by metal atoms has been recently shown to exhibit exotic electronic properties, such as half-metallic and topological insulating states. Here, using first-principles calculations, we investigate systematically the structural, electronic, and magnetic properties of such HOMFs containing 3d transition metal (TM) series (Sc to Cu). Two types of structures are found for these HOMFs: a buckled structure for those made of TMs with less half-filled 3d band and a twisted structure otherwise. The HOMFs show both ferromagnetic and antiferromagnetic properties, as well as nonmagnetic properties, due to the electronic configuration of the TM atoms. The V, Mn, and Fe lattices are ferromagnetic half metals with a large band gap of more than 1.5 eV in the insulating spin channel, making them potential 2D materials for spintronics application.
KeywordsHalf metal 2D hexagonal organometallic framework Triphenyl-transition metal Spintronics device
Half metals, which act as conductor for electrons of one spin orientation, but as insulator for those of opposite spin orientation, have attracted much recent interest. Due to their 100% spin polarization near the Fermi level, the half metals can provide purely spin-resolved electric current, holding great promise for future spintronics and nanoelectronic devices. Most known half metals are ferromagnets, with a few exceptions of half-metallic antiferromagnets; on the other hand, most magnets are not half metals. Although some types of magnets have been theoretically predicted to be half metals, most of them do not have high enough spin polarization at room temperature for device applications. As such, much effort has been devoted to searching for new half-metallic materials, including organic half metals and half-metallic nanostructures.
In the last decade, 2D materials made of a single atomic layer have drawn much interest from both fundamental and practical points of view. Magnetic as well as half-metallic properties of 2D materials are especially attractive topics of study, because they can be tuned by manipulating geometry[5, 6], doping, and adsorption[7–17]. One way to achieve magnetic 2D materials with half metallicity is via self-assembled growth of organometallic frameworks using organometallic compounds. For example, the single layer organometallic framework of transition metal (TM)-phthalocynine has been synthesized, and first-principle calculations demonstrated its half metallicity. Recently, the 2D lattice of hexagonal organometallic frameworks (HOMFs)[20–22] has been proposed theoretically by bridging the triphenyl-TM molecules with TM atoms, which show many exotic electronic properties, including topological insulating states[20, 21], and particularly, one such lattice made of triphenyl-Mn is shown to be a half metal.
In the present work, using first-principle calculations, we systematically studied the structural, magnetic, and electronic properties of the 2D free-standing HOMFs of triphenyl-TM lattices for all the 3d TMs from Sc to Cu. We found that the structures of this family of HOMFs have two types: buckled structure and twisted structure. Different TM elements lead to distinctively different electronic behavior, such as metallic vs. insulating and magnetic vs. nonmagnetic. Most interestingly, a few of them turn out to be half metals.
Our first-principle calculations were based on the spin-polarized density functional theory (DFT) using the generalized gradient approximation (GGA) in the form proposed by Perdew, Burke, and Ernzerhof (PBE) as implemented in the Vienna ab initio simulation package (VASP) code. The projected augmented wave (PAW) method[25, 26] with a plane wave basis set was used. We applied periodic boundary conditions with 20-Å periodicity in the z-direction to avoid interaction between two 2D lattices in the neighboring unit cells. An energy cutoff of 400 eV and a 5 × 5 × 1 k-point mesh were adopted for optimization and total energy calculation. During structural relaxation, we set the convergence criteria for energy and force to be 10-4 eV and 0.01 eV/Å, respectively.
Results and discussion
Summary of the structure feathers of the triphenyl-TMs lattices
We also did molecular dynamics simulation at room temperature to check the stability of such HOMF. For a free-standing triphenyl-TM lattice, the lattice is rather stable; only the benzene ring can rotate along the TM-benzene-TM axis, which may be impeded when the lattice is laid on a substrate.
The magnetic behavior of these HOMFs may be understood from simple electron counting. For Sc-Cr, both 4s and 3d electrons are involved in forming ‘covalent’ bond with three benzene rings. For Sc, all three valence electrons (two 4s and one 3d) are used to form bonds with three benzene rings, leaving no unpaired electron, so that Sc holds no magnetic moment. For Ti, V, and Cr, in addition to three valence electrons forming bonds, there are one, two, and three unpaired 3d electrons per TM atom, respectively, which give rise to 1, 2, and 3μB magnetic moments on each Ti, V, and Cr atom, respectively. This simple model agrees well with actual calculation results (Figure 2) which produce non-integer magnetic moments due to broadening of the atomic orbital into bands from TM-C bonding.
For Mn and Fe, the situation is different. Two 4s electrons are not involved in the bonding, only the 3d electrons bond to benzene rings. Due to local crystal field of C3 symmetry, the five 3d orbitals split into two groups: d xy , d yz ,, and d zx orbitals belong to E representation of C3 group, while belongs to A representation. For triphenyl-Mn lattice, the d xy , d yz ,, and d zx orbitals of the Mn atom have lower energy, leading to parallel spin alignment of two extra 3d electrons that are not involved in bonding, giving rise to 2μB on each Mn atom and 4μB per unit cell. For triphenyl-Fe lattice, the orbital of Fe atom has lower energy, two of the three un-bonded 3d electrons will occupy the two antiparallel x orbital, leaving behind one unpaired 3d electron occupying the higher level, leading to 1μB on each Fe atom and 2μB per unit cell.
An interesting property of these half-metallic HOMFs(V, Mn, and Fe) is that they have both Dirac bands, with Dirac points at K and K′, and flat band, due to the underlying hexagonal lattice symmetry[21, 22]. We note that inclusion of spin-orbital coupling will open small band gaps but without affecting magnetic properties, as shown before for Mn lattice[21, 22], and similar behavior is found here for Fe and Cr lattice. Depending on the location of the Fermi level, quantum anomalous Hall effect may be realized for these systems at low temperatures[21, 22].
In summary, we systematically studied the structural, magnetic, and electronic properties of a kind of HOMF containing 3d transition metals, using first-principle calculations. We found that their structures can be divided into two groups: buckled versus twisted, probably due to the degree of 4s-3d orbital hybridization involved in forming bonds with benzene rings. Some of these HOMFs (V, Mn, and Fe) favor ferromagnetic coupling and show half metallicity with the size of the band gap in the insulating spin channel larger than 1.5 eV, which makes these HOMF half metals potential 2D spin-injection or spin-detection materials. Combining with a good spin-conducting material, such as graphene, we also propose a possible design of 2D spintronic device.
This work is supported by NSF MRSEC (Grant No. DMR-1121252). HH acknowledges the Startup Funding from FIST, Xi’an Jiaotong University. FL acknowledges support from DOE-BES (Grant No. DE-FG02-04ER46148). We thank also the DOE-NERSC and the CHPC at the University of Utah for providing the computing resources.
- Katsnelson MI, Irkhin VY, Chioncel L, Lichtenstein AI, de Groot RA: Half-metallic ferromagnets: from band structure to many-body effect. Rev Mod Phys 2008, 80: 315–378. 10.1103/RevModPhys.80.315View ArticleGoogle Scholar
- Nie YM, Hu X: Possible half metallic antiferromagnet in a hole-doped perovskitecuprate predicted by first-principles calculations. Phys Rev Lett 2008, 100: 117203.View ArticleGoogle Scholar
- Galanakis I, Dederichs PH: Half-Metallic Alloys-Fundamentals and Applications. Berlin: Springer; 2005.View ArticleGoogle Scholar
- Geim AK: Graphene: status and prospects. Science 2009, 324: 1530–1534. 10.1126/science.1158877View ArticleGoogle Scholar
- Yu D, Lupton EP, Liu M, Liu W, Liu F: Collective magnetic behavior of graphene nanohole superlattices. Nano Res 2008, 1: 56–62. 10.1007/s12274-008-8007-6View ArticleGoogle Scholar
- Yu D, Lupton EP, Gao H, Zhang C, Liu F: Unified design rule for nanomagnetism in graphene. Nano Res 2008, 1: 497–501. 10.1007/s12274-008-8053-0View ArticleGoogle Scholar
- Kan E, Hu W, Xiao C, Lu R, Deng K, Yang J, Su H: Half-metallicity in organic single porous sheets. J Am Chem Soc 2012, 134: 5718–5721. 10.1021/ja210822cView ArticleGoogle Scholar
- Krasheninnikov AV, Lehtinen PO, Foster AS, Pyykkö P, Nieminen RM: Embedding transition-metal atoms in graphene: structure, bonding, and magnetism. Phys Rev Lett 2009, 102: 126807.View ArticleGoogle Scholar
- Huang B, Liu F, Wu J, Gu BL, Duan W: Suppression of spin-polarization in graphene nanoribbon by edge defect and impurity. Phys Rev Biol 2008, 77: 153411.View ArticleGoogle Scholar
- Uchoa B, Rappoport TG, Castro Neto AH: Kondo quantum criticality of magnetic adatoms in graphene. Phys Rev Lett 2011, 106: 016801.View ArticleGoogle Scholar
- Pi K, McCreary KM, Bao W, Han W, Chiang YF, Li Y, Tsai SW, Lau CN, Kawakami RK: Electronic doping and scattering by transition metals on graphene. Phys Rev Biol 2009, 80: 075406.View ArticleGoogle Scholar
- Brar VW, Decker R, Solowan HM, Wang Y, Maserati L, Chan KT, Lee H, Girit CO, Zettl A, Louie SG, Cohen ML, Crommie MF: Gate-controlled ionization and screening of cobalt adatoms on a graphene surface. Nat Phys 2010, 7: 43–46.View ArticleGoogle Scholar
- Cretu O, Krasheninnikov AV, Rodríguez-Manzo JA, Sun L, Nieminen RM, Banhart F: Migration and localization of metal atoms on strained graphene. Phys Rev Lett 2010, 105: 196102.View ArticleGoogle Scholar
- Rodríguez-Manzo JA, Cretu O, Banhart F: Trapping of metal atoms in vacancies of carbon nanotubes and graphene. ACS Nano 2010, 4: 3422–3428. 10.1021/nn100356qView ArticleGoogle Scholar
- Zan R, Bangert U, Ramasse Q, Novoselov KS: Metal-graphene interaction studied via atomic resolution scanning transmission electron microscopy. Nano Lett 2011, 11: 1087–1092. 10.1021/nl103980hView ArticleGoogle Scholar
- Du A, Chen Y, Zhu Z, Amal R, Lu GQ, Smith SC: Dots versus antidots: computational exploration of structure, magnetism, and half-metallicity in boron-nitride nanostructures. J Am Chem Soc 2009, 131: 17354–17359. 10.1021/ja9071942View ArticleGoogle Scholar
- Kan EJ, Xiang HJ, Wu F, Tian C, Lee C, Yang JL, Whangbo MH: Prediction for room-temperature half-metallic ferromagnetism in the half-fluorinated single layer fo BN and ZnO. Appl Phys Lett 2010, 97: 122503. 10.1063/1.3491416View ArticleGoogle Scholar
- Abel M, Clair S, Ourdjini O, Mossoyan M, Porte L: Single layer of polymeric Fe-Phthalocyanine: an organometallic sheet on metal and thin insulating film. J Am Chem Soc 2011, 133: 1203–1205. 10.1021/ja108628rView ArticleGoogle Scholar
- Zhou J, Sun Q: Magnetism of phthalocyanine-based organometallic single porous sheet. J Am Chem Soc 2011, 133: 15113–15119. 10.1021/ja204990jView ArticleGoogle Scholar
- Wang ZF, Liu Z, Liu F: Organic topological insulators in organometallic lattices. Nat Commun 2013, 4: 1471.View ArticleGoogle Scholar
- Liu Z, Wang ZF, Mei JW, Wu YS, Liu F: Flat chern band in a two-dimensional organometallic framework. Phys Rev Lett 2013, 110: 106804.View ArticleGoogle Scholar
- Wang ZF, Liu Z, Liu F: Quantum anomalous hall effect in 2D organic topological insulators. Phys Rev Lett 2013, 110: 196801.View ArticleGoogle Scholar
- Perdew JP, Burke K, Ernzerhof M: Generalized gradient approximation made simple. Phys Rev Lett 1996, 77: 3865–3868. 10.1103/PhysRevLett.77.3865View ArticleGoogle Scholar
- Kresse G, Furthmuller J: Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev Biol 1996, 54: 11169–11186. 10.1103/PhysRevB.54.11169View ArticleGoogle Scholar
- Blochl PE: Projected augmented-wave method. Phys Rev Biol 1994, 50: 17953–17979. 10.1103/PhysRevB.50.17953View ArticleGoogle Scholar
- Kresse G, Joubert D: From ultrasoft pseudopotential to the projector augmented-wave method. Phys Rev Biol 1999, 59: 1758–1775. 10.1103/PhysRevB.59.1758View ArticleGoogle Scholar
- Tombros N, Jozsa C, Popinciuc M, Jonkman HT, van Wees BJ: Electronic spin transport and spin precession in single graphene layers at room temperature. Nature 2007, 448: 571–574. 10.1038/nature06037View ArticleGoogle Scholar
- Castro Neto AH, Guinea F: Impurity-induced spin-orbit coupling in graphene. Phys Rev Lett 2009, 103: 026804.View ArticleGoogle Scholar
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