Surface-Enhanced Infrared Absorption for the Periodic Array of Indium

Feb 3, 2018 - y directions; and m1 and m2 are the diffraction orders, respectively.44 Figure 3(c and d) clearly show the peak shifting along the diffr...
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Surface-enhanced infrared absorption for the periodic array of indium tin oxide and gold microdiscs: Effect of in-plane light diffraction Ryosuke Kamakura, Tomoki Takeishi, Shunsuke Murai, Koji Fujita, and Katsuhisa Tanaka ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01265 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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ACS Photonics

Surface-enhanced infrared absorption for the periodic array of indium tin oxide and gold microdiscs: Effect of inplane light diffraction Ryosuke Kamakura,† Tomoki Takeishi,† Shunsuke Murai,†‡* Koji Fujita,† and 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: Surface-enhanced infrared absorption (SEIRA) is an important phenomenon to achieve nondestructive, simplified, and in situ high-sensitivity infrared (IR) sensors. Conventionally, metal structures with nanogaps are employed to realize the high sensitivity owing to the extremely strong field enhancement in the hot spot. Although a library of surface modifiers has been developed, the manipulation of nanogaps and immobilization of target molecules in the hot spot are still complicated. In addition, target molecules immobilized at the positions other than the hotspot have relatively low sensitivity. A periodic array with pitch comparable to the wavelength of interest is an alternative structure in which the coupling of the plasmonic mode to in-plane light diffraction provides the hybrid mode accompanied by an enhanced electric field. Although the field enhancement by the hybrid mode depends on matching between localized surface plasmon resonance (LSPR) and diffraction, the contribution of the matching to SEIRA enhancement has never been clarified. In this work, we fabricated periodic arrays of indium tin oxide (ITO) and Au microdiscs (pitch: 3 µm, diameter: 2 µm) to analyze the contribution of the hybrid mode through varied LSPR and diffraction conditions. As a result, the ITO and Au arrays demonstrate a similar plasmonic—photonic hybrid mode in the mid-IR region despite the different excitation frequency of LSPR. To estimate the effect of the hybrid mode on SEIRA enhancement, the incident angular profiles of IR spectra of the polymer layer on the ITO and Au arrays were measured. The SEIRA enhancement factors for ITO and Au arrays are comparable in the measurement IR region (2200–1400 cm-1). Our results verify that the plasmonic−photonic hybrid mode is very efficient for SEIRA enhancement and the periodic array of microdiscs is very suitable for this application. KEYWORDS: indium tin oxide, localized surface plasmon resonance, periodic disc array, surface-enhanced infrared absorption. Surface-enhanced infrared absorption (SEIRA) has promising applications in plasmonics to achieve high-sensitivity infrared (IR) sensors for detection of biological molecules1-3. In SEIRA measurement, the signals of molecular vibrations are enhanced strongly owing to the intense electric field caused by the excitation of localized surface plasmon resonance (LSPR), which is the collective oscillation of free electrons on a metal surface coupled to electromagnetic waves4-8. SEIRA measurement requires only an incoherent IR light source and has advantages such as nondestructiveness and a simplified measurement system, as well as in situ measurement in liquid13-19. In this sense, SEIRA has advantages over surface-enhanced Raman scattering (SERS)9-12, which is the most prevalent application in plasmonics. For SERS measurement, a laser is required to obtain high resolution, which can lead to the decomposition of target molecules. For a conventional SEIRA study, noble metals, especially Au, have been used exclusively. Au is a good plasmonic material to obtain a highly enhanced electric field owing to the large carrier density, while the ohmic loss derived from the electron–electron scattering becomes larger in the IR region20,21. Low ohmic loss is required to effectively use the light power in the IR region. Indium tin oxide (ITO), which is most frequently used as a transparent conductive oxide in the electronic industry22, is one of the promising plasmonic materials in the IR region20,23-28. Figures 1(a) and (b) show the real (ε') and imaginary (ε'') parts of the dielectric function for Au and ITO (electrical resistivity: 139.37 µΩ·cm, GEOMATECH, Japan) thin films in the IR region. The ITO thin film grows

with preferred orientation on a silica glass substrate (Fig. S1). The magnitudes of ε' and ε'' for ITO are smaller than those of Au because of the lower carrier density. To compare the usefulness between ITO and Au for plasmonic applications in the IR region, the figures of merit (FOMs) are estimated by dividing −ε' by ε'' as shown in Fig. 1(c)26. The FOM of ITO is comparable with that of Au in the IR region because of the smaller ε'' of ITO than Au as shown in Fig. 1(b). The FOM of ITO indicates that ITO is suitable as an alternative plasmonic material for noble metals in the IR region. Additionally, the optical properties of ITO are tunable in the IR region by changing the dopant concentration and the dopant species such as oxygen deficiency and F23,25,29-33. In addition, ITO has specific properties important for the development of a novel plasmonic platform for biosensors. ITO can capture selectively carboxylic and phosphonic acids on the surface34-36, and shows adsorption and desorption of DNA owing to the surface charge36. For these tunable optical properties and selective adsorptions, ITO is a promising plasmonic material for biosensors in the IR region. Various designs of metallic structures such as nanorod arrays37, fan-shaped antennas16, and roughened metal surfaces are in development to enhance the SEIRA factor1-3. Conventionally, metal structures with nanosized gaps are fabricated to obtain the extremely strong electric field concentrated in the gaps called hot spots, leading to the enhancement of vibrational signals of molecules captured in the hot spots13,14. For commercial applications, the local enhancement has the disadvantages that the fabrication for the nanogaps is complex and the selective immobilization of the target molecule in the hot

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spot is difficult in spite of the recent progress of surface modifiers16,17,39. Recently, a periodic array of metal bars40-44 has been employed to achieve a high SEIRA enhancement factor. In such a metal array, a plasmonic−photonic hybrid mode, which is the in-phase oscillation of LSPRs through diffraction in the plane of the array, leads to field enhancement and the spatial distribution of the near-field owing to LSPRs45-48. The SEIRA effect due to the hybrid mode is comparable with the hot spot despite the long distances between the individual nanostructured metals46. In previous reports, the effects of the hybrid mode were verified by changing the periodicity and size of the array, but the relation between LSPR frequency and angular dependence of diffraction mode in SEIRA enhancement is unclear46. The relation should be clarified to precisely evaluate the SEIRA enhancement by the hybrid mode.

Fig. 1 (a) Real (ε') and (b) imaginary (ε'') parts of dielectric function of ITO and Au thin films. (c) Figure of merit for LSPR, calculated by –ε'/ε'', of ITO and Au. These functions were obtained from the Drude-Lorentz model fitted to the spectroscopic ellipsometry data.

In this study, we have fabricated the periodic arrays of ITO and Au microdiscs (pitch: 3 µm, diameter: 2 µm) to compare their SEIRA enhancement caused by the hybrid mode. To analyze the relation between the LSPR and diffraction, the

effect of hybrid mode for SEIRA enhancement was evaluated through the varied angle of incidence. We experimentally show that the excitation wavenumbers of LSPRs for ITO and Au discs are different, whereas similar angular dependence is observed for both the ITO and Au disc arrays. The ITO disc array demonstrates SEIRA enhancement factors comparable to those of the Au disc array. Additionally, the SEIRA enhancement factors for ITO and Au disc arrays obviously show the incident angular dependencies corresponding to the diffraction conditions. The present study experimentally demonstrates the immense contribution of the hybrid mode to SEIRA enhancement.

SAMPLE FABRICATION An ITO thin film (thickness: 150 nm) was grown on a silica glass substrate by using the DC magnetron sputtering method (GEOMATECH, Japan). The ITO thin film showed metallic behavior in the IR region as shown in Figs. 1 (a) and (b). The thin film was then patterned to a periodic microdisc array by using direct laser lithography (DWL 2000, Heidelberg) and reactive ion etching (RIE) (RIE−101iPH, Samco). The process flow is described in Fig. 2 (a). A photoresist (THMR-iPE1800) was coated on the thin film and prebaked for 90 s at 90 °C. The photoresist was then irradiated with a laser diode (wavelength = 405 nm) to pattern the periodic disc array (pitch: 3 µm, diameter: 2 µm). The sample was then dipped in a standard 2.38 wt% aqueous developer of tetramethylammonium hydroxide (developer NMD-3, TOKYO OHKA KOGYO CO., Japan) for 65 s to remove the irradiated part of the photoresist. The ITO disc array was structured by RIE under a chamber pressure of 3 Pa, radio frequency power of 200 W, and Cl2 and BCl3 gas flowrates of 4 and 16 cm3/min, respectively49. A Au disc array was fabricated by using the conventional lift-off process as shown in Fig. 2 (b). A periodic hole array (pitch: 3 µm, diameter: 2 µm) of a photoresist (THMRiPE1800) on the silica glass substrate was fabricated with the same procedure as the ITO disc array. A Au thin film (thickness: 150 nm) was deposited onto the patterned hole array by using electron beam deposition. After deposition, the photoresist was removed by dipping into piranha solution (H2SO4aq:H2O2aq = 3:1). For the demonstration of SEIRA enhancement, the poly(methyl methacrylate) (PMMA) containing pentyl-cyanobiphenyl (5CB) was coated on the samples. The PMMA (88.8 mg) and 5CB (74 mg) were dissolved in chloroform (30 ml) to prepare the thin PMMA + 5CB layer (85 nm) by the spincoating. The PMMA + 5CB layers were spin-coated on the disc arrays of ITO and Au. The PMMA + 5CB film deposited on the flat ITO thin film (thickness: 150 nm) on a silica glass substrate was used as the reference for the evaluation of SEIRA enhancement of ITO and Au disc arrays.

CHARACTERIZATION The surface structures of ITO and Au disc arrays were examined with scanning electron microscopy (SEM) (SU8000, Hitachi). The optical reflectance was measured as a function of the incident angle (θin) and wavenumber (ω) by using Fourier transform-IR (FT-IR) spectrometry (Nicolet 6700, Thermo Fisher). The intensity ratio of s- and p-polarized components of light source for FT-IR spectrometry was approximately 2:1. The refractive index n and the extinction coefficient k of ITO

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ACS Photonics and Au were obtained from the Drude-Lorentz model fitted to the spectroscopic ellipsometry data (FE–5000, Otsuka Japan, for Au and SENTECH SE 850 DUV combined with Agilent technologies Cary 660 FT–IR spectrometer, for ITO, respectively).

2 2    2π   2π   2π  2 2 2 2   k =k +m + m   + 2k m   out inc 1a  2 a  inc 1 a   x  x  y

(1) 

SIMULATION MODEL To analyze the experimental spectra, we conducted a simulation using the finite elemental method (COMSOL Multiphysics). The simulation model is composed of the following elements from bottom to top: the silica glass substrate (refractive index, n = 1.24)50, ITO disc (height: 150 nm, diameters on top and bottom: 1640 nm and 2000 nm, respectively, as shown in Fig. S2), or Au disc (height: 150 nm, diameter: 2 µm). The values of n and k of ITO and Au were obtained from the ellipsometory data. The size of the unit cell was 3 × 3 × 14 µm in the x, y, and z directions, respectively, with periodic boundary conditions applied to the x and y directions, the port node on the top, and the perfectly matched layer on the bottom. The coordinate axes were selected in accordance with the experiment (see the SEM image in Figs. 2 (c) and (d)). The incident light was linearly polarized in the y direction and was projected from the top.

RESULTS AND DISCUSSION SEM images of ITO and Au disc arrays. The SEM images of fabricated ITO and Au disc arrays are shown in Figs. 2 (c) and (d), respectively. These disc arrays have the same geometries with a diameter of 2 µm and height of 150 nm arranged in a square lattice with a pitch of a = 3 µm.

Fig. 2 Schematic flows for the fabrication process of (a) ITO and (b) Au disc arrays. SEM top-view images of (c) ITO and (d) Au disc arrays. The insets of (c) and (d) show the appearances of ITO and Au disc arrays.

FT-IR angular profiles for ITO and Au disc arrays. Figures 3 (a) and (b) show the reflectance spectra of ITO and Au disc arrays at θin = 10–50º, respectively. The reflectance peak positions of ITO and Au disc arrays are shifted obviously to lower ω with the increase in θin. To analyze the peak shifting, the color plot of reflectance spectra for ITO and Au disc arrays are displayed as a function of θin in Figs. 3 (c) and (d), respectively. The black dotted lines in Figs. 3 (c) and (d) are the in-plane diffraction conditions calculated from the Rayleigh anomalies with the refractive index of the silica glass (n = 1.24). The Rayleigh anomalies satisfy the condition

where kout and kinc [= sin ] are the wave vectors of the  scattered and incident light; ax and ay are the pitches in the x and y directions; and m1 and m2 are the diffraction orders, respectively44. Figures 3 (c) and (d) clearly show the peak shifting along the diffraction condition. The peak position of the ITO disc array is located at lower ω than that of the Au disc array because of the lower free electron density of ITO than that of Au. The peak positon of ITO and Au disc array disagrees with the diffraction condition at θin = 10°, indicating that the reflectance peak at θin = 10° is dominantly derived from the excitation of LSPR on each disc. With the increase of θin, the peak position of reflectance and the diffraction condition tend to agree as shown in Figs. 3 (a) and (b), indicating that those peak shifts of ITO and Au disc arrays are derived from the coupling of the individual LSPRs via in–plane diffraction leading to the plasmonic–photonic hybrid mode41-44. In addition, the width of the reflectance peak for ITO is broader than that of the Au array because the frequencies of the LSPR and the in-plane diffraction for the ITO disc array are coincident. This overlap in frequency causes the strong interaction of the LSPRs with the in-plane diffraction to facilitate the hybridization of the two modes and leads to increasing width of resonance43. Simulated reflectance spectra. To analyze the experimental spectra of ITO and Au disc arrays, the simulated reflectance spectra at θin = 10–50° were obtained as shown in Figs. 4 (a) and (b), respectively. The simulation reproduces well the peak shifting of experimental spectra with θin, but there are discrepancies between the experiments and simulations for ITO and Au disc arrays. Two origins for discrepancies would be expected as follows. First, in the simulation, we assumed that the refractive index of the silica glass substrate is a constant (n = 1.2450 and k = 0 (non-absorption)), causing the discrepancy from the experiment. Second, the incident light for the simulation model contains only s-polarization while that for the experiment contains p-component as well as scomponent although it is relatively small. Nonetheless, the simulated reflectance reproduces the experimental spectra for ITO and Au arrays qualitatively. This indicates that the excitation of LSPR and in-plane diffraction by s-components is dominant in the experiment. This simulation model can be applied to the analysis of electric field distribution for ITO and Au disc arrays without losing the physics involved in the experiment.

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Fig. 3 Reflectance spectra of (a) ITO and (b) Au disc arrays at θin = 10–50°. The diffraction conditions of the Rayleigh anomaly for the refractive index of a silica glass substrate in this frequency range (n = 1.24) at θin = 10–50° are displayed as vertical dotted lines with same color as each reflectance spectrum. Color plots of reflectance spectra of (c) ITO and (d) Au disc arrays as a function of θin. The black dotted lines are the conditions of the Rayleigh anomaly for the refractive index of a silica glass substrate (n = 1.24) 50.

Electric field distribution. To analyze the angular dependencies of reflectance spectra for ITO and Au disc arrays, the electric field distributions were evaluated at ω = 2000 cm-1 and θin = 10, 20, and 30° as shown in Figs. 4 (c)-(h). At θin = 10°, the electric field is concentrated around the discs of Au and ITO as shown in Figs. 4 (c) and (f). For the ITO disc array at θin = 20° as shown in Fig. 4 (g), the electric field is accumulated not only around the disc but also between the discs, whereas the enhanced electric field is still restricted within a region around the disk for the Au disc array at θin = 20° as shown in Fig. 4 (d) (Fig. S3 highlights the difference of field distribution between Au and ITO disc arrays at θin = 20°). This result indicates that the LSPRs on ITO discs couple more effectively with the in-plane diffraction than the Au discs owing to the suitable matching between the frequencies of LSPRs and the diffraction. For the Au and ITO disc arrays at θin = 30° as shown in Figs. 4 (e) and (h), respectively, when the LSPR is matched with the in-plane diffraction condition at θin = 30°, the electric field is extended in the plane of the arrays owing to the coupling between the LSPRs and the in-plane diffraction, leading to the asymmetric shaping of optical spectra as shown in Figs. 4 (a) and (b)41-44. It should be noted that the carrier density of ITO is far smaller than that of Au, whereas the enhancements of the electric field for the ITO and Au disc arrays are almost the same values due to the presence of LSPR at the frequency of interest.

Fig. 4 Simulated reflectance spectra of (a) ITO and (b) Au disc arrays normalized by those of the silica glass substrate (n = 1.24) at θin = 10–50°. The vertical lines are drawn at ω = 2000 cm-1, corresponding to the wavenumber to simulate the spatial field distribution. Calculated spatial distribution of the normalized light intensity, |E|2/|E0|2, of (c)–(e) Au and (f)–(h) ITO disc arrays in the xy-plane at z =150 nm. The conditions of irradiation are ω = 2000 cm-1 at θin = 10° (c)(f), 20° (d)(g), 30° (e)(h).

SEIRA enhancement factor. Figures 5 (a) and (b) display the FT-IR spectra of the PMMA + 5CB layer on ITO and Au disc arrays and ITO thin film at θin = 10–50° after subtraction of the broad reflectance peak derived from the excitation of LSPRs (the original FT-IR spectra are displayed in Fig. S4). In Figs. 5 (a) and (b), four absorption bands derived from the molecular vibrations are clearly observed at 1490 (ring CC stretch (ν19a)), 1610 (ring CC stretch (ν8a)), 1700 (carbonyl bond: CO stretch), and 2200 (CN stretch) cm-1 15,51. The circles describe the diffraction condition at each θin. The absorption intensities except CN vibration (2200 cm-1) for ITO and Au disc arrays are enhanced compared to the absorption intensities for ITO thin film. Because the reflectance peak positions due to the excitation of LSPRs for ITO and Au are different (ITO: 1700 cm-1, Au: 2100 cm-1), the shape of the absorption bands is symmetric for the ITO disc array but asymmetric for the Au disc array owing to the Fano resonance induced by the interference between the molecular vibration and LSPR38. In addition, the enhancement factors for ITO and Au disc arrays clearly show the characteristic angular dependencies for each molecular vibration as shown in Fig. 5 (c). This clear angular dependence indicates the minor effect of the surface roughness of the PMMA + 5CB layer on the spectra. The film is thin (= 85 nm) compared to the wavelength of interest so that it did not give scattering that disturbed the in–plane diffraction. The enhancement factors were estimated as the signal intensities for the arrays divided by those for the ITO thin film. To clarify the origin of those angular dependencies, the enhancement factors are displayed as a function of the differences in ω of the diffraction condition (ωdiff) deduced from eq. (1) and the molecular vibrations (ωvib) (∆ω = ωvib – ωdiff) in Fig. 5 (d). It can be seen that the enhancement factor increases with the increase from negative ∆ω, hits a local maximum at ∆ω ≈ -160

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ACS Photonics cm-1, and then decreases with a further increase in ∆ω. Among the four different vibrational modes appearing in the range of measurement, the largest enhancement as high as 8 times is attained for the carbonyl bond stretching at ωvib = 1700 cm-1. The comparable enhancement factor for the ITO disc array with that for the Au disc array is consistent with the calculation of FOMs for ITO and Au as shown in Fig. 1 (c). It is worth noting that these enhancement factors for ITO and Au disc arrays at 1700 cm-1 are comparable with a previous report for the array of the ITO bar (750 nm × 80 nm × 80 nm) having 100 nm spacing between each ITO bar47.

Fig. 5 ∆Reflectance spectra, where the broad reflectance peak derived from the excitation of LSPR was subtracted from the spectra as a baseline to clarify the absorptions due to CN, CO, and ring CC stretches (ν8a and ν19a), for (a) ITO and (b) Au disc arrays embedded in the PMMA + 5CB layer. The incident angle was changed as θin = 10–50º. The spectra were shifted vertically for the sake of clarity. ∆Reflectance of the layer on the ITO thin film is displayed on the bottoms of (a) and (b). The circles in (a) and (b) describe the diffraction condition at each θin. The vertical dotted line shows the peak position of LSPR at θin = 10º. The enhancement factors of molecular vibrations as a function of (c) θin and (d) difference in wavenumber of molecular vibrations (ωvib) and the diffraction condition (ωdiff) with refractive index of a silica glass substrate in this frequency range (n = 1.24) (∆ω= ωvib – ωdiff).

We now briefly summarize the contribution of the plasmonic—photonic hybrid mode to realize the SEIRA enhancement. With increasing ∆ω = ωvib – ωdiff from negative by changing the angle of incidence, the near-field of LSPRs on ITO and Au disc arrays are enhanced and distributed into the plane of the array owing to the excitation of the in-plane diffraction, leading to the enhancement of the signal from molecular vibrations45-47. The maximum enhancement factor attained is as high as 8 times at ∆ω ≈ -160 cm-1, which is slightly lower than that for the diffraction condition (∆ω ≈ 0 cm-1). This difference is reproduced well by the simulation using the same model used to derive Figs. 4(a) and (b), as shown in Fig. S5. The difference stems from the difference between the enhancements of the near- and far- fields—i.e., the maximum near-field enhancement occurs at a smaller ω than the enhancement of the far-field40-44. As ∆ω crosses -160 cm-1, the near-field enhancement decreases because the collective oscil-

lation of LSPRs diminishes through in-plane diffraction. With increasing ∆ω from 0 to positive, the near-field enhancement decreases further as shown in Fig. S5. In addition, the incident light is diffracted into the substrate side, leading to the decreased total signal of reflectance as shown in Figs. 3 (c) and (d).

CONCLUSION We have fabricated periodic arrays of ITO and Au microdiscs to elucidate the potential of ITO and the plasmonic— photonic hybrid mode for SEIRA applications. The ITO disc array clearly demonstrates a SEIRA enhancement comparable to the Au disc array. In addition, both the ITO and Au disc arrays clearly show SEIRA enhancement accompanying the plasmonic—photonic hybrid mode in the IR region, which supports SEIRA enhancement even if the frequencies of molecular vibration disagree with those of the LSPRs. This result suggests that periodic disc arrays, which are simpler and easier to fabricate than the nanostructures with nanogaps, are useful for plasmonic applications in the IR region. It is noteworthy to mention that the magnitude of hybrid mode becomes strong by extending 2–dimensional structure into 3–dimensional structure27,28,40, indicating that the SEIRA factor by the hybrid mode can be enhanced by the optimization of metal structure such as shape and height of disc. Our results show that the plasmonic— photonic hybrid mode is a powerful tool to achieve high sensitivity in the SEIRA technique and encourages the use of various emerging materials including transparent conductive oxides in the IR region.

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 ITO thin film, Enlarged SEM images and simulation model of ITO disc, FT-IR spectra of the ITO and Au disc array embedded in the PMMA + 5CB layer, Electric field distribution of ITO and Au disc arrays, and Simulated reflectance and field enhancement around ITO and Au disc arrays, Simulated reflectance and field enhancement around ITO disc arrays for s– and p– polarized components. (PDF).

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

Notes The authors declare no competing financial interest.

Acknowledgments This work was partly supported by the Nanotechnology Hub, Kyoto University and 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 is acknowledged. SM gratefully acknowledges the support from “the construction project for the

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consortium of the fostering of science and technology personnel,” Nanotech Career-up Alliance (Nanotech CUPAL). The authors would like to thank Enago (www.enago.jp) for the English language review.

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