scattering-based hole burning in y 3 al 5 o 12 :ce 3+ monoliths with hierarchical porous structures...

Published: August 02, 2011

r 2011 American Chemical Society 17676 dx.doi.org/10.1021/jp204594c | J. Phys. Chem. C 2011, 115, 17676–17681

ARTICLE

pubs.acs.org/JPCC

Scattering-Based Hole Burning in Y3Al5O12:Ce3+ Monoliths with

Hierarchical Porous Structures Prepared via the Sol�Gel RouteShunsuke Murai,† Koji Fujita,*,†,‡ Koji Iwata,† and Katsuhisa Tanaka†

†Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan‡PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

’ INTRODUCTION

Morphology in an optical length scale affects optical proper-ties of materials because light strongly interacts with structures ofsizes comparable to its wavelength. In particular, complexmaterials with an inhomogeneous morphology in optical lengthscale are referred to as randommedia, in which lightwaves undergomultiple scattering and interference to bring about unconventionaloptical functions into the material.1 For example, the combina-tion of light scattering and optical gain yields random lasers,2�5

where optical feedback is given by randomly distributed scat-terers instead of predefined cavities. On the other hand, thecombination of light scattering with photoreaction causes ascattering-based hole burning effect6 because the interferencepattern of multiple-scattered light varies sensitively with thewavelength and incident angle of the light beam and can bememorized inside the medium via photoreactions. This memoryeffect is applicable to high density optical storage.

As a randommedium that strongly interacts with light, porousmaterials are promising since scattering strength can be tuned viaporosity and pore size of the media. The tunability in scatteringstrength is very important since light transport in a randommedium drastically varies with the scattering strength, and itinevitably affects the functionality of random media from anapplication point of view. Properties of random lasers, such aslasing threshold and emission directionality, critically depend onthe scattering strength. In the case of scattering-based holeburning, the density of memory, i.e., the number of data thatcan be memorized in unit volume, is dominated by the scattering

strength. Many macroporous materials including titanium diox-ide (TiO2),

7�11 silicon dioxide (SiO2),12 and gallium phosphide

(GaP),13,14 have been prepared and utilized as random media.These studies experimentally clarified that the scattering strengthof the medium can be controlled from very weak to extremelystrong through the fine-tuning of morphology in the opticallength scale. As a result, a diversity of light transport has beenrealized, from ballistic transport to near-localization.

Among several pore�forming techniques, the sol�gel meth-od accompanied by phase separation is a versatile way offabricating porous materials from liquid phase.15 One distinctfeature of this technique is that elaborate control over thesynthesis conditions allows fabrication of hierarchically porousstructures; macropores are formed as a result of phase separation,while interstices between the grains constituting the gel skeletonsbecome nanoscale pores. The drying process is especially im-portant for the formation of nanoscale structures; while conven-tional evaporative drying of solvent generates the capillary forcethat collapses the nanostructures, supercritical drying preservesthe nanoporous structure by minimizing the capillary force.16

Thus, controlling the drying process enables us to prepareporous materials having similar macrostructures but differentnanostructures from an identical set of starting reagents.Although nanopores have not been actively utilized to tune the

Received: May 17, 2011Revised: July 29, 2011

ABSTRACT: We have synthesized hierarchically porousY3Al5O12 (YAG) ceramics doped with trivalent cerium (Ce3+)ions (YAG:Ce) by the sol�gel process and examined theiroptical properties including scattering strength. Starting fromthe ionic precursors, i.e., AlCl3 3 6H2O, YCl3 3 6H2O, CeCl3 3 7H2O, and poly(ethylene oxide) (PEO) dissolved in a mixture ofwater and ethanol, monolithic wet gels are synthesized usingpropylene oxide as a geletion initiator. PEO induces phaseseparation parallel to the gelation in the sol�gel system to formbicontinuous morphologies consisting of gel- and fluid-phases.The wet gels thus obtained are subjected to either evaporative or supercritical drying, and then heat-treated to crystallize the gelskeletons to form YAG phase. The drying process critically affects the nanostructures of YAG skeletons after heat treatment; thesupercritical drying results in skeletons consisting of fine grains (<50 nm), while evaporative drying yields skeletons with largergrains. The scattering strength of the heat-treated samples is evaluated by using coherent backscattering measurements andscattering-based hole burning, both of which clarify that the samples prepared via evaporative drying scatter visible light much morestrongly than that prepared via supercritical drying.

17677 dx.doi.org/10.1021/jp204594c |J. Phys. Chem. C 2011, 115, 17676–17681

The Journal of Physical Chemistry C ARTICLE

scattering strength thus far, tailoring nanostructure would giveadditional degree of freedom in controlling the scatteringstrength of macroporous materials.

In the present study, we have demonstrated the control ofscattering strength by tuning nanoscale structures of macro-porous yittrium aluminum garnet doped with cerium ions(Y3Al5O12:Ce

3+, YAG:Ce) fabricated via the sol�gel route. Byadopting either the supercritical- or evaporative-drying process,we can prepare YAG:Ce ceramics with similar macroporousstructures but different nanostructures. The scattering propertiesof the samples in the visible range have been studied by usingcoherent backscattering (CBS) measurements. Our materialsexhibit photobleaching of Ce3+ in response to the irradiationwith intense blue laser light, and we utilize the reaction toperform the scattering-based hole burning. Both results of CBSand scattering-based hole burning effect clarify that the scatteringstrength of the evaporation-derived sample is higher than that ofthe supercritically derived one, although the macroporous struc-tures in optical length scale are similar to each other. The resultsare discussed in terms of subwavelength structures of the samplesthat vary largely depending on the drying process.

’EXPERIMENTAL SECTION

Sample Preparation. Gel samples were prepared by themetal�salt-derived sol�gel process as reported previously.17�19

YCl3 3 6H2O, AlCl3 3 6H2O, and CeCl3 3 7H2O were utilized assources of yittrium, aluminum, and cerium, respectively, and amixture of distilled water and ethanol was used as a solvent.Propylene oxide (PO) was added to initiate the condensationreaction, and poly(ethylene oxide) (PEO) having viscosity-averaged molecular weight of 1 000 000 was used as a phase-separation inducer. Nominal composition of the gel was(Y0.995Ce0.005)3Al5O12, i.e., 0.5 mol % of Y3+ was replaced byCe3+. Undoped gel samples having a nominal composition ofY3Al5O12 were also prepared for the CBS measurement. Thedetails of the gel preparation are as follows: First, AlCl3 3 6H2O(7.90 g), YCl3 3 6H2O (5.92 g), CeCl3 3 7H2O (0.037 g), and PEO(0.15 g) were dissolved in amixture of water (20.0 g) and ethanol(15.0 mL). PO (9.10 mL) was then added to the transparentsolution under ambient conditions (25 �C). After stirring for1 min, the resultant homogeneous solution was transferred into aglass tube. The tube was sealed and kept at 40 �C for gelation.After gelation, the obtained wet gels were aged for 24 h at 40 �C,and then processed either by supercritical drying or evaporativedrying. For supercritical drying, the wet gels were subjected tosolvent exchange with 2-propanol. Supercritical drying wascarried out in a custom-built autoclave (Mitsubishi MaterialTechno Co., Japan) using supercritical carbon dioxide (CO2)at 80 �C and 14.0 MPa to form aerogels. On the other hand,evaporative drying was performed at 60 �C after aging to formxerogels. The aero- and xerogels thus obtained were heat-treatedat various temperatures for 10 h in air. Hereafter, the areogels(xerogels) heat-treated at a temperature T is referred to asAerogel-T (Xerogel-T), e.g., an aerogel heat-treated at 1000 �Cis called Aerogel-1000.Structural Characterization. A field-emission scanning elec-

tron microscope (FE-SEM; JSM-6700F, JEOL, Japan) was used toobserve the macroscopic morphology of the samples. X-ray diffrac-tion (XRD) analysis with Cu KR radiation (RINT2500, Rigaku,Japan) was performed to identify the crystalline phases. XRDmeasurements were carried out for the powder samples prepared

by grinding the macroporous monoliths. Porous structures in thenanoscale were investigated by nitrogen(N2) adsorption�desorp-tion apparatus (ASAP 2010, Micromeritics, USA). The pore sizedistribution was calculated from the adsorption branch of theisotherm by the Barrett�Joyner�Halenda (BJH) method.Optical Measurements. To evaluate the scattering strength of

the sample, CBS was measured by the off-centered rotationtechnique using a 496 nm line from an Ar+ laser as a lightsource.20 The measurements were carried out using the undopedmacroporous YAG in order to avoid the optical absorption andemission due to Ce3+. For the Ce3+-doped YAG samples, fluores-cence and excitation spectra were measured at room temperaturewith a spectrophotometer (Hitachi 850, Japan) using a xenon lampas a light source. Photobleaching ofmacroporous YAG:Ce sampleswas explored at room temperature by using a continuous-wave(cw) He�Cd laser (Kinmon, Japan, wavelength =442 nm, outputpower =10 mW) as an excitation light source. During the irradia-tion with the laser light, the yellow fluorescence due to the 5d�4ftransition of Ce3+ was detected using a photomultiplier tubecombined with glass filters so that the fluorescence intensity wasrecorded as a function of irradiation time.Scattering-based hole burning was measured by using a 442 nm

light from a cw He�Cd laser as both writing and reading beams.For the schematics of the experimental procedure, see ref 6. First,the sample was placed on a rotatable stage and irradiated with awriting beam (1 mW) for about 30 s to induce photobleaching ofCe3+ within an interference pattern. In a reading process, the laserbeam was attenuated by a factor of 200 (5 μW) and used as areading beam to probe the hole without causing additionalphotoreactions. The spot that was irradiated with the writing beamprior to reading was excited by a reading beam, and the fluores-cence intensity was plotted as a function of the difference inincident angle between writing and reading beams (Δangle).

’RESULTS

Figure 1 shows the XRDpatterns for Aerogel-800, -900, -1000,and -1100 as well as Xerogel-1000. For Aerogel-800, a halopattern ascribed to the amorphous phase is recognized. Bycontrast, diffraction lines ascribed to the YAG crystalline phaseappear in the XRD patterns of Aerogel-900, -1000, and -1100.With an increment of the heat treatment temperature, the peakintensity increases and the line width becomes narrow, indicatingthe growth of YAG crystals in the skeleton. The crystallite sizes

Figure 1. XRD patterns of Aerogel-800, -900, -1000, and -1100, andXerogel-1000.



are estimated to be about 17 (Aerogel-900), 23 (Aerogel-1000),and 24 nm (Aerogel-1100) by the Scherrer equation using thewidth of a 420 diffraction line (2θ = 33.2�). It is found that theXRD patterns of Aerogel-1000 and Xerogel-1000 are very similarto each other; all peaks in both patterns are ascribed to the YAGcrystalline phase, and the crystallite sizes are estimated to beabout 23 nm.

Figure 2 depicts the FE-SEM images of Xerogel-1000 (a) andAerogel-1000 (b). Even after the heat treatment, the intercon-nected macropores and skeletons can be well recognized in asubmicrometer length scale (see the left-side images). Thestructures in optical length scale are almost identical to eachother. However, a close look at the surface of skeletons reveals adifference in morphology. The images on the middle and right-side of Figure 2, corresponding to magnifications of the left-sideimages, reveal that although the skeletons consist of fine grainsfor both samples, the grain size of Xerogel-1000 is larger than thatof Aerogel-1000.

The difference in the nanoporous structure is quantitativelycharacterized by using N2 adsorption�desorption measure-ments. Figure 3 shows the nanopore size distributions of Aero-gel-1000 and Xerogel-1000. The volume of nanosized pores inboth samples is large, as expected from the FE-SEM observa-tions. For Xerogel-1000, the median pore size lies around 60 nmand cumulative pore volume V = 0.12 cm3/g. On the otherhand, Aerogel-1000 possesses the median pore size of 90 nm andV of 0.18 cm3/g. The porosity P of the skeleton was calculatedby using the relation P = V/(V + 1/d), where d is the densityof the skeletons and set to the value of bulk YAG crystal (d =4.55 g/cm3). We obtained P = 0.42 and 0.51 for Xerogel-1000and Aerogel-1000, respectively.

Figure 4 shows the CBS cones of Xerogel-1000 and Aerogel-1000 derived from undoped gels. The line width of the cone forXerogel-1000 (denoted by solid circles) is obviously larger thanthat for Aerogel-1000 (open circles), indicating that the formerexhibits stronger scattering. We fit the CBS data with a theore-tical curve based on diffusion theory21 to estimate the transportmean free path l, the length after which the propagation directionof the light is randomized. In the fitting process, the reflectivity ofthe surface was calculated using the Maxwell-Garnett effective

medium theory in order to correct for the overestimation of l dueto internal reflection. After the correction, the l values wereestimated to be 1.8 and 5.5 μm for Xerogel-1000 and Aerogel-1000, respectively.

Figure 2. FE-SEM images of Xerogel-1000 (a) and Aerogel-1000 (b). Images on the middle and right-side columns are the magnifications of theleft-side images. Scale bars: 1 μm, 100 nm, and 100 nm (from left to right).

Figure 3. Pore size distribution curves calculated by the BJH methodusing the adsorption branch of the N2 adsorption�desorption iso-therms. Solid and open circles denote the data for Xerogel-1000 andAerogel-1000, respectively.

Figure 4. CBS cones of undoped Xerogel-1000 (denoted by solidcircles) and undoped Aerogel-1000 (open circles). Solid curves repre-sent the theoretical fits based on the light diffusion theory.



As long as the YAG phase is precipitated by heat treatment atelevated temperatures, both the xerogel- and aerogel-derivedsamples exhibit fluorescence upon irradiation with UV light. Theinset of Figure 5 shows the typical excitation and fluorescencespectra for Aerogel-1000. Excitation spectrum exhibits two bandsat 340 and 460 nm, which are assigned to the electron transitionsfrom the 4f1 ground state to the lower two energy levels of 5d statesof Ce3+ in the YAG lattice. The emission spectrum consists of abroad band with a maximum at 530 nm and a broad shoulder at575 nm, due to the transitions from the lowest 5d level to the 4f1

states of Ce3+ (2F5/2 and2F7/2).

22 The spectra of the heat-treatedxerogels are similar to those displayed here, as reported in aprevious paper.19 We found that the fluorescence intensity of theYAG:Ce sample decreases with the elapse of time when theexcitation intensity is high enough.23 Figure 5 shows the depen-dence of fluorescence intensity on time of laser irradiation(10 mW, 442 nm) for Xerogel-1000 and Aerogel-1000. Theintensity monotonically decreases to 43 (Xerogel-1000) and 24%(Aerogel-1000) of the initial intensity after the irradiation for 3000 s.

Figure 6 depicts the typical hole profiles registered in the angledomain for Xerogel-1000 and Aerogel-1000. The decrease in thefluorescence intensity is clearly observed when the incident angles ofwriting and reading beams are identical, i.e., Δangle =0 mrad. Thehole is wider for Xerogel-1000 than for Aerogel-1000. The holeprofiles obtained are analyzed on the basis of the intensity correlationbetween the writing and reading beams inside themedium using thediffusion theory.24 In the fitting process, we adopted l obtained bytheCBSmeasurements, i.e., l = 1.8 and 5.5μm for Xerogel-1000 andAerogel-1000, respectively. A single fitting parameter used was theabsorption length la, the distance over which the amplitude of thelight wave decays by a factor e�1 due to absorption. The results offitting by using la = 1500 μm are superimposed on the experimentaldata in Figure 6. The experimental hole profiles can be reproducedwell by the theoretical curves.

’DISCUSSION

Formation of YAG:Ce Having Hierarchical Porous Struc-tures.Preparation of bulk gels in sol�gel systems containingmetalsalts and epoxides such as PO was first reported by Gash et al.25

The epoxide acts as an irreversible proton scavengers throughprotonation of the oxygen in epoxide and subsequent ring-openingreactions. As a result, a moderate and uniform increase in solutionpH allows the homogeneous hydrolysis and condensation reac-tions to produce monolithic gels in various sol�gel systems.26,27

Tokudome et al. combined this method with phase separation andsucceeded in synthesizing macroporous alumina (Al2O3) by theaddition of PEO into the starting system.18 The metal salt-derivedsol�gel process accompanied by phase separation is furtherextended to multicomponent oxide systems such as YAG17 andcalcium hydrogenphosphate (CaHPO4).

28

The drying process has significant impacts on the nanostruc-ture of the gel. The wet gel contains fine structures that canbecome nanosized pores after drying. During the evaporation ofsolvents, such fine structures are subjected to considerableshrinkage and deformation by the capillary force. Since themagnitude of capillary force exerted on the pore is inverselyproportional to the radius of the pore, the nanostructures cannotbear the large force and mostly collapse during the evaporativedrying. On the other hand, supercritical drying allows wet gels toretain the nanoporous structure even after the drying process,because the process is virtually free from capillary force; thesupercritical fluid CO2 sublimes directly to the gas phase withouttransforming into liquid state. As a result, the nanoporousstructure survives the drying process. Thus, the as-dried nano-structure is quite different between xero- and areogels, asdemonstrated previously for hierarchically porous Al2O3 gels.

29

In the present study, the nanostructure formed in the dryingstages affects the final structures after the heat treatment. Duringthe heat-treatment process, the cations constituting the gelskeleton migrate to form the YAG crystalline phase. Nanoporespresent in the gel skeleton limit the migration paths of ions andsuppress the growth of YAG crystal, which eventually results in alarger volume of interstices in the skeleton of the heat-treatedaerogel. The presence of nanopores also alters the temperature atwhich the YAG crystals start to precipitate. As shown by the XRDpatterns in Figure 1, the crystalline phase appears after the heattreatment of aerogels at and above 900 �C. The temperature ofcrystallization is a little higher than that for the xerogels, wherethe YAG phase precipitates at 800 �C.19Scattering Properties of Macroporous YAG. The CBS of

light is a universal phenomenon that refers to an increase in theintensity of light reflected from a random medium at exactly the

Figure 5. Time evolution of the fluorescence intensity for the Xerogel-1000 (denoted by solid circles) and Aerogel-1000 (open circles). Thefluorescence intensity under irradiation with a 442 nm laser light(10 mW) was collected with a photomultiplier tube combined with glassfilters. The inset shows excitation (dashed curve) and fluorescence (solidcurve) spectra of the Aerogel-1000. The excitation spectrum wasmonitored at 530 nm, and the fluorescence spectrum was measuredupon excitation at 442 nm.

Figure 6. Dependence of emission intensity on incident angle ofreading beam for Aerogel-1000 (upper) and Xerogel-1000 (lower).The holes were burned at 0 mrad under irradiation with a writing laserbeam (wavelength = 442 nm). Bold solid curves represent theoretical fitsbased on diffusion approximation. Inset shows a schematic illustration ofthe measurement.



backscattering direction.30�32 The enhancement of backscatter-ing intensity originates from constructive interference betweentime-reversed optical paths and is observed as a cone in the plotof backscattered intensity versus scattering angle. The width ofthe cone is inversely proportional to l; a wide (narrow) conemeans a short (long) l or a strong (weak) scattering. The CBSresults in Figure 4 show the notable difference in scatteringstrength between Xerogel-1000 and Aerogel-1000, although themorphology on the optical length scale is similar to each other asrevealed by FE-SEM observation (Figure 2). This indicates thatlight recognizes the subwavelength structure of the skeletons, i.e.,the fine grains that constitute the skeletons. The scattering byparticles far smaller in size than the wavelength of incident light isdenoted as Rayleigh scattering, in which the scattering strength isproportional to the sixth power of the particle size.33 Accordingly,larger grains in the skeletons of Xerogel-1000 scatter visible lightmuch stronger than the grain of Aerogel-1000. In addition, asrevealed by the N2 adsorption measurements (the porosity of theskeleton P = 0.51 and 0.42 for the aero- and xerogels, respectively),the skeleton of Aerogel-1000 possesses the larger volume ofinterstices between grains, which lowers the refractive index contrastbetween the macropores and skeleton to reduces the scatteringstrength. Due to the structural differences in the grain size and apacking density of grains, the distinctive variation in scatteringstrength is observed between Xerogel-1000 and Aerogel-1000.Photobleaching and Scattering-Based Hole Burning in

Macroporous YAG:Ce.The occurrence of photobleaching in theYAG:Ce crystal, as shown in Figure 5, suggests the presence ofsome trapping sites; 4f electrons of Ce3+ ions are captured by thosesites either with direct excitation or via excitation into somedelocalized states such as the conduction band of YAG. Possibletrapping sites are lattice defects or imperfections such as oxygenvacancy and Ce4+ that can accommodate electrons. Although rarelyreported for bulk materials, photobleaching was observed in severalYAG:Ce nanocrystals prepared by liquid phase routes.34,35 The factthat photobleaching is observed only in nanosized YAG:Ce suggeststhat some defects located on the surface are plausibly the stage of theelectron trapping. Namely, because the skeletons in our materialsconsist of fine grains having large surface to volume ratio, manysurface defects, which play a role as an electron trap center, areexpected to be present. A difference in the decrease rates of thefluorescence intensity (Figure 5) may reflect the variation in grainsizes between Xerogel-1000 and Aerogel-1000.The formation of holes by the combination of multiple scatter-

ing and photoreaction was first reported by Kurita et al. using zincsulfide (ZnS) nanoparticles doped with Sm2+.6 In that case, a ZnSnanoparticle acts as a scatterer, and photobleaching of Sm2+ isutilized as the photoreaction to memorize interference patterns.Afterward, many combinations of scattering materials and photo-reactions were employed as the stage of scattering-based holeburning, including Sm2+ with ground glasses36 or with macropor-ous aluminosilicate glasses,37 ZnSe0.15S0.85 with ground glasses,24

spiropyran with TiO2 nanoparticles,38 and fulgide with polystylene

powers.39 In the present study, we have utilized the photoreactionof Ce3+ in the macroporous YAG for the observation of scattering-based hole burning.The appearance of a hole in Figure 6 means that interference

pattern created by the incidence of 442 nm light is memorized inthe medium by the photobleaching of Ce3+. When Δangle =0mrad, where the incident angles of writing and reading beams areidentical to each other, the reading beam makes an interferencepattern that is exactly the same as that created by the writing

beam. Ce3+ ions within the interference pattern have alreadybeen photobleached by the irradiation with the intense writingbeam, so that the dip appears in the fluorescence intensity profileand it is recognized as a hole. The difference in hole widthbetween two samples in Figure 6 reflects the difference inscattering strength. The success of theoretical fitting using ldeduced from the CBS measurement, with la being the only onefitting parameters, confirms that the scattering strength variesdrastically, depending on the subwavelength nanostructures. It isalso noted that la = 1500 μm corresponds to the absorptioncoefficient of 6.7 cm�1, which is smaller than the reportedvalues40,41 probably because of the photobleaching.By irradiating the sample with writing beams at different

incident angles, multiple holes can be burned within the samespot, as seen from Figure 7. The formation of multiple holesmeans that at each incident angle, the interference patternspecific to the angle is created and memorized separately bythe photoreactions. From the point of view of application,multiple hole production is favorable for high-density opticalstorage because multiple sources of information can be registeredin a single volume.

’CONCLUSION

In summary, we have prepared hierarchically porous YAG:Cemonoliths via sol�gel method accompanied by phase separationand compared the structural and photochemical properties of thesamples derived via two different drying processes. The dryingprocess plays a crucial role on determining the nanotexture of thegel skeleton, which affects the final morphology of YAG:Ceceramics obtained after heat treatment. FE-SEM observationsand N2 adsorption�desorption measurements show that theskeleton of heat-treated xerogels becomes denser and consists oflarger grains compared to those of heat-treated aerogels. The CBSmeasurements and scattering-based hole burning effects clarify thestronger scattering in the heat-treated xerogels than in the heat-treated aerogels, although the morphologies in the optical lengthscale are similar to each other. The experimental results manifestthat the nanostructure of the skeletons crucially affects the scatteringstrength, i.e., tailoring the nanostructure gives a degree of freedom incontrolling the scattering strength of the porous material.

’AUTHOR INFORMATION

Corresponding Author*TEL: +81 (0)75 383 2432. FAX: +81 (0)75 383 2420. E-mail:[email protected].

Figure 7. Demonstration of the burning of three holes in a singlevolume. The holes were burned in the domain of incident beam angles at�50, 0, 50mrad using Aerogel-1000. Arrows indicate the incident anglesof writing beams.



’ACKNOWLEDGMENT

The authors are grateful to K. Kanamori and K. Nakanishi forthe sample preparation using supercritical drying and measure-ments of N2 adsorption�desorption profiles. This study wassupported by a Grant-in-Aid for Young Scientist (B, Nos.22750188 and 22760512) from the Ministry of Education,Culture, Sports, Science, and Technology (MEXT), Japan.

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