Single Molecule Conductance through Rigid Norbornylogous Bridges

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J. Phys. Chem. C 2008, 112, 9072–9080

Single Molecule Conductance through Rigid Norbornylogous Bridges with Zero Average Curvature Wenrong R. Yang,† Michael W. Jones,† Xiulan Li,‡ Paul K. Eggers,† Nongjian Tao,*,‡ J. Justin Gooding,*,† and Michael N. Paddon-Row*,† School of Chemistry, The UniVersity of New South Wales, Sydney, NSW 2052, Australia, and Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State UniVersity, Tempe, Arizona 85287 ReceiVed: March 17, 2008; ReVised Manuscript ReceiVed: March 25, 2008

A new homologous series of norbornylogous (NB) bridges has been synthesized in which the average curvature of the bridges is very small. The molecules are rigid and have two thiol moieties at each end to allow them to form self-assembled monolayers (SAMs) and to connect to two gold electrodes to form a molecular junction. The SAMs formed were characterized using electrochemistry to determine the surface coverage of molecules on gold surface and to provide an indication of the packing of the NB bridges while ellipsometry and X-ray photoelectron spectroscopy (XPS) were used to provide an indication of the SAM thickness and orientation. Single molecule conductance of NB bridges was measured as a function of the molecular length. The conductance was found to decrease exponentially with the length with a decay constant that is exactly correlated with photoelectron transfer and other studies at the multiple molecule level. The molecule-electrode contact resistance was determined and compared with that of related alkanedithiol molecular junctions. 1. Introduction Understanding how electrons are transmitted through single molecules is important in a number of fields including the fabrication of nanoelectronic devices,1,2 sensors,3 and biosensors,4–6 and investigating electron transfer (ET) in covalently linked donor-bridge-acceptor molecules7 and biological molecules.8 Pivotal to the rational design and construction of molecular electronic devices is the need to understand, at a fundamental level, electron transport characteristics of a single molecule, or a small collection of identical molecules, connected to two electrodes.9,10 Many elegant studies have provided important insights into the magnitude and distance dependence of ET dynamics through various organic molecular bridges in metal-bridge-metal (M-B-M) systems. Bridges that have been studied include saturated alkanes,2 oligocyclohexylidenes,11 oligo-p-phenyleneethynylenes, oligo-p-phenylenevinylenes,12–14 and oligo-p-phenylenes.15 Notwithstanding the usefulness of saturated alkane bridges, they suffer from the disadvantage that, being conformationally flexible, measurements on self-assembled monolayers (SAMs) of these bridges carry an element of uncertainty regarding bridge conformation. Moreover, it is difficult to study, unambiguously, the effect of bridge conformation on ET dynamics in these bridges. This is an important issue since it is known from ET studies in solution that changes in bridge geometry can have an order-of-magnitude effect on ET rates.16 There is, therefore, a compelling case for studying M-B-M systems containing a rigid hydrocarbon bridge of well-defined geometry and variable length. An ideal candidate is the norbornylogous (NB) bridge (Chart 1a), on account of its singular role in providing unequivocal confirmation of the importance of the superexchange mechanism in mediating long* To whom correspondence should be addressed. E-mail: justin.gooding@ unsw.edu.au (J.J.G.);[email protected] (N.T.); m.paddonrow@ unsw.edu.au (M.N.P.-R.). † The University of New South Wales. ‡ Arizona State University.

CHART 1

range ET processes in solution and how this mechanism depends on bridge length and bridge configuration.17–22 Initial electrochemical studies on SAMs fabricated from bisthiol-terminated NB bridges, bearing a redox group at the other end (Chart 1b) demonstrated that such bridges, with both thiol feet presumably attached to the gold surface, were very efficient mediators of ET, much more so than single alkyl chain cognates.23,24 That both thiol feet of the NB bridge are, in fact, attached to the gold surface was confirmed by extensive physical characterization of SAMs made from tetrathiol-terminated NB bridges of various lengths, the longest member of the series is shown in Figure 1.25 This study did reveal, however, an unwanted complication with SAMs made from the standard NB bridge, namely, that the innate curvature of the bridges gave rise to SAMs having a mixture of the desired upright and hairpin conformations (Figure 1) and that the amount of hairpinning increased with increasing bridge length. The purpose of this paper is 3-fold. First, we present a new series of NB bridges that possess little overall curvature and,

10.1021/jp802328b CCC: $40.75  2008 American Chemical Society Published on Web 05/28/2008

NB Bridges with Zero Average Curvature

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9073

Figure 1. Profiles of the B3LYP/6-31G(d) optimized geometry of one of the tetrathiols constructed from the original curved NB bridges. This particular molecule is capable of forming SAMs displaying both normal, upright attachment to the gold surface and hairpinning.

therefore, should not produce SAMs contaminated by hairpinning. Second, SAMs fabricated from these new bridges are characterized by a variety of techniques that do, indeed, confirm the absence of hairpinning. Third, charge transfer, at the single molecule level on these SAMs, is investigated from which the distance dependence of conductance with the bridge length is determined. 2. New NB Bridges. The new tetrathiol-terminated NB bridges, 2-4 are shown in Figure 2, together with the shortest member 1, which is also common to the original “curved” series.25 The new NB bridges differ from the original series by possessing an anti,anti-bisnorbornene dimer unit (Figure 2, inset) within the bridge. The two wings of the bridge emanating from this unit necessarily have opposite curvatures, thereby tending to produce small curvature averaged over the entire bridge. This effect is clearly evident in the profiles of the B3LYP/6-31G(d) optimized geometries of 1-4 (Figure 3). In particular, 4 (C18 bridge) displays negligible curvature, which is in marked contrast to the strong curvature displayed by the bridge of similar length (C19) from the old series (Figure 1). Hairpinning in SAMs made from 4 should be absent. A comment on the stereochemistry of the methylenethiol (CH2SH) substituents is warranted. Each pair of terminal substituents are trans with respect to each other, as depicted in Figure 2. However, the synthetic procedure leads to the formation of an approximate 50:50 inseparable mixture of two diastereomers for each tetrathiol, as illustrated by 1 in Chart 2 (each diastereomer is a racemic modification). Consequently, the SAMs derived from 1-4 comprise a mixture of these diastereomers, although it is expected that both diastereomers should bond in identical fashion to the gold surface. However, the two diastereomers might display slightly different chargetransfer characteristics due to the fact that, although both diastereomers possess two gauche arrangements of vicinal C-C bonds in the chains of carbon atoms connecting the distal thiol groups, they are distributed differently between the two chains (see Chart 2). In diastereomer 1a, both gauche arrangements occur in one chain, the other chain possessing an all-trans arrangement of single bonds. In contrast, each of the two chains in diastereomer 1b has a single gauche arrangement. Naı¨ve application of the all-trans rule of through-bond coupling26,27

Figure 2. Structures of the norbornylogous tetrathiols 1-4. Only a single diastereomeric form of each molecule is shown. Cn refers to the number, n, of carbon atoms in each of the two chains of atoms connecting the distal sulfur atoms and m σ-bonds refers to the number, m, of σ-bonds in each chain.

Figure 3. Profiles of B3LYP/6-31G(d) structures of the norbornylogous tetrathiols 1-4, together with the mean direct (through-space) distance, R(S-S), between the two pairs of distal sulfur atoms.

would suggest that, since both 1a and 1b have two gauche conformations in their chains, then the through-bond (i.e., superexchange) coupling should have the same strength in both diastereomers, so both 1a and 1b should display similar chargetransfer dynamics. This reasoning may not be correct, but, even in that case, the coupling strength should be similar in both diastereomers. Finally, the NB frames (minus the CH2SH substituents) of 1, 2, and 4 each has a plane of symmetry bisecting the bridge. Consequently the same SAM is produced, irrespective of which end bonds to the gold surface. This is not the case for 3 because the pair of intrabridge CH2OMe substituents is located closer to one terminus. As a result, it is

9074 J. Phys. Chem. C, Vol. 112, No. 24, 2008 CHART 2

likely that any SAM fabricated from 3 will comprise a mixture of two adsorbents, differing by the gold being bonded to either the proximal or distal bisthiol groups. The relative position of the CH2OCH3 substituent, relative to gold in SAMs formed from 3, should not significantly affect charge-transfer characteristics. Nevertheless, this assertion needs to be verified experimentally, and a new synthetic strategy is being developed in the UNSW laboratory that will lead to the formation of both “proximal” and “distal” SAMs. 3. Experimental Method A. Sample Preparation. Synthetic schemes for the formation of the norbornylogous molecules 2-4 can be found in the Supporting Information. The synthesis of 1 has been described elsewhere.25 Gold substrates were prepared by thermally evaporating 130 nm of gold on mica in an ultrahigh-vacuum chamber.28 Prior to each experiment, the gold substrates were briefly annealed in a hydrogen flame. The self-assembled monolayers of 1-4 were obtained by immersing freshly annealed gold substrates in ultrapure dichloromethane containing 1 µM of 1-4 for 1 h. After thorough rinsing with dichloromethane, the films were transferred to dichloromethane and incubated at room temperature for 24 h. The resulting films were carefully rinsed with dichloromethane and dried using a nitrogen stream before measurements. XPS data were obtained at a takeoff angle of 90° with a VG ESCALAB 220iXL spectrometer (U.K.) that focused monochromatic Al KR X-rays (1486.6 eV) onto the sample with a spot size of approximately 0.6 × 0.3 mm2 at a pressure of less than 10-8 mbar. Ellipsometric measurements were made using Gaertner-Stokes ellipsometer LSE. The He-Ne laser (632.8 nm) light was incident at 70° on the sample. Data were taken on three to four spots in different regions of a given sample and then averaged. An index of refraction (nf) of 1.55 was assumed for all the film thickness calculations. B. Electrochemistry. Voltammograms to obtain the surface coverage of molecules by reductive desorption were recorded in 0.5 M KOH using a Ag/AgCl electrode as the reference electrode and a platinum wire as the counter electrode. The gold substrate was mounted at the bottom of a cone-shaped cell using an O-ring and a clamp. The exposed area of the working electrode was 0.152 cm2 (4.5 mm diameter circle) with average roughness of 1.2. The solution in the cell was deaerated by bubbling H2O-saturated argon for 30 min. Voltammograms were recorded on a BAS 100B electrochemical analyzer. C. Conductance Measurements. The molecular conductance was measured by repeatedly forming and breaking Au point contacts in an environment of molecules with a modified scanning tunneling microscope (STM). The setup and method have been described in detail elsewhere.29 The STM-break junction setup was a modified Pico-STM (Molecular Imaging)

Yang et al. using a Nanoscope IIIa controller. The gold STM tip was prepared by cutting a 0.25 mm gold wire (99.998%, Alfa Aesar). The Teflon STM cell was cleaned with piranha solution and then sonicated in Milli-Q water (>18MΩ) water three times. (Caution: Piranha solution reacts Violently with most organic materials and must be handled with extreme care.) The Au substrate was prepared by thermally evaporating 130 nm of gold (99.999%, Alfa Aesar) on freshly cleaved mica slides (Ted Pella, Inc.) in an ultrahigh-vacuum chamber (∼5 × 10-8 torr).30 Prior to each experiment, the substrate was briefly annealed in a hydrogen flame to remove possible contamination and to form an atomically flat surface and then immediately immersed into NB solution for conductance measurements.31 The conductance measurements were carried out in a solution containing ∼1 µM of molecules in toluene using an Au(111) substrate and Au STM tip. The basic procedures are similar to previous work.32,33 Briefly, the first step was to image the substrate in the regular STM mode. Images showing clear and sharp atomic steps are good indications of a clean substrate and a sharp tip. After surveying the substrate and tip conditions, the tip was parked typically near the center of an atomically flat terrace, and the STM feedback loop was turned off. At the same time a Labview program was used to move the tip into and out of contact with the substrate at a typical rate of 20-40 nm/s. During the contacting process, molecules can bridge the tip and substrate via thiol groups on both ends of a molecule bonding to the gold surfaces. After reaching a preset value, the tip was pulled back until the current dropped to zero. The above processes were repeated automatically at each bias voltage. To cover a wide current range, the current was recorded using two or three scales simultaneously, differing by 2 or 3 orders of magnitude for each current preamplifier, and the measurements were repeated using preamplifiers with different gains. Since the steps in the individual current traces fluctuate in both current value and length, a large number (∼1500) of current traces were collected consecutively without selection and then statistical analyses were performed on the recorded current traces. Three types of transient curves were observed. The first type is a smooth exponential decay due to electron tunneling between the tip and the substrate without an intervening molecule. The percentage of these decay curves was 15-30% when performing the experiment in toluene containing ∼1 µM of a given NB molecules. The exponential decay curves were usually rejected (either not recorded or removed after recording) in the construction of the conductance histogram because these curves contributed a large background peak near zero current (or conductance). The second type of transient curve was noisy and rapidly varying, probably due to mechanical vibrations, acoustic noise, and impurities in the sample cell. The percentage of the noisy curves was around 20-30%, and they were also rejected when constructing the conductance histogram. The remaining 40-50% was curves with clear conductance steps, which were used to construct the conductance histogram. The conductance histograms were constructed by counting the number of times each step occurs and weighting that number by the time duration of the step. Examples of the three types of transient curves and detailed discussions are given in ref 33. 4. Results and Discussion The contact of molecules with substrate is a critical factor in single molecule electrical studies based on SAMs.34–36 As molecules 1-4 were completely new molecules, prior to conducting STM break junction experiments, it was important

NB Bridges with Zero Average Curvature

Figure 4. Reductive desorption peaks for 2 SAMs (black line) and decanedithiol SAMs (red line) coated with Au(111) surface. All recorded in 0.1 M KOH at a scan rate of 100 mV/s.

to characterize the self-assembled monolayers that they formed. This was necessary as with these rigid molecules it cannot be assumed that they behave on the surface in a similar manner to flexible alkanethiol monolayers. Specifically, it was necessary to first ascertain that only one end of the molecule had bound to the flat gold electrode surface and that the thickness of the monolayer scaled with the length of the molecules. This is required as with the previously studied homologous series,25 the thickness of the SAM did not scale with the length of the molecules across the whole series due to some hairpinning of the molecules onto the electrode surface. To address these issues, we used electrochemistry to determine the surface coverage of molecules on the electrode surface and to provide an indication of the packing of the NB bridges, ellipsometry and X-ray photoelectron spectroscopy (XPS) are used to provide information on the SAM thickness, and XPS is used to determine the percentage of hairpinning. A. Reductive Desorption: Monolayer Surface Coverage. The first step in characterizing SAMs formed from NB bridges was to determine the surface coverage of the NB bridges on the gold electrodes. This was achieved by reductive desorption of the NB bridges from the electrode surface. The charge passed during the reductive desorption is related to the amount of thiols bound to the surface. The assumption was made that both thiols on one end of an NB bridge were bound to the electrode and hence two electrons were transferred per NB bridge. Figure 4 shows a typical linear sweep voltammogram (LSV) for the reductive desorption of an NB bridge monolayer for 2 as well as the equivalent LSV for the reductive desorption of decanedithiol adsorbed onto evaporated gold surfaces which are composed predominantly of Au(111). The peak potential (Ered.) for reductive desorption and the calculated surface coverage of NB bridges 1-4 are summarized in Table 1. The surface coverage of NB bridge 2 was (1.71 ( 0.5) × 10-10 mol cm-2. In contrast the surface coverage obtained for decanedithiol was (4.92 ( 0.5) × 10-10 mol cm-2, and the theoretical maximum surface coverage of alkanethiols on an Au(111) surface is 7.6 × 10-10 mol cm-2. Considering NB bridges have two thiol feet and are rigid, the molecules are expected to have a footprint at least twice the area of an alkanethiol and hence the surface coverage for 2 indicates a monolayer or submonolayer of 2 on the surface. STM measurements of the same family of bridges with ferrocene molecules attached to the distal end (not shown) suggest well-ordered monolayers.

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9075 An important observation is the surface coverage of NB bridges decreases slightly, but consistently, as the length increases from 1 to 4. Also correlating with length and surface coverage is a slight anodic shift in the potential required for reductive desorption. The potential of reductive desorption is a reflection of the strength of the monolayer on the gold surface and is related to the energy of the gold-thiol bond and van der Waals forces between chains.37,38 Hence typically the peak potential for reductive desorption shifts cathodically with longer chain alkanethiols38–41 and smoother gold surfaces,42 where chain-chain interactions are maximized. The anodic shift with longer NB bridge is quite surprising but reflects the rigid NB chains not being as closely packed together, and hence weaker van der Waals interactions between the molecules in the SAM, compared with alkanethiols. As the bridges get longer, the van der Waals interactions between molecules become weaker still (consistent with the lower surface coverage with longer bridges). In the case of NB bridges 3 and 4 there is also the possibility of some repulsive interactions between molecules due to the presence of methoxy species on the backbone. Supportive of this proposition that the SAMs of the NB bridges are not as densely packed as alkanethiols comes from the full width at half-maximum (Efwhm) of the reductive desorption peak. A small Efwhm ) 45 mV, such as that observed for decanedithiol, has been interpreted to be the result of interchain attractive interactions between van der Waals-contacting molecules.41 The Efwhm of NB bridge 1 was 100 mV indicative of weaker van der Waals interactions. B. Monolayer Thickness and Orientation. To gain a clearer picture of the SAMs formed from NB bridges, XPS43,44 and ellipsometry were used. High-resolution scans of the S 2p region are shown in Figure 5. For each of the four SAMs of 1-4, sulfur exists in two different states: bound due to two sulfur atoms at one end binding to the gold (BE ∼ 162 eV) and unbound due to the two sulfurs on the distal end of the NB bridges (BE ∼ 163.5 eV). No oxidized sulfur (BE ∼ 168 eV) was observed in contrast to our previous report for a homologous series of curved NB bridges.25 On the basis of the ratio of the area of the two peaks, taking into account the attenuation of photoelectrons through the monolayer from the bound sulfurs, the percentage of hairpinning of the different NB bridges can be calculated. Such a calculation is important since this new series of NB bridges was explicitly designed to prevent hairpinning which was observed for the longest of the curved NB bridges explored previously.25 Table 1 shows that for all four molecules there was virtually no hairpinning which means that for all four molecules in the series the distal thiols are accessible for connecting to the STM tip in breakjunction experiments. The attenuation of photoelectrons by the monolayer, specifically the Au 4f electrons, can also be used to provide an estimate of the thickness of the NB bridge SAMs45,46 according to

I Z ) exp Io λ cos θ

(

)

(1)

where Z is the film thickness, I and I0 are averages of the intensities of the Au 4f5/2 and 4f7/2 peaks after and before monolayer growth, respectively, θ is the angle of photoelectron detection (measured with respect to the surface normal); and λ is the effective attenuation length of the photoelectron through the carbon chain. In these experiments, θ was 45° and the effective attenuation length of photoelectrons was taken to be 34.63 Å.45,46 Monolayer thicknesses were determined by XPS

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TABLE 1: Surface Coverage and Thickness of SAMs of NB Bridges molecules

Ered. (V)

coverage (Γ × 10-10mol cm2)

molecular length (Å)

1 2 3 4

-1.03 -0.95 -0.89 -0.86

1.92 ( 0.2 1.71 ( 0.5 1.55 ( 0.6 1.46 ( 0.9

10 14 19.2 24.2

monolayer thickness by XPS (Å) 11.2 ( 1.6 13.5 ( 2.1 16.0 ( 1.4 21.5 ( 2.0

monolayer thickness by ellipsometry (Å)

tilt anglesa

percent hairpinned (%)

10.2 ( 1.2 12 ( 1.1 16.9 ( 2.4 20.3 ( 2

0 31 28 33

-3.5 -1.5 -3.8 -4.4

a Tilt angle was calculated by average monolayer thickness by ellipsometry and molecular length with optimized geometries by the B3LYP/ 6-31G(d).

Figure 5. High-resolution XPS scans of the S 2p region in SAMs of 1-4, showing the presence of three different types of S atoms: bound (∼162 eV) and unbound (∼163.5 eV) in each SAM.

as shown in Figure 6. The XPS results clearly correlate with the ellipsometric results, and the thicknesses of SAMs for 1-4 determined by ellipsometry are also shown in Figure 6. The thickness of SAMs of 1-4 increase linearly with molecular length as expected for molecules that do not hairpin. On the basis of the thickness of the SAMs of the different NB bridges, as determined by XPS and ellipsometry, and the molecular length, the angle from the normal that the different NB bridges project from the surface can be estimated. For these straight NB bridges 1-4 the tilt angle from the normal to the substrate surface was found to be 0, 31, 28, and 33°, respectively. The tilt angles of 2-4 are consistent with those observed for n-alkanethiolates of 30°,47 but the 0° tilt angle of 1 is quite different. (In our previous study of similar NB bridges,25 the molecular length of 1 is not correlated with geometry.) Note however that an angle of approximately 30° is within experimental uncertainty for 1. By comparing the peak area of S 2p under the bound peak with that of the unbound peak, it is possible to estimate if 1-4 are standing up or lying down. The % hairpinned ) (peak areabound - peak areaunbound)/(total peak area), where peak areabound is corrected for attenuation.25 The small negative value shown in Table 1 indicates that none of the four molecules are hairpinning to any appreciable extent. This is an important result as it indicates the molecular design, with a view to preventing

Figure 6. Average of thickness measurements obtained by ellipsometry and XPS increasing monotonically with molecular contour length for SAMs of 1-4. Error bars represent one standard deviation from the mean. Also shown are the thicknesses of four individual SAM samples (one sample of each chain length) determined by XPS (dots) and ellipsometry (filled squares) for a direct comparison of the thickness between the two techniques. Good correlation is shown between the thickness obtained by XPS and the thickness obtained by ellipsometry.

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Figure 7. (a) Typical individual current traces of 2 with different current window. (b) Conductance histograms of SAMs of 2 built by statistically analyzing individual current traces. Three sets of conductance values are revealed from the conductance histograms, which are marked as HC, MC, and LC. The solid lines are the Gaussian fits. The bias voltage was 0.1 V in the measurement.

TABLE 2: Conductance of NB Bridges Compared to High Conductance (HC), Medium Conductance (MC), and Low Conductance (LC) of Alkanedithiola molecules 1,6-hexanedithiol

HC

92 nS 1.2 × 10-3 G0 1,10-decanedithiol 1.54 nS 2 × 10-5 G0 1 65 nS 8.8 × 10-4 G0 2 3 nS 3.8 × 10-5 G0 3 0.077 nS 1 × 10-6 G0

MC

LC

24 nS 3.1x 10-4 G0 0.3 nS 4 × 10-6 G0 7 nS 9 × 10-5 G0 0.3 nS 3.8 × 10-6 G0 -

2.5 nS 3.2 × 10-5 G0 0.08 nS 1.3 × 10-6 G0 -

a Conductance values are also described in the unit of G0, where G0) 2e2/h ≈ 77.5 µS, with e as the electron charge and h as the Planck constant.

the hairpinning observed with the longest member of the previous generation of NB bridges, has been successful. The lack of hairpinning also means the molecules in the new NB series will be available for forming molecular junctions. C. Charge Transport Measurements. We determine the conductance of NB molecules by performing statistical analysis of the conductance traces showing clear steps. It is notable that more steps are displayed in the conductance traces for NB molecules than is the case with alkanedithiols.33 Figure 7a shows typical conductance transient traces for 2 recorded with different current (or conductance) scales. Statistical analysis of the curves with steps was then carried out by constructing conductance histograms. Figure 7b displays the three conductance histograms for 2 obtained from ∼400 individual conductance curves as shown in Figure 7a. Three sets of conductance peaks are revealed at integer multiples of 3 ( 1, 0.3 ( 0.1, and 0.08 ( 0.04 nS. The error bars are determined from fwhm (full width at half-maximum) of the first peak in conductance histograms. The three sets of conductance values are referred to as HC (high conductance), MC (medium conductance), and LC (low conductance) junctions. The pro-

nounced first peak of each set is attributed to a single molecular junction. Multiple conductance peaks have been reported previously for alkanedithiols.33,48–50 The origin of the multiple conductance steps is not completely understood at this time. Variation in the metal-molecule contact geometry has been proposed as an explanation.33 This explanation is supported by theoretical work by Muller35 who demonstrated that zero-voltage conductance is strongly dependent on the details of the atomic configuration of the electrodes using first-principles quantummechanical TRANSIESTAC calculations. Other theoretical work51 has also found the strong dependence of molecular conductance on the contact geometry. Isometric structure48,49 has been considered as an alternative contribution for the multiple conductance values. Recently, Wandlowski and his coworkers50 carried out systematic experimental studies and quantum chemistry calculations on alkanedithiol molecules. Their work confirms both the existence and interpretation of the multiple conductance values. NB molecules studied in the present work have two thiol groups on each side. Furthermore, they comprise 50:50 mixtures of two diastereomers, which may have behave differently in electron transfer.52 Multiple conductance values due to variations in both molecule-electrode contact geometry and molecular conformation are possible. We then carried out the same measurements on 1 and obtained also three sets of conductance peaks in the conductance histograms, which are located at 65 ( 26 (HC), 7 ( 2.5 (MC), and 2.5 ( 1 nS (LC), respectively. The probabilities of finding MC and LC are similar, but HC occurs less frequently. The ratios of HC, MC to LC for 1 are similar to the corresponding ratios for 2. For 3, we found only the HC value. This is likely due to the molecule being long and their MC and LC values, as extrapolated from the MC and LC values of 1 and 3, are below the noise level of our current preamplifier. The same reason prevents us from obtaining the conductance of 4. The conductance values of each NB molecule are summarized in Table 2; the result from alkanedithiols is also listed as comparison.

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Figure 8. Experimental I-V curves of 2 for both MC (solid squares) and LC (solid circles) from the positions of the peaks in the conductance histograms. Fitting to the Simmons model with R ) 0.5, φ ) 1.83 is given by solid lines. The error bars are determined from fwhm of the first peak in the conductance histograms.

Figure 9. Natural logarithm of conductance versus N (number of carbon atoms in the bridge molecules) for both NB bridges and alkanedithiol. The solid lines are fitting curves based on eq 3 with fixed decay constant βN ) 1. The error bars are determined from fwhm of the first peak in the conductance histograms. The bias voltage was 0.1 V in the measurements.

Current-voltage (I-V) characteristics can provide important insight into the electron transport through molecules. To obtain I-V curves, we measured the current histograms at different bias voltages and plotted the current extracted from the position of the first peak vs bias (solid squares and circles in Figure 8). We have performed nonlinear least-squares fitting of the measured I-V curves for MC and LC conductance values of 2 using the Simmons model, which expresses the tunneling current through a barrier in the tunneling regime of eV < φ, where φ is the barrier height, as

I)

(

qA 4π2pL2

){ (

φ-

(

φ+

[ [

1⁄2

2(2me) qV exp 2 p

)

1⁄2

2(2me) qV exp 2 p

)

(

R φ-

(

R φ+

qV 2

eV 2

1⁄2

)

1⁄2

)

] ]}

s -

L

(2)

where me is the electron mass, A the cross-sectional area of molecules, L the width of the tunneling barrier, φ the barrier

height (relative to the Fermi level of the gold), and V the applied tip bias. Reasonable fits to the data can be obtained with fitting parameters, φ ) 1.83 and R ) 0.5. R is an adjustable parameter which may be interpreted to account for either nonideal tunneling barrier geometry or for an effective mass of the tunneling electrons,25,53,54 which provides qualitative support for nonresonant tunneling as the electron-transfer mechanism. Both φ and R observed here are smaller than those found for decanedithiol SAMs of the same length (φ ) 1.83 and R ) 0.6),33,55 which is expected since both types of bridge consist of the same types of C-C σ and σ* orbitals. Two factors may be responsible for the observed enhanced conduction. First, the NB systems are bound to gold by two thiol groups, each making a separate contribution to the overall strength of the electronic coupling. Second, there is cogent experimental evidence suggesting that electronic coupling through NB bridges is intrinsically stronger than that through single-chain alkanes.19 The good fit of Simmons model with reasonable parameters further indicates the tunneling or superexchange as the conduction mechanism in NB compounds. Another important indicator of the electron tunneling mechanism (superexchange) is the exponential decrease of the conductance with increasing molecular length.56,57 Figure 9 plots HC, MC, and LC values versus molecular contour lengthsrepresented by the number, N, of carbons in the chain(s) connecting the distal thiol groupssfor SAMs of 1-3. HC and MC of alkanedithiol versus molecular length are also plotted as comparison. The error bars are determined from fwhm of the first peak in each conductance histogram. Although there are only two data points for MC and LC, the two conductance values have the same trend as HC. The length dependence of the conductance, G, is well-described by

G ) A exp(-βNN)

(3)

where N is the number of carbon atoms in each of the two C-C chains, A is a constant determined by the molecule-electrode coupling strength, reflecting the contact resistance, and βN is the tunneling decay constant. βN is about 1.0 ( 0.1 or ∼0.80 Å-1 (Table 3). The value is in good agreement with values determined for saturated alkane chains in molecular junctions.21 It is also consistent with the decay constant estimated with the Simmons model,



β)2

2mφ p2

using φ determined from I-V curves. Although we have previously reported the study of charge transport through similar NB bridges on gold using conducting probe AFM with a similar β value,25 the current study unambiguously demonstrates a nonresonant tunneling mechanism at single molecular level. The similarity of values between the norbornyl and alkyl systems suggests that the electronic details of molecular junctions formed from SAMs of each type of molecule (i.e., charge transfer at

TABLE 3: β, the Decay Constant, and A, the Contact Resistance For NB Bridges and Alkanedithiolsa norbornylogous HC av (βNb) av (βb) AN

1.02 ( 0.18 G0

alkanedithiol

MC

LC

0.091 G0

1 0.8 0.032 G0

HC

MC

0.56 ( 0.19 G0

0.12 ( 0.03 G0

Contact resistance A was extracted from eq 3 with fixed decay constant βΝ ) 1. av(βN ) and av(β ) denote the average decay constant per -CH2 unit and angstrom, respectively. a

b

a

a

NB Bridges with Zero Average Curvature the metal-molecule interface, the relative position of the molecular energy levels with respect to the system Fermi level, and the potential for structural rearrangements of these interfaces) are also similar. This tendency is in agreement with conducting AFM measurements. The contact conductance, AN, was also extracted by fitting the length-dependent conductance with eq 3. As shown in Table 3, the HC contact conductance for NB bridges is larger than the corresponding contact conductance of alkanedithiols. Chen et al.58 have measured the conductance of alkane chains terminated with dithiol, diamine, and dicarboxylic acid groups, and found that the contact conductance increases with the binding strengths of the linker groups to Au electrodes. The large contact conductance observed here for NB is consistent with the observation. Furthermore, since NB bridges have two thiols on each side, we may expect that the contact conductance of NB bridges is twice that of alkanedithiols. However, the expectation is based on the assumption that the two thiols take two equivalent binding sites on Au, which cannot be validated at this point. Direct comparison of the MC and LC of NB bridges with that of alkanedithiols is also complicated by the lack of detailed binding geometry knowledge. 5. Conclusions We have synthesized a new series of NB bridges, which are rigid and terminated with two thiol groups on each side for binding to gold electrodes. This series offers a significant advantage over its predecessors25 in that the average curvature of these bridges is very small. Electrochemical reductive desorption measurement indicates that NB bridges form monolayers or submonolayers on a gold surface. Ellipsometry and X-ray photoelectron spectroscopy were used to determine the thickness and orientation of NB SAMs, which demonstrates that our new design provides straight packed and prevents hairpinning configuration. We have measured charge transport through these NB bridges using the STM break junction method. Both the length dependence of the measured conductance and I-V curves indicate that electron tunneling along the NB bridges is the charge-transport mechanism. Three sets of conductance values were observed in the statistical analysis of repeated formed molecular junctions. Each set of conductance values is found to decay exponentially with the molecular length with a similar decay constant, β ) 0.80 Å-1, but a drastically different contact conductance. These multiple sets are attributed to the existence of different molecule-electrode contact geometries and molecular conformations. Acknowledgment. The Australian group gratefully acknowledges funding from the Australian Research Council (ARC), M.N.P.-R. thanks the Australian Partnership for Advanced Computing (APAC) for allocation of computing time. The U.S. group acknowledges support from the National Science Foundation (NSF). We thank Paulo Da Silva for synthetic support (UNSW), Till Bo¨cking (UNSW) for XPS measurements, and Jin He (ASU) for ellipsometry measurements. Supporting Information Available: Experimental details for the synthesis of compounds 1-4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liu, Z. M.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science 2003, 302, 1543–1545. (2) Akkerman, H. B.; Blom, P. W. M.; de Leeuw, D. M.; de Boer, B. Nature 2006, 441, 69–72.

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