Observation of Single Dinuclear Metal-Complex Molecules Using

candidate for use in molecular quantum-dot cellular automata (QCA) devices. Isolated ... multiple identical cells, with each cell capable of assuming ...
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J. Phys. Chem. B 2006, 110, 21846-21849

Observation of Single Dinuclear Metal-Complex Molecules Using Scanning Tunneling Microscopy Zhongqing Wei, Song Guo, and S. Alex Kandel* Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670 ReceiVed: June 13, 2006; In Final Form: August 29, 2006

We report a scanning tunneling microscopy (STM) investigation of a dinuclear organometallic molecule, trans-[Cl(dppe)2Ru(C≡C)6Ru(dppe)2Cl] (Ru2), absorbed on a Au(111) surface; this molecule is a potential candidate for use in molecular quantum-dot cellular automata (QCA) devices. Isolated Ru2 molecules were observed under ultra-high-vacuum conditions. Submolecular structure was clearly discernible in the STM images, with a bright feature corresponding to each of the two Ru-ligand complexes within the Ru2 molecule. Rotation and translation of the Ru2 molecules were observed to be induced by the STM tip under some tunneling conditions.

Introduction Research in molecular electronics is receiving ever-increasing interest as the continued miniaturization of conventional microelectronics approaches the limits of what is physically possible and financially feasible.1-11 There are many possible approaches for designing single-molecule-based devices; of these, we have focused on a molecular implementation of quantum-dot cellular automata (QCA).12,13 In QCA, devices are constructed from multiple identical cells, with each cell capable of assuming at least two internal configurations of electrostatic charge. Information is transmitted and computation is performed through the Coulomb interactions between adjacent cells.7 Organometallic molecules containing multiple metal centers have been proposed as potential single-molecule QCA cells. In principle, if the metal centers have similar or identical electron affinities, the molecule will have two or more near-degenerate or degenerate electron configurations corresponding to different oxidation states on each metal center. Switching between these states would provide the basis for QCA functionality. This charge-configuration switching has been demonstrated in bulk samples.14,15 Scanning tunneling microscopy (STM) is a powerful tool for the investigation of the topographic and electronic structure of molecules and is widely used for the direct testing of individual components for molecular electronics.16-27 The earliest applications of STM investigated clean surfaces and small inorganic or organic molecules; however, an increasing range of large molecules have also been studied by this technique. These include carbon nanotubes,28-36 DNA,19,37 C60 and its derivatives,38-41 and polycyclic organic molecules.26,42,43 Large metalcomplex molecules such as phthalocyanines and porphyrins have also been extensively investigated by STM on Au,44-47 HOPG,48 Cu,49,50 Ag,51 NiAl(100),52 and H-passivated Si(100) substrates.53,54 In this article, we report an STM investigation of a QCA candidate molecule, referred to as Ru2, or trans-[Cl(dppe)2Ru(C≡C)6Ru(dppe)2Cl], where dppe ) diphenylphosphinoethane. We recorded STM images under ultra-high-vacuum (UHV) conditions of isolated molecules showing clear submolecular * Corresponding author. E-mail: [email protected]. Tel.: (574) 631-7837. Fax: (574) 631-6652.

resolution of each ruthenium-ligand complex, as well as molecular rotation and translation induced by scanning of the STM tip. This work is a starting point for the detailed investigation of the structural and electronic properties of Ru2 at the single-molecule scale, aimed at determining its feasibility for use in molecular-electronic applications. Experimental Section 1. Synthesis of Ru2 Powder and Preparation of Ru2 Solution. The structure of trans-[Cl(dppe)2Ru(C≡C)6Ru(dppe)2Cl] is shown in Figure 1. This molecule contains a rigid 12-carbon-atom linker (six alkyne groups) between the two metal Ru centers. The molecule was synthesized by our collaborators (Dr. Roger Nassar and Prof. Thomas P. Fehlner) following literature procedures.55,56 Ru2 powder was dissolved in toluene solvent to prepare a solution (0.5 mg in 5 mL); this process was carried out in a nitrogen-purged drybox to avoid possible degradation of the air- and water-sensitive solution. 2. Sample Preparation. Sample preparation for STM plays a key role in obtaining high-quality images. STM imaging of isolated molecules requires a reliable sample preparation method for depositing target molecules onto a well-defined surface without aggregation. Thermal evaporation is the most widely used method for depositing molecules under UHV conditions. Thermal evaporation in UHV is not generally applicable, however, to heat-sensitive and low-vapor-pressure molecules. The alternative, solution-based sample preparation, has other drawbacks: in particular, exposing a sample to solution potentially introduces impurities onto the surface, and solution-based deposition techniques (dip-, drop-, or spin-casting) can facilitate molecular aggregation on the surface. An attractive and effective method for depositing higher-molecular-weight molecules on substrates is to inject a sample solution into vacuum in microliter quantities through a pulsed nozzle. This pulse-injection technique37,57,58 has been used to deposit large molecules, such as conducting polymers,59 C60 derivatives,41 double-helix DNA,37 and carbon nanotubes.60 In this work, the pulse-injection technique was used to prepare samples for STM imaging. Au(111) on mica (Molecular Imaging) was used as the substrate for deposition of the target

10.1021/jp0636928 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006

Observation of Single Dinuclear Molecules Using STM

Figure 1. Molecular structural formula of Ru2, trans-[Cl(dppe)2Ru(C≡C)6Ru(dppe)2Cl].

molecules. Prior to deposition, the gold substrate was annealed under a hydrogen flame for 10 min; the annealed gold was then transferred to a sample preparation station installed with a pulsed solenoid valve (Parker Instrumentation 9-Series, 0.5-mm nozzle diameter, IOTA One driver) positioned 8 cm from the sample. Then, 0.3 mL of Ru2/toluene solution prepared in a drybox was injected into the pulsed valve with a syringe. Under vacuum, the Ru2 solution was pulsed onto an annealed Au surface from the solenoid valve; typically, eight pulses of 1-ms duration were delivered. Pulse deposition delivers molecules to a small region of a fairly large sample. In addition, the physical processes involveds droplet formation and evaporationsare complex and typically result in molecular coverage that varies widely over the length scales relevant for STM. We found that, by translating the sample slightly between injected pulses, we could produce a more evenly covered surface. However, most experiments required us to scan multiple areas over the surface in order to find one with the desired molecular coverage. The resulting Ru2/Au sample was imaged at room temperature using an Omicron scanning tunneling microscope (LTSTM). The base pressure of the STM chamber was kept in the 10-10 Torr range throughout imaging. The image in Figure 2 is low-pass-filtered, and the images in Figure 4 are 3 × 3 medianfiltered. The image in Figure 3 shows raw, unprocessed data. Results and Discussion Figure 2A shows an STM image of isolated Ru2 molecules on Au(111). Some blurring and streaking is evident in this image; this is typical of our observations of Ru2, and we will discuss these effects presently. The most prominent features in the image, however, are bright, circular “dots”, with an apparent topographical height of 1.5-1.7 Å. These are largely found in pairs, and the symmetrical nature of these features leads us to assign each paired-dot structure as a single Ru2 molecule. The paired-dot features appear with similar heights, contrasts, and center-center spacings in all areas of the image, indicating that they correspond to a single molecular structure with a narrow range of adsorption geometries. In addition, multiple orientations

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21847 are observed, which definitively rules out STM tip convolution effects as the origin of the submolecular structure. Two such geometries are shown in the enlarged area in Figure 2B. We propose that the molecules are lying flat, with their long axes parallel to the surface. Each bright feature corresponds to one of the ruthenium atom-ligand systems. The two ligand systems are clearly and separately resolved in the STM image, and the internal 12-carbon-atom linker is invisible with the imaging conditions used. A space-filling model of Ru2, drawn to scale, is presented in Figure 2C; the close correspondence with the image in panel B provides support for this assignment. Figure 3A shows a higher-resolution STM image. Cross sections for six molecules are shown in Figure 3B, which demonstrates that the observed separation between bright features does not vary appreciably from molecule to molecule. We observed an average lateral spacing of 17.5 Å, calibrated using a separate measurement of a close-packed octanethiol selfassembled monolayer. This spacing is exceptionally close to the theoretical value of 17.7 Å for the Ru-Ru intramolecular spacing in Ru2;14 certainly, these values agree within the 5-10% measurement error we expect from our microscope. The apparent topographic height of each Ru2 molecule in Figure 3A is 1.5-1.7 Å, which is much smaller than the calculated value of 12.9 Å.14 STM images of molecules on metal surfaces reflect a convolution of molecular geometrical and electronic structure, and the disparity between actual and observed molecular heights indicates that the molecule is largely insulating; that is, there is very little electronic-state density between the Fermi level and imaging bias.61,62 Closer inspection of Figure 3A shows that there are preferred orientations for the six labeled molecules. Molecules labeled 2, 4, and 5 are all aligned in virtually the same direction with respect to the substrate; similarly, molecules 1 and 6 align along the same axis. Molecule 3 aligns along a third direction. The initial explanation is that the molecular axis must be registered to the hexagonally symmetric Au(111) surface, which is the only source of directionality on this length scale. We would expect Ru2, as a large molecule, to be flexible enough to average out any strong surface corrugation effects; this makes the observed alignment a moderately surprising result. Future work will be focused on acquiring more statistics to quantify this effect. In Figures 2A and 3A, the areas around molecular features are slightly blurred and shadowed. These are most likely due to imaging artifacts resulting from STM tip convolution, the effect of the geometrical shape of the tip and its microstructure on STM images. Tip convolution is difficult to eliminate entirely when imaging features with very high aspect ratios, particularly

Figure 2. (A) 455 × 280 Å STM image of Ru2 molecules on Au(111). Each single molecule is imaged as a feature consisting of two closely paired bright dots. Imaging conditions are It ) 20 pA and Vt ) 0.5 V, at room temperature. (B) Expanded view of two Ru2 molecules. (C) Space-filling model of two Ru2 molecules, drawn to the same scale as the image shown in panel B.

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Figure 3. (A) STM image (225 × 315 Å) of Ru2 molecules deposited on a Au(111) surface by pulse injection. Each pair of bright dots is assigned to one Ru2 molecule. Imaging conditions are It ) 20 pA and Vt ) 0.5 V, at room temperature. (B) Height profiles for each of six molecules indicated by the red lines drawn in the image of panel A; an average height profile for all six molecules is shown in red. The average spacing between observed peaks is 17.5 Å. A molecular model of Ru2 is drawn to scale.

Figure 4. STM images (265 × 140 Å) showing STM-induced motion of Ru2 molecules near a substrate step. STM conditions: (A) Vt ) 1.0 V and It ) 20 pA, (B) Vt ) 1.0 V and It)100 pA.

when the apparent topographic height falls far short of the actual molecular size, as is the case here. We find that a majority of our best-resolved Ru2 images have some noticeable tip convolution. An additional possibility to consider is raised by previous studies, which attribute distorted features to contamination during pulse injection.63,64 For our experiments, possible contaminants include residual toluene or products of air- or water-induced Ru2 decomposition; the latter process might also

be responsible for the occasional unpaired bright feature observed on the surface. The horizontal streaks in Figure 3A are along the STM fastscanning direction; this type of “noise” is commonly attributed to molecular motion occurring on the time scale of imaging. Indeed, although we found that Ru2 molecules were relatively stable on Au(111) at room temperature given the gentle imaging conditions (specifically, low tunneling currents), we occasionally observed larger-scale molecular motion while scanning the STM tip. This molecular manipulation can be enhanced by adjusting the magnitude and direction of the force exerted on the adsorbed molecules.65 Increasing the tunneling current or reducing the voltage applied between the tip and the sample will decrease the distance and increase the interaction between the tip and molecules. As a result, the tip can push or drag molecules across a surface. Figure 4 shows STM-induced motion of Ru2 molecules: after the current increases from 20 to 100 pA (while the bias voltage is kept constant at 1.0 V), the configurations of molecules in Figure 3A changes to those in Figure 3B. Comparing these images, molecule 1 rotates anticlockwise by approximately 60°; during this rotation, one of the Ru centers maintains its position, indicating that the molecule-substrate interaction is governed by the dppe ligands underneath the metal center. In addition to this rotation, molecule 3 translates toward molecule 2. The observation that rotation and translation leave the structure unchanged provides strong confirmation that the paired-dot features do correspond to single-molecule features.66 At this stage, we cannot determine the specific nature of the interaction responsible for molecular manipulation.27,67 However, the results reported here suggest the possibility of using the STM tip to manipulate large dinuclear organometallic molecules on surfaces.

Observation of Single Dinuclear Molecules Using STM Conclusion In this article, we report the use of a pulse-injection technique to deposit the QCA candidate molecule Ru2 on a Au(111) surface. Ultra-high-vacuum STM was used to investigate the molecular structure and imaging properties of the deposited molecules. The isolated molecules were clearly observed with a submolecular “dumbbell”-like structure in high-resolution STM images. Translation and rotation of individual molecules with the STM tip were also demonstrated. This investigation provides a step forward in the characterization of QCA molecules at the molecular level. Acknowledgment. This work was performed under NSF NIRT Grant 0403760. The authors acknowledge Dr. Roger Nassar and Prof. Thomas P. Fehlner, who synthesized the Ru2 molecules used in this study. References and Notes (1) Jortner, J.; Ratner, M. A. Molecular Electronics; Blackwell Science: Cambridge, MA, 1997. (2) Tour, J. M. Molecular Electronics: Commercial Insights, Chemistry, DeVices, Architecture and Programming; World Scientific: Singapore, 2003. (3) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. Y. J. Phys. Chem. B 2003, 107, 6668. (4) Feldheim, D. L.; Keating, C. D. Chem. Soc. ReV. 1998, 27, 1. (5) James, D. K.; Tour, J. M. Chem. Mater. 2004, 16, 4423. (6) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (7) Lent, C. S. Science 2000, 288, 1597. (8) Tseng, G. Y.; Ellenbogen, J. C. Science 2001, 294, 1293. (9) Reed, M. A. Proc. IEEE 1999, 87, 652. (10) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y. X.; Jagessar, R. C.; Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C. W.; Chen, J.; Wang, W. Y.; Campbell, I. Chem.-Eur. J. 2001, 7, 5118. (11) Ellenbogen, J. C.; Love, J. C. Proc. IEEE 2000, 88, 386. (12) Lent, C. S.; Tougaw, P. D.; Porod, W.; Bernstein, G. H. Nanotechnology 1993, 4, 49. (13) Lent, C. S.; Tougaw, P. D.; Porod, W. Appl. Phys. Lett. 1993, 62, 714. (14) Qi, H.; Gupta, A.; Noll, B. C.; Snider, G. L.; Lu, Y. H.; Lent, C.; Fehlner, T. P. J. Am. Chem. Soc. 2005, 127, 15218. (15) Qi, H.; Sharma, S.; Li, Z. H.; Snider, G. L.; Orlov, A. O.; Lent, C. S.; Fehlner, T. P. J. Am. Chem. Soc. 2003, 125, 15250. (16) McCarty, G. S.; Weiss, P. S. Chem. ReV. 1999, 99, 1983. (17) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (18) Kong, J.; LeRoy, B. J.; Lemay, S. G.; Dekker, C. Appl. Phys. Lett. 2005, 86, 112106. (19) Porath, D.; Bezryadin, A.; de Vries, S.; Dekker, C. Nature 2000, 403, 635. (20) Fan, F. R. F.; Yang, J. P.; Cai, L. T.; Price, D. W.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y. X.; Rawlett, A. M.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 5550. (21) Dhirani, A. A.; Zehner, R. W.; Hsung, R. P.; GuyotSionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (22) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (23) Lieber, C. M. MRS Bull. 2003, 28, 486. (24) Zhong, Z. H.; Wang, D. L.; Cui, Y.; Bockrath, M. W.; Lieber, C. M. Science 2003, 302, 1377. (25) Albrecht, T.; Moth-Poulsen, K.; Christensen, J. B.; Guckian, A.; Bjornholm, T.; Vos, J. G.; Ulstrup, J. Faraday Discuss. 2006, 131, 265. (26) Hietschold, M.; Lackinger, M.; Griessl, S.; Heckl, W. M.; Gopakumar, T. G.; Flynn, G. W. Microelectron. Eng. 2005, 82, 207. (27) Moresco, F. Phys. Rep. 2004, 399, 175. (28) LeRoy, B. J.; Kong, J.; Pahilwani, V. K.; Dekker, C.; Lemay, S. G. Phys. ReV. B 2005, 72, 075413.

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