Plasmon-Induced Conductance Switching of an Electroactive

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Plasmon-Induced Conductance Switching of an Electroactive Conjugated Polymer Nanojunction Yong Ai,† Van Quynh Nguyen,†,‡ Jalal Ghilane,† Pierre-Camille Lacaze,† and Jean-Christophe Lacroix*,† †

Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, Université Paris Diderot, 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France ‡ Department of Advanced Material Science and Nanotechnology, University of Science and Technology of Hanoi (USTH), Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam S Supporting Information *

ABSTRACT: A plasmonic molecular electronic device, consisting of poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires bridging an ultramicroelectrode and an indium tin oxide (ITO) substrate covered by gold nanoparticles (Au NPs), has been developed. Light irradiation of this device has a dramatic impact on its conductance. Polymer strands, maintained electrochemically in their oxidized, conducting state, reversibly switch to their insulating state upon irradiation by visible-wavelength light, resulting in a sharp decrease in the conductance. The high-conductance state is restored when the light is turned off. Switching depends on the wavelength and the intensity of the incident light. It is due to reversible reduction of the nanosized region of PEDOT nanowires in contact with a gold NP and is attributed to plasmon-induced hot-electron injection into the PEDOT. The high/low conductance ratio can be as great as 1000, and switching requires low light intensity (220 W/m2). These results could open the way to the design of a new family of optoelectronic switches. KEYWORDS: plasmonics, molecular electronics, nanojunctions, switching, polyethylene dioxythiophene, scanning electrochemical microscopy (SECM) detectors16,17 and photovoltaic18−20 and electroluminescent devices.21,22 Molecular electronics is another research field that exploits charge transfer at the nanoscale.23,24 The most studied device is a two-terminal molecular junction that can be fabricated at the single-molecule level25 or as large-area devices.26,27 In both cases, one of the main functions of interest is switching between two states of different conductance for logic or memory applications. Various examples of light-induced switching devices based on photoconductive, photochromic, or cis− trans isomeric effects have been proposed and were recently reviewed.28 Among the most promising devices is a flexible, nonvolatile, optical, multilevel-memory, thin-film transistor with over 256 distinct levels29 and a single diarylethene molecule junction, exhibiting an unprecedented on/off ratio of 100 between high and low photoconductance states.30 Optoelectronic devices are intrinsically faster than electronic devices, but they suffer from the severe drawback that they cannot be smaller than the light wavelength used, which strongly limits their integration. In this context, the development of plasmonic devices is seen as a way of combining the advantages of optoelectronics with those of nanoelectronic

1. INTRODUCTION Light irradiation of metallic nanoparticles (NPs) of Ag, Au, or Cu induces coherent but confined oscillations of the quasi-free electrons in the conduction bands. When the characteristic frequency of these oscillations coincides with that of the light, the response of the NPs becomes resonant. A strong absorption in the visible and near-infrared range occurs (called localized surface plasmon resonance, LSPR), and the electric field very close to the NPs is strongly enhanced.1 LSPs can act as nanoantennas running at optical frequencies and can also work as nanosensors,2 adapted to trace analysis in the case of surfaceenhanced Raman spectroscopy.3 Besides, as the LSPR occurs at frequencies corresponding to typical electronic excitations of molecules, there are strong interactions between LSPs and molecular systems in the vicinity of the metals.4 This makes active molecular plasmonic devices possible5 and also provides a new approach for studying the electronic properties of organic compounds.6 Another property of plasmonic nanostructures is their ability to produce hot charge carriers,7 and their interaction with adsorbed molecular compounds constitutes a new means of driving photocatalysis,8 as shown in particular in solar water splitting.9,10 Plasmonic hot carriers resulting from multiple combinations of materials (graphene,11 various semiconductors,12,13 and phase-change materials14) and nanostructures are now increasingly involved in the conception and improvement of photoelectric devices,15 such as photo© XXXX American Chemical Society

Received: April 3, 2017 Accepted: August 2, 2017 Published: August 2, 2017 A

DOI: 10.1021/acsami.7b04695 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) SEM image of Au NPs on ITO. Inset: Extinction spectrum of AuNPs@ITO electrode. (b) Electrochemical setup used to bridge the gap between the tip (ultramicroelectrode, UME) and the substrate by PEDOT and to study the PEDOT junction. The NP array may be irradiated (yellow arrow). (c) A few nanowires are in contact with one or a few NPs and control the transport properties of the whole device.

circuitry.31 As a consequence, merging molecular plasmonics and molecular electronics is of tremendous interest. Few steps have already been done in this direction. Plasmon−exciton coupling plays a central role in scanning tunneling-luminescence spectroscopy;32 molecular electronic control over plasmons has been reported,33,34 and plasmon-enhanced conductance has been demonstrated in single-molecule junctions.35 In this work we develop a plasmonic molecular electronic device that uses plasmon-induced phenomena and the production of hot charge carriers. We demonstrate that a PEDOT nanojunction, held electrochemically in its conducting state and connected to gold NPs, can be switched between high and low conductance states (with an on/off ratio > 1000) through light irradiation of the NPs.

(SEM), the NPs are randomly distributed and separated by distances of 15 min when the light is switched on and off every 100 s. The same behavior is observed for another junction with a gold plate instead of ITO (Figure S3). This result proves that induced photoconduction is negligible in the present case. Light has no effect on the conductance of an oxidized PEDOT molecular junction when ITO or a gold plate is used as the substrate. The situation is different when the ITO is covered with Au NPs. Localized plasmons are produced, and the region of the C


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Figure 3. Light wavelength effect on PEDOT conductance switch. Transport current ISD vs time of three PEDOT junctions submitted to light switching with different filters: (a) white light; (b) blue-filtered light; and (c) orange-filtered light. (d) Transport current ISD vs time of PEDOT junctions submitted to continuous irradiation through different filters. Electrolytic medium: 0.1 M TBAPF6 + 10 mM EDOT in acetonitrile. White light intensity, 260 W/m2; tip/substrate bias, 0.1 V, tip gated at 0.5 V.

from a plasmonic effect. Further evidence for the plasmonic effect is found by comparing the cyclic voltamograms recorded at the tip electrode when its potential is swept from −0.3 to 0.6 V with a constant 0.1 V tip/substrate bias with continuous irradiation or without (Figure 2e and f, respectively). Under irradiation (Figure 2e) the transport current ISD is so weak that Itip vs VG only reflects the classical cyclic voltametric redox curve of a PEDOT polymer. In other words, upon irradiation electrochemical current (IEC) is much higher than the transport current (ISD), and more importantly, the electrochemical reaction driven by the applied potential is not capable of switching the whole bridge to its conductive state. This means that a part of it remains in the insulating state and inhibits transport from tip to substrate. On the contrary, when the light is off, and when the sweep voltage of the tip is started again from −0.3 V, as soon as the potential reaches 0.2 V, ISD through the doped PEDOT wires becomes more important than IEC, as evidenced in Figure 2f. The usual PEDOT voltammetric signature disappears and is replaced by the transport current curve with ISD ≈ 70 nA at 0.6 V. These highly reproducible results (>95% of the junctions show similar response; see Supporting Information file for a discussion on the remaining 5%) confirm those obtained with larger junctions. Unique and reversible photoswitching of the transport current from on to off through the PEDOT molecular junctions is obtained when small PEDOT nanowires contact Au NPs attached to the electrode surface. These results suggest that hot electrons, generated on the Au NPs, reduce the PEDOT contact and maintain it in its reduced state despite an applied electrochemical potential that should drive it to its conductive state. To confirm the correlation between photoreduction and the formation of localized plasmons at the surface of the Au NPs, several PEDOT junctions were irradiated with filtered white light providing different cutoff wavelengths, one overlapping the plasmon resonance that lies between 520 and 650 nm

bridge in contact with the Au NPs is subjected to their action. Indeed, after a 150 s delay in the dark, with the PEDOT junction maintained in its oxidized state (tip potential VG = 0.5 V, transport current ISD = 2 μA, with 0.1 V bias), ISD drops abruptly when the light is turned on and stabilizes to a background value ∼2 nA. When the light is turned off (after a 150 s delay), it returns to the same level of 2 μA as before irradiation. Good reproducibility is obtained when the light is switched on and off with the same junction. Similar features were observed with junctions of lower conductance (1 μS and 200 nS), i.e., junctions with fewer oligomer strands in the nanowire in contact with the gold NP (Figure 2c and Figure S4). It is important to note that the PEDOT bridge has the properties of a conducting polymer and thus behaves like a metal in its conducting state. This can be seen in Figure 2d (black curve), which shows the I(V) curve of the PEDOT junction in the dark. Good linear behavior is observed when the tip potential is swept from 0.35 to 0.45 V/Ag while the AuNPs@ITO substrate is polarized at 0.4 V/Ag. On the contrary, the flat response of the same junction under irradiation clearly shows its resistive behavior (Figure 2d, red curve). The on/off conductance ratio resulting from light switching is high (>1000), but the delay for the recovery of the initial conductivity after the light is turned off is rather long (∼12 s), in contrast to the conductive-to-insulating transition when the light is turned on, which occurs more rapidly. Such a delay can probably be attributed to the memory effect often observed when a conductive polymer is polarized for various times in its undoped state49,50 and may be attributed to moving front phenomena in the switching of conductive polymers.51 Compared with the control experiment, where the junctions were made with bare ITO or bare gold electrodes, this demonstrates that negative photoswitching (conductance decreasing under light irradiation) is related to the Au NPs attached to the electrode surface and suggests that it results D


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Figure 4. Energy levels of Au NPs and PEDOT in the dark and after irradiation. (a) Oxidized PEDOT in the dark. (b) Production of hot electrons and holes with light and transfer of electrons to the polaron level P+; holes are evacuated in the external circuit. (c) Energy levels after reduction of a small thickness of PEDOT in contact with the NPs depicted as a step of the highest occupied molecular orbital (HOMO) energy level in contact with gold.

(orange filter with cutoff below 520 nm) and two others outside the plasmon resonance (black filter with cutoff below 700 nm and a blue filter with cutoff above 550 nm) (Figure S5, Supporting Information). No significant switching occurs when the wavelength range of the incident light is 700 nm (black filter, Figure 3d), for which the transmitted light is outside the plasmon resonance (Figure 1a) On the contrary, as for white light (Figure 3a), an abrupt decrease in the transport current ISD is observed when the orange filter is used (Figure 3c), for which the transmitted light wavelength above 520 nm is compatible with the LSPR. These findings are confirmed when the junction is successively submitted to illumination through different filters (Figure 3d). As previously, it is worth noting that the on/off switching of light is accompanied by a reversible ISD switching of the PEDOT junction, which recovers its initial conductivity every time the illumination is off and confirms that the bridge does not break when the light is on, despite the small number of PEDOT strands involved in the nanowires Overall, these experiments confirm without ambiguity that PEDOT molecular junctions are switched to an insulating state, only if there is resonant plasmonic excitation of Au NPs connected to the bridge. The light intensity also has a strong impact on the excitation of plasmon and, therefore, on the flow of hot electrons.52,53 It can be predicted that plasmon-induced conductance switching will only be observed if the rate of production of hot electrons is high enough to overcome the electrochemical oxidation rate. Thus, increasing the light intensity will trigger resistive switching of the junction at a threshold value for which the two competing rates, i.e., electron transfer from the electrochemical process and that from the plasmonic process, are in balance. Such thresholds are always observed. As an example, when a PEDOT junction with 12 μS conductance is used, plasmon-induced conductance switching just appears at 220 W/m2 (Figure S8) and is characterized by Itip oscillations with maxima at 600 and 800 nA, indicating that the electron flow is not strong enough to fully reduce PEDOT in its insulating state. Note that, when the potential is swept as high as 1 V/Ag with light intensities > 220 W/m2, the plasmon is still able to reduce the PEDOT nanowires as no transport currents are observed in Figure S8a and d. On the contrary, with light intensities < 220 W/m2, Itip reaches a maximum value of ∼1200 nA, indicating that the whole PEDOT bridge remains in its conductive state and that electron flow from the

plasmonic process is too low to induce partial reduction (Figure S8c, 120 W/m2 and below). Hot-carrier (electrons and holes) devices have been demonstrated, with hot electrons or hot holes, generated by LSPR being injected into other materials.11−13,54 Hot carriers generated from plasmonic nanostructures can also have an important catalytic effect.16−23,26,55 It must be recalled that, after excitation of a plasmonic NP by a light pulse, the LSPR decays rapidly with a lifetime of ∼0−100 fs. This decay occurs through two mechanisms: elastic radiative reemission of photons and nonradiative Landau damping associated with the production of athermal electron−hole pairs in the NP. Their decay occurs through reemission of photons or through carrier multiplication resulting from electron−electron interactions; it is followed by their relaxation in a time range of 100 fs to 1 ps and then thermal dissipation between 100 ps and 1 ns.56 However, when an adsorbate is present on the surface of the NPs, an ultrafast dephasing pathway can occur in a time of ∼5 to 100 fs, corresponding to the injection of hot electrons into the unpopulated adsorbate states. This process, known as chemical interface damping (CID),8,57,58 must be taken into account in our system and explains the switching of PEDOT junctions from conductive to insulating states. In our case, it is likely that hot electrons generated from Au NP plasmons are trapped in the oxidized PEDOT in contact with gold and, therefore, reduce it to its insulating neutral state. It must be recalled that the energy of hot carriers generated by a plasmonic effect depends on the size of the NPs.7 Owing to the average size of Au NPs (90 ± 10 nm),36 the density of hot electrons is high but their energy remains close to the Fermi level EF of the Au NPs. The ease with which they induce the reduction of the bridge is due to the absence of a contact potential and to the quasi-metallic character of the oxidized PEDOT, as shown previously. Their transfer toward the oxidized PEDOT occurs at the polaron level, which is also the Fermi level of the doped PEDOT and, therefore, is aligned with that of gold (Figure 4a).59,60 The work function (WF) of doped PEDOT with polystyrene sulfonate as counterion is 5 to 5.2 eV,61 and small variations may occur depending on the doping counterion. The polaron P+, which is equivalent to an impurity level inside the band gap of PEDOT, lies 0.4−0.5 eV above the HOMO, and the band gap of PEDOT ranges between 1.2 and 1.7 eV, the smallest value being obtained for a conjugated chain of infinite length.62 The gold WF is generally estimated to be E


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ACS Applied Materials & Interfaces 4.5 ± 0.1 eV in ambient atmosphere rather than 5.2 eV (in vacuum).63 Thus, when the junction is irradiated, electrons reduce a small amount of PEDOT while the holes are neutralized in the external circuit (Figure 4b). A thin dedoped insulating layer in contact with the NPs is formed and produces a barrier that prevents further electron transfer (Figure 4c). As illustrated in Figure 5, it is likely that most of the PEDOT connected to the tip remains in the conducting state after

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discussion about reproducibility, and effect of light intensity on the negative photoswitching effect (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jean-Christophe Lacroix: 0000-0002-7024-4452 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. John Lomas for editing our manuscript.

Figure 5. Switching PEDOT junction by electrochemistry or plasmonics. Schematic interpretation of a gated PEDOT junction switched by plasmonic hot electrons, showing that only a small part of the bridge is reduced by hot electrons.

illumination, whereas a small part, the end connected to the Au NP, is reduced to the insulating state. The reduction of this small part of the PEDOT by hot electrons is enough to limit charge transport through the whole junction to a very low level.

4. CONCLUSION PEDOT nanojunctions, electrochemically established between a UME tip and Au NPs deposited on ITO, exhibit highly reproducible resistive switching from high to low conductive states when irradiated (negative photoswitching). This is observed above a light intensity threshold, depends on the light wavelength, and occurs only under the resonant excitation of the plasmon of the NPs. It can be attributed to the injection of hot electrons into the doped PEDOT at the polaron energy level, resulting in the reduction of the small part of the bridge in contact with the NPs. To the best of our knowledge, this is the first time that a bistable resistive system induced by a plasmonic effect has been observed with an electroactive polymer junction. This work provides a new understanding of the plasmonic effect on the transport current through molecular devices and underlines the fact that it does not always increase the current in molecular junctions. We anticipate that it can be extended to a solid-state device and could thus open the way to the design of a new family of optoelectronic switches.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04695. Chemical reagents and instrumentation, electrochemical and electronic properties of PEDOT junctions, morphology of the PEDOT junction, blank experiment showing that no plasmonic effect occurs with gold and an ITO plate, photoeffect on a gated PEDOT nanojunction, transmission spectra of blue, black, and orange filters, F


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