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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43838−43845
Ultrathin Polydiacetylene-Based Synergetic Composites with Plasmon-Enhanced Photoelectric Properties Anastasiia L. Dubas,† Alexey R. Tameev,† Alexandra I. Zvyagina,† Alexander A. Ezhov,‡,§ Vladimir K. Ivanov,∥,⊥ Burkhard König,# Vladimir V. Arslanov,† Oxana L. Gribkova,† and Maria A. Kalinina*,† †
A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31 Leninsky prospect, Moscow 119071, Russia ‡ Faculty of Physics, M. V. Lomonosov Moscow State University, 1-2 Leninskiye Gory, GSP-1, Moscow 119991, Russia § A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky prospect, Moscow 119991, Russia ∥ N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninsky prospect, Moscow 119991, Russia ⊥ Faculty of Materials Science, M. V. Lomonosov Moscow State University, 1-73 Leninskiye Gory, GSP-1, Moscow 119991, Russia # Institut für Organische Chemie, Universität Regensburg, D-93040 Regensburg, Germany S Supporting Information *
ABSTRACT: Fabricating plasmon-enhanced organic nanomaterials with technologically relevant supporting architectures on planar solids remains a challenging task in the chemistry of thin films and interfaces. In this work, we report a bottom-up assembly of ultrathin layered composites of conductive polymers with photophysical properties enhanced by gold nanoparticles. The polydiacetylene component was formed by photopolymerization of a catanionic mixture of pentacosadiynoic surfactants on a surface of citrate-stabilized gold hydrosol monitored by a fiber optic spectrometer. Microscopic examination of the 3 nm thick solid-immobilized film showed that gold nanoparticles (AuNPs) do not aggregate within the monolayer upon polymerization. This polydiacetylene/AuNPs monolayer was coupled with 60 nm thick polyaniline-based layer deposited atop. The resulting polymer composite with an integrated 4-stripe electric cell showed nonadditive electric behavior due to the formation of electron−hole pairs with increased charge carrier mobility at the interface between the polymer layers. Under visible light irradiation of the composite film, a plasmonic effect of the gold nanoparticles was observed at the onset of photoconductivity, although neither polydiacetylene nor the polyaniline component alone are photoconductive polymers. The results indicate that our bottom-up strategy can be expanded to design other plasmon-enhanced ultrathin polymer composites with potential applications in optoelectronics and photovoltaics. KEYWORDS: polydiacetylene, gold nanoparticles, ultrathin films, Langmuir−Blodgett technique, plasmon coupling, plasmon-enhanced properties, photoconductivity, photovoltaics
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tures into ultrathin films, which can be transferred onto solids by conventional dip coating techniques.13,14 A combination of interfacial geometry and energetics favors the formation of inplane arrangements of noble metal nanoparticles via Langmuir−Blodgettry, a term suggested by Yang for creating well-defined colloidal structures in two dimensions by using nanosized objects and a Langmuir trough.15 During the past decade, this strategy has been widely applied for creating optically tuned functional surfaces presenting differently
INTRODUCTION Plasmonic nanostructures, a special class of metal nanomaterials, are of paramount importance for modern nanoscience and nanotechnologies both in practical and fundamental aspects. Plasmonic nanoparticles are widely applied in electronics,1,2 optics,3−5 chemical sensing,6,7 medicine,8,9 and many other fields of modern industry. As assemblies associated with thin organic films on solid surfaces, plasmonic structures can tailor the optics of interfaces in a desirable fashion that complements both the tendencies of optoelectronic devices to miniaturization and their compatibility with bottom-up fabrication strategy.7,10−12 The air/water interface provides a convenient platform for ordering of various organic-stabilized nanostruc© 2017 American Chemical Society
Received: August 14, 2017 Accepted: November 29, 2017 Published: November 29, 2017 43838
DOI: 10.1021/acsami.7b12156 ACS Appl. Mater. Interfaces 2017, 9, 43838−43845
Research Article
ACS Applied Materials & Interfaces
conductive polymers such as PANI:PAMSA and PEDOT:PSS in two different supporting architectures: a typical sandwich structure of solar cells and a lateral four-probing layout. We showed that the formation of the interface between two different polymer layers results in a synergy of their electric properties in the laterally ordered architecture, whereas in the sandwich structure this interface prevents efficient charge transfer to the electrode. Finally, we demonstrate the plasmonassociated onset of photoconductivity of polydiacetylene films with polyaniline-based ultrathin layers upon their irradiation with visible light.
ordered planar assemblies of various nanostructures. The method for preparing these 2D structures is a capping of plasmonic nanoparticles with self-assembled monolayers of long-chain or branched-chain thiols to add hydrophobicity to metal clusters and thereby assist their spreading on the surface of water.13,16−20 After transferring onto solids, such monolayers may yield long-range ordered tightly packed assemblies or linear patterns depending on the kinetics of their deposition. The optical properties of the resulting solid-immobilized structures, that is, the spectral position of the characteristic wavelength of localized surface plasmons (LSP) are primarily controlled by the size and nature of surfactant stabilizer.14,21,22 Another approach to form ultrathin plasmonic films uses electrostatic interactions between opposite charged nanoparticles and monolayers of fatty surfactants.23 For their catanionic mixtures, highly tunable plasmonic arrangements can be fabricated through substrate-mediated condensation (SMC) on a surface of hydrosols of plasmonic particles.24 The condensation occurs due to the strong interactions between oppositely charged surfactants, which form readily a crystalline solid phase at high surface concentrations. The arrangement of nanoparticles entrapped in this solidified transparent matrix is kinetically controlled and it may vary from separated particles and short chains to laterally extended networks.25 Some researchers have used the interactions between nanoparticles and biological templates such as DNA26 or polypeptides to control the spectral behavior of resulting 2D plasmonic assemblies.27−29 Finally, plasmonic structures can be stabilized by polymers,30,31 either already synthesized before spreading or formed through polymerization of monomers in the monolayer,32 to provide steric control of the assembly of metal nanoparticles in the floating monolayers.33,34 However, in most studies the surfactants or polymers are utilized only as an optically inert matrix for plasmonic particles. These particle-capping materials are rarely used to add some new functionalities35,36 or synergistic properties to the resulting hybrid plasmonic surface coating. Herein, we describe the formation of ultrathin composite materials, in which the plasmonic properties of gold nanoparticles are coupled with the photophysical properties of the stabilizing 2D polymer. An approach for fabricating such a structure resembles to a certain degree the SMC method described above. We used the catanionic mixture of surfactant diacetylene monomers, which are capable of polymerization at the air/water interface37−39 to limit lateral mobility of 18 nm gold nanoparticles (AuNPs) within the polymer matrix and to prevent their aggregation. Pentacosadiynoic monomer derivatives are well-studied polymerizable surfactants, which offer a combination of properties that make them especially useful in plasmonic-associated composites. First, their polymers absorb visible light at wavelengths matching the excitation range of localized surface plasmon (LSP) of gold nanoparticles (520− 600 nm).40−42 Second, when polymerized, they form a polydiacetylene film, which possess comparatively high conductivity.43,44 By using a combination of fiber-optic spectrometry and microscopy methods, we studied the formation and properties of photopolymerized mixed monolayers with entrapped AuNPs to relate their optical properties to the morphology of the resulting films on solids. The monolayers of polydiactylenes, integrated with AuNPs, might initiate photoelectrical effects. We explored this by using the combinations of polydiacetylene/AuNPs films and other
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MATERIALS AND METHODS
Materials. Amphiphilic monomer 10,12-pentacosadiynoic acid (PDA; reagent grade, Fluka) was used as received. 10,12Pentacosadiynamine (PDAM) was prepared by activation of the carboxylic acid group of PDA with thionyl chloride and subsequent reaction with aqueous ammonia. The compound was purified by preparative HPLC. Monolayers based on PDA, PDAM, and equimolar PDA/PDAM mixtures with concentration of 1 × 10−4 M were formed from solutions in chloroform containing methanol (0.1%; solvents of special purity grade were used). The monolayers were formed immediately after the preparation of individual and mixed solutions of PDA and PDAM. Gold hydrosol was synthesized by a common procedure for citrate reduction of HAuCl445 (Acros Organics) in water deionized to 16 MΩ cm resistivity. For as-prepared nanoparticles, the surface plasmon band appears at 519−520 nm in the UV−vis adsorption spectra (Figure S5). The synthetic yield corresponds to a number of nanoparticles of ∼1015 L−1. The pH of gold hydrosol was 6.7 ± 0.1. Solution of conductive polymer poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT−PSS) (Orgacon IJ-1005, SigmaAldrich) was used as received (0.8 wt % in H2O). Conductive polyaniline and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PANI−PAMPSA) complex was synthesized46 using PAMPSA (Mw ≈ 2 000 000, 15 wt % in H2O, Sigma-Aldrich). Regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT, 4002-EE; Rieke Metals) and fullerene derivative PC71BM (Sigma-Aldrich) were used as received to prepare photovoltaic (PV) solar cells. 1.1 mm thick glass plates coated with ITO (indium and tin oxides) of 15 Ohm/sq. (KINTEC) served as conductive transparent substrates for the PV cells. Ultrathin Films Fabrication. Monolayer compression isotherms (dependences of surface pressure π on surface area A per molecule) were obtained with the Langmuir balance (KSV Instruments Ltd.) and a Teflon trough. The two-dimensional pressure in a monolayer was measured by a platinum Wilhelmy plate. Immediately before an experiment, the Teflon trough was successively washed with acetone, chloroform, and distilled water. Barriers made of Delrin polyacetal resin were wiped with ethanol and washed with distilled water. An examined solution was spread onto a citrate-stabilized AuNPs hydrosol with a DISTRIMAN automatic micropipette (Gilson) by 5 μm droplets in a chessboard pattern to uniformly distribute a substance over the subphase. The monolayers were exposed for 20 min until the solvent completely evaporated; then, they were compressed at a rate of 5 mm min−1. All experiments were carried out at 25 ± 1 °C. For polymerization, the monolayers were compressed to a selected surface pressure and equilibrated at this pressure and a constant surface area for 20 min. Then, the monolayers were polymerized by irradiation with a VL-6.LC UV lamp (Vilber Lourmat, 265 μW/cm2 at a distance of 10 cm, 254 nm). The spectral characteristics of a monolayer were measured directly during polymerization of pentacosadiyne monomers on the surface of an aqueous subphase with an AvaSpec-2048 UV−vis fiber-optic spectrometer (Avantes) by recording variations in the intensity of relevant absorption bands with time at a constant surface pressure. The spectra were measured with intervals of 1 min, with the first one being recorded 0.5 min after the UV lamp was switched on. 43839
DOI: 10.1021/acsami.7b12156 ACS Appl. Mater. Interfaces 2017, 9, 43838−43845
Research Article
ACS Applied Materials & Interfaces For preparing the ultrathin hybrid films polymerized monolayers with integrated AuNPs were transferred onto silicon and glass substrates in the vertical manner at a velocity of 1 mm min−1 using an automatic dipper. The layer of PANI−PAMPSA was deposited onto the PDA/PDAM/AuNPs film by drop-casting as described in.46,47 Scanning Electron Microscopy (SEM). Scanning electron microscopy images of polymerized films were obtained using a NVision 40 workstation (Carl Zeiss) at 1 and 5 kV accelerating voltages using secondary electron (SE2 and InLens) detectors. The silicon-supported film surfaces were scanned at an electron beam accelerating voltage of 1 kV. Atomic Force Microscopy (AFM). Atomic force microscopy images were obtained by the scanning probe microscopes SOLVER P47-PRO and NTEGRA Prima (NT-MDT). Semicontact mode was used. High resolution noncontact/semicontact “Golden” silicon AFM probes NSG01 series (NT-MDT) were used. The amplitude of the “free air” probe oscillations was from 20 to 25 nm (peak-to-peak). Confocal Laser Scanning Microscopy (CLSM). Confocal laser scanning microscopy images and spectra were measured by CLSM FluoView FV1000 (Olympus) equipped with spectrometer confocal unit. This equipment allows getting the local emission spectra with spatial resolution down to square micron in order of magnitude. Conductivity Measurements. The electrical conduction of thin films was measured using the four-probe technique in the 4-stripe layout (Figure S1). A film of thickness t was formed on a glass substrate with aluminum electrodes preliminary thermally deposited using a BOC Edwards Auto 500 Evaporation System. The narrow electrodes were made as parallel stripes of 10 mm in width (W) with equal distance (d) of 0.7 mm between neighbors. The outer and inner pairs of the electrodes served as current and voltage contacts, respectively. Electrical measurements were carried out in a glovebox with Ar atmosphere at room temperature using Keithley 236 and 2400 source-meter units and a standard probe station setup. The conductivity was measured in darkness and under illumination at wavelength of 532 nm (DPSS laser, 5 mJ). The relative error of conductivity measurements is 15%. PV Cell. Organic PV cells of the architecture of ITO/ABL/ P3HT:PC71BM/LiF/Al were prepared as follows. ITO-glass substrates were cleaned in isopropanol in an ultrasonic bath for 10 min and then in an oxygen plasma. A 3÷6 nm layer of the PDA/PDAM and AuNPs blend was deposited onto an ITO-glass substrate as described earlier. On the top of the layer, a PEDOT−PSS layer of 40 nm in thickness was spin-coated at 2000 rpm onto an ITO-glass substrate for 1.5 min and annealed at 80 °C for 15 min. These both layers or one of them served as the anode buffer layer (ABL). Then a photoactive layer of ∼150 nm in thickness was spin-coated at 900 rpm from a solution in chlorobenzene of a blend (1:0.8 wt.) of P3HT and PC71BM. The samples were then transferred into a glovebox (MBraun MB200MOD) with argon atmosphere dried during 24 h at r.t. then annealed at 90 °C for 5 min and at 140 °C for further 5 min. LiF (0.9 nm) and Al cathode (60 nm) were deposited sequentially on the P3HT:PC71BM layer by vacuum (∼10−6 mbar) thermal evaporation in Edwards Evaporator Auto500. Testing of the PV cells was performed inside the glovebox. J−V characteristics were recorded by a SMU Keithley 2400 at AM1.5G conditions provided by a solar simulator (Xe lamp 150 W Oriel Solar Simulator, Newport Corp.). The relative error of J−V measurements is 5%.
Figure 1. Schematically illustrated process for the fabrication of PDA/ PDAm monolayer with incorporated AuNPs: (a) the mixture PDA/ PDAm was spread onto a surface of gold hydrosol, (b) the compressed monolayer was irradiated by UV light and measured by a fiber optic spectrometer, (c) the PDA/PDAm mixture converted into the 2D polymer film with entrapped AuNPs, and (d) the polymerized PDA/ PDAm film with incorporated AuNPs was vertically deposited onto the solid support.
two diacetylene surfactants compressed on the surface of gold colloidal solution followed the same pattern previously observed for the mixed monolayer of their single-bond saturated analogs (see Figure S2, curve 3, and Figure S3, curve 3).39 For polymerization, the monolayers were compressed to a selected surface pressure and equilibrated at a constant surface area of 35 Å2 molecule−1 for 20 min. Then, the monolayers were polymerized by irradiation with a VL-6.LC UV lamp (Vilber Lourmat, 265 μW/cm2 at a distance of 10 cm, 254 nm). The spectral characteristics of a monolayer were measured directly during polymerization of pentacosadiyne monomers on the surface of an aqueous subphase with an AvaSpec-2048 UV− vis fiber-optic spectrometer (Avantes) by recording variations in the intensity of relevant absorption bands with time at a constant surface pressure. The spectra were measured with intervals of 1 min, with the first one being recorded 0.5 min after the UV lamp was switched on. The polymerized monolayers with integrated AuNPs were vertically transferred onto silicon wafers and glass substrates at a velocity of 1 mm min−1 using an automatic dipper. The colloid-induced condensation effect, which is an indication of adsorption of AuNPs on the monolayer, was observed as a continuous decrease of surface pressure, when the area of condensed monolayer was kept constant (Figure S4). However, the spectra of the compressed mixed monolayer monitored in the course of photopolymerization (Figure 2) differ significantly from those of single-component monolayers of PDA and PDAM (Figures S6 and S8). The diacetylene surfactants in their nonpolymerized forms do not absorb light in the range 300−800 nm, and they gave no spectral response before polymerization. At the initial stage of the process (0÷1 min), the extinction increased in a comparatively narrow range of wavelengths with maximal absorbance at 540 nm. This band
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RESULTS DISCUSSIONS To study the influence of gold nanoparticles on the properties of the monolayers of diacetylene surfactants, the surface pressure isotherms for single-component monolayers of 10,12-pentacosadiynoic acid (PDA) and 10,12-pentacosadiynamine (PDAM) as well as that of their mixture on a surface of gold hydrosol (Figure 1) were recorded and compared (for details on the monolayers of individual components, see Figure S2). We found that the phase behavior of the mixture of these 43840
DOI: 10.1021/acsami.7b12156 ACS Appl. Mater. Interfaces 2017, 9, 43838−43845
Research Article
ACS Applied Materials & Interfaces
Figure 2. UV−vis spectra for a monolayer of an equimolar PDA/ PDAM mixture recorded on a surface of the citrate-stabilized gold hydrosol as depending on the time of the UV-induced (254 nm) polymerization. The spectra were measured with intervals of 1 min, with the first one being recorded 30 s after the UV lamp was switched off. The increase in time is denoted by the arrow.
was coupled with a less intense absorption at 507 nm, both suggesting the formation of a mixed polymer in its red form (see also SI−S2 and Figure S10). During further polymerization, the extinction increased sharply and dramatically with a simultaneous broadening of the spectral band. The high intensity and the broadness of the resulting band are significantly larger than these parameters for the monomolecular films of polydiacetylenes without AuNPs.39 The maximal extinction of the monolayer exceeded that of the polymer film on the aqueous subphase by more than two times (Figure S10). These spectral characteristics, which are more typical for gold colloidal 2D assemblies, are an indication of the impact of plasmonic nanostructures on the spectral properties of photopolymerized PDA/PDAM film. Total extinction measured for this film is a combination of absorbance of the polymer and a sum of absorbance and scattering of the filmentrapped AuNPs. The most remarkable aspect of this system is the stability of the position of the extinction maximum in the course of polymerization. We did not observed a red shift of the band at 540 nm that might occur due to a plasmon coupling in aggregated gold colloid.48 This stability suggests that the tightly packed polymer matrix prevents aggregation of AuNPs at the interface (for the spectra of the polymer film with aggregated AuNPs and formed at low surface pressure, see Figure S11). SEM investigation of the films transferred onto the silicon wafers complemented our spectral observations. Figure 3 shows the fragments of the PDA/PDAM film compressed up to 30 mN m−1, then equilibrated for 20 min, and polymerized on the surface of gold hydrosol in a dark room. The resulting structure is an extended organic surface coating presenting numerous and rather closely packed nanoparticles. The particles are separated from each other by a distance comparable with the size of a single AuNP. In a low-voltage scanning regime, it is possible to resolve a number of round-shape hole-like defects with similar size, which most likely formed due to the squeezing out of AuNPs from the diacetylene monolayer, because of its shrinking upon polymerization. AFM investigation resolved the same hole-like defects (Figure 3c,d), although not in the vicinity of the AuNPs because of the AFM probe radius. The minimal thickness of the polymer film with embedded AuNPs measured by AFM was about 3 nm (Figure S13). The formation of such empty holes can be potentially useful as a route to fabricate 2D polymer nanosieves with an adjustable diameter of the defects.
Figure 3. (a, b) SEM images of the PDA/PDAM polymer matrix with incorporated AuNPs transferred onto the silicon wafer. The area of a green square in (a) is equal to a scanned area shown in (b) with higher resolution. For a large-scale SEM image and the AFM image with corresponding surface profile, see Figure S12; (c, d) AFM images and corresponding surface profile of the PDA/PDAM polymer matrix with integrated AuNPs transferred onto the silicon wafer (after Z-scale reduction for better resolution of the hole-like defects; for the original image see Figure S14b). An area of the green square in (c) is equal to a scanned area shown in (d) with a larger resolution.
Another remarkable aspect of this system is that the number of particles packed within the polymer matrix can be kinetically controlled. Packing density for AuNPs in the resulting film depends on the time allowed for monolayer equilibration prior to photopolymerization on the surface of gold colloidal solution. For 5 min of monolayer exposition, the number of particles in the resulting structure on the solid surface is rather low, whereas for 20 min equilibration the amount of film-bound particles increased significantly. We found, however, that further increase of the number of particles does not lead to the formation of an aggregated colloidal phase in the polymer film, which prevents such aggregation irrespective of the time allowed for the their adsorption (Figure S14). Instead, the high loading with AuNPs results in the simultaneous increase of the number of empty holes within the PDA/PDAM monolayer. Because both the particles themselves and their empty vacancies are the structural defects within the polydiacetylene matrix, they make the film less firm and initiate extensive rupturing of the monolayer under mechanical stress experienced by the film during its transfer onto the solid support. When taken together, the microscopic observations confirmed that the UV-irradiation of a diacetylene mixture on the surface of the aqueous gold colloid makes it possible to incorporate 20 nm large AuNPs into the ultrathin polymer matrix yielding uniform nanostructured material. The presence of the red form of a PDA/PDAM polymer matrix with integrated AuNPs on the silicon substrate was also confirmed by the CLSM results (Figure S15). A photoluminescence spectrum of the PDA/PDAM/AuNPs film was very similar to that previously published for the red form of PDA.49 To assess photoelectrical properties of the polymer layers, I− V curves were measured in the dark and under light irradiation at a wavelength of 532 nm by using the four-probe method 43841
DOI: 10.1021/acsami.7b12156 ACS Appl. Mater. Interfaces 2017, 9, 43838−43845
Research Article
ACS Applied Materials & Interfaces
doping has been observed, which was 3 orders of magnitude higher than that of the nondoped PDA thin films.55 In our study with 4-probe techniques, the reasonable concentration of charge carriers was maintained by the charge injection (“doping”) from the external electrodes. The relatively high effective conductivity of the chemically nondoped PDA/PDAM is therefore a result of the high charge mobility in the ordered PDA/PDAM thin layer. Significantly enhanced electric properties were achieved for the both PDA/PDAM and PDA/PDAM/AuNPs thin layers coated by a relatively thick layer (40 nm) of PANI−PAMPSA complex (Table S1 and Figure 4). Electric response for these systems was about twice greater than that of the PANI− PAMPSA layer (the electrical conductivity of the PANI− PAMPSA material was studied in details in refs 46 and 47). This synergetic effect of the combination of the PDA/PDAM and the PANI−PAMPSA layers originates, most likely, from the formation of electron−hole pairs along the interface of the layers, which are both electron acceptor and donor materials for each other.56 Even at the low concentration of the electron− hole pairs, the charge carrier mobility can increase due to the reduction of the energy barrier for hopping of charge carriers by Coulomb attraction.57 Most important characteristic of the as-prepared PDA/ PDAM/AuNPs/PANI−PAMPSA layered films was their response to visible light irradiation of 532 nm wavelength. Unlike the gold-doped PDA/PDAM polymers, which showed only a minor enhancement of the conductivity under light irradiation, the layered composite exhibited significant difference in the electric conductivity measured in the dark and lightirradiation regimes (Figure 4 and Table S1). Irradiated light can increase the conductivity of the PDA/PDAM/AuNPs/PANI− PAMPSA film by 75%, whereas the photoconductivity for the PDA/PDAM/AuNPs and the PDA/PDAM/PANI−PAMPSA layers did not exceed 10%. We note especially, that the PANI− PAMPSA film does not absorb light in the range of absorption for polydiacetylenes or that for the excitation of localized surface plasmons (LSP). The results suggest that all three components (PDA/PDAM, AuNPs and PANI−PAMPSA) are responsible for a significant photoconductivity. We suggest the following possible mechanism of this plasmon-enhanced photoconductivity in such a polymer nanocomposite (Figure 5). The energy of photons of a wavelength of 532 nm corresponds to 2.33 eV. This energy is close to that of the resonant frequency of LSP excited in AuNPs, which can capture light with a frequency corresponding to its LSP resonance. The energetic electron−hole pairs are thus generated due to the decay of plasmons.58,59 The intraband transitions in the sp-band of AuNPs generate continuous energy distributions of electrons and holes extending from zero to the plasmon energy relative to the Fermi level.60 It is generally accepted that for the red form of PDA the absorption peaks at 490÷560 nm (2.53÷2.22 eV) represent the π−π* transitions in the π-conjugated system of PDA. Nevertheless, these transitions do not induce free charge carriers and photoconductivity because of large exciton binding energy.41 The poor increase of the concentration of mobile charge carriers only by 10% in the PDA−PDAM/AuNPs film under illumination is therefore a consequence of the recombination of charge carriers. This is why the PANI− PAMPSA layer needs to be applied to extracts holes from the PDA−PDAM/AuNPs film. Because the presence of the PANI/ PAMSA layer inhibits the recombination of charge carriers in
(see: paragraph 2.6 and Figure S1). The relationship between the injected current I and the voltage drop ΔV of the inner probes was nonohmic, so we had to consider a parameter denoted as the effective conductivity σeff derived from the injected current of 1÷2 nA in accordance to the expression σeff =
I d ΔV Wt
The obtained data are summarized in Figure 4 (for related digital data, see Table S1).
Figure 4. Effective conductivity and photoconductivity measured for the ultrathin polymer layers studied in this work. (*) Au nanoparticles were incorporated only into the bottom monolayer, on which five monolayers without Au NPs were deposited.
Although the polymerized PDA/PDAM monolayers showed certain electrical response, it was low and instable, because of a comparatively low conductivity of the red form of the polymer with respect to the blue one44,50−52 and the extremely thin film. There was no significant difference between the conductivity of dark and illuminated films with and without AuNPs (Table S1). This observation is in agreement with a large number of holes and gap-like defects, which the PDA film form around AuNPs. For about 50% of the runs, we were, however, unable to obtain electric responses at all. Poor reproducibility was, most likely, due to the damage of the film structure. Every particle introduces a defect into the polymer structure that causes extensive rupturing of the deposited film on the edges of 60 nm thick electrode stripes. Better reproducibility for such thin layers was achieved for thicker LB films, in which the bottom polymer layer with entrapped AuNPs was then coated with five monolayers of nondoped polymerized layers of the PDA/PDAM mixture (Table S1). An average thickness of the polymer phase in such a structure was about 10 nm (Figure S16; note that the particles introduce gap-like defects around them within a polymer film). The effective conductivity values for the PDA/PDAM layers are of (1.3÷2.3) × 10−3 S cm−1 which are about 3 orders of magnitude larger than those previously reported for an LB film of polymerized 10,12-nonacosadiynoic acid.43 Polydiacetylene single crystals are known as nearly complete insulators, although they exhibit high electron mobility of about 10 cm2 V−1 s−1.53 By increasing the concentration of charge carriers through either chemical doping or ion implantation, the conductivity of the crystals can be enhanced up to 10 S cm−1.54 For polydiacetylene thin films, a significant improvement of the conductivity up to 3 × 10−3 S cm−1 by iodine 43842
DOI: 10.1021/acsami.7b12156 ACS Appl. Mater. Interfaces 2017, 9, 43838−43845
Research Article
ACS Applied Materials & Interfaces
Table 1. Photovoltaic Characteristics of the PV Cells of the Structure ITO/Sublayer/PEDOT−PSS/P3HT:PC71BM/ LiF/Al N
sublayer
Jsc, mA/cm2
Voc, V
FF
PCE, %
1 2 3
PDA/PDAM PDA/PDAM/AuNPs without sublayer
4.21 6.18 8.65
0.570 0.502 0.585
0.20 0.42 0.50
0.47 1.31 2.55
characteristics than nondoped polymerized PDA/PDAM. This relative increase in efficiency is most likely related to the presence of a highly conductive metal phase at the interface between ITO and the layer of ionomers. When compared to nonmodified conventional architecture, the PV cell with integrated nanostructured diacetylene showed, however, significantly poorer performance. As the diacetylene polymer possesses only lateral conductivity along the conjugated electronic system of double and triple bonds, such film forms an insulating layer decreasing the interfacial charge transfer and thereby leading to the loss of the overall PV efficiency. The PDA sublayer forms a barrier for the hole transfer from the PEDOT−PSS layer to the ITO anode, because the workfunction values for both ITO (4.8 eV) and PEDOT−PSS (4.65−5.05 eV62) are located above the PDA HOMO level.63 Based on these observations, it is reasonable to conclude that the application of plasmon-enhanced ultrathin conductive polymers requires laterally ordered architectures of light convertors that would complement the functional structure of the polymer.
Figure 5. (a) Schematically illustrated structure of the planar polymerAuNPs composite and (b) energy level diagram explaining the plasmon-induced lateral photoconductivity of PDA−PDAM/AuNPs films coated with the PANI−PAMPSA layer.
the PDA−PDAM/AuNPs, the concentration of free charge carriers increases by 75%. For PANI−PAMPSA, the photon energy of 2.33 eV is less than the energy required to reach the lowest unoccupied molecular orbital (LUMO; or conductive band) because the forbidden gap is of 3.9 eV.61 The hole generated from the sp-band of AuNPs is able to occupy the HOMO (valence band) of PANI−PAMPSA with the edge at −4.8 eV.62 The concentration of the charge carriers is therefore increased by electrons excited from the sp-band to the LUMO of PDA and the holes generated from the sp-band of AuNPs to the HOMO of PANI. The excited electron−hole pairs on PDA dissociate resulting in a transfer of holes from PDA directly to the HOMO of PANI. These results provide evidence for plasmon-enhanced photoconductivity in a polymer composite with plasmonic Au nanoparticles. Although the detailed mechanism of this enhancement is disputable and needs further investigations, it is most likely related to plasmon resonance induced energy transfer between metal and polymer components absorbing light in the same spectral range. By supplying additional energy for the excitation of the conjugated system within the focused electromagnetic field of polymer-integrated AuNPs, the plasmonic component increases the total efficiency of charge transfer along the polymer chains. Photoelectric properties of the obtained ultrathin composite layers might be useful for some applications in photoelectric devices. To integrate thin layers into such devices, one needs to adapt their fabrication and deposition to the typical sandwich structure of photoelectronic cells. We integrated a sublayer of the PDA/PDAM/AuNPs into the architecture of an organic PV cell comprising conventional PEDOT−PSS as an anode buffer layer and P3HT:PC71BM forming a bulk heterojunction as a photoactive layer. The power conversion efficiency (PCE) and general characteristics of the PV cells are given in Table 1, where Jsc, Voc, and FF are short-circuit current density, open circuit voltage, and fill factor, respectively. When incorporated into organic PV solar cells, the PDA/ PDAM/AuNPs polymer monolayer showed better photovoltaic
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CONCLUSIONS In summary, we showed that photopolymerization of mixed monolayers of diacetylene surfactants, pentacosadyinic acid and amine, on a surface of gold hydrosol yields nanostructured polymer films with entrapped gold nanoparticles. The number of particles and their packing density within the ultrathin polymer may vary depending on time allowed for their adsorption on the monolayer prior to polymerization. A combination of polymer diacetylene monolayer with a layer of polyaniline-based polymer showed strong synergetic behavior due to the formation of the interface between two conductive materials with different electrical properties. Because gold nanoparticles are assembled in a nonaggregated well-defined 2D-arrangement within polydiacetylene monolayer, the composite exhibits plasmon-enhanced photoconductivity upon irradiation with visible light. This property is important for fabricating ultrathin and flexible optoelectronic elements from organic materials. For applying these nanostructured and photoactive surface coatings in ultrathin solar cells, a specific lateral architecture to maintain polymer functioning remains to be developed replacing the typically used sandwich-like systems; our group works actively toward this goal.
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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.7b12156. Π-A isotherms of PDA, PDAM, and the PDA/PDAM mixture at the air/hydrosol and air/water interface; stability isotherm for PDA/PDAM monolayer at the surface of gold hydrosol; UV−vis spectra of gold hydrosol, PDA, PDAM, the PDA/PDAM mixture, and 43843
DOI: 10.1021/acsami.7b12156 ACS Appl. Mater. Interfaces 2017, 9, 43838−43845
Research Article
ACS Applied Materials & Interfaces
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the PDA/PDAM/AuNP hybrid system on the surface of gold hydrosol; fluorescence spectra, CLSM, SEM, and AFM images of hybrid systems PDA/PDAM/AuNP on solid support, conductivity, and photoconductivity measurements. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Alexandra I. Zvyagina: 0000-0002-1213-815X Vladimir K. Ivanov: 0000-0003-2343-2140 Burkhard König: 0000-0002-6131-4850 Maria A. Kalinina: 0000-0003-2934-9284 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Russian Foundation for Basic Research (Grant No. 16-29-05284-ofi-m). REFERENCES
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