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Effect of Uniaxial Strain on the Electrical and Magnetic Property of a One-Dimensional Bimetallic Sandwich Molecular Wire (FeCpVCp)∞ Haixia Da,†,‡ Hong Mei Jin,‡ Shuo-Wang Yang,*,‡ and Kok Hwa Lim*,† School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore, 637459, Singapore, and Institute of High Performance Computing (IHPC), 1 Fusionopolis, Way, #16-16 Connexis Singapore 138632, Singapore ReceiVed: August 2, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009
We employed density functional theory (DFT) calculations to explore the manipulation of the electrical and magnetic properties of one-dimensional (1D) bimetallic sandwich molecular wire, (FeCpVCp)∞, under uniaxial strain. It is predicted that (FeCpVCp)∞ undergoes ferromagnetic-semiconducting, metallic, and antiferromagnetic-semiconducting transitions under compressive strain, but remains a semiconductor under tensile strain. Detailed analysis revealed that such phenomena are caused by electron transfer and redistribution between metal atoms. Meanwhile, we calculate magneto-crystalline anisotropy values and predicted that the magnetization direction will change along the external strains applied on the wire. Introduction One-dimensional (1D) organometallic sandwich molecular wires (SMWs) recently have attracted considerable attention both experimentally and theoretically because of their unique electrical and magnetic properties and potential applications in molecular electronics and spintronics.1-11 In contrast with extensively studied bulk materials, such as magnetoelectric Cr2O3 and some perovskites, such as La1-xSrxMnO3 and mutilferric BiFeO3,12-14 1D molecular magnetizms have unique advantages, such as small size, large magnetic moment (MM) and multiple electrical and magnetic functions. For example, (FeCp)∞ (Cp ) C5H5) SMW was theoretically predicted to be half-metallic (HM) with excellent spin filter and negative differential resistance (NDR) effects simultaneously.5 Experimentally, 1D europium (Eu)-cyclooctatetraene (COT ) C8H8) sandwich molecular clusters, EunCOTn+1 (n ) 1-18) were observed to be extremely stable and showed huge MMs.9,10 In addition, the magnetization direction of an Eu2COT3 cluster was predicted to be switched from in-plane (parallel to the COT ring) to out-of-plane (perpendicular to the COT ring) by changing its oxidation states.3 All aforementioned characteristics make 1D SMWs or molecular clusters to be very promising candidates as high-density storage media in modern electronics or spintronics, and open a new window for molecular magnetic/ electrical transport applications. Today, most of the theoretical and experimental works on SMWs’ properties only consider the static or invariable states. However, sometimes it is desirable to have materials with convertible or switchable properties, such as metallic to semiconducting, semiconducting to insulating transitions, or ferromagnetic (FM) to antiferromagnetic (AFM) transition, to accommodate the specific requirements and adapt various devices. Such physical property controls have been widely reported for other materials: for example, a pressure-induced metal-to-semiconductor transition in lithium;15 a metal-toinsulator16 or metal-to-semiconductor transition for graphene17-20 * Corresponding author. Tel: +65-6514 1909. Fax: +65- 6794 7553. E-mail:
[email protected] (K.H.L.);
[email protected] (S.-W.Y.). † Nanyang Technological University. ‡ Institute of High Performance Computing.
and a semiconductor to HM transition for graphene nanoribbons as well as carbon nanotube dots.21,22 It has also been reported that biaxial compressive strain can enhance the ferroelectric properties of BaTiO3 thin films;23 uniaxial strain can modify the Raman spectra and band gap of graphene24 and theoretically predict band structure manipulation for various nanowires and nanotubes.25-27 In fact, it is always desired to manipulate the electrical and magnetic properties of molecular wires or clusters via simple but efficient external forces, such as pressure and strain. Unfortunately, there are very few reports on molecular wire strain effects.1,4 Xiang et al. examined strain effects on (VBz)∞ and (MnBz)∞, (Bz ) benzene) and found that their physical properties remains largely unchanged under compression conditions, but (VBz)∞ turns into HM from quasi HM1 when elongated by 3% to 12%. In a similar study by Maslyuk et al., it has also been reported that (VBz)∞ remains as an HM until ∼12% elongation, before undergoing a low-spin to high-spin transition.4 This implies that unveiling the mechanism of strain effect is very important, which allows us to clearly understand the intrinsic physics behind such a phenomenon. In this work, we perform density functional theory (DFT) calculations to investigate the manipulation of electrical and magnetic properties for a 1D bimetal sandwich molecular wire (BSMW), (FeCpVCp)∞, by external uniaxial strain. (FeCpVCp)∞ was chosen because the respective molecular clusters, Vn(FeCp2)(n+1), (n ) 1-3), are the only successful synthesized bimetal multidecker molecular clusters28 and they exhibit large MM enhancement and unique charge distribution, which can be presented as (Fe0Cp-V2+Cp-)∞.29 We predict that its electrical and magnetic properties can be manipulated by external strains. It is an FM semiconductor in nature and exhibits 5.0 µB MM per unit, but becomes metallic when it is compressed by 7.5%. Interestingly, it becomes an AFM semiconductor when it is compressed beyond 15% and exhibits MM of 1.0 µB. In contrast, the wire is found to be able to maintain its FM semiconductor feature until 18% tensile strain. We uncover the mechanism behind such strain-dependence on the electrical, and magnetic characteristics by detailed electron transfer analysis and individual local electron configurations on each metal atom. In
10.1021/jp907427j 2009 American Chemical Society Published on Web 11/24/2009
Effect of Uniaxial Strain on (FeCpVCp)∞
Figure 1. Uniaxial strain effects on bonding energy, total MM, spinpolarization energy (∆E ) EFM - EAFM), and MCA of the BSMW, (FeCpMCp)∞. Here SC and MT stand for semiconductor and metallic, respectively.
addition, the magnetization direction of this BSMW is also found to be switchable along the external strain with very small energy difference, providing potential new application for molecular spintronics. Computational Models and Methods The BSMW, (FeCpVCp)∞, is placed within a 15 × 16 × c Å3 unit cell (see Figure 1), where c is the lattice constant. Structure optimizations and band structure calculations are performed using the Vienna ab initio simulation package (VASP).30 The generalized gradient approximation (GGAPBE)31 scheme is implemented for electron exchange and correlation, while the frozen-core projector augmented wave (PAW) method is used to describe the interaction between ions and electrons.32 The cutoff energy is set at 400 eV with a 1 × 1 × 45 k-point mesh. The full structure was optimized until the force acting on each atom was less than 0.01 eV/Å. The uniaxial strain is introduced by changing the lattice constant, c. At each fixed lattice constant, structural optimization is performed without any constraints. Spin-polarization is considered for all the calculations. The binding energies are calculated using the following equation:
EB ) Eunit - (EFe + EV + 2ECp) Here, Eunit is the optimized total energy for an unit cell under a specific uniaxial strain; ECp, EFe, and EV denote the energies of the isolated radical Cp ring, single Fe, and V atoms, respectively. Meanwhile, we use ε to denote strain extent, which is defined as ε )(cs - c)/c. cs and c denote the lattice constants with and without strain, respectively. Results and Discussions The optimized lattice constant, c, is 7.38 Å at equilibrium state where ε equals to zero. The strain effects on the wire
J. Phys. Chem. C, Vol. 113, No. 51, 2009 21423 properties are schematically presented in Figure 1. It can be seen that (FeCpVCp)∞ is very flexible along the c direction, and shows extreme resistibility to the uniaxial strain. On the basis of the binding energy calculated, its structure is stable and retains its sandwich feature from compressive 18% to tensile 20% regions. Meanwhile, the ground states were examined via energy comparison of various magnetic states, i.e. paramagnetic, FM (where the spins of Fe and V atoms align in parallel within the same, as well as adjacent units) and AFM states (the spins of Fe and V atoms are opposite to the same elements in the adjacent unit). It is found that the FM states are energetically more favorable compared to paramagnetic and AFM states within the region ε of -15% to +15%, indicating that the FM states are the robust ground states, and (FeCpVCp)∞ exhibits excellent spin stability. The energy difference (∆E) between FM and AFM states is above 500 meV in the tensile region and compressive region before ε extends to -7.5%. However, when ε extends through -7.5%, ∆E decreases steeply, suggesting electron reconfiguration and charge transfer between the metal atoms, which lead to changes to its electrical and magnetic properties. At the strain-free condition, i.e., the equilibrium ground state, (FeCpVCp)∞ is a FM semiconductor with band gap of 1.40 eV for spin-up band and 0.87 eV for spin-down band. Its total MM is 5 µB (see Figure 2) and the local MMs on Fe and V atoms are 2 µB and 3 µB respectively. It remains FM semiconductor in compressive condition until ε to -7.5%, after that it becomes metallic within region ε of -7.5% to -15%, where the local MMs on Fe and V atoms among (FeCpVCp)∞ are decreased smoothly along continuously compression, and the total MM decreases as well. For example, total MM is 3.6 µB at ε of -10%, meanwhile, the local MMs on Fe and V atoms are 1.3 µB and 2.2 µB, respectively. The noninteger MM values imply that the highest occupied bands are partially filled and cross over the Fermi level, which is the metallic characteristics. Further compression beyond ε of -15% converts (FeCpVCp)∞ to an AFM semiconductor, with total MM of 1 µB located entirely on the V atom. At this stage, the local electron configurations on each metal atom are totally different from that of original strain-free state (see Figure 3). In contrast, under tensile range, (FeCpVCp)∞ maintains its FM semiconductor characteristic and its total MM as well as the local MMs on each metal atom remain unchanged as that in equilibrium state. All aforementioned electrical and magnetic property changes along the external uniaxial strain are presented in Figure 1, where we differentiate the compressive region into three zones: (I) FM semiconducting, (II) FM metallic, and (III) AFM semiconducting; meanwhile, the tensile region was denoted as (IV) FM semiconducting. Here, it can be clearly seen that the electrical and magnetic properties of this BSMW can be manipulated by uniaxial strain. To deeply understand these manipulated electrical and magnetic behaviors and uncover the mechanism behind them, we study the electron configurations on individual metal atoms among (FeCpVCp)∞ since they predominate the electrical and magnetic properties of it.29 (FeCpVCp)∞ wire has D5h symmetry, according to the crystal field theory, the 3d orbitals of Fe and V atoms will split into a dz2 (dσ) and two sets of doubly degenerate dxz, dyz (dπ) and dxy, dx2-y2 (dδ) orbitals under a D5h ligand environment. It has been unveiled in our previous study7 that, at the equilibrium state, both Fe and V atoms donate one electron to neighboring Cp rings respectively. Meanwhile, an unusual electron transfer occurs from the V atom to the Fe atom via a spin-flipping mechanism, i.e., one electron transfers from
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Figure 2. The schematic spin densities and local MMs on Fe and V atoms at different strain states. The possible charge distributions within (FeCpMCp)∞ wire are also presented beside each state.
the V minority spin (dσ) to the Fe majority spin (dπ). Such electron transfer eventually produces a charge redistribution that can be presented as (Fe0Cp-V2+Cp-)∞. It is a semiconductor based on its spin-polarized band structures (Figure 3), where the local electron configurations of Fe and V atoms are given. Actually, such electron transfer results in an MM enhancement and an increase in the total MM to 5 µB.7 Applying further compressive uniaxial strain on this BSMW moves the majority-spin (spin-up) states toward the Fermi level; meanwhile, minority-spin (spin-down) states are stabilized (Figure 3). The band gaps become smaller, but the wire remains semiconducting in Zone (I) until ε reaches -7.5%. Before that, the local electron configurations on Fe and V atoms and the charge distribution among the wire remain the same as that in strain-free conditions, i.e., (Fe0Cp-V2+Cp-)∞. Therefore, the total and local MMs on Fe and V atoms remain unchanged. With further compression, the Fe-dδ band energy continuously increases and finally crosses over the Fermi level (zone II). Meanwhile, the lowest unoccupied spin-down band, V-dδ, energy decreases and crosses the Fermi level, meaning that it starts to be partially filled by the electrons transferred from the
Fe-dδ band via the spin-flipping mechanism. During the compressive and tensile procedure, Hu¨ckel rule is still valid for Cp rings, which retain their 4n + 2 electron aromatic configurations with negatively charged (Cp-) ionic states. The net electron transfer occurs from metal atoms to organic Cp rings. After ε is less than -7.5%, Fe atom becomes more positively charged from its neutral state and charges on the V atom are less than the 2+ state. The wire becomes FM metallic, and the magnetization drops sharply since the spin-flipping electron transfer greatly quenches the total MM as well as the local MMs on metal atoms. Higher compressive strain means more electrons spin-flipping transfer from spin-up Fe-dδ to spin-down V-dδ bands. The local electron configurations of Fe and V atoms change from 3d84s0 and 3d34s0 to 3d74s0 and 3d44s0, respectively (see Figure 3). Thereby, the total MM decreases from 5 µB to 3 µB, and approaches 1 µB, and the charge distribution changes from (Fe0Cp-V2+Cp-)∞ to (Fe1+Cp-V1+Cp-)∞. Continuous compression forces more electrons to transfer from Fe to V atoms. When ε is less than -15%, all the electrons in spin-up Fe-dδ are transferred into the spin-down V-dδ band, and (FeCpVCp)∞ becomes an AFM semiconductor (zone III).
Effect of Uniaxial Strain on (FeCpVCp)∞
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Figure 3. Spin-polarized band structures of (FeCpVCp)∞ at (a) equilibrium state; (b) 10% compressive state (the hollow arrows indicate partially filled electrons); (c) 15% compressive state; and (d) 15% tensile state. The local electron configurations on Fe and V atoms are given as insets in. The red hollow arrow means partially filled.
For example, it shows a 0.76 eV band gap at ε of -17.5% where the spin-up Fe-dδ band is completely above the Fermi level, and the spin-down V-dδ band is below the Fermi energy. Within this region, the local electron configurations for Fe and V atoms are 3d64s0 and 3d54s0 (Figure 3), and the local MMs on Fe and V atoms are 0 µB and 1 µB respectively. Furthermore, the charge distribution within the wire can be presented as (Fe2+Cp-V0Cp-)∞. The charge states on Fe and V atoms are reversed when compared to the strain-free state.
Compared to compressive conditions, tensile effect seems quite monotone in zone IV. (FeCpVCp)∞ remains an FM semiconductor, and the total MM of the wire as well as the local MMs on Fe and V atoms are also unchanged. When the tension increases, the binding energy becomes moderately weaker but still exhibits a quite strong bound structure. The binding energy still has -11.16 eV up to ε of 15% (-12.71 eV at original state), indicating this BSMW is very flexible toward tensile strain. Meanwhile, the energy difference
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between FM and AFM, ∆E is almost constant at about 50 meV, meaning that the FM spin is stable within this region. It reveals that the original electron distribution, i.e., (Fe0Cp-V2+Cp-)∞ is robust against elongation. This is because the V atoms lose electrons much more easily compared to the Fe atoms. More importantly, the electron spin-polarization splitting for V atoms shows a dramatically larger difference between two opposite spin states, which results in all spin-down bands of V atoms residing above the Fermi level. The external tension further pushes these spin-down bands up. We believe this is the intrinsic physical mechanism for the BSMW toward tension. Besides studying the manipulation of (FeCpVCp)∞’s electrical and magnetic properties, we have also investigated the effect of strain on its magnetization direction. It has been reported that the magnetization direction of the sandwich molecule Eu2(C8H8)3 can be controlled via oxidation-reduction reactions.3 This is because the electron distributions have changed before/after these reactions, which should significantly affect the spin-orbit interaction (SOI), and subsequently breaks the environment symmetry and splits the Kondo resonance.33,34 In fact, we have noted that, along the compressive procedure, significant charge transfers occurred in zones II and III, i.e., electrons have been redistributed under the higher crystal field. It implies that the magnetization direction may be switchable for BSMW. Therefore, we further investigate the effect of strain on magnetocrystalline anisotropy (MCA) with SOI, which is the criterion to determine the energy necessary to deflect the MM direction. Noncollinear magnetic calculations are carried out to obtain the energy difference of the two magnetization directions [MAE ) E(0,0,1) - E(1,0,0)] for a series of strained structures: one is in-plane, and the other is out-of-plane. The strain effect on the magnetization direction for (FeCpVCp)∞ has been illustrated as a function of MAE versus ε (Figure 1). Our calculations show that the magnetization direction prefers in-plane at the strain-free condition. It remains unchanged for the extension region (zone IV) as well as weak compressive strain (in zone I), where MAE exhibits positive values. It means that there is no significant charge transfer or electron redistribution within the above areas, which could not affect the SOI much. This conclusion is actually consistent with the earlier analysis of this work. The BSMW shows slight resistibility to compression; on the other hand, it seems that tensile strain has minimal effect on the SOI, and MAE remains the same in the tensile region. However, under compressive condition, significant charge transfers are observed together with MMs changes (in zones II and III). Subsequently, it leads to a dramatic change in SOI, which eventually changes the MAE sign,35 and the magnetization direction becomes out-of-plane in zone II, but turns back to in-plane again in zone III. Unfortunately, the MAE values are quite small, which shows instability of magnetic direction for BSMW. In fact, the reported Eu2(C8H8)33 molecule was also predicted to have small MAEs compared to thermo-perturbation at room temperature (about 25 meV/mol). In conclusion, we use DFT calculations to explore the manipulation of a (FeCpVCp)∞ wire’s electrical and magnetic properties by uniaxial strain, and found the molecular wire can be manipulated to be FM semiconducting, metallic, or even AFM semiconducting along the compressive strain increase, but it is immune to the tensile strain and remains a semiconductor. In addition, it is predicted that the magnetization direction of the wire is also switchable, but it is difficult to control due to
Da et al. very small energy difference between MAEs. Our study focuses on the mechanism investigation behind the above phenomena and indicates that the wire electrical and magnetic property changes arise from the electron distributions within the wire. Specially, electrons mainly transfer or redistribute between the metal atoms in (FeCpVCp)∞ but do not affect the Cp- rings, which is totally different from that of 1D Eu-COT systems, where the external charge changes only affect the COT ring but not the lanthanide elements.10 We believe these results are important toward understanding the strain effects on molecular wires/clusters, and toward application in the areas of molecular electronics and spintronics. Acknowledgment. This research is supported by MOE AcRF Tier 1 (RG28/07). References and Notes (1) Maslyuk, V. V.; Bagrets, A.; Meded, V.; Arnold, A.; Evers, F.; Brandbyge, M.; Bredow, T.; Mertig, I. Phys. ReV. Lett. 2006, 97, 097201. (2) Mokrousov, Y.; Atodiresei, N.; Bihlmayer, G.; Blugel, S. Int. J. Quantum Chem. 2006, 106, 3208. (3) Atodiresei, N.; Dederichs, P. H.; Mokrousov, Y.; Bergqvist, L.; Bihlmayer, G.; Blugel, S. Phys. ReV. Lett. 2008, 100, 117207. (4) Xiang, H.; Yang, J.; Hou, J. G.; Zhu, Q. J. Am. Chem. Soc. 2006, 128, 2310. (5) Zhou, L.; Yang, S.-W.; Ng, M.-F.; Sullivan, M. B.; Tan, V. B. C.; Shen, L. J. Am. Chem. Soc. 2008, 130, 4023. (6) Wang, L.; Cai, Z.; Wang, J.; Lu, J.; Luo, G.; Lai, L.; Zhou, J.; Qin, R.; Gao, Z.; Yu, D.; L, G.; Mei, W.; Sanvito, S. Nano. Lett 2008, 11, 3640. (7) Shen, L.; Yang, S.-W.; Ng, M.-F.; Ligatchev, V.; Zhou, L.; Feng, Y. P. J. Am. Chem. Soc. 2008, 130, 13956. (8) Sairam, S.; Mallajosyula, P. P.; Swapan, K. P. J. Mater. Chem. 2009, 19, 1761. (9) Hosoya, N.; Takegami, R.; Suzumura, J.; Yada, K.; Koyasu, K.; Miyajima, K.; Mitsul, M.; Knickelbein, M. B.; Yabushita, S.; Nakajima, J. Phys. Chem. A 2005, 109, 9. (10) Zhang, X.; Ng, M.-F.; Wang, Y.; Wang, J.; Yang, S. W. ACS Nano 2009, 3, 2515. (11) Wu, J.-C.; Wang, X,-F.; Zhou, L.; Da, X.; Lim, K. H.; Yang, S. W.; Li, Z. Y. J. Phys. Chem. C 2009, 113, 7913. (12) Folen, V. J.; Rado, G. T.; Stalder, E. W. Phys. ReV. Lett. 1961, 6, 607. (13) Fa¨th, M.; Freisem, S.; Menovsky, A. A.; Tomioka, Y.; Aarts, J.; Mydosh, J. A. Science 1999, 285, 1540. (14) Kornev, I. A.; Bellaiche, L. Phys. ReV. B 2009, 79, 100105. (15) Matsuoka, T.; Shimizu, K. Nature 2009, 458, 186. (16) Castro, E. V.; Novoselov, K. S.; Morozov, S. V.; Peres, N. M. R.; Lopes dos Santos, J. M. B.; Nilsson, J.; Guinea, F.; Geim, A. K.; Castro Neto, A. H. Phys. ReV. Lett. 2007, 99, 216802. (17) Oostinga, J. B.; Heersche, H. B.; Liu, X.; Morpurgo, A. F.; Vandersypen, L. M. K. Nat. Mater. 2008, 7, 151. (18) Zhou, S. Y.; Gweon, G. H.; Fedorov, A. V.; First, P. N.; de Heer, W. A.; Lee, D. H.; Guinea, F.; Castro Neto, A. H.; Lanzara, A. Nat. Mater. 2007, 6, 770. (19) Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Science 2006, 313, 951. (20) Zhou, S. Y.; Siegel, D. A.; Fedorov, A. V.; Lanzara, A. Phys. ReV. Lett. 2008, 101, 086402. (21) Son, Y. W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347. (22) Hod, O.; Scuseria, G. E. ACS Nano 2008, 2, 2243. (23) Choi, K. J.; Biegalski, M.; Li, Y. L.; Sharan, A.; Schubert, J.; Uecker, R.; Reiche, P.; Chen, Y. B.; Pan, X. Q.; Gopalan, V.; Chen, L.-Q.; Schlom, D. G.; Eom, C. B. Science 2004, 306, 1005. (24) Ni, Z. H.; Yu, T.; Lu, Y. H.; Wang, Y. Y.; Feng, Y. P.; Shen, Z. X. ACS Nano 2008, 2, 2301. (25) Wang, Z.; Zu, X.; Xiao, H.; Gao, F.; Weber, W. J. Appl. Phys. Lett. 2008, 92, 183116. (26) Hong, K.-H.; Kim, J.; Lee, S.-H.; Shin, J. K. Nano Lett. 2008, 8, 1335. (27) Lyons, D. M.; Ryan, K. M.; Morris, M. A.; Holmes, J. D. Nano Lett. 2002, 2, 811. (28) Nagao, S.; Kato, A.; Nakajima, A.; Kaya, K. J. Am. Chem. Soc. 2000, 122, 4221. (29) Shen, L.; Xi, H.-W.; Jin, H.; Ligatchev, V.; Yang, S.-W.; Sullivan, M. B.; Lim, K. H.; Feng, Y. Nano Lett., submitted for publication.
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