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Enhanced photoluminescence from organic dyes coupled to periodic array of zirconium nitride nanoparticles Ryosuke Kamakura, Shunsuke Murai, Koji Fujita, and Katsuhisa Tanaka ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00320 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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ACS Photonics

Enhanced photoluminescence from organic dyes coupled to periodic array of zirconium nitride nanoparticles Ryosuke Kamakura,† Shunsuke Murai,†‡* Koji Fujita,† Katsuhisa Tanaka† †

Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 6158510, Japan ‡ PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan Supporting Information ABSTRACT: Noble metals, particularly gold, have been conventionally used for their suitable optical properties in the field of plasmonics. However, gold has a relatively low melting temperature, especially when nanosized, and the abundance of gold in the earth’s crust is low. These material-related limitations hinder the exploration of the use of plasmonics in several application areas. Transition metal nitrides are promising material alternatives because of their high mechanical and thermal stabilities, in addition to their acceptable plasmonic properties in the visible spectral region. Zirconium nitride (ZrN) is one such promising alternative owing to a higher carrier density than that of titanium nitride (TiN), which has been the most studied complementary material to gold. In this study, we have fabricated periodic arrays of ZrN nanoparticles and found that the ZrN array enhances the photoluminescence from an organic dye on the array; the photoluminescence intensity is increased by as much as 9.7 times in the visible region. This result experimentally verifies that ZrN is useful as an alternative material to gold, to further develop plasmonics and mitigate the conventional material-related limitations. KEYWORDS: zirconium nitride, localized surface plasmon resonance, periodic nanoparticle array, photoluminescence enhancement. Localized surface plasmon resonance (LSPR) is the collective oscillation of free electrons on a metal nanostructure that resonates upon light illumination, and is an important player in the field of nanophotonics1-3. The plasmonic properties are deeply dependent on their electrical properties. The carrier concentration determines the plasmon frequency and the mobility dominates the optical loss. The visible region has been the most widely targeted in current plasmonics; the high photon energy can apply to various applications such as biosensors, photoreactions, solar cells, and light-emitting devices4-6. Noble metals such as gold and silver have been used almost exclusively for plasmonic applications in this region due to their suitable optical properties. Their other properties, such as their thermal, mechanical, and chemical (except gold) stabilities, are insufficient to satisfy the compatibility requirements with semiconductor technologies and high-power laser applications for non-linear photonics. As such, these material-related limitations hinder the further development of plasmonics. Transitional metal nitrides, such as titanium nitride (TiN) and zirconium nitride (ZrN), are emerging alternative materials to noble metals in plasmonics711 . These nitrides show not only metallic behavior in the visible region but also good mechanical, thermal, and chemical stabilities12-16. In addition, the work functions of TiN and ZrN (TiN: 4.2-4.5 eV, ZrN: 4.6 eV) are lower than those of Au, which results in Ohmic contacts to typical photocatalysts such as TiO2. This allows for the excitation of photoelectrons without a Schottky barrier at the interfaces17-20. For these advantages, nitrides have been considered as alternative and complementary plasmonic materials for noble metals. Figures 1 (a) and (b) show the real (ε′) and imaginary (ε′′) part of the dielectric functions of Au, TiN, and ZrN thin films, respectively. The Au thin film was deposited on a silica glass substrate using electron-beam deposition (EB1200, Canon ANELVA), and the TiN and ZrN thin films were grown on a silica glass substrate using the DC magnetron sputtering meth-

od (GEOMATECH, Japan)21. The wavelengths at which ε′ crosses zero (λ0) for Au, ZrN, and TiN are λ0 = 330 nm, 390 nm, and 510 nm, respectively. ZrN possesses a λ0 value comparable to that of Au, and it starts to behave as a metal at a wavelength shorter than that of TiN. On the other hand, the ε′′ of TiN and ZrN is higher than that of Au because of their high optical loss, which is one of their demerits for plasmonic applications. Utilizing periodic arrays of metal nanoparticles is a sensible way to reduce energy dissipation21-25. In such periodic arrays, light diffraction in the plane of the array mediates radiative coupling between the LSPRs on each nanoparticle, and the coherent oscillation of LSPRs yields a collective response that is stronger than the simple sum of each LSPR. This lightdiffraction-mediated collective response is referred to as a plasmonic—photonic hybrid mode, and it can mitigate the energy dissipation to the metal because the light energy is not only localized in the vicinity of the nanoparticles but is also extended in the plane of the array26-30. Owing to this unique spatial distribution of light energy, the array can enhance the photoluminescence (PL) from good emitters, or the emitters with high quantum yield. This is in sharp contrast to most plasmonic nanostructures, where the energy transfer to metals quenches the PL from the emitters. Thus it is generally difficult to achieve PL enhancement for emitters with high quantum yield. PL enhancement by metallic nanostructure has been a topic in plasmonics and it is accepted that the enhancement stems from three contributions; absorption, quantum yield, and outcoupling.31,32 Our group previously reported the PL enhancement of rhodamine 6G (R6G), an orange dye with high quantum yield, using the hybrid mode on the periodic array of TiN nanoparticles in the visible region21. However, the enhancement factor is 2.8 times at most, because the plasmonic effect is not strong enough in the excitation and emission spectral regions of the organic dye used.

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In this study, as an alternative material, we selected ZrN to realize PL enhancement in the visible region. Compared to TiN, ZrN has a larger carrier density and thus behaves as a metal at shorter-wavelength regions as shown in Fig. 1 (a). First principle density functional theory-based calculations also suggested that ZrN is more suitable as a plasmonic material than TiN16. We fabricated the square arrays of ZrN nanoparticles with pitches of 350 and 400 nm, which will be referred to as ZrN350 and ZrN400, respectively. We placed a polymer layer containing R6G on the top of the arrays for PL measurements. The two arrays were designed to show identical absorption at the excitation wavelength of R6G (λex = 473 nm), while the spectral position of hybrid modes outside (ZrN350) and inside (ZrN400) of the PL spectrum of R6G. As a result, the ZrN400 achieved a PL enhancement as high as 9.7 times by the combination of the absorption and outcoupling by LSPR (3.88 times) and outcoupling by the plasmonic — photonic hybrid mode (2.50 times)). It is noted that the R6G in polymer is a good emitter with high quantum yield and usually difficult to enhance PL intensity by plasmonic nanostructures.

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PMMA + R6G layer. Before the layer deposition, the spectra show a broad dip in the measured region because of the excitation of LSPR on the ZrN nanoparticles. In addition, a kink is observed at λ = 510 (580) nm for ZrN350 (ZrN400), which corresponds to the in-plane diffraction conditions calculated using the refractive index of a silica glass substrate (n = 1.46), as indicated by vertical dashed lines. The in-plane diffraction conditions can be described as Rayleigh anomalies, which satisfy the following relationship at θin = 0° in the square lattice 2 2  2π   2π  k 2 = m2  + m2  , (1) out

1 a 

2 a 

where kout is the wave vector of the scattered light; a is the pitch; and m1 and m2 are the diffraction orders34. After the layer deposition, the transmittances clearly show peaks at λ = 510 and 580 nm for ZrN350 and ZrN400, respectively. The transmittance is higher and the kink is better defined compared to those before the layer deposition. Note that the transmission is calculated using bare glass as a reference for the sample before deposition, while the PMMA + R6G layer on the glass is used after the deposition. The presence of the layer changes the light-energy distribution around the ZrN nanoparticle, leading to the enlarged transmittance. The better defined kink indicates the stronger light diffraction after the layer deposition.

Figure 2 (a) Fabrication process flow schematic of the periodic arrays of ZrN nanoparticles. SEM top-view images of (b) ZrN350 and (c) ZrN400. The insets in (b) and (c) show the enlarged and oblique (20° to vertical) images.

Figure 1 (a) Real (ε′) and (b) imaginary (ε′′) part of the dielectric functions for Au, TiN, and ZrN thin films. These data are obtained from the ellipsometry measurements.

RESULTS AND DISCUSSION The ZrN film on a silica glass substrate shows the preferred orientation in X-ray diffraction pattern, as shown in Fig. S1. Figures 2 (b) and (c) show the SEM images of the ZrN350 and ZrN400 arrays, respectively. The diameters of the nanoparticle are 280 and 370 nm for ZrN350 and ZrN400, respectively, and the height is 150 nm for both. Figures 3 (a) and (b) show the transmittance spectra of ZrN350 and ZrN400 before and after the deposition of the

The arrows in Fig. 3 denote the wavelength of excitation, λex = 473 nm, at which the transmittance is very similar to one another for both arrays. In order to estimate the absorption enhancement by the array, we plotted the transmittance normalized to the incident light in Fig. S2. Figure S2 shows that the transmittance at λex = 473 nm for ZrN350 and ZrN400 is reduced by 7.1 and 6.9 times, respectively, compared to the reference. This reduction indicates that the number of excited R6G molecules is larger for array samples, which is one factor of the PL enhancement. Figures 4 (a) and (b) show the difference in transmittance between the arrays before and after the layer deposition (∆T). The black and red lines denote the experimental and simulation results, respectively. Although the magnitude in ∆T is different between the experiment and the simulation, the simulation reproduces qualitatively the experimental result without

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ACS Photonics losing the physics involved in the experiment. Deviation from the experiment would be caused by the disagreement between the real and simulated structures such as surface roughness of the PMMA + R6G layer and geometry of ZrN nanoparticles. We highlighted the spectral region of PL for R6G as a colored area to clarify the diffraction condition is out of the spectral area for ZrN350. Figures 4 (c) and (d) ((e) and (f)) show the simulated light-energy distribution of ZrN350 (ZrN400). When the specimens are irradiated with a light with λ = 650 nm, the light energy is accumulated on the ZrN nanoparticle owing to the excitation of LSPR for both ZrN350 and ZrN400 (Figs. 4 (d) and (f)). On the other hand, at the peak position in ∆T, λ = 515 nm and 580 nm for ZrN350 and ZrN400, respectively, the light energy is concentrated in the PMMA + R6G layer indicating that the layer acts as a waveguide to trap the incident light(Figs. 4 (c) and (e)).

ZrN400 is different from that for ZrN350. The PL intensity for ZrN350 is enhanced in a broad wavelength range corresponding to the PL of R6G (λ = 530 – 650 nm), whereas the PL intensity for ZrN400 is remarkably enhanced in a narrower range of around λ = 550 – 580 nm, with a peak being shifted to of λ = 570 nm.

Figure 4 Experimental and simulated difference between the transmittance spectra before and after the deposition of the PMMA + R6G layer on (a) ZrN350 and (b) ZrN400. The spectral region of PL for R6G (530 – 650 nm) is highlighted in (a) and (b) as a yellow region. Panels (c) and (d) ((e) and (f)) show the calculated spatial distributions of the normalized intensity, |E|2/|E0|2, in the zy-plane at position x intersecting the center of a nanoparticle for ZrN350 (ZrN400). The irradiation conditions are λ = 515 nm (c) and λ = 650 nm (d) for ZrN350, and λ = 580 nm (e) and λ = 650 nm (f) for ZrN400. The white lines in the panels represent the boundaries of the materials. The diameter of the ZrN nanoparticle for ZrN350 and ZrN400 is 280 and 370 nm, respectively. The incident light is linearly polarized in the y-direction, i.e., the electric field oscillates in the y-direction.

Figure 3 Transmittance spectra of (a) ZrN350 and (b) ZrN400 before and after the deposition of the PMMA + R6G layer. The vertical lines display the diffraction conditions from eq. (1), reflecting the refractive index of the silica glass substrate (n = 1.46). The arrows denote the excitation wavelength (λex = 473 nm).

Figure 5 (a) depicts the PL spectra from the PMMA + R6G layer on a silica glass substrate as a reference as well as on ZrN350 and ZrN400. The reference shows the PL spectrum with a peak at λ = 565 nm, which is typical for R6G35. The PL intensities from ZrN350 and ZrN400 are obviously enhanced compared to that from the reference. The enhancement factors (EFs) of ZrN350 and ZrN400 are obtained by dividing the PL intensity from ZrN350 and ZrN400 by that from the reference as shown in Fig. 5 (b). At the peak wavelength, the EFs of ZrN350 and ZrN400 are as high as 3.7 and 9.7 times, respectively. It should be noted that the spectral shape of the PL for

In Fig. 5(c), the EF of ZrN400 (EFZrN400) divided by that of ZrN350 (EFZrN350) is shown. The value of EFZrN400/EFZrN350 is more than unity for λ = 550 – 580 nm, and has a peak as high as 2.5 times at λ = 570 nm. Also plotted is the transmittance of ZrN400 embedded in the PMMA + R6G layer. The kink in transmittance due to a Rayleigh anomaly (λ = 580 nm) lies in a wavelength range slightly longer than the peak in the EFZrN400/EFZrN350 ratio (λ = 570 nm). The discrepancy is due to the spectral shift between the far-field and near-field, and is reproduced qualitatively by the simulation. Figure 6 (a) shows the simulated transmittance and the integrated light energy in the PMMA+R6G layer (thickness: 600 nm) on ZrN400. The peak of the integrated light energy (λ = 575 nm) is slightly blueshifted from the peak of the transmittance (λ = 580 nm). Figures 6 (b) and (c) display the light-energy distribution under the irradiation of λ = 575 nm light in the zy- and zx-planes, respectively. The spatial distributions show a concentration of light energy in the PMMA + R6G layer28,36. This simulation indicates the reciprocity of the outcoupling from the emitting layer to air, i.e., when the PL is produced in the layer, the PL is preferentially coupled out and radiated into the normal direction.

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Figure 5 (a) PL spectra of the PMMA + R6G layer on a silica glass substrate (Reference), ZrN350, and ZrN400. (b) PL enhancement factor (EF) for ZrN350 and ZrN400, obtained by dividing the PL intensity of the ZrN arrays by the PL intensity of the reference. (c) Transmittance spectra of ZrN400 embedded in the PMMA + R6G layer, the same spectrum as that shown in Fig. 3(b), and the EF of ZrN400 divided by that of ZrN350. The vertical line denotes λ = 570 nm)

We further investigate the effect of layer thickness. We simulated the integrated light energy in the PMMA+R6G layer with a thickness from 200 to 1000 nm on ZrN400. The peak of the integrated light energy in the PMMA+R6G layer is plotted in Fig. 6(d). As increasing the thickness from 200 nm, the energy increases, hits a maximum at the thickness = 800 nm, and then decreases. This indicates PL enhancement would occur for layers thicker than that used experimentally in this study. This calculation is in agreement with a previous study for plasmonic array of Al nanocylinders,32 where the PL enhancement effect is obtained much thicker layer up to several micrometers, and suggests that local light energy accumulation by LPSR is not a single reason for PL enhancement in plasmonic arrays, but light trapping in the light emitting layer by hybrid mode contributes significantly.

Figure 6 (a) Simulated transmittance spectra and integrated light energy in the PMMA + R6G layer (thickness: 600 nm) on the ZrN array (pitch: 400 nm) normalized by that of the layer on the substrate, ʃ |E|2 dV/ʃ |Eref|2 dVref, at θin = 0°. The vertical blue line is drawn at λ = 575 nm. Calculated spatial distribution of the normalized intensity, |E|2/|E0|2, in the (b) zy- and (c) zx-planes at λ = 575 nm. (d) The peak value in ʃ |E|2 dV/ʃ |Eref|2 dVref as a function of the layer thickness. The dotted line is a guide for eyes.

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ACS Photonics show a reduced quantum yield40. The present value is also close to the combination with a silver nanoparticle array (11 times in the visible region)41. From previous studies, it should be noted that the presence of a dielectric spacer between the emitters and the gold mitigates the energy dissipation and increases the PL enhancement significantly37,38. ZrN nanoparticle arrays have achieved comparable PL enhancement even in the absence of a spacer, suggesting that the surface state of the ZrN nanoparticles is not critical for the PL enhancement in plasmonic arrays. This is partly because of the spatial distribution of the light energy that is away from the nanoparticles (see Figs. 6(b) and (c))29. Figure 7 PL decay curves (λem = 565 nm, λex = 470 nm) for the PMMA + R6G layer on a silica glass substrate (Reference), ZrN350, and ZrN400.

The plasmonic structure can contribute to the PL enhancement in three ways, i.e., the enhancement of absorption at λex, quantum yield at λem, and outcoupling at λem. To speculate the change in quantum yield, we measured the PL decay time. Figure 7 shows the time-resolved PL decay curves for the reference, the PMMA + R6G layer on ZrN350 and the layer on ZrN400. Their PL lifetimes, which are calculated from the fit using a single exponential curve, are 2.46, 1.85, and 1.90 ns for the reference, ZrN350, and ZrN400, respectively. The difference in lifetime from that of the reference is 25 and 23 % for ZrN350 and ZrN400, respectively. The lifetime of the ZrN arrays is shorter than that of the reference. This change could be due to the increase in the radiative decay rate by the Purcell effect and/or the increase in the non-radiative decay rate by energy dissipation from R6G to ZrN34. Since, according to previous works6,21, the Purcell effect is not observed in the plasmonic arrays embedded in a dye layer, we assume that the decrease of decay time is as a result of the energy dissipation. The PL enhancements are observed for the array samples in spite of energy dissipation that reduce the PL intensity, which indicates that the effects of absorption and outcoupling outweigh this negative effect. Let us summarize the origin of enhancement. ZrN350 shows the PL enhancement (as large as EFZrN350 = 3.70, see Fig. 5(b)) that results from the excitation of LSPR at both λex and λem. Spectrally broad LSPRs (see Fig. 3) contribute to both absorption and outcoupling. The values of EFZrN350 (=3.70) is smaller than the reduction in transmittance (7.1 times at λex = 473 nm, see Fig. S2) because not all of the reduction is the result of the absorption by the R6G; part of the light is scattered and/or absorbed by the ZrN array. The larger EF by ZrN400 comes from an additional term of the outcoupling by the hybrid mode at the enhancement peak (EFZrN400 /EFZrN350 = 2.50 at λ = 570 nm, see Fig. 5(c)), in addition to the excitation of LSPRs at λex and λem. Here, the contribution of LSPRs is calculated to be 3.88 by dividing the total EFZrN400 (=9.70, see Fig. 5(b)) by the enhancement factor of the hybrid mode(=2.50). The EF as large as 9.7 is comparable to the values reported for the combination of R6G or other high quantum yield dyes with gold nanostructures such as nanorod arrays with a spacer (PL enhancement of 3.5 times in the visible region), metalcore/silica-shell nanoparticles (6.8 times in the visible region), and nanobar arrays (6–11 times in the near–IR region)28,37-39. A very large enhancement (>100) was reported for R6G in gold nanohole array, where dye molecules could be aggregated to

CONCLUSION We have fabricated periodic arrays of ZrN nanoparticles to verify the PL enhancement experimentally, in the visible region. The ZrN arrays clearly enhance the PL from the dyecontaining layer deposited by as much as 9.7 times. This enhancement is the result of the absorption and outcoupling enhancements by the LSPRs and the outcoupling of hybrid mode, while PL decay data indicate the decrease in quantum yield of the dye by energy dissipation to ZrN. This is the first demonstration of PL enhancement by a ZrN array in the visible region. This result clarifies that the performance of a ZrN nanoparticle array is better than that of TiN for PL enhancement in the visible region and is comparable to that of noble metals. It is worth mentioning that compared to silver and gold, ZrN has higher mechanical and thermal stability, which would be favorable in nanostructuring and applications to electronics and photonics, allowing the material-related limitations imposed on plasmonics to be overcome. METHOD

Experimental. A ZrN thin film (thickness: 150 nm) was grown on a silica glass substrate using DC magnetron sputtering (GEOMATEC, Japan). The thin film was then patterned with a combination of nanoimprint lithography (EntreTM3, Obducat) and reactive ion etching (RIE) (RIE101iPH, Samco) to structure the ZrN nanoparticle arrays. Figure 2 (a) shows the process flow schematic. First, the resist (TU2-170, thickness: 200 nm) was coated onto the ZrN thin film and prebaked for 5 min at 95 °C. As a master mold for the nanoimprint lithography, Si molds consisting of square arrays of nanopillars (diameter: 150 nm; height: 200 nm; pitch: 350 nm (ZrN350) and 400 nm (ZrN400)) were fabricated by electron-beam lithography (F7000sKYT01, Advantest) and silicon deep etching (RIE-800iPBKU, Samco). Then, the surface structures of the Si molds were transferred to the resist by nanoimprint lithography. The square arrays of ZrN nanoparticles were structured by RIE under a chamber pressure of 2 Pa, a radio-frequency power of 700 W, and Ar, BCl3, and Cl2 gas flow rates of 8, 5, and 15 cm3/min, respectively33. For the demonstration of PL enhancement, poly(methyl methacrylate) (PMMA) (average molecular weight of 120,000) containing 0.8 wt% R6G was spin coated on the ZrN arrays and a silica glass substrate as a reference. This light emitting layer will be referred to as the PMMA + R6G layer. The thickness of the PMMA + R6G layer was measured by surface profiler to be approximately 600 nm. Simulation model. To analyze the experimental results, we conducted simulations using the finite element method

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(COMSOL Multiphysics). The simulation model was composed of the following elements from bottom to top: a silica glass substrate (refractive index n = 1.46), the ZrN particle (height: 150 nm, diameter: 370 nm for ZrN400 and 280 nm for ZrN350), and the PMMA + R6G layer. The values for n and the absorption coefficient k of ZrN and the PMMA + R6G layer were obtained from spectroscopic ellipsometry data. The size of the unit cell was 400 × 400 × 2300 nm for ZrN400 and 350 × 350 × 2300 nm for ZrN350 in the x, y, and z directions, respectively, with the periodic boundary conditions being applied to the x and y directions, the port node on the top, and the perfectly matched layer on the bottom. The projected light was polarized in the y direction. Characterization. The surface structures of the ZrN arrays were examined using scanning electron microscopy (SEM) (SU8000, Hitachi). The dispersive refractive index of the ZrN film and the PMMA + R6G layer were examined using a spectroscopic ellipsometry setup (FE-5000, Otsuka Electronics Inc.). The optical transmittance spectra were measured using a Si-based detector at incident angle (θin) = 0°. The zeroth-order transmittances were obtained by normalizing the transmission intensity of the array sample to that of a silica glass substrate. For the transmittance of the array embedded in the PMMA + R6G layer, the transmission intensity was normalized to the transmission of the PMMA + R6G layer on a silica glass substrate. The PL spectra were measured as follows: a diodepumped solid state laser (excitation wavelength λex = 473 nm, Shanghai Dream laser) was incident on the backside of the array tilted by 5° from the normal, and PL spectra were collected at emission angle (θem) = 0°. The PL decay curve at emission wavelength λem = 565 nm was measured using a time-correlated single—photon counting module (Quantaurus— Tau, Hamamatsu Photonics). The excitation pulse at λex = 470 nm of the system was used. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.XXXXXXX X-ray diffraction pattern of ZrN thin film and Transmittance spectra of the PMMA + R6G layer on a silica glass substrate, ZrN350, and ZrN400 (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. M.) ORCID

Ryosuke Kamakura 0000-0002-5859-6850 Shunsuke Murai: 0000-0002-4597-973X Koji Fujita 0000-0002-1700-0889 Author Contributions S.M conceived and designed the project. R.K. fabricated the arrays, measured PL, and carried out the optical simulations. K.F. and K.T. assisted in manuscript preparation. R.K. and S.M. analyzed the data and wrote the manuscript. All authors discussed the progress of research and reviewed the manuscript.

Acknowledgments This work was partly supported by the Nanotechnology Hub, Kyoto University and the National Institute for Material Science (NIMS) Nanofabrication Platform in the “Nanotechnology Platform Project” sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Financial support from Grants-in-Aid for Scientific Research (B, No.16H04217) from MEXT and the Asahi Glass Foundation are acknowledged. SM gratefully acknowledges the support from “the construction project for the consortium of the fostering of science and technology personnel”, Nanotech Career-up Alliance (Nanotech CUPAL), and Precursory Research for Embryonic Science and Technology (PRESTO, JPMJPR131B) from Japan Science and Technology Agency.

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Notes The authors declare no competing financial interest.

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