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A sensitive photodetection with photomultiplication effect in an interfacial Eu complex on a mesoporous TiO film 2+/3+

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Ayumi Ishii, Tatsuro Sakai, Riku Takahashi, Shuhei Ogata, Kazuki Kondo, Takahiro Kondo, Daichi Iwasawa, Soichi Mizushima, Koushi Yoshihara, and Miki Hasegawa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18200 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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ACS Applied Materials & Interfaces

A sensitive photodetection with photomultiplication effect in an interfacial Eu2+/3+ complex on a mesoporous TiO2 film Ayumi Ishii,*,†,‡ Tatsuro Sakai,† Riku Takahashi,† Shuhei Ogata,† Kazuki Kondo,† Takahiro Kondo,† Daichi Iwasawa,† Soichi Mizushima,† Koushi Yoshihara,† and Miki Hasegawa*,† †College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan ‡JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan KEYWORDS. Photodetection, Photomultiplication, Interfacial complex, Europium, TiO2

ABSTRACT. A simple device structure composed of an interfacial Eu2+/3+ complex on a mesoporous TiO2 film is developed by a solution process, and acts as the high performance photodetector with photomultiplication phenomena. The electron transfer from the photo-excited organic ligand 2,2’:6’,2’’-terpyridine (terpy) as a photosensitizer to TiO2 is accelerated by the reduction level of Eu3+/2+ ions chemically bonding among terpy and TiO2, resulting in the generation of a large photocurrent. It is worthy of note that its external quantum efficiency is in excess of 105% under the applied reverse bias. The corresponding responsivity of the device is also determined to be 464 A/W at an irradiation light intensity of 0.7 mW/cm2 (365 nm), which

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is more than three orders of magnitude larger than those of inorganic photodetectors. A dark current of the device can be reduced to 10-9 A/cm2 by introducing a Eu oxide thin film layer as a carrier blocking layer at the interface between TCO and the TiO2 layer, and the specific detectivity reaches 5.2 × 1015 Jones at 365 nm with -3 V. The performance of our organicinorganic hybrid photodetector surpasses those of existing ultraviolet photodetectors.

1. INTRODUCTION Photodetectors based on organic semiconductors have attracted more interest because of the good photophysical properties of organic materials such as high absorption coefficients and narrow and adjustable band gaps,1-5 which become key components for advanced optical communication,6,7 high-resolution imaging,8 single photon counting,9 and biochemical sensing.10,11 The performance of such photoelectric conversion systems has advanced rapidly due to improvements in the device structures, photostability, and conversion efficiency.12,13 Recent years have seen significant developments in the application of organic compounds as substitutes for inorganic semiconductors in such systems. For highly sensitive photodetection, photoelectron emission or the avalanche effect must be available in inorganic photodetectors, which needs a sufficiently strong external electric field (~100 V)14,15 and cannot occur in organic semiconductors with a large exciton binding energy.16 Recently, photomultiplication phenomena have been noted to obtain highly sensitive photodetectors composed of organic or organicinorganic hybrid systems.17-19 This phenomenon is explained by hole (electron) trapping in the active layer under light irradiation and large external electron (or hole) injection. Thus, the external quantum efficiency (EQE) of organic photodetectors with photomultiplication can exceed 100%. However, the large charge injection, meaning the relatively smaller shunt


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resistance, will result in a large dark current.20 The large dark current can lower the sensitivity of the photodetector. A highly sensitive photodetector with high gain and low noise is required to increase the shunt resistance and reduce the dark current. In this investigation, we have developed a highly sensitive photodetector based on photomultiplication phenomena accelerated by a nano-ordered coordination interface in a coreshell structured inorganic-organic hybrid structure. The simple device structure is composed of a mesoporous TiO2 film coordinated with europium (Eu) complexes at the interface, which can be fabricated by a low-cost and environmental-friendly solution process. Eu ions have a relatively lower reduction potential (-0.36 V vs. NHE.21) than those of other lanthanide (Ln) ions that adopt the stable trivalent state, and exhibit two redox active states with characteristic emissions: divalent (Eu2+) and trivalent (Eu3+). Eu2+ generates a broad emission band in the blue wavelength region, which is assigned to an electric dipole d-f transition. The emission of Eu3+ is observed in the red spectral region and is assigned to an electric dipole forbidden (Laporte forbidden) transition of the inner-shell 4f orbitals. In a previous study, we found that Eu ions can be reduced upon forming an interfacial complex with phen on SiO2 nanoparticles.22-23 The reduction occurs at the interface between organic ligands (1,10phenanthroline; phen) and inorganic SiO2 nanoparticles through the colloidal suspension process in ethanol. Remarkably, the reduction of the Eu ion only requires mild conditions (reaction in ambient air and at low temperature), resulting in a drastic change in the emission color from red (Eu3+) to blue (Eu2+). This work demonstrates that a specific interface of an organic-inorganic hybrid nanostructure is possible to induce novel photochemical and electrochemical phenomena for a wide range of applications.


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The reduction potential of Eu2+/3+ is located near the conduction band position of main semiconductors such as TiO2, CdS, and ZnO.24 TiO2 is one of the most promising photoanodes in dye-sensitized solar cells25-28 and for solar-hydrogen conversion by water splitting.29-31 Here, we propose a simple device structure composed of interfacial Eu2+/3+ complexes on a mesoporous TiO2 film for a high-performance photodetector with photomultiplication phenomena, in which 2,2’:6’,2’’-terpyridine (terpy) is used as a ligand for the interfacial complex. 2. EXPERIMENTAL SECTION 2.1. Device fabrication Transparent conducting oxide (TCO) glass (10 Ω/sq, Geomatic Co., Ltd.) was cleaned ultrasonically in acetone, isopropanol, and ultra-pure water sequentially for 10 min each, dried by N2 gas, and heated at 110 °C. For fabrication of the mesoporous TiO2 layer, a commercial TiO2 paste (PST-21 NR, JGC Catalysts and Chemicals, Ltd.) diluted in ethanol (TiO2 paste : ethanol = 1 : 2.5 wt%) was spin-coated on TCO glass at 2000 rpm for 30 s, and gradually sintered at a temperature from 150 to 500 oC for 45 min. The TiO2 electrode was immersed in a 1 mM ethanol solution of EuCl3·6H2O (Kanto Chemicals Co., Inc.) at 70 °C for 1 h, and sintered at 200 °C for 1 h. The TiO2/Eu film was immersed in a 1 mM ethanol solution of 2,2’:6’,2”terpyridine (Sigma-Aldrich Co., LLC.) at 70 °C for 1 h, and sintered at 110 °C for 15 min. For the counter electrode, a commercial Ag paste (FA-333, Fujikura Kasei Co., Ltd.) was coated and sintered at 120 °C for 15 min. The Eu oxide thin film layer was introduced at the interface between TCO and the TiO2 mesoporous layer by spin-coating of a 10 mM ethanol solution of EuCl3·6H2O and sintering at 200 °C for 1 h. 2.2. Measurements


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SEM images were obtained on a Zeiss Ultra 55 microscope equipped with a secondary in-lens electron detector, together with a Bruker-Quantax detector for EDS studies. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD equipped with a monochromatic Al-Kα X-ray source (1253.6 eV); the binding energies were calibrated at the Au 4f level (84.0 eV). Synchrotron X-ray diffraction (XRD) patterns were obtained with a large Debye-Scherrer camera installed at the SPring-8 BL02B2 beamline, using an imaging plate as the detector and an incident X-ray wavelength of 0.99885 Å. The film thickness was measured by the stylus surface profiling system (Dektak XT, Ulvac, Inc.). Electronic absorption and luminescence spectra were recorded on a Shimadzu UV-3100 with an absolute specular reflectance attachment and a Horiba Jobin-Ybon Fluorolog 3-22. Fluorescence quantum yields were measured using a C9920-02 Absolute PL Quantum Yield Measurement System (Hamamatsu Photonics K. K.). The ionization potential (UPS) in the air was estimated by a photoemission yield spectrometer (AC-2, Riken Keiki Co., Ltd.). Photocurrent density–voltage (J–V) curves were measured by a computer-controlled digital source meter (Keithley 2400) under irradiation by a Xe lamp (λex = 365 nm, 0.7 mW/cm2). Impedance spectra were obtained on a Hioki IM3590 chemical impedance analyzer. 3. RESULTS AND DISCUSSION 3.1. Interfacial complexation and device structure The process for complexation at the interface and device fabrication is schematically depicted in Figure 1. The cross-sectional scanning electron microscopy (SEM) image of the device in Figure 1 shows the 200 nm thick flat interface of the active layer. In this device, a photoanode was prepared by coating a thin mesoporous TiO2 film on a transparent conducting oxide (TCO)


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surface of a glass substrate. Ag paste was used as a counter (cathodic) electrode. The active area of the device that receives incident light was 2×2 mm2.

Figure 1. Schematic illustration of the device fabrication for an interfacial Eu complex on a mesoporous TiO2 film and cross-sectional SEM image. A mesoporous TiO2 film coated with Eu complexes (TiO2/Eu-terpy) works as a photoanode. Ag paste was used as a counter (cathodic) electrode. The active area of the device that receives incident light was 2×2 mm2.

To coat Eu ions on the photo-anode, the mesoporous TiO2 film was immersed in an ethanol solution of EuCl3 and sintered at 200 °C for 30 min. SEM observation of the Eu-coated TiO2 film (Figure S1) indicates that the average TiO2 particle size of 20 nm does not change, which confirms that the ions formed a nano-ordered thin film layer. The energy dispersive X-ray spectroscopy (EDS) results given in Figure S2 confirm the presence of Si, O, and Eu. The lack of a Cl peak around 2.6 keV indicates that Eu oxides or hydroxides were formed on the TiO2 mesoporous film.


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XPS measurements reveal chemical bonds between Ti and Eu ions. In Figure 2a, TiO2 exhibits Ti 2p3/2 and 2p1/2 bands at 458.8 and 464.6 eV, respectively. After reaction with Eu ions, the Ti 2p XPS bands on the TiO2 surface can be divided into two species by a Gaussian function. New bands are observed at the lower energy side of 458.0 and 463.7 eV with respect to those of pure TiO2. Therefore, the electron density of Ti ions is increased because Ti ions on the surface bond to Eu ions through oxygen atoms. The chemical bonding at the interface between Eu and Ti ions is also confirmed by the Eu 3d XPS band shift to the higher energy side than that for Eu2O3

Figure 2. a) Ti 2p XPS bands of TiO2 film (dotted lines) coated with Eu ions (solid lines). The solid line could be divided into the red and dotted lines by a Gaussian function. b) Eu 3d XPS bands of Eu ions on TiO2 film (solid line) and Eu2O3 (dotted line). c) N 1s XPS bands of terpy (dotted line) coordinated with Eu ion on TiO2 film (solid line). d) Synchrotron XRD patterns of TiO2/Eu-terpy film (solid line) and TiO2 film (gray line) (λ = 0.99885 Å).


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powder samples (Figure 2b), which confirms that Eu ions are bonded to oxygen atoms on the surface of TiO2. The Eu-coated TiO2 film was immersed in an ethanol solution of terpy to form a red-emissive film, as shown in the inset of Figure 1. The N 1s XPS band of pure terpy in Figure 2c is shifted to the higher energy side, which indicates that terpy is coordinated to the Eu ions on the TiO2 film. The band position of the terpy ligand is not significantly shifted by reaction with TiO2 (Figure S3); therefore, the presence of Eu ions on the surface leads to interfacial complexation. Based on synchrotron X-ray diffraction (XRD) measurements (Figure 2d), the TiO2 mesoporous film has an anatase-type structure. The diffraction pattern is essentially unchanged by the addition of Eu ions and terpy ligands at the surface, which indicates the complex forms an amorphous-like thin film layer. 3.2. Luminescence properties of the interfacial Eu complex The hybrid device with the interfacial Eu2+/3+ complexes shows bright luminescence under UV light at 0 V. Figure 3 shows the excitation and luminescence spectra of the TiO2/Eu-terpy film. Excitation at 340 nm under 0 V generates sharp emissions that originate from the f-f transitions of Eu3+ at 579.9, 592.0, 617.2, 651.0, and 698.7 nm, which are assigned to 5D0 → 7F0, 5D0 →7F1, 5

D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions, respectively (Figure 3b). The absolute emission

quantum yield was estimated to be 3%. The excitation spectrum corresponds to the ππ* absorption bands of terpy coordinated with Eu3+ (Figure S4), which indicates that the emission occurs through the energy transfer from terpy to Eu3+ at the interface of the Eu-coated TiO2 film. Under application of a voltage, the red emission of the Eu ion becomes significantly weaker than


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that at 0 V, especially for the reverse bias direction, as shown in Figure S5. This indicates that the excitation energy of the Eu ion is transferred to the TiO2 photoanode under applied voltage. The emission decay curve in Figure 3c shows three luminescent components at 0 V. A luminescence lifetime τ of 1.15 ms is observed (< 10%), which corresponds to that of a powder Eu complex with terpy (Figure S6). The two shorter components (τ = 0.33 and 0.13 ms) may indicate the existence of energy deactivation pathways from the excited state of the Eu ion to TiO2. In addition, application of a negative voltage in the reverse bias direction causes the

Figure 3. Luminescence properties of TiO2/Eu-terpy film. a) Excitation spectrum monitored at 618 nm under 0 V. b) Luminescence spectrum excited at 340 nm under 0 V. c) Luminescence decay curves under 0 V (black line) and -5 V (blue line) monitored at 618 nm (λex = 340 nm).


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electron transfer from Eu to TiO2 to be accelerated. Under application of -2 V, the emission decay becomes relatively short with lifetimes of 0.26, 0.05, and 0.02 ms, which indicates that the excitation photons on Eu ions are changed to electrochemical energy at the TiO2 surface. On the other hand, the decay curve is almost unchanged under application of a positive voltage, although the emission intensity is slightly decreased because the excitation energy from terpy is transferred to the cathodic electrode. In other words, a part of the excitation energy transferred to the Eu ion is not reversed to terpy in the excited state under application of 5 V. 3.3. Photoelectric conversion properties The current density versus voltage (J-V) characteristics of the TiO2/Eu-terpy device were measured under UV light irradiation (λex = 365 nm, 0.7 mW/cm2) and under dark conditions, as shown in Figure 4a. This solid-state device exhibited photoelectric conversion properties. Under a reverse bias voltage, the dark current is between 10-4 and 10-3 A/cm2, which is then significantly increased over 10-1 A/cm2 at -1 V by UV light irradiation. Figure 4b shows the relationship between the incident photons and current; a photocurrent is generated in the absorption wavelength region of the terpy ligand at around 350 nm and not in the absorption range of TiO2 around 300 nm. Thus, photoelectric conversion occurs by the coordination of terpy at the solid state surface of the Eu-coated TiO2 film. From the I-T curve of the device as shown in Figure S7, the photocurrent is saturated within 0.3 s after UV light irradiation. On the other hand, the Eu-free film composed of TiO2 and terpy shows no photoelectric conversion properties (Figure S8), which indicates that the presence of Eu accelerates electron transfer from terpy to TiO2 at the interface. To clarify the electron transfer from Eu ions on the surface of TiO2, a Eu ion-doped TiO2 mesoporous film was prepared and the photoelectric


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conversion properties were evaluated. As shown in Figure S9, the photocurrent is not different from that for the dark current. This result indicates that the presence of Eu ions at the interface between TiO2 and ligand induces electron transfer. It should be noted that electron transfer at the interface can be enhanced only with Eu ions and not with other lanthanide ions such as Tb or Gd ions which are redox inactive in this potential range. Thus, the electron transfer from terpy to TiO2 occurs through the reduction level of the Eu ion.

Figure 4. Photoelectric conversion properties of the TiO2/Eu-terpy device. a) J-V curves under UV light irradiation (red line, λex = 365 nm, 0.7 mW/cm2) and dark conditions (black line). b) Incident photon-to-current conversion spectra. c) EQE vs. voltage curve under the reverse bias direction. 3.4. Photomultiplication properties Interestingly, the EQE values of the TiO2/Eu-terpy device exceed 100% even at -0.1 V. The EQE value is calculated according to eq. 1:32 EQE = Jphhc / Pineλ = (Jlight-Jdark)hc / Pineλ

(1)

where Jph is photocurrent density, Jlight is current density under light irradiation, Jdark is dark current density, and Pin is the incident light power. λ, e, h and c are the incident wavelength, electron charge, Planck constant and speed of light, respectively. The EQE value of the TiO2/Eu-


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terpy device is saturated at a bias of -4 V as shown in Figure 4c, and the peak value is estimated as 1.67 × 105%. The value increases quickly with increasing negative bias above -0.1 V, which is consistent with the rapid increase of photocurrent under the reverse bias direction. The corresponding responsivity, which is an important parameter for photodetectors, describes the ability of a device to respond to optical signals. It can be calculated as the ratio of the photocurrent and the incident light power (Pin) by eq. 2:33 R = Jph / Pin = (Jlight-Jdark) / Pin

(2)

The responsivity of the device has been determined to be 464 A/W at an irradiation light intensity of 0.7 mW/cm2 (365 nm), which is more than three orders of magnitude larger than those of commercial inorganic photodetectors (GaN or SiC, < 0.2 A/W).34 The value of EQE and responsivity of the TiO2/Eu-terpy device are among the highest reported for organic-based photodetectors under low applied bias.16,35,36 The electrochemical impedance measurement has clarified the role of the coordination interface in the device for a low dark current and large photoconductivity as a highly sensitive photodetector. Figure S10 shows the impedance spectra given as the Nyquist plot of the TiO2/Eu-terpy device under UV light irradiation and dark conditions. On the basis of the equivalent circuit, RS corresponds to the series resistance from conductive wire or TCO, while R1 is associated with the resistance from the interface of TiO2/Eu-terpy.37,38 The interface resistance (R1) takes on an extremely high value of 203 kΩ under the dark condition, meaning that the interfacial complex increases the shunt resistance and reduces the dark current. It is noteworthy that the interface resistance of the device is significantly reduced to 434 Ω by UV light irradiation due to the increased photoconductivity. These results confirm that a high performance


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for the photodetector, combining high gain and low noise, is successfully achieved by the interfacial complex layer. A schematic energy diagram for the TiO2/Eu-terpy device is depicted in Figure 5. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the terpy ligand were determined from optical absorption edge and photoemission spectroscopy measurements (Figure S11). The LUMO level of the terpy ligand prevents electron injection from the Ag electrode under the dark condition because of the very large electron injection barrier (> 2 eV), resulting in the low dark current of the device. The terpy ligands are excited by UV light irradiation, and their electrons transfer to the reduction level of the Eu ion under the applied reverse bias while the holes remain trapped at the interfacial complex of Eu-terpy. The trapped level arises from the valence band of the Eu oxide on the surface of TiO2 located at -5.5 eV as shown in Figure S10 of the photoemission spectrum.39 The trapped holes are accumulated

Figure 5. Schematic energy diagram of the TiO2/Eu-terpy device. a) The energy levels under 0 V. b) The applied reverse bias in dark conditions, and c) under UV light irradiation (left: hole trapping, right: electron injection).


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at the interface between the Eu-terpy and Ag electrode, which results in build-up of a high electric field at the interface. Finally, a large external tunnelling injection of electrons occurs from the Ag electrode, and the EQE of the device exceeds 105% even under low applied bias. The photomultiplication phenomenon is surely induced by the specific interface composed of the Eu-terpy complex thin film layer. 3.5. Specific detectivity for a photodetector To further reduce the dark current, which dominates the noise-current of the photodetector, a carrier blocking layer was introduced at the interface between TCO and the TiO2 mesoporous layer of the TiO2/Eu-terpy device. For the purpose of carrier blocking from the electrode, a Eu oxide thin film layer was spin-coated on the TCO glass substrate (Figure S12). The thickness of the Eu oxide layer is less than 10 nm and the initial oxidation state is +3, as determined from SEM observations and the Eu 3d XPS band of the Eu oxide layer shown in Figures S13 and S14. The J-V curve of the device with the Eu oxide thin film layer is shown in Figure 6. The dark current is significantly decreased to 10-9 A/cm2 under a reverse bias, while the photocurrent is maintained, as compared with the initial state. This indicates that the trivalent Eu oxide layer acts as a charge carrier blocking layer under dark conditions and does not interfere with the light current generation. A dark current as low as 10-9 A/cm2 provides a very low shot noise. To evaluate the signal-tonoise performance of the photodetector, wherein the shot noise from the dark current is the major contribution, the specific detectivity (D*) can be calculated by eq. 3:40 D* = R / (2eJdark)1/2

(3)


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The D* of the TiO2/Eu-terpy device with the Eu2O3 layer for carrier blocking is 5.2 × 1015 Jones (1 Jones = 1 cm Hz1/2/W) for illumination at 365 nm with a bias at -3 V. This specific detectivity of the device is three orders of magnitude larger than those of inorganic ultraviolet photodetectors like silicon and GaN.41 Thus, our inorganic-organic hybrid photodetector with the interfacial Eu complex, which shows the highest level of performance of all existing ultraviolet photodetectors, has great potential not only for replacing inorganic photodetectors but also for opening a new field in photosensing applications.

0

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-1 0 1 2 Voltage (V)

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Figure 6. J-V curves under UV light irradiation (red line, λex = 365 nm, 0.7 mW/cm2) and dark conditions (black line) of the TiO2/Eu-terpy device with the Eu oxide thin film layer.

4. CONCLUSION In conclusion, a highly sensitive photodetector based on photomultiplication phenomena has been developed with an interfacial Eu complex on a mesoporous TiO2 film. The reduction level of Eu ions contributes electron transfer from terpy to TiO2 at the interface, which results in the generation of a large photocurrent with the disappearance of the red emission from Eu3+. The


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EQE values of the TiO2/Eu-terpy device exceeded 100% even at -0.1 V, and the peak value was estimated as 1.67 × 105% at -4 V. The corresponding responsivity of the device was determined to be 464 A/W at an irradiation light intensity of 0.7 mW/cm2 (365 nm), which is more than three orders of magnitude larger than those of commercial inorganic photodetectors. The dark current was also reduced by introducing a Eu oxide thin film layer as a carrier blocking layer at the interface between TCO and the TiO2 mesoporous layer of the TiO2/Eu-terpy device. The dark current was significantly decreased to 10-9 A/cm2 under a reverse bias, while the photocurrent was maintained. The specific detectivity of the TiO2/Eu-terpy device with the Eu oxide was 5.2 × 1015 Jones for illumination at 365 nm with a bias at -3 V. This is three orders of magnitude larger than those of inorganic semiconductor-based ultraviolet photodetectors. The performance of our organic-inorganic hybrid photodetector, fabricated by an environmental-friendly solution process, shows great potential for surpassing existing ultraviolet photodetectors. Further research is now underway to stabilize the visible light sensitivity and optimize the device configuration.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. SEM images, EDS pattern, absorption and luminescense spectra, luminescence decay curve, IV curves, I-T curves, impedance spectra, UPS, and XPS (PDF)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A.I.) *E-mail: [email protected] (M.H.) Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Dr. Kunihisa Sugimoto and Dr. Shogo Kawaguchi of the Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8 for conducting the quality synchrotron XRPD measurements. Synchrotron radiation experiments were performed at the BL02B2 beamline at SPring-8 with the approval of JASRI (Proposal Nos. 2015A1552, 2015A1862, 2015B1353, 2016A1333, 2016A1336, 2016B1342, 2016B1706, 2017A1648, and 2017A1380). This work was supported by JSPS KAKENHI Grant Number JP14453489 (AI, Grant-in-Aid for Young Scientists B), JST PRESTO Grant Number JP17941016 (AI), the Iketani Science and Technology Foundation (AI), the Sumitomo Foundation (AI), the Izumi Science Foundation (MH), Aoyama Gakuin Soken Project (MH), Grant-in-Aid for Scientific Research on Innovative Areas “Soft Crystals: Area Number 2903” (No. 17H06374, MH), and the Supported Program for


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the Strategic Research Foundation at Private Universities (MEXT, MH) via a matching fund subsidy.

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