Elise Villemin

Baptiste Lemarque

Thiêt Thiêt Vũ

Gaelle Van Quynh Nguyen

Pascal Trippé-Allard

Pierre-Camille Martin

Jean-Christophe Lacaze

Lacroix

Baptiste Lemarque #

Van Quynh Nguyen

Gaelle Trippé-Allard #

Pascal Martin

Pierre-Camille Lacaze

Jean-Christophe Lacroix

Improved adhesion of poly(3,4-ethylenedioxythiophene) (PEDOT) thin film to solid substrates using electrografted promoters and application to efficient nanoplasmonic devices

Keywords: conjugated polymers, poly(3, 4-ethylenedioxythiophene), adhesion, electrografting, electropolymerization, electrochemical switching, plasmonic devices, localized surface plasmon resonance

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Among the commercialy available conducting polymers, poly(3,4ethylenedioxythiophene) (PEDOT) is probably one of the most used due to its high chemical and thermal stability. [START_REF] Winter | The thermal ageing of poly(3,4-ethylenedioxythiophene). An investigation by X-ray absorption and X-ray photoelectron spectroscopy[END_REF][START_REF] Heywang | Poly(alkylenedioxythiophene)s-new, very stable conducting polymers[END_REF] Its low oxidation potential, allowing it to switch from an insulating to a highly conductive state (more than 3000 S cm -1 under specific conditions [START_REF] Kim | Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution-Processed Crystallization[END_REF][START_REF] Cho | Single-Crystal Poly(3,4-ethylenedioxythiophene) Nanowires with Ultrahigh Conductivity[END_REF]) and the possibility of coating it as a thin film on various substrates [START_REF] Fabretto | Polymeric Material with Metal-Like Conductivity for Next Generation Organic Electronic Devices[END_REF][START_REF] Jonas | Conductive modifications of polymers with polypyrroles and polythiophenes[END_REF] make PEDOT and its derivatives [START_REF] Kerszulis | Tuning the painter's palette: subtle steric effects on spectra and colour in conjugated electrochromic polymers[END_REF][START_REF] Beaujuge | Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices[END_REF][START_REF] Roncali | 3,4-Ethylenedioxythiophene (EDOT) as a versatile building block for advanced functional $π$-conjugated systems[END_REF] suitable for various applications. Energy conversion and energy storage, organic light-emitting diodes, electrochemical transistors, thermoelectrical materials and sensing devices [START_REF] Sun | Review on application of PEDOTs and PEDOT:PSS in energy conversion and storage devices[END_REF][START_REF] Wei | A review on PEDOT-based counter electrodes for dyesensitized solar cells[END_REF][START_REF] Koussi-Daoud | Gold nanoparticles and poly(3,4-ethylenedioxythiophene) (PEDOT) hybrid films as counter-electrodes for enhanced efficiency in dye-sensitized solar cells[END_REF][START_REF] Groenendaal | Poly(3,4ethylendioxythiophene] and Its Derivatives : Past , Present , and Future[END_REF][START_REF] Li | PEDOT-based thermoelectric nanocomposites -A mini-review[END_REF] and, more recently, medical applications such as neural prosthesis, [START_REF] Williamson | Controlling epileptiform activity with organic electronic ion pumps[END_REF][START_REF] Green | Conducting Polymers for Neural Prosthetic and Neural Interface Applications[END_REF][START_REF] Gkoupidenis | Neuromorphic Functions in PEDOT:PSS Organic Electrochemical Transistors[END_REF][START_REF] Richardson-Burns | Polymerization of the conducting polymer poly(3,4ethylenedioxythiophene) (PEDOT) around living neural cells[END_REF][START_REF] Zhou | Poly(3,4ethylenedioxythiophene)/multiwall carbon nanotube composite coatings for improving the stability of microelectrodes in neural prostheses applications[END_REF][START_REF] Moulton | Organic bionics: A new dimension in neural communications[END_REF][START_REF] Ouyang | In vivopolymerization of poly(3,4-ethylenedioxythiophene) in the living rat hippocampus does not cause a significant loss of performance in a delayed alternation task[END_REF] cardiovascular implants for tissue regeneration, [START_REF] Karagkiozaki | Bioelectronics meets nanomedicine for cardiovascular implants: PEDOT-based nanocoatings for tissue regeneration[END_REF] antitumor compounds for metastatic cell adhesion [START_REF] Puiggalí-Jou | Fibrin Association at Hybrid Biointerfaces Made of Clot-Binding Peptides and Polythiophene[END_REF] and PEDOT hybrid-based electronic biomedical devices are 2 the multiple domains where PEDOT is involved. However, its low adhesion to various inorganic solid substrates constitutes a serious drawback and its frequent delamination from the substrate decreases the device lifetime. [START_REF] Dupont | Interlayer adhesion in roll-toroll processed flexible inverted polymer solar cells[END_REF]25] Various processes have been used to limit the cracking and delamination of the PEDOT layer. Silane linkers bearing various aromatic units (pyrrole, thiophene, aniline) [START_REF] Inaoka | Chemical and Electrochemical Polymerization of 3-Alkylthiophenes on Self-assembled Monolayers of Oligothiophene-Substituted Alkylsilanes[END_REF][START_REF] Simon | Synthesis and characterization of a new surface derivatizing reagent to promote the adhesion of polypyrrole films to Ntype silicon photoanodes: N-(3-(trimethoxysilyl)propyl)pyrrole[END_REF][START_REF] Wu | Electroless surface polymerization of polyaniline films on aniline primed ITO electrodes: a simple method to fabricate good modified anodes for polymeric light emitting diodes[END_REF][START_REF] Carli | Conductive PEDOT Covalently Bound to Transparent FTO Electrodes[END_REF] have been used to form covalent bonds with metals, indium tin oxide (ITO) or various inorganic oxide substrates. [START_REF] Awada | Surface-initiated polymerization of A-A/B-B type conjugated monomers by palladium-catalyzed Stille polycondensation: towards low band gap polymer brushes[END_REF][START_REF] Awada | Biomimetic Mussel Adhesive Inspired Anchor to Design ZnO @ Poly ( 3-Hexylthiophene ) Hybrid Core @ Corona Nanoparticles[END_REF] According to this concept, the preparation of adhesive conducting PEDOT films was fulfilled by anchoring an EDOT vinyl derivative onto a disilane-functionalized free-radical initiator followed by copolymerization with EDOT. [START_REF] Sadekar | Robust PEDOT films by covalent bonding to substrates using in tandem sol-gel, surface initiated free-radical and redox polymerization[END_REF] Electrostatic interactions between carboxylic acids functionalized by EDOT and inorganic solid subtrates were also found to enhance PEDOT adhesion. [START_REF] Wei | Significant Enhancement of PEDOT Thin Film Adhesion to Inorganic Solid Substrates with EDOT-Acid[END_REF] In recent years, the electrografting process, where electron transfer is followed by the formation of covalent metal-organic bonds between a substrate and a molecular compound, has been considered as a novel means of improving the adhesion of polymer films. [START_REF] Bélanger | Electrografting: a powerful method for surface modification[END_REF][START_REF] Adenier | Electrochemical Oxidation of Aliphatic Amines and Their Attachment to Carbon and Metal Surfaces[END_REF] Electrografting presents several advantages, such as a high selectivity, high versatility and a significant economy of time. It has been applied to several thiophene derivatives, mixed with various co-monomers [START_REF] Labaye | Full Electrochemical Synthesis of Conducting Polymer Films Chemically Grafted to Conducting Surfaces[END_REF][START_REF] Ouhib | Electrografting onto ITO substrates of poly(thiophene)-based micelles decorated by acrylate groups[END_REF] and recently to generate on various electrodes a thick (a few µm) nonconjugated, cross-linked, and adherent P(EDOT-NH2)-based polymer coating through electrochemical oxidation of the amine. This thick layer did not block charge transport through the interface, and subsequent electrochemical PEDOT deposition onto P(EDOT-NH2)-modified electrodes was possible. It was shown that with P(EDOT-NH2) as the anchoring layer, PEDOT coating adhesion was greatly enhanced. [START_REF] Ouyang | Enhanced PEDOT adhesion on solid substrates with electrografted P(EDOT-NH 2 )[END_REF] Diazonium salt electroreduction [START_REF] Bélanger | Electrografting: a powerful method for surface modification[END_REF][START_REF] Delamar | Covalent modification of carbon surfaces by grafting of functionalized aryl radicals produced from electrochemical reduction of diazonium salts[END_REF] is another effective way of electrografting organic compounds onto a surface, and has been widely used by our group to synthesize adherent thin films of conjugated oligomers on various conductive substrates. [START_REF] Stockhausen | Grafting Oligothiophenes on Surfaces by Diazonium Electroreduction: A Step toward Ultrathin Junction with Well-Defined Metal/Oligomer Interface[END_REF][START_REF] Santos | Host -Guest Complexation : A Convenient Route for the Electroreduction of Diazonium Salts in Aqueous Media and the Formation of Composite Materials[END_REF] The technique consists in preparing from an aminophenyl derivative a diazonium salt whose electroreduction leads to the formation of extremely reactive phenyl radicals which, in turn, react with the substrate to form a covalent metal-organic bond. By this technique it is possible generate homogeneous, ultrathin (less than 10 nm in contrast with reference 38). compact and pinhole-free films. We have shown that this latter property is particularly useful and essential for the design of molecular electronic devices. [42-45][46] Several examples of the diazonium electrografting technique used for enhancing the adhesion of organic films on solid substrates have been described. In the first example a thin layer of diphenylamine (DPA) was grafted by the electrochemical reduction of the 4aminodiphenylamine (ADPA) diazonium salt. The DPA-grafted layer did not impeded aniline oxidation, and polyaniline (PANI) was easily deposited on the DPA-modified electrodes. The PANI films generated on these electrodes showed electrochemical behavior similar to that of PANI electrodeposited on bare carbon electrodes but exhibited better chemical stability and high resistance to aggressive environments, that is, high temperatures and ultrasonic etching. [START_REF] Santos | Electrografting Polyaniline on Carbon through the Electroreduction of Diazonium Salts and the Electrochemical Polymerization of Aniline[END_REF] Electrografting of 4-nitrobenzene groups onto a gold surface and onto a porous silicon substrate using the 4-nitrophenyl diazonium salts, followed by the electroreduction of the nitro group to amine, has also been reported as a means of increasing the adhesion of PANI. [START_REF] Vacca | Coating of gold substrates with polyaniline through electrografting of aryl diazonium salts[END_REF][START_REF] Tiddia | Porous silicon-polyaniline hybrid composites synthesized through electroreduction of an aryldiazonium salt: preparation and photocurrent properties[END_REF] In-situ generated pyrrole diazonium derivatives have been electrografted onto nickel surfaces. [START_REF] Jacques | Grafting of 4-pyrrolyphenyldiazonium in situ generated on NiTi, an adhesion promoter for pyrrole electropolymerisation?[END_REF][START_REF] Jacques | Electrografting of in situ generated pyrrole derivative diazonium salt for the surface modification of nickel[END_REF] Diazonium-modified flexible substrates have been prepared for the growth of adherent silver/polypyrrole nanocomposite films using surface-confined UV photopolymerization. [START_REF] Samanta | In Situ Diazonium-Modified Flexible ITO-Coated PEN Substrates for the Deposition of Adherent Silver-Polypyrrole Nanocomposite Films[END_REF] In this paper, we used a two-step strategy (Figure 1) to enhance PEDOT adhesion to an ITO surface. The first step consists in electrografting, by diazonium salt electroreduction, ultrathin adhesion layers based on EDOT or thiophene moieties; the second step consists in the electropolymerization of EDOT monomer onto the primer layer. We demonstrate the efficiency of BTB (1-(2,2'-bisthienyl)benzene) and ETB (4-(5-(2,3-dihydrothieno [3,4b][1,4]dioxin-5-yl)thiophen-2-yl)benzene) as adhesion promoter layers to improve the adherence of PEDOT thin films on ITO. We prove with electrochemical and optical analysis that the particular electronic and optical properties of PEDOT are maintained in the presence of these adhesion-promoting ultrathin layers. Then, we report the plasmonic properties of gold nanoparticles (AuNPs) surrounded by ETB/BTB-PEDOT block polymer as a function of the electronic state of the polymer. We demonstrate that this strategy is an effective way of increasing PEDOT adhesion to plasmonic electrodes and is a useful means of fabricating robust optoelectronic devices. Figure 1. Two-step strategy to enhance PEDOT adhesion to solid substrates: The first step consists in the grafting of an ultrathin layer (5-7 nm) (here oligo(ETB)) acting as an adhesion layers. The second step is the PEDOT deposition from EDOT electropolymerisation on the ETB modified ITO electrode. Note that ETB can be replaced by BTB.

Experimental details

Chemicals and apparatus

1-(2,2'-Bisthienyl)-4-aminobenzene (BTAB) and 4-(5-(2,3-dihydrothieno [3,4-b][1,4]dioxin-5-yl)thiophen-2-yl)aniline (ETAB) precursors were synthesized according to published procedures. [START_REF] Trippé-Allard | Synthesis of nitro-and amino-functionalized πconjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units[END_REF] All chemicals were used as received. tert-Butylnitrite (tBuO-NO), ferrocene (Fc), potassium gold(III) chloride (KAuCl4) and sodium carbonate (Na2CO3) were purchased from Sigma-Aldrich. Lithium perchlorate (LiClO4) and tetra-n-butylammonium tetrafluoroborate (Bu4NBF4) from Aldrich were used as supporting electrolytes. 3,4-Ethylenedioxythiophene (EDOT) from Bayer was distilled before use. Acetone, acetonitrile (ACN) and absolute water-free ethanol were purchased from VWR.

For electrochemical experiments, a conventional three-electrode cell was employed with a CHI 760C potentiostat (CH Instruments, Austin, TX). The auxiliary electrode was a stainless steel grid, and a SCE (4M KCl) was used as reference electrode. Indium tin oxide (ITO)coated glass plates with resistance of 30-50 Ω/square were purchased from Solems and were used as working electrodes. Before modification, the ITO-coated glass slides were ultrasonically cleaned (BRANSON 1510 ultrasonic cleaner) in acetone, ethanol and deionized water, each for 15 min, then carefully rinsed with distilled water and dried in a stream of N2 before use.

Electrografting of adhesion promoters (BTB and ETB)

The in situ formation of the diazonium cation from (BTAB) or (ETAB) was performed under acid-free conditions, by adding tert-butylnitrite (tBuO-NO). [START_REF] Fluteau | Tuning the thickness of electrochemically grafted layers in large area molecular junctions[END_REF][START_REF] Stockhausen | Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction[END_REF] Briefly, an acetonitrile solution containing 5.10 -4 M BTAB or 5.10 -4 M ETAB and Bu4NBF4 (0.1 M) as supporting electrolyte was prepared and degassed by argon bubbling for at least 10 min. Then tBuO-NO was added in excess to the solution (30 equiv.). As the reaction between an amine and tertbutylnitrite is relatively slow, grafting started only after an incubation time of 5 min. After modification the subtrates were rinsed with ACN to remove any residual physically adsorbed moieties from the ITO surface and they were dried in a stream of N2.

Electrochemical deposition of PEDOT on bare ITO and on ITO modified by Oligo-BTB and Oligo-ETB layers

PEDOT thin films were deposited at constant current (0.01 mA/cm 2 ≤ I ≤ 0.2 mA/cm 2 ) and for a time calculated to deliver a constant coulombic charge of 20 mC/cm 2 corresponding to a PEDOT thickness of around 100 nm. It was carried out in aqueous medium (2.10 -2 M EDOT and 0.1 M LiClO4) and in the presence of a surfactant (0.1 M sodium dodecyl sulfate-SDS) according to a previous procedure. [START_REF] Sakmeche | Improvement of the Electrosynthesis and Physicochemical Properties of Poly ( 3 , 4ethylenedioxythiophene ) Using a Sodium Dodecyl Sulfate Micellar Aqueous Medium[END_REF][START_REF] Santos | Electrosynthesis of well-organized nanoporous poly(3,4ethylenedioxythiophene) by nanosphere lithography[END_REF]

AFM Characterization

AFM measurements were performed on a Keysight system (Picoscan 500). AFM images were recorded in the tapping or contact mode with a tip from Budgetsensor (AFM tap 300 G, resonance frequency 300 kHz, force constant 40 nN/m). Experiments were performed to estimate the thickness of the organic layers by the "scratching" procedure. [START_REF] Anariba | Mono-and Multilayer Formation by Diazonium Reduction on Carbon Surfaces Monitored with Atomic Force Microscopy "Scratching[END_REF] A statistical procedure was devised to average the trench depth over a large fraction of the scratch area. [START_REF] Yan | Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions[END_REF]

Preparation of plasmonic devices

AuNPs were deposited on ITO electrodes at a constant potential according to published procedures [START_REF] Nguyen | Comparing plasmonic electrodes prepared by electron-beam lithography and electrochemical reduction of an Au (iii) salt: application in active plasmonic devices[END_REF][START_REF] Nguyen | Large-area plasmonic electrodes and active plasmonic devices generated by electrochemical processes[END_REF] ITO substrates were dipped into a stirred aqueous solution of 2.10 -3 M KAuCl4 and 0.25 M Na2CO3 as supporting electrolyte, and a controlled total charge of 22 mC/cm 2 was applied. Then, after modification, the plasmonic samples were inserted in a home-made planar electrochemical cell consisting of a standard three-electrode configuration (with a stainless steel counter-electrode grid and a silver (Ag/AgCl) pseudo-reference electrode), the whole placed under an Olympus microscope. All potentials are referenced to this Ag/AgCl electrode. The electrolyte solution was carefully degassed prior to the introduction into the cell. Modulation of LSPR signals was checked by switching the applied potential between a cathodic (-1 V/Ag) and an anodic value (0.5 V/Ag).

Adhesion of PEDOT tested by ultrasonic stirring in water

Unmodified and modified electrodes, covered with a PEDOT layer, were immersed in a 25 mL glass vial filled with deionized water. The vials were placed in a BRANSON 1510 ultrasonic cleaner with a power of 80-143 W (frequency 40 kHz) for different times (from a few seconds to 15 min).

Results and discussion

The electrografting of adhesion promoters (BTB and ETB) was performed by cyclic voltammetry from the electroreduction of the corresponding diazonium compounds. With the ETB-diazonium compound, [START_REF] Stockhausen | Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction[END_REF] a first irreversible reduction wave, starting at 0.4 V/SCE with a peak current centered at 0.15 V/SCE (Figure 2a), yields ETB radicals which react with the ITO surface to form a covalent bond. This first ETB monolayer does not impede the reduction of other diazonium salts which then react with the ITO-bonded species to form a multilayer corresponding to an ETB oligomer grafted onto the ITO surface. As of the second cycle, the intensity of the cathodic current decreases and the reduction wave totally vanishes after several cycles. This behavior is associated with the formation of an ultrathin insulating organic layer (less than 10 nm) blocking the surface in the +0.4 V to -0.5 V potential range. Similar results are obtained with BTB-diazonium compounds whose reduction starts at a less positive potential than previously (0.2 V/SCE) with a peak current for the first cycle centered also at a more negative potential, -0.15 V/SCE (Figure S1). The AFM image confirms that the films are homogeneous with no visible pinholes (at the AFM resolution). [START_REF] Martin | Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions[END_REF][START_REF] Fluteau | Tuning the thickness of electrochemically grafted layers in large area molecular junctions[END_REF] The thicknesses of the ultrathin films formed range from 5 to 7 nm (Figure S2). These oligo(BTB) and oligo(ETB) layers grafted onto ITO can switch to a conductive state at a specific potential corresponding to an oxidized-p-doped state of the oligomers. [START_REF] Stockhausen | Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction[END_REF][START_REF] Fave | Electrochemical Switches Based on Ultrathin Organic Films: From Diode-like Behavior to Charge Transfer Transparency[END_REF] This is revealed by the behavior of the redox probes on the modified electrodes. [START_REF] Fave | Tunable electrochemical switches based on ultrathin organic films[END_REF] As an example, Figure 2b and Figure S1 compare the voltammograms of 5.10 -3 M ferrocene in ACN with 0.1 M Bu4NBF4, obtained on ITO, ETB-ITO and BTB-ITO electrodes. Ferrocene oxidation currents are maximum at 0.63 V/SCE and 0.95 V/SCE on ETB-ITO and BTB-ITO electrodes, respectively, instead of 0.37 V/SCE on bare ITO. The oxidation peak of ferrocene on BTB-ITO and ETB-ITO electrodes is thus strongly shifted to more positive potentials than that observed on bare ITO, confirming that these layers are conductive when the applied potential is above an anodic polarization threshold but insulating below this threshold. Furthermore, the replacement of a thiophene unit in BTB by the electron-rich 3,4-EDOT unit lowers the insulating-conducting switching potential.

Electrooxidation of EDOT was then carried out on the modified ITO electrodes and compared to that on bare ITO. In a first series of experiments PEDOT film was electrodeposited by sweeping the potential from -0.75 V/SCE to 1 V/SCE. Figure 3a andS3 shows the first electrochemical cycle of EDOT oxidation on ITO electrodes modified by ETB and BTB ultrathin layers, respectively. The CVs on the modified electrodes are similar to those on bare ITO and demonstrate that the presence of ultrathin ETB (or BTB) adhesion layers on ITO does not prevent EDOT oxidation and the deposition of PEDOT film. However, on bare ITO the current intensity grows abruptly at 1 V/SCE while on the ETBmodified electrode the intensity grows more smoothly but at a lower potential (onset at 0.75 V/SCE). All these data confirm that the electropolymerization of the EDOT remains possible on the ultrathin switchable ETB or BTB adhesion layer deposited on ITO.

Note that in a previous study, a minimal amount of EDOT was oxidized on ultrathin BTB layers (by sweeping the potential at the foot of the EDOT oxidation signal). We demonstrated that grafting only a few (less than 3) nanometers of EDOT moieties (5 to 6 units) onto the switchable BTB layer completely changes its switching properties. A range of In the present studies, we deposited much thicker PEDOT layers on the ETB-modified ITO-or BTB-modifies ITO substrate. To do so, EDOT was electropolymerized at constant current (with intensities between 10 -2 mA/cm 2 and 2.10 -1 mA/cm 2 ) and for deposition times calculated to obtain a charge deposition (Qdeposition) of 20 mC/cm 2 corresponding to a PEDOT thickness of about 100 nm. Typical chronopotentiometric curves recorded during PEDOT deposition on ITO and ETB-modified ITO electrodes are shown in Figure S4. The potential stabilizes at a constant value during the whole deposition process and the initial potential observed in the first seconds is smaller on modified ETB-ITO electrodes. Then, the electroactivity of the PEDOT films was characterized by cyclic voltammetry (Figure 3b and S5). The same ranges of faradic and capacitive currents are observed for PEDOT deposited on bare ITO and PEDOT deposited on ETB modified ITO, confirming that the same amount of PEDOT is deposited on both electrodes. The PEDOT film on the ETB-modified electrode is characterized by reversible oxidation/reduction peaks centered at -0.50 V/SCE and -0.70 V/SCE, respectively (Figure 3b), while on the BTB-modified electrode a similar reversible oxidation/reduction peak occurs at slightly less negative potentials of -0.40 V/SCE and -0.50 V/SCE, respectively (figure S6).

The charge required to dope the PEDOT film, named Qdoping i.e. the charge for the oxidation of the deposited PEDOT film between -1 and 0.4 V, was measured from the cyclic voltammograms (Figure 3b) of the PEDOT deposited on the various electrode (ITO,ETBmodified ITO and BTB-modified ITO electrodes). The Qdoping/Qdeposition ratio was then estimated. For most conductive polymer and when the deposition yield is close to 100% this ratio is around 10%, since for each monomer 2 electrons are removed in the electropolymerization while 0.2 to 0.3 electrons are used to dope the growing film. As a consequence Qdoping/Qdeposition is between 9% (0.2/2.2) and 13% (0.3/2.3) [START_REF] Camalet | Aniline electropolymerization on platinum and mild steel from neutral aqueous media[END_REF][START_REF] Lacroix | Polyaniline -a very fast electrochromic material[END_REF]. This is clearly observed for all PEDOT films generated in this study as long as the current intensities used during EDOT oxidation are above 2.10 -2 mA/cm 2 . (Table 1), which indicates that deposition is quantitative. However, this ratio drops to 6-10% for lower current intensities (10 -2 mA/cm 2 ) which indicates a deposition yield below 100% under these conditions. PEDOT coatings synthesized on ITO, BTB-or ETB-modified ITO electrodes were ultrasonicated to estimate the adhesion strength of the PEDOT layer. These PEDOT films are about 100 nm thick, corresponding to a charge deposition of about 20 mC and their adhesion is estimated by the Q'doping/Qdoping ratio where Qdoping and Q'doping are the charges corresponding to the doping yield before and after ultrasonication, respectively. This ratio gives an estimate of the percentage of PEDOT which remains attached to the substrate. Poor adhesion was found for PEDOT film deposited on bare ITO (Q'doping/Qdoping less than 30%). After a few seconds of ultrasonication, the PEDOT film was partially delaminated; this was established by the sharp decrease in the current density and the disappearance of the capacitive and faradic currents (Figure 4a). On the contrary, the PEDOT films electrodeposited on the ETB-or BTB-modified electrodes remained intact after 15 min of ultrasonication. Their electrochemical activity (Figure 4b) exhibits capacitive and faradic currents similar to those before ultrasonication. Similar behaviors are observed for all current densities but, for the largest current density (10 -1 mA/cm 2 ), the PEDOT film on the BTBmodified electrode appears to be less resistant (Q'doping/Qdoping = 78%) than at lower current densities, where Q'doping/Qdoping is about 90-95% (Table 1). In contrast, the same PEDOT film formed on ITO-ETB at 10 -1 mA/cm 2 remains adherent after sonication (Q'doping/Qdoping = 94%). Figure 4c summarizes these results and compares Qdoping before and after ultrasonication for ITO-PEDOT and ITO-ETB-PEDOT electrodes, while Figure 4d shows images of the electrodes. Both figures indicate that PEDOT grown on ETB-modified electrodes are strongly adherent to the underlying ITO substrate while PEDOT grown directly on ITO is not. Similar curves are shown in Figure S5 and S6 for ETB-and BTB-modified electrodes covered by PEDOT deposited at different current densities.

Overall, these results demonstrate that the grafted BTB and ETB layers considerably increase the adhesion of PEDOT film on the ITO electrode. This strong adhesion increase can be directly used to enhance the stability of an active plasmonic device [START_REF] Lacroix | Tailored Surfaces/Assemblies for Molecular Plasmonics and Plasmonic Molecular Electronics[END_REF] consisting of a PEDOT film deposited on a plasmonic electrode (Au NPs deposited on ITO). [START_REF] Stockhausen | Giant Plasmon Resonance Shift Using Poly(3,4ethylenedioxythiophene) Electrochemical Switching[END_REF] The plasmonic electrodes were prepared by electrochemical reduction of a gold salt which has been described elsewhere. [START_REF] Nguyen | Large-area plasmonic electrodes and active plasmonic devices generated by electrochemical processes[END_REF] These substrates show strong extinction spectra around 600 nm due to localized surface plasmon resonance (LSPR). The LSPR is very sensitive to the dielectric function of the surrounding medium. As a consequence, when PEDOT is deposited on Au NPs, the LSPR can be controlled by the electrochemical reversible redox switch of the PEDOT film, which is accompanied by changes in its color and dielectric function. [START_REF] Sakmeche | Improvement of the Electrosynthesis and Physicochemical Properties of Poly ( 3 , 4ethylenedioxythiophene ) Using a Sodium Dodecyl Sulfate Micellar Aqueous Medium[END_REF][START_REF] Dietrich | Electrochemical and spectroscopic characterization of polyalkylenedioxythiophenes[END_REF][START_REF] Baba | Optical Properties of Ultrathin Poly(3,4ethylenedioxythiophene) Films at Several Doping Levels Studied by In Situ Electrochemical Surface Plasmon Resonance Spectroscopy[END_REF][START_REF] Nguyen | Highly Resolved Nanostructured PEDOT on Large Areas by Nanosphere Lithography and Electrodeposition[END_REF] However, delamination of PEDOT from Au NPs or from ITO decreases the lifetime of the ITO-AuNPs-PEDOT active plasmonic devices. The use of BTB and ETB as adhesion promoters followed by PEDOT electrodeposition on Au NPs is thus a means of fabricating robust plasmonic devices.

The extinction spectrum of a AuNPs-PEDOT(100 nm) plasmonic device obtained without an adhesion promoter is shown in Figure 5a. The PEDOT film was polarized at -1.0 V/SCE in aqueous LiClO4 solution and dried in air before its spectrum was recorded. A redshift of 34 nm of the LSPR relative to that of AuNPs in air is clearly observed. Then the device was ultrasonicated for a few minutes and its extinction spectrum recorded again (Figure 5a, red curve). Neither extinction nor absorption of light was measured, which demonstrates that the PEDOT layer and the gold NPs were totally removed by ultrasonication. These results clearly illustrate the poor adhesion of PEDOT on ITO and gold substrates.

The same experiments were then performed on a plasmonic device with the ETB adhesion promoter. ETB was first grafted onto ITO-AuNPs as described above and 100 nm of PEDOT film was then deposited on the ITO-AuNPs-ETB system. With PEDOT in its neutral state (applied potential-1 V/SCE but experiments performed in air) the LSPR is red-shifted by 30 nm, which is similar to that obtained without the ETB layer. The same behavior has already been reported for plasmonic devices with AuNP nanoarrays coated with an ultrathin BTB layer [START_REF] Schaming | Tunable Plasmon Resonance of Gold Nanoparticles Functionalized by Electroactive Bisthienylbenzene Oligomers or Polythiophene[END_REF] while evidence of the deposition of BTB directly on gold was also directly evidenced by SEM [START_REF] Nguyen | Plasmon-Induced Nanolocalized Reduction of Diazonium Salts[END_REF]. The presence of an ultrathin layer of ETB (5-10 nm) on the surface of the AuNPs does not significantly modify the optical properties of the plasmonic device formed by an assembly of Au NPs coated with PEDOT. Then the device was ultrasonicated for 30 minutes and its extinction spectra recorded (Figure 5b, red curve). It shows a strong LSPR band, indicating that the whole coating is conserved. These results clearly illustrate the strong adhesion of PEDOT on ETB-modified ITO and gold substrates. of AuNPs in air (black line), coated with ETB-PEDOT before (green line) and after (red line) sonication. All spectra have been shifted on the y axis for better comparison.

The LSPR properties of the ITO-AuNPs-ETB-PEDOT device were then switched. Extinction spectra were recorded in situ while the electrode was polarized in 0.1 M LiClO4 aqueous electrolyte. Figure 6 shows when the polarization is changed at 0.5 V (doped state, green curve) and -1 V/SCE (undoped state, blue curve) the plasmon band is considerably redshifted, from 589 nm to 711 nm, The plasmonic response of the device is reversible and its properties are maintained over successive polarization cycles between 0.5 V and -1 V and after several ultrasonication cycles. Moreover, this LSPR shift for a disordered assembly of Au NPs is comparable to that obtained with an ordered array of PEDOT-coated Au nanoplots (192 nm). [START_REF] Stockhausen | Giant Plasmon Resonance Shift Using Poly(3,4ethylenedioxythiophene) Electrochemical Switching[END_REF] Figure 6. Normalized extinction spectra of AuNPs in air (black line), coated with ETB-PEDOT and polarized at 0.5 V/Ag (green line) and -1.0 V/Ag (blue line) in aqueous electrolyte (0.1 M LiClO4). All spectra have been shifted on the y axis for better comparison.

CONCLUSION

Ultrathin layers of oligo(ETB) or or oligo(BTB) generated by electroreduction of diazonium salts considerably enhance PEDOT adhesion to ITO and gold substrates without altering the electrochemical and optical properties of the electrodeposited PEDOT film. This enhancement of PEDOT adhesion on solid substrates can be explained by the formation of covalent bonds at the electrode/oligo(BTB) or oligo(ETB) interface and between oligo(promoter) and the PEDOT layers, giving new structured block polymers firmly anchored to the substrate. This strategy has been used for the preparation of plasmonic devices with AuNPs surrounded by an electrochromic PEDOT film. The plasmonic device is destroyed by ultrasonication when PEDOT is directly deposited on ITO and Au NPs whereas it resists ultrasonication when a BTB or ETB adhesion layer is used. The large reversible shift of the LSPR of the ITO-AuNPs extinction spectra between the reduced state and the oxidized state of ETB-PEDOT film is comparable to that observed on unmodified ITO-AuNPs-PEDOT AuNPs in air AuNPs-ETB-PEDOT -1.0 V AuNPs-ETB-PEDOT +0.5 V plasmonic electrode. This procedure should be an efficient way of preparing robust optoelectronic devices.

From a more general point of view, we anticipate that the two-step method developed in this study to promote PEDOT adhesion will impact the development of organic electronic and bioelectronic research fields, as well as many technologies which use PEDOT.

ASSOCIATED CONTENT

Supporting information

The Supporting Information is available free of charge on X website at DOI X.

BTB electrodeposition on ITO and ferrocene signal on BTB-ITO electrode, thickness measurement of BTB layer on ITO using a scratch experiment, EDOT electrodeposition on bare ITO and on BTB-modified ITO for current density of 0.05 mA/cm 2 and using cyclic voltammetry (first cycle). Cyclic voltammograms of a PEDOT film deposited on ITO or on ETB-modified electrode at 2.10 -1 mA/cm 2 ; CVs are given before and after sonication. Cyclic voltammograms of PEDOT films deposited on ITO and BTB-modified electrode at 10 -1 mA/cm 2 before and after sonication

Figure 2 .

 2 Figure 2. a) Electroreduction by cyclic voltammetry of 5.10 -4 M ETB-diazonium on ITO electrode in ACN with 0.1 M Bu4NBF4. Scan rate 0.1 V/s. b) Cyclic voltammograms of ferrocene redox probe on bare ITO electrode (black line) and on ETB-ITO electrode (red line).

  Figure 3. a) First voltammogram of EDOT oxidation on bare ITO electrode (black lines) and on ETB-modified ITO electrode (red line) b). Cyclic voltammograms of PEDOT deposited on bare ITO electrode (black lines), and on ETB-modified ITO-electrode (red line), PEDOT was deposited at constant current (5.10 -2 mA/cm 2 ).

Figure 4 .

 4 Figure 4. Cyclic voltammograms of PEDOT films obtained at constant current intensity (10 -2 mA/cm 2 ) a) on bare ITO electrode before (black line) and after ultrasonication (red line), b) on ETB-modified electrode before (black line) and after ultrasonication (red line). c) Faradic and capacitive charges involved in the doping of the PEDOT film before (black) and after ultrasonication (red) for PEDOT on ITO and PEDOT on ETB-ITO electrodes. d) Optical images of PEDOT films on ITO (left) and ETB-ITO (right) before and after ultrasonication.

Figure 5 .

 5 Figure 5. a) Normalized extinction spectra of AuNPs in air (black line), coated with a PEDOT film before (green line) and after (red line) ultrasonication. b) Normalized extinction spectra

Table 1 .

 1 Comparison of Qdoping, Qdoping/Qdeposition and Q'doping/Qdoping for PEDOT deposited on bare ITO, BTB-modified ITO and ETB-modified ITO electrodes before and after ultrasonication.

		I (mA/cm 2 )	Deposition time (s)	Q	before (mC/cm 2 ) sonication	*	%	(mC/cm 2 ) Q after sonication	(%)
	PEDOT	1.10 -2	2000		1.88	9.5		0.16	8.5
	deposited bare ITO	2.10 -2	1000		2.56	12.9		0.70	27.3
		5.10 -2	400		2.63	13.2		0.35	13.3
		1.10 -1	200		2.75	13.9		0.96	34.9
	PEDOT	1.10 -2	2000		1.32	6.6		1.30	98.4
	deposited on BTB modified	2.10 -2	1000		2.20	11.1		2.00	90.9
	ITO electrodes
		5.10 -2	400		2.07	10.4		1.89	91.3
		1.10 -1	200		2.20	11.1		1.72	78.1
	PEDOT	1.10 -2	2000		1.68	8.5		1.60	95.2
	deposited on	2.10 -1	80*		1.70	10.6		1.60	94.1
	ETB modified
	ITO-electrodes
	(*) Qdeposition = 16 mC/cm 2

ACKNOWLEDGMENTS

This work was supported by the Agence Nationale de la Recherche ANR (France) (ANR-15-CE09 0001-01)

AUTHOR INFORMATION

Corresponding Author *Email: lacroix@univ-paris-diderot.fr

Notes

The authors declare no competing financial interest.