An electrically driven and readable molecular monolayer switch using a solid electrolyte
چکیده
The potential application of molecular switches as active elements in information storage has been demonstrated through numerous reported works. Importantly for devices, such switching capability has also been observed on self-assembled monolayers (SAMs). SAMs of electro-active molecules have been exploited in the last few years as electrochemical switches. Typically, the state of these switches could be read out through their optical and/or magnetic responses. These output reading processes are difficult to integrate into devices and in addition there is a need to use liquid environments to switch the redox-active molecular systems. Here, we overcome both these challenges using an ion-gel as electrolyte medium achieving an unprecedented solid state device based on a single molecular layer. Further, electrochemical impedance has been successfully exploited as the output of the system. Bistable molecules that can be externally interconverted between two or more states with different properties have been known for a long time and their potential as active elements in information storage in future emerging devices recognized.. In order to move towards device miniaturization, single molecules or a monolayer of molecules have been employed as active components in switches. In such systems, the input and output signals can be of different nature such as chemical, optical, magnetic or electrical. However, electrical signals are particularly appealing since they can be more easily integrated with current technologies. Single electroactive molecules have been incorporated in three terminal junctions, where an electrical field generated by applying a voltage to a gate terminal was employed to modulate the molecular charge, and the current through the molecule connected between two contacts was used as readout mechanism. Although scientifically extremely motivating, these devices are technologically very demanding and currently show poor fabrication yields and device reproducibility. The preparation of hybrid surfaces that consist of inorganic conducting supports functionalized with a chemically bonded self-assembled monolayer (SAM) offers a clear alternative platform for the preparation of molecular devices. The preparation of SAMs from solution, which is based on the spontaneous surface self-organization of the molecules, represents a very low cost as well as powerful and versatile tool for the modification of surfaces at the molecular level. SAMs of electroactive molecules have been exploited in the last few years as electrochemical switches. Typically, the state of these switches could be read out through their optical and/or magnetic responses. Despite the extremely encouraging results achieved so far, two main bottlenecks have been clearly identified which seriously hamper the progress in the field towards real applications: i) the reading processes that the output signals require are difficult to integrate into devices, and ii) in order to switch between the different accessible redox states of molecular systems there is an obvious need to use solutions containing an electrolyte to stabilize the charged states (i.e., ion pairing) and to ensure ionic conductivity in the electrochemical cell. Here, we overcome both these challenges employing SAMs with an electroactive ferrocene (Fc) unit and following a two-fold approach. First, electrochemical impedance spectroscopy (EIS) has been applied to measure the capacitance of the SAM and exploited as output signal of the switch. Further, a solid state device based on such a single molecular layer as active component has been successfully achieved using a solid electrolyte medium. These results represent a step forward in the field raising the perspectives of employing hybrid surfaces as molecular switches for their future integration in electronic applications. SAMs on gold of ferrocenylalkylthiol derivatives have been established as reference systems in electrochemical studies of surface confined molecules. For this reason, SAMs of 11(ferrocenyl)undecanethiol (FcC11SH) were employed here as a switchable redox unit, since ferrocene exhibits two accessible redox states (Figure 1a). A home-made electrochemical cell was fabricated for this purpose which is shown in Figure 1b. Three coplanar gold electrodes (counter (CE), working (WE) and reference (RE)) were thermally evaporated through a mask on a glass substrate and a silicon mold or a Teflon ring were used to delimit a fixed area of the electrolyte in contact with the electrodes. The Au WE was modified with the ferrocene SAM by its immersion in a 1 mM solution of the FcC11SH in ethanol for 18 hours (Figure S1). The electrochemical properties of the SAMs were first investigated using an ionic liquid (IL) as electrolyte. Figure 1c shows the CV of Fc-SAM using 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide (EMIM-TFSI) as electrolyte and employing the electrochemical cell described above. A redox peak at E= 0.23 V (vs Au at a scan rate of 200 mV/s) is observed corresponding to the Fc/Fc pair. The almost insignificant voltage difference observed between the cathodic and anodic peak (ΔE= 22 mV, at 200 mV/s) is characteristic of surface confined molecules, which is further confirmed with the observed linear dependence of the current density of the redox peak (Jp) with the scan rate in the range 50-500 mV/s (Figure S2). A surface coverage of 3.7 x 10 mol/cm was estimated which is slightly lower than the reported theoretical value (4.5 x 10). The observed smaller number of electroactive sites within the SAM could be explained by the large size of the TFSI anion, that determines the ion pairing formation and, as reported, if it cannot fully compensate the positive charges of all Fc, some of the Fc units would be apparently none-electroactive. [a] E. Marchante, Dr. N. Crivillers, M. Buhl, Prof. J. Veciana, Dr. M. Mas-Torrent Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and CIBER-BBN, Campus de la UAB, 08193, Bellaterra, Spain E-mail: mmas@icmab.es Figure 1. a) Scheme of a molecular switch based on a ferrocene SAM on gold. b) Home-made electrochemical cell with three coplanar gold electrodes. c) Cyclic votammograms of the Fc-SAM at different scan rates, 100, 200, 300, 400 and 500 mV/s using the modified Au as working electrode (WE) and gold as reference and counter-electrode using the ionic liquid EMIM-TFSI. Impedance spectroscopy (EIS) is a powerful tool to characterize electrical interfacial properties. [11] [12] Here, EIS is exploited to measure the capacitance of the different redox states of the switchable SAM. In an ideal situation, the gold metal surface would act as one of the capacitor plate and the physisorbed ions at the SAM/electrolyte interface would act as the other capacitor plate. EIS measurements were performed at two different applied DC bias, before (V0 = + 10 mV) and after (V1 = + 450 mV) the redox peak, which define the state 0 and state 1 of the switch (see supporting information for experimental details). Figure 2a shows the Bode magnitude plot (impedance modulus (|Z|) vs. frequency). It is clearly seen that for both states at low frequencies the plot consists of two parallel straight lines with a slope of around -0.9. This behavior is characteristic of capacitorlike systems and the deviation from a slope of -1 can be explained by capacitance dispersion due to the adsorption of ions on the electrodes and the roughness of the electrode surface. Accordingly, in the Bode phase plot (φ vs f) the phase angle slightly deviates from 90° at low frequency, which would be expected for an ideal ionic insulator (Figure S3a). Remarkably, a significant decrease in |Z| of one order of magnitude is observed between state 0 and state 1 at this low frequency region, indicating that the performed impedance measurements can be successfully used as a readout of the molecular switch. As a reference, the same experiments were performed in undecanethiol SAMs but no impedance differences were observed when different DC bias were applied to the functionalized WE at this frequency region (Figure S4). This indicates that the changes observed in |Z| for the Fc-SAMs do not come by the simple polarization of the electrode/electrolyte interface by the applied voltage but, instead, the redox state of the electroactive species is playing a crucial role. At higher frequencies (f>10 KHz), the frequencyindependent impedance (Figure 2a) and phase angle close to zero (Figure S3a,c) indicate that the system behaves as a resistor, that is, the total impedance is dominated by the solution resistance. This is due to the fact that at high frequencies, the capacitor behaves a short-circuit element since there is no time to be charged, allowing the AC current to pass. The EIS measurements can also be presented in the form of real or imaginary part of the interfacial complex capacitance as a function of frequency. The real part of capacitance per unit area (Cre) were calculated by using the following expression: CCCCCC (ff) = 1 2ππff ZZZZZZ [ZZCCCC 2+ZZZZZZ 2] 1