Molecular Wire Formation from Viologen Assemblies - Langmuir (ACS

The adsorption behavior of viologen α,ω-dithiols (viologen dithiols) on gold has been investigated. At short exposures, a low-coverage phase consist...
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Molecular Wire Formation from Viologen Assemblies Wolfgang Haiss, Harm van Zalinge, Horst Ho¨benreich, Donald Bethell, David J. Schiffrin, Simon J. Higgins, and Richard J. Nichols* Centre for Nanoscale Science, Chemistry Department, University of Liverpool, Liverpool L69 7ZD, UK Received March 30, 2004. In Final Form: May 19, 2004 The adsorption behavior of viologen R,ω-dithiols (viologen dithiols) on gold has been investigated. At short exposures, a low-coverage phase consisting of flat-lying molecules has been determined by STM and IR spectroscopy. In contrast, multilayer films are formed after long adsorption times. Single molecular wires could be formed between a gold STM tip and a surface with a low coverage of the adsorbed dithiols, and their electrical behavior was investigated. Molecular conductivity was determined either by the repeated measurement of I(s) curves or by recording I-V curves for different tip-sample separations. These methods concurred in producing a value of (0.5 ( 0.1) nS for the single-molecule conductivity of the R,ω-viologen dithiol molecule HS-6V6-SH. The high conductivity of HS-6V6-SH, as compared to that of HS-C12SH, may be related to the low-lying LUMO, which provides a barrier indentation for electron transport in a two-step electron-transfer mechanism.

1. Introduction The assembly of organic molecules into two- and threedimensional networks has received great attention in recent years as an approach for developing molecularbased electronic devices. In this respect, self-assembled monolayers (SAMs) have been widely employed for attaching molecular functionalities to metal surfaces in a well-defined manner. Such structures can also serve as a motif for the fabrication of nano-assemblies with potential applications including electronic, optical, and sensor devices. In particular, R,ω-dithiol SAMs can be used to attach metal nanoparticles to gold surfaces, and these structures can exhibit single electron charging phenomena as observed by scanning tunnelling microscopy (STM).1 This ability of SAMs to act as a linker between surfaces and metal nanoparticles has also been exploited for the measurement of single-molecule conductivity.2-4 A diverse range of molecular functionalities has been incorporated into alkanethiol- and alkanedithiol-based SAMs at particular sites along the alkyl chain allowing chemical, photochemical, or electrochemical control of the behavior of these thin films. The resulting nonlinear electrical behavior has been exploited by sandwiching them between electrical contacts in both micro-machined and nanoparticle-based devices. Using molecular selfassembly, Chen et al. produced micro-machined devices based on nitroaniline SAMs that possess outstanding switching properties.5 Gittins et al. showed that redoxactive groups can be incorporated into SAMs to act as * Corresponding author. E-mail: [email protected]. (1) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (2) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571. (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) Rawlett, A. M.; Hopson, T. J.; Nagahara, L. A.; Tsui, R. K.; Ramachandran, G. K.; Lindsay, S. M. Appl. Phys. Lett. 2002, 81, 3043. (5) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550.

linkers to metal nanoparticles and can offer possibilities for reversible switching of nanoscale structures.6,7 There has been a recent focus on the measurement of single-molecule properties. This has been facilitated by the application of probe microscopy, that allows, for instance, the study of the mechanical properties of singlemolecule polymer strands8 or the conductivity of single atoms, molecules, and nanostructures.2,9-14 These methods, as well as allowing the quantification of the intrinsic conductivity of a single molecule, permit the examination of charge-transport mechanisms and discrimination between single-molecule and ensemble contributions. Such measurements underpin the development of molecularand nanoscale-based electronics. In this Article, we describe an approach for forming molecular wires between a gold surface and a gold STM tip for the determination of single-molecule conductivity. The method is based on the formation of a low-coverage phase of the analyte molecules from which single molecular wires can be readily formed with the aid of an STM tip. Our interpretation for the formation of molecular wires between the gold STM tip and surface is underpinned by surface characterization of the R,ω-viologen dithiol adsorbate. These molecules consist of a 4,4′-bipyridinium moiety, commonly referred to as viologen (V), incorporated into the alkyl chain, which is itself terminated at both ends by thiol groups. The bipyridinium moiety is electrochemically active and can be reversibly reduced between its dication and cation radical state with resulting changes in the electronic (6) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. J. Mater. Chem. 2000, 10, 79. (7) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67. (8) Rief, M.; Gautel, M.; Oesterhelt, F.; Fernandez, J. M.; Gaub, H. E. Science 1997, 276, 1109. (9) Xu, B. Q.; Tao, N. J. J. Science 2003, 301, 1221. (10) Leatherman, G.; Durantini, E. N.; Gust, D.; Moore, T. A.; Moore, A. L.; Stone, S.; Zhou, Z.; Rez, P.; Liu, Y. Z.; Lindsay, S. M. J. Phys. Chem. B 1999, 103, 4006. (11) Haiss, W.; van Zalinge, H.; Higgins, S. J.; Bethell, D.; Ho¨benreich, H.; Schiffrin, D. J.; Nichols, R. J. J. Am. Chem. Soc. 2003, 125, 15294. (12) Datta, S.; Tian, W. D.; Hong, S. H.; Reifenberger, R.; Henderson, J. I.; Kubiak, C. P. Phys. Rev. Lett. 1997, 79, 2530. (13) Collins, P. G.; Zettl, A.; Bando, H.; Thess, A.; Smalley, R. E. Science 1997, 278, 100. (14) Yazdani, A.; Eigler, D. M.; Lang, N. D. Science 1996, 272, 1921.

10.1021/la049183j CCC: $27.50 © 2004 American Chemical Society Published on Web 07/28/2004

Molecular Wire Formation from Viologen Assemblies

Figure 1. Molecular structure of HS-6V6-HS.

structure of the molecule. These changes have been proposed to constitute the basis of a nanoscale redox switch.7 The structure of one of the molecules employed in this study, HS-6V6-SH, is shown in Figure 1 (see Experimental Section for an explanation of the abbreviations employed). This study takes an electrochemical surface science approach by first characterizing the self-assembly of the dithiol viologen SAMs before discussing the formation of molecular wires from a low-coverage adsorbed phase. There is a considerable body of published work dealing with the electrochemistry and structural aspects of film formation for both dialkyl viologens and viologen monothiols, while, in contrast, there is relatively little data available for viologen dithiol SAMs. Viologens have a rich electrochemical behavior that has been widely investigated.15 Three redox states are readily accessible in an aqueous environment. The first redox transition (V2+ f V+•) is fully reversible and occurs at -0.42 V vs the saturated calomel electrode (SCE), while the second redox reaction (V+• f V0) at -0.9 V is quasi-reversible. The cation radical form of N,N′-dialkyl viologens is water insoluble for alkyl chains longer than C4 in the presence of the bromide ion, producing permanent films on electrode surfaces that can be cycled between redox states, although not always fully reversibly.15 Better defined, “self-assembled” viologen thin films have been produced by a number of methods including condensation of long-chain N,N′-dialkyl viologen derivatives at electrode surfaces,16 theirinterdigitationintoalkanethiolmonolayers,17-20 or direct chemisorption of viologen-thiol derivatives.21-25 In addition, multilayered structures have been fabricated in a layer-by-layer fashion from metal nanoparticles and viologen dithiol linkers.6,26,27 Viologen monothiol SAMs have been investigated by a number of methods, including cyclic voltammetry, quartz crystal microbalance, electroreflectance, and infrared and Raman spectroscopy.21-25,28-31 Coverage of viologen monothiol SAMs on metal surfaces has generally been estimated from cyclic voltammetry of the V2+ f V+• reaction.21-25 It has been concluded that fairly close-packed monolayer SAMs are formed, although the polymethylene chains are (15) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49. (16) Bae, I. T.; Huang, H.; Yeager, E. B.; Scherson, D. A. Langmuir 1991, 7, 1558. (17) Creager, S. E.; Collard, D. M.; Fox, M. A. Langmuir 1990, 6, 1617. (18) John, S. A.; Ohsaka, T. J. Electroanal. Chem. 1999, 477, 52. (19) John, S. A.; Kitamura, F.; Tokuda, K.; Ohsaka, T. J. Electroanal. Chem. 2000, 492, 137. (20) John, S. A.; Ohsaka, T. Electrochim. Acta 1999, 45, 1127. (21) Tang, X. Y.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921. (22) Hiley, S. L.; Buttry, D. A. Colloid Surf., A 1994, 84, 129. (23) Delong, H. C.; Buttry, D. A. Langmuir 1992, 8, 2491. (24) Sagara, T.; Maeda, H.; Yuan, Y.; Nakashima, N. Langmuir 1999, 15, 3823. (25) Sagara, T.; Tsuruta, H.; Nakashima, N. J. Electroanal. Chem. 2001, 500, 255. (26) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. Adv. Mater. 1999, 11, 737. (27) Chen, S.; Deng, F. J. Langmuir 2002, 18, 8942. (28) Delong, H. C.; Buttry, D. A. Langmuir 1990, 6, 1319. (29) Tang, X. Y.; Schneider, T.; Buttry, D. A. Langmuir 1994, 10, 2235. (30) Ock, J. Y.; Shin, H. K.; Kwon, Y. S.; Song, S. H.; Chang, S. M.; Qian, D. J.; Miyake, J. Mol. Cryst. Liq. Cryst. 2003, 407, 525. (31) Li, J. H.; Cheng, G. J.; Dong, S. J. Thin Solid Films 1997, 293, 200.

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disordered.22,29 It has also been concluded from SERS and infrared spectroscopic data that the bipyridinium group maintains an upright or tilted orientation, as evidenced by observation of viologen ring stretching modes.29 In these SAMs, the V2+ f V+• electrochemical transition is (quasi)reversible, although strongly anion dependent.22-25,28 This sensitivity of the electron-transfer behavior of surface immobilized viologens to the nature of the electrolyte ions has been taken as an indication of the significance of anion binding to the viologen moiety.22-25,28 In this study, several complementary methods have been used to characterize self-assembled viologen dithiol structures, including electrochemical investigations, scanning tunneling microscopy, ellipsometry, and surface infrared spectroscopy. This combined approach has allowed the determination of adsorption behavior and adlayer structures. It is shown that by controlling the surface coverage single molecular wires can be subsequently formed using a gold STM tip. Single molecular conductivity measurements can then be made on these molecules. 2. Experimental Section 2.1. Compounds Used and Synthesis. The synthesis of the viologen R,ω-dithiols has been described elsewhere.26,32 To avoid excessive use of chemical formulas, these compounds have been abbreviated as HS-xVy-SH, where x and y denote the length of the polymethylene chains and V refers to the bipyridinium (viologen) moiety. For example, HS-6V6-SH (bromide) refers to the compound HS-C6H12V2+C6H12-SH‚2Br-. 2.2. Voltammetry. Cyclic voltammetry was used to measure the formal potential for the V2+ T V+• couple and to estimate the adlayer coverage from charge integration under the redox peak. Although the peak separation for the V2+ T V+• surface redox process depends strongly on sweep rate, the mean peak potential (E0 ) (Epa + Epc)/2) remains almost constant ((5 mV), and this mean peak potential is quoted. Epa and Epc are the anodic and cathodic peak potentials. All potentials are referred to the SCE electrode. All cyclic voltammetry (CV) experiments were carried out using an AutoLab PGSTAT 20 (Eco Chemie, The Netherlands) computer-controlled instrument. 2.3. Ellipsometry. Ellipsometric measurements were conducted with a computer-controlled Gaertner L126 ellipsometer using a helium-neon laser (λ ) 632.8 nm) at an angle of incidence of 70°. The optical constants for the clean flame annealed goldon-glass substrate were measured before each experiment and averaged over four scans, while n ) 1.50 and k ) 0 were taken for the organic film. 2.4. Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). PM-IRRAS spectra were recorded with a Bruker IFS 66v/S infrared spectrometer equipped with a PMA 37 polarization modulation module. Spectra were recorded in a grazing incidence configuration (ca. 85° angle of incidence). 2.5. Sample Preparation. Gold slides (Dr. Schro¨er, Werther, Germany) were used for all experiments. These had been prepared by evaporating approximately 200 nm of gold onto glass slides (Tempax AF45) that had been previously coated, also by vacuum evaporation, with a thin Cr layer (∼2 nm). Prior to use, the goldcoated slides were flame annealed in a Bunsen burner until they showed a slight orange hue. This procedure was repeated several times, and after cooling in air for a short time the slides were quenched in ultrapure water. This treatment is known to produce a flat gold surface with strong Au(111) characteristics.33 Adsorption of the viologen R,ω-dithiols was in all cases from methanolic solution. 2.6. STM and STS Measurements. STM imaging and I-V spectroscopy were performed with a Pico2000 system using PicoScan 4.19 software (Molecular Imaging Corp.). These mea(32) Haiss, W.; Nichols, R. J.; Higgins, S. J.; Bethell, D.; Ho¨benreich, H.; Schiffrin, D. J. Faraday Discuss. 2004, 125, 179. (33) Haiss, W.; Lackey, D.; Sass, J. K.; Besocke, K. H. J. Chem. Phys. 1991, 95, 2193.

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Haiss et al. Table 1. Dependence of the V2+ f V+• Reduction Potential on Electrolyte for HS-11V11-SH (Adsorbed from the Tosylate Salt) on Au(111)a electrolyte (all 0.1 M)

V2+ f V+• reduction potential (vs SCE)

phosphate buffer (pH 7) sodium nitrate sodium perchlorate

-0.393 ( 0.009 -0.436 ( 0.016 -0.485 ( 0.014

a Adsorption was for 5 min from 2 mM methanolic solutions, followed by rinsing with ethanol and Milli-Q water.

Figure 2. Thickness, calculated from ellipsometric data, of a HS-10V10-SH (bromide salt) film adsorbed on Au(111) from a 2 mM methanolic solution. surements were carried out in air. For the STM imaging experiments, STM tips were mechanically fabricated from PtIr wire (80:20 Pt-Ir wire, 0.25 mm diameter) by sharpening them on a polishing wheel and then polishing with increasingly fine alumina powder until a conical tip apex was observed through a high magnification optical microscope. Imaging was performed in constant current mode. The formation of molecular wires and the tunneling spectroscopy measurements were performed with gold wire STM tips to facilitate thiol attachment to the tip. In all cases, Ut refers to the tip potential.

3. Results and Discussion 3.1. Ellipsometry. The dependence of film thickness on adsorption time for HS-10V10-SH (bromide salt) on Au(111) from a 2 mM methanolic solution is shown in Figure 2. Because the optical constants of the film are not precisely known and may vary with film structure and solvent content, the thickness should be taken as an estimate. The film thickness is approximately 1.7 nm after a 10-min adsorption time and increases slightly over the next hour. Very long adsorption times of between 10 and 100 h lead to the formation of rather thick films, up to approximately 6 nm thick. A similar behavior has been observed for HS-11V11-SH (tosylate salt) and HS-6V6SH (tosylate salt), although the C11 derivative produces thicker films than the shorter C6 derivative after long adsorption times. Structural changes during film formation are described in more detail below using IR spectroscopy and STM. 3.2. Electrochemical Investigation of Viologen Dithiol SAMs. For comparison, the electrochemical data for the adsorbed monothiol 12V11-SH is briefly presented, before discussing the electrochemical behavior of the dithiol, HS-11V11-SH. For the tosylate-containing salt of 12V11-SH, coverage was determined by cyclic voltammetry after 5 min, 14 h, and 3 days adsorption. These results showed that a limiting coverage of (2.2 ( 0.5) × 10-10 mol cm-2 was rapidly reached for this monothiol and did not increase with long adsorption times. In comparison, Delong and Buttry23 have obtained a coverage of 3.9 × 10-10 mol cm-2 for 10V10-SH (perchlorate salt), while Tang et al.21 obtained a coverage of 4.5 × 10-10 mol cm-2 for 1V12-SH (nitrate salt). Delong and Buttry noted that the cross-sectional area of the viologen group plus counterions is much greater than that of the alkyl chains (ca. 20 Å2) and therefore the anion would be expected to influence the film structure.23 The coverage obtained with

the tosylate salt is about half that quoted in the literature for the monothiol viologen derivative with Cl-, ClO4-, or nitrate counterions.21,23 Sagara et al. have also quoted low-coverage values in the range (1.8-2.8) × 10-10 mol cm-2 for 5V12-SH with PF6- counterions.24 These results demonstrate that the anion can play a structuredetermining role in monolayer formation and can lead to dramatically lower coverage, which is particularly apparent in the case of the relatively bulky tosylate anion. The behavior of the viologen dithiol derivatives is markedly different from that of the viologen monothiol derivatives. For the latter, cyclic voltammetric and ellipsometric data concur in the conclusion that a monolayer film is formed, with no evidence for multilayer formation even after lengthy adsorption times. In the case of 12V11SH, a layer thickness of approximately 3 nm was estimated from ellipsometry, consistent with the formation of a monolayer of thiols in an upright orientation. On the other hand, the ellipsometric data presented in the previous section for the viologens dithiol clearly point to multilayer film formation after long adsorption times. This conclusion is supported by voltammetric measurements for a HS12V11-SH (tosylate salt) SAM, for which a coverage of 1.7 × 10-10 mol cm-2 is estimated after 5 min of adsorption, which doubles after 15 h of adsorption. As well as affecting the assembly and coverage of adsorbed layers, it is recognized that anion binding to the viologen moiety strongly influences the first reduction potential of monothiol viologen films.22,24,25A similar effect has also been noted for viologen dithiol, as summarized in Table 1 for three different electrolytes. This table presents data for the tosylate salt of HS-11V11-SH adsorbed on Au(111) from a 2 mM methanolic solution. The voltammetric data were recorded for adsorption times of 5 min, which is sufficiently short to avoid multilayer formation. Following adsorption, cyclic voltammetry measurements were conducted in 0.1 M phosphate buffer (pH 7), sodium nitrate, and sodium perchlorate to assess whether the anion has an effect on the voltammetric response of the viologen dithiol monolayer. It has been well-established from voltammetric and quartz crystal microbalance studies that there is anion ingress/egress during the redox reaction of viologen monothiols.23,24,28 For instance, it has been shown that F- in the monolayer can be completely exchanged by PF6- within a few seconds.24 Therefore, although the viologen was adsorbed from its tosylate salt, reduction and oxidation in the voltammetric experiments of the adsorbed layer in the aqueous electrolyte will lead to replacement of tosylate by the electrolyte anion. Indeed, such anion replacement is verified by the voltammetric results presented in Table 1. Previous studies of monothiol viologen adsorbates have linked the strong dependence of the V2+ f V+• reduction potential to the strength of anion binding to the viologen moiety.22,24,25 The V2+/V+• reduction takes place at increasingly negative potentials as the interaction of the anion with the V2+ moiety increases.16,17 It has been further

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Figure 3. STM images of a Au(111) surface after various immersion times in a 2 mM methanolic solution of HS-10V10-SH. (a) and (b) 300 s, (c) 10 min, and (d) 24 h.

argued that anions which bind their solvation shells least tightly (generally softer anions) will form the strongest ion pairs with the V2+ group, while those that bind their solvation shells most tightly will form the weakest ionpairing. Indeed, Hiley and DeLong22 and Sagara et al.25 have observed that generally softer ions show more negative formal potentials, indicating stronger ion-pairing with the viologen. On the other hand, harder ions or more strongly hydrated anions generally display less negative reduction potentials, which has been taken as an indication of weaker ion-pairing with the viologen moiety. The values shown in Table 1 also indicate a similar trend. ClO4- has a notably smaller hydration energy, leading presumably to stronger electrostatic interactions through ion-pairing and a consequently relatively more negative V2+ f V+• reduction potential. 3.3. Scanning Tunneling Microscopy. Cyclic voltammetry can be readily used to estimate the coverage obtained with both viologen monothiol and dithiol SAMs, but it cannot give direct structural information about the SAMs and cannot be readily used to assess whether the assembly proceeds through ordered intermediate phases. STM can be used to image adlayer structures for different immersion times and in the case of viologen dithiol SAMs has shown the presence of intermediate low-coverage adlayers (vide infra). Scanning tunneling microscopy was used to image adlayers for different immersion times in 2 mM methanolic solutions of viologen dithiols. Short attachment times of typically less than 300 s for HS-10V10-SH (bromide salt) gave STM images with a characteristic striped appearance

(Figure 3a and b). The stripes in this phase, which run roughly horizontal in Figure 3b, have a periodicity of (6 ( 1) nm, while the vertical corrugation of the structure is approximately 1 nm. Because the HS-10V10-SH molecule has a sulfur-to-sulfur distance of nearly 3 nm, each strip would be consistent with a pair of HS-10V10SH molecules aligned flat on the substrate surface. A longer attachment time of 10 min leads to the appearance of the high contrast features in the STM image presented in Figure 3c. This is taken as an indication of the reorientation of the phase to a higher-coverage adlayer. Figure 3d shows an image after 24 h of immersion. The surface morphology is markedly different from the images acquired for short attachment times. Resolution in this case is limited, perhaps as a result of the conformational mobility of the alkyl chains in the multilayer film or the dynamics of the anions present within the film. Note that the IR data presented later on point to conformational disorder in the methylene chains which could account for the limited resolution. However, some molecular resolution could be achieved, with the feature separations being consistent with the imaging of molecules standing upright. In contrast to the bromide-containing salts, SAMs formed from the HS-11V11-SH and HS-6V6-SH tosylates do not form ordered structures that can be imaged by STM. The observation that the STM results show an ordered phase with the bromide ion, but no such ordered phase for the tosylate anion, demonstrates the importance of the anion in the formation and stabilization of the lowcoverage structures. Indeed, more strongly coordinating anions have also been shown to affect markedly the

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Figure 4. PM-IRRAS spectra of HS-10V10-SH on Au(111) for 300 s (a) and 48 h (b) adsorption times from 2 mM methanolic solutions. For comparison, a spectrum was also obtained by the slow evaporation of a droplet of a 0.1 mM HS-10V10-SH solution (in methanol) to give a 10 nm film (c).

molecular and crystallographic structures of dialkylbipyridinium compounds.34,35 3.4. Polarization Modulation Infrared Spectroscopy. Figure 4 shows polarization modulation infrared spectra (PM-IRRAS) for HS-10V10-SH (bromide salt) adsorbed on Au(111) at different adsorption times. These spectra correspond to the low-coverage phase (Figure 4a) formed after 5 min of immersion and higher-coverage films formed after 24 h of immersion (Figure 4b). For comparison and in contrast to the spectra for the self-assembled monolayers, a spectrum was also obtained of a Au(111) surface on which a droplet of a 0.1 mM HS-10V10-SH solution in methanol had been evaporated is shown in Figure 4c. The film thickness of this sample was approximately 10 nm as determined by ellipsometry. The most prominent peaks in Figure 4c in the 18001000 cm-1 spectral region are located at 1644, 1514, 1497, 1465, 1244, 1180, and 1040 cm-1. A number of spectroscopic bands associated with vibrational modes of the bipyridinium rings fall within this region.36,37 The fact that the bands contain information from more than one mode precludes a detailed quantitative orientational analysis in this case. The 1645 and 1180 cm-1 bands have been assigned to the asymmetric in-plane bipyridinium ring vibrations (B2u and/or B3u modes in D2h symmetry).37,38 These modes are clearly present in the 10 nm film (Figure 4c). In contrast, these bands are absent in the low-coverage phase (Figure 4a). The surface selection rule indicates that in the low-coverage phase the plane of the bipyridinium ring lies close to parallel to the gold surface. A similar conclusion is made from spectra for short immersion times for adsorption of the tosylate salt of the viologen dithiol (data not shown), where additional spectroscopic bands are observed in the mid-IR arising from the tosylate anion. The methylene C-H stretching modes can be clearly observed in all three spectra in Figure 4, and these bands increase in intensity for higher coverage. For all spectra, (34) Polishchuk, I. Y.; Grineva, L. G.; Polishchuk, A. P.; Chernega, A. N. Zh. Obshch. Khim. 1996, 66, 1530. (35) Grineva, I.; Krainov, I.; Polishchuk, A.; Tolmachev, A. Mol. Cryst. Liq. Cryst. 1992, 211, 397. (36) Ghoshal, S.; Lu, T. H.; Feng, Q.; Cotton, T. M. Spectrochim. Acta, Part A 1988, 44, 651. (37) Hester, R. E.; Suzuki, S. J. Phys. Chem. 1982, 86, 4626. (38) Osawa, M.; Yoshii, K. Appl. Spectrosc. 1997, 51, 512.

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νa(CH2) lies in the range of 2927-2929 cm-1, while νs(CH2) falls between 2854 and 2856 cm-1. The positions of these aliphatic C-H stretches provide a measure of the degree of order in the methylene chains. These peak positions are known to depend on the degree of crystallinity of the methylene chain environment, which has been characterized most extensively for self-assembled alkanethiol monolayers on Au. As discussed by Porter et al.,39 the peak position for νa(CH2) of a crystalline polymethylene chain (2920 cm-1) is 8 cm-1 lower than that for the liquid state (2928 cm-1). Likewise, νs(CH2) of a crystalline polymethylene chain (2850 cm-1) is lower than that of the liquid (2856 cm-1). Comparison of these values with those for the spectra presented in Figure 4 reveals that the methylene chain environment is “liquidlike”. This implies that there is a considerable degree of conformational disorder in the chains. This is not surprising given the relatively larger cross-sectional area of the viologen group and of the counterions (>40 Å2) as compared to that of the alkyl chains (ca. 20 Å2).23 Indeed, Hiley et al. have made the same observation for monothiol viologen SAMs on the basis of the methylene stretching region, concluding that the viologen monolayer is more disordered than simple alkanethiol monolayers.22 It is concluded that such conformational disorder is apparent in both low-coverage viologen dithiol SAMs, as well as for higher-coverage phases and multilayer structures. The appearance of a band at 1040 cm-1 is also noted (Figure 4b and c) which is absent in the low-coverage phase (Figure 4a). This band does not correspond to a vibrational mode of the viologen,40 but is consistent with the C-O(H) stretching mode of methanol, implying that methanol is incorporated into the higher-coverage films during assembly. This is not surprising given the lack of close packing of the methylene chains and their conformational disorder. 3.5. Molecular Wire Formation. Figure 5 shows an STM image of the low-coverage phase of HS-10V10-SH in which the tunneling resistance was changed during imaging. The image was acquired with the slow scan direction running from top to bottom. In the center of the image, as marked by “arrow 1”, monoatomic steps of Au(111) (0.24 nm height) are visible after the tunneling resistance is decreased by 1-2 orders of magnitude. In the lower part of the image, the tunneling resistance has been increased back to its original value (arrow 2), and the adsorbate layer is visible again. This shows that lowering the tip toward the surface can promote strong tip-adsorbate interactions. This property was used as the basis for the formation of molecular wires. To achieve the best chance of obtaining single molecular wires between the Au(111) substrate and the gold STM tip, the concentration of the derivatization solution was reduced to 5 × 10-5 M and the attachment time was reduced to 100 s. As demonstrated below, these conditions enabled the formation of single wires with a high probability. Measurements were performed on HS-6V6-SH (iodide salt) because this gave conductivity values that could be easily addressed at low noise within the dynamic range of the current amplifier employed. Current-distance (I(s); s ) relative tip-sample distance) spectroscopy measurements were performed under the conditions schematically depicted in Figure 6. To attach a molecule to the tip, the tip was lowered toward the surface by fixing the tunneling current I0 at relatively high values and then lifting it while keeping a constant (39) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (40) Regis, A.; Corset, J. J. Chim. Phys. Phys.-Chim. Biol. 1981, 78, 687.

Molecular Wire Formation from Viologen Assemblies

Figure 5. STM image of the low-coverage phase of HS-10V10SH on Au(111) formed after 300 s immersion time in a 2 mM methanolic solution. The image was scanned with the slow scan running from top to bottom. The upper arrow marks the point at which the tunneling resistance was decreased by 1-2 orders of magnitude. Following this, in the center of the image, monoatomic steps of Au(111) (0.24 nm height) are visible. The lower arrow marks the point at which the tunneling resistance was increased to its original value and the adsorbate layer is visible again.

position in the x-y plane. The current decay was found to follow two distinctive forms as illustrated in Figure 7. Curve A shows a fast exponential decay typical of electron tunneling between a metal STM tip and the surface. On the other hand, curve B shows a markedly different response with a much slower decay of the tunneling current, over several nanometers, followed by a current plateau. As was discussed previously, this behavior has been related to electron tunneling through molecular wires bridging the STM tip and the substrate surface.11 At sufficiently large tip-sample displacements, the current decreases to zero as the chemical contacts joining the molecular wires to the STM tip and the surface are broken. In reaching these conclusions, it is useful to compare the control experiment (Figure 7, curve A, no wires present) with the slower decay observed in the presence of molecular wires (Figure 7, curve B). This comparison

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discounts the idea that the response observed could arise from faradaic or ionic processes in the thin water layer that may be present on the surface in air. Similar arguments may also be used to interpret I(s) curves recorded in electrolyte solution in the presence of molecular wires.11 3.6. Single-Molecule Conductivity Measurements. The markedly nonexponential decay of the current in the presence of molecular wires, as featured in Figure 7b, has been characterized by two parameters, the current plateau value, Iw, and the distance, s1/2, for which the current falls to half its value at the plateau (I ) Iw/2). The dependence of Iw on s1/2 is shown in Figure 8 for some 100 I(s) scans taken at different locations of the sample. Each I(s) scan is represented by a vertical bar that indicates its corresponding s1/2 and Iw values. The average s1/2 value was (2.4 ( 0.6) nm. Taking a realistic estimate of s0, this places the end of the plateau at a tip-substrate separation of approximately 2.5 nm. Molecular modeling shows that the distance between the two sulfur cores is 2.4 nm for trans-oriented methylene chains in HS-6V6-SH. This observation supports the assumption that the molecules are in an upright position prior to bond breaking as shown in the schematic illustration in Figure 6. Figure 8 shows that the current plateau values form distinctive groups of events, labeled GR1, GR2, and GR3. It can also be noted that GR1 and GR2 events are more probable than GR3 for these experimental conditions. Iw for GR1 events cluster around (98 ( 16) pA, while GR2 and GR3 events occur at integer multiples of this value. As previously discussed, the lowest conductivity unit has been assigned to conduction through a single molecule, while GR2 and GR3 are assigned to conduction through two and three molecules, respectively.11 From these results, the conductivity of a single molecule (σM) at Ut ) 0.2 V was determined as (0.49 ( 0.08) nS. For comparison, the conductivity of HS-6V6-SH was measured by using a variation of the gold cluster method developed by Cui et al.2 In this “matrix isolation” method, R,ω-dialkanethiols were incorporated at relatively high dilution into an alkanethiol matrix on Au(111). The dithiol molecules were assumed to form thiol-bonded chemical contacts to both the Au substrate and gold clusters adsorbed on top of the matrix, while the monothiols could not chemically bind to the Au clusters. Conducting AFM was used to measure the current response of these “wired”

Figure 6. Schematic illustration of the STM method of forming molecular wires. A low coverage of the analyte molecule is formed on the Au(111) surface, and the set-point current is increased (A). Attachment of the molecule at one end to the Au STM tip is achieved, and the tip is then retracted from the surface while recording the current (B). At sufficiently large tip sample displacements, chemical contacts joining the molecular wires to the STM tip and the surface are broken (C).

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Figure 7. Current decay curves for a clean Au(111) substrate (A) and HS-6V6-SH on Au(111) in air (B).

Figure 8. A plot of s1/2 versus Iw for about 100 I(s) scans taken at different locations of the sample, obtained from I(s) scans using a Au STM tip. Each I(s) scan is represented by a vertical bar which indicates its corresponding s1/2 and Iw values. Data obtained for HS-6V6-SH on Au(111) substrate, with adsorption for 100 s from a 5 × 10-5 M methanolic solution. Iw is the current plateau value, and s1/2 is the distance for which the current falls to half its value at the plateau (I ) Iw/2).

assemblies, and discrete current steps corresponding to conduction through one, two, three.... dithiol molecules were observed.2 In the present work, the “matrix isolation” technique was employed using an STM rather than a conducting AFM. Similar to Cui et al.,2 current values were recorded on top of isolated Au clusters adsorbed on Au(111) covered with a mixed monolayer of HS-6V6-SH and hexanethiol (HT). For these “matrix isolation” experiments, the mixed monolayer (“matrix”) was formed from a solution containing 10 mM HT and 0.005 mM HS-6V6-SH to ensure sufficient dilution (2000:1) of the analyte molecule in the monolayer. Several experiments with different ratios of dithiols to monothiols in methanolic solutions were performed before it was decided to use this dilution. The experiments with different dilutions showed that if the dithiol/monothiol ratio was 1: 100, the whole surface was covered with Au nanoparticles after the sample was immersed into the nanoparticle solution directly following a brief exposure to the dithiol/monothiol-containing solution (1 min of immersion followed by thorough rinsing with methanol). However, if the dithiol/monothiol ratio

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Figure 9. An I(s) spectrum obtained by the method of Cui et al.2 for an isolated gold nanoparticle (Au-NP) immobilized on a mixed monolayer (“matrix isolation” method) on Au(111). The mixed monolayer was formed by 100 s immersion in a methanolic solution of 10 mM hexanethiol + 5 × 10-6 M HS6V6-SH (iodine salt), which was then blown dry in a stream of nitrogen. The Au-NPs, (6 ( 1) nm in size, were then adsorbed on the monolayer by 15 min of immersion in 18 nM Au-NP solution in toluene, and the electrode was then dried in a nitrogen stream. The tunneling spectroscopy settings were as follows: Ut ) -0.2 V, I0 ) 10 pA; scan duration 20 ms; the scan was performed in the downward direction. Iw values determined from such curves taken on different clusters are shown in the histogram in Figure 10a.

was 1:2000 (10 mM hexanethiol + 5 × 10-6 M HS-6V6SH), the surface was partially covered with Au nanoparticles when the sample was immersed into the nanoparticle solution directly after a brief exposure to the dithiol/ monothiol-containing solution (1 min of immersion followed by thorough rinsing with methanol). Isolated gold nanoparticles could be found routinely on these samples, and these isolated nanoparticles were then used for the conductivity measurements. It is also important to note that clustering of like molecules in the two-component monolayer was found if the samples were kept in air for 24 h after the immersion in the dithiol/monothiolcontaining solution and prior to the immersion in the nanoparticle solution. Samples immersed in the dithiol/ monothiol-containing solution for 24 h exhibited groups of gold nanoparticles and large uncovered regions after immersion in the nanoparticle solution. This indicates clustering of like molecules in the monolayer. These samples were unsuitable for conductivity measurements because they exhibited large conductivities resulting from the attachment of many dithiolated molecules. Following the formation of the mixed thiol matrix and disperse adsorption of Au nanoparticles on this monolayer, the conductivity measurements were performed. As the STM tip was lowered toward the cluster, the current attained a limiting value, which did not significantly increase as the tip approached further. This was then taken to signify the point at which the tip touches the cluster. An example of an I(s) spectrum recorded by this method is presented in Figure 9. A limiting current value is attained upon contact between the STM tip and Au cluster from which the molecular conductivity was calculated. Similarly, in the results reported by Cui et al.,2 the conductivity was found to fall into discrete groups, the lowest of which was taken as the conductivity of a single HS-6V6-SH molecule within the HT matrix. A histogram of current values obtained by this “matrix

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Figure 10. (a) Histogram of Iw values determined from I(s) curves taken on gold nanoparticles chemically attached to matrix isolated 6V6 molecules on Au(111) (see, for example, Figure 9). (b) Histogram of Iw values determined from I(s) spectra in which HS-6V6-SH molecular wires were formed directly between the tip and the surface.

isolation” method is shown in Figure 10a. For comparison, a histogram showing Iw values obtained from the STM method of forming HS-6V6-SH molecular wires directly between the STM tip and surface is shown in Figure 10b. Both cases are characterized by groups of current values in the histogram, the lowest denomination of which is taken to represent conduction through a single HS-6V6SH molecule. From these histograms, the single-molecule conductivity of HS-6V6-SH determined by the “matrix isolation” method is (0.56 ( 0.03) nS, while that obtained by direct “wiring” between the Au tip and surface is (0.49 ( 0.08) nS. The similarity between the single-molecule conductivity values obtained for these two techniques is reassuring. Perhaps surprisingly, the molecular wires formed by appending single molecules between the surface and the STM tip are sufficiently stable to allow current-voltage spectroscopy measurements (I-V curves) at different values of s, that is, for different tip-sample separations. Figure 11a shows the results of such measurements. This figure shows a typical I-V curve taken in the plateau region, and the resulting potential dependence of the integral and differential conductivities are shown in Figure 11b. A clear potential dependence of the differential conductivity on the applied voltage is observed. Using the I-V method, we could determine the conductivity for Ut ) 0 (σM0) for different values of s from the dependence of σM on Ut. The dependence of σM0 on s is shown in Figure 11c. In addition, the value of σM0 measured for events in which no molecular wire was present are shown in this figure. These results are very similar to those obtained from the direct measurement of the current flowing through individual molecular wires (Figure 7). We now turn to an analysis of the conductivity values measured for HS-6V6-SH. The measured conductivity of a single 6V6 molecule at 0 V (0.43 ( 0.04) nS can be compared to that of a single dodecanedithiol molecule (0.12 ( 0.01) nS determined by Cui et al. using the “matrix isolation” method.41 This comparison shows that the conductivity of a single 6V6 is considerably more than that of a single dodecanedithiol molecule. This implies that conduction through the HS-6V6-SH molecule is mediated through the bipyridinium group, which can be considered as a moiety present at the center of a dodecanedithiol chain. The bipyridinium group has a lowlying LUMO close to the Fermi level that actually corresponds to the V2+/V+• redox level of the molecule. In a simple barrier penetration model of conduction, this

Figure 11. (a) Typical I(V) spectrum of a single HS-6V6-SH molecule taken at a tip sample displacement (s - s0) of 2.1 nm. The data were averaged over seven data points. (b) Differential conductivity (dI/dUt) calculated from the data in (a) (upper curve in (b)); the integral conductivity (I/V) is also shown (lower curve in (b)). (c) Conductivity at Ut ) 0 V plotted against the tipsample separation for the case of a single HS-6V6-SH molecular wire (B) and in the absence of a molecular wire (n).

(41) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Nagahara, L. A.; Lindsay, S. M. J. Phys. Chem. B 2002, 106, 8609.

level could be described as providing a barrier indentation that facilitates electron transport across the molecule.

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4. Conclusions We have presented a study of the behavior of adsorbed films assembled on Au(111) from viologen R,ω-dithiols. A number of complementary characterization methods have been used to define the adlayers, including electrochemical investigations, scanning tunneling microscopy (STM), ellipsometry, and surface infrared spectroscopy. In the case of low coverages, it is demonstrated that single molecular wires of viologen R,ω-dithiols can be formed between a gold STM tip and surface and the electrical properties can be interrogated in this configuration. The major results of this study are summarized below: The adsorption behavior of the R,ω-viologen dithiols is more complex than that of the monothiol analogues that have been previously studied. A low-coverage phase has been identified by STM and IR spectroscopy, in which viologen R,ω-dithiols form a dimer row structure in which the bipyridinium group is oriented roughly parallel to the surface. After long adsorption times, multilayer films are produced, possibly through the formation of disulfide bridges. As for monothiol viologens, the V2+ f V+• reduction potential for the viologen R,ω-dithiols is dependent on the counterion and follows the trend E(V2+ f V+•) perchlorate < nitrate < phosphate. More negative reduction potentials have been related to stronger anion-viologen ion-pairing. The methylene chains of the viologen R,ω-dithiols are in a conformationally disordered state, with νa(CH2) and νs(CH2) values typifying those of methylene chains in a liquidlike environment. This conformational disorder in the chains is not surprising given the relatively larger cross-sectional area of the viologen group plus counterions as compared to that of the alkyl chains.23 There is also spectroscopic evidence for the incorporation of methanol during the self-assembly process. The disordering or disturbance of the low-coverage phase is observed as an STM scanning tip approaches the surface. This strong tip-adsorbate interaction is used to form molecular wires bridging the surface and Au STM tip, where the molecule is presumably bonded by thiol-Au chemisorption at these two “contacts”.

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Single molecular wires could be formed with high probability between the Au(111) substrate and the gold STM tip for reduced adsorption times and concentrations of attachment solutions. The I(s) curves corresponding to molecular attachment between the STM tip and sample were characterized by a markedly nonexponential decay, showing a current plateau that reached to about the length of the fully extended configuration of the analyte molecule. The current plateaus for the I(s) scans were seen to form distinctive groups in histogram plots. The lowest group has been assigned to conduction through a single molecule. The conductivity of a single HS-6V6-SH molecule was determined as (0.49 ( 0.08) nS. This was checked using the method developed by Cui et al., which gave a value of (0.56 ( 0.03) nS, demonstrating the similarity of the values obtained by the two methods. The molecular wires formed by appending single molecules between the surface and STM tip are sufficiently stable to allow current-voltage spectroscopy measurements (I-V curves) at different values of s, that is, for different tip-sample separations. The single-molecule conductivity values obtained by the I-V method are very similar to those obtained from the direct measurement of the current flowing through individual molecular wires. The relativity high conductivity of HS-6V6-SH, when compared to HS-C12-SH, shows that the bipyridinium moiety plays a major role in electron transport through the molecule. In a simple model, the bipyridinium group, with its low-lying V2+/V+• redox state, can be envisaged as a barrier indentation available for the mediation of electron transport in a two-step mechanism. Acknowledgment. This work was carried out under the Nanoscale Switch-Project supported by EPSRC, grant GR/R07684/01. H.H. would like to thank the European Union for the award of a Socrates grant. H.v.Z. would like to thank the European Union for support under the SUSANA project. LA049183J