Broad-Range Electrically Tunable Plasmonic Resonances of a

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Broad-Range Electrically Tunable Plasmonic Resonances of a Multilayer Coaxial Nanohole Array with an Electroactive Polymer Wrapper Ziwei Zhou,†,# Ye Yu,†,‡,# Ningwei Sun,§ Helmuth Möhwald,⊥ Panpan Gu,† Liyan Wang,*,† Wei Zhang,† Tobias A. F. König,‡,∥ Andreas Fery,*,‡,∥ and Gang Zhang*,†

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†

State Key Lab of Supramolecular Structure and Materials and §Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, China ‡ Leibniz Institut für Polymerforschung Dresden e.V, Institute of Physical Chemistry and Polymer Physics, Hohe Str. 6, D-01069 Dresden, Germany ⊥ Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany ∥ Cluster of Excellence Centre for Advancing Electronics Dresden (CfAED), Technische Universitat Dresden, D-01062 Dresden, Germany S Supporting Information *

ABSTRACT: Plasmonic assemblies featuring high sensitivity that can be readily shifted by external fields are the key for sensitive and versatile sensing devices. In this paper, a novel fast-responsive plasmonic nanocomposite composed of a multilayer nanohole array and a responsive electrochromic polymer is proposed with the plasmonic mode appearance vigorously cycled upon orthogonal electrical stimuli. In this nanocomposite, the coaxially stacked plasmonic nanohole arrays can induce multiple intense Fano resonances, which result from the crosstalk between a broad surface plasmon resonance (SPR) and the designed discrete transmission peaks with ultrahigh sensitivity; the polymer wrapper could provide the sensitive nanohole array with real-timevaried surroundings of refractive indices upon electrical stimuli. Therefore, a pronounced pure electroplasmonic shift up to 72 nm is obtained, which is the largest pure electrotuning SPR range to our knowledge. The stacked nanohole arrays here are also directly used as a working electrode, and they ensure sufficient contact between the working electrode (plasmonic structure) and the electroactive polymer, thus providing considerably improved response speed (within 1 s) for real-time sensing and switching. KEYWORDS: nanohole array, surface plasmon, Fano resonance, electrochromic polymer, tunable plasmonic behavior

1. INTRODUCTION

resultant plasmonic resonances over a broad range from the visible to near-infrared region in a set of similar nanostructures. This can benefit numerous applications such as multicolor displays, solar cells, and highly integrated bio/chemical sensors.4−8 Further, the tailored control over the line shape of the optical response, owing to the interaction between different plasmonic modes via appropriate structural dimen-

Surface plasmons have received focused attention over many decades because of their designed manipulation of the light− matter interaction in nanometric volumes.1−3 The surface plasmon resonance (SPR) wavelength is extremely reliant on the geometry of metal structures and the dielectric surroundings. Engineering SPR behavior demands a precise control of the structure but also promises a sensitive measure of the small changes of interactions. With the rapid development of nanofabrication technologies, high precision of plasmonic structures could be achieved, enabling vigorous control of the © 2017 American Chemical Society

Received: July 27, 2017 Accepted: September 19, 2017 Published: September 19, 2017 35244

DOI: 10.1021/acsami.7b11139 ACS Appl. Mater. Interfaces 2017, 9, 35244−35252

Research Article

ACS Applied Materials & Interfaces sions, is essential for practical applications.9−11 For example, a sharp plasmonic signal is advantageous for chemical sensing and biosensing, while solar cell applications need a broad absorption band.6,8,12 On the other hand, in order to achieve tailored optical performance of the plasmonic nanostructures to meet multidirectional needs, it is vital to easily tune the resonance position, line width, and amplitude.13 One of the most common strategies is to alter structural configurations to match the desired plasmonic behavior. While effective, it still requires parallel design and the fabrication of certain plasmonic nanostructures, which can be time-consuming and expensive. Whereas, for a fixed nanostructure, changing the angle of incidence is an effective mean to tune the plasmon.14−16 Further, for a fixed nanostructure with a fixed incidence, an alternative is to consider the active and dynamic manipulation of the responsive near-field dielectric properties of the surroundings, including deposition of pH/temperature-sensitive polymers,17,18 photochromic molecules,19,20 and electrochromic polymers16,17,21 onto plasmonic components to tailor the plasmonic resonances, among which an electric field would be the most convenient and accurate trigger even upon remote control. In order to gain a maximum SPR shift upon electrostimulation as we described above, a resonance possessing a high sensitivity and improved quality factor (defined as the resonance frequency divided by the width of the resonance) is obviously preferred.22 However, conventional planar nanosensors usually own a modest quality factor owing to the weak confinement of electromagnetic fields and strong coupling to vacuum. Plasmonic Fano resonance, induced by the interaction between a bright continuum and a discrete state, exhibits ultrahigh environmental sensitivity in a narrow line shape for applications including photoswitching, narrowband filters, and high-sensitivity chemical sensing and biosensing.9,12,23−25 Hence, designing nanostructures to support Fano resonances is one way to achieve high sensitivity in nanosensors.26−28 Yet, inducing plasmonic Fano resonances usually requires a structural symmetry break along lateral dimensions,29−36 which is in most cases is realized via direct-writing technologies such as electron-beam lithography and focused ion-beam lithography.29−34 The nature of the direct-writing process limits the resulting nanostructures to provide high precision only over very small areas. The ability of upscaling is also vital in terms of real-life applications. Bottom-up patterning strategies, for example, colloidal self-assembly, provide the property of abundant nanoarrays over the centimeter and even meter scale.37,38 And one can precisely control the plasmonic behavior by tuning the separation of deposited plasmonic material along the vertical direction over nanometric length scales.39 Further, using electroactive polymers to dynamically and reversibly tune the plasmonic behaviors without alteration of the structural dimensions or incidence is quite efficient13,17,21 and highly promising in biomimetic active hybrid systems.40,41 Yet, as so far shown, the plasmonic tunability of electroactive polymers did not cover the collective optical effect,13,17,21 which largely limited their downstream practical applications. Accordingly, our strategy is to incorporate ordering of the plasmonic assembly and thus realize the collective optical behavior, such as diffraction, further providing more possibilities to design and fabricate nanostructures with desired Fano resonances to achieve higher sensitivity and larger tuning range.

In this paper, we focus on the nanoscale insulator spacer inserted in between two nanohole arrays to create horizontal nanogaps, readily inducing controlled crosstalk between the bisected upper and lower nanohole arrays. On the contrary to our previous work, where vertical Au-air-Au nanogaps were fabricated to induce intense electromagnetic field enhancement,3 the nanohole array layers feature profound Fano resonances when separated on a nanometric length scale. These resonances can be readily tailored by tuning the periodicity, spacer thickness, and hole diameter. Furthermore, we present an ITO-omitted SPR electrotuner, via an electrochromic polymer wrapped MIM (metal−insulator−metal) nanohole array. The coated electrochromic polymer film acts as a dynamic tuner of the surrounding refractive indices, enabling stable and extensive plasmonic tuning with a pronounced spectral shift. In comparison of the absorption change of the used polymer upon electric stimuli, the shift of plasmonic resonances is much greater and therefore easier to detect. To our knowledge, this hybrid material provides the largest pure electrotuning of SPR peaks owing to the ultrasensitivity of the designed Fano resonance. Also, the multilayer nanohole array can directly be used as electrode due to its high conductivity, ensuring sufficient contact between the working electrode (plasmonic structure) and the electroactive polymer to efficiently shorten the response time, enabling realtime switching applications. This largely tunable and fastresponsive plasmonic hybrid nanosystem combing the areas of plasmonics, color displays, and electric devices will promote development of this interdisciplinary work.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Multilayer Coaxial Nanohole Array Films. The PS sphere (700 nm in diameter) monolayers were prepared on the precleaned glass substrate through the interface method.42 Oxygen-reactive ion etching (RIE), impinging upon the close-packed PS colloidal crystals using a Plasmalab Oxford 80 Plus system (ICP 65) (Oxford Instrument Co., UK) with a pressure of 35 mTorr, a flow rate of 50 sccm, a radio frequency (RF) power of 30 W, and an inductively coupled plasma (ICP) power of 300 W was applied for 1− 2 min, eliminating the PS sphere diameter to different degrees. After that, the samples were mounted in a deposition chamber to form the Ag-insulator (SiO2)−Ag sandwich. After the capped PS spheres were removed by brief sonication in toluene, a MIM multilayer coaxial nanohole array located on a glass substrate was fabricated. The use of larger PS spheres (1 μm in diameter) would yield larger periodic nanohole arrays. Replacing Ag by Au would yield gold coaxial nanohole arrays with better environmental resistance. Also, increasing the number of deposition cycles contributes to an even thicker multilayer coaxial nanohole array. 2.2. Synthesis of Au Nanohole Array/Polymer Nanocomposites. The electrochromic polymer TPA-PA (triphenylamine-based polyamide) was synthesized according to reported methods.43 The diamine with a methoxy group was synthesized by Pd/C-catalyzed reduction of the dinitro group, which resulted from the nucleophilic displacement reaction of 4-methoylaniline and 4fluoronitrobenzene. Then the polyamide with the methoxy group was prepared through condensation polymerization from the synthesized diamine and trans-1,4-cyclohexanedicarboxylic acid. Dissolving 200 mg of as-prepared TPA-PA into 4 mL of dimethylacetamide (DMAC) produced 50 mg/mL polymer solution, which was then spin-coated onto the multilayer gold nanohole array at a speed of 1500 rpm for 1 min. The resulting nanohole-polymer composite was then heated at 90 °C for 3 h for thorough solvent evaporation before use. 2.3. Characterization. Scanning electron microscopy (SEM) images were taken with a JEOL JSM 6700F field emission scanning electron microscope with a primary electron energy of 3 kV. A 35245


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ACS Applied Materials & Interfaces Shimadzu 3600 UV-vis-NIR spectrophotometer was used to measure the nanohole transmission spectra. Electrochemistry measurements were performed on a CHI 660e electrochemical analyzer using a threeelectrode cell. The nanohole/TPA-PA nanocomposite was used as a working electrode (the area of polymer films was ∼0.5 × 0.8 cm2). The electrolyte used for electrochemical measurements was a solution of 0.1 M tetrabutylammonium perchlorate (TBAP)/acetonitrile (CH3CN). A platinum wire was used as a counter electrode and an Ag/AgCl, KCl (sat.) electrode was used as a reference electrode. UVvis-NIR spectra at different potentials were in situ recorded using a Shimadzu UV 3600 spectrophotometer in combination with the CHI 660e electrochemical analyzer. In the measurement, the applied potential was gradually changed to a target value via a cyclic voltammetry method at a speed as low as 50 mV/s to avoid abrupt current changes. The spectra were recorded 10 s after applying the potential. All of the electromeasurements were carried out in air. 2.4. Finite-Difference Time-Domain (FDTD) Simulations. FDTD simulations were carried out with commercial software, FDTD Solutions (v8.17.1072, Lumerical Solutions, Inc.). The simulated structures with different layers of the stacked nanohole arrays were constructed based on the experimental sample parameters. Periodic boundary conditions were applied on lateral dimensions, while for the vertical dimension, 64-layered perfectly matched layers (PML) were employed in order to increase the stability of the simulations. The mesh size was chosen to be 2 nm along all three dimensions. The mesh accuracy was set as 3 for accurate results. The overall hole structure was normally illuminated with an incident plane wave source. The optical parameters of gold were taken from Palik’s handbook; the CRC model was used for silver, and the refractive index of silica was set to 1.45.

Figure 1. Transmittance simulations of the single nanohole, nanohole array, and stacked nanohole array with a periodicity p, hole size d = 0.85p, Ag thickness t = 50 nm, and spacer thickness g = 40 nm. The material is set to be Ag. The dielectric constant of the spacer is set to be 1.45 (SiO2), and the refractive index of the surrounding medium varies from 1−1.6 in steps of 0.2 with the line color fading. (A) Single nanohole under different surrounding medium, exhibiting one broad transmittance peak for the dipolar mode of the hole. (B) Singlelayered nanohole array in different surrounding medium, showing an obvious Fano dip (green arrow) due to the discrete scattering state from the periodicity, resulting in two narrow peaks. (C) MIM nanohole array under different surrounding medium, showing two more featured Fano dips (crimson arrows) within the original wide peak due to the strong coupling of the upper and lower nanoholes.

3. RESULTS AND DISCUSSION The sensitive SPR nanostructure used in this study to generate giant spectral shifts in response to a small change of environment is related to the designed multiple Fano resonances of nanoholes. As a proof of principle, we study a series of nanohole arrangements as shown in Figure 1. The nanohole material is initially assumed to be Ag, the refractive index of the spacer is 1.45 (SiO2), and the refractive index for the surrounding medium varies from 1−1.6 at a step of 0.2. It has been widely known that a subwavelength nanohole milled opaque metal film could enable extraordinary optical transmission (Figure 1A), which shows one broad and flat peak across the spectrum, resulting from the plasmonic dipolar mode; whereas, if the nanoholes arrange into a hexagonal array (Figure 1B), the discrete scattering induced by a Bragg grating would interact with the continuum dipolar mode to form an obvious Fano dip, splitting into two narrowed and profound peaks with improved refractive index sensitivity.1 Further, introducing a nanoscale dielectric spacer plane into the singlelayer nanohole array to construct a novel MIM nanohole array (Figure 1C) leads to two more Fano dips associated with the strong interaction between the upper and lower nanohole arrays through a cavity mode. The original SPR peak continues to be sharply narrowed down by the increased number of Fano dips, which may provide improved accuracy and high sensitivity for sensors.26−28 A set of gold nanohole arrays could also show similar Fano performances (Figure S1). Based on the theoretical designs we presented above, we first fabricated samples featuring Fano-resonance-induced, narrowed SPR transmission peaks. The target multilayer coaxial nanohole arrays over a large area were fabricated through colloidal lithography on a transparent glass substrate, combining simple etching and successive Ag/SiO2/Ag deposition processes (Figure 2).

Figure 2. Scheme of fabrication of a multilayer coaxial nanohole array. Close-packed PS spheres are prepared on a glass substrate, followed by O2 plasma treatment to generate non-close-packed arrays. After that, successive normal evaporation of a metal/SiO2/metal structure is performed. Then the capped PS spheres are removed through mild ultrasonic treatment to form the MIM nanohole array.

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Our previous work had shown that when plasmonic nanostructures are vertically separated by a length scale of a few tens of nanometers, the collective optical behavior will be drastically changed as a result of strong coupling between different layers.5 A representative transmittance spectrum of the prepared silver MIM nanohole arrays (p = 700 nm, d1 = 560 nm, d2 = 640 nm, t = 50 nm, g = 40 nm) recorded at normal incidence is presented in Figure 4A. The black line presents the transmittance of the conventional single-layered nanohole array with two transmittance peaks located at about 800 and 1200 nm. However, when two such nanohole arrays are coaxially stacked and bisected by a nanoscale insulator nanohole array along the vertical direction, to form a MIM nanohole array, the transmission spectrum shows two remarkable Fano-like dips within the parent resonance peaks of the conventional nanohole array (red line, Figure 4A). The resonance positions fit well with the corresponding simulations (Figure 4B). The discrepancy between experiments and simulations can be attributed to the inevitable oxidation of silver layers and some disordering in the fabricated MIM nanohole arrays compared with the perfect model used in simulations. From the large scale SEM image in Figure S5, we could see several kinds of disordering induced by the fabrication process based on colloidal lithography, such as crystal defects, a residual PS sphere mask, cracks of the MIM film, and hole-size variation. These reasons may lower the maximal transmission intensity but lift the minimal intensity and smooth the sharp peaks and dips, finally bedimming simulated spectra as the experiments presented (detailed discussion of the discrepancy between experiments and simulations could be found in SI). The two parent resonance peaks are marked as I and II, while the two emerging dips in the MIM nanohole spectrum are marked as III and IV. The electric-field distribution and the charge distribution of the four resonances are plotted in Figure 4C−F, and the corresponding distributions of the Poynting vector and electric field vector were plotted in Figure S6. Mode I of the single-layered nanohole array belongs to the (1,0) transmission peak at the metal/air interface, which is a quadrupolar resonance according to the charge distribution in Figure 4C. Mode II is attributed to the (1,0) transmission peaks at the metal/glass interface, which is a dipolar resonance (Figure 4D). When another nanohole array is stacked upon the original one, the narrow gap in between induces magnetic cavity modes, which further interact with the discrete diffraction and form a pronounced Fano line shape. The corresponding cavity modes are marked as III and IV.46,47 For the Fano dip III (at 780 nm) as shown in Figure 4B, a hybridized mode is generated through the coupling between the dipolar mode of the lower nanohole and the quadrupolar mode of the upper nanohole, with the electric field distributed within the SiO2 spacer (Figures 4E and S6E). Similarly, as to the strong Fano dip located at 1060 nm and labeled by IV, a hybridized mode is produced by the interaction between the dipolar modes of the lower and upper nanoholes with the electric field confined at the edges of the spacer and the metal−substrate interface (Figures 4F and S6F). Unlike the dipolar modes of peak II, modes III and IV could both generate a circular current and an out-of-plane magnetic-dipole moment (Figures 4E,F and S6). And the Fano resonances induced by the “dark” modes usually show a large dependence on the surrounding refractive index, rendering this MIM nanohole array promising in sensing and plasmonic tuning.26−28

With the further deposition upon the upper hemisphere of the polystyrene (PS) spheres, the deposited material accumulates along the sphere equator, gradually increasing the diameter of the hole mask. As a result, the multilayer nanoholes we fabricate here are always slightly tapered, as illustrated in Figure 3A. The schematic shows a unit cell

Figure 3. (A) Parameters of a repeat unit of the MIM nanohole array. (B) SEM image of the fabricated nanohole array (p = 1 μm, d1 = 650 nm, d2 = 740 nm, t = 50 nm, g = 40 nm, scale bar: 1 μm). (C) Magnification of the nanohole array (scale bar: 200 nm); the red and yellow arrows indicate the lower and upper metal nanoholes. (D) SEM image of the cross section of the nanohole array; the blue dotted line indicates the outline of each hole layer (scale bar: 200 nm).

marked by the main structural parameters. During the fabrication process, the materials, periodicity (p), hole size (d1 for the lower smaller hole and d2 for the upper larger hole), the metal thickness (t) and the spacer thickness (g) could be precisely controlled. Compared with conventional methods to fabricate MIM nanostructures using electron beam lithography (EBL) or ion milling,44,45 the simple colloidal-lithographybased approach is highly versatile and capable of large-area patterning in parallel at low cost. Representative SEM images of the resultant multilayer nanohole arrays are shown in Figure 3B−D. The SEM image at lower magnification in Figure 3B indicates the capability of building multilayer nanohole arrays over large areas with welldefined order. The edges of the lower and upper holes are clearly distinguished as indicated by red and yellow arrows in Figure 3C. The blue dotted line in Figure 3D indicates a slight gradient resulting from the expanded equator during deposition. Changing the diameters of the PS spheres would result in different periodicities of the nanohole arrays. (In this paper, we used two diameters of PS spheres, 700 nm and 1 μm as examples, with more detailed information in Supporting Information (SI).) A longer etching duration would produce smaller PS spheres, resulting in smaller nanoholes (as shown in Figure S2). The thickness of each film could be well tuned (Figures S3 and S4) to obtain precisely tailored nanohole arrays by taking advantage of the vertically nanometric resolution of evaporation/sputtering deposition. Moreover, improving the cycle number of the alternate deposition would easily generate novel metal−insulator−metal−insulator−metal (MIMIM) nanohole arrays (Figure S2). In addition, altering the deposited metal toward gold would yield gold multilayer nanohole arrays with a smoother surface (Figure S4), illustrating the strong versatility for tailoring multilayer nanohole arrays. 35247


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Figure 4. (A,B) Experimental and calculated transmission spectra of a conventional single-layered nanohole array (black lines, p = 700 nm, d = 560 nm, t = 50 nm) and a MIM nanohole array (red lines, p = 700 nm, d1 = 560 nm, d2 = 640 nm, t = 50 nm, g = 40 nm). The SPR modes are marked as I−IV. (C−F) Simulated electric-field distribution (upper panel) and charge distribution and schematic surface electric field direction (lower panel) of the excited SPR modes. I: quadrupolar mode of a single-layered nanohole array; II: dipolar mode of a single-layered nanohole array; III: Fano resonance due to interaction between the dipolar mode of the power nanohole and the quadruple mode of the upper nanohole; IV: Fano resonance due to interaction between the dipolar modes of the upper and lower nanoholes.

Further, in order to find a threshold for activation of the Fano resonance, we analyze the resonance modes by tuning the spacer thickness and the hole diameter through FDTD simulations, respectively (Figure S7). With the spacer thickness increasing from 20 to 55 nm, all resonances blue-shift due to the increased cavity path between the two nanohole arrays.48 With higher energy overlap, more pronounced Fano dips are generated; for the one at longer wavelength, the blue-shifted Fano resonance is passing through its parent resonance. Hence, if the energy levels coincide the most (g ≈ 35−40 nm), the Fano dip reaches the extremum (Figure S7A). On the other hand, an enlarged hole-diameter also enables a blue-shift of the Fano resonances (Figure S7B). When the Fano resonance is located nearest to the parent mode, the strongest Fano resonance would be excited. Thus, the Fano resonances can be

readily optimized by tuning the spacer thickness and the hole diameter. To demonstrate the versatility of this simple method to generate strong dual magnetic-based Fano resonances, we fabricated MIM nanohole arrays with an altered periodicity (p = 1 μm) and a different material (gold) and also obtained strong Fano resonances (Figure S8), satisfying multiple application criteria. The low-loss Fano resonance with a narrow line width is ultrasensitive to structural parameters and even the smallest change of the refractive index of the surrounding environment.26−28 To obtain a required Fano-featured SPR with a tailored resonance position, line width, and amplitude, most reports focused on the parameter alteration of the plasmonic nanostructures.30,36,47 In contrast, here we consider active electroregulation of the dielectric environment via an electro35248


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Figure 5. (A) Electrochromic process in TPA-PA. The insets show the appearance of color changes of the polymer-nanohole composite. (B) Schematic of the in situ electrotuned Fano resonance device based on a three-electrode system with the MIM nanohole array (p = 700 nm, d1 = 440 nm, d2 = 530 nm, t = 30 nm, g = 10 nm) wrapped by an electrochromic polymer (∼100 nm) acting as a working electrode. (C) In situ transmittance spectra of the polymer-coated nanohole array upon different applied potentials. The insets show the magnified spectral changes in the dotted windows. The thick arrows indicate the polymer absorption peaks, and the thin arrows show the SPR resonances. (D) Dynamic monitoring of the reversibility of SPR shifting through an electrochromic modulator. The periodicity of the square-wave voltage is 10 s between 0 and 0.8 V. The black line presents the change of the charge density, and the red and blue lines show the reproducibility of transmittance switching for the Fano-featured SPR (1280 nm) and the characteristic polymer absorption (780 nm). (E) CV measurement for 30 cycles of the nanostructure-polymer composite.

in this part an Au−SiO2−Au MIM nanohole array was invoked to obtain an electrotunable Fano-featured SPR. As discussed above, the Fano resonance located at the NIR region owns the stronger interaction due to the larger overlap with the parent

chromic polymer coating, whose molecular combination and absorption vary largely with applied potential,43,49,50 to achieve for the first time real-time and reversible tuning of the Fanofeatured SPR. As gold possesses better electrochemical stability, 35249


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with the polymer peaks and are much easier to identify. Notably, the sharp Fano-featured SPR peak at about 1200 nm undergoes a pronounced red-shift of 72 nm from neutral to oxidized states, which is more than two times larger than a previously reported electroplasmoinc range of 27 nm;21 specifically, from 0.6 to 0.8 V, this Fano-featured SPR redshifts by 55 nm (for reference, this fast-changed range is 23 nm from 0 to 0.2 V in ref 21), enabling a high electrosensitivity of 275 nm/V, which is more than doubling that of the reference (115 nm/V in ref 21). This is the largest range of pure electrotuning of SPR ever since that study, resulting from the ultrasensitivity of the designed Fano resonance. In reference, the broad SPR peak at the longer wavelength of about 1520 nm has a smaller tuning range of about 45 nm during the whole electrochemical redox process. The reversibility of the Fano-featured SPR shifting through an electrochromic modulator is illustrated in Figure 5D. A dynamically reversible switching behavior was detected for more than 10 cycles (see the monitored movie in SI). The current density and the transmittance at the characteristic polymer peak (780 nm) and the Fano-featured SPR (1260 nm) are extremely stable with excellent reproducibility. Importantly, the response time (3.4/1.9 s for the charging/discharging process, defined as the time needed for 90% of full switching) was reduced compared to the ITO-using electrical devices (4.5/ 1.5 s for the charging/discharging process),43 which may result from the improved immediate contact between the working electrode and the surrounding polymer. Also, to consider the MIM nanohole array solely as the electrode, its stability and response time can be further optimized. When increasing the nanohole periodicity (p = 1000 nm, d1 = 650 nm, d2 = 740 nm, t = 30 nm, g = 10 nm), the response time was reduced to

Broad-Range Electrically Tunable Plasmonic Resonances of a

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