Active Tuning of Strong Coupling States between Dye Excitons and

Article Cite This: ACS Photonics 2018, 5, 788−796

Active Tuning of Strong Coupling States between Dye Excitons and Localized Surface Plasmons via Electrochemical Potential Control Fumiya Kato,† Hiro Minamimoto,† Fumika Nagasawa,† Yuko S. Yamamoto,‡ Tamitake Itoh,‡ and Kei Murakoshi*,† †

Department of Chemistry, Faculty of Science, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan Nano-Bioanalysis Research Group, Health Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Takamatsu, Kagawa 761-0395, Japan

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‡

S Supporting Information *

ABSTRACT: Here we report the tuning of a number of excited dye molecules that were strongly coupled with the localized surface plasmons (LSPs) of Au nanostructures by electrochemical potential control. Using the redox-state-tuned dye molecules and several types of metal nanostructures with distinct LSP energies, active control of the high coupling strength was achieved via an electrochemical potential-based control method. One interesting finding of the present work is that the parabolic behavior of the coupling strength in the range between 0.10 and 0.27 eV is dependent on the electrochemical potential; this has not been observed previously. Anticrossing plots showing the energies of the upper and lower states of the coupling to the LSP energy suggest that the number of dye molecules contained in the cavity-confined LSP field is controlled not only by the redox states of the dye molecules but also by the interactions between the dyes and the metal surfaces. The present finding provides a novel route to control light−matter interactions regarding the energy of electrons in metals and molecules, defined by absolute potential, i.e., electrochemical potential. KEYWORDS: strong coupling state, localized surface plasmon, dye exciton, electrochemical potential control

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and chemical reactivity modifications, have been reported.7,17−20 These characteristics of systems in strong coupling states are attracting considerable attention because of their potential for the development of methods to manipulate energy on the nanoscale. The Rabi splitting energy serves as a measure of the optical properties in the strong coupling state. The ratio of the vacuum Rabi frequency (Ω) to the frequency of the cavity (ω0) that is coupled to the light is used to characterize the strong coupling regime. As the Ω/ω0 value increases from 10−2, the spontaneous emission rate increases because of the modification of the Purcell effect, and the rate is saturated when Ω/ω0 becomes close to 1. This saturated Ω/ω0 region is often called the ultrastrong coupling state.21,22 In the region where Ω/ω0 > 1, the light emission rate decreases because of decoupling.23 Based on these specifications, the establishment of a method to control the strong coupling regime should be considered to be very important because it will allow desirable properties to be obtained in the strong coupling regime. To control the strong coupling state, it is necessary to control the plasmon or exciton energy precisely. At present, the

ocalized surface plasmons (LSPs), which are excited by illumination onto metal nanostructures, produce a confined light field that provides a highly polarized electromagnetic field with a specific energy that can effectively interact with materials located in the vicinity of the metal nanostructures.1−3 When the exciton energies of molecules are close to the plasmon energy, strong interactions between the excitons of dye molecules and the plasmons lead to the formation of new hybrid energy states in which energy is coherently or reversibly exchanged between the excitons and the generated electromagnetic field.4−7 These new hybridized energy states, which are separated by Rabi splitting, are recognized as strong coupling states. Recently, various systems have been established for the formation of strong coupling states with various Rabi splitting energy (ℏΩ) values using structures such as optical microcavities, layered semiconductors, and metamaterials operating in the terahertz region.8−12 A strong coupling state with a splitting energy of more than 100 meV was also observed in a metal colloidal solution.13 In the strong coupling regime, it is known that both the radiative and nonradiative losses can be suppressed, which result in interesting responses that are beyond the optical properties of the original system that was composed of metal nanostructures and dye molecules.14−16 Several extreme phenomena, including enhancement of the exciton conductance, extremely long energy transfer processes, © 2018 American Chemical Society

Received: July 28, 2017 Published: January 7, 2018 788

DOI: 10.1021/acsphotonics.7b00841 ACS Photonics 2018, 5, 788−796

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Figure 1. (a) SEM and (b) 3D-AFM images of Au nanostructures with heights of 70 nm. (c) Extinction spectra of the Au-NSL substrates that were obtained in air. The heights of each of the Au nanostructures are indicated in the spectrum. (d) Absorption spectrum of a 5.6 μM HITC aqueous solution. The vertical broken lines at 735 nm (1.69 eV) in both (c) and (d) represent the exciton energy of the HITC molecules.

control were evaluated using in situ extinction measurements. The dependence of the electrochemical potential on the coupling strength is discussed based on comparisons of the number of dye molecules with the LSP energies of metal nanostructures of specific sizes.

means of control seem to be limited to changing the metal structures, changing the angle of light incidence, or appropriate selection of the dye molecules.24−27 In our previous work, we have also successfully tuned a strong coupling state that was excited on Ag dimer structures with the coverage of dye molecules by changing the height of the structure, leading to tuning of the plasmon energy.28 Polarized Raman measurements proved that the interactions between the dye molecules with well-defined orientations and the localized electromagnetic field are most important in the control of the coupling strength. Measurement of a single molecule in the strong coupling regime also indicates the importance of molecule orientation against the polarization direction of the LSP field.29 While there have been encouraging reports about control of the strong coupling regime, the active tuning of this regime remains difficulty because the LSP energy is influenced with high sensitivity by both the numbers and the polarization directions of the excitons in the ultrasmall spaces used to confine the electromagnetic fields, which could also be affected by slight differences in the shapes and sizes of the nanostructures. If a highly reproducible method for active tuning of the coupling strength could be established, the systems in strong coupling states would have the possibility of manipulation of the light energies and the electrons in materials in very sophisticated ways. In this work, we have attempted to tune the electrochemical potential of a metal nanostructure for the control of the strong coupling states between LSP and molecule exciton. It has been reported that the resonant energy and the localization of LSP can be controlled by tuning the potential energy of electrons in a metal nanostructure via an electrochemical method.30−34 The redox state of the molecules on a metal nanostructure can also be tuned by the electrochemical potential of a metal nanostructure. The structures of an electrochemical double layer, which are defined by the orientation of solvent, ions, and dye molecules, could also be affected by the alternation of the electrochemical potential.35−38 The changes in the optical properties of the system under electrochemical potential

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RESULTS AND DISCUSSION Plasmon-active Au nanostructures (Au-NSLs) were prepared by an angle-resolved nanosphere lithography (NSL) method.39 To enable the electrochemical measurements, a transparent indium tin oxide (ITO) substrate was used. Polystyrene (PS) bead (diameter: φ = 350 nm) monolayer was deposited on ITO substrates, Au single-layer deposition was performed at an angle that was vertical to the substrate, and the layer thicknesses were adjusted to 20, 30, 40, 60, and 70 nm by control of the evaporation time. The PS mask was then removed by sonication in ultrapure water (18 MΩ cm−1), which was prepared by Milli-Q integral. The resulting Au nanostructures on the ITO glass substrate, which were denoted by Au20, Au30, Au40, Au60, and Au70 to indicate their respective heights, were confirmed using an atomic force microscope (AFM, Nanoscope-IIIa, Digital Instruments) in air and a scanning electron microscope (SEM, JSM-6700FT, JEOL Ltd.). The dye used in the system was 1,1′,3,3,3′,3′-hexamethylindotricarbocyanine perchlorate (HITC). The HITC molecules were purchased from Aldrich and were used without any additional purification processes. After deposition of a 6 μM dye aqueous solution on the substrate, any residual dye molecules were rinsed using Milli-Q water. Dye-coated Au nanostructures or bare Au nanostructures on the ITO substrate were used as working electrodes. The counter electrode and the reference electrode were a Pt plate and a Ag/AgCl reference electrode, respectively. An illustration of the resulting electrochemical cell is shown in Figure S1. The electrochemical potential region was defined to be in the double-layer region of the Au electrode to avoid water oxidation and reduction reactions (see SI, Figure S2) Figure 1(a) and (b) show SEM and 3D AFM images of typical Au nanostructures that were prepared by the NSL 789


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nanostructures. These splitting energy saturations reflect the limitations in the number of dye molecules that can interact with the highly localized electromagnetic mode of the LSP. According to the previous report, the coupling strength, g, which serves as an index for the evaluation of the strong coupling state, can be expressed using the following equation:37

method with a height of 70 nm. Extinction spectra for the Au nanostructures with various heights and the absorption spectrum of HITC are shown in Figure 1(c) and (d), respectively. The broken lines at 735 nm in both Figure 1(c) and (d) correspond to the maximum absorption wavelength of HITC (ωΗΙΤΧ = 1.69 eV, λex = 735 nm). The strong optical field would be generated at each edge of the triangle.40 The plasmon resonant wavelength (λLSP) in air appeared at 811, 795, 776, 757, and 720 nm for the Au20, Au30, Au40, Au60, and Au70 structures, respectively. The small shoulder that appeared at approximately 600 nm can be attributed to the band separation caused by the use of a substrate with a high refractive index.41 It should be noted here that this plasmon mode is too weak and is too far from the exciton energy to form a strong coupling state. Generally, the increase in the volume for the nanostructure leads to the red-shift of λLSP. In the case of the NSL structure, as the height of the Au nanostructures increases, the corner of the triangle gradually becomes rounded, leading to the λLSP blue-shift.40,42−44 Our present observation agrees well with these previous reports. These differences in the LSP energies of the Au nanostructures could then lead to distinct coupling with the excitons of the HITC molecules. To prepare the system for the strong coupling state, a dye molecule layer was deposited on top of the prepared nanostructures. Figure 2(a) shows the extinction spectra for

g∝

nf V

(1)

where n, f, and V are the number of molecules in the strong coupling regime, the oscillator strength of the dye molecule, and the LSP cavity volume, respectively. The loading quantity for saturation given above of 0.27 nmol cm−2 corresponds to full coverage of the substrate surface (as estimated from extinction spectrum measurements using the absorption coefficient of 1.9 × 105).45 The multilayered dyes located a certain distance from the metal surface interact weakly with the LSP for the coupling. The information presented above indicates that the number of dye molecules at the surface of the Au nanostructures is a critical factor that determines the coupling strength. Previously documented results reported that the successful observation of the formation of the strong coupling of single Au nanorods and HITC molecules with welldefined orientation.39 In this report, the peak splits into three peaks due to the interaction of Au nanorods with both monomer and H-aggregate absorption bands. At the present system, HITC is randomly dispersed on the Au-NSL, thus the amount of H-aggregation of HITC on Au-NSL would be much less compared to the previous system. As a result, the present extinction spectra show two peaks. Before control of the strong coupling states, electrochemical in situ extinction measurements were performed for the bare Au nanostructures to evaluate changes in the LSP energies that are dependent on the electrochemical potential. The potential was scanned from 0.3 (bottom) to −0.8 V (top) at intervals of 0.1 V, with results as shown in the upper column of Figure 3. In the case of the bare Au20 structure, λLSP was shifted from 885 to 843 nm (from 1.40 to 1.47 eV) by the electrode potential change as reported priviously.31 In all the Au nanostructures, λLSP was shifted linearly to shorter wavelengths by approximately 40 nm with the negative potential polarization (Figure S3). We often observed the nonlinearity tendencies of changes in λLSP at the negative potential region below −0.3 V, as shown in Figure S3. This characteristic change at the negative potential region in solutions has been explained based on the change in the solvation structure of the electrochemical double layer.46 These changes in the LSP energies ωLSP of the bare Au nanostructures offer control of the coupling strengths of the LSPs with the dye excitons via electrochemistry. The electrochemical tuning of the plasmonic property has been discussed through the experimental results and theoretical simulations.30,31,47−49,32 Recently, responses of injected charged metal nanoparticles in a vacuum have been discussed based on the time-dependent density functional theory.33,34 According to calculations, additional charge is accumulated at the interface of the structures. This interpretation could add a different point of view compared with the previous interpretation of the shift of metal bulk density of states relative to the vacuum level with the electrochemical polarization.30 In the present experiment, the electrochemical potential region corresponds to the double-layer region where the Faradaic current is not observed (see SI, Figure S2).

Figure 2. (a) Extinction spectra of HITC-supported Au60 substrates with various amounts of HITC coverage. The number of depositions of 625 μL of the 3.6 μM HITC aqueous solution is increased from 0 (bottom) to 6 (top). The broken vertical line indicates the exciton energy of the HITC molecules (735 nm, 1.69 eV). (b) Energy splitting estimated from Figure 2(a) as a function of the number of HITC molecules on the ITO glass substrate per unit area. The broken line serves as a guide for the eye.

the Au60 structure with the dye molecules after repeated depositions. The bottom spectrum shown in Figure 2(a) is that of the bare Au60 structure. Spectra after repeated depositions are also shown in Figure 2(a), ranging from the second bottom to the top lines. The optical characteristics of the metal nanostructures changed dramatically, showing the occurrence of energy splitting with the deposition of an increasing number of molecules. The triangles in Figure 2(a) indicate the two peak positions corresponding to the energies of the upper and lower branches, ωUB and ωLB. The values were estimated by the fitting of a Lorentzian function (see SI). Figure 2(b) shows the relationship between the deposited number of molecules and the splitting energy obtained from the energy difference between ωUB and ωLB. The energy splitting is saturated at 0.29 eV with deposition of more than 0.27 nmol cm−2. It should be noted that this tendency was observed in all Au 790


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Figure 3. Electrochemical extinction spectra of the bare (upper column) and HITC-supported Au-NSL substrates (bottom coulmn) obtained in a 0.1 M NaClO4 aqueous solution. The heights of the Au nanostructures are (a) 20, (b) 30, (c) 40, (d) 60, and (e) 70 nm. The electrode potential was scanned from +0.3 V (bottom) to −0.8 V (top). The vertical broken lines indicate the exciton energy of the HITC molecules (735 nm, 1.69 eV).

as the potential scans from 0.3 to −0.3 V. As a consequence, the splitting energy increased from 0.15 to 0.28 eV. At negative polarizations more than −0.4 V, however, the maximum splitting energy gradually decreased, reaching 0.15 eV at −0.8 V. The electrochemical potential dependence of the splitting energy was confirmed in respective systems covered by HITC molecules. The maximum splitting energies of 0.18 eV for Au30@HITC and 0.22 eV for Au70@HITC were relatively small compared with those of 0.27 eV for Au40@HITC and 0.28 eV for Au60@HITC. The differences occurred because the LSP energies of the Au60 and Au40 structures are closer to the energy of the HITC exciton than those of the Au30 or Au70 structures. In the Au20@HITC case, the changes that were dependent on the electrochemical potential were not significant. This insensitivity to electrochemical potential of Au20@HITC could be because of the relatively large mismatch between ωLSP = 1.53 eV in air for Au20 and ωex = 1.69 eV for HITC for the formation of strong coupling states. For a more detailed analysis of the coupling strength characteristics, a

Although the effect of the change in the potential of the Fermi level and localization of electrons on ωLSP shift cannot be critically discussed in the present system, it is expected that the electrochemical potential control of metal nanostructures could define both energy and spatial distribution of the LSP field, which may interact with excitons of dye molecules located at the metal/electrolyte interface. In situ observations of the electrochemical extinction spectra of the Au-NSL structures coated with 0.27 nmol cm−2 HITC were also carried out to characterize the coupling strength dependence on the electrochemical potential (the bottom column of Figure 3). The same as the case for Figure 2, the triangles indicate the peak top positions estimated by the Lorentz fitting (see the SI). Spectral changes were observed as changes in the split of the extinction maximum peaks in all spectra. The splitting energy reflects the change in the coupling strength that is dependent on the electrochemical potential. In the Au60@HITC case shown at the bottom of Figure 3(d), ωUB and ωLB shift toward lower and higher energy, respectively, 791


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Figure 4. Dispersion relations of ωUB and ωLB versus ωLSP; the LSP energy (diagonal line), the HITC molecules’ exciton energy (horizontal line, 1.69 eV). The red, orange, green, blue, and purple plots correspond to the structure heights of 20, 30, 40, 60, and 70 nm, respectively. The plots of filled circles, blank circles, filled squares, and blank squares indicate independent measurements using distinct substrates. The gray colors indicate calculated ωUB and ωLB by varying g from 0.50 to 0.35 eV.

quantitative comparison between ωLSP and ωex is therefore essential to clarify the contributions of the dye molecules. The difference between the value of ωLSP in air and that in an electrolyte solution should be also considered, because the dielectric constant of the medium surrounding metal nanostructures affects the plasmon property.31,50 The effect on ωLSP of HITC-supported Au-NSLs is estimated in the present analysis (see the SI). The strong interaction between the LSPs and the excitons of the HITC molecules can be described as the formation of a hybridized excited state that demonstrates the anticrossing behavior of the polariton energy as a function of ωLSP.51 Therefore, an analysis using the anticrossing plot is effective for comparison of the coupling strength’s dependence on the difference between ωLSP and ωex. Based on the results of electrochemical extinction measurements for these Au nanostructures with HITC, ωUB and ωLB were estimated to plot these values as functions of ωLSP (see Figure 4). All plotted values of ωUB and ωLB for respective structures are obtained by the Lorentzian fitting of the obtained spectra at the respective potential. A detailed description of the determination of ωLSP is provided in the Supporting Information. The horizontal and tilted black lines correspond to the exciton energies of ωex and ωLSP, respectively. In the Au60 case, ωLSP was estimated to be 1.69 eV at an electrode potential of −0.5 V. Because of the potential-dependent properties of ωLSP, the anticrossing plots of ωUB and ωLB show their variations as functions of ωLSP at the various Au@HITC structures, as shown in Figure 4. The variations of ωUB and ωLB shown in the anticrossing plots were observed for all structures, demonstrating that the strong

coupling states can be actively tuned by the electrochemical potential control method. Very interestingly, parabolic behavior was observed for the changes in ωUB and ωLB, indicating that the splitting values reached maxima around the middle potential of −0.4 V over the electrochemical polarization region from 0.1 to −0.8 V. The Au@HITC structures presented here provide ωLSP values between 1.54 and 1.80 eV, meaning that ωUB varies between 1.67 and 1.85 eV and ωLB varies between 1.46 and 1.67 eV. The estimated maximum splitting energy value is 0.28 eV at ωLSP = 1.68 eV. In previous reports, as mentioned in the introduction section, several efforts to control the splitting energy were described, with methods that included the use of photochromic molecules, changes in the angle of light incidence, and changes in the metal structures. The strength of the strong coupling has also been characterized using the value of Ω/ω0.23 While variable tuning of the strong coupling states has not yet been achieved, the electrochemical system presented here realizes active tuning of the strong Ω coupling states in the 0.05 < ω < 0.2 region at near-infrared 0

and visible light wavelengths. The observed variation in the splitting energy is a unique characteristic of the presented system. Quantitative estimation of the coupling strength was performed to enable discussion of the origin of the observed variations of ωUB and ωLB and their dependence on the electrochemical potential. In cavity quantum electronic dynamics, the coupling between the plasmons and the excitons has been evaluated using a classical-mechanical model. The results showed good agreement with the results of finitedifference time-domain calculations. We therefore used the 792


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Figure 5. (a) Estimated g values versus the electrode potential. (b) Normalized numbers of HITC molecules coupled with the LSP as a function of electrode potential. The broken vertical line at −0.7 V indicates the redox potential of the HITC molecules. The red, orange, green, blue, and purple plots correspond to structure heights of 20, 30, 40, 60, and 70 nm, respectively. (c) Cyclic voltammogram of HITC molecules in a nonaqueous solution. (d) Change of the relative number of molecules adsorbed at the metal surface estimated from in situ spectroelectrochemical absorption measurement (see SI, Figure S7).

splitting.53 In Figure 4, the theoretically estimated g values with respect to the plasmon energy are indicated using a gray scale. The gray scale corresponds to g values ranging from 0.05 to 0.35 eV. The coupling strength values at the respective potentials of the Au@HITC structures were varied over this range of g. The dependence of g on the electrochemical potential is shown in Figure 5(a). As shown in all structure cases, g increased as the potential was scanned from 0.3 to −0.4 V and subsequently decreased with further negative potential scanning down to −0.8 V. In the Au60@HITC case, which shows the highest value of g = 0.27 eV for the strong coupling state, the variation of g was most significant when compared with the variations of the other structures, such as that of Au20@HITC. The minimum value of g = 0.16 eV was observed at 0.3 V and the maximum at −0.4 V (g = 0.27 eV). This value then decreases to 0.16 eV at −0.8 V. The g value of 0.19 eV for the Au40@HITC structure increased to 0.25 eV at −0.2 V and decreased to 0.12 eV at −0.8 V. For the Au20@HITC structure, the g value was not significantly dependent on the potential, showing variations between 0.15 and 0.20 eV. The g

classical coupled-oscillator model to analyze the spectral features. Under resonant conditions, ωLSP and ωHITC become the same value of ω0, and the extinction cross section can thus be described as follows:52 Cext(ω) ⎡ ⎛ 1 ω0 − ω − iγHITCω /2 ∝ ωIm⎢ ⎜ 2 ⎢ 2ω g 2 − (γ − γ ) /4 ⎝ ω − Ω+ ⎣ 0 LSP HITC −

⎞⎤ 1 ⎟⎥ ω − Ω− ⎠⎥⎦

(3)

where γLSP and γHITC are the coordinates of the plasmonic and HITC electronic oscillations, respectively, and Ω± = ω0 + i(γLSP + γHITC)/4 (a more detailed de± g 2 /4 − (γLSP − γHITC)2 /16 scription is provided in the SI). In the case where g ≫ (γLSP − γHITC)/2, hybridization results in the formation of two modes with different frequencies, and this is attributed to Rabi 793


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In addition to the changes in g at negative potential due to electrochemical reduction of HITC molecules, a slight increment of g in potential scanning to positive potential from 0.3 to −0.4 V could also be an interesting characteristic of the present active tuning of a strong coupling state. The HITC molecule does not change electrochemical redox state in this positive potential region. Therefore, we may consider different contributions other than that of the electrochemical reaction to alter g. We may consider two possible contributions to this change. Equation 1 shows that both n and V dominate the coupling strengths of these systems. It should be noted that the amount of HITC molecule loading is controlled to give the same n for all Au-NSLs in the present system. Therefore, any changes in Δn would reflect the potential-dependent change in n in the cavity at the positive potential region. As one possible contribution, change in the molecule orientation depending on the electrode potential could be considered. Potential-dependent surface charges as well as the structure of the electrochemical double layer may affect the orientation and the density of HITC adsorbed on the surface of a metal nanostructure. These changes affect the number of HITC molecules interacting in the LSP field at the interface, leading to the change of n. As the second, it should be noteworthy that V is dependent on the electron density of the Au nanostructure with regard to the LSP strong coupling system. The estimated decrease in V of the present system over the corresponding potential change is approximately 0.2 (see the SI).54 A reduction in V could result in a reduced n, then showing a smaller Δn, if we assumed a constant spatial distribution for the dye molecules in the vicinity of the metal nanostructure. The observed change in Δn may reflect the increase in V that leads to the enhanced n in the strong coupling states. Even though the change in g by the potential scanning at the relatively positive potential region is not so significant, these possibilities prove the importance of active tuning by the electrochemical potential in the absence of a redox reaction. Control of n and V by LSP electrochemical tuning could also be a variable for the application using the system with a tailored polarization direction of the electromagnetic field of the LSP.54 Recent nanoscale electrochemistry has demonstrated the difference of the single-molecule electrochemical response from the bulk electrochemistry. For example, it was found that the different adsorption configuration for single molecules leads to different electrochemical properties; that is, the reduction potential value changes because the shorter distance between the molecule and the metal favors charge transfer and vibration damping.55−57 In our system, it can be recognized that the optical measurement provides information on the entire molecule existing on the Au surface as an ensemble. Therefore, at the present stage, detailed information on a few molecules in the strong coupling state is not clear. However, the change in the distance or the configuration for the few molecules at the Au surface depending on the electrode potential could be detected, if the present method is applied to a well-defined single metal nanostructure coupled with a small number of dye molecules. Moreover, it is also possible to detect an optical force being applied to the molecules, because of the sensitivity to the electrochemical potential dependence of n at the surfaces of the LSP-active metal nanostructures.58 Consequently, to clarify the origin of the apparent change Δn, a full evaluation of the molecular behavior in the strong coupling regime should be performed using a variety of techniques, such as spectroscopic measurements, in the near future.

(4)

where g and gmax correspond to the values of g at the electrode potential and at the maximum, respectively, as obtained from Figure 5(a). When eq 1 is considered, the number of molecules that contribute to the strong coupling state, n, could reach a maximum at gmax. Δn was then plotted as a function of the electrode potential as shown in Figure 5(b). We focus on the dependence of the electrochemical potential on Δn in the negative potential region below −0.4 V. In order to understand the electrochemical property of HITC molecules, we have performed cyclic voltammetry (CV) measurements and electrochemical extinction measurements of HITC and, then, determined the redox potential of HITC is around −0.4 V (Figure 5(c)). In the CV measurements, we have confirmed the electrochemical reversible reaction of HITC at the potential value of −0.4 V, corresponding to the onset value for the decrease of g. Additionally, from the electrochemical extinction measurements (see the SI), the change in the relative numbers of molecules adsorbed at the Au-NSL surface can be plotted as a function of the electrode potential and shows Nernstian behavior, as indicated by the black bold line in Figure 5(d). The potential dependence of Δn between −0.4 and −0.8 V agrees well with that derived from the electrochemical redox reaction of HITC (Figure 5(b)). Consequently, the tendency of g to decrease is considered to be caused by the electrochemical reduction reactions of the HITC molecules at the surface of Au-NSL. Transparent reduced HITC molecules with small f values reduce the coupling with the LSP, leading to a smaller number of apparent molecules n. It could therefore be concluded that the active tuning of g in the negative potential region may be a consequence of the number of reduced HITC molecules in the LSP field at the surfaces of the Au nanostructures. The present observation shows hysteresis dependence on the potential scanning direction and electrochemical potential limit (see the SI). Multiple potential scanning shows that the change in g contains the information on structural deformation of a metal nanostructure as well as that on irreversibility of the electrochemical reaction of the HITC molecule. At the positive potential region from 0.3 to −0.4 V, slight deformation of the metal nanostructure at the edge results in the shift of ωLSP to higher energy, leading to the shift of g to higher for the case in which the initial ωLSP is smaller than ωHITC (Figure S9(a)). At more negative polarization to −0.8 V, a decrease in a number of electrochemically active molecules due to the formation of the irreversibly reduced radical form results in a decrease in g after the scanning (Figures S8 and S9(b)). Although these changes in metal nanostructure and the amount of dye molecules during electrochemical potential scanning should be carefully considered for the quantitative analysis of g, parabolic behavior of the change in g with negative potential scanning is clearly observed to be the same as in Figure 5(a). 794


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CONCLUSION Using electrochemical potential control, active tuning of the strong coupling regime has been achieved. The value of g that corresponds to the coupling strength could be changed significantly by electrode potential scanning while using the same system with the same structure. The successful preparation of the anticrossing plot over a wide region proves that the present method is available for the tuning of the Ω coupling strength in the 0.05 < ω < 0.2 region at near0

infrared and visible light wavelengths. The changes in the coupling in the present systems reflect the number of molecules in the highly localized electromagnetic field with the LSP energy of the metal nanostructures, critically defined by the energy of electrons in the system referred to as the absolute potential, i.e., electrochemical potential. We believe that the results presented here improve the possibility for the application of the strong coupling state with tailored optical properties showing controlled light absorption, scattering, and emission as well as desirable photoelectrochemical reactions.

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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/acsphotonics.7b00841. Additional information about the electrochemical cell for the present measurement, electrochemical measurements for Au, electrochemical tuning of the plasmonic property, estimation of the plasmon energy, determination of the upper and lower branches for the strong coupling state, theoretical simulation of g values, the reversibility of the present active tuning method, and the cavity volume change depending on the electrode potential (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kei Murakoshi: 0000-0003-4786-0115 Author Contributions

K.M., F.N., and H.M. designed the research; F.K. and F.N. prepared the samples and performed experiments; F.K., F.N., H.M., Y.Y., and T.I. analyzed and interpreted the results; H.M., F.K., F.N., and K.M. developed the model and prepared the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 26248001, 15H05988, and 16K17848), from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, especially that of Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation” (16H06506).

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