1. Introduction

Electronic reflective displays represent one of the most important technologies of dynamic visual displays, which are widely used in daily applications, such as mobile phones, e-readers, electronic paper and billboards. Compared with other emissive techniques based on light-emitting diodes or liquid crystals, reflective displays are of lower energy consumption and higher visibility free from the influences of the viewing angle and the ambient light intensity [1]. The reconfigurable control of the reflectively displayed patterns relies on the dynamic modulation of colored pixels by manipulating their electrochemical or electromechanical processes [2–4]. The pixels can be made of dye pigments, photonic crystals, plasmonic nanostructures and so on. Among them, pixels based on plasmonic structures are more preferable because their colors are immune to chemical bleaching and they can be manufactured in higher spatial resolutions [5,6].

The modulation of plasmonic colors can be realized by (i) overlaying dynamic transmission or polarization modulators made of stimuli-responsive materials to filtrate the structural colors or (ii) using stimuli-responsive materials as the composition of plasmonic nanostructured systems to directly modulate the resonances by controlling the factors, such as the morphology, arrangement, material and the environmental refractive index of the nanostructure units, for the purpose of the intrinsic variation of the structural colors. For example, liquid crystals (LCs) represent some of the most typical stimuli-responsive materials whose transmission and refractive index can be changed by external stimulations, such as acoustic waves [7], voltages [8], light fields [9,10] and so on. LCs can serve as filters, templates for ordering metal nanocrystals and dynamic media, leading to the effective modulation of plasmonic colors. However, LC-based plasmonic modulation suffers from low color brightness because LCs are polarization selective materials [1].

In addition, some metals can serve as switchable components in plasmonic structures to realize the reconfigurable morphology variation or the reversible change of the refractive index by using chemical reactions, such as the reversible electrochemical deposition approach of silver [11–13]. This strategy breaks the limit of polarization dependency in the case of LC-based systems. However, both LC- and switchable-metal-based pixels cannot be realized with an ultrathin thickness or a relatively high resolution because relatively large-scale LCs and macroscale electrochemical deposition cells are needed to compose the pixels, restricting their application and integration in miniaturized optical systems.

Hybridizing electrochromic polymers with plasmonic metasurfaces is a feasible solution. Electrochromic polymers themselves, such as polypyrrole, polythiophene, polyaniline and poly(3,4-ethylenedioxythiophene) (PEDOT), are dynamic color generators, whose intrinsic color, refractive index and transparency depend on their redox states and can be tuned by driving different voltages [14,15]. However, macroscale electrochromic polymers are difficult to process into high-resolution pixels because their sufficient color saturation and contrast demand large thicknesses, which also leads to a high response time to complete the redox reaction of the entire volume of the polymers. Instead, their hybridization with plasmonic metasurfaces is more preferable, in which the electrochromic polymer serves as the dynamic medium around the plasmonic metasurface. In this case, the pixel size depends on the micro-/nanoscale metasurface, which can be lowered and precisely fabricated. The thickness of the polymer can reach down to tens of nanometers, resulting in fast modulation.

Effective color changes and high contrast modulation can still be realized by electrically tuning the plasmonic resonances. The commonly used structures of plasmonic metasurfaces are gratings [16,17], nanoholes [18–22], nanomeshes [23], nanoparticles and nanocrystal assemblies [24–28]. For the purpose of a high resolution and well-defined morphology, most of these structures are fabricated using high cost and time-consuming techniques, such as electron beam lithography or focused ion beam lithography, limiting their area and practical applicability. Developing a low-cost fabrication process of a large-area metasurface hybridized with electrochromic polymers to realize effective plasmonic modulation remains challenging.

In this work, we present a PEDOT-coated gold nanodisk (PCGN) metasurface, in which the ordered-arranged and well-defined gold (Au) nanodisk array is fabricated through a low-cost and large-area process based on the anodic aluminum oxide (AAO) template assisted deposition. An ultrathin layer of PEDOT is in situ synthesized on the Au nanodisk array by the electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) monomers. The reflective intensity and the plasmonic resonant wavelength of the PCGN metasurface can be shifted by tuning the applied voltages to realize fast and reversible electrochromic modulation. Numerical simulation results demonstrated that the modulation was due to environmental refractive index changes around the plasmonic Au nanodisks caused by the ultrathin PEDOT coating in different redox states stimulated by the voltages.

2. Methods

Materials: 3,4-ethylenedioxythiophene (EDOT), acetonitrile, acetone and tetrabutylammonium hexafluorophosphate (TBAPF₆) were purchased from Aladdin and used as received. 0.1 M TBAPF₆-acetonitrile solution was prepared and then EDOT was dissolved in the TBAPF₆-acetonitrile solution. Commercial ITO glass slide (length of 2.5 cm and width of 1 cm) was cleaned with deionized water before used. The high purity gold particles (99.999%) for the use of PVD were purchased from ZhongNuo Advanced Material (Beijing) Technology Co. Ltd., China. The AAO template was homemade.

Fabrication of AAO templates: The AAO templates were prepared in a two-step anodization process. Briefly, polished Al foils were anodized in a 0.3 M oxalic acid solution at 40 V in 2–6 °C for 4 h. Then, the anodic oxide layer was removed in a mixture of H₃PO₄ (6 wt.%) and H₂CrO₄ (1.8 wt.%) at 60 °C. It was anodized again using the same solution and voltage. The thickness of the AAO was controlled by anodizing time. Subsequently, a polymethyl methacrylate (PMMA) layer was coated on AAO from a PMMA/methylbenzene solution. The Al layer was removed in mixture of CuSO₄ and HCl solutions. The removal of the thin barrier layer was carried out in H₃PO₄ solution (5 wt.%) at 30 °C. PMMA/AAO sheets were put in acetone so that PMMA was dissolved leaving AAO membrane suspending in acetone. The AAO was mounted on the substrates in the liquid and attached on them with the quick drying of the acetone.

Nanofabrication: Films of Au (200 nm) with Ti (5 nm) adhesion deposited on ITO glass slide substrates surface using sputtering. AAO template with nanohole periodicity of 470 nm and hole diameter of 260 nm was transferred onto surface of the gold/ITO glass substrates in acetone solution at room temperature. After that, the thickness of 40 nm Au film deposited into the nanohole of AAO template with a method of electron beam evaporation without sample stage rotation. The deposition rate for Ti and Au is R_Ti ≈ 0.016 nm/s and R_Au≈0.05 nm/s, respectively. A gold nanodisk array was obtained after removing the AAO templates.

Spectroelectrochemistry Setup: Reflection spectra were detected by a UV-Vis-NIR fiber optic spectrometer (PG2000-Pro back-thinned spectrometer, Ideaoptics, China) with a light source (IDH2000, Ideaoptics, China). Angle-resolved spectrum system (R1, angle-resolved spectrum system, Ideaoptics, China) was used to ensure that the incident angle of light was 0°. The PEDOT/Au nanodisk array/Au/ITO glass substrate was clamped against a homemade cell with an O-ring silicone gasket preventing leakage of electrolyte solution. Then the substrate was mounted the sample stage of the spectrometer. Surface plasmons were excited at the periodic nanostructure–dielectric interface and Reflection-SPR signals were detected by fiber optic spectrometer.

Electrochemical measurements were carried out using a three-electrode system with the electrochemical workstation (CHI660E, Shanghai Chenhua, China). Au nanodisk array/ITO glass substrate served as the working electrode with an area of 2 cm². The counter electrode was a platinum wire about 2 cm and an Ag|AgCl|KCl electrode was used as the reference electrode. PEDOT polymer films were obtained on the surface of periodic nanostructure at a constant potential about 1.22 V lasting 20 s in an acetonitrile solution with 0.1 M TBAPF₆ containing 0.02 M EDOT monomer. Then, the substrates were transferred to a monomer-free acetonitrile solution with 0.1 M TBAPF₆ by cyclic voltammetry to stabilize the polymer films. Combining the fiber optic spectrometer with electrochemical workstation, SPR response of Au nanodisk array was studied by applying the different potential from −1 to 1 V.

3. Results and discussion

Figure 1(a) illustrates the fabrication process of the PCGN metasurface. It is fabricated based on the template-assisted physical vapor deposition (PVD) technique. A layer of 200 nm Au was first deposited onto the indium tin oxide (ITO) glass substrate with an adhesive layer of 5 nm Ti between them. Subsequently, an AAO membrane with a thickness of 500 nm was transferred onto the Au layer in acetone. The large area AAO membrane was perforated by many ordered-arranged nanoholes, and it served as the template during the PVD process. The period and the size of the nanoholes are 470 and 260 nm, respectively. Afterwards, 40 nm thick Au was deposited into the holes of the AAO template and located on the underlying Au substrate. Because of the sufficient difference between the thicknesses of the AAO template and the Au deposited in the nanoholes, the Au nanodisk array can remain on the substrate after the mechanical peeling-off of the AAO template. Figure 1(b) shows the scanning electron microscopy (SEM) image of the ordered-arranged and well-defined Au nanodisks. The period and diameter of the gold nanodisks were the same as those of the AAO nanoholes, resulting from the conformal deposition of Au into the nanoholes during the PVD process. Finally, the synthesis of the PEDOT coating was carried out at the surface of the Au nanodisk array in situ. The electrochemical polymerization reaction happened in a conductive solution, where 0.02 M EDOT monomers and 0.1 M TBAPF₆ were distributed in acetonitrile. The mixed solution was simultaneously connected with the Au nanodisk array, a Pt wire and an Ag/AgCl electrode, which served as the working, counter and reference electrodes, respectively. The three-electrode system was driven and monitored using an electrochemical workstation. In this experiment, the PEDOT coating was fabricated and driven by a constant voltage of 1.22 V (vs. the potential of the Ag/AgCl electrode) for 20 s. As shown in Fig. 1(c), after the electrochemical polymerization, the diameter of the nanodisks was slightly increased, which demonstrated the formation of the PEDOT coating. The average thickness of the PEDOT coating was 27 nm. In addition, because the AAO template was fabricated based on a bottom-up technique, both the Au nanodisk array and the PCGN metasurface could be of large areas. The photographs of the Au nanodisk array and the PCGN metasurface with a ∼1 cm² area are shown in the insets of Figs. 1(b) and (c), respectively.

 

Fig. 1. (a) Schematic illustration of the fabrication process of the PCGN metasurface. (b) and (c) SEM images of the Au nanodisk array and the PCGN metasurface, respectively. The scale bars stand for 500 nm. Insets of (b) and (c) are the optical photographs of the Au nanodisk substrate and the PCGN metasurface, the scale bars stand for 2 mm.

Download Full Size | PDF

To demonstrate the modulation capability of the PCGN metasurface, we built a spectroelectrochemistry setup, in which the metasurface can be electrically driven and simultaneously optically characterized. The setup was the combination of a reflective spectral system and the aforementioned three-electrode electrochemistry system. Differently, the metasurface was immersed in the conductive solution without EDOT monomers. Figure 2(a) shows the reflection spectra of the Au nanodisk array in air, the PCGN metasurface in air and the PCGN metasurface in the conductive solution. For each one of the three spectra, a major reflection dip occurred at short wavelength. The dip wavelengths were 550, 615 and 678 nm, respectively. All the three reflection dips were generated from the absorption caused by the plasmonic resonance of Au nanodisks. The PEDOT coating and conductive solution increased the environmental refractive index around the Au nanodisks, resulting in the red shift of the resonant wavelength.

 

Fig. 2. (a) Reflection spectra of the Au nanodisk substrate in air, the PCGN metasurface in air, and the PCGN metasurface in the conductive solution. (b) Electrochromic modulation of the PCGN metasurface: Reflection spectra of the PCGN metasurface with different driven voltages. (c) Reflection spectra of PEDOT film on planar Au substrate driven by voltages ranging from −1 to 1 V. (d) Time resolved electrochromic modulation: the resonant wavelength of the PCGN metasurface with the driven varied voltages switching between 1 and −1 V.

Download Full Size | PDF

The analogous mechanism could be used to dynamically modulate the plasmonic resonances. In detail, both the intensity and the wavelength of the plasmonic resonance induced absorption could be controlled by adjusting the dielectric constants of the PEDOT coating, which was realized by driving different voltages on it. Figure 2(b) shows the reflection spectra of the PCGN metasurface under different driven voltages. With the voltages ranging from 1 to −1 V, the resonant wavelength shifts from 653 to 718 nm, and the lowest reflectance decreases from 53% to 48%. Varying the driven voltage can lead to the change of the chemical properties of PEDOT, such as the redox state, the molecular composition and configuration, the doping level of ions or electrons and so on [15]. Consequently, the complex refractive index of PEDOT is also changed [29]. With the voltage decreases from 1 V to −1 V, the real part of the refractive index increased, resulting in the red shift of the resonant wavelength. Furthermore, the change of the complex refractive index of PEDOT also leads to the decrement of the effective impedance of the PCGN metasurface at the corresponding resonant wavelengths, which results in a higher impedance matching between air and the PCGN metasurface and generating lower reflection. It is worth mentioning that the spectral modulation of the PCGN metasurface was based on the hybrid contribution of the plasmonic Au nanodisks and the electrochromic PEDOT rather than solely that of the PEDOT. This can be proved by the reflection spectra of a reference sample, i.e., a PEDOT coated planar Au film, as shown in Fig. 2(c). The variation of the driven voltage mainly caused the change of the reflection intensity at the wavelength near to 600 nm, but the dip wavelength shifted slightly.

We also investigated the response time of the electrochromic modulation of the PCGN metasurface. It was characterized by recording the real-time resonant wavelength under an alternating voltage that is periodically switched between 1 and −1 V, as shown in Fig. 2(d). The resonant wavelength reversibly fluctuated between 652 and 719 nm. Right after the switching of the voltage, the resonant wavelength was changed immediately and dramatically. Then, when it got close to the opposite value, the variation slowed down. This phenomenon was related to the ratio of the electrochemical redox reactions. It was dominated by the reactant concentration, which was highest right after the switch and then lowered down. Herein, we define the response time of the electrochromic modulation as the duration from the beginning of the switching to the moment when the wavelength reaches 95% of the entire shift, which is ∼10 s. This short response time could be attributed to the small thickness of the PEDOT coating, which was only 27 nm. The thin film facilitated the redox reactions due to (i) the high surface area which provided plentiful sites for the exchange of electrons or ions, and (ii) the low amount of reactants per unit area.

To interpret the mechanism of the plasmonic modulation using the PCGN metasurface, we carried out numerical simulations using the full-wave 3D finite-different time-domain method. The model was established according to the realistic microscopic morphology of the PCGN metasurface. The refractive index of the conductive solution was measured using a refractometer, which was 1.33. The refractive indices of Au and PEDOT were acquired from the reported results [30]. Accordingly, we calculated three samples, which, respectively, corresponded to the Au nanodisk array in air, the oxidation-state PCGN metasurface in the conductive solution under a 0.5 V driven voltage and the reduction-state PCGN metasurface in the conductive solution under a −1.0 V driven voltage.

Illuminated by a normal incident plane wave, the reflection spectra and the near-field distributions of the three samples were recorded. Figure 3(a) shows the three simulated reflection spectra, whose resonant wavelengths match well with the ones in the experimentally measured results, i.e., 550 nm for the Au nanodisk, 675 nm for the oxidation-state PCGN metasurface and 716 nm for the reduction-state PCGN metasurface. This demonstrated the validity of the simulations. Figure 3(b) shows the electric field (E-field) distributions of the three samples at the resonant wavelengths. No matter with or without the PEDOT coating, the highly enhanced E-fields always located at the opposite edges of the Au nanodisks, featuring the dipole mode of the localized surface plasmon resonance (LSPR) of the Au nanodisk. Namely, the electrochromic modulation was based on the same LSPR mode. The maximum E-field enhancements (E/E₀, where E stands for the local E-field and E₀ stands for the incident E-field) for the Au nanodisk, the oxidation-state PCGN and the reduction-state PCGN are 17, 47, and 78, respectively. It demonstrated that the PCGN metasurface was of the capability of higher near-field localization, which could be attributed to its higher absorption. The PEDOT coating served as an impedance matching layer between air and the Au nanodisk substrates, preventing a larger amount of light from being reflected [31]. The shift of the resonance wavelength was due to the change of the refractive indexes the PEDOT coating, which increased during the transition from the oxidation state to the reduction state [29]. As a result, the electrochromic modulation of both the reflection intensity and the resonant wavelength was realized.

 

Fig. 3. Simulated (a) reflection spectra and (b) electric field distribution plots of the Au nanodisk substrate in air, the oxidation-state PCGN metasurface in the conductive solution, and the reduction-state PCGN in the conductive solution.

Download Full Size | PDF

4. Conclusions

In conclusion, the PCGN metasurface was fabricated using a low cost and large area process based on the AAO template assisted PVD and electrochemical polymerization. The ultrathin PEDOT coating served as a dynamic medium surrounding the plasmonic Au nanodisks. By driving different voltages on the PCGN metasurface, the refractive index of PEDOT could be tuned resulting in the shift of the resonant wavelength and the variation of the reflective intensity of the metasurface. As a result, fast and reversible electrochromic modulation of plasmonic resonances was achieved on the large area metasurface. With further explorations, we believe this technique is promising for high-yield manufactures of devices applied in dynamic reflective displays.