Reversible Spin Polarization at Hybrid OrganicâFerromagnetic

Letter pubs.acs.org/JPCL

Reversible Spin Polarization at Hybrid Organic−Ferromagnetic Interfaces Yan Wang,† J. G. Che,‡ J. N. Fry,§ and Hai-Ping Cheng*,† †

Department of Physics and Quantum Theory Project, University of Florida, Gainesville, Florida 32611, United States Surface Physics Laboratory (National Key Laboratory) and Department of Physics, Fudan University, Shanghai 200433, China § Department of Physics, University of Florida, Gainesville, Florida 32611, United States ‡

S Supporting Information *

ABSTRACT: We report a first-principles study of magnetic properties of organic− ferromagnetic interfaces between photoswitchable azobenzene molecules and Fe/ W(110) surfaces. Our calculations demonstrate that the magnetic properties of the hybrid interface, such as the local magnetic moment and spin polarization, change significantly as the azobenzene molecule switches reversibly between the trans to the cis form. The molecule−surface interaction, which determines the feasibility of isomerization of the azobenzene on the surface, can be altered by chemical modification of the molecule. This study suggests a new pathway to manipulate magnetism and spin polarization at organic−ferromagnetic hybrid interfaces.

SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

C

In this Letter we introduce the concept of reversible surface magnetic modulation by examining hybrid organic-ferromagnetic interfaces. Such an interface has magnetic states switchable via azobenzene isomerization. We discover that, on a ferromagnetic surface, the azobenzene molecule switching has a large impact on the magnetic properties, leading to a significant change of local magnetic moment and spin polarization. The effect is attributed to the broken symmetry between two phenyl rings in cis azobenzene adsorbed on the ferromagnetic surface. Interfacial hybridization of the π-type molecular orbitals with the Fe d states leads to symmetric spin polarizations in the trans form but asymmetric ones with intramolecular inversion in the cis form. Furthermore, chemical functionalization of azobenzene provides a possible path for tuning the interaction between the molecule and the ferromagnetic surface while maintaining large spin polarization of the hybrid states. We expect that these findings will have significant impact on understanding fundamental physical properties of organic molecule−ferromagnetic surface interactions and on molecular-level engineering of spin states, and will bring new functionalities to spintronics applications. Calculations are performed using density functional theory (DFT) implemented in the plane-wave-basis-set Vienna ab initio simulation package (VASP).20 Projector augmented wave potentials with a kinetic energy cutoff of 500 eV are employed.

urrent interest in organic−magnetic hybrid systems is largely driven by the vision of molecular spintronics and promises of new functionalities emerging from organic molecule−magnetic material heterostructures. In the past few years, understanding the properties of organic−magnetic interfaces has come to the spotlight. One of the primary objectives and also a major challenge is to manipulate the spin functionality at such interfaces.1−5 As one example, spinpolarized hybrid states at the interface6−8 having spin polarization opposite to the underlying ferromagnetic substrate were discovered. These states determine the spin injection into the organic molecules and can be used as a spin filter.4 Azobenzene and its derivatives, the most frequently studied candidates among photochromic molecules for reversible photoswitches, have attracted extensive attention in molecular electronics9−13 and energy applications.14 Azobenzene molecules have a unique property that allows conformational change from a thermodynamically stable trans configuration to an isomer cis configuration in response to an external stimulus such as UV light,15 and vice versa upon exposure to visible light or by thermal excitation.16 Isomerization of these molecules has been widely studied in both gas and solution phases, and significant effort has been made to switch the molecule on an underlying substrate. Studies have been carried out on nonmagnetic metals, Au(111)17,18 in particular, or on semiconductor19 substrates, but so far azobenzene on ferromagnetic metal surfaces is unexplored by either experimental or theoretical studies. © 2013 American Chemical Society

Received: August 22, 2013 Accepted: September 26, 2013 Published: September 26, 2013 3508

dx.doi.org/10.1021/jz401800m | J. Phys. Chem. Lett. 2013, 4, 3508−3512

The Journal of Physical Chemistry Letters

Letter

For the exchange and correlation functional, we utilize the Perdew−Burke−Ernzerhof generalized gradient approximation (PBE-GGA).21 As ferromagnetic substrate we chose Fe/ W(110), a surface intensively investigated both experimentally and theoretically,22,23 which exhibits stable and reproducible magnetic properties in experiments.24 The Fe/W(110) substrate is modeled by a slab consisting of three W(110) layers and a Fe(110) layer with a 5 × 7 surface unit cell. The molecule−surface supercell thus contains 164 atoms, consisting of 35 atoms per W (or Fe) layer and 24 atoms for the azobenzene molecule. A vacuum spacing of 15 Å is used to ensure decoupling between adjacent slabs. The bottom three W(110) layers are fixed at their bulk-like positions with a lattice constant of 3.16 Å, and the top Fe(110) layer as well as the molecule adsorbate are allowed to relax during geometry optimization and transition state search. In all calculations, a dipole correction is applied. Geometries are optimized until the remaining force on each atom falls below the convergence criterion of 0.02 eV/Å. To find the most stable adsorption geometry for the azobenzene on the Fe/W(110) surface, we have examined all possible initial positions based on symmetry considerations (see Supporting Information). The lowest energy structures of trans and cis azobenzene on Fe/W(110) are shown in Figure 1.

calculation has a negligible effect on the GGA optimized structure (∼0.02 Å change of the C−Fe and N−Fe bond length). We also find the trans azobenzene on Fe/W(110) to be more stable, by about 0.81 eV, than the cis configuration, which is slightly larger than the value for a gas phase molecule.26 It should be noted that, compared to the nonplanar configuration of the isolated gas phase molecule, after adsorption the cis azobenzene orients parallel to the surface with all atoms in the molecule bonded with Fe atoms, similar to the trans configuration. This is quite different compared to the results from previous studies of azobenzene on nonmagnetic coinage-metal surfaces such as Cu, Ag, and Au(111),26−28 in which only the N pair form covalent bonding with the surface while the two phenyl rings experience a substantial Pauli repulsion, resulting in a nonflat, butterfly-like configuration for both cis and trans azobenzene. Bader analysis based on the realspace charge density29 shows a large charge transfer from Fe/ W(110) substrate to the molecule, 2.36 and 2.50 |e| for the trans and cis states, respectively. In both cases, azobenzene adsorption leads to a reduction of the magnetic moment on the interface Fe atoms below the molecule, from 2.40 μB to a value between 1.4 to 2.2 μB, depending on the location of the underlying Fe atom. Not surprisingly, upon adsorption, the molecule becomes magnetic and antiferromagnetically ordered with respect to the Fe substrate, similar to previous studies of benzene molecules on the Fe/W(110) surface.6 The size of the magnetic moment is 0.22 and 0.16 μB for trans and cis molecules, respectively, mostly located at the C atoms of the molecule (see Supporting Information). The effect of hybridization at the interface between the azobenzene and Fe/W(110) surface can be analyzed in terms of the spin-resolved density of states projected (PDOS) onto the C and N atoms in the molecule as well as the Fe atoms, as shown in Figure 2. Compared to the bare Fe surface, the spindown DOS of an Fe atom below the molecule after azobenzene adsorption is greatly decreased, especially for the d-states, leading to a reduced spin polarization, consistent with a decreased magnetic moment found for the Fe atoms. At the molecule side, we can see that both σ (s + px + py) and π (pz) type molecular orbitals are strongly mixed with the d states of Fe of the same symmetry, forming broadened spin-dependent states. A general feature for both trans and cis molecules is that, at and just above the Fermi level EF the pz states with large weight are in the spin-up channel; in contrast the Fe d-states have a larger weight in the spin-down channel. This holds true also for the pz states of trans azobenzene just below EF, and the spin-down channel becomes dominant only for states with energies lower than −0.5 eV with respect to EF. However, for cis azobenzene the situation is much more complicated. Interestingly, the asymmetric adsorption geometry of cis azobenzene on an Fe surface introduces an asymmetry to the molecule, which strongly influences its density of states just below the EF, and results in two distinct types of spin polarization (SP) for the two phenyl rings. The solid line and dashed lines in Figure 2 show the PDOS for C atoms in the two phenyl rings of the cis azobenzene (noted as CA and CB), respectively. Looking at the pz states just below EF for CA, it is dominated by contributions from the spin-down channel, in contrast to the states of CB where the contribution from spinup channel is more pronounced. Consequently, the SP reverses its sign at one-half of the molecule with respect to the other within this energy range.

Figure 1. (a) DFT optimized structure of gas phase azobenzene molecule. (b) Side and (c) top views of an azobenzene molecule adsorbed on the Fe/W(110) substrate with the preferred geometry. Top and bottom panels represent the trans and cis configurations, respectively. C, N, H, Fe, and W atoms are colored red, blue, white, light gray, and dark gray, respectively. Bottom W atoms are not shown in panel c.

In the trans form, the molecule adsorbs with its N pair on top of a Fe atom and aligned with the surface [11̅0] direction, and the adsorption sites of the two phenyl rings are symmetric. However, for cis azobenzene we find the most stable adsorption position to be the hollow site in which the N pair bonds with three Fe atoms aligned with the surface [11̅3] direction. In this case, the two phenyl rings are differentone is more centered in a hollow site on the Fe surface, and the other one is offcenterand this breaks the symmetry of the molecule. The asymmetric adsorption in the cis case is likely to be a geometric mismatch that one phenyl ring is adsorbed on a favored position with its center on top of a hollow site,6 but for the other ring such preferred adsorption is unattainable. Both trans and cis molecules adsorb about 1.9 Å above the Fe surface. We have verified that inclusion of van der Waals (vdW) interactions using a vdW density functional optB88-vdW25 3509



Letter

Figure 3. The calculated spin polarization at 3.0 Å above the (a) trans and (b) cis azobenzene adsorbed on 1 ML Fe/W(110) surface with an area of 19 × 19 Å for unoccupied [EF,EF + 0.5 eV] (left panels) and occupied [EF − 0.5 eV,EF] (right panels) energy intervals around the Fermi level. CA and CB are marked as the two C atoms in the two phenyl rings in the cis azobenzene.

the broken symmetry between the two phenyl rings of the cis molecule adsorbed on the Fe surface. The asymmetric SP pattern shows clearly positive values for one phenyl ring (CA) and negative values at another (CB), correlating well with the calculated PDOS for the C atoms in the two parts of the cis molecule in Figure 2. The sign reversal of the SP at the Fe surface in the two energy ranges is attributed to the competition between the contributions of Fe d and sp states to the SP above the surface. Although the Fe d states are the dominant states and primarily spin-down around EF, the s and pz states are primarily spin-up in [EF − 0.5 eV,EF], as shown in Figure 2. At a distance above the Fe surface, the s and pz states make more of a contribution to the spin polarization, and this concurs with the interpretation that a much lower decay rate for these states in the vacuum should be expected as compared to the Fe d state (see Supporting Information for more details about the height dependence of the calculated SP on the surface). Such an effect is reminiscent of the same behavior investigated by electron transport of a tunneling electron from an Fe electrode into a vacuum barrier in which tunneling of sp states is more efficient.31 We also performed calculations of minimum energy pathways for azobenzene isomerization in the electronic ground state on the Fe/W(110) surface using the climbing-image nudged elastic band method.32 The switching of azobenzene from trans to cis configuration on the surface is actually realized by a translation of trans azobenzene from an adsorption site atop an Fe atom to a hollow site, then followed by a trans-to-cis isomerization. Here we show the result only for the trans-to-cis transformation, as the translation energy barrier is found to be only 0.21 eV from the top site to the hollow site, and 0.10 eV vice versa. Nine intermediate images are used to interpolate the inversion isomerization pathway between the trans and cis states, and for the rotation pathway 19 intermediate images are used. The inversion mechanism involves a succession of inplane intermediate geometries in which both phenyl rings are oriented parallel to the Fe surface. The rotation pathway occurs by an out-of-plane torsion of the azobenzene, and in its transition state one of the two phenyl rings remains on the

Figure 2. Spin-resolved PDOS of an Fe atom of the bare Fe/W(110) surface (upper left), an Fe atom below the azobenzene molecule (upper right) after adsorption, and trans-azobenzene (lower left) and cis-azobenzene molecules (lower right) adsorbed on the Fe/W(110) surface. The solid and dotted lines in the PDOS of C atoms for cis azobenzene represent the PDOS of the C atoms in its two phenyl rings (noted as CA and CB), respectively.

Within a given energy interval [E,EF], the molecule−surface mixed states have an unbalanced electronic charge in the spinup and spin-down channels. Iso-charge surfaces above the azobenzene on the Fe/W(110) substrate can be extracted from the energy integrated local density of states (LDOS), and such surface images mimic the spin-polarized scanning tunneling microscope (SP-STM) experimental situation of a local and spin-sensitive tip probing the charge density above the surface at a constant height.30 The relation between the two spin channels can be quantitatively described by the value of the SP defined as P = (n↑ − n↓)/(n↑ + n↓), where n↑ and n↓ stand for the spin-up and spin-down LDOSs. In Figure 3 we show the calculated SP at 3.0 Å above the azobenzene adsorbed on the Fe/W(110) surface with an area of 19 × 19 Å2 for unoccupied [EF, EF + 0.5 eV] and occupied [EF − 0.5 eV, EF] energy intervals (the original spin and charge LDOSs can be found in the Supporting Information). These intervals corresponding to a tip bias of 0.5 V and −0.5 V. A height of 3.0 Å relative to the molecule is used to calculate the SP in order to account for the tip position in vacuum. For both trans and cis molecules within the energy interval above EF, the spin polarization at the Fe surface is negative and we find an inversion of the SP occurs at the molecular site with respect to the Fe surface, which is in line with recent studies of hybrid organic−ferromagnetic interfaces.6,23 For an energy interval below the Fermi level, however, the SP at the Fe surface becomes positive, and the image shows not a simple red or blue contrast but a mixed pattern for the SP at the molecule site. For the trans azobenzene, at the molecule site the SP is mostly positive, and it is symmetric between the two phenyl rings. In the cis case, we find the SP pattern reflects 3510



Letter

Figure 4. (a) Azobenzene trans−cis isomerization on the Fe/W(110) surface. Energy profile of the (b) inversion and (c) rotation pathway of azobenzene isomerization in the electronic ground state. The switching of azobenzene from trans to cis configuration requires translation between different adsorption sites in addition to the isomerization, which is not shown in the figures (see text for details).

the trans and 0.27 eV for the cis . Moreover, we find that, although the isomerization energy barrier for the fluorinated azobenzene on Fe/W(110) remains unchanged in the inversion pathway, it is greatly reduced in the rotation pathway, as shown in Figure 4 (1.88 eV for the trans-to-cis pathway and 0.98 eV for cis-to-trans). As a consequence, by adequate chemical functionalization of the azobenzene, one could tailor the molecule−metal coupling, thus making the switching more feasible. We note that the calculated energy barriers in the electronic ground state may be applicable only to the heatinduced azobenzene isomerization. Further studies are needed to identify the isomerization barrier in the excited states for ultraviolet-photon15 or transmitting-electron17 induced isomerization. In summary, we propose here a new direction of combining molecular switching and ferromagnetism to achieve tunable spin polarization at hybrid organic-ferromagnetic interfaces. Using first-principles calculations we have shown that the local spin polarization can be significantly changed by azobenzene molecular switching on the Fe/W(110) surface. From azobenzene trans-to-cis isomerization we clearly find an intramolecular inversion of the molecular spin polarization. A detailed analysis of the PDOS reveals that such inversion is due to a broken symmetry between two phenyl rings in the cis molecule adsorbed on the Fe/W(110) surface. These findings open up new concept for future design of molecular spintronics, toward the possible engineering of spin polarization by organic−ferromagnetic hybridization effects, especially in view of recent experiments on successful observation of local spin polarization on ferromagnetic surfaces using advanced SPSTM techniques.

surface while the second one is pointing upward. The two processes are illustrated in Figure 4a. The calculated energy profile for the inversion and rotation pathways are presented in Figure 4b and (c), respectively. For the inversion pathway the activation energy barrier is found to be 1.16 eV for the trans-tocis transform and 0.22 eV for cis-to-trans transform. For the rotation pathway the barrier is much higher, 2.18 eV for the trans-to-cis transformation and 1.24 eV for cis-to-trans transformation. It is obvious that the Fe surface stabilizes the transition state geometry and lowers its energy barrier for the inversion pathway but not for the rotational isomerization. The difference arises because the rotational transformation requires one of the two phenyl rings to stand upright (perpendicular to the surface), resulting in a weak interaction between the molecule and the Fe surface, leading to a loss of adsorption energy and thus a gain in the total energy, whereas in the inversion pathway the closer proximity between the molecule and the surface maintains its adsorption energy. This is quite different from the case that azobenzene on a gold surface,33 in which the rotation barrier is only slightly lower than the inversion barrier, with both remaining close to the calculated barrier for gas-phase isomerization.34 The isomerization energy barrier relates to the feasibility of switching of azobenzene on the surface. The strong coupling between the ferromagnetic surface and the molecule may hamper the switching. One possibility to solve this dilemma is introducing a molecule somewhat weakly bound to the surface. This can be achieved by chemical functionalization of the azobenzene molecule. It is shown that the interaction between a benzene molecule and an Fe surface can be specifically tuned by substituting for the H atoms more electronegative atoms such as Cl and F, while retaining large spin polarizations.23 Here our calculated surface SP images confirm that the findings above of the magnetic switch effect are still valid for the surface adsorption of fluorinated azobenzene molecules, in which two H atoms are replaced with F atoms (see Supporting Information). The adsorption energies of the fluorinated azobenzene on the Fe/W(110) are decreased by 0.21 eV for

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ASSOCIATED CONTENT

S Supporting Information *

Details of calculations including adsorption site and adsorption energy, magnetic properties, local density of states, and spin polarization, and fluorinated azobenzene on an Fe/W(110) 3511



Letter

Excitation: A Femtosecond Fluorescence Anisotropy Study. J. Am. Chem. Soc. 2004, 126, 10109−10118. (16) Ikegami, T.; Kurita, N.; Sekino, H.; Ishikawa, Y. Mechanism of Cis-to-trans Isomerization of Azobenzene: Direct MD Study. J. Phys. Chem. A 2003, 107, 4555−4562. (17) Choi, B.-Y.; Kahng, S.-J.; Kim, S.; Kim, H.; Kim, H. W.; Song, Y. J.; Ihm, J.; Kuk, Y. Conformational Molecular Switch of the Azobenzene Molecule: A Scanning Tunneling Microscopy Study. Phys. Rev. Lett. 2006, 96, 156106. (18) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Fréchet, J. M. J.; Trauner, D.; Louie, S. G.; et al. Reversible Photomechanical Switching of Individual Engineered Molecules at a Metallic Surface. Phys. Rev. Lett. 2007, 99, 038301. (19) Pechenezhskiy, I. V.; Cho, J.; Nguyen, G. D.; Berbil-Bautista, L.; Giles, B. L.; Poulsen, D. A.; Fréchet, J. M.; Crommie, M. F. Selfassembly and Photomechanical Switching of an Azobenzene Derivative on Gaas (110): Scanning Tunneling Microscopy Study. J. Phys. Chem. C 2011, 116, 1052−1055. (20) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (22) Pratzer, M.; Elmers, H. J.; Bode, M.; Pietzsch, O.; Kubetzka, A.; Wiesendanger, R. Atomic-Scale Magnetic Domain Walls in QuasiOne-Dimensional Fe Nanostripes. Phys. Rev. Lett. 2001, 87, 127201. (23) Atodiresei, N.; Caciuc, V.; Lazić, P.; Blügel, S. Engineering the Magnetic Properties of Hybrid Organic−Ferromagnetic Interfaces by Molecular Chemical Functionalization. Phys. Rev. B 2011, 84, 172402. (24) Miesch, S.; Fognini, A.; Acremann, Y.; Vaterlaus, A.; Michlmayr, T. Fe on W(110), a Stable Magnetic Reference System. J. Appl. Phys. 2011, 109, 013905−013905. (25) Klimes, J.; Bowler, D. R.; Michaelides, A. van der Waals Density Functionals Applied to Solids. Phys. Rev. B 2011, 83, 195131. (26) McNellis, E.; Meyer, J.; Baghi, A. D.; Reuter, K. Stabilizing a Molecular Switch at Solid Surfaces: A Density Functional Theory Study of Azobenzene on Cu(111), Ag(111), and Au(111). Phys. Rev. B 2009, 80, 035414. (27) McNellis, E. R.; Meyer, J.; Reuter, K. Azobenzene at Coinage Metal Surfaces: Role of Dispersive van der Waals Interactions. Phys. Rev. B 2009, 80, 205414. (28) Mercurio, G.; McNellis, E. R.; Martin, I.; Hagen, S.; Leyssner, F.; Soubatch, S.; Meyer, J.; Wolf, M.; Tegeder, P.; Tautz, F. S.; et al. Structure and Energetics of Azobenzene on Ag(111): Benchmarking Semiempirical Dispersion Correction Approaches. Phys. Rev. Lett. 2010, 104, 036102. (29) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360. (30) Zhou, L.; Meier, F.; Wiebe, J.; Wiesendanger, R. Inversion of Spin Polarization Above Individual Magnetic Adatoms. Phys. Rev. B 2010, 82, 012409. (31) Zhang, X.-G.; Butler, W. H. Band Structure, Evanescent States, and Transport in Spin Tunnel Junctions. J. Phys.: Condens. Matter 2003, 15, R1603. (32) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points And Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901. (33) Chapman, C.; Paci, I. Conformational Behavior of Chemisorbed Azobenzene Derivatives In External Electric Fields: A Theoretical Study? J. Phys. Chem. C 2010, 114, 20556−20563. (34) Crecca, C. R.; Roitberg, A. E. Theoretical Study of The Isomerization Mechanism of Azobenzene and Disubstituted Azobenzene Derivatives. J. Phys. Chem. A 2006, 110, 8188−8203.

surface. This material is available free of charge via the Internet http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: hping@ufl.edu. Phone: +1 (352)392-6256. Fax: +1 (352)392-8722. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by US/DOE/BES/DE-FG0202ER45995. The authors acknowledge the DOE/NERSC and UF-HPC centers for providing computational resources.

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REFERENCES

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Reversible Spin Polarization at Hybrid OrganicâFerromagnetic

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