Toward Tailored All-Spin Molecular Devices - ACS Publications

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Towards tailored all-spin molecular devices Maciej Bazarnik, Bernhard Eberhard Christian Bugenhagen, Micha Elsebach, Emil Sierda, Annika Frank, Marc Heinrich Prosenc, and Roland Wiesendanger Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04266 • Publication Date (Web): 24 Dec 2015 Downloaded from http://pubs.acs.org on December 27, 2015

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TOWARDS TAILORED ALL-SPIN MOLECULAR DEVICES

Maciej Bazarnik1,3*, Bernhard Bugenhagen2, Micha Elsebach1, Emil Sierda3, Annika Frank2, Marc H. Prosenc4, Roland Wiesendanger1 1

Dept. of Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany

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Dept. of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany

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Institute of Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland

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Dept. of Chemistry, Technical University of Kaiserslautern, Erwin-Schrödinger-Str. 52, D-67663

Kaiserslautern, Germany *[email protected] Molecular based spintronic devices offer great potential for future energy-efficient information technology as they combine ultimately small size, high-speed operation, and low-power consumption. Recent developments in combining atom-by-atom assembly with spin-sensitive imaging and characterization at the atomic level have led to a first prototype of an all-spin atomic-scale logic device, but the very low working temperature limits its application. Here, we show that a more stable spintronic device could be achieved using tailored Co-Salophene based molecular building blocks, combined with in situ electrospray deposition under ultra-high vacuum conditions as well as control of the surface-confined molecular assembly at the nanometer scale. In particular, we describe the tools to build a molecular, strongly bonded device structure from paramagnetic molecular building blocks including spin-wires, gates, and tails. Such molecular device concepts offer the advantage of inherent parallel fabrication based on molecular self-assembly as well as an order of magnitude higher operation temperatures due to enhanced energy scales of covalent through-bond linkage of basic molecular units compared to substrate-mediated coupling schemes employing indirect exchange coupling between individual adsorbed magnetic atoms on surfaces. Keywords: Ullmann coupling, spintronic, scanning tunneling microscopy, electrospray deposition, molecular self-assembly, salene-complex

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The development of all-spin based devices requires basic building blocks such as spin information transmitting wires and junctions. On the atomic scale, a purely spin-based logic device consisting of two antiferromagnetically (AFM) coupled spin chains and a triangular atomic junction has recently been reported [1]. A schematic representation of such a device is shown in Figure 1 (a). The inputs are realized by small magnetic clusters (such as the ones shown in [2]), from which spin information is transmitted by AFM-coupled Fe atoms to a final atomic triangle. Here, based on magnetic frustration of the readout atom, the logic operation can be performed. Although the utilization of magnetic moments of single atoms truly represents the ultimate limit of miniaturization, it is necessary to precisely put each single atom at a particular place to achieve the desired indirect magnetic exchange interaction mediated by the substrate’s electrons. Moreover, all atoms have to stay at the designated positions. Therefore, the structure is stable only at sufficiently low temperatures where thermal diffusion of the weakly bonded atoms is absent. Additionally, the substrate mediated indirect magnetic exchange (RKKY) coupling between the atoms is very weak, on the order of J = 0.3 meV [3], therefore the device can only work at temperatures as low as 300 mK [1]. To obtain a higher thermal stability, molecular structures offer an interesting alternative. By depositing magnetic molecules [4, 5] on a surface, i.e. metal-organic coordination compounds instead of single atoms, their self-assembling properties can be exploited and even tailored by the choice of the ligands [6, 7, 8]. Using the right functional groups as substituents for the ligands, the molecular arrangements can even be stabilized by chemical cross-linking [9-18], thereby achieving more rigid structures as well as opening up additional, through-bond magnetic interactions, e.g. superexchange couplings, which are usually much stronger than a spin-dependent coupling via the substrate [17,19]. In a recent work [17], we employed the Ullmann [20] reaction to oligomerize paramagnetic (S=1/2) 5,5'dibromosalophenato-cobalt(II) monomers (later referred to as Br2Co) on a Au(111) surface. In these oligomeric chains, measurements of the Kondo temperature in dependence of the chain length, as well as theoretical calculations, indicated that the cobalt atom sites are AFM-coupled, as depicted in Figure 1 (b). These chains have chemically active endings enabling a selective linkage to other molecules, clusters, or islands. Considering the components needed for a logic device, these AFM coupled chains can in principle act as spin-based leads. Next, we focus on a potential candidate for the missing building block of a spin logic device, i.e. a junction. To fit our needs, such a molecular junction has to fulfill several requirements: First, by design, it must be able to covalently link two or more molecular spin chains. Second, it needs to conduct the spin-


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information between the connected oligomeric chains. Third, it has to be compatible with ultra-high vacuum (UHV) preparation methods. The tri-bromo-triplecobaltsalophene (Br3Co3) molecule exhibits C3 symmetry and is nearly planar, therefore the metal center is easily accessible by STM-methods. The complex is sufficiently stable to allow for UHV-based deposition techniques. In Figure 1 (c) a density functional theory (DFT) optimized structure of a gas-phase molecule is depicted. The cobalt atom sites in the molecule form an equilateral triangle with a side length of about 730 pm. It can also be seen that the Co atoms fit neatly into the N2O2 entity and are embedded in the ligand's π-system. This should provide uncompensated spins in the molecule and in principle assures an efficient coupling between Co atoms. The bromine atoms at the outermost sites of the molecule are a prerequisite for the above mentioned selective on-surface coupling of single molecules via the Ullmann reaction in a well-defined way. However, once the thermal chemical reaction is triggered by heating of the sample it is difficult to control and quite extended covalently linked structures are usually obtained (see supplementary Figure S10a). Two strategies can be employed to avoid this: First, on-surface chemistry activated by femtosecond laser pulses with counter cooling would induce limited diffusion and chemical reactions [21-23], thereby leading to the formation of isolated supramolecular structures. However, this strategy is experimentally very demanding and challenging. Therefore, we favor a different approach, i.e. the use of chemically designed terminators. For such a terminator the complex 5-bromo-5'-H-salophenato-cobalt(II) BrCo (Figure 1 (d)) was synthesized which bears only one Br-substituent and thus, functions as terminator complex in the surface oligomerization of the cobalt complexes. In Figure 2 (a) the two-step synthesis of Br3Co3 from a 4-bromo-monoimine and phloroglucinoltrialdehyde is drawn. This method follows a synthesis from Richthofen et al. [24] and was modified by the choice of solvents and the work-up procedure (see details in Supplementary Information). In step 1, the mono-brominated ligand Br3H6 was obtained as an orange solid and could be isolated in analytically pure form without further purification. In step 2, Br3Co3 was obtained by reacting Br3H6 with cobalt acetate tetrahydrate in N,N-dimethylformamide as a solvent. Triethylamine was added to the reaction mixture to provide a basic reaction environment since the imine-groups of Br3H6 are sensitive to acids. After purification, Br3Co3 was obtained in analytically pure form in an overall yield of 45 %. Due to the poor solubility of both Br3H6 and Br3Co3 in standard organic solvents, the metalation-step had to be performed in purified N,N-dimethylformamide. In fact, the resulting complex Br3Co3 proved to be soluble only in dimethyl sulfoxide (DMSO) and pyridine. The low vapor pressure of the complex tri-bromo-


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triplecobaltsalophene Br3Co3 prevented an efficient sublimation. In fact, we were able to completely carbonize the compound while trying to sublime it under UHV conditions. However, high-resolution electrospray ionization mass spectrometry (ESI-MS) proved Br3Co3 to be ionizable without major decomposition of the molecules (see supplementary material, Figure S2), thus Br3Co3 was finally deposited by electrospray methods. Synthesis of the chain-terminator (BrCo) has been achieved in a template-based one-pot procedure [25] (Figure 2 (b)). The 4-bromo-monoimine was dissolved in methanol and trimethylamine, treated with one equivalent of cobalt-acetate-tetrahydrate and afterwards with one equivalent of a salicylaldehyde (Figure 2). During this procedure, the aldehyde-group of the salicylaldehyde and the amine-group of the preformed monoamine complex are in close vicinity and readily undergo a Schiff-base reaction building the final salophene complex.

A typical scanning tunneling microscopy (STM) image after the deposition of tri-bromotriplecobaltsalophene molecules onto the Au(111) substrate held at room temperature is depicted in Figure 3 (a). Individual and well separated Br3Co3 molecules are randomly distributed over the surface. However, not all molecules survive the impact on the surface when the energy of the molecular beam is 25.8±3.1 eV per charge (see supplementary material Figure S6a and b). We therefore developed a softlanding scheme to slow down the molecules to impact energies of 5 eV. As a result, we obtained the highest rate of intact molecules on the surface at a level of 80 % which is sufficient for further studies. A zoom-in STM image of a single complex Br3Co3 with a DFT model overlaid on top is depicted in Figure 3 (b). Upon closer inspection the molecule does not appear completely C3-symmetric on the surface. The cobalt atom marked by 1 appears slightly higher. This can be explained by simple geometric considerations of a DFT model of the molecule overlaid on top of the Au(111) fcc lattice (Figure 3 (c)). The equilateral triangle formed by Co atoms does not fit to the underlying atomic lattice of the Au(111) substrate. When two Co atoms are in hollow or bridge sites, the third Co atom is located at the on-top position. This leads to an inequivalency of the three Co-sites upon adsorption of the molecule, thereby lowering the C3-symmetry. As a result, one can expect a different hybridization for the different adsorption sites. It is indeed evident from the scanning tunneling spectroscopy (STS) data presented in Figure 3 (d) that the on-top Co atom shows a very pronounced spectroscopic feature at 500 mV, while the same state is barely visible on the other two Co sites. This inequivalency among the three cobalt


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atoms upon surface adsorption is very important as this inequivalency is expected to slightly modify the magnetic coupling between the Co centers, making the spin state of the output-state magnetic atom tunable by an external magnetic field similar to the case of the all-spin atomic-scale logic device [1]. The molecules adsorb with a preference of one of the bromine atom positions either on an elbow site of the Au(111) herringbone reconstruction or on the hcp part of the Au(111) surface. The tri-bromotriplecobaltsalophene complex adsorbs in two enantiomer configurations. An STM image of a hydrogen terminated molecular chain built from three salophenato-cobalt(II) building blocks, overlaid with a structure model, is depicted in Figure 4 (a). At low bias a single salophenatocobalt(II) molecule is imaged as three protrusions, two corresponding to carbon rings at the ends of the molecule, and one to the N2C6H4 bridge. When two molecules form a chemical bond, one of the end protrusions of the first molecule combines with that of a second molecule, thereby forming an elongated ridge. Therefore, a covalently bound dimer is actually imaged with five elevations in topography instead of six. Consequently, a trimer is imaged with seven protrusions, two of them are elongated because of the covalent bond between the carbon rings. Figure 4 (d) depicts this scheme for a larger molecular assembly. An additional intensity in STM constant-current images arises at a cobalt site if the molecule adsorbs with this particular Co-atom at an on-top position. This behavior is very similar to that of a single complex Br3Co3. An STM image of the final product of an Ullmann reaction of subsequently deposited tri-bromotriplecobaltsalophene,

5,5'-dibromosalophenato-cobalt(II),

and

5-bromo-salophenato-cobalt(II)

molecules on the Au(111) surface in a 1:6:3 ratio is shown in Figure 4 (b). The supramolecular structure consists of a Br3Co3 molecule, one two-block long chain in the upper part of the image, one three-block long chain on the left, and a four-block long chain extending to the right. As in the case of simple molecular wires, two covalently bound carbon rings form an elongated ridge. This is also the case for the Br3Co3 molecule bound to the molecular chains. Therefore, at low bias a triplecobaltsalophene molecule embedded in a supramolecular structure is imaged as a central round protrusion, three elevations in topography are corresponding to N2C6H4 bridges, and three elongated ridges are corresponding to outermost carbon rings bound to side rings of salophenato-cobalt(II). By changing the ratio of the tribromo-triplecobaltsalophene, 5,5'-dibromosalophenato-cobalt(II), and 5-bromo-salophenato-cobalt(II) molecules, the shape and extension of the resulting molecular structure can be tailored. The molecular structure model is shown in Figure 4 (d).


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The complexes used can be deposited onto the surface without decomposition and can even be selectively coupled to form chains of defined lengths. By using the triplesalophene complex a branch is introduced which allows to link molecular spin chains in a defined manner. Due to the expected coupling of spins in the molecular junction the information can be processed which is a prerequisite for future spintronic devices. To investigate this spin coupling within the molecular junction, we now turn to DFT calculations. Geometry optimization of the complexes Br3Co3 and (Br3Co3 + 2BrCo) revealed geometric parameters listed in Table S2 in the supplementary materials. Exchange parameters between individual Co atoms calculated for the complex Br3Co3 are J = +4.6 meV, i.e. a ferromagnetic interaction between the unpaired electrons in the three nuclear complex, and J = -7.9 meV, corresponding to an antiferromagnetic coupling, for the linear molecular extension, in accordance with literature values [17]. For the complex Br3Co3 the unpaired electrons are located on the Co-atoms in a 3dxz type orbital, similar to the case of the complex Br2Co as reported previously [17]. The spatial distribution of the spin densities of the lowest energy state are depicted in Figure 4(c). Yellow and blue colors represent the different spin states, visualizing the antiferromagnetic arrangement of neighboring nodes (C atoms). The iso-values of the spin densities are chosen such that they best depict the localization of the spin, though there is a considerable pπ – dπ overlap driving the superexchange mechanism. As previously shown the spincarrying exclusively couples through the p-system and not via the surface (RKKY) [17]. Motivated by the concept of an all-spin based atomic-scale logic device [1], we designed a novel molecular spintronic device with tailor-made connections between spin-carrying metal centers, allowing for spin information transfer based on the tailored spin-spin coupling between those metal centers. This molecular spintronic device has the advantage that an order of magnitude higher spin coupling energies are achieved compared to the previously reported all-spin atomic-scale logic device, which allows operating the molecular device at considerably higher temperatures. The biggest advantage, however, is that thanks to the superexchange mechanism the spin information is conducted via the chain (lead) of complexes directly to the junction. Thus it does not interfere with neighboring chains or devices which would be the case for an RKKY driven device, if two or more of them are in close range. Moreover, the enhanced thermal stability of the molecular array up to room temperature [17] promises interesting future applications of such devices. In summary, we have demonstrated the design of basic building blocks for an all-spin based molecular logic gate. We have identified necessary components and synthesized appropriate molecules. Moreover,


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we have successfully deposited these molecules on a Au(111) surface and characterized the products of on-surface Ullmann reactions by STM. Finally, we have constructed and imaged a prototype supramolecular structure fulfilling all needs for an all-spin based molecular device.


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Methods Experiment All experiments were performed in two UHV systems equipped with either a low temperature STM or a variable temperature STM and preparation chambers for substrate cleaning, CVD growth, and molecule as well as metal deposition. Electrochemically etched tungsten tips were used as STM probes and cleaned by standard procedures. The Au(111) crystals were cleaned by repeated cycles of Ar+ sputtering (0.8 kV, 5·10-6 mbar) and annealing at 670 K. The surface quality was directly assured by atomicresolution STM characterization. Br3Co3 molecules were deposited using the ESD method (Elion Pure Jet, see supplementary material for details). BrCo and Br2Co molecules were deposited from thoroughly outgassed sources by thermal sublimation under UHV onto the sample held at room temperature in the preparation chamber. The Ullmann reaction was triggered by annealing at 420 K for 10 min. Finally, the samples were transferred in vacuo to the STM and cooled to measurement temperatures. All images were recorded in constant-current mode and processed using the WSxM software [26]. Computational Details For all calculations on the density functional theory (DFT) level the program system TURBOMOLE was used employing the program RI-DFT [27]. Energies and gradients were calculated on the non-local level of theory according to Becke [28] and Perdew [29]. For all atoms the split valence triple zeta basis set def2-TZVP was used. The exchange coupling constant J was calculated by the broken symmetry method [30] following the scheme of van Wüllen [31, 32]. The high-spin wave function was calculated and the initial low-spin configurations (guess) were generated from the localized spin orbitals. Energies were calculated and subtracted according to JAB = -(EHS-EBS)/(HS-BS) [33]. Acknowledgements The authors are grateful to Jens Brede for fruitful discussions. We gratefully acknowledge financial support from the DFG via SFB668. MP gratefully acknowledges financial support from the DFG via SFB/TRR 88 3MET. Author Contribution M.B. and B.B. equally contributed to the manuscript. M.B. and B.B. conceived the experiment. B.B. carried out the chemical synthesis and characterization. A.F. synthesized the Br3H6 ligand. M.B. and M.E. setup the ESD experiment. M.B. carried out the experiment and analyzed the data. E.S. supported the


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experiments. B.B. and M.P. carried out the DFT calculations. M.B., M.P. and R.W. wrote the manuscript. All authors discussed the results and commented on the manuscript. Supporting Information for Publication Additional information concerning: details of chemical synthesis and material characterization; details of electrospray deposition experiments; Ullman reaction of Br3Co3 molecules only, mixture of Br3Co3 and Br2Co molecules; DFT calculations of single Br3Co3 molecule. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest. Bibliography 1. Khajetoorians A.A.; Wiebe J.; Chilian B.; Wiesendanger R.; Science 2011, 332, 1062-1064 2. Khajetoorians A.A.; Baxevanis B.; Hubner C.; Schlenk T.; Krause S.; Wehling T.O.; Lounis S.; Lichtenstein A.; Pfannkuche D.; Wiebe J.; Wiesendanger R.; Science 2013, 339, 55-59 3. Zhou L.; Wiebe J.; Lounis S.; Vedmedenko E.; Meier F.; Blügel S.; Dederichs P.H.; Wiesendanger R.; Nature Physics 2010, 6, 187-191 4. Miller J. S.; Inorg. Chem. 2000, 39, 4392-4408 5. Gatteschi D.; Sessoli R.; Angew. Chem. 2003, 115, 278 6. Kuck S.; Chang S.H.; Klöckner J.P.; Prosenc M.H.; Hoffmann G.; Wiesendanger R.; Chem. Phys. Chem. 2009, 10, 2008-2011 7. Barth J.V.; Ann. Rev. Phys. Chem. 2007, 58, 375–407 8. Barth J.V.; Surf. Sci. 2009, 603, 1533–41 9. Grill L.; Dyer M.; Lafferentz L.; Persson M.; Peters M.V.; Hecht S.; Nature Nano. 2007, 2, 687-91 10. Lafferentz L.; Ample F.; Yu H.; Hecht S.; Joachim C.; Grill L.; Science 2009, 323; 1193-1197 11. Lipton-Duffin J.A.; Ivasenko O.; Perepichka D.F.; Rosei F.; Small 2009, 5, 592-597 12. Gutzler R.; Walch H.; Eder G.; Kloft S.; Heckl W.M.; Lackinger M.; Chem. Commun. 2009, 6, 4456-4458 13. Bieri M.; Treier M.; Cai J.; Aït-Mansour K.; Ruffieux P.; Gröning O.; Gröning P.; Kastler M.; Rieger R.; Feng X.; Müllen K.; Fasel R.; Chem. Commun. 2009, 45, 6919-6921 14. Bieri M.; Nguyen M.T.; Gröning O.; Cai J.; Treier M.; Aït-Mansour K.; Ruffieux P.; Pignedoli C.A.; Passerone D.; Kastler M.; Müllen K.; Fasel R.; J. Am. Chem. Soc. 2010, 132, 16669-16676


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15. Lackinger M.; Heckl W.M.; J. Phys. D.: Appl. Phys. 2011, 44, 464011 16. Zhang Y.-Q.; Kepčija N.; Kleinschrodt M.; Diller K.; Fischer S.; Papageorgiou A.C.; Allegretti F.; Björk J.; Klyatskaya S.; Klappenberger F.; Ruben M.; Barth J.V.; Nature Commun. 2012, 3, 1286 17. DiLullo A.; Chang S.H.; Baadji N.; Clark K.; Klöckner J.P.; Prosenc M.H.; Sanvito S.; Wiesendanger R.; Hoffmann G.; Hla S.W.; Nano Lett. 2012, 12, 3174-3179 18. Nacci C.; Ample F.; Bleger D.; Hecht S.; Joachim C.; Grill L.; Nature Commun. 2015, 6, 7397 19. Kanamori J.; J. Phys. Chem. Solids; 1959, 10, 87-98 20. Ullmann F.; Bielecki J.; Chem. Ber. 1901, 34, 2174 21. Gawronski H.; Mehlhorn M.; Morgenstern K.; Angew. Chem. Int. Ed. 2010, 49, 5913-6 22. Mehlhorn M.; Gawronski H.; Morgenstern K.; Phys. Rev. Lett. 2010, 104, 076101 23. Mehlhorn M.; Carrasco J.; Michaelides A.; Morgenstern K.; Phys. Rev. Lett. 2009, 103, 026101 24. Frhr. v. Richthofen C.-G.; Stammler A.; Bögge H.; Glaser T.; Eur. J. Inorg. Chem. 2011, 2011, 49-52 25. Kleij A.W.; Kuil M.; Tooke D.M.; Lutz M.; Spek A.L.; Reek J.N.H.; Chem. Eur. J. 2005, 11, 4743-4750 26. Horcas I.; Fernandez R.; Gomez-Rodriguez J.; Colchero J.; Gomez-Herrero J.J.; Baro A.M.; Rev. Sci. Instrum. 2007, 78, 013705 27. Eichkorn K.; Treutler O.; Öhm H.; Häser M.; Ahlrichs R.; Chem. Phys. Lett. 1995, 242, 652-660 28. Becke A.D.; Phys. Rev. A 1988, 38, 3098 29. Perdew J. P.; Phys. Rev. B 1986, 33, 8822 30. Noodleman L.; J. Chem. Phys. 1981, 74, 5737 31. Schmitt S.; Jost P.; van Wüllen C.; J. Chem. Phys. 2011, 134, 194113 32. Kessler E.M.V.; Schmitt S.; van Wüllen C.; J. Chem. Phys. 2013, 139, 184110 33. Soda T.; Kitigawa Y.; Onishi T.; Takano Y.; Shigeta Y.; Nagano H.; Yoshioka Y.; Yamaguchi K.; Chem. Phys. Lett. 2000, 319, 223-230


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Concept of the molecular device design. (a) Scheme of an all-spin based logic gate built of single Fe atoms and small magnetic clusters as realized by Khajetoorians et al. [1] (b) Molecular spin lead: ball-and-stick model of a molecular chain with antiferromagnetically coupled Co centers. (c) Prototype of a molecular junction: ball-and-stick model of tri-bromo-triplecobaltsalophene; yellow background highlights a single salophenato-cobalt(II) subunit. (d) Chain terminator: ball-and-stick model of a 5-bromo-salophenatocobalt(II) molecule. 1169x703mm (90 x 90 DPI)


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Schematics of chemical synthesis of (a) tri-bromo-triplecobaltsalophene (Br3Co3) and (b) 5-bromosalophenato-cobalt(II) (BrCo). 119x65mm (300 x 300 DPI)


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Individual Br3Co3 molecules adsorbed on Au(111). (a) STM image of Br3Co3 molecules deposited onto Au(111) using the electrospray deposition technique. (b) STM image of a single Br3Co3 molecule revealing its internal structure. A ball-and-stick DFT model is overlaid on top of the STM image. (c) A ball-and-stick DFT model extracted from (b) overlaid on top of the Au(111) atomic lattice. (d) dI/dU spectra taken over the three cobalt sites and over the middle carbon ring of the molecule. 84x83mm (300 x 300 DPI)


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Design of an all-spin based logic gate built of covalently bound molecules. (a) STM image along with a balland-stick model of a molecular chain with antiferromagnetically coupled Co centers and hydrogen terminations at both ends. (b) STM image of a molecular arrangement. (c) Ball-and-stick model of the DFT calculated structure with spatial distribution of the spin densities plotted on top. Blue and yellow colors correspond to opposing spin directions. (d) Ball-and-stick model of a molecular device structure corresponding to the STM image in (b). Dotted circles and ellipses depict parts of molecules from which high intensities in the STM image arise. 533x359mm (300 x 300 DPI)


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TOC 34x13mm (300 x 300 DPI)


Toward Tailored All-Spin Molecular Devices - ACS Publications

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