Active Tuning of Strong Coupling States between Dye Excitons and

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Active Tuning of Strong Coupling States between Dye Excitons and Localized Surface Plasmons via Electrochemical Potential Control Fumiya Kato, Hiro Minamimoto, Fumika Nagasawa, Yuko Yamamoto, Tamitake Itoh, and Kei Murakoshi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00841 • Publication Date (Web): 07 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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Active Tuning of Strong Coupling States between Dye Excitons and Localized Surface Plasmons via Electrochemical Potential Control Fumiya Kato†, Hiro Minamimoto†, Fumika Nagasawa†, Yuko 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

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 0.1 and 0.27 eV is dependent on the electrochemical potential; this has not been observed previously. Anti-crossing

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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 of lightmatter interaction 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

Localized surface plasmon (LSP), which are excited by illumination of 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


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suppressed, which results 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, 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 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 (ω଴ ) that is coupled to the light is used to characterize the strong coupling regime. As the Ω/߱଴ value increases from 10−2, the spontaneous emission rate increases because of modification of the Purcell effect, and the rate is saturated when Ω/߱଴ becomes close to 1. This saturated Ω/߱଴ region is often called the ultra-strong coupling state.21,22 In the region where Ω/߱଴ > 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 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 also successfully tuned a strong coupling state that was excited on

Ag dimer structures with dye molecule coverage 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


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the strong coupling regime also indicates the importance of molecule orientation to 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 difficult because the LSP energy is influenced with high sensitivity by both the numbers and the polarization directions of the excitons in the ultra-small 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 can be established, then systems in strong coupling states could offer 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 meatal nanostructure via 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 electrochemical double layer, which are defined by the orientation of solvent, ions, and dye molecules, could be also affected by the alternation of the electrochemical potential.35–38 The changes in the optical properties of the system under the electrochemical potential 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.

Results and Discussion


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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) monolayers were deposited on ITO substrates and 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 ultra-pure 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, JSM6700FT,

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 the 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 Fig. S1. Electrochemical potential region was defined to be in the double layer region of Au electrode to avoid water oxidation and reduction reactions (see SI, Fig. S2) Fig. 1(a) and (b) show SEM and 3D AFM images of typical Au nanostructures that were prepared by the NSL method with a height of 70 nm. Extinction spectra for the Au nanostructures of various heights and the absorption spectrum of HITC are shown in Fig. 1(c) and (d), respectively. The broken lines at 735 nm in both Fig. 1(c) and (d) correspond to the


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maximum absorption wavelength of HITC (ωΗΙΤΧ = 1.69 eV, λex = 735 nm). The strong optical field would be generated at the edge of each triangle shown.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 band separation caused by 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 nano-structure 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 a rounded shape leading to the λLSP blue shift.40,42–44 Our present observation agrees well with this previous report. 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. Fig. 2(a) shows the extinction spectra for the Au60 structure with the dye molecules after repeated depositions. The bottom spectrum shown in Fig. 2(a) is that of the bare Au60 structure. Spectra after repeated depositions are also shown in Fig. 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 increasing a number of molecules. The triangles in Fig. 2(a) indicates 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 Lorentzian function (see SI). Fig. 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


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0.27 nmol cm−2. It should be noted that this tendency was observed in all Au 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 a previous report, the coupling strength, g, which serves as an index for evaluation of the strong coupling state can be expressed using the following equation: 37 g∝ට

௡௙ ௏

(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 presence of multi-layered dyes located certain distance from the metal surface interacts 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 the successful observation of the formation of the strong coupling of single Au nanorods and HITC molecules with well-defined 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. In this study, HITC is randomly dispersed on the Au-NSL. The amount of H-aggregation of HITC on Au-NSL would be much less compared to the previous system. As the result, the present extinction spectra show two peaks. Before control of the strong coupling states was attempted, electrochemical in situ extinction measurements were performed for the bare Au nanostructures to evaluate changes in the LSP energies that were dependent on the electrochemical potential. The potential was scanned from


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0.3 (bottom) to −0.8 V (top) at intervals of 0.1 V, with results as shown in the upper column of Fig. 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 (Fig. S3). We often observed the nonlinearity tendencies of changes in λLSP at the negative potential region below −0.3 V as shown in Fig. S3. This characteristic change at 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 plasmonic property has been discussed through the experimental results and theoretical simulations.30,31,47–49,32 Recently, responses of injected charged metal nano-particles in vacuum have been discussed based on the time-dependent density functional theory.33,34 According to calculation, the additional charge is accumulated at the interface of the structures. This interpretation could add different point of view compared with previous interpretation of the shift of metal bulk density of states relative to the vacuum level at the electrochemical polarization.30 In the present experiment, the electrochemical potential region corresponds to the double layer region where net Faradaic current is not observed (see SI, Fig. S2). Although the effect of the change in the potential of the Fermi level and localization of electron on ωLSP shift cannot be critically discussed in the present system, it is expected that the electrochemical potential control of metal nanostructure could define both energy and spatial distribution of the


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LSP filed which may interact with exciton 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 Fig. 3). As same as the case for Fig. 2, the triangles indicates the peak top positions estimated by the Lorentz fitting (see 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 in the bottom of Fig. 3(d), ωUB and ωLB shift toward lower and higher energy, respectively, 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 the HITC molecules. The maximum splitting energies of 0.18 eV for Au30@HITC and 0.22 eV for Au70@HITC were relatively small when 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 quantitative comparison between ωLSP and ωex is therefore essential to clarify the contributions


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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 NSL is estimated in the present analysis (see 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 anti-crossing 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 of these Au nanostructures with HITC, ωUB and ωLB were estimated to plot these values as functions of ωLSP (see Fig. 4). All plotted values of ωUB and ωLB for respective structures are obtained by the Lorentzian fitting of the obtained spectra at 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 potentialdependent properties of ωLSP, the anti-crossing plots of ωUB and ωLB show their variations as functions of ωLSP at the various Au@HITC structures, as shown in Fig. 4. The variations of ωUB and ωLB shown in the anti-crossing plots were observed for all structures, demonstrating that the strong coupling states can be actively tuned by the electrochemical potential control method. Very interestingly, the 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 V to −0.8 V. The Au@HITC structures presented here provide ωLSP values between 1.54 eV and 1.80 eV, meaning that ωUB varies


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between 1.67 eV and 1.85 eV, and ωLB varies between 1.46 eV 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 introductory 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 been also characterized using the value of Ω/߱଴ .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 coupling states in the 0.05

Active Tuning of Strong Coupling States between Dye Excitons and

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