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Enantiospecific Spin Polarization of Electrons Photoemitted Through Layers of Homochiral Organic Molecules Miguel Ángel Niño, Iwona Agnieszka Kowalik, Francisco Jesús Luque, Dimitri Arvanitis, Rodolfo Miranda, and Juan José de Miguel*
The long sought-after connection between the symmetry lowering associated with enantiomerically pure chiral organic molecules and electron spin polarization is demonstrated by spin-resolved photoemission experiments. Macroscopic spin polarizations are observed in electron currents photoemitted from molecular films adsorbed on single-crystal surfaces. These measurements demonstrate that the different enantiomers of the same molecule can produce spin polarizations oriented along different directions in space; it is proposed that this unexpected effect results from enantiospecific adsorption geometries of the molecules at the surface. The spin polarization can be observed at room temperature even for thicknesses as low as a molecular bilayer, thus setting the basis for their possible use as efficient and versatile spin filters for spintronic applications. Furthermore, these results could shed some new light onto other profound enigmas such as the general chiral molecular asymmetry encountered everywhere in Nature. Already Louis Pasteur, right after his discovery of molecular chirality based on their optical activity,[1] speculated about a possible influence of magnetism. Although the existence of a relationship between these two effects was later ruled out by Lord Kelvin based on symmetry arguments,[2] an increasing amount of evidence has accumulated in recent times strongly suggesting that both fields are indeed somehow related. The
Dr. M. Á. Niño, Prof. R. Miranda IMDEA-Nanoscience Cantoblanco 28049, Madrid, Spain Dr. I. A. Kowalik Institute of Physics Polish Academy of Sciences 02668 Warsaw, Poland F. J. Luque, Prof. R. Miranda, Dr. J. J. de Miguel Dpto. Física de la Materia Condensada Univ. Autónoma de Madrid Cantoblanco 28049, Madrid, Spain E-mail:
[email protected] Dr. D. Arvanitis Department of Physics and Astronomy Uppsala University 75237 Uppsala, Sweden Prof. R. Miranda, Dr. J. J. de Miguel Condensed Matter Physics Center (IFIMAC) and Instituto de Física de Materiales “Nicolás Cabrera” Univ., Autónoma de Madrid Cantoblanco 28049, Madrid, Spain
DOI: 10.1002/adma.201402810
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asymmetric absorption of photons by chiral media depending on their propagation direction with respect to an applied magnetic field was first predicted theoretically[3] and later confirmed by experiments.[4] This so-called magnetochiral dichroism effect[5] has played an important role in the reawakening of the interest on this highly suggestive area of research, culminating in the recent spectacular finding of up to 60% spin polarization of electrons originating right below the Fermi level of Au substrates and transmitted through layers of chiral double-stranded DNA.[6] Our present results confirm and go beyond the findings of that pioneering work. In our experiments we have been able to study the effect of the two chiral forms of one and the same molecule, namely 1,2-diphenyl-1,2-ethanediol (C6H5-CHOHCHOH-C6H5, DPED). This compound possesses two chiral centers, located at the two C atoms of the ethane chain. These enantiomers are designated as (R,R) and (S,S) in reference to the handedness at each chiral center (R ≡“Rectus”; S ≡ “Sinister”); they are shown in Figure 1A and 1B, respectively. We then demonstrate that substituting one enantiomer by its specular image does not simply revert the sign of the spin polarization of the transmitted electrons but rather can rotate the spatial orientation of that polarization. We also show that strong spin asymmetries, comparable at least to those offered by a typical ferromagnet, can be obtained with films of not more than double molecular thickness at room temperature and without the need for a high degree of long-range structural order within it. Ultrathin Co films epitaxially grown on a Cu(001) single crystal were chosen as the substrate for the adsorption of the enantio-pure chiral molecular layers for several reasons. Co/ Cu(001) has been extensively studied in the past as a model system for two-dimensional magnetism;[8] since its spinresolved band structure has been determined in photoemission experiments analogous to ours,[9,10] a measurement of the spin polarization of our Co films serves both as a calibration of the spin detector and as an in situ reference for the magnitude of the polarization induced by the chiral molecular layers. Besides, the use of a single-crystal substrate allows us to precisely control the magnetic anisotropy axes within the Co film,[11] which is important in order to have the film magnetization properly aligned with respect to the Mott detector –see Figure 1C and the corresponding explanation in the Experimental Section. And finally the DPED molecules, which show negligible adhesion at room temperature on clean Cu, rapidly stick to the bare Co surface covering this substrate with a stable layer of well defined thickness. After that the adhesion probability of the molecules decreases sharply, and hence this preparation
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23 eV photon energy and vertical polarization with respect to the sample surface. The bright feature close to the Fermi level corresponds to the d-band emission of Co. The peak at 3.35 eV below EF is the Cu d-band, with the secondary electrons appearing at higher binding energy. By taking a cut along the blue line marked in the figure one can obtain the linear spectrum corresponding to k = 0, which is depicted in Figure 2C and 2D together with the corresponding spin analysis for the in-plane (Y) and out-of-plane (Z) components, respectively; these axes are identified in Figure 1C. The Co film was magnetized parallel to the surface during the experiment. An in-plane spin polarization of up to 20% can be detected in the Co d electrons right below EF; no significant spin polarization appears perpendicular to the surface, also according to the expectations.[9,10] A more detailed description of the spin polarization measurements and further data on the clean Cu(001) substrate are presented online as Supporting Information. Having characterized our substrate we proceed to study the effect of the adsorption of the chiral molecules on the spin polarization of the photoemitted electrons. A double molecular layer of the pure (R,R)(+)-DPED enantiomer was adsorbed at 300 K on an 8 ML Co/Cu(001) substrate analogous to the one described above. The weak (1×1) LEED pattern with high diffuse background depicted in Figure 3A reveals a laterally disordered distribution of the adsorbed molecules; one should remark here that LEED patterns at a fixed energy such as this one do not offer information about the arrangement of the molecules in the direction perFigure 1. The molecular configurations of the two enantiomers used in this study: (a) (R,R)pendicular to the surface. The corresponding (+)-DPED and (b) (S,S)-(−)-DPED, as derived from DFT calculations using the StoBe package.[7] 2D-ARUPS spectrum is presented in panel (c) Schematic representation of the spin-resolved photoemission experiment (see the Sup3B: The emission from the Co d-band is porting Information for a detailed description). still visible near EF, although weakened due to the dispersion of the photoemitted electrons by the molecular overlayer. The peaks at 3.7 and 6.5 eV procedure allows us to easily obtain reproducible samples. A binding energy originate at least partially from the molecular calibration of the adsorbed film thickness as a function of expobands of (R,R)-(+)-DPED with predominantly π character as sure, obtained by means of x ray photoelectron spectroscopy, is given in the Supporting Information; it shows that after the inicross-checked with DFT calculations.[7] From an estimate of the tial rapid uptake a nearly stable thickness of approximately 5 Å attenuation of the electrons upon crossing the molecular layer is reached, roughly corresponding to a double molecular layer. we conclude that the peak at 3.7 eV contains a contribution of The results of our measurements on an 8-ML-thick Co film approximately 68% from the d-band of the buried Cu substrate; on Cu(001) are summarized in Figure 2. The low energy elecsince there are no significant emissions from either Cu or Co at tron diffraction (LEED) pattern displayed in Figure 2A, with 6.5 eV binding energy, this latter peak must belong entirely to sharp spots and low background, demonstrates that the Co films the molecules. The tail of secondary electrons appears around grow pseudomorphically on Cu(001) with good crystallinity and 9 eV. As in the previous case, the sample was magnetized inlow roughness, as expected.[12] Figure 2B shows the two-dimenplane in several occasions during the measurements, and the in-plane and out-of-plane spin polarizations of the k = 0 specsional (2D) angle resolved ultraviolet photoemission (ARUPS) spectrum (photoemission intensity as a function of both energy trum are displayed in Figure 3C and 3D respectively. For this and parallel momentum) acquired at normal emission with sample we find a roughly uniform polarization independent
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Figure 2. (a) The LEED pattern measured at 133 eV primary beam energy for an 8 ML Co film grown on Cu(001) shows good crystallinity. (b) Energyand momentum-resolved ARUPS spectrum of this sample. The intense emission near the Fermi level is the Co d-band, with the Cu d-band appearing at 3.35 eV. (c) and (d) In-plane and out-of-plane spin polarization spectra measured at k = 0 (along the blue line displayed in B) and 300 K. The sample was magnetized parallel to the surface, and the in-plane spin polarization of the Co d-electrons can be clearly observed below EF.
of the binding energy within the surface plane with negligible values for the perpendicular direction. Our lower bound estimate for the spin polarization induced by the chiral layer in this case is about 5% at 300 K and for a molecular layer only about 5 Å thick. The real value, however, could be larger because our measurement only probes the in-plane projection of the spins along the Y axis, as schematically indicated in Figure 1C; the spin components parallel to the X direction are not detected in this experiment. In any case the spin polarization detected is similar in magnitude to that associated to the in-plane magnetized Co film, which is now embedded within the energy-independent signal created by the chiral film. The data presented in Figure 3 thus show at least some analogy with the report by Göhler et al.[6] that the electrons transmitted through the chiral molecular layer do indeed emerge spin-polarized. Nevertheless, the effect of substituting one enantiomer by its mirror counterpart could not be directly tested by these authors since they employed chiral doublestranded DNA chains with a single handedness. Although it may seem reasonable to expect that the sign of the spin polarization might be reversed, a direct proof of this hypothesis has been lacking so far. In this work we have tried to answer this question by depositing analogous films of the other enantiomer, (S,S)-(−)-DPED on the same substrate, Co/Cu(001), and directly measuring the resulting spin polarization. These films were grown on similar 8 ML Co/Cu(001) substrates as for the previous enantiomer, and also at 300 K. The LEED pattern shown in Figure 4A reveals an analogous absence of in-plane long-range order within the molecular layer as for
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(R,R)-(+)-DPED; the features appearing in the ARPES spectrum of panel 4B are also practically identical to those shown in Figure 3B for that enantiomer. An estimate analogous to the one performed for (R,R)-DPED indicates that also in this case the contribution from the Cu substrate's d-band to the peak appearing at 3.7 eV binding energy amounts to about 71% of the measured intensity; the similarity of these two quantities further confirms that the adsorbed films of both enantiomers have also nearly the same thickness. As above the sample was magnetized in-plane for these measurements. However, the spin analysis presented in graphs 4C and 4D reveals a totally unexpected behavior: rather than a simple inversion of the sign of the spins what is found here is a rotation of their direction, yielding a practically constant polarization of about 8% perpendicular to the surface throughout the whole energy range probed in the experiment. At the same time, and what appears even more surprising, the in-plane magnetization of the Co d-electrons is still clearly visible close to the Fermi level. Again in this case the magnitude of the spin polarization induced by the chiral layer can best be evaluated by comparing it with the one stemming from the underlying Co film. The smaller value of this latter when compared to the uncovered Co sample presented in Figure 2 suggests that the adsorption of the (S,S)-(−)DPED layer may have altered the coercivity and, consequently, also the domain structure of the ferromagnetic Co film, thus reducing its remanent magnetization. Such an effect could be related to the charge transfer between the adsorbed molecules and the Co film surface (see Figure S1 in the Supporting Information for more details).
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COMMUNICATION Figure 3. (a) LEED pattern at 133 eV for a (R,R)-(+)-DPED layer grown on 8 ML Co/Cu(001); the absence of fractional order peaks and the strong diffuse background indicates that the DPED molecules adsorb on the Co surface with no long-range lateral order. (b) Energy- and momentumresolved ARUPS spectrum. The intensity from the Co d-band appears attenuated by the molecular overlayer, which has additional emissions at 3.7 and 6.5 eV. The tail of secondary electrons appears around 9 eV. (c) and (d) In-plane and out-of-plane spin polarization spectra measured at k = 0 (blue line depicted in B) and 300 K, with the sample magnetized in-plane. An approximately energy-independent, in-plane spin polarization can be observed in the photoemitted electrons.
The asymmetric inner potentials created by strain-induced deformations in graphene sheets are predicted to create pseudomagnetic fields more intense than 300 Tesla.[13] It appears that a similar effect might be involved in the origin of the spin polarization in chiral molecules such as those employed in our study. Some recent reports suggest that the polarization measured in the experiment of Göhler et al. might be caused by the effective magnetic field felt by the electrons as they propagate through the helicoidal electronic potential created by the chiral media.[14–16] Our results lend some support to these models, since the magnitude of the polarization measured in our experiments is practically constant in the whole energy range explored and therefore it does not seem to be related to any particular electronic state either from the substrate or within the molecular layer. Nevertheless, our ultrathin DPED films obviously do not present helicoidal symmetry; hence, the details of the interaction of the electrons as they travel through the potentials created by the different types of molecules may be different. Further work will be needed to enlarge the experimental database and to provide more guidance for the theoretical interpretation of these phenomena. It is important to stress that growing the molecular films on a ferromagnetic surface, beyond its usefulness for self-calibration purposes, does not play a significant role in our results. The spectrum of Figure 4D demonstrates that the effect induced by the chiral layer is the same for all the electrons analyzed independently of their
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origin (the Co d-band or the molecular orbitals) and their initial state of polarization. The observation that the different enantiomers of a same molecule do not react equally to linearly polarized ionizing radiation, in terms of emission of photoexcited electrons and their spin polarization, is reported here for the first time. We suggest that this behavior may be related to differences in the structure of the adsorbed films: Since the two enantiomers have different spatial configurations, one can easily imagine that upon their adsorption on a solid surface the arrangement and relative alignment of the enantiomers may also be different, particularly for compact layers when moleculemolecule interactions also play an important role. Based on this hypothesis we have also characterized the adsorption geometry of the DPED molecules on the Co surface by analyzing the angular dependence of the measured intensities in x ray absorption experiments.[17] These results, which will be reported in full detail elsewhere, show how the two chiral enantiomers adopt different orientations when adsorbed on Co: while (R,R)-(+)-DPED lies with the phenyl rings parallel to the surface, (S,S)-(−)-DPED binds with those rings in upright position. Further work will also be needed to determine whether this is a frequent behavior among adsorbed enantiomers. The importance of the structural ordering of the adsorbed molecules for the achievement of significant spin
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Figure 4. (a) LEED pattern at 133 eV for a (S,S)-(–)-DPED layer grown on 8 ML Co/Cu(001). As in Figure 3A, the weak integer-order diffracted spots with high background indicate molecular adsorption with no long-range ordering within the surface plane. (b) Energy- and momentum-resolved ARUPS spectrum, similar to the one depicted in Figure 3B for the (R,R)-(+)-DPED enantiomer. (c) and (d) In-plane and out-of-plane spin polarization spectra at k = 0 (along the blue line marked in B) and 300 K. In spite of having the sample magnetized in-plane as in the previous experiment with (R,R)(+)-DPED, the photoemitted electrons are spin-polarized out-of-plane in this case, with a magnitude also independent of their binding energy; the Co d-electrons maintain their in-plane polarization though.
polarization is also an interesting issue. Göhler et al. report in their study that the spin polarization observed in highly ordered layers of double-stranded DNA (ds-DNA) disappeared when those molecules were substituted by chains of singlestranded DNA (ss-DNA) which are much more flexible. We suggest that their result could be related to the higher tendency of ss-DNA to establish cross-links and tangle up when adsorbed in a close-packed arrangement, which might disrupt any type of preferential alignment or orientation of the chains. In our experiment, on the contrary, although our LEED measurements indicate that the molecules do not adsorb at the surface forming well-ordered superstructures with long-range periodicity, the average orientation revealed by x ray absorption and mentioned above might be enough to obtain the spin polarization. These findings are important for several reasons. Firstly, they may add considerable flexibility for practical applications such as the fabrication of spin filters for detectors or for use in spintronics devices.[18–20] But they can also shed some light onto the basic questions about the origin of the chiral asymmetry in the Cosmos.[21,22] The organic molecules detected in outer space are found mostly adsorbed on interstellar dust particles and subject to the effect of electromagnetic radiation. An enantio-selective enhancement of reaction rates induced by spin polarized secondary electrons has recently been proposed as the possible source of enantiomeric
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excesses.[23] Our work now provides the first direct proof that the enantio-selective response of adsorbed chiral molecules to irradiation could be a general phenomenon, even without the need for circularly polarized light. The observed strong asymmetry in the ability of the two enantiomers to filter and rotate the two spin components could translate into different secondary electron emission rates and eventually in an increased probability of destruction of one particular enantiomer over the other one. In conclusion, the spin polarization induced in electron currents emitted through layers of enantiopure chiral molecules adsorbed on a Co surface has been measured for the two enantiomers of 1,2-diphenyl-1,2-ethanediol. A clear spin polarization has been detected at room temperature in both cases, of magnitude comparable to the one yielded by the ferromagnetic Co substrate. Most significantly, the effect of substituting one chiral enantiomer by its mirror image is not to simply revert the sign of the polarization but rather to change its direction. Our results also suggest that this behavior might be related to differences in the adsorption geometry of the enantiomers at the metal surface. We expect that these results may create opportunities for applications not only in organic-based molecular spintronics but also in other fields such as asymmetric chemical synthesis, as well as provide some insight into the origins of the so-far unexplained chiral asymmetries found in Nature.
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Experimental System: The measurements reported in this paper were carried out at the solid-state station of the I3 beamline of synchrotron MAX-lab, in Lund (Sweden). The preparation chamber of this system possesses an ion gun for sample cleaning, LEED optics for structural characterization and crystallographic alignment of the sample and a residual gas analyzer. Several ports are available for mounting evaporators and other auxiliary equipment. The analysis chamber is equipped with a Scienta R4000 electron analyzer allowing for highresolution angular resolved photoemission spectroscopy combined with a Mott detector for spin measurements (see Figure 1C).[24] The base pressure in the analysis chamber is typically in the low 10−10 mbar range. Sample Preparation: The Cu(001) crystal was cleaned by repeated cycles of Ar+ sputtering (2 µA cm−2, 1 keV) and annealing (900 K). Co was deposited at 300 K from a high-purity (99.995%) rod heated by electron bombardment. The deposition rate was approximately 0.5 monolayers per minute. The DPED molecules were purchased from Sigma-Aldrich and evaporated from Knudsen cells equipped with glass crucibles directly heated by DC current. The melting temperature of DPED is ca. 423 K. The crucible temperature was monitored by means of a K-type thermocouple attached to the crucible; at 330 K the pressure in the ultra high vacuum (UHV) chamber typically rose up to the low 10−8 mbar range. All evaporators used were equipped with water-cooled shrouds. Spin-Resolved Photoemission: At the I3 beamline an “apple-type” undulator provides ultraviolet (UV) photons of energies between 5 and 50 eV and variable polarization. The UV beam incides on the sample at a grazing angle of 17 degrees with respect to the surface plane. The substrate azimuth was aligned relative to the incident photon beam with the aid of LEED. The easy axes of magnetization of fcc-Co(001) correspond to the [110] and [1–10] directions;[11] thus, in order to prevent the magnetization of the Co films from lying parallel to the X direction which is invisible for the spin detector under the experimental geometry used (see Figure 1C) our sample was oriented with the [100] axis parallel to the incident photon beam. All the samples containing Co films were magnetized every few hours during the spin measurements, by applying an in-plane magnetic field by means of a retractable permanent NdFeB magnet. Also, in order to minimize the possible radiation damage to the molecular layers the incidence point of the photon beam at the surface was changed periodically during the experiments. A lower density of data points was used in the regions of the spectrum between the most significant features with the purpose of reducing the acquisition time and improving the statistics.
Additional financial support from the Polish National Science Centre under Grant Nr. 2011/03/D/ST3/02654, the Swedish STINT Foundation, the Swedish VR Baltic Science Link project and the Carl Tryggers foundation for Science Research is gratefully acknowledged. Received: June 24, 2014 Revised: July 23, 2014 Published online: September 2, 2014
[1] [2] [3] [4] [5] [6] [7]
[8] [9]
[10] [11] [12]
[13] [14] [15] [16] [17]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
[21] [22] [23]
Acknowledgements The authors are grateful to the MAX-lab staff for their help with the execution of these experiments. We also thank Dr. J. Lobo for his kind assistance during the measurements. The main part of this work has been supported by the Spanish MINECO through Grant FIS2010–18531.
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[18] [19] [20]
[24]
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