Broad-Range Electrically Tunable Plasmonic Resonances of a

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Broad-Range Electrically Tunable Plasmonic Resonances of Multilayer Coaxial Nanohole Array with Electroactive Polymer Wrapper Ziwei Zhou, Ye Yu, Ningwei Sun, Helmuth Möhwald, Panpan Gu, Liyan Wang, Wei Zhang, Tobias A.F. Koenig, Andreas Fery, and Gang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11139 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Applied Materials & Interfaces

Broad-Range Electrically Tunable Plasmonic Resonances of Multilayer Coaxial Nanohole Array with Electroactive Polymer Wrapper ⊥

Ziwei Zhou,†# Ye Yu,†,‡# Ningwei Sun,§ Helmuth Möhwald, Panpan Gu,† Liyan Wang,†,* Wei Zhang,† ∥ ∥ Tobias A. F. König,‡, Andreas Fery, ‡, ,* Gang Zhang†,* †

State Key Lab of Supramolecular Structure and Materials, 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 § Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, China ⊥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 # These authors contributed equally. * Email: [email protected]; [email protected]; [email protected] ABSTRACT: Plasmonic assemblies featuring high sensitivity that can be readily shifted by external fields are keys for sensitive and versatile sensing devices. In this paper, novel fast-responsive plasmonic nanocomposite composed of multilayer nanohole array and responsive electrochromic polymer is proposed with 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; while the polymer wrapper could provide the sensitive nanohole array with real-time-varied surroundings of refractive indices upon electrical stimuli. Therefore, a pronounced pure electro-plasmonic shift up to 72 nm is obtained, which is the largest pure electro-tuning SPR range to our knowledge. The stacked nanohole arrays here is also directly used as a working electrode and ensures 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

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1. INTRODUCTION Surface plasmons have received focused attention over decades because of their designed manipulation of 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 small changes of interactions. With the rapid development of nanofabrication technologies, high precision of plasmonic structures could be achieved, enabling vigorous control of the resultant plasmonic resonances over a broad range from 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 dimensions, is essential for practical applications.9-11 For example, a sharp plasmonic signal is advantageous for chemical/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 fabrication of certain plasmonic nanostructures, which can be time-consuming and high-cost. While, 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 molecules19,20 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 on remote control. 2


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In order to gain a maximum SPR shift upon electro-stimuli 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 ranging from photoswitching, narrowband filters to high-sensitivity chem-/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 realized via direct-writing technologies such as electronbeam 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 ability of abundant nanoarrays over 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 electro-active 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 bio-mimetic active hybrid systems.40,41 Yet so far shown, electro-active polymer plasmonic tunability 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.

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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 with 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 electro-tuner, 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. Comparing 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 electro-tuning of SPR peaks owing to the ultra-sensitivity of the designed Fano resonance. Besides, 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 real-time switching applications. This largely-tunable and fast-responsive plasmonic hybrid nanosystem combing the areas of plasmonics, color displays and electric devices will promote development of this interdiscipline. 2. EXPERIMENTAL SECTION 2.1. Fabrication of multilayer coaxial nanohole array films. The PS sphere (700 nm in diameter) monolayers were prepared onto the pre-cleaned 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 4


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samples were mounted in a deposition chamber to form the Ag-insulator (SiO2 here)-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. Using 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 methoxy group was synthesized by Pd/C-catalyzed reduction of the dinitro group, which resulted from the nucleophilic displacement reaction of 4-methoylaniline and 4-fluoronitrobenzene. Then the polyamide with methoxy group was prepared through condensation polymerization from the synthesized diamine and trans-1,4-cyclohexanedicarboxylic acid. Dissolving 200 mg as-prepared TPA-PA into 4 ml dimethylacetamide (DMAC) produced 50 mg/ml polymer solution, which was then spin-coated on the multilayer gold nanohole array at a speed of 1500 rpm for 1 min. The resulting nanohole-polymer composite was then heated at 90 oC 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 primary electron energy of 3 kV. A 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. UV-vis-NIR spectra at different potentials were in situ recorded using a Shimadzu UV 3600 spectrophotometer 5



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combing 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 the electromeasurements were carried out in air. 2.4. Finite-difference time-domain (FDTD) simulations. FDTD simulations were carried out with a 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; especially the CRC model was used for silver, and the refractive index of silica was set to 1.45. 3. RESULTS AND DISSCUSSION 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 initially assumed is 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 sub-wavelength 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; while, 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 re6


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fractive index sensitivity.1 Further, introducing a nanoscale dielectric spacer plane into the single-layer 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 sharply narrow 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 (Supporting Information, Figure S1).

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 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) Single-layered 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. 7



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Based on the theoretical designs we presented above, we first fabricated samples featuring Fanoresonance-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 Ag/SiO2/Ag successive 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. 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 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

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milling,44,45 the simple colloidal-lithography-based approach is highly versatile and capable of largearea patterning in parallel at low cost.

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). 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 well-defined 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 invoked two diameters of PS spheres of 700 nm and 1 µm as examples, more detailed information in Supporting Information). Longer etching duration would produce smaller PS spheres, resulting in smaller nanoholes (as shown in Figure S2). Taking advantage of the vertically nanometric resolution of evap9



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oration/sputtering deposition, the thickness of each film could be well tuned (Figure S3, S4) to obtain precisely tailored nanohole arrays. 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 towards gold would yield gold multilayer nanohole arrays with smoother surface (Figure S4), illustrating the strong versatility for tailoring multilayer nanohole arrays. 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 nm 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, residual PS sphere mask, cracks of the MIM film and hole-size variation. These reasons may lower the maximal transmission intensity, meanwhile lift the minimal intensity and smooth the sharp peaks and dips, finally bedimming simulated spectra as the experiments presented (detailed dis-

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cussion of the discrepancy between experiments and simulations could be found in Supporting Information).

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. 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 accord11



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ing 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 quadruplar mode of the upper nanohole, with electric field distributed within the SiO2 spacer (Figure 4E, S6-E). Similarly, as to the strong Fano dip located at 1060 nm 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 spacer and metal-substrate interface (Figure 4F, S6-F). Unlike the dipolar modes of peak II, mode III and IV could both generate a circular current and an out-of-plane magnetic-dipole moment (Figure 4E-F, 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 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; while for the one at longer wavelength, the blue-shifted Fano resonance is passing through its parent resonance. Hence, if the energy levels coincide most (g ≈ 35-40 nm), the Fano dip reaches the extremum (Figure S7-A). On the other hand, enlarged hole-diameter also enables a blueshift of the Fano resonances (Figure S7-B). 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. 12


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To demonstrate the versatility of this simple method to generate strong dual magnetic-based Fano resonances, we fabricated altered periodicity (p = 1 µm) and different material (gold) MIM nanohole arrays and also obtained strong Fano resonances (Figure S8), satisfying multiple application criteria. The low-loss Fano resonance with 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 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 electro-regulation of the dielectric environment via an electrochromic 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 Fano-featured SPR. As gold possesses better electrochemical stability, in this part an Au-SiO2-Au MIM nanohole array was invoked to obtain an electro-tunable 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 resonance, thus providing more interesting properties such as ultrasensitive detection of changes of invisible light. Therefore, in order to observe pure electro-tuning of the NIR Fano-featured SPR, an electrochromic polymer, responsible for dynamic control of the surrounding refractive indices, should show nearly no absorption across the NIR region during an electro-chemical redox process. This would ensure decoupling of the polymer absorption peak from the NIR Fano-featured SPR. To meet this need, we chose and synthesized a short-conjugated electrochromic, triphenylamine-based polyamide (TPA-PA)43 as the refractive index modulator. TPA-PA possesses a transmitting neutral state, and with electrochemical oxidation it turns into bright green resulting from the molecule variation in the redox reaction. Specially, TPA-PA is bistable, meaning removing the potential, the redox state remains. (Figure 5A) The electro-tunable Fano-featured SPR device consisted of a classical three-electrode system with the polymer wrapped MIM nanohole array acting as both the Fano-featured SPR exciter and the working 13



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electrode (Figure 5B). Unlike many electro-modulated SPR systems, where the plasmonic nanoparticles are always discrete without macroscopic conductivity and thus highly dependent on an ITO substrate for integration into an electro-tuner device, the as-prepared MIM nanohole array is a continuous metal phase enabling good macroscopic conductivity, in which the two stacked nanohole arrays could be regarded as parallel circuits. So this novel electro-SPR tuner device omits ITO as a conductive medium, ensuring sufficient contact between the working electrode (plasmonic structure) and the electro-active polymer.

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 electro-tuned 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 14


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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 squarewave voltage is 10 s between 0 V and 0.8 V. The black line presents the change of the charge density, and the red and blue line show the reproducibility of transmittance switching for the Fano-featured SPR (1280 nm) and the polymer characteristic absorption (780 nm). (E) CV measurement for 30 cycles of the nanostructure-polymer composite. Cyclic voltammetry (CV) (Figure 5E), shows little damping of the current density and good electrochemical stability of the nanocomposite. The open-circuit voltage was approximately 0.55 V (vs Ag/AgCl reference, the same below), obviously lower than typical for an ITO-polymer composite (0.62 V),43 due to the excellent conductivity of the MIM nanohole array. The applied potential for full oxidation in a supporting electrolyte is +0.8 V, which is also lower than that of an ITO-polymer composite (+0.95 V).43 The in situ transmittance measurements upon different applied potentials are presented in Figure 5C. Without applying a voltage, the nanocomposite shows four Fano-featured SPR peaks indicated by the colored thin arrows (black line in Figure 5C). The two peaks in the visible region resulting from the resonance at the Au/polymer interface become inconspicuous after polymer coating due to the high refractive index of the surrounding polymer environment; while another two Fano-featured SPR at the NIR region attributed to the Au/glass interface remain strong. Therein, the peak at about 1200 nm is sharp and more obvious as simulated, which is selected to evaluate the sensitivity. As the applied voltage exceeds the open-circuit voltage of 0.55 V and reaches 0.6 V (red line), the transmittance spectrum changes considerably. The polymer characteristic absorptions at 385 nm (purple thick arrow) and 780 nm (pink thick arrow) increase with the appearance of light green color; the NIR Fano-featured SPRs undergo an obvious red-shift due to the change of refractive index, which originally results from the change of the molecular combination of TPA-PA. With the applied potential increased to 0.8 V to achieve full oxidation, the two polymer absorptions approach the extremum with the apparent color changing to bright green. The SPR peaks located at the visible region couple with the polymer absorption; in contrast, the NIR peaks show nearly no overlap with the polymer peaks and are much easier to identify. Notably, the sharp Fano-featured SPR peak at about 1200 nm undergoes a pronounced 15



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red-shift of 72 nm from neutral to oxidized states, which is more than two times larger than a previously reported electro-plasmoinc range of 27 nm;21 specially, from 0.6 V to 0.8 V, this Fano-featured SPR red-shifts by 55 nm (for reference, this fast-changed range is 23 nm from 0 V to 0.2 V in ref. 21), enabling a high electro-sensitivity of 275 nm/V, which is over doubling to the reference (115 nm/V in ref. 21). This is the largest range on pure electro-tuning of SPR ever since, resulting from the ultrasensitivity of the designed Fano resonance. In reference, the broad SPR peak at 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 Supporting Information). The current density and the transmittance at the polymer characteristic 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 with 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 working electrode and the surrounding polymer. Besides, 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 < 1 s (0.85/0.41 s for the charging/discharging process, as shown in Figure S9), and the millisecond response times enable exactly real-time switching.51,52 The low-loss Fano-featured SPRs with high sensitivity and narrow linewidths therefore enable the measurement of tiny spectral shifts caused by even the smallest change of refractive index of the surrounding environment, promoting various kinds of sensing through the refractive indices of media, for example the direct refractive index sensing, bio-sensing and temperature sensing.48,53,54 The broadly 16


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electro-tuned Fano-featured resonance presented here will also in turn provide high potential in small voltage sensing. 4. CONCLUSION In summary, we introduced a simple and versatile approach to fabricate large-area MIM nanohole arrays with tailored geometry. Multiple pronounced Fano resonances were observed and investigated in experiments and simulations to optimize narrowed and sensitive transmission peaks. Notably, without structural alteration, the sensitive Fano-featured SPRs can be dynamically and reversibly tuned via an electrochromic polymer coating with a pronounced spectral shift of ~72 nm. The stacked nanohole arrays in the electro-plasmonic device not only excite strong plasmonic coupling but also act as a working electrode, enabling obviously improved response speed for real-time control. We expect that the fastresponsive MIM-nanohole-array/TPA-PA nanocomposites introduced in this work are promising in diverse applications including tunable lasers, optical modulators, sensors, and fast-responsive electric papers. Acknowledgements This work was supported by the National Natural Science Foundation of China (51673085, 51373066, 51173068), “111” project (B06009), National Basic Research Program of China (2013CB834503) and Center for Advancing Electronics Dresden “CfAED”. Dr. Y. Yu was supported by Alexander von Humboldt Foundation. Competing Financial Interests statement The authors declare no competing financial interests. Supporting Information The supporting information contains the simulated transmittance spectra of Au nanoholes in different arrangements, and SEM images of various multilayer nanohole arrays. Simulations of the Poynting vec-

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Table of Contents

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Broad-Range Electrically Tunable Plasmonic Resonances of a

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