J. Phys. Chem. C 2010, 114, 21705–21707
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Half-Metallic Spintronic Switch of Bimetallic Sandwich Molecular Wire via the Control of External Electrical Field Haixia Da,†,‡ Hong Mei Jin,‡ Kok Hwa Lim,*,† and Shuo-Wang Yang*,‡ School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore, 637459, Singapore, and Institute of High Performance Computing, 1 Fusionopolis, Way, #16-16 Connexis Singapore 138632, Singapore ReceiVed: June 16, 2010; ReVised Manuscript ReceiVed: October 21, 2010
On the basis of systematic density functional theory calculations, we investigate the electrical and spintronic multifunctional switch through the control of transverse external electrical field (EEF) on a one-dimensional bimetallic sandwich molecular wire, (CpFeCpV)∞. The wire shows robust ferromagnetism under applied EEFs of up to 1.0 V/ Å. By switching the EEF on and off at 1.0 eV/ Å, the molecular wire can be converted between semiconducting and half-metallic states, and it is also possible to change its magnetization direction simultaneously. This finding opens up the potential applications of one-dimensional bimetallic sandwich molecular wire as spintronic materials. Introduction With the rapid miniaturization in the semiconductor industry, molecular-sized components will be used in hybrid electronics or spin electronics (spintronics) devices in the foreseeable future.1,2 The unique half-metallic (HM) property predicted by Groot et al. in 19833 and proven experimentally five years later4 meets all the requirements for application in spintronics.5 During the past two decades, much effort has been invested to search for new HM materials as potential new spintronic materials. For examples, some specific/modified graphene and its analogue boron nitride (BN) sheets are reported to be HM.6 Recently, one-dimensional (1D) organometallic sandwich molecular wires (SMW) have also attracted extensive attention because of their reported unique properties of HM, high spin filter, and negative differential resistance (NDR) effects.7-12 Many reports on various 1D SMWs or related linear clusters have also been made, in particular, the (M-benzene)∞, [M-(C5H5)]∞, [M-anthracene]∞, double-metal metallocene nanowires,13 and even lanthanide rareearth wire [Eu(C8H8)]∞ and its cluster.14 From the application point of view, materials with switchable electric and magnetic properties are desirable as electronic and spintronic device materials. Sometimes, these properties can be achieved via structural modifications such as chemical doping, creation of defects, and anchor substitutions.15-18 Unfortunately, these approaches are not always practical for SMWs. Recently, we have demonstrated that 1D bimetallic SMW (BSMW, FeCpVCp)∞, [Cp ) C5H5], can be switchable between semiconducting and HM states by well designed oxidation-reduction reactions.19 However, such reactions are not easy to implement in material design; therefore, a physical method becomes a viable option. In fact, external electric fields (EEFs) have been shown to influence the electrical and magnetic properties of some nanostructures, where the charge distribution and electron spin are tunable under the amplitude and directions of the applied EEF.6,20-25 Son et al. have reported that the zigzag graphene nanoribbons (GNRs) are convertible between semi* Corresponding authors. E-mail:
[email protected] and yangsw@ ihpc.a-star.edu.sg. † Nanyang Technological University. ‡ Institute of High Performance Computing.
conducting and HM states by changing the EEF.20 In addition, it was reported that magnetic properties of BN nanoribbons are sensitive to the transverse EEFs, and can be converted between semiconducting, metallic, and HM states by different applied EEFs.6 Electrical properties of bilayer graphene have also been reported to be tunable under EEFs.21-23 The EEF-based switchable properties among aforementioned low-dimensional systems offer great potential applications in the areas of nano-electronics and spintronics materials. We have previously worked on several SMWs materials;26,27 however, to the best of our knowledge, there has been no report on switchable properties of SMWs under different applied EEFs yet. In 2D GNRs or BN sheets, the frontier orbitals are primarily pz, which are perpendicular to the sheets and are very sensitive to EEF. In contrast, the frontier orbitals in SMWs are mainly the d orbitals from metal atoms, which make up the major contribution to their electrical and magnetic properties; therefore, the nature of their interactions with EEF are more complex compared to those of the 2D systems. In the current work, we have carried out systemic DFT calculations to investigate the electrical and magnetic properties of BSMWs under applied transverse EEF. It is found that (FeCpVCp)∞ can be switched from semiconducting to HM states and vice versa by switching on and off the EEF. In addition, for the first time, we have also found that the magnetization orientation of BSMW can be converted from the z direction (BSMW extension
Figure 1. (Color online) 1D BSMWs (FeCpVCp)∞ in transverse EEF.
10.1021/jp105532n 2010 American Chemical Society Published on Web 11/16/2010
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J. Phys. Chem. C, Vol. 114, No. 49, 2010
Da et al.
TABLE 1: Calculated Optimized Distances between Fe/V and Cp (dFe/V-Cp), Lattice Constant (LC), Binding Energies (Eb), Spin-Polarization Energies (∆E ) EFM - EAFM), Total Magnetic Moments (MM), Electronic Ground States (EGS), and MAE ) [E(0,0,1) - E(1,0,0)] of (FeCpVCp)∞ Under Applied EEF ) 0.0, 0.5, and 1.0 V/ Å electric field (V/Å)
dFe/V-Cp
LC, Å
Eb, eV
EFM, eV
EAFM, eV
∆E (meV)
MM (µB)
EGS
MAE (meV)
0.0 0.5 1.0
1.79/1.90 1.76/1.91 1.73/1.94
7.38 7.35 7.34
-12.71 -12.85 -14.24
-281.44 -281.77 -283.98
-280.89 -281.25 -283.53
-550 -520 -450
5.0 5.0 4.0
sc FM sc FM HM
1.28 -6.05 -32.0
direction) to the xy plane (perpendicular to the wire) during the process (see Figure 1). Such multifunctional molecular switch that can be easily controlled by transverse EEF would open up new potential applications in molecular electronics and spintronics.
indicates a stable structure. The optimized lattice constants, energies, and electrical and magnetic properties under various EEF conditions are summarized in Table 1. Results and discussion
Method The (FeCpVCp) unit was modeled in a 15 × 16 × c Å3 periodic cell shown in Figure 1, where c is the lattice constant along the z direction, and the vacuum distances along the x and y directions are about 20 Å. A homogeneous EEF is applied perpendicular to the molecular wire’s axis with two specific magnitudes of 0.5 or 1.0 V/ Å, respectively. Geometry optimizations are performed by using the Vienna ab initio simulation package (VASP) without any constraints for all the calculations.28 Upon consideration of the calculation results of various methods, the generalized gradient approximation (GGA-PBE)29 scheme for electron exchange and correlation is selected, and the frozen-core projector-augmentedwave (PAW) method is employed to describe the interaction between ions and electrons.30 A gamma-centered 1 × 1 × 21 k-point grid is used for integration over the Brillouin zone, and all atomic coordinates are optimized until the force of each atoms is less than 10-2 eV/ Å. The applied EEF is simulated via an artificial dipole sheet in the middle of the vacuum among the periodic supercell.31 The binding energies are defined by the following equation,
EB ) Eunit - (EFe + EV + 2ECp) Here, Eunit is the optimized energy for one unit of (CpFeCpV)∞ with or without EEF; ECp, EFe, and EV represent the energies of the isolated Cp radical, single Fe atom, and V atom at EEFfree states, respectively. With this definition, a negative value
The optimized lattice constant (c) of (CpFeCpV) is 7.38 Å under the EEF-free state, and it is contracted by 0.41% (0.03 Å) and 0.54% (0.04 Å) under applied EEF ) 0.5 and 1.0 V/ Å, respectively. Among the studied states, the wire remains a stable ferromagnetic (FM) semiconductor (see Figure 2a).19,26,27 Detailed structural analysis shows that, with increased EEF, the bond distances between Fe and Cp rings decreased, and those between V and Cp rings increased (see Table 1). Compared to the EEF-free state, the binding energies increase by 1.1 and 12.0% at EEF ) 0.5 and 1.0 V/ Å, respectively. This is an indication that EEF has enhanced the electron coupling within the wire and hence increased the bonding strength of the BMSW. According to our previous study,26 such small LC contraction (
