Structural, Electronic, and Magnetic Properties of One-Dimensional

Article pubs.acs.org/JPCC

Structural, Electronic, and Magnetic Properties of One-Dimensional Organic Bimetal-Naphthalene Sandwich Nanowires Xiuyun Zhang,*,† Jiu Han,† Yongjun Liu,† and Jinlan Wang*,‡ †

Department of Physics, Yangzhou University, Yangzhou, 225009, China Department of Physics, Southeast University, Nanjing, 211189, China

‡

S Supporting Information *

ABSTRACT: We systematically investigate the structural, electronic, and magnetic properties of one-dimensional bimetallic naphethalene sandwich nanowires, [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe, Np = C10H8, naphethalene) by employing ab initio calculations. Three structures with different alignments of TM atoms (isomer-I, -II, -III) are considered, and they are all of high stability with exceptions of [Np2V2Mn2]∞ (isomer-II, -III) and [Np2V2Fe2]∞ (isomer-III). Furthermore, the electronic and magnetic properties of [Np2V2TM2]∞ show clear dependence on chemical component and geometries. Most sandwich wires favor ferromagnetic coupling, while [Np2V2Ti2]∞ (isomer-I, -III) shows antiferromagnetic ground states. Interestingly, [Np2V2Cr2]∞ (isomer-II, -III), [Np2V2Mn2]∞ (isomer-I), and [Np2V2Fe2]∞ (isomer-I) are found to be robust ferromagnetic half-metals, and [Np2V2Cr2]∞ (isomer-I) is a ferromagnetic quasi-half metal.

I. INTRODUCTION Metal−ligand sandwich complexes are attracting great attention due to their intriguing electronic, magnetic, and optical properties and are expected to be potential candidates for future application in spintronics devices.1−48 To date, many organometallic sandwich complexes have been studied, such as benzene ligand, TM n Bz m , 2−22 cyclopentadienyl ligand, TMnCpm,23−36 and cyclo-octatetraene ligand, LnnCOTm,37−48 (TM = transition metal, Bz = C6H6, Cp = C5H5, Ln = lathanide, COT = C8H8), to name a few. Of which, the magnetic moments of VnBzm,6−11 TMn(FeCp2)m (TM = Ti, V, Mn),35,36 and EunCOTm42−44,48−50 increase linearly with cluster size; VnBzm, EunCOTm, and Vn(FeCp2)m coupled to certain electrodes show good spin filtering.17,21,34,50 Moreover, onedimensional (1D) infinite organometallic sandwich nanowires also show intriguing electronic and magnetic properties. For example, 1D (TMBz)∞16−21 (TM = V, Mn), (TMCp)∞ (TM = V, Cr, Fe),29,30 (BzVCpV)∞,37 [CpTiCpTM]∞ (TM = Cr, Fe), [CpCrCpTM]∞ (TM = Fe, Co), and [CpFeCpCo]∞36 are theoretically predicted to be ferromagnetic (FMs) half-metals (HMs). Furthermore, the electronic and magnetic properties of these organometallic sandwich complexes can be effectively engineered by carefully choosing the metal elements as well as the organic ligands.32−36 For example, the magnetic moment of TMn(FeCp2)m and its corresponding 1D [TM(FeCp2)]∞ (TM = Ti, Sc, V, and Mn) nanowires vary with the metal elements.35 Moreover, [CpTiCpV]∞ is an antiferromagnetic (AFM) metal, while [CpTiCpTM]∞ (TM = Cr, Fe) and [CpCrCpTM]∞ (TM = Fe, Co), [CpFeCpCo]∞ are ferromagnetic (FM) HMs.37 In addition, the magnetic moments as well as the © 2012 American Chemical Society

ferromagnetic stability of VnBzm are found to be significantly enhanced upon the replacement of Bz by Cp.20,22 On the other hand, organic-polycyclic-hydrocarbons ligand sandwich compounds, composed of metal atoms and a few fused polycyclic hydrocarbons rings, such as TM2nNpn+1 (Np = C10H8, naphthalene), TM2Pn2 ((Pn = C8H6, Pentalene), TM2nAntn+1 (Ant = C14H10, anthracene), as well as their derivatives, have also been investigated.51−54 Experimental explorations have revealed that TM2Np2 (TM = Ni, Co) and Mo2[C8H4(1,4-SiPri3)2]2 are AFM.51,52 First-principle calculations have predicted that 1D [PnMn2]∞ is FM, while 1D [NpTM2]∞ (TM=V, Mn, Ti, Nb) and [PnTM2]∞ (TM=V, Cr, Co, Ni) are AFM.53,54 However, compared with organic monocyclic sandwich complexes, the study on the organic polycyclic sandwich complexes is rather limited, and further understanding of the bonding characteristics and the magnetic mechanism are needed. In this work, we perform a systematic theoretical study on the structural, electronic, and magnetic properties of 1D infinite organic bimetallic Np-ligand sandwich nanowires (OBNSWs), [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe). Our results show that most 1D [Np2V2TM2]∞ wires are thermodynamically stable and their electronic and magnetic properties are strongly dependent on the chemical component and geometric structures. Interestingly, four wires, [Np2V2Cr2]∞ (isomer-II, -III), [Np2V2Mn2]∞ (isomer-I), and [Np2V2Fe2]∞ (isomer-I), are found to be robust FM HMs, and [Np2V2Cr2]∞ (isomer-I) is a FM quasi-HM. Received: November 27, 2011 Revised: February 6, 2012 Published: February 6, 2012 5414

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Figure 1. (a) Three initial structures considered for the [Np2V2TM2]∞ OBNSWs: isomer-I, two V and two TM atoms sit in the same column; isomer-II, two V and two TM atoms sit in the same row; isomer- III, V and TM atoms sit alternately in the same row and in the same column. (b) Top view of three isomers.

Table 1. Lattice Constant (c), the C−C (RC−C) and C−H (RC−H) Bond Lengths of Np Molecules, the Distances of TM−TM (RTM−TM) Atoms and TM Atoms to the Mass Centers of Their Facing Hexagonal Rings (RTM−R), and the Binding Energies (Eb) per Unit Cell of the [Np2V2TM2]∞ Wires structure

system

c/Å

RC−C (Å)

RC−H (Å)

RTM−TM (Å)

RTM−R (Å)

Eb (eV)

isomer I

V−Ti V−Cr V−Mn V−Fe V−Ti V−Cr V−Fe V−Ti V−Cr

7.15 6.82 7.16 6.94 7.2 6.89 7.19 7.25 7.05

1.444−1.489 1.444−1.488 1.442−1.479 1.440−1.502 1.454−1.500 1.455−1.486 1.443−1.469 1.451−1.490 1.446−1.453

1.089 1.092 1.092 1.089 1.091 1.092 1.090 1.089 1.091

2.803 2.797 2.699,2.962 2.976 2.774, 2.66631 2.824, 2700 2.626, 3.124 2.835 2.631, 2.926

1.791, 1.794 1.709, 1.711 1.692−1.943 1.739, 1.787 1.754, 1.851 1.659−1.791 1.778, 1.844 1.777−1.823 1.717−1.817

23.824 20.532 20.354 20.148 24.206 20.802 20.717 24.037 20.533

isomer II

isomer III

II. COMPUTATIONAL METHOD

energetically preferred states, the magnetic moment is first allowed to optimize freely to the favored spin state (Sz), and then, we also consider the neighboring spin states (Sz ± 2) and optimize them by fixing the magnetic moment.

All calculations are performed within the framework of spinpolarized density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP).55,56 The exchange−correlation potentials are treated by the generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerholf (PBE).57 The interaction between valence electrons and ion cores is described by the projected augmented wave (PAW)58,59 method. The combination of PBE/PAW has been commonly exploited to describe the interaction of metal atoms and organic molecules in previous literatures.29,30,32,36,37 Three possible configurations (isomer-I, -II, -III) are considered for 1D OBNSWs as shown in Figure 1. Periodic boundary condition is applied along the TM-Np principal axis with the unit cell containing four metal atoms and two Np molecules. The vacuum spaces different along TM-Np axes are set as 20 Å to ensure that interaction between the supercells negligible. The energy cutoff for the plane-wave function is 400 eV and the ions are allowed to fully relax until the force acting on each atom is less than 0.01 eV/Å. Reciprocal space is sampled by 1 × 1 × 15 grid meshes using the Monkhorst-Pack scheme for relaxation and much denser k-point grid (1 × 1 × 45) is used for the electronic structure calculations. Furthermore, for each conformer, different spin states are considered. To ensure the obtained spin state is the most

III. RESULTS AND DISCUSSION Structures and Stabilities. The structural information and binding energies of 1D OBNSWs, [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe) are summarized in Table 1, and the optimized structures are displayed in Figure S1 in the Supporting Information. Our calculations identify six normal sandwich structures, such as: [NpV 2 NpTi 2 ] ∞ (isomer-I, -II), [Np2V2Cr2]∞ (isomer-I, -II, -III), and [Np2V2Fe2]∞ (isomerI). Slight deformations are observed in [Np2V2Cr2]∞ (isomerIII), [Np2V2Fe2]∞ (isomer-II), and [Np2V2Mn2]∞ (isomer-I). In contrast, [Np2V2Mn2]∞ (isomer-II, -III) and [Np2V2Fe2]∞ (isomer-III) (see Figure S1 of Supporting Information) are largely structurally distorted in that some metal atoms sit in distance from the mass centers of hexagonal carbon rings or that some Np molecules are largely deformed, and they are no longer plane, showing that these three structures are not good building blocks for forming 1D sandwich molecular chains. Thereby, we only focus on the structural, electronic, and magnetic properties of the other nine OBNSWs with perfect or near-perfect sandwich configurations in the following discussion. Among these 1D OBNSWs, [Np2V2Ti2]∞ (isomer-III) 5415

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showing that no covalent bonding exist between them, therefore, we will not consider the interaction of layered TM atoms in following discussion. For [Np2V2Ti2]∞, no charge density is found between TM atoms in the same layer for isomer-I (2a), while it increases a bit larger for isomer-II (2b) and becomes the largest for isomer-III (2c), showing that the TM−TM covalent interaction in isomer-III is the strongest. On the other hand, the largest charge density between TM and nearby Np is found for isomer-II (2b). As a result, the stability of [Np2V2Ti2]∞ follows the order: Eb(II) > Eb(III) > Eb(I). Similarly, for the case of [Np2V2Cr2]∞, the charge density between the TM atoms in the same layer and TM−Np in isomer-II (2e) is a bit larger than that of isomer-I (2d) and isomer-III (2f), where almost zero TM−TM and equivalent TM−Np covalent interaction are found. Therefore, [Np2V2Cr2]∞ (isomer-II) has the largest binding energy, while isomers-I/-III almost have same stability. Furthermore, interesting magnetic properties are observed in these OBNSWs. The total/local magnetic moments and the energy differences between FM and AFM states of [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe) are summarized in Table 2 and parts a and b of Figure 3. Their spin isosurface electron densities are also displayed in Figure 3c. The information of other different spin states is presented in Table S1 in the Supporting Information. Clearly, [Np2V2Ti2]∞ (isomers-I, -III) have AFM ground states with local magnetic moments of 0.9942/1.324 μB, −0.9942/−1.324 μB, 0.4832/ 0.755 μB, −0.4832/−0.755 μB for V, V, Ti, Ti atoms, respectively; the AFM ground states are 0.116 and 0.097 eV lower in energy than their corresponding FM states, respectively. In contrast, the FM ground state of [Np2V2Ti2]∞ (isomer-II) is energetically more stable than the AFM state by about 0.164 eV lower in energy. As for [Np2V2Cr2]∞ (isomer-I, -II), they favor FM states and the AFM states are less stable by 76 and 31 meV in energy, respectively. Differently, the metal atoms in [Np2V2Cr2]∞ (isomer-III) display a ferrimagnetic (FIM) coupling, where small negative magnetic moments for one V (−0.344 μB) and Cr (−0.077 μB) atom and larger positive magnetic moments for another V (1.266 μB) and Cr (3.015 μB ) atoms are observed. The metal atoms in [Np2V2Mn2]∞ (isomer-I) and [Np2V2Fe2]∞ (isomer-I, -II) all are FM coupled with large magnetic moments of 6, 6, and 8 μB per unit cell, respectively; their AFM states are quite unstable with 0.767, 0.245, and 0.117 eV higher in energy than their FM states, respectively. Furthermore, our results show that the chemical component ef fects can effectively tune the magnetic properties of monometallic [Np2V4]∞ (AFM), e.g., doping Ti atoms to [Np2V4]∞ can stabilize the AFM state ([Np2V2Ti2]∞, isomer-I, -III), while doping other metals (Cr, Mn, Fe) can switch it to be FM materials. Indeed, the chemical component effects can be analogue to the reported carrier-tunable magnetic effect54 (the TM atom in [Np2V2TM2]∞ having more valence electrons than V can be regarded as doping electrons to [Np2V4]∞ and vice versa), where the AFM ordering of [Np2V4]∞ can be stabilized by injecting holes and it is switched to FM ordering by injecting electrons, due to the insufficient electron carriers of [Np2V4]∞ itself. The magnetic coupling mechanism of these OBNSWs can be understood from the band structures and local density of states (LDOS) shown in Figure 4 and Figures S2−S3 in the Supporting Information. Similar to the monocyclic TMBz16−22 or TM-Cp29−37 sandwich systems, the magnetic

has the largest lattice parameter of 7.25 Å, in contrast to the smallest one (6.82 Å) of [Np2V2Cr2]∞ (isomer-I). Besides, the C−C bond lengths of [Np2V2TM2]∞ are around 1.440−1.502 Å, and the C−H bond lengths are about 1.089−1.092 Å. The distances of metal atoms to the mass centers of their nearby hexagonal carbon rings are in the range of 1.659−1.943 Å, and the TM−TM distances in the same layer vary from 2.626 to 2.976 Å. To evaluate the structural stability of the wires, we calculate the binding energy per unit cell of [Np2V2TM2]∞, defined as Eb = −{2E(Np) + 2E(V) + 2E(TM) − E([Np2V2TM2]∞ )}

(1)

where E[·] is the total energy of wires, Np molecule, and TM atoms, respectively. Compared with the binding energy (Eb) of monometal [Np2V4]∞ (16.879 eV), the stability of these bimetallic wires is largely enhanced (Eb > 20 eV, see Table 1), showing that they are thermodynamically stable enough again decomposing into small fragments. Indeed, the stability of these OBNSWs is found to associate with the metal elements (chemical component ef fects) as well as their geometric conformations (structural ef fects). For example, 1D [Np2V2Ti2]∞ (isomer-II) has the largest binding energy of 24.206 eV per unit cell, which is about 0.382/0.169 eV lower in energy than isomers-I/-III, respectively. In contrast, [Np2V2Mn2]∞ (isomer-I) has the smallest stability with the binding energy of 20.354 eV per unit cell. Besides, isomer-I and isomer-III of [Np2V2Cr2]∞ have almost same binding energies (20.532 eV vs 20.533 eV, respectively), which both are less stable than isomer-II by about 0.27 eV. The structural effects to the stability of these [Np2V2TM2]∞ wires can be qualitatively understood from the TM−TM and TM−Np interaction. We take [Np2V2Ti2]∞ and [Np2V2Cr2]∞ as examples and plot their charge densities in Figure 2. As the

Figure 2. (a−f) Charge densities of three different structures of [Np2V2Ti2]∞ and [Np2V2Cr2]∞.

distances between TM atoms above and below the Np molecule (layered TM atoms) are larger than 3.5 Å (see Table 1), much larger than the same-layer TM−TM atoms and atomic interaction distance of V−TM dimers (less than 2 Å); therefore, the direct coupling between them should be basically minimal. Furthermore, the weak interaction of layered TM atoms can also be clearly seen from the charge density profiles (see Figure 2), where almost zero charge density is found, 5416

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Table 2. Lattice Parameters (c), Electronic State (ES), Energy Difference between FM and AFM states (ΔE), Total/Local Magnetic Moments (MMs), and Ground states (GS) of [Np2V2TM2]∞ Wires individual MMs (μB) C (Å)

ES

ΔE(eV)

MMs (μB)

V1

V2

TM1

TM2

V−Ti

7.15

V−Cr

6.82

V−Fe

6.94

V−Mn

7.16

V−Ti

7.2

V−Cr

6.89

V−Fe

7.19

V−Ti

7.25

V−Cr

7.05

AFM FM FM AFM FM AFM FM AFM FM AFM FM NM FM AFM AFM FM FIM AFM

0 0.116 0 0.076 0 0.245 0 0.767 0 0.164 0 0.031 0 0.117 0 0.097 0 0.124

0 4 2 0 6 0 6 0 2 0 2 0 8 0 0 4 4 0

0.994 1.696 0.873 −0.149 1.450 1.532 0.995 1.720 0.921 0.029 0.929 0 1.680 −0.647 1.324 0.847 −0.344 −0.555

−0.994 1.696 0.873 −0.161 1.450 1.412 1.155 1.720 0.919 −0.027 0.930 0 1.680 −0.763 −1.324 0.501 1.266 0.555

0.483 1.005 0.204 0.150 1.787 −1.663 3.557 −1.862 0.094 0.043 0.058 0 2.405 1.382 0.755 1.152 3.015 0.049

−0.483 1.005 0.204 0.150 1.787 −1.367 0.465 −1.855 0.075 −0.044 0.058 0 2.405 0.425 −0.755 1.016 −0.077 −0.049

system isomer-I

isomer-II

isomer-III

GS AFM

metal

FM

metal

FM

HM

FM

HM

FM

metal

FM

HM

FM

SC

AFM

metal

FIM

HM

states, and their TM−Np interaction follows double-exchange mechanism as discussed previously.53,54 Since the superexchange and double-exchange mechanisms both include a intermediary part (Np) besides the layered TM atoms, indicating again that interaction of layered TM atoms is rather weak and is the determining one mediated by the Np ligands. On the other hand, the magnetic moments of these [Np2V2TM2]∞ wires also exhibit evident dependence on the geometric structures: the magnetic moments of [Np2V2Ti2]∞ (isomer-I, -III) is 0 μB, contrast to the 2 μB of isomer-II; the magnetic moment of [Np2V2Cr2]∞ (isomer-III) is 4 μB, larger than that of isomer-I and isomer-II (2 μB, 2 μB); the magnetic moment of [Np2V2Fe2]∞ (isomer-I) is 6 μB, different from 8 μB of isomer-II. Similarly, the different structural effects can also be understood from their different TM−TM and TM−Np coupling. By use of [Np2V2Ti2]∞ for example, the TM−TM distances in isomer-II are not uniform, e.g., 2.774 Å for V−V and 2.666 Å for Ti−Ti, as compared with those in isomers- I/III, which results in asymmetric TM−TM and TM−Np interaction. As a result, two minority bands (black arrows in Figure 4a) composed of dz2 and dx2−y2 states from V and Ti atoms located below the Fermi level of isomer-I are shifted above the Fermi level in isomer-II (Figure 4e), the total magnetic moment of the latter thus increases to 2 μB. Similarly, for [Np2V2Cr2]∞ (isomer-III), the asymmetric TM−TM and TM−Np interaction flips one minority dxz band composed of V/Cr (isomer-I, -II, blue arrow) (parts b and g of Figure 4) into the majority one (Figure 4f), as a result, the total magnetic moment of [Np2V2Cr2]∞ (isomer-III) is increased by 2 μB. Similar explanation can also be made for [Np2V2Fe2]∞ (isomerI, -II) (parts d and h of Figure 4d, h). Finally, these 1D OBNSWs display rich electronic properties. Four wires, [Np2V2Cr2]∞ (isomers-II, -III), [Np2V2Mn2]∞ (isomer-I), and [Np2V2Fe2]∞ (isomer-I), are identified to be FM HMs (see parts c−f of Figure 4), e.g. they are semiconducting on one spin channel, while they are metallic on the other one. Besides, the [Np2V2Cr2]∞ (isomer-I) (see Figure 4b) can be regarded as a quasi-HM having a small hole at the X point near the Fermi level on the minority channel.

Figure 3. (a) Magnetic moments, (b) energy differences (ΔE) of FM and AFM states, and (c) spin densities of the [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe) wires per unit cell.

coupling of these 1D OBNSWs can be regarded as a result of the interplay between the spin splitting and the crystal-field splitting, and differently, the lower symmetry of Np splits the d orbitals of TM atoms into five nondegenerate ones. Spin density and LDOS shown in Figure 3c and Figure S3 of Supporting Information reveal that the magnetic moments of these wires are mainly contributed by the 3d states of metal atoms, while those of Np ligands are negligible. Moreover, the exchange splitting of these OBNSWS shows a clear dependence on the metal elements. For example, almost no exchange splitting is found for [Np2V2Ti2]∞ (isomer-I, -III), where identical valence electron densities are found on the majority and minority channel (see Figures 3c, 4a, and S2 of Supporting Information); therefore, their ground states favor AFM coupling and the V−Np/Ti−Np interaction is through superexchange mechanism.60,61 Differently, large exchange splitting is found for the majority and minority bands of all the other wires, which are responsible for their FM ground 5417

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Figure 4. (a−h) Band structures of [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe) OBNSWs at different structures.

per unit cell of [Np2V2TM2]∞ OBNSWs (Figure S1); band structures of [Np2V2Ti2]∞ for isomer-III (Figure S2). Local density of states of [Np2V2Cr2]∞ (isomer-I, -II, -III) (Figure S3) are in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

These (quasi) HMs have nearly 100% spin polarization and can act as perfect spin transport devices. On the other hand, the half-metal gaps37 defined as the difference between the Fermi level and the topmost occupied band in the semiconductor channel of above HM wires are 0.45/0.15, 0.42, and 0.84 eV for TM = Cr (isomer-II, -III), Mn (isomer-I), and Fe (isomer-I), respectively, and the largest HM gap of [Np2V2Fe2]∞ (isomerI) is due to the stronger covalent hybridization of Fe and V atoms with the Np ligands.

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*E-mail: [email protected] (X.Z.); [email protected] (J.W.).

IV. CONCLUSION The structural, electronic, and magnetic properties of quasione-dimensional bimetallic Naphthalene OBNSWs, [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe, Np = C10H8) have been systematically studied by employing spin-polarized density functional theory method. The electronic and magnetic properties of [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe) OBNSWs can be effectively tailored by chemical component and geometric structures. Most of the wires studied here favor FM coupling with exceptions of [Np2V2Ti2]∞ (isomers-I, -III) having AFM ground states. Furthermore, [Np 2 V 2 Cr 2 ] ∞ (isomers-II, -III), [Np2V2Mn2]∞ (isomer-I), and [Np2V2Fe2]∞ (isomer-I) wires are robust FM HMs, showing that they might be served as ideal materials for promising molecular spintronics. The tunable electronic and magnetic properties of these bimetallic naphthalene sandwich complexes might open new ways to nanomaterials with required properties.

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AUTHOR INFORMATION

Corresponding Author

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the National Nature Science Foundation of China (No.1110424, 11074035). The authors thank the computational resource at Department of Physics, YZU, and Department of Physics, SEU.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Energy differences of different spin states of [Np2V2TM2]∞ (TM = Ti, Cr, Mn, Fe) wires (Table S1); optimized structures 5418

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp211419a | J. Phys. Chem. C 2012, 116, 5414−5419