Electrochemical Fine Tuning of Plasmonic Property for Au Lattice

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Electrochemical Fine Tuning of Plasmonic Property for Au Lattice Structures Hiro Minamimoto, Shunpei Oikawa, Takahiro Hayashi, Alice Shibazaki, XiaoWei Li, and Kei Murakoshi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01495 • Publication Date (Web): 28 Apr 2018 Downloaded from http://pubs.acs.org on April 29, 2018

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The Journal of Physical Chemistry

Electrochemical Fine Tuning of the Plasmonic Properties of Au Lattice Structures Hiro Minamimoto, Shunpei Oikawa, Takahiro Hayashi, Alice Shibazaki, Xiaowei Li, and Kei Murakoshi* Department of Chemistry, Faculty of Science, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan.

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ABSTRACT

We tuned the plasmonic properties of the Au lattice structure by electrochemical potential control. Au lattice structures with different values of the spacing, diameter, and height show characteristic optical properties determined by the surface lattice resonance of the localized surface plasmon mode. Electrochemical potential control can change the metal structures through metal dissolution, as well as the energy of the electrons in metals. In situ real time observation of the optical properties of Au lattice structures by electrochemical dark-field scattering microscopy shows the fine tuning of the plasmonic properties with characteristic resonance energy and controlled spectral width. By controlling surface dissolution of the Au lattice structure at a rate of a few nanometers per minute, we tuned the plasmonic properties and achieved a spectral width of 0.145 eV with a resonance maximum of 1.74 eV (714 nm).


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INTRODUCTION Localized surface plasmon resonance (LSPR), which is the collective oscillation of free electrons in metal nanostructures induced by light illumination onto the metal nanostructure, can realize nanoscale confinement of light energy.1–4 This confined light leads to formation of a strong optical field, and, especially for the dimer case, it can produce intensity magnification of up to ~105.5,6 The excited plasmonic optical properties of the metal nanostructure are sensitive to the choice of the metal, shape, and size.1,5,7–9 Owing to the improvement in various nanoscale fabrication techniques, such as electron beam lithography,10–12 the plasmonic properties can be precisely controlled for various applications, such as biochemical sensors and surface-enhanced Raman scattering measurements.13–15 To achieve efficient use of the nanoscale confined light, further suppression of light scattering and a well-tuned energy with extended lifetime of the excited states are required to make use of the highly confined light energy. Regarding the plasmonic properties, dephasing of the coherent oscillation is induced by electronic/vibrational excitation, radiation damping (energy loss by scattering), and damping owing to surface collisions.16–19 For nanostructures with diameters larger than 20 nm, radiation damping is predominant.16 To discuss the lifetime of the plasmonic excited state, evaluation of the linewidth, which is defined as the full width at half maximum (FWHM), is effective because the linewidth depends on nonradiative and radiative damping of the plasmon modes.20 Many measurements using different sized and/or shaped structures have revealed that typical plasmon lifetimes are 2–10 fs.19,21 One approach to obtain a longer lifetime of the excited state is to use the Fano resonance, which results from interaction of the narrow dark modes with the broad bright modes.22–24 For excitation of the Fano resonance, overlap of the broad dipolar mode with the narrow dark mode is required. Because of this coupling, the Fano resonance exhibits a distinct


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asymmetric shape, leading to apparent simultaneous excitation of both the bright and dark modes. The nonradiative dark mode can be used as a propagating mode in the wave guide to confine the light energy in an ultrasmall space beyond the diffraction limit. Recently, two-dimensionally arranged metal nanoparticles have received much attention as another approach to control plasmon lifetimes.25–29 In such a two-dimensional (2D) square grating of metal nanoparticles (lattice structure), the scattered light from one particle is absorbed by neighboring nanoparticles to excite the plasmon, resulting in a transformation from radiative to evanescent. By this suppression of radiative loss, strongly coupled nanoparticles in the lattice structures show the surface lattice resonance (SLR) mode with an extremely narrow linewidth ( 400 nm indicate improved lifetime of the plasmons in the lattice structure.27 The extinction spectra of Au lattice structures with different D values (a = 400 nm and h = 100 nm) are shown in Fig. 2b. The D value was changed from 60 to 140 nm. As D decreases from 140 to 60 nm, the SLR maximum shifts from 718 to 634 nm (middle part of Fig. 2b) and the extinction intensity decreases. From the difference in the intensity, dipolar coupling in the 2D structures decreases for smaller particles. For D = 140 nm, the relatively weak shoulder peak at 560 nm could be derived from the LSPR mode. The coupling mode is strongly correlated with the LSPR character. Generally, increasing the volume of a single particle leads to a red shift in the LSPR resonance wavelength.8 Thus, the present result for the red shift of the SLR is reasonable. With the red shift of the resonance wavelength, the FWHM value also decreases, as shown in the bottom part of Fig. 2b. For D = 100 nm, the relatively small FWHM value (0.16 eV) indicates an extended lifetime of plasmons in the lattice structure with a = 400 nm. In addition to the structural parameters a and D, the effect of h on the lattice plasmon modes was also investigated by varying h from 50 to 100 nm, as shown in Fig. 2c. Compared with the changes in the spectral characteristics by changing a and D, the extinction intensities of


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the coupling modes of the structures do not drastically change. The maximum resonance wavelength changes for h < 70 nm. In the size range 70–100 nm, there is no significant shift in the maximum resonance wavelength. The dependence of h on the FWHM is not so apparent compared to the that for the case of a or D. There is a slight increase from ~0.20 eV at h = 60 nm to ~0.22 eV at h = 50 nm. The scattered values of the FWHM around h = 90 nm and the increase in the maximum resonance wavelength with decreasing h below h = 70 nm could be because of the difference in the sphericity of a single disk unit. The disk structure with a flat top becomes round shaped as h decreases.9 The present observations indicate the importance on the top structure of the disk, which is sensitive to the lattice plasmon modes. The results obtained by varying a, D, and h confirm the importance of structural control of the lattice and disk unit in the SLR properties.25,27,28 As an additional factor for fine tuning the SLR mode, we introduced electrochemical potential control into the system. In situ electrochemical dark-field scattering microscopic measurements were performed by changing the electrochemical potential (f) of the Au lattice structure from −0.8 to 0.4 V, as shown in Fig. 3a. This potential region corresponds to the double-layer region of the Au electrode, where no Faradic redox reactions occur at the surface. As shown in the extinction spectra (Fig. 3a), the potential scan results in a wavelength shift from 730 to 734 nm, while the FWHM remains almost constant. According to previous studies, this SLR shift can be considered to be the result of fine tuning of the LSPR by the potential scan, which causes a change in the electrochemical potential of the electrons in the Au structure.33,35 The changes in the density and spatial localization of electrons by the electrochemical potential are considered to be the origin of the blue shift in the maximum wavelength of the LSPR at negative electrochemical potential polarization. The observed blue shift of the SLR in Fig. 3(b)


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corresponds to LSPR of the single Au particle, as shown in Fig. S1. This result is an interesting observation of the electrochemical potential effect on the SLR. By varying f of the Au lattice structure, we can achieve fine tuning of the SLR without a significant change in the FWHM. In addition to control of the electrochemical potential of the structure, the electrochemically controlled metal dissolution reaction was introduced to tune the lattice plasmon modes via fine structural control of a single disk unit. The extinction spectra during electrochemical potential polarization of the Au lattice structure (a = 400 nm, D = 140 nm, and h = 100 nm) at f = 0.8 V for 600 s in 0.1 M NaClO4 containing 10 mM KBr are shown in Fig. 4a. The significant change in the extinction spectrum indicates decreases in the extinction intensities for both SLR and LSPR. SEM and AFM images of the Au lattice structures after polarization for 0, 120, and 600 s are shown in the top and middle rows of Fig. 4b. From the SEM image after polarization for 120 s, homogeneous dissolution is confirmed. This fact would indicate that only changing D and h results in a volume change, which could be the main factor for the change in the plasmonic properties. At 600 s, extensive dissolution is the main reason for the higher FWHM and lower scattering intensity. The cross-section views of the AFM images (bottom row of Fig. 4) show that D and h simultaneously decrease by 25 and 33 nm after polarization for 600 s, respectively. For f = 0.8 V, oxidative dissolution of Au occurs at the Au surface, leading to formation of dissolved AuBr%& anions. Thus, during the in situ observations, the homogeneous decrease in the volume of a single disk unit via Au dissolution results in a change in the lattice plasmon modes. In the present system, the estimated rate of surface dissolution is a few nanometers per minute, which can be easily controlled by changing f. Figure 5a shows the maximum wavelength of SLR plotted against the electrochemical potential polarization time. For a polarization time of 600 s, the blue shift is ~50 nm (from 729 to


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680 nm). In the present case, both the height and diameter simultaneously decrease because of the homogeneous dissolution process. Nevertheless, the trend of the wavelength shift is close to the case with only a lattice spacing change (Fig. 2). This is because the lattice spacing, diameter, and shape mainly affect the in-plane dipolar interaction, while the height of the structure contributes less to the in-plane mode.28 In the present observation system, the in-plane mode is mainly observed because a Fano-type asymmetric peak-and-dip spectrum profile derived from interference between the out-of-plane and in-plane modes could appear if the out-of-plane is mainly observed. Consequently, the trend in Fig. 5a can be easily understood. The FWHM is plotted against the polarization time in Fig. 5b. The FWHM decreases from an initial value of 0.165 eV to a minimum of 0.145 eV after polarization for 120 s, and it then increases to above 2.0 eV. At the FWHM minimum, the SLR resonance maximum is 714 nm (1.74 eV). This characteristic change in the FWHM suggests that that the lifetime of the excited SLR mode can be precisely tuned by decreasing D and h at a rate of a few nanometers per minute. This decrease in the FWHM could originate from the changes in the LSPR and lattice spacing optimizing the transformation from radiative to evanescent, leading to a confined light field and long lifetime excited state. It should be emphasized that the absolute minimum FWHM value achieved in the present system is comparable with those in previous studies.41,42 Real-time observation of the FWHM associated with controlled surface dissolution is important to obtain a lattice structure with tailored plasmonic properties for an extended lifetime. The advantages of the present electrochemical tuning method are that both the resonance wavelength shift and FWHM can be precisely controlled. Consequently, the present method can produce a highly confined light field with a relatively long lifetime excited state tuned to the target energy of the plasmon.


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CONCLUSIONS By applying the electrochemical method to the Au plasmonic lattice structure system, we have succeeded in tuning both the resonance wavelength and FWHM for the SLR mode. By controlling the electrochemical potential of the Au lattice, the resonance wavelength can be tuned with precision below a few nanometers, which cannot be achieved by structural control techniques. Furthermore, we have succeeded in fine tuning the FWHM by controlling the electrochemical metal dissolution reactions. By controlling surface dissolution of the Au lattice structure at a rate of a few nanometers per minute, we achieved a minimum spectral width of 0.145 eV at a resonance maximum of 714 nm (1.74 eV). This result indicates the possibility of application of the method to control plasmonic light confinement in the lattice structure. Although there is still room for improvement in the observation technique and analysis, such as detail assignment of the observed modes and theoretical simulation of the present results, the present method is a novel optimization technique for controlling the lattice plasmon modes.

ASSOCIATED CONTENT Supporting Information Details about the electrochemical potential control of the LSPR properties of a single Au nanodisk.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Author Contributions K.M., H.M. and S.O. designed the research. A.S., T.H., and S.O. prepared the samples and performed the experiments. A.S., T.H., S.O., X.L., and H.M. analyzed and interpreted the results. H.M., S.O., and K.M. developed the model and prepared the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The present 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 “NanoMaterial Optical-Manipulation” (16H06506).

Supporting Information. The detail about the electrochemical potential control of the LSPR property for the single Au nano-disk is supplied as Supporting Information


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Figures

Fig. 1 | (a) SEM images, (b) dark-field scattering images and (c)extinction spectra of Au lattices with a = 360 (bottom), 400 (middle), and 440 nm (top) (h = 100 nm and D = 140 nm). All of the scale bars in (a) are 200 nm.


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Fig. 2 | Extinction spectra of Au lattice structures with different (a) a, (b) D, and (c) h values. The respective parameters of a, D, and h were changed from 300 to 460 nm, 60 to 140 nm, and 50 to 100 nm, respectively. The middle and bottom rows are the maximum resonance wavelength and FWHM plotted against (a) a, (b) D, and (c) h.


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Fig. 3 | In situ electrochemical extinction spectra of the Au lattice structures. The electrochemical potentials were kept at −0.8, −0.4, 0, and 0.4 V (from bottom to top) in 0.1 M NaClO4. The values of a, D, and h are 400, 140, and 100 nm, respectively. (b) and (c) Plots of the maximum resonance wavelength and FWHM as a function of the electrochemical potential, respectively.


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Fig. 4 | (a) In situ extinction spectra of the Au lattice structures polarized at 0.8 V for 600 s in 0.1 M NaClO4 containing 10 mM KBr. The values of a, D, and h are 400, 140, and 100 nm, respectively. (b) SEM (top row) and AFM images (middle row) obtained after polarization times of 0 (left), 120 (center), and 600 s (right). The scale bars in the SEM images are 200 nm. The bottom row shows a cross-section view of a single Au disk shown in each AFM image.


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Fig. 5 | Plots of the (a) maximum resonance wavelength and (b) FWHM of the extinction spectra in Fig. 4 as a function of the electrochemical potential polarization time.

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Fig. 1 | (a) SEM images, (b) dark-field scattering images and (c) extinction spectra of Au lattices with a = 360 (bottom), 400 (middle), and 440 nm (top) (h = 100 nm and D = 140 nm). All of the scale bars in (a) are 200 nm. 119x101mm (300 x 300 DPI)


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Fig. 2 | Extinction spectra of Au lattice structures with different (a) a, (b) D, and (c) h values. The respective parameters of a, D, and h were changed from 300 to 460 nm, 60 to 140 nm, and 50 to 100 nm, respectively. The middle and bottom rows are the maximum resonance wavelength and FWHM plotted against (a) a, (b) D, and (c) h. 203x211mm (300 x 300 DPI)



Fig. 3 | In situ electrochemical extinction spectra of the Au lattice structures. The electrochemical potentials were kept at −0.8, −0.4, 0, and 0.4 V (from bottom to top) in 0.1 M NaClO4. The values of a, D, and h are 400, 140, and 100 nm, respectively. (b) and (c) Plots of the maximum resonance wavelength and FWHM as a function of the electrochemical potential, respectively. 127x104mm (300 x 300 DPI)


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Fig. 4 | (a) In situ extinction spectra of the Au lattice structures polarized at 0.8 V for 600 s in 0.1 M NaClO4 containing 10 mM KBr. The values of a, DD, and h are 400, 140, and 100 nm, respectively. (b) SEM (top row) and AFM images (middle row) obtained after polarization times of 0 (left), 120 (center), and 600 s (right). The scale bars in the SEM images are 200 nm. The bottom row shows a cross-section view of a single Au disk shown in each AFM image. 151x254mm (300 x 300 DPI)



Fig. 5 | Plots of the (a) maximum resonance wavelength and (b) FWHM of the extinction spectra in Fig. 4 as a function of the electrochemical potential polarization time. 127x61mm (300 x 300 DPI)


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Electrochemical Fine Tuning of Plasmonic Property for Au Lattice

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