Cyclic Conductance Switching in Networks of Redox-Active Molecular

Cyclic Conductance Switching in Networks of Redox-Active Molecular Junctions .... Journal of the American Chemical Society 2014 136 (47), 16497-16507...
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Cyclic Conductance Switching in Networks of Redox-Active Molecular Junctions Jianhui Liao,† Jon S. Agustsson,† Songmei Wu,† Christian Scho¨nenberger,† Michel Calame,*,† Yann Leroux,‡ Marcel Mayor,‡,| Olivier Jeannin,§ Ying-Fen Ran,§ Shi-Xia Liu,*,§ and Silvio Decurtins§ †

Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056, Basel, Switzerland, ‡ Department of Chemistry, University of Basel, St. Johanns Ring 19, CH-4056 Basel, Switzerland, and § Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ABSTRACT Redox-active dithiolated tetrathiafulvalene derivatives (TTFdT) were inserted in two-dimensional nanoparticle arrays to build interlinked networks of molecular junctions. Upon oxidation of the TTFdT to the dication state, we observed a conductance increase of the networks by up to 1 order of magnitude. Successive oxidation and reduction cycles demonstrated a clear switching behavior of the molecular junction conductance. These results show the potential of interlinked nanoparticle arrays as chemical sensors. KEYWORDS Molecular electronics, nanoelectronics, molecular switch, nanoparticle arrays

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make it particularly attractive in the perspective of realizing molecular scale logics.34 Lastly, while the fabrication of individual junctions is essential for fundamental studies, an implementation at large scale is a prerequisite for the realization of functional devices.35,36 For this purpose, nanoparticles have recently been identified as ideal candidates to build stable and upscalable devices.37 Obtaining a welldefined spatial organization of the nanoparticles with good control over the formation of molecular junctions between neighboring particles is however a delicate task. Recently, we have shown that nanoparticle arrays represent an interesting platform for molecular electronics, offering upscaling possibilities while based on a simple self-assembly and microcontact printing approach.38–40 For this study, we have inserted redox-active dithiolated tetrathiafulvalene (TTFdT) derivatives in nanoparticles arrays and investigated the arrays’ electrical response to oxidizing and reducing agents. We demonstrate here that the conductance of a nanoparticle array interlinked by TTFdT derivatives can be modulated by exposing the array to oxidizing and reducing agents. The change in oxidation state of the TTF unit leads to a rearrangement of the TTFdT molecular orbitals, causing the conductance of the array to change by up to 1 order of magnitude. The high- and low-conductance states are stable enough to allow intermediate electrical characterization between successive oxidations and reductions. Figure 1a shows a scanning electron microscopy (SEM) image of two as-prepared devices in parallel. The line width of the nanoparticle monolayer and the distance between two neighboring contact pads are labeled w and l, respectively. On the top left contact, the edges of the nanoparticle monolayer are highlighted by white lines (dashed when below the contact pad). These devices are stable for several weeks at room temperature in air. In addition, they can resist

unctionality is a central question in molecular-based electronic junctions and devices1–4 and has ever been so since early works in the field back in the 1970s.5 If any practical electronic function is to be expected from a molecular device, it will have to be the result of a thorough design and implementation. The first aspect consists in engineering active molecules providing the desired functionality. To date, a variety of molecular compounds have been devised, which exhibit switching behavior.6 That alone is not enough without further demonstrating that the molecular function is preserved when the compounds are tested in a junction geometry. Contacting individual molecules to characterize their transport properties requires dedicated techniques.7,8 Extensive experimental work has been performed to investigate conductance modulation effects in molecular junctions, including electrostatic backgate,9–13 electrochemical gate,14–21 incident light,22,23 and chemical reactions.24 Organic compounds containing redox centers are particularly attractive candidates for molecular electronics since the current through the junctions can be influenced either by electrochemical gating or by the presence of oxidizing or reducing agents. Several redox units, such as viologen15,20,25,26 aniline,16,27 ferrocene,28 or thiophene29 have been considered so far. Tetrathiafulvalene (TTF) compounds have however been seldom incorporated in metalmolecule-metal junctions,30,31 despite extensive chemical studies.32,33 The unique redox properties of TTF resulting in well-defined and stable species at different oxidation states

* To whom correspondence should be addressed, [email protected] and [email protected]. | Karlsruhe Institute of Technology, Institute for Nanotechnology, P. O. Box 3640, D 76021 Karlsruhe, Germany. Received for review: 06/23/2009 Published on Web: 02/02/2010

© 2010 American Chemical Society

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FIGURE 1. (a) SEM image of two devices (top and bottom) each of which comprises two Au contact pads deposited on a one monolayer thick nanoparticle array (stripe) between them. The width (w) and the length (l) of the nanoparticle array are indicated on the image. (b) Highmagnification SEM image of an alkanethiol-covered nanoparticle monolayer prepared by a combination of self-assembly at the air/water interface and microcontact printing. (c) Schematic representation of a nanoparticle array (yellow spheres) after insertion of redox-active molecules (blue rods) via molecular exchange. The alkanethiol molecules are depicted as gray rods. Typically, one-third of the alkane chains are displaced upon exchange.39 Neighboring nanoparticles can be interlinked by the dithiolated molecules (white dashed arrows).40 (d) Structure of the redox-active dithiolated tetrathiafulvalene (TTFdT) studied. In presence of iron chloride or ferrocene, the TTFdT compounds can be oxidized or reduced in situ.

common organic solvents, such as ethanol, acetone, or tetrahydrofuran. A high-magnification SEM image of a nanoparticle array is shown in Figure 1b, where the hexagonal close-packed ordering of the stamped nanoparticles appears. The separation between neighboring Au particles results from the alkanethiol coating. The SEM images reveal that nanoparticle arrays are homogeneous and free of multilayer regions, with only few defects. As our devices are stable upon immersion in common organic solvents, we can perform place-exchange to insert new molecules into the arrays. Figure 1c shows a schematic representation of an interlinked array after a place-exchange reaction. Alkanethiols and dithiolated molecules are depicted by gray and blue rods, respectively. As shown in a previous work,39 optical measurements reveal that after placeexchange, 20%-40% alkanethiols have been displaced by the new dithiolated molecules. As a result, we obtain a mixture of alkanethiols and dithiolated molecules in the arrays. More importantly, some of the dithiolated molecules will interlink neighboring nanoparticles to form a molecular junction,40 as schematically shown in Figure 1c (arrows). In each device, we have typically ∼106 molecular junctions. The molecule investigated in this study is depicted in Figure 1d. It carries a redox-active TTF group in the center © 2010 American Chemical Society

and three methylene units on each side. At both ends, each molecule has one thiol anchor group, which can covalently bond to a gold surface. Note that, as depicted, two isomers were present in the solution. As oxidant and reductant, we used iron chloride and ferrocene, respectively. We first demonstrate reversible oxidation and reduction of the TTFdT compound in solution via iron chloride and ferrocene using optical spectroscopy (Figure 2) and by cyclic voltammetry (Figure S2 in Supporting Information). The TTFdT compound behaves similarly to the simple TTF molecule.34 The blue curve in Figure 2 shows the UV-visible absorption spectrum of the acetyl-protected TTFdTs dissolved in acetonitrile, with a strong absorption band at 320 nm and a small shoulder around 370 nm. The oxidation state of the TTF groups can be controlled by changing the amount of the added oxidant. Adding 1:1 (molar ratio, FeCl3:TTFdT) iron chloride produces the radical cation state (TTF•+), and the absorption spectrum is shown by the black dotted curve in Figure 2. For the radical cation state, two absorption peaks appear at 450 and 700 nm. The dication state (TTF2+) can be obtained by adding 2:1 (FeCl3:TTFdT) or more iron chloride. The resulting absorption spectrum, with only one peak centered at 550 nm, is displayed by the red curve in Figure 2. Ferrocene was then used to reduce the TTF groups 760

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FIGURE 3. (a) Sheet conductance of 24 devices from one sample measured at various stages: as-prepared (9), after TTFdT exchange (b), after oxidation with excess iron chloride (2), and after reduction with excess ferrocene (1). (b) Histogram of the sheet conductance values shown in (a). FIGURE 2. UV-visible absorption spectra of acetyl-protected TTFdT measured at different oxidation states in solution. The succession of oxidation and reduction processes is depicted at the top of the figure. The absorption of the original TTFdT dissolved in acetonitrile (ACN, CH3CN) is shown in blue. The radical cation state (TTF•+) is produced by adding iron(III) chloride (FeCl3·6H2O) at a 1:1 molar ratio with TTFdT (black curve). Adding 2:1 (FeCl3:TTF) or more iron chloride results in the dication state (TTF2+), as shown by the red curve. Ferrocene (Fe(C5H5)2) is used to reduce the oxidized TTF groups back to a radical cation state (green curve) and a neutral state (orange curve).

stages. We report the sheet conductance G0 ) Gl/w obtained from the measured device conductance G, where l and w are the length and width, respectively, of the nanoparticle array between two neighboring contact pads (Figure 1a). Figure 3a shows the sheet conductance of 24 devices measured on one silicon chip in the as-prepared state, i.e., with C8 molecules only (gray squares), after TTFdT exchange (blue circles), after immersion in excess iron chloride (red up triangles), and after immersion in excess ferrocene (orange down triangles). A histogram of the sheet conductances is also shown (Figure 3b). The sheet conductance of all devices increased on average by a factor of 20 after immersion in iron chloride and decreased again by about an order of magnitude after immersion in ferrocene. To produce the radical cation state (TTF•+), we should add a 1:1 molar ratio of FeCl3 to TTFdT. In our device geometry, this is however delicate to control since, by adding a limited amount of the oxidizing agent, we cannot exclude the presence of oxidized and neutral compounds in the arrays. This is unacceptable since it would lead to an uncontrolled final conductance of the device. We therefore used an excess of iron chloride, to ensure that the oxidation state of most TTFdT compounds changed from neutral to the dication state. It is tempting here to attribute this remarkable change in conductance to the oxidation and reduction of the TTF groups. Indeed, the UV-vis spectra of TTFdT in its neutral and dicationic states clearly differentiate the underlying molecular orbital situations.41 To rule out possible effects other than the oxidation of the TTF unit, we performed additional experiments. We first carried out the same oxidation-reduction experiment in the absence of TTFdT compounds, i.e., for C8-coated nanoparticles only (Figure S5 in Supporting Information). As a second control, we repeated a similar experiment after inserting a conjugated compound for which we did not expect any effect of the oxidizing or reducing agents. For this purpose, dithiolated oligo(phenylene vinylene) (OPVdT) was inserted in the arrays (Figure S6 in Supporting Information). The results of these experiments are summarized in Table 1. G0i corresponds to the averaged conductance measured after

from the dication state to the radical cation state (green dotted curve) and neutral state (orange curve). Note that, in the cationic state, the two isomers can interconvert during the redox cycle. The small peak around 620 nm in the orange curve can be assigned to the absorption of oxidized ferrocene (Figure S2 in Supporting Information). From these absorption spectra, we conclude that iron chloride and ferrocene are appropriate oxidizing and reducing agents to control the oxidation state of the TTFdT compound in solution. For the sake of completeness, we also tried to investigate the redox properties of surface-bound TTFdT molecules via cyclic voltammetry (CV). The measurements for TTFdT selfassembled monolayers (TTFdT-SAM) on gold surfaces are shown in Figure S3 (Supporting Information). Compared to the CV of the dissolved species (Figure S1 in Supporting Information), functionalized Au electrodes displayed an increase in complexity with respect to their redox activity. In particular, scan number dependent peak positions and intensities pointed toward considerable rearrangements of the SAM during the electrochemical experiments. We attribute this to the larger number of geometrical arrangements that the TTFdT molecules can take over a flat Au surface. In the particular case of nanoparticles arrays, the TTFdT compound is inserted within a pre-existing alkanethiol layer via molecular exchange which drastically reduces the possibilities of, for instance, flat-lying molecules. For the next characterization step, we inserted TTFdTs into C8-capped gold nanoparticle arrays to examine the influence of the oxidation state of the TTF groups on the device conductance. For this purpose, we measured the conductance of the devices at different experimental © 2010 American Chemical Society

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TABLE 1. Comparison of the Sheet Conductance for Arrays with Different Molecules during Oxidation and Reduction Processesa molecule

G0i (nS)

G0ox/G0i

G0red/G0i

TTFdT C8 OPVdT

0.34 0.14 0.65

20 ( 4 1.0 ( 0.2 2.2 ( 0.2

1.7 ( 0.3 1.2 ( 0.6 2.3 ( 0.3

a G0i is the initial conductance of the arrays, either as stamped (C8) or immediately after exchange of the dithiolated compound (TTFdT, OPVdT). G0ox and G0red are the conductances after oxidation and reduction, respectively. Note that the second and third columns indicate a conductance ratio, namely, the conductance after oxidation or reduction normalized by the initial conductance.

place exchange, when dithiolated molecules were present (TTFdT and OPVdT) or directly after stamping, for the asprepared sample when only C8 molecules were present. G0ox and G0red correspond to the average conductances measured after oxidation and reduction, respectively. For all three experiments, we characterized more than 20 devices. The data in Table 1 show that arrays containing TTFdT compounds exhibit on average a conductance change by 1 order of magnitude while, for devices containing only C8 molecules, the conductance remains constant. For the devices containing OPVdTs, the conductance increases slightly after immersion in the oxidant solution (roughly by a factor of 2), while no change after reduction is observed. We will come back to this point below. Globally, the experiments demonstrate that the conductance changes by about 1 order of magnitude for compounds with a redox-active unit while no or little change is observed in the control experiments. To assess the observed conductance change, we can first consider a commonly used simple single-step tunneling model accounting for the charge transport through the molecular junction. The conductance can be written G ) Gc exp(-2(2Φm)1/2d/p) where Gc is the contact conductance determined by the coupling between the molecule and the metal electrodes, Φ is the barrier height, m is the electron mass, and d the length of the molecule. The barrier will be taken here as half of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).42 This is a relatively crude approximation since we can expect that one of the molecular orbitals, usually the HOMO, will lie closer to the Fermi level of the electrodes. However, given the presence of the alkane spacers between the TTF unit and the -SAc binding group, we do not expect a substantial charge transfer between the TTF unit and the Au particles upon binding. We can therefore anticipate that the molecular orbital levels of the TTF unit will be only weakly affected by the Au terminals. At equilibrium and for a symmetric junction, the Fermi level can, to a good approximation, be assumed to lie in the middle position of the HOMO-LUMO gap EHL, with deviations of the order kBT/EHL, depending on the degeneracy of the HOMO and LUMO levels. When the redox molecule is brought into the dication state, we are still in a situation where the molecular orbitals are filled with a globally neutral π-system. The situation is therefore analogous to the neutral © 2010 American Chemical Society

FIGURE 4. (a) Average ratio G0TTF/G0C8 for 24 devices during four cycles of oxidation-reduction. The conductance ratio is plotted as a function of process step, starting from the array immediately after TTFdT insertion (b), followed by successive oxidations (2) and reductions (1). Inset: Schematics for charge transport across a molecular junction in a resonant tunneling model.

case, and the Fermi level is also expected to lie approximately in the middle of the HOMO-LUMO gap. The HOMO-LUMO gap EHL of TTFdT was determined from the absorption spectra shown in Figure 2 for the dicationic state. The spectrum of the dicationic molecule is dominated by a broad absorption band centered at 550 nm which reflects basically a HOMO-LUMO transition. The onset around 700 nm gives a good estimate for the optical HOMO-LUMO gap. For the neutral state, the most active optical transition does however not correspond to the HOMO-LUMO gap and we therefore take the gap value from a TD-DFT calculation.43 Note also that the LUMO state is out of the potential window in cyclic voltammetry experiments, which prevents an estimate of the HOMO-LUMO gap via electrochemistry. Overall, this results EHL ) 3.7 eV for neutral TTFdT and EHL ) 1.8 eV for TTF2+. Using d ) 2.2 nm for the length of the molecule, the conductance ratio is GTTF2+/GTTF = 1.1 × 104. (The energy of molecule was minimized using the MM2 force field in ChemDraw 3D and the length estimated as the distance between the terminal sulfur atoms.) This simple tunneling model provides an estimate almost 3 orders of magnitude larger than the ratio observed experimentally (=20). The reasons could be the following. First, the approximation of the effective tunneling barrier height is relatively crude and does not take into account any possible charging effect and reorganization of the molecular orbitals upon contacting.44 Second, a two-step tunneling model could be more appropriate considering the molecular system investigated. The alkane linkers were actually added to prevent a too strong coupling of the central redox unit to the metallic reservoir electrodes (one can expect a HOMO-LUMO gap reaching typically 7 eV for a short alkane chain45). This is essential to ensure a stable oxidation and reduction of the molecule. Such a configuration therefore suggests that a description via a resonant tunneling picture might be more appropriate (inset Figure 4). The central TTF unit plays here the role of a weakly 762

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coupled quantum dot. At low bias voltages, the conductance can be approximated by

G)

via one anchor group, leaving the other end-thiol group free as shown by spectroscopic characterization (see, e.g., ref 47). This situation is schematically depicted in Figure 1c. It can therefore be expected that, during immersion of the sample in the oxidation solution, the free thiol groups will coordinate with the iron species present, thereby altering the overall conductance of the array. In the case of C8, the situation is different since no free thiols are available. Here, we do not observe a conductance change after immersion in the oxidation solution. To examine the reversibility of the conductance change upon exposure to the oxidant and reductant, we repeated the process several times. Figure 4a shows the average ratio G0TTF/G0C8 of 24 devices at different stages during four oxidation-reduction processes. The red up triangle data points correspond to the oxidized, dication state (TTF2+) while the orange down triangle data points correspond to the neutral state (TTF). Our experimental data demonstrate that the conductance of the devices can be reproducibly switched between a high conductance level (after oxidation) and a low conductance level (after reduction). Note that the switching amplitude decays with the number of repeated cycles. This feature might originate from the breaking of Au-S bonds between the TTFdT compound and the nanoparticles by the oxidant and the coordination of the unbound end groups with iron ions of the oxidant. We also cannot exclude the formation of dimers or oligomers via disulfide bond formation during the redox cycles. Such effects will result in a lower maximum conductance and higher minimum conductance. In conclusion, we have successfully inserted molecules with tetrathiafulvalene redox units into two-dimensional nanoparticle arrays to form functional networks of molecular junctions. The conductance of the networks could be repeatedly switched between a high conductance level and a low conductance level by means of chemical oxidation and reduction. The change in conductance arises due to the reorganization of the TTFdT molecular orbitals upon oxidation, as supported by UV-vis spectroscopy. The high- and low-conductance states are stable enough to permit electrical characterization after each oxidation or reduction step. Our experiments not only demonstrate the efficient modulation of the conductance of molecular junctions by up to 1 order of magnitude but also raise interesting perspectives for chemical sensing based on networks of active molecular junctions. The high surface area provided by the nanoparticles array combined with the chemical tunability of the interlinking elements can provide specific and selective reactions or interactions with particular species.37 As shown here, an interlinked monolayer of Au nanoparticles exhibits a substantial change of conductance upon reaction with oxidative species. The system appears therefore promising in the perspective of building molecular circuits com-

Γ1Γ2 2e2 2 h ε + (Γ + Γ )2 /4 1 2

where Γ1 and Γ2 are the electronic couplings between the molecule and the left and right electrodes, respectively, and ε is the energy difference between the closest molecular orbital and the Fermi level of the reservoirs. Since the molecule is symmetric, we can assume Γ1 ) Γ2 ) Γ. In the weak coupling limit where Γ , ε, the conductance ratio GTTF2+/GTTF can be expressed as (εTTF/εTTF2+)2 = 4.2. This second estimate is now much closer to the experimentally observed conductance change, underestimating it by about a factor of 5. The weak coupling approximation can be justified by the presence of the alkane linkers. In a similar study on an asymmetric C60-based system with a slightly different but comparable linker, we found a coupling constant verifying Γ j 5 meV.46 For ε, we use the values of the HOMO-LUMO gaps for the neutral and dication states given above. This is a rough oversimplification since the distance between the Fermi energy and the closest molecular orbital relevant for transport might not be simply a fraction of EHL. We indeed do not account for charging effects and molecular levels broadening here. A spectroscopic characterization of the compounds would provide additional insight here44 but remains practically very delicate to implement for such devices having to face liquid immersion and operation at room temperature. While phenomenological, the above estimate remains reasonable and provides a value for the conductance ratio in relatively good agreement with the observed data. We can see from Figure 3 that the conductance of the devices measured after the reduction by ferrocene (orange down triangle) does not exactly go back to the conductance value reached immediately after molecular exchange (b) but remains about 1.7 times larger. Interestingly, this behavior is also observed in control experiments with OPV molecules while it is imperceptible for the C8 reference sample (see Figures S6 and S5, respectively, in Supporting Information). Notice here that the C8 compound carries only a single thiol anchor group while both TTFdT and OPVdT bear two. We attribute the effect described above to the presence of free thiol groups after molecular exchange for the TTFdT and OPVdT compounds. As mentioned previously, we can expect that 20%-40% octanethiols have been displaced by the incoming dithiolated molecules during the course of molecular exchange. However, only a fraction of these incoming molecules have a chance to form two bonds with the gold surfaces available and effectively interlink neighboring nanoparticles. For geometrical reasons, a large part of the dithiolated molecules can only bind to the gold surface © 2010 American Chemical Society

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bining fluidic channels to deliver substances to be analyzed with addressable, small nanoparticle arrays. Acknowledgment. We gratefully acknowledge Martin Bryce for fruitful discussions. We thank Viviana L. Horhoiu for providing the oligo-phenylene vinylene control molecules. This work was supported by the European Commission (EC) FP7 ITN “FUNMOLS” project no. 212942, the EC FP7 “FUNMOL” project no. 213382, the Swiss NCCR on Nanoscale Science, the Swiss NSF, and the Gebert Ru¨f Foundation. Supporting Information Available. Synthesis details, materials and methods, and additional control experiments. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

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DOI: 10.1021/nl902000e | Nano Lett. 2010, 10, 759-–764