Ionic Liquid Based Approach for Single-Molecule Electronics with

DOI: 10.1021/la503077c. Publication Date (Web): November 4, 2014. Copyright © 2014 American Chemical Society. *Phone: ++44-(0) 151 794 3533. E-mail: ...
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Ionic Liquid Based Approach for Single-Molecule Electronics with Cobalt Contacts Samantha R. Catarelli,† Simon J. Higgins,† Walther Schwarzacher,‡ Bing-Wei Mao,§ Jia-Wei Yan,§ and Richard J. Nichols*,† †

Chemistry Department, University of Liverpool, Liverpool L69 7ZD, United Kingdom H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom § State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China ‡

S Supporting Information *

ABSTRACT: An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane(di)thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions.

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INTRODUCTION

controllable break junctions an ultrathin gold wire or bridge is broken, giving a nanogap into which molecules can then attach. Metal contacts other than gold have been investigated, but to a much lesser extent. Gold is relatively straightforward to use as a contact since it is relatively easy to clean and use under ambient conditions as its surface is oxide-free. Many molecular anchoring groups have been shown to be suitable for attaching the molecules between gold contacts, with thiol, pyridyl, and amine terminal groups being the most widely deployed. Other electrodes have also been investigated with STM-based methods for determining single-molecule conductance, including Pt, Pd, and indium tin oxide (ITO). For instance, Kiguchi et al. have used the in situ BJ method to investigate a series of 1,4disubstituted benzene derivatives with thiol, isocyanide, and amine groups used respectively to attach to the Pt contacts.24,25 As a transparent and conductive oxide, ITO presents a very different substrate type, but it has still been shown to be suitable for single-molecule conductance studies using a scanning probe microscope.26,27 In this case carboxylic acid terminal groups have been used to link to the surface of the ITO. In 2008 Mao et al. introduced a new STM break junction technique which they called the “jump-to-contact” method.28 This approach is based on the electrochemical nanostructuring

A variety of techniques have emerged in recent years for studying the electrical properties of junctions containing a small number of molecules or even single molecules. These have commonly employed a scanning tunneling microscope,1−3 a conducting atomic force microscope,4 or electrical break junctions (BJs).5−10 These techniques have in common that molecule(s) are trapped within the nanoscale gap between two proximal electrodes. Lithographically fabricated nanogaps have also been employed in two-electrode and three-electrode (gated) platforms.11−13 These methods have given great impetus to the field of molecular electronics since they have inspired detailed studies of the physics of charge transport in single-molecule junctions. These methods have also enabled structure−property relationships to be determined for diverse families of molecular wires and bridges.14−16 These singlemolecule techniques have been applied in a wide variety of environments, including ultrahigh vacuum (UHV),17,18 organic liquids,1 aqueous electrolytes,1,2 and also ionic liquids (ILs).19,20 New techniques have also been developed on the basis of these methods, such as the determination of thermoelectric properties of molecular junctions21,22 and simultaneous force−conductance determination.23 By far the most widely studied contact is gold; in the case of scanning tunneling microscopy (STM)-based implementations, this means a gold substrate and gold tip, while for mechanically © 2014 American Chemical Society

Received: August 1, 2014 Revised: November 3, 2014 Published: November 4, 2014 14329

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Ni in which transfer through air prior to alkanethiol adsorption is avoided. Instead, alkanethiol adsorption is achieved directly from the aqueous electrolyte under electrochemical potential control of the Ni substrate.36,37 The resulting alkanethiol multilayer is then rinsed to leave high-quality oxide-free nickel substrates covered by the alkanethiol monolayer. Once formed, these surfaces were seen to exhibit very good resistance to oxidation in air. Hoertz et al. have compared the quality of alkanethiol monolayers formed on cobalt following ambient self-assembly with those formed by self-assembly under an inert atmosphere and by self-assembly on electrochemically reduced surfaces.38 The resulting monolayers were examined by XPS, cyclic voltammetry (CV), contact angle measurements, and surface infrared spectroscopy. Both the electrochemically reduced and the inert-atmosphere- prepared SAM-covered Co surfaces showed low oxide coverage. However, the electroreduced surfaces were deemed superior, because the oxide was found to return more slowly upon exposure of the SAMcovered surface to air. Electrochemical reduction of the Co surface was performed in aqueous electrolytes, and careful control of this step was necessary to avoid etching of the surface.38 For this reason, in the present work, we have chosen to avoid aqueous electrolytes for preparing alkanethiol and alkanedithiol monolayers on cobalt, since in the absence of water the potentially deleterious influence of hydrogen evolution can be circumvented. We achieve both electrochemical reduction of the oxide film on the electroplated cobalt surface and the adsorption of the alkanethiol or alkanedithiol in situ in the ionic liquid, avoiding air contact during these procedures. It is noted that ionic liquids have been shown to promote the formation of more ordered SAMs than traditional solvents.39 It is believed that this arises because of their steric bulk and high viscosity, which impede their intercalation into the forming monolayers and thereby promote a more ordered assembly. Our ionic liquid based approach for single-molecule electronics with cobalt contacts involves a number of steps. First, the (111) textured gold on glass samples are electroplated with cobalt from an aqueous electroplating solution. These are then rinsed and transferred through air to an electrochemical cell containing the room temperature ionic liquid BMIM-OTf (1-butyl-3-methylimidazolium trifluoromethanesulfonate) and a low concentration of the target (di)thiol. Electrochemical reduction of the oxide and adsorption of the (di)thiol are achieved under cathodic polarization in the ionic liquid electrolyte. A 1 h adsorption period was used. These monolayer-coated samples were then characterized using electrochemical methods and surface infrared spectroscopy, as described later in the text. Single-molecule electrical measurements were achieved using the scanning tunneling microscope in an ionic liquid environment.

of electrodes with an electrochemical scanning tunneling microscope, in which metal previously electrodeposited onto the STM tip is transferred to the substrate as the tip is driven toward the surface; this transfer of metal from tip to surface results in a so-called jump-to-contact. This results in small clusters of the electrodeposited metal on the substrate which then, together with the electrodeposit on the STM tip, provide the molecular break junction for single-molecule measurements. This method is well suited to metals which can oxidize under ambient conditions such as Cu, Ag, Fe, Co, Ni, etc., and it provides a convenient capability for extending in situ STM− BJ methods to a wide range of metallic contacts under ambient (electrochemical) conditions. The technique has been used, for example, to measure the single-molecule conductance of succinic acid with Cu and Ag contacts.29 The jump-to-contact technique is of course restricted to the relatively small metallic cluster contacts that are formed during STM nanostructuring, but it nevertheless provides a very powerful platform for singlemolecule electrical measurements. In this study, we demonstrate a complementary electrochemical method in which large-area ferromagnetic substrates can be prepared and subjected to single-molecule electrical measurements. We demonstrate this for cobalt substrates, but our method could be readily applied to other ferromagnetic metals through adaptation of the electroplating protocol. Our new method relies on electroplating of cobalt followed by “passivation” of the substrate by a self-assembled molecular monolayer (SAM), consisting of the target molecule for the subsequent molecular conductance measurements with the scanning tunneling microscope. A key feature is that the molecular monolayer helps to protect the electroplated cobalt substrate from surface oxidation, giving a pristine oxide-free cobalt substrate for single-molecule measurements even under non-UHV conditions. The passivation of easily oxidizable ferromagnetic surfaces with SAMs has been well studied. The term “passivation” is used here since the SAMs protect the surface from oxidation and can thereby provide corrosion protection, which has been viewed as a potential application. The Mekhalif group has published a number of detailed studies on the formation of alkanethiol monolayers on Fe,30 Ni,31−33 and Co.34 The approach of Mekhalif et al. involves first electroreducing the surface oxide on the metal substrate, followed by a rapid transfer of the electrochemically reduced samples from the electrochemical cell to the alkanethiol adsorption solution. In the case of cobalt, relatively concentrated ethanolic solutions of the alkanethiol were found to give the best results.34 They used X-ray photoelectron spectroscopy (XPS) as a primary tool to characterize the monolayers and analyze the chemical state of the metallic surface. In their method the metal is first electrochemically reduced prior to formation of the alkanethiol monolayers through standard self-assembly methods. It should be noted that, even in the absence of full electrochemical reduction of the surface oxide, oxide-free or reduced-oxide-coverage metal may still result after exposure to thiol since the alkanethiol can be oxidized.30,35 A mechanism suggested in the literature for this has been referred to as “chemical cleaning” (of the oxide).30 It is suggested that the alkanethiol is first oxidized to a variety of products which are more weakly adsorbed, and these are subsequently displaced by fresh alkanethiol, ideally resulting in an oxide-free alkanethiol-covered surface.30,35 Bengio et al.36 and Sadler et al.37 have used a method different from that of Mekhalif et al.31−33 for forming SAMs on

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EXPERIMENTAL SECTION

Cobalt Electroplating. Cobalt was electrochemically deposited onto a Au(111) textured slide from an aqueous pH 4.5, 0.2 M CoSO4/ 0.5 M B(OH)3 solution as described in detail in the Supporting Information. Electroplating was achieved potentiostatically and stopped once a charge of 290 mC cm−2 had been passed, and then the slide was rinsed with Milli-Q water only and dried immediately with nitrogen. Assuming 100% faradaic efficiency, this corresponds to a minimum of 500 monolayer equivalents of cobalt on the gold slide. Visual examination confirmed that the deposited film was shiny and cobalt-colored. 14330

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Monolayer Formation. The ionic liquid BMIM-OTf was dried for 18 h under vacuum at 120 °C, after which Karl Fischer titration gave a water content of ∼100 ppm (Karl Fischer titration is an established method for determining the water content of ILs40,41). Thiol or dithiol adsorption was conducted under electrochemical potential control in a 1 mL capacity three-electrode cell with 0.5 mm Pt wire counter and quasi-reference electrodes. The cell was prepared and filled with ionic liquid in an oxygen-free nitrogen-filled glovebox, before being sealed and removed from the glovebox. The reduction of any oxide on the cobalt surface was achieved by cycling the electrode potential between −1.28 and −2.28 V vs Fc/Fc+. The electrode potential was cycled during this process so that any changes in the electrochemical behavior could be observed. The electrode potential was held negative of the oxide reduction potential, at −1.85 V vs Fc/Fc+, for 1 h. The monolayer-coated cobalt sample was then removed from the ionic liquid solution and rinsed for 1.5 min with ethanol to remove any physisorbed alkane(di)thiol. The sample was then rinsed for 30 s with Milli-Q water and dried under a stream of nitrogen before further characterization. Electrochemical Studies. The monolayer-covered cobalt surfaces were studied by CV and impedance spectroscopy with a focus on examining the degree to which they passivate the surface (in other words, the integrity of the monolayer). Details of the surface preparation for the electrochemical studies, drying of the ionic liquids, and preparation of the solutions and the equipment are given in the Supporting Information. Surface Infrared Spectroscopy. Surface infrared spectra of the freshly prepared octanethiol monolayers on cobalt were recorded using polarization modulation infrared reflection−adsorption spectroscopy (PM-IRRAS) using a Bruker IFS 66v/S spectrophotometer. Further details are given in the Supporting Information. Molecular Conductance Measurements. Single-molecule conductance was measured for the octanedithiol (ODT)-functionalized surfaces with a scanning tunneling microscope in BMIM-OTf solutions. These measurements were performed with an Agilent Technologies 2500 or 5500 controller, PicoScan 5.0 software, and a scanner with 10 nA/V sensitivity. Measurements were performed in a nitrogen inert atmosphere in an environmental STM chamber to exclude water and oxygen. The Teflon STM fluid cell was filled with vacuum-dried BMIM-OTf and loaded into the environmental chamber, which was purged for 1 h prior to use. Purging was continued throughout the measurements. The monolayer-covered cobalt samples were prepared as described above. Cobalt STM tips were prepared by electrochemical etching as described in the Supporting Information. Au/octanedithiol/Au junctions were also investigated in control experiments, and experimental details for these measurements are also given in the Supporting Information.

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Figure 1. Cyclic voltammogram recorded for the electrodeposited cobalt film in the in situ oxide reduction/thiol adsorption ionic liquid electrolyte. The electroplated cobalt was transferred to BMIM-OTf containing 10−2 M octanethiol. The first CV cycle between −1.28 and −2.28 V vs Fc/Fc+ exhibited a weak peak at ca. −1.8 V (black curve) vs Fc/Fc+, which was attributed to oxide reduction. On the second scan (red) no peak was seen at this potential. The CV sweep rate was 0.2 V s−1.

end of the sweep the current starts to rise rapidly; this is likely to result from the limit of the usable potential range of the ionic liquid being reached. This ionic liquid has a usable negative limit of around −2 V vs Fc/Fc+ for a gold working electrode before the onset of large cathodic currents. Self-assembled monolayers of octanethiol and octanedithiol were formed on the electroplated Co from 10 mM BMIM-OTf solutions under potential control at −1.85 V vs Fc/Fc+. This adsorption potential is negative of the potential for the reduction of surface oxide on the cobalt surface in the ionic liquid, and it avoids the need for any sacrificial thiol step in which thiol adsorbs, reduces the surface oxide as it is oxidized itself, and is then displaced by fresh alkanethiol. The risk with such a cathodic potential is that the thiol can reductively desorb, leading to poor surface coverage. However, subsequent electrochemical and spectroscopic characterization (quod vide) establish that this is not the case and that a uniform and compact self-assembled monolayer results. Electrochemical Characterization. Electrochemistry provides a means of indirectly characterizing the cobalt film surfaces with and without self-assembled monolayer coverage. Two different experiment types are presented here. The first type of experiment uses cyclic voltammetry to probe electron transfer between the clean oxide-free (electroreduced) cobalt surface and a redox probe dissolved in the ionic liquid solution. This is used to examine whether the electron transfer is well behaved and characteristic of a clean metal surface, thereby verifying the cobalt film preparation and electroreduction and oxide removal in the ionic liquids. Such characterization is performed for N,N′-dimethyl-4,4′-bipyridinium dichloride (“paraquat”) dissolved in the ionic liquid and for ferrocene dissolved in the ionic liquid; the former shows two single electron reduction redox reactions, while the latter exhibits a single oxidation redox reaction. The second type of experiment examines electron transfer between the monolayer-passivated surface and a redox probe in the ionic liquid electrolyte. This experimental type is sensitive to the integrity of the monolayer. A well-ordered monolayer would be expected to act as a barrier to electron transfer for an approaching redox-active molecule in solution, while defective monolayers, especially those with significant pinholes, would not.

RESULTS AND DISCUSSION

Monolayer Preparation. Following electrodeposition of cobalt onto the Au(111) textured slides as described in the Experimental Section, we first attempted monolayer formation on the electroplated cobalt surfaces from dilute aqueous solutions of octanethiol. Evidence for surface damage was apparent as surface discoloration, notably pink-colored spots on the surface. The lack of success with monolayer formation from the aqueous solutions led us to assembly from ionic liquid. Following electrodeposition of cobalt onto the Au(111) textured slides, the samples were transferred to the ionic liquid and thiol adsorption solution. Cyclic voltammograms were recorded following this transfer. The first cycle shows a weak cathodic peak at ca. −1.8 V vs Fc/Fc+ for a voltammetric scan between the limits shown in Figure 1. This peak has disappeared on the second scan and may be attributed to removal of oxide from the surface, although potentials for this process could not be found in the literature. At the cathodic 14331

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the bare cobalt surface (black curve), but obviously very effectively blocked for the monolayer-covered surface. Paraquat and its radical cation are both charged. The ability of the octanethiol-covered cobalt surface to block electron transfer to the somewhat smaller and uncharged ferrocene probe molecule has also been examined. A similar result is obtained, with the Fc/Fc+ redox wave being clearly apparent on the bare cobalt surface in BMIM-OTf, but fully blocked at the octanethiolcovered cobalt surface (see the Supporting Information). An octanedithiol-covered cobalt surface also afforded a high degree of surface passivation (see the Supporting Information). The stability of the octanethiol monolayers on Co upon 1 h exposure to the ambient atmosphere was assessed by first transferring the sample to air and then transferring it back to 5 mM Fc in BMIM-OTf. From this experiment it was seen that the redox waves of Fc/Fc+ did not return, implying that the monolayer is stable to air exposure for 1 h, since extensive oxidation would have been expected to result in pinholes in the monolayer and a resulting voltammetric response. The octanethiol monolayer could, however, be reductively desorbed by potential excursion to very negative values. Clear reductive desorption was not apparent in cyclic voltammetry even on bringing the potential to −2.74 V vs Fc/Fc+. However, when these potential excursions were performed with paraquat in solution, the redox properties of the probe paraquat were seen to return, indicating the removal of the octanethiol monolayer. The passivation properties of the octanethiol monolayer on electroplated cobalt have also been examined by electrochemical impedance spectroscopy (EIS) and compared to those of analogous SAMs formed on gold. Such experiments probe charge transfer across the monolayer film to a redox species in solution. Charge transfer on bare cobalt and gold surfaces is also determined in control experiments. In the case of the monolayer-covered films, a good blocking of the surface by a well-ordered octanethiol monolayer with good integrity will be reflected by high values of charge transfer resistance determined from the data. Such experiments were conducted in solutions of 5 mM ferrocene in BMIM-OTf. Nyquist plots of the data are shown in Figure 4. The Randles equivalent circuit is commonly used to model simple electron transfer at electrode surfaces, with characteristic semicircles featured in Nyquist plots. The semicircle fitted to the data intersects the Z′ axis at two points. The low Z′ intersection, which corresponds to the extrapolation to f → ∞ (f = frequency) gives the uncompensated solution resistance (Ru), while the high Z′ intersection gives Ru + Rct, where Rct is the charge transfer resistance. The semicircle fitting for the Nyquist plots of the monolayer-covered surface would seem to indicate that the mechanism of electron transfer involves a single activation energy process.42 As expected, for both systems Rct is much higher for the octanethiol-monolayercovered gold and cobalt surfaces than for the respective bare surfaces. Since the charge transfer resistance is large, the charge transfer can be considered to be kinetically slow, as would be expected for an electron transfer blocked by a passivating layer on the electrode. Importantly, the Rct value for the octanethiolcovered cobalt surface is even higher than for the gold surface. These EIS measurements have shown the monolayers formed on Co with electrochemical potential control in BMIM-OTf are of at least a comparable quality to those formed on Au by standard self-assembly methods, as they give rise to highly passivating layers as characterized by the higher Rct value for the octanethiol-covered Co samples (10.6 kΩ) than for the equivalent Au system (5.5 kΩ). This further confirms the

Figure 2 shows cyclic voltammograms recorded at different sweep rates for the bare electrodeposited cobalt surface in a 7.5

Figure 2. Cyclic voltammograms of 7.5 mM paraquat in BMIM-OTf solution on the oxide-free cobalt surface. Cyclic voltammograms for sweep rates ranging from 0.1 to 1.0 V s−1 (at 0.1 V s−1 intervals) are shown.

mM solution of paraquat in BMIM-OTf. Any oxide was first removed by cathodic polarization in the ionic liquid. Two clear redox waves are seen corresponding to the first and second reduction processes of paraquat. Equivalent experiments on surfaces where the oxide had not been intentionally electroreduced showed poorly defined peaks which may be attributed to more complex and poorly defined electrochemistry at the oxide-covered surface. By contrast, the two pairs of redox waves seen in Figure 2 are well-defined and attest to the straightforward electrochemical response of the clean metallic surface. The first reduction wave corresponds to the reduction of the paraquat dication to its cation radical, while the second reduction wave corresponds to reduction of this cation radical to the neutral species. The well-known electrochemical reaction scheme for viologen reduction is shown in the Supporting Information. We now turn to the octanethiol-covered cobalt surfaces in ionic liquids and examine how the monolayer is able to passivate the surface. This is analyzed with cyclic voltammetry with a redox-active molecular probe in the ionic liquid solution. Since paraquat was used in the case of the bare cobalt surface, it is also examined here for the octanethiol-monolayer-covered surface in BMIM-OTf. Figure 3 shows this comparison, focusing on the second reduction peak for paraquat. The redox wave corresponding to V•+ to V0 is clearly apparent for

Figure 3. Cyclic voltammograms of bare Co (black) and octanethiolmonolayer-passivated Co (green) in 7.5 mM paraquat in BMIM-OTf, showing the second redox process, V•+ to V0, which is clearly blocked for the octanethiol-covered cobalt surface. Sweep rate 0.5 V s−1. 14332

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Figure 5. Ex situ PM-IRRAS spectrum of an octanethiol monolayer formed on Co under electrode potential control in BMIM-OTf. Spectral collection was optimized at a wavelength of 2850 cm−1, and a background of BMIM-OTf on Co was subtracted from the corrected spectra. Spectral assignments are from refs 48 and 49. Figure 4. Electrochemical impedance spectra (Nyquist plots) for octanethiol on gold and cobalt systems. (a) shows Nyquist plots of bare Au (black points) and octanethiol-monolayer-coated Au (red points) in 5 mM ferrocene in BMIM-OTf. (b) shows Nyquist plots of the bare Co (black points) and octanethiol-monolayer-coated Co (red points) in 5 mM ferrocene in BMIM-OTf. All Nyquist plots have been fitted with a semicircle (solid lines) so that Rct can be determined.

monolayer,48,49 respectively, although due to the nature of the preparation there may be a contribution from BMIM-OTf. Single-Molecule Conductance Measurements. We use the I(s) STM method developed by Haiss et al., which has been widely used to determine the conductance of single molecules adsorbed on surfaces and also molecules assembled in monolayers.50−53 The adaptation by Sek et al.50−52 of this method involves measurement on compact molecular monolayers rather than low-coverage adsorbate films, with the STM tip first being lowered to penetrate into the molecular monolayer. We used the Sek adaptation of the I(s) method to measure the conductance of octanedithiol on cobalt surfaces covered with a full self-assembled monolayer. These measurements were made in BMIM-OTf in a nitrogen atmosphere to further protect the Co surface, since burrowing of the STM tip into the molecular monolayer may cause partial exposure of the surface at that point. Figure 6 shows examples of I(s) curves (I = current versus s = distance) recorded for various sample types, namely, a gold substrate, an ODT monolayer, and a gold STM tip (Au/ODT/ Au, red curves), an electrodeposited cobalt substrate, an ODT monolayer, and a gold STM tip (Co/ODT/Au, green curves),

general quality of the monolayer formed on the electroplated cobalt surface. Surface Infrared Spectroscopy. The octanethiol monolayer formed on cobalt from the ionic liquid was characterized by ex situ surface infrared spectroscopy (PM-IRRAS). Before the ex situ spectra were recorded, the octanethiol monolayer on cobalt was removed from the ionic liquid solution and rinsed thoroughly with ethanol and water. Nevertheless, as well as the expected C−H stretching and bending modes of the octanethiol alkane chain, an additional peak is seen at 1189 cm−1, which can be attributed to an asymmetric C−F stretch (vas(C−F))43 of the ionic liquid anion, OTf−, which is not completely removed in the rinsing process, either on top of the monolayer or intercalated at defects in the monolayer. This would imply that cations also remain on the monolayer even following rinsing to ensure neutrality. It has been proposed that the cation of the ionic liquid, particularly ones like BMIM, can intercalate into the alkanethiol monolayers through their alkyl chains.44−47 The presence of the cation is difficult to ascertain from IR spectra since it has CH peaks in the same location as those of the octanethiol monolayer. Due to the persistence of features from BMIM-OTf in the monolayer spectra, the spectra were referenced to a background spectrum of a cobalt sample prepared and rinsed in the same manner as BMIM-OTf/ octanethiol/Co, but without octanethiol adsorption. The BMIM-OTf/Co reference spectrum was subtracted from the BMIM-OTf/octanethiol/Co spectrum, and the resulting spectrum is shown in Figure 5. There is no peak clearly apparent at 1189 cm−1, indicating that this contribution from the ionic liquid has been largely removed in the background subtraction. Clear peaks are seen around the 3000−2800 and 1500−1350 cm−1 spectral regions which can be attributed to C−H stretching and bending modes of the self-assembled

Figure 6. Example I(s) curves for (a) a junction in which no molecule spans the gap (black curve) and (b) junctions in which octanedithiol spans the gap: red, Au/ODT/Au; green, Co/ODT/Au; magenta, Co/ ODT/Co. I(s) curves are staggered along the x axis. The set-point current is 20 nA, and Vbias = −0.6 V. 14333

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assembled monolayers of ODT on gold and cobalt. These measurements on SAMs in Figure 7 show a broad tail in the conductance histogram to high values, which may be an indication of double and multiple molecular junctions promoting the formation of this high-conductance tail. Nevertheless, the conductance peak is close to that obtained previously for single molecular junctions, which shows that they are more prevalent. The conductance values measured for the three junctions types (Au/ODT/Au, Co/ODT/Au, Co/ODT/ Co) are similar. This may be related to the broadly similar work functions of cobalt (5.0 eV) and gold (5.1−5.47 eV depending on crystallographic orientation)57 and the relatively large gap between the metal Fermi energy and the frontier orbitals of ODT (the frontier orbitals are those molecular orbitals which span the molecular bridge and lie closest to EF of the contacts).

and an electrodeposited cobalt substrate, an ODT monolayer, and a cobalt STM tip (Co/ODT/Co, magenta curves). The plateaus and steps seen in the I(s) curves correspond to stretching and cleavage of the molecular junctions. These curves are recorded by first setting the set-point conditions such that the STM tip penetrates into the molecular film. This is achieved by setting the tunneling resistance parameters of the scanning tunneling microscope to greatly exceed the molecular conductance. The tip is then rapidly withdrawn. Plateaus and steps are seen when molecular junctions are formed and broken during this retraction process. Many of these are collected together in conductance histograms, and these histograms are shown in Figure 7 for Au/ODT/Au, Co/ODT/Au, and Co/

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CONCLUSIONS In summary, we have presented a new method for forming cobalt contacts for molecular electronics measurements. The use of ionic liquids has been central to this study, for providing an environment for preparing oxide-free surfaces and also as a medium for adsorbing the target molecules and performing molecular electronics characterization. Such substrates could find application in future room temperature molecular spintronics studies.

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ASSOCIATED CONTENT

S Supporting Information *

Further experimental details of Co electroplating, electrochemical measurements, surface infrared spectroscopy, and STM molecular conductance measurements and additional cyclic voltammetry data for octanethiol and octanedithiolcovered electroplated cobalt surfaces in ionic liquids with ferrocene redox probes in BMIM-OTf solution. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. Histograms used to determine the conductance of the (a) Au/ODT/Au, (b) Co/ODT/Au, and (c) Co/ODT/Co junctions. All histograms were formed from more than 500 scans. The set-point current is 20 nA, and Vbias = −0.6 V.

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AUTHOR INFORMATION

Corresponding Author

*Phone: ++44-(0) 151 794 3533. E-mail: [email protected].

ODT/Co junctions. These conductance histograms correspond to data recorded at Vbias = −0.6 V, which is within the linear I− Vbias range for alkanedithiols.20 Measuring the peak values of the histograms gives the following respective conductance values: 1.5 nS for Au/ODT/Au, 1.5 nS for Au/ODT/Co, and 1.5 nS for Co/ODT/Co. The conductance histogram peak values at 1.5 nS can be compared to reported literature values for the single-molecule conductance of octanedithiol for the A or low-conductance group, with values of about 1 nS,3,54,55 0.9 nS,56 and 1.2 nS20 reported in the literature. As described in the literature, measurements with the I(s) technique on relatively smooth gold substrates favor observation of the A (low) group, rather than the B (medium) and C (high) conductance groups which are favored for rougher junctions or junctions formed using break junction methods in which there is first metallic contact between the gold STM tip and the substrate followed by cleavage of the metal−metal contact.54 Kay et al. have previously reported a value of 1.2 nS for the low-conductance group of ODT measured in BMIM-OTf, which is consistent with the value reported here given the relatively broad spread of the conductance peak.20 The data reported by Kay et al. were for low-coverage adsorbate layers of ODT on gold, while measurements reported here are for full (high-coverage) self-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) under Grants EP/ H001980/1 and EP/H002227/1 and National Natural Science Foundation of China (NSFC) (Grants 21033007 and 20911130235).

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REFERENCES

(1) Xu, B. Q.; Tao, N. J. J. Measurement of single-molecule resistance by repeated formation of molecular junctions. Science 2003, 301, 1221−1223. (2) Haiss, W.; van Zalinge, H.; Higgins, S. J.; Bethell, D.; Hobenreich, H.; Schiffrin, D. J.; Nichols, R. J. Redox state dependence of single molecule conductivity. J. Am. Chem. Soc. 2003, 125 (50), 15294− 15295. (3) Haiss, W.; Nichols, R. J.; van Zalinge, H.; Higgins, S. J.; Bethell, D.; Schiffrin, D. J. Measurement of single molecule conductivity using the spontaneous formation of molecular wires. Phys. Chem. Chem. Phys. 2004, 6, 4330−4337. (4) 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.

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