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2007, 111, 7561-7564 Published on Web 05/10/2007
Origin of Current Enhancement through a Ferrocenylundecanethiol Island Embedded in Alkanethiol SAMs by Using Electrochemical Potential Control Yasuyuki Yokota,†,‡,§ Ken-ichi Fukui,† Toshiaki Enoki,*,† and Masahiko Hara‡,| Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan, Local Spatio-Temporal Functions Laboratory, Frontier Research System, RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan, and Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8502, Japan ReceiVed: March 21, 2007; In Final Form: April 30, 2007
Electroactive ferrocenylundecanethiol islands embedded in an n-decanethiol SAM matrix were studied under potential control using in situ scanning tunneling microscopy (STM). Contrary to previous reports, the positive charges on ferrocene moieties are not a prerequisite to enhancing the current through ferrocenylundecanethiol on gold. Rather, conduction paths are opened at determined potentials for both neutral and monocationic ferrocene moieties. In addition, although stable and reproducible images were obtained under potential control, conventional STM measurements under N2 atmosphere were sometimes unstable and irreversibly changed the sample, indicating that current measurements of ferrocenylundecanethiol are much easier under an electrochemical environment.
Electroactive molecules have attracted much attention as a component of molecular devices because their well-defined and readily accessible redox states are thought to nonlinearly change the resistance of metal-molecule-metal junctions.1 Since Gorman et al. have reported negative differential resistance (NDR),2,3 some groups have studied ferrocenylalkanethiol on gold to elucidate underlying mechanisms by using scanning probe microscopy. Tivanski et al. have shown that positive charges injected into the ferrocene moiety are responsible for the high conducting state of the junctions, thus invalidating the simple model of resonant tunneling proposed by Gorman et al.4 He et al. have reported that the high conducting charged state is electrochemically generated (though the details are unknown) and is irreversibly destroyed by ambient oxygen at high bias voltage.5 However, controversial issues remain: Why does the positive charge enhance the current? Why did Tivanski et al. not observe NDR at negative sample bias? Because these experiments were performed in ambient or nonpolar solvents without potential control, the oxidation state of the ferrocene moiety is unknown. As a result, the junctions are not well-defined and the experimental observations are inconsistent and irreproducible. We therefore think that electronic characterization with precise potential control must be performed. * Corresponding author. E-mail:
[email protected]. Tel & Fax: +81-3-5734-2242. † Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology. ‡ Local Spatio-Temporal Functions Laboratory, Frontier Research System, RIKEN (The Institute of Physical and Chemical Research). § Present address: Surface Chemistry Laboratory, RIKEN, Wako, Saitama 351-0198, Japan. | Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology.
10.1021/jp072249+ CCC: $37.00
CHART 1: Molecular Structures Used in This Work
In the present paper, we show that positive charges are not a prerequisite to enhancing the current through ferrocenylundecanethiol (FcC11H22SH) on gold. Rather, conduction paths are opened at determined potentials for both neutral and monocationic ferrocene moieties. The electronic properties of the islands were studied with and without potential control by using in situ scanning tunneling microscopy (STM) and conventional STM, respectively. Because it is well-known that FcC11H22SH self-assembled monolayers (SAMs) exhibit redoxinduced orientational changes,6,7 we fabricated the FcC11H22SH islands embedded in shorter C10H21SH SAMs to reduce the structural degree of freedom in the alkyl chain part (Chart 1).8 FcC11H22SH was purchased from DOJINDO Laboratories, Japan. An evaporated gold film on mica with a (111) oriented surface was used as the substrate. Au(111) substrates were annealed in a butane flame and immersed in 1 mM C10H21SH ethanol solution for at least 24 h. Full-coverage C10H21SH SAMs were rinsed with pure ethanol and dried with N2 gas. The FcC11H22SH islands were fabricated by immersing full-coverage C10H21SH SAMs into 0.1 mM FcC11H22SH acetone solution for 10-40 min, rinsing with pure acetone, and drying with N2 gas. For cyclic voltammetry measurements of full-coverage FcC11H22SH SAMs, freshly annealed Au(111) substrates were immersed in 0.1 mM FcC11H22SH acetone solution for at least 12 h, rinsed with pure acetone, and dried with N2 gas. The © 2007 American Chemical Society
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Figure 1. Cyclic voltammograms of full-coverage FcC11H22SH SAMs on Au(111) electrodes in a 0.05 M HClO4 solution at scan rates of 0.1 (blue), 0.05 (red), and 0.02 V/s (black). The vertical dotted lines and the upward arrows indicate the sample and tip potentials, respectively, at which in situ STM measurements were performed. Three peaks, P1, P2, and P3, originate from the different microenvironments of the ferrocene moiety.
experiments of cyclic voltammetry and in situ STM measurements have been described previously.9 Both measurements were performed in N2 atmosphere. All electrochemical potentials are presented with respect to the Au/AuOx reference electrode. Figure 1 shows typical cyclic voltammograms of full-coverage FcC11H22SH SAMs in a 0.05 M HClO4 solution. A redox wave due to the one-electron oxidation processes (Fc T Fc+) appears at about -0.6 V. This peak position is within the ranges of the redox potentials of ferrocene SAMs reported previously.10 The peak current was found to be proportional to the scan rate, indicating a surface-wave response. The peak shape of the present study is slightly different from the previous reports.6,7,11-14 According to the recent systematic study by Lee et al., P1 and P3 in Figure 1 are assigned to ferrocene moieties in “isolated” and “clustered” states, respectively.15 Although P2 has not been reported, to the best of our knowledge, its origin can be deduced easily from the previous studies of SAM-modified electrodes, for example, the reductive desorptions of the n-alkanethiol SAMs. There is a consensus that the multiple peaks observed in the reductive desorptions at polycrystalline electrodes are derived from the difference of the adsorption sites of the n-alkanethiols.16-18 Briefly, because cyclic voltammograms taken at the polycrystalline electrodes can be regarded as the superposition of the (111), (100), and (110) surfaces, the multiple peaks are thought to be a reflection of the different local potential of zero charge (in other words, work function, the differences are a few hundred mV).19 Hence, we tentatively assigned P2 to the redox peak that originated from the local (100) or (110) sites with different effective potentials due to the use of quasi (111) electrodes. Systematic studies using
Letters single-crystal electrodes will easily elucidate this origin, but it is not the scope of this paper. It is noted that the redox states of FcC11H22SH islands during in situ STM measurements were deduced from these cyclic voltammograms. We believe that the microenvironment of the ferrocene moieties of FcC11H22SH islands resembles that of P3 due to the clustered feature, but it does not significantly affect the following results. Figure 2 shows typical in situ STM images of FcC11H22SH islands embedded in C10H21SH SAMs in a 0.05 M HClO4 solution (positive sample bias). The typical island size is 3-12 nm (an island is indicated by a circle). As shown clearly, the apparent height of FcC11H22SH islands against C10H21SH SAMs was reversibly changed with the oxidation state of the ferrocene moieties. In the case of Fc+ (Figure 2C), the islands were observed as protrusions of ∼0.5 nm, whereas they were imaged as depressions in the case of Fc (Figures 2A and 2A′). The former happens to be close to the physical height difference between FcC11H22SH and C10H21SH (0.5 nm; calculated using standard bond lengths and angles and assuming a 30° tilt of the molecules on the Au substrate). At near formal potential (Figure 2B), where the oxidation state of the ferrocene moieties fluctuates between Fc and Fc+, the apparent height was ∼0.2 nm. The apparent height change of the islands was reversible against potential changes (Figure 2A′). This oxidation-statedependent behavior seems to be consistent with the results of Tivanski et al.4 and He et al.5 When the current direction was changed by changing the tip potential to -0.3 V (negative sample bias), however, the series of apparent height contrasts was inverted, as shown in Figure 3. For example, although the oxidation states of ferrocene moieties were Fc+ in both Figures 2C and 3C, the apparent height was inverted (protrusions and depressions, see also Figures 2A (2A′) and 3A (3A′)). The STM contrast is generally influenced by the geometric structure and electronic state, but these results indicate that the contrast is not derived from the orientational changes of the islands, but from the electronic origin. Furthermore, our results were not consistent with the previous model of positive charge-induced high conducting state because the apparent height was enhanced at a neutral state of the ferrocene moieties (Figures 3A and 3A′). It should be noted that the apparent height is not correlated with the island size, irrespective of the oxidation state. As such, the intermolecular conduction path (i.e., hopping or electrochemical exchange reaction between ferrocene moieties) is not a major conduction mechanism for this system. This is quite contrary to the case of tetrathiafulvalene (TTF) derivative islands (TTF derivatives have been known as typical organic conductors), where the tunneling current was enhanced by intermolecular conduction path.20 We therefore think that the conduction carrier does not stay in the ferrocene moiety for
Figure 2. Typical in situ STM images in the region of 137 × 137 nm2 for FcC11H22SH islands embedded in C10H21SH SAMs at a tip potential of -0.9 V. The potentials of the sample were (A) -0.8, (B) -0.6, (C) -0.4, and (A′) -0.8 V, respectively. These images were consecutively obtained in the order of left to right. The tunneling current was 20 pA.
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Figure 3. Typical in situ STM images in the region of 156 × 156 nm2 for FcC11H22SH islands embedded in C10H21SH SAMs at a tip potential of -0.3 V. The potentials of the sample were (A) -0.8, (B) -0.6, (C) -0.4, and (A′) -0.8 V, respectively. These images were consecutively obtained in the order of left to right. The tunneling current was 20 pA.
Figure 4. Relationship between the electrochemical potential of the STM tip, the ferrocene moiety, and gold substrate. The electrochemical potential of the ferrocene moiety is regulated at a formal potential (-0.6 V). Arrows indicate the direction of possible electronic transport and do not represent the energy decay for actual paths.
enough time to travel to other ferrocene moieties but pass one electrode to another via the energy level of the ferrocene moiety by fast processes. Tran et al. have reported that the electrochemical exchange reaction between electroactive moieties tethered in each electrode leads to current amplification under potential control.21 In their Hg/SAM//SAM/Hg system, the electroactive SAMs must be tethered in both Hg electrodes to amplify the current by electrochemical exchange reactions. They suggested that electron transfer between an electrode and its electroactive moieties on the same electrode is always faster than electron transfer between an electrode and the redox centers on another electrode. This is not consistent with our result because the relative rates of electron transfer depend on the tip and sample potentials, as mentioned later. Different microenvironments and/ or different alkyl chain lengths may lead to the different mechanisms of the electron transfer. Figure 4 shows a schematic diagram of the proposed mechanism according to our observation. The electrochemical potential of the ferrocene moiety is regulated at a formal potential by the reference electrode. As can be seen in the diagram, our results suggest that the uphill electron transfer is forbidden and that only electrochemically allowed paths conduct the electron. It should be noted that the electrochemically allowed paths do not require the positive charge in the ferrocene moiety (Figure 4b). Again, the paths do not mean a two-step electrochemical reaction (between the tip and the ferrocene moiety, and the ferrocene moiety and the substrate).22 In such a case, reorganization of the surrounding water molecules raises or lowers the energy levels of the ferrocene moiety, making the forbidden paths available. Direct current measurements with controlled STM tip position (i.e., electrochemical distance tunneling spectroscopy as well as voltage tunneling spectroscopy) will enable us to discuss more quantitatively.23
Figure 5. STM images (68 × 68 nm2) in N2 atomosphere for FcC11H22SH islands embedded in C10H21SH SAMs. (a) Initial state. (b) Typical STM images taken under unstable STM imaging. (c) After unstable imaging, there were irreversible contrast changes in the FcC11H22SH islands. Typical irreversible change is indicated by a circle. The tunneling current and sample bias were 10 pA and 1.0 V, respectively.
We performed STM measurements in N2 atmosphere to compare the measurements with and without potential control. Figure S1 shows STM images of FcC11H22SH islands at positive biases. The apparent heights of the FcC11H22SH islands against C10H21SH SAMs were increased with the sample bias, as reported by Gorman et al.3 It is obvious from the comparison of Figures 2C and S1A (same bias) that the potential control amplifies the electronic conduction through FcC11H22SH islands by modulating the electronic levels of ferrocene moieties. In the case of negative sample biases, the same kinds of images were obtained (Figure S2). These results indicate that the current amplification occurs irrespective of potential control, although the oxidation state is unknown in the case of N2 atmosphere. It is noted that although stable and reproducible images were obtained under potential control, STM measurements under N2 atmosphere were sometimes unstable during a usual experimental duration (∼1 h) and irreversibly changed the apparent height of the islands, as shown in Figure 5, where the imaging quality of the underlying C10H21SH SAMs did not change (we definitely observed these phenomena in several experiments). This might be caused by a trace amount of water and/or oxygen, as systematically studied by He et al.5 Despite a lack of direct evidence, we believe that these irreproducible phenomena lead to inconsistent results between different groups. Finally, we
7564 J. Phys. Chem. C, Vol. 111, No. 21, 2007 emphasize that STM measurements under potential control are much easier than those under N2 atmosphere. In summary, we have demonstrated that a positive charge is not a prerequisite to amplifying the STM current through electroactive FcC11H22SH islands and that current measurements of electroactive molecules under potential control are much easier than those under N2 atmosphere. We believe that these results are key factors in designing molecular devices based on the use of the electroactive molecules. Acknowledgment. We thank Prof. T. Kakiuchi and Prof. M. Yamamoto for fruitful discussions. This work was financially supported by Grants-in-Aid (No. 15073211, No. 17034014, and 21st Century COE Program “Creation of Molecular Diversity and Development of Functionalities”) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Y.Y. thanks RIKEN for the JRA fellowship. Supporting Information Available: STM images taken in N2 atmosphere. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (2) Gorman, C. B.; Carroll, R. L.; Fuierer, R. R. Langmuir 2001, 17, 6923. (3) Wassel, R. A.; Credo, G. M.; Fuierer, R. R.; Feldheim, D. L.; Gorman, C. B. J. Am. Chem. Soc. 2004, 126, 295. (4) Tivanski, A. V.; Walker, G. C. J. Am. Chem. Soc. 2005, 127, 7647.
Letters (5) He, J.; Lindsay, S. M. J. Am. Chem. Soc. 2005, 127, 11932. (6) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (7) Yao, X.; Wang, J.; Zhou, F.; Wang, J.; Tao, N. J. J. Phys. Chem. B 2004, 108, 7206. (8) Yokota, Y.; Miyazaki, A.; Fukui, K.; Enoki, T.; Tamada, K.; Hara, M. J. Phys. Chem. B 2006, 110, 20401. (9) Yokota, Y.; Miyazaki, A.; Fukui, K.; Enoki, T.; Hara, M. J. Phys. Chem. B 2005, 109, 23779. (10) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335. (11) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (12) Rowe, G. K.; Creager, S. E. Langmuir 1994, 10, 1186. (13) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 290. (14) Valincius, G.; Niaura, G.; Kazakevicˇiene˘ ; B.; Talaikyte˘ , Z.; Kazˇeme˘ kaite˘ , M.; Butkus, E.; Razumas, V. Langmuir 2004, 20, 6631. (15) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438. (16) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (17) Yoshimoto, S.; Yoshida, M.; Kobayashi, S.; Nozute, S.; Miyawaki, T.; Hashimoto, Y.; Taniguchi, I. J. Electroanal. Chem. 1999, 473, 85. (18) Arihara, K.; Ariga, T.; Takashima, N.; Arihara, K.; Okajima, T.; Kitamura, F.; Tokuda, K.; Ohsaka, T. Phys. Chem. Chem. Phys. 2003, 5, 3758. (19) Lipkowski, J.; Stolberg, L.; Yang, D.-F.; Pettinger, B.; Mirwald, S.; Henglein, F.; Kolb, D. M. Electrochim. Acta 1994, 39, 1045. (20) Yokota, Y.; Fukui, K.; Enoki, T.; Hara, M. J. Am. Chem. Soc. In press. (21) Tran, E.; Rampi, M. A.; Whitesides, G. M. Angew. Chem., Int. Ed. 2004, 43, 3835. (22) Alessandrini, A.; Corni, S.; Facci, P. Phys. Chem. Chem. Phys. 2006, 8, 4383. (23) Schindler, W.; Hugelmann, M.; Hugelmann, Ph. Electrochim. Acta 2005, 50, 3077.
