J. Phys. Chem. C 2009, 113, 7913–7916
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Manipulating Spin Transport via Vanadium-Iron Cyclopentadienyl Multidecker Sandwich Molecules Jian-Chun Wu,‡,§,| Xue-Feng Wang,⊥ Liping Zhou,‡ Hai-Xia Da,§ Kok Hwa Lim,†,§ Shuo-Wang Yang,| and Zhen-Ya Li*,‡ Department of Physics and Jiangsu Key Laboratory of Thin Films, Soochow UniVersity, Suzhou, 215006, China, School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore 637459, Singapore, Institute of High Performance Computing, 1 Fusionopolis Way, No. 16-16 Connexis, Singapore 138632, Singapore, and School of Material Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore ReceiVed: March 26, 2009
Electronic spin transport through (CpFeCpV)n multidecker wire sandwiched between magnetic nickel (Ni) electrodes is simulated in the linear response regime based on DFT. We studied the effects of the molecule-electrode contact and molecule wire length on its spin filter behavior. The amplitude and the sign of the spin filter efficiency can be manipulated by choosing the contact condition (e.g., anchoring groups, absorbing positions on Ni electrodes surface). The performance of the spin filter can be further manipulated by adjusting the length of the molecule wire. Various ways to realize nearly perfect spin-filter are illustrated. Introduction Recently, molecular spintronics has attracted much attention due to the rapid development in bottom-up technologies and its promising application in high-speed and low-energy-consuming quantum electronic devices.1-5 It has been predicted that the one-dimensional (1D) magnetic organometallic molecules like vanadium-iron-cyclopentadienyl, [VCpFeCp]n (n ) 1-3, Cp ) C5H5), can be used as a perfect spin filter 6-9 with a mechanism similar to the 1D magnetic semiconductor waveguide.10 Compared to the conventional metals or semiconductor nanostructures, one of the advantages of organic molecular devices is its longer spin-coherence distance and time due to weaker spin-orbit and hyperfine interactions. On the other hand, one of the challenges in molecular electronics is the complicated effects of contact between the molecules and the electrodes. It has been shown experimentally and theoretically that the transport characteristics between the left and right electrodes strongly depend on local orbital matching at the contacts.11-16 For example, molecular adsorption geometry may affect the device behavior substantially. Synthesized in 2000,8 [VCpFeCp]n has been predicted to be a ferromagnetic semiconductor for infinite n and 92% spin filter efficiency can be achieved for spin transport through V(CpFeCp)2 sandwiched between nonmagnetic metal (Au) electrodes.9 In this paper, we report a systematic simulation of spin transport via a finite (CpFeCpV)n multidecker wire connected to two magnetic fcc nickel (Ni) electrodes on their (001) surfaces as illustrated in Figure 1. Computational Models and Methods The spin transport simulation is carried out with the help of the Atomistix Toolkit17 after a structure optimization for the * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Soochow University. § Nanyang Technological University. | Institute of High Performance Computing. ⊥ Nanyang Technological University. † E-mail:
[email protected].
Figure 1. CpFeCpVCpFe multidecker molecule connected to the (001) surfaces of two Ni electrodes. The vertical dotted lines denote the central scattering region of the system. Green sphere: Ni atom; orange sphere: Fe atom; gray sphere: C atom; white sphere: H atom.
system is performed by the Vienna ab initio simulation package (VASP)18 to estimate the structure parameters. The system is described by density functional theory (DFT) with the standard nonlocal norm-conserving pseudopotential and the quantum spin transmission is evaluated by the nonequilibrium Green’s function (NEGF) technique.17,19,20 The wave functions are expanded on a numerical basis set of double-ζ plus polarization (DZP) for Fe and V and single-ζ plus polarization (SZP) for other atoms. The local spin density approximation (LSDA) with the Perdew-Zunger parametrization of the correlation energy is used for the exchange-correlation functional. Convergent results are achieved by using the Monck Horst-Pack grid with 100 k-points in the 1D Brillouin zone. The spin filter efficiency in the linear response regime is then calculated from the transmission by its definition
SFE )
Tv(EF) - TV(EF) × 100% Tv(EF) + TV(EF)
where Tv(EF) and TV(EF) are the transmission coefficient of spinup and spin-down channel, respectively. Results and Discussions On the basis of the experimental observation, (CpFeCpV)n can be terminated with a Cp ring or a metal atom, V.8 For the current study, we have also terminated the molecule either with metal atoms (V or Fe) or the Cp ring. This allows us to choose
10.1021/jp902718h CCC: $40.75 2009 American Chemical Society Published on Web 04/13/2009
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TABLE 1: The Spin Filter Efficiency and the Total Conductance for Different Contact Types and Molecular Lengths contact type
molecule
SFE (%)
G (µs)
I (Cp · · · Cp)
CpFeCpVCp CpFeCpVCpFeCpVCp CpFeCpV CpFeCpVCpFeCpV CpFeCpVCpFe CpFeCpVCpFeCpVCpFe FeCpVCpFeCpVCpFe VCpFeCpVCpFeCpV FeCpVCpFeCpV
20.1 10.9 33.3 27.9 -72.5 -62.5 -94.7 77.9 -41.5
36.5 6.17 16.64 2.55 13.57 3.95 23.21 0.52 3.18
II (Cp · · · V) III (Cp · · · Fe) IV (Fe · · · Fe) V (V · · · V) VI (Fe · · · V)
the preferred anchoring groups and absorbing positions on the electrode surface when connecting the molecule to the Ni electrodes. The contact effect then offers us a freedom for manipulating the performance of the molecular spin filter. Due to favorable energetics,21 the Cp rings are usually placed on the hollow sites of the electrode surfaces while the metallic atoms prefer to anchor on the top site. In Table 1, we show the spin filter efficiency (SFE) and the total linear conductance (G ) (e2/h)T) for molecular wires with different combinations of anchoring groups at their two ends. These configurations are sorted into two major categories: (a) classes I, II, and III for those having a Cp anchoring end and (b) classes IV, V, and VI for those having only a metal anchoring end. As shown in Table 1, for molecules of similar length, the general characteristics of the spin transport are the following: (i) higher SFE in cases with metal anchoring groups; (ii) symmetric double metal anchoring further increasing the SFE; (iii) the system with Fe atoms anchoring at the two ends (class IV) having the highest SFE (-94.7%) as well as big conductance (23.2 µS); and (iv) Fe anchoring usually resulting in negative SFE. From the above observations, we conclude that, for electrons near the Fermi energy, the Cp-Ni contact is strong for both the spin-down and spin-up transport, the V-Ni contact is weak and prefers the spin-up transport, and the Fe-Ni contact is strong but prefers the spin-down transport. To further understand the characteristics of spin transport in the system, we show in Figures 2 and 3b the transmission spectra for the contact configurations listed in Table 1 with Fermi energy as the reference energy (set to zero). Compared to systems with only metal anchoring, the transmission spectra of systems having Cp anchoring show wider peaks with the same order of spinup and spin-down transmissions at the Fermi energy. This indicates that the Cp-Ni coupling is in general strong for both spins and explains why the linear SFE is low when the Cp anchoring is used. However, at some energies away from but near the Fermi energy, the spin-down transmission can almost reach zero. This suggests that high SFE with large conductance may be realized in Cp anchoring systems by shifting the Fermi level to these energies with the help of gate voltage on the molecule. In the system of double Fe-Ni contacts, both spin transmission spectra have only narrow peaks as illustrated in Figure 2f. Fortunately, one spin-down peak is located at the Fermi energy, which leads to the high negative SFE with big linear conductance. In the system with double V-Ni contacts, on the contrary, the transmission spectra of both spins have wider peaks or higher average transmission compared to those for Fe-Ni contact but the Fermi level is located in a wide gap of the transmission spectra as shown in Figure 2g. As a result the linear conductance becomes very small in systems having V anchoring. For each class of contact type, we can vary the molecule length in the simulation and study the length effect on spin
transport. Some typical results are shown in Table 1 for classes I, II, and III. We find both the SFE and the conductance decrease with the molecule length. As we know, the transmission in the 1D periodic system is enhanced for electrons in propagating states but suppressed for electrons in localized states as the system length increases.10 The above observed length dependence of spin transport then suggests that the Fermi level in the molecule wires is located in a energy gap between extended states. This coincides with the finding that long (CpFeCpV)n is intrinsically a semiconductor.9 As a result, the electrons transport in the molecule wires by tunneling among localized states and SFE and conductance decreases with the increased disorder in longer molecule wires. This picture is also implied by the transmission spectra in Figure 2. In short molecule cases, electrons can travel across the molecule through either localized or extended states, the transmission is then energy insensitive and the transmission spectra have broad peaks as shown in Figure 2a,c. When the molecule length increases, electrons are harder to diffuse via localized states but easier via extended states. Consequently, the transmission increases at energies with extended states and decreases otherwise10 and the transmission peaks become sharp as shown in Figure 2a,b. From the above analysis, we can see that the spin polarized electrons traveling across the Ni-molecule-Ni device are controlled by the molecule-Ni contact as well as the length of the molecule. The contact can control the current amplitude and the spin polarization of the injected electrons22 while the length of the wire manipulates the transmission channel according to the localization degree of the states at the Fermi level. In the double Fe-Ni contact case as shown in Figure 2f, the spindown state at the Fermi level is extended in the molecule wire and is also a favorite of the Fe-Ni contact. As a result, both high SFE and large conductance are observed in this rather long system. Besides the preferred top absorbing positions discussed above for metal anchoring groups on the Ni surface, there are a few other energetically stable positions and one of them is the hollow position. We expect the absorbing position will also influence spin filter efficiency significantly. As an example, we simulate the spin transport through a Ni-CpFeCpVCpFe-Ni structure when the anchoring Fe atom is absorbed on the hollow or the top position. On the top position, we obtain a SFE value of -72.5%, as shown in Table 1, but on the hollow position the SFE decreases to -9.1%. The change comes mainly from the spin-up transmission and the spin-down one remains almost intact. In the transmission spectra corresponding to the hollow position as plotted in Figure 3a, mainly peak p1 and p2 contribute to the spin-up and spin-down transmission at the Fermi level, respectively. However, peak p1 almost disappears when Fe is on the top position while peak 2 changes only slightly as shown in Figure 3b. How the molecule-electrode coupling for spin-up electrons becomes weak is illustrated in Figure 4 by showing the variation of the isosurfaces of the spinup HOMO-1 and HOMO orbitals of the molecular projected self-consistent Hamiltonian (MPSH). The energies of the two orbitals are indicated by the two vertical solid lines below the Fermi level (dotted line) in the upper parts of panels a nd b of Figure 3. The spin-up HOMO orbital is always localized in both cases as shown in Figure 4 and does not contribute to the transport. In the hollow position case, the spin-up HOMO-1 orbital extended into both electrodes and induces transmission peak p1, which spreads to the Fermi level.23 In the top position case, however, this orbital becomes localized and disconnected from the left electrode. The decoupling between the molecule
Electronic Spin Transport through (CpFeCpV)n Multidecker Wire
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Figure 2. Spin-up (red) and spin-down (blue) transmission spectra for systems listed in Table 1 (excluding CpFeCpVCpFe). The energy is measured from the Fermi level.
Figure 3. Transmission spectra of the CpFeCpVCpFe molecule sandwiched between Ni electrodes on hollow and top absorbing positions. The solid vertical lines show the spin resolved molecular projected self-consistent Hamiltonian (MPSH) eigenstates. The green vertical dotted lines show the Fermi levels.
and the left electrode breaks the transport channel for spin-up electrons and results in the disappearance of peak p1 in Figure 3b. In contrast, the spin-down HOMO and LOMO MPSH orbitals, which contribute to the spin transmission at the Fermi level, remain extended into both electrodes when the anchoring
Fe atom moves from the hollow to the top position as shown in the third and fourth panel columns of Figure 4. Then the linear spin-down conductance remains almost unchanged. Therefore, we can come to the conclusion that contact effects can effectively manipulate the spin filter effects. Other factors,
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Figure 4. Isosurfaces of MPSH obitals for configurations of CpFeCpVCpFe hollow and top absorbing on the Ni (001) surface. HOMO-1 (up), HOMO (up), HOMO (down), and LOMO (down) represent the spin-up highest-1 occupied molecular orbital, the spin-up highest occupied molecular orbital, the spin-down highest occupied molecular orbital, and the spin-down lowest unoccupied molecular orbital, respectively.
such as higher order effects and thermal and vibronic effects, are not taken into account in the study and they may slightly modify the efficiency of the spin filter in some situations.24 For strong electric coupling between the molecule and the electrode contact, a higher order tunneling process becomes important, leading to the Kondo effect.24,25 These phenomena deserve our future research. Summary In summary, we have investigated the contact effects of the anchoring group and its absorbing position on the spin transport through the derived molecule wires of (CpFeCpV)n sandwiched between magnetic Ni electrodes in the low bias linear regime. Our result shows that the contact effect can manipulate the performance of spin filters made from this molecule, including the amplitude and the sign of the spin filter efficiency and the total conductance. For the same type of contact, increasing molecule length results in decreasing spin filter efficiency and conductance in most of the studied cases where the electronic states at the Fermi energy is usually localized. One of the exceptions is the molecules with Fe atoms anchoring at both ends, Fe(CpVCpFe)n, where the spin-down states at Fermi energy can extend to long-range along the wire and can penetrate the Fe-Ni contact into the electrodes. This leads to an almost perfect spin filter effect with high conductance in long molecule devices. With the help of the spin-resolved transmission spectra and the MSPH eigenstate orbital scheme, we show that the localization and matching of electronic states plays a critical role in the contact effect as well as the molecule length dependence. Other than the contact effect, temperature, vibration, and higher order tunneling effects can also affect the spin filter efficiency in real systems. Acknowledgment. The authors would like to express their thanks to Mr. L. Shen and Dr. H.-W. Xi for simulating scientific discussions. This project was supported by the National Natural Science Foundation of China (grant Nos. 10774107 and 10804080) and the Doctoral Program of High Education (No. 20060285003). Financial support from NTU grant M58120016 is acknowledged.
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