Single Molecular Switches: Electrochemical Gating of a Single

Article pubs.acs.org/JPCC

Single Molecular Switches: Electrochemical Gating of a Single Anthraquinone-Based Norbornylogous Bridge Molecule Nadim Darwish,† Ismael Díez-Pérez,‡,§ Shaoyin Guo,‡ Nongjian Tao,*,‡ J. Justin Gooding,*,† and Michael N. Paddon-Row*,† †

School of Chemistry, The University of New South Wales, Sydney, NSW, 2052 Australia Centre of Bioelectronics Biosensors, Biodesign Institute, Arizona State University, Tempe, Arizona, 85287, United States § Department of Physical Chemistry, University of Barcelona & Institute for Bioengineering of Catalonia, Barcelona 08028, Spain ‡

S Supporting Information *

ABSTRACT: Herein we report the electrochemical gating of a single anthraquinone-based molecule bridged between two gold electrodes using the STM break-junction technique. Once a molecule is trapped between the STM gold tip and the gold substrate, the potential is swept in order to alternate between the oxidized anthraquinone (AQ) and the reduced hydroanthraquinone (H2AQ) forms. It is shown that the conductance increases about an order of magnitude with a net conversion from the oxidized AQ form to the reduced H2AQ form. The results obtained from sweeping the potential (dynamic approach) on a single molecule are compared to those obtained from measuring the conductance at several fixed potentials (static approach). By comparing the static and dynamic approach, qualitative information about the kinetics of the redox conversion was achieved. The threshold potential of the conductance enhancement was found to shift to more negative potentials when the potential is swept at a single molecule. This shift is attributed to a slow redox conversion between the AQ and the H2AQ forms. The hypothesis, of slow redox kinetics being responsible for the observed differences in the single-molecule conductance studies, was supported by electron transfer kinetics studies of bulk self-assembled monolayers using both cyclic voltammetry at different sweeping rates and electrochemical impedance spectroscopy.

1. INTRODUCTION There is intense ongoing interest in understanding charge transport through single molecules, both from a fundamental point of view and for applications as nanodevices.1−7 The commonly used technique for measuring the conductance of a single molecule is the scanning tunneling microscopy (STM) break junction technique8,9 in which an STM tip is brought in and out of contact with molecules adsorbed on gold substrates. During the contact process, the adsorbed molecules can bridge the tip and the surface electrode via the thiol linkers at both ends of the molecules. Plateaus appear in the current vs distance profile. These plateaus are attributed to a single molecule event, and statistics are built by breaking and reforming the junction thousands of times. Grouping the measured conductance step values into histograms gives the most frequently observed molecular conductance.8 An important goal in molecular electronic devices is to control the electron transport process through a single molecule. This can be achieved using redox molecules that can be switched between high-conducting and low-conducting states using electrochemical gating in which a counter electrode and a reference electrode act as the “gate” for the tunneling current.10−12 The other two electrodes, the STM tip and the © 2012 American Chemical Society

gold surface, act as contacts to the molecules and represent the source or the drain of a single-molecule device. We have recently demonstrated the operation of an electrochemically controlled single-molecular switch in a rigid anthraquinone-based norbornylogous bridge (5AQ5, Figure 1), with a conductance on/off ratio of an order of magnitude.13 This difference in conductance between the two redox states is attributed to destructive quantum interference effects14 operating in the cross-conjugated anthraquinone (AQ) form which is absent in the linear-conjugated reduced hydroanthraquinone (H2AQ) form.13,15−18 In the H2AQ form, the direct conjugation pathway in the three aromatic rings is not broken, while, in the AQ form, the linear conjugation is broken by the CO groups. This is because the two aromatic rings in the AQ form are linearly conjugated to the CO groups but not to each other. The approach used in that previous study was to build up histograms at different gating potentials.13 When the potential is fixed at positive potentials vs Ag, 5AQ5 is in its oxidized form. As the potential is shifted to more negative Received: July 4, 2012 Revised: August 24, 2012 Published: September 11, 2012 21093

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Figure 2. (a) Conductance histograms of 5AQ5 at different gating potentials. (b) Evolution of the conductance of 5AQ5 with changing gate potential. Figure 1. Sketch diagram of the electrochemical switching of 5AQ5 to 5H2AQ5. 5AQ5 is a completely rigid molecule possessing five sigma bonds on each side and an AQ moiety in the center.

potentials, the conductance increases and reaches an order of magnitude higher conductance when the 5AQ5 is reduced to the more conducting 5H2AQ5 form.13 The static approach, however, is limited to determining the conductance at only few fixed gating potentials, making it difficult to perform a dynamic analysis of the switching behavior. The purpose of this paper is to demonstrate dynamically the switching of a single 5AQ5 molecular junction. In order to achieve this goal, once a step was detected in the transient current curves, the tip was held still while the gate voltage was swept to produce source-drain current vs gate voltage characteristics of a single 5AQ5 molecule, which can be translated into conductance vs gate (G−V) curves. Unlike the fixed potential approach, this method sweeps the potential on a single molecular bridge. The G−V curves were then compared to the “static potential” approach. This comparison suggests the kinetics of the conversion between the AQ and H2AQ forms is a rate limiting process in the dynamic single molecule measurements. The single-molecule measurements were then compared to those obtained from SAMs formed from 5AQ5 using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Figure 3. (a) Typical current trace for 5AQ5; the red dot represents the position where the individual G−V curve was measured and (b) corresponding G−V curve. The potential was swept in both positive and negative directions.

for the forward scan and the monotonic decrease in conductance for the reverse scan indicate that the change in conductance can be attributed to a net conversion between the low conducting 5AQ5 and the high conducting 5H2AQ5 forms. When comparing the static potential approach with the dynamic G−V curves of single 5AQ5 molecules, both methods showed approximately a similar magnitude of the increase in conductance. The conductance increases from (2.5 ± 2.0) × 10−4 Go at +300 mV to (6.2 ± 5.0) × 10−3 Go at −700 mV. This order of magnitude increase is attributed to the molecule being in its high-conducting reduced form (5H2AQ5) at −700 mV, whereas, at +300 mV, the lower conducting oxidized form (5AQ5) prevails. As a control experiment, G−V curves obtained in the absence of a bridged 5AQ5 molecule showed no change in the conductance with the potential sweep (see Figure S3 of the Supporting Information). In the individual G− V curves, the potential sweep cycle was performed at 4000 mV/ s. This specific high sweep rate was chosen because the lifetime of a single 5AQ5 molecule bridged between the two electrodes is relatively short. We have estimated a lifetime of ca. 0.5 s for a single 5AQ5 molecular bridge using a blinking technique (see the Supporting Information). Scan rates slower than 4000 mV/ s led to breakage of the junctions before a complete potentialsweep cycle. Scan rates higher than 4000 mV/s were not attempted, as at higher potential sweeps the required potential window would exceed the potential window over which the gold−thiol bonds are stable. The maximum conductance value in the G−V curves is reached at ca. −700 mV, about 300 mV more negative than that reached with the static potential method (ca. −400 mV). This

2. RESULTS AND DISCUSSION Figure 2a shows the conductance histograms using the static gating approach. When the potential is fixed at +300 mV vs Ag, at which the 5AQ5 molecule is in its oxidized form, the conductance is (2.4 ± 1.2) × 10−4 Go. When the potential is changed to more cathodic potentials of −400 mV, the conductance increases and reaches a value of ca. (3.2 ± 1.0) × 10−3 Go. In order to study the dynamics of the electrochemically gated conductance, a different approach to that used for the previous histogram analysis was implemented. When a step is detected in the current trace curves, the tip position was held fixed and the current was recorded while sweeping the substrate potential versus the reference electrode. Figure 3a shows a typical current trace for 5AQ5 with a plateau observed at ca. 2 × 10−4 Go, and Figure 3b presents the corresponding G−V curve. The G−V curves are rather noisy, but the current change with the potential is reproducibly observed (Figure 4). The monotonic increase in conductance 21094

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Figure 4. Typical G−V curves obtained by recording the source-drain current while sweeping the gate voltage versus the reference electrode in 0.5 M phosphate buffer, pH 3.

discrepancy can be attributed to the slow kinetics of the redox conversion between 5AQ5 and 5H2AQ5 such that the observed reduction of the 5AQ5 is shifted to more negative potentials due to the fast changes in the applied voltage. Another indication of kinetic effects that can be inferred from the individual G−V curves is the presence of hysteresis between the forward and the reverse scan. Although there is variation in the degree of hysteresis between the individual G−V curves (Figure 4), the hysteresis appears to be centered around −300 mV which is a potential close to the redox formal potential of the 5AQ5 obtained from cyclic voltammetry (CV) and AC voltammetry of a self-assembled monolayer formed from 5AQ5 (see the Supporting Information). This suggests that both the reduction step (forward scan) and the oxidation step (reverse scan) are slow processes. This conclusion is supported by CVs performed on a SAM formed from 5AQ5 at different scan rates, as illustrated by the CVs at 5 and 4000 mV/s, shown in Figure 5. It is important to note here that, as these are single molecules, there is naturally considerable variation between single molecule measurements. If quantitative kinetic data could be extracted from single-molecule measurements, it would require many hundreds of these scans over a range of scan rates. More stable surface constructs are required before these measurements could be performed over the range of scan rates required. The reduction wave in the CV at 4000 mV/s ends at ca. −750 mV. This potential is close to the potential where the maximum value of conductance was observed in the individual G−V curves (−700 mV, Figure 4). However, with a slow sweep rate of 5 mV/s, the reduction wave ceases at around −500 mV which is closer to that observed for the maximum value of

Figure 5. CVs at (a) 5 mV/s and (b) 4000 mV/s of a SAM formed from 5AQ5 in 0.5 M phosphate buffer, pH 3.

conductance obtained from the static potential approach (−400 mV, Figure 2b). These results suggest that, in the static potential approach, the system is given enough time to respond to the applied potential, while sweeping the potential at high scan rates on a single molecule leads to a delay in the conductance increase. The slow kinetics of the redox reaction was further confirmed using electrochemical impedance spectroscopy (EIS). By applying a potential equal to E1/2 to the electrode, the redox-active SAM is forced to be at equilibrium. 21095

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(dynamic approach) is compared to conductance measurements at several fixed potentials (static approach), the potential range of the conductance enhancement is shifted to more cathodic potentials. This shift is attributed to the kinetics of the redox reaction between the AQ and H2AQ forms. The slow kinetics of an ensemble of many 5AQ5 molecules in a selfassembled monolayer was confirmed using cyclic voltammetry at different sweeping rates and using electrochemical impedance spectroscopy.

Furthermore, perturbing this equilibrium with a small (15 mV) AC excitation signal at different frequencies allows investigation of the kinetics of the charge-transfer reaction.19 EIS data is often analyzed via a complex-plane plot of the imaginary component vs the real component of the impedance, i.e., Nyquist plot. This plot is interpreted by curve fitting the data to equivalent circuit models using the complex nonlinear leastsquares (CNLS) technique of Macdonald and Potter.19 The circuit in Figure 6 (inset) is the Randles circuit which is

4. EXPERIMENTAL SECTION 4.1. Sample Preparation. Gold substrates were prepared by thermally evaporating ca. 100 nm of gold (99.999% Alfa Aesar) on freshly cleaved mica slides (Ted Pella, Inc.) in an ultrahigh-vacuum chamber (∼5 × 10−8 Torr). 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 a 10 μM 5AQ5 solution in dichloromethane (DCM). The substrate was left in the modification solution for 3 h after which it was removed, washed thoroughly with DCM, dried under argon, and used for the measurements. 4.2. Electrochemistry. The redox electrochemistry of SAMs formed on freshly annealed single crystal Au(111) substrates of compound 5AQ5 were studied by cyclic voltammetry using a BAS 100B electrochemical analyzer. The counter electrode was a platinum mesh, and the reference electrode was a silver wire. The electrolyte used was 0.5 M phosphate buffer using NaH2PO4/H3PO4 at pH 3. The same setup was used to record the impedance data with a Solartron Impedance/Gain-Phase Analyzer. The AC amplitude was 15 mV. Data analysis was carried out using the program Z view by Scribner Associates Inc. 4.3. Conductance Measurements. The STM-break junction setup was a modified Pico-STM (Molecular Imaging) using a Nanoscope IIIa controller. The setup and method have been described in detail elsewhere.8 The SAM modified Au(111) substrate was placed in a Teflon STM cell, and the surface was covered with phosphate buffer. The STM tip was prepared by cutting a 0.25 nm gold wire (99.999%). It was then coated with Apiezon wax in order to reduce ionic conduction and polarization. The leakage current, due to ionic conduction and polarization, was on the order of pA, much smaller than the electron transport current through the 5AQ5 molecule. The molecular conductance was measured by repeatedly forming and breaking Au point contacts using the STM gold tip. Images showing clear and sharp atomic steps in the regular STM mode are a good indication of a clean substrate and a sharp tip. After surveying the substrate and confirming the tip condition, the tip was fixed at the center of an atomically flat terrace and the STM feedback loop was turned off. Consequently, a Lab View program was used to move the tip into and out of contact with the substrate at a typical rate of 40 nm/s. During the contact process, molecules can bridge between the tip and the surface via the thiol linkers at the distal end of the molecules. In the typical “static electrochemical-gating” STM break junction experiment, a large number of conductance traces are recorded (at fixed gating potential) and are grouped into histograms from which the conductance value of a single molecule can be obtained.13 In contrast, in the dynamic method, once a step is detected in the transient current curves, the STM tip was held still and the gate voltage was swept at 4000 mV/s to produce source-drain current vs gate voltage for single molecules from

Figure 6. Complex plane plots of the impedance of a SAM formed from 5AQ5. The frequency range was 0.1 Hz to 100 kHz, and the applied potential was −305 mV vs reference. This value represents the E1/2 which was obtained from the peak maximum in an alternating current voltammogram (see the Supporting Information). Squares represent experimental data, and the line represents the fit. The fit is performed by using the model in the inset.

appropriate for modeling a redox-active monolayer on an electrode surface where different components contributing to the overall impedance are represented by discrete circuit elements.20 The summary of best-fit values for the circuit elements is presented in Table S1 (Supporting Information). The relationship between the circuit elements (Cdl, doublelayer capacitance; Cads, adsorption pseudocapacitance; Rct, charge-transfer resistance) has been previously described, and the electron-transfer rate constant can be expressed as ket = (2RctCads)−1.21,22 We note that both Cdl and Cads did show a frequency-dependent capacitive behavior and were therefore treated as constant-phase elements (CPEs) for which the impedance is equal to 1/C(jω)α. C is the capacitance, ω is the angular frequency, and α is an exponential term with a value between 0 and 1. Using the refined Rct and Cads values, we obtained an electron-transfer rate constant of ca. 0.3 s−1. This low rate constant agrees with those reported for SAMs formed from quinone-terminated bridges of comparable length to 5AQ5 in acidic electrolytes as in the case of the present study.23 The slow kinetics is attributed to the occurrence of a stepwise mechanism according to a nine-membered square scheme involving a 2e−/2H+ reduction process of the AQ moiety to the H2AQ form.23−25

3. CONCLUSION We have applied electrochemical gating to a single AQ-based molecule, 5AQ5, bridged between two gold electrodes using the STM break-junction technique. It is shown that the conductance increases about an order of magnitude, indicating a net conversion from the oxidized AQ form to the reduced H2AQ form. When sweeping the potential on a single molecule 21096

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(24) Finklea, H. O. J. Phys. Chem. B 2001, 105, 8685−8693. (25) Laviron, E. J. Electroanal. Chem. 1983, 146, 1−13.

which conductance vs gate (G−V) curves were plotted. The values of the conductance obtained from the individual G−V curves were an average of 25 G−V curves ± standard deviation.

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ASSOCIATED CONTENT

* Supporting Information S

Conductance measurements using the blinking technique, G−V curves in the absence of a molecular junction, and the fitting parameters of the EIS and AC voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.T.); Justin.gooding@unsw. edu.au (J.J.G.); [email protected] (M.N.P.-R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.J.G. and M.N.P.-R. thank the Australian Research Council for support. N.T. thanks the National Science Foundation (CHE1105588) for support.

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REFERENCES

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