研究者詳細 - 磯部　徹彦

論文 - 磯部　徹彦

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Effects of Halide Composition on the Self-Recovery of Photodegraded Cesium Lead Halide Perovskite Nanocrystals: Implications for Photoluminescence Applications

Miyashita K., Kidokoro K., Iso Y., Isobe T.

ACS Applied Nano Materials （ACS Applied Nano Materials） 4 （ 11 ） 12600 - 12608 2021年

研究論文（学術雑誌）, 共著, 査読有り

Photodegraded CsPbBr3 perovskite nanocrystals (NCs) can exhibit complete self-recovery following storage in the dark. This behavior results from the readsorption of surface ligands that experienced photoinduced desorption under UV light excitation. In the present work, mixed-halide CsPb(Cl0.4Br0.6)3, CsPb(Cl0.2Br0.8)3, and CsPb(Br0.7I0.3)3 NCs were prepared via ion exchange to evaluate the influence of halide composition on the photodegradation and self-recovery of NCs. Partially substituting Cl- for Br- enhanced the photoactivation, which improved the photoluminescence properties by optimizing the adsorption state of the surface ligands during light excitation. In contrast, partially substituting I- for Br- reduced the stability of the cubic crystal structure, thus subjecting the NCs to irreversible degradation (likely including a partial phase transition) and preventing their self-recovery. Cycle tests confirmed the reversibility of the photodegradation/self-recovery of CsPbBr3 NCs; however, this behavior was disturbed by irreversible surface oxidation reactions when the sample was exposed to ambient air. Therefore, it was concluded that ambient air must be excluded to effectively take advantage of the self-recovery phenomenon for practical applications.

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Feasibility of Emission-Enhanced CsPbCl<inf>3</inf>Quantum Dots Co-Doped with Mn2+and Er3+as Luminescent Downshifting Layers in Crystalline Silicon Solar Modules

Song P., Hase S., Zhao S., Xu Z., Iso Y., Isobe T.

ACS Applied Nano Materials （ACS Applied Nano Materials） 5 （ 2 ） 2522 - 2531 2021年

研究論文（学術雑誌）, 共著, 査読有り

In order to enhance the photoelectric conversion efficiencies of crystalline silicon (c-Si) solar cells, CsPbCl3 quantum dots (QDs) codoped with Mn2+ and Er3+ (CsPbCl3:Mn2+, Er3+ QDs) were mixed with ethylene-(vinyl acetate) (EVA) to form a film which was used as a luminescent down-shifting (LDS) layer. The LDS layer effectively improved the low utilization of near-ultraviolet light of c-Si solar cells. These CsPbCl3:Mn2+,Er3+ QDs were synthesized via a conventional high-temperature injection method. Mn2+ is the luminescence center, and the incorporation of Er3+ greatly enhances the luminescence intensity of Mn2+. The absolute photoluminescence quantum yield of the QDs dispersed in toluene reached 79.5% when the QDs were synthesized under the optimum conditions, that is, an injection temperature of 180 °C and Pb:Mn:Er preparation molar ratios of 6:4:4. The EVA film embedded with QDs at the optimum concentration (0.9 wt %) was used as an LDS layer for c-Si solar module. The short-circuit current (ISC) and the photoelectric conversion efficiency (η) were increased by 3.42% and 4.02%, respectively, owing to the LDS layer. Moreover, a luminescent solar concentrator (LSC) which was another application of luminescent materials was also demonstrated. For LSC, the relative changes in ISC and η by using the QDs-dispersed EVA film were +14.9% and +18.0%, respectively. These results indicate a feasible application of luminescent downshifting films in solar modules.

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1. Particle characteristics and measurement 1.12 size effect of particle properties 1.12.1 nanophosphors: Fluorescent semiconductor quantum dots

Iso Y., Isobe T.

Journal of the Society of Powder Technology, Japan （Journal of the Society of Powder Technology, Japan） 57 （ 12 ） 641 - 6465 2020年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 03866157

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Photoluminescence properties of L-cysteine-derived carbon dots prepared in non-aqueous and aqueous solvents

Yoshinaga T., Akiu M., Iso Y., Isobe T.

Journal of Luminescence （Journal of Luminescence） 224 2020年08月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00222313

© 2020 Elsevier B.V. Fluorescent carbon dots (CDs) can be produced from amino acids such as L-cysteine by an intermolecular dehydration reaction between NH2 and COOH groups, followed by carbonization. This dehydration reaction may be hindered in aqueous media because the amino acid molecules are surrounded by water; therefore, we used a non-aqueous reaction media to facilitate the dehydration reaction. In the present study, L-cysteine was heated at 230 °C for 30 min in a non-aqueous solvent, diphenyl ether, which has a high boiling point of 258 °C, to yield CDs-NA. For comparison, we also hydrothermally prepared CDs-A from L-cysteine dissolved in water, with the use of an autoclave and microwave heating system under the same temperature and time conditions. The maximum photoluminescence (PL) intensity of the water dispersion of CDs-NA was 1.5 times as high as that of the CDs-A. The absolute PL quantum yield (QY) of CDs-NA was 4.2%, which was 1.4 times as high as that of CDs-A, 3.0%. Characterization of the elemental compositions, particulate properties, and chemical bonding states confirmed that the non-aqueous solvent not only facilitated CD formation via the dehydration reaction but also contributed to the formation of sulfonic acid groups by oxidation of thiol groups. The rapid particle growth and formation of the additional functional groups might contribute to the increased PL intensity and absolute PLQY of CDs-NA. Through the optimization of the synthesis conditions in a non-aqueous solvent, the absolute PLQY of CDs-NA was increased from 4.2% to 10.2%.

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Bandgap-Tuned CuInS<inf>2</inf>/ZnS Core/Shell Quantum Dots for a Luminescent Downshifting Layer in a Crystalline Silicon Solar Module

Nakamura Y., Iso Y., Isobe T.

ACS Applied Nano Materials （ACS Applied Nano Materials） 3 （ 4 ） 3417 - 3426 2020年04月

研究論文（学術雑誌）, 共著, 査読有り

Copyright © 2020 American Chemical Society. CuInS2 (CIS) quantum dots (QDs) were investigated as a luminescent downshifting (LDS) material that converts near-ultraviolet (UV) light to visible light and were applied to a single crystalline silicon (c-Si) solar module that has no spectral sensitivity in the near-UV region. The bandgap of the CIS/ZnS core/shell QDs was successfully adjusted to &tild;3 eV, which was adequate for the LDS layer in solar devices, by changing the molar ratio of Cu/In. CIS/ZnS/ZnS core/shell/shell QDs with 59.9% absolute photoluminescence (PL) quantum yield were prepared by the hot-injection method and embedded in ethylene-vinyl acetate copolymer (EVA) resin to fabricate QD@EVA films as the LDS layer. The PL intensity of the QD@EVA films under near-UV excitation monotonically increased with increasing QD concentration. The films were attached to a commercial single c-Si solar module. The advantages and drawbacks of the films were discussed based on the results of incident photon-to-electron conversion efficiency and current-voltage curve measurements.

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Surface modification strategy for fluorescence solvatochromism of carbon dots prepared from: P -phenylenediamine

Sato K., Sato R., Iso Y., Isobe T.

Chemical Communications （Chemical Communications） 56 （ 14 ） 2174 - 2177 2020年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 13597345

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Critical Review - Photostable Fluorescent Cd-Free Quantum Dots Transparently Embedded in Monolithic Silica

Iso Y., Isobe T.

ECS Journal of Solid State Science and Technology （ECS Journal of Solid State Science and Technology） 9 （ 1 ） 2020年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 21628769

© The Author(s) 2019. Published by ECS.. Fluorescent Cd-free quantum dots (QDs) such as InP/ZnS and CuInS2/ZnS have attracted attention for use as color converters for blue light emitting diodes. To protect QDs from oxygen, which degrades them by photo-oxidation during excitation, we fabricated a transparent monolithic silica composite containing the QDs using an aqueous solution of tetramethylammonium silicate (TMAS) as a silica source. The TMAS solution readily dispersed the negatively charged QDs prepared by ligand exchange of 1-dodecanethiol for 3-mercatopropionic acid (MPA). A highly transparent monolithic TMAS-derived silica composite containing the MPA-capped QDs was obtained from the QD dispersion through a sol-gel process. Improved photostability of QDs was explained by the TMAS-derived silica acting as a gas barrier to protect the embedded QDs from photo-oxidation by oxygen in air.

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Effective Stabilization of Perovskite Cesium Lead Bromide Nanocrystals through Facile Surface Modification by Perfluorocarbon Acid

Sato D., Iso Y., Isobe T.

ACS Omega （ACS Omega） 5 （ 2 ） 1178 - 1187 2020年

研究論文（学術雑誌）, 共著, 査読有り

© 2020 American Chemical Society. CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) have attracted much attention as promising materials for next-generation optoelectronic applications. However, improvement of their low stabilities against heating and humidity is needed for practical use. In this work, we focused on perfluorodecanoic acid (PFDA) as a surface ligand and investigated the thermal and chemical stabilities of the photoluminescence (PL) properties of CsPbBr3 NCs. Oleic acid (OA) adsorbed on the NCs was exchanged for decanoic acid (DA) and PFDA. OA-modified and DA-modified NCs exhibited drastic fluorescence quenching to 12.9 and 21.1% of their initial PL intensities, respectively, after heating at 100 °C for 4 h. In contrast, the PFDA-modified NCs maintained 92.1% of their PL intensity after the same heating. Furthermore, the polar solvent resistance was also improved by PFDA modification. These improvements can be attributed to the strong adsorptivity and high chemical stability of the PFDA ligand.

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Musa AAA and Jatropha curcas L. sap mediated TiO<inf>2</inf> nanoparticles: Synthesis and characterization

Wagutu A., Yano K., Sato K., Park E., Iso Y., Isobe T.

Scientific African （Scientific African） 6 2019年11月

研究論文（学術雑誌）, 共著, 査読有り

© 2019 Titanium dioxide (TiO2) nanoparticles were synthesized using East African highland banana (Musa AAA) and Jatropha curcas L. sap. The nanoparticles were characterized with X-ray diffractometry (XRD), Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM). FT-IR peaks show presence of amino, carboxyl and hydroxyl groups on the crystal surface of the nanoparticles. XRD and Raman characteristic indicate anatase phases. The average crystallite size calculated from XRD peak width using Scherrer equation are 5.5 nm and 6.1 nm for Jatropha and Musa AAA sap TiO2 particles respectively. SEM images show polydispersed spherical clusters of agglomerated nanoparticles. The nanoparticles showed antimicrobial activity against gram-negative bacteria when irradiated in natural sunlight for 2 h. The sap extracts provide a simple, cheaper and more environmentally benign reducing and stabilizing agents for transforming transition metal salts into nano-scale particles, which have wide range of industrial applications.

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Optimizing the microwave-assisted hydrothermal synthesis of blue-emitting L-cysteine-derived carbon dots

Yoshinaga T., Iso Y., Isobe T.

Journal of Luminescence （Journal of Luminescence） 213 6 - 14 2019年09月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00222313

© 2019 Elsevier B.V. We synthesized carbon dots (CDs)from biocompatible L-cysteine, containing N and S, through an eco-friendly, facile hydrothermal reaction. These L-cysteine-derived CDs are produced by pyrolysis and carbonization after dehydration between NH 2 and COOH groups of L-cysteine. To optimize the microwave-assisted hydrothermal synthesis of the blue-emitting L-cysteine-derived CDs, we varied the hydrothermal temperature, hydrothermal duration, synthesis pH, and L-cysteine concentration. We synthesized CDs from 17 to 133 mmol L −1 aqueous solutions of L-cysteine whose pH values were 1, 7, or 13 through hydrothermal treatment at 110–230 °C for 5–120 min using a microwave heater. The absorbance of transitions related to C[dbnd]C, C[dbnd]O, and C[dbnd]N bonds increased through hydrothermal treatment at increasing temperature and for increasing duration. At the same time, the photoluminescence (PL)intensity and PL quantum yield (QY)of blue emission from the CDs under near-UV excitation increased. We propose that higher temperature and longer duration promote the carbonization, oxidation, and doping of nitrogen into CDs, resulting in the formation of π levels, surface state levels related to functional groups, and N-doped levels, respectively. The absorbance and PL intensity of the CDs prepared at pH 7 were the highest among the pH values 1, 7, and 13. The protonation of NH 2 groups at pH 1 and the deprotonation of COOH groups at pH 13 suppressed dehydration between the NH 2 and COOH groups of L-cysteine. In contrast, rapid dehydration at pH 7 induced carbonization, contributing to the maximized absorbance and PL intensity. The absorbance, PL intensity, and PL QY increased through hydrothermal treatment at increasing L-cysteine concentration. Higher L-cysteine concentration promoted dehydration between molecules and the subsequent carbonization, improving the optical properties.

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Electrophoretically Deposited Y<inf>2</inf>O<inf>3</inf>:Bi3+,Yb3+ Nanosheet Films as Spectral Converters for Crystalline Silicon Solar Devices

Watanabe R., Iso Y., Isobe T.

ACS Applied Nano Materials （ACS Applied Nano Materials） 2 （ 6 ） 4009 - 4017 2019年06月

研究論文（学術雑誌）, 共著, 査読有り

© 2019 American Chemical Society. Transparent Y2O3:Bi3+,Yb3+ films that exhibit near-infrared (NIR) emission under near-ultraviolet (NUV) excitation are candidates for the spectral converters in crystalline silicon solar devices. In this work, NIR-emitting films were fabricated by the aqueous electrophoretic deposition of Y2O3:Bi3+,Yb3+ nanosheets, and their photoluminescence (PL) properties were enhanced by a postcalcination process. Positively charged Y2O3:Bi3+,Yb3+ nanosheets with adsorbed polyethylenimine (PEI) were deposited as a uniform and dense micrometer thick layer on a transparent conductive substrate by applying an electric field to an aqueous nanosheet dispersion. The resulting nanosheet film showed NIR emission from Yb3+, realized through energy transfer from Bi3+, when excited under NUV light. The NIR PL intensity of the film was improved by calcination at 700-1000 °C under a flow of air. This NIR PL enhancement was explained by the calcination process increasing the crystallinity of the Y2O3:Bi3+,Yb3+ nanosheets and removing adsorbed hydroxyl groups that quenched the NIR emission of Yb3+. Furthermore, the increased energy transfer efficiency of the Bi3+ → Yb3+ transition with increasing calcination temperature also contributed to the observed NIR PL enhancement of the nanosheet film. The NIR PL intensity of the as-deposited nanosheet film did not change under continuous NUV excitation, whereas a gradual increase in the PL intensity was observed for the calcined nanosheet films. This increase in the PL intensity of the calcined films would be caused by the photooxidation of the Bi and Yb ions, which were partially reduced by PEI during the calcination process.

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Complete self-recovery of photoluminescence of photodegraded cesium lead bromide quantum dots

Kidokoro K., Iso Y., Isobe T.

Journal of Materials Chemistry C （Journal of Materials Chemistry C） 7 （ 28 ） 8546 - 8550 2019年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 20507534

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Effects of oleic acid on the stability of perovskite CsPbBr <inf>3</inf> quantum dot dispersions

Kosugi T., Iso Y., Isobe T.

Chemistry Letters （Chemistry Letters） 48 （ 4 ） 349 - 352 2019年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 03667022

© 2019 The Chemical Society of Japan. We investigated the effects of addition of oleic acid (OA) on the stability of CsPbBr 3 quantum dot (QD) dispersions in toluene during storage and light irradiation. Excess amounts of OA degraded the fluorescent perovskite CsPbBr 3 QDs because of promoted crystal growth through dissolution and reprecipitation. In contrast, a suitable amount of OA (0.2 vol%) improved their photostability possibly due to re-adsorption of OA on the exposed CsPbBr 3 crystal surface after the photoinduced desorption of OA.

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Green Photoluminescence of Perovskite CsPb(Br<inf>1- x</inf>I<inf>x</inf>)<inf>3</inf> Nanocrystals for Wide Color Gamut Displays

Miyata S., Iso Y., Isobe T.

ACS Omega （ACS Omega） 4 （ 12 ） 15067 - 15073 2019年

研究論文（学術雑誌）, 共著, 査読有り

© 2019 American Chemical Society. All-inorganic mixed-halide CsPb(Br1-xIx)3 perovskite nanocrystals (NCs) are excellent candidates for green-emitting phosphors in wide color gamut displays; however, a detailed investigation of their photoluminescence (PL) properties based on the halide composition has been missing. In this work, we report a fundamental investigation of the changes in the PL properties of CsPb(Br1-xIx)3 NCs. The PL color of the NCs, which were prepared by a hot-injection method, changed from green to red with increasing iodide composition (x). Almost ideal green emission close to the chromaticity coordinates of the green vertex of the BT.2020 standard was achieved by appropriately substituting iodide ions for bromide ions in monohalide CsPbBr3 NCs. However, the PL peak width of the mix-halide NCs at x ∼0.5 was 0.127 eV, which was broader than the 0.105 eV peak width of the monohalide CsPbBr3 NCs. This phenomenon should be due to the compositional inhomogeneity among the individual CsPb(Br1-xIx)3 NCs. On the other hand, the PL quantum yield (PLQY) for the monohalide CsPbBr3 NCs decreased from 70 to 25% as x increased to 0.5. This result may be attributed to lattice distortion by the difference in the ionic radii of bromide and iodide. Improvements in the compositional inhomogeneity and lattice distortion would enhance the color purity of the green emission and the PLQY, respectively, of the CsPb(Br1-xIx)3 NCs.

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Fluorescence Solvatochromism of Carbon Dot Dispersions Prepared from Phenylenediamine and Optimization of Red Emission

Sato R., Iso Y., Isobe T.

Langmuir （Langmuir） 2019年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 07437463

© 2019 American Chemical Society. Fluorescent carbon dots (CDs) are of interest as a promising alternative to quantum dots, partly because they do not include heavy metals. However, most CDs exhibit blue or green emission, while red-emitting CDs are required for a variety of applications. In the present work, CDs were synthesized by refluxing three phenylenediamine (PD) isomers with amino groups at different positions (o-PD, m-PD, and p-PD) in diphenyl ether at 250 °C for 4 h. Upon dispersing the resulting CDs in eight solvents with different polarities, emission colors ranging from green to red were observed. Among these CDs, p-PD-derived CDs exhibited both the longest emission wavelength range, from 538 to 635 nm, and the highest absolute red photoluminescence quantum yield (PLQY) of 15%. Herein the results are discussed based on a comparison of the polymerization processes of o-PD, m-PD, and p-PD. This work demonstrated that the optimum reaction time was 2 h, which yields a p-PD-derived CD dispersion in methanol with red emission and an absolute PLQY as high as 18%. Additionally, the use of 1-decanol and deuterated methanol in place of methanol improved the maximum absolute PLQY to 25% and 36%, respectively. These improved values are attributed to reduced concentration quenching by suppression of Ï€-πstacking interactions and inhibition of the nonradiative relaxation process through the vibration of OH groups, respectively.

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Ligand exchange of core/shell CuInS2/ZnS quantum dots for preparation of their homogeneous ink

Shiraishi M., Iso Y., Isobe T., Seshimo T., Liao Y.C., Noda K., Shiota D.

Proceedings of the International Display Workshops （Proceedings of the International Display Workshops） 2 803 - 805 2019年

研究論文（学術雑誌）, 共著, 査読有り

Preparation and characterization of Y<inf>2</inf>O<inf>3</inf>:Bi3+,Yb3+ nanosheets with wavelength conversion from near-ultraviolet to near-infrared

Fujita K., Watanabe R., Iso Y., Isobe T.

Journal of Luminescence （Journal of Luminescence） 198 243 - 250 2018年06月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00222313

© 2018 Elsevier B.V. Y2O3:Bi3+,Yb3+ nanophosphors, which exhibit near-infrared (NIR) emission under near-ultraviolet (near-UV) excitation, are promising candidates for spectral converters in crystalline silicon solar devices. Herein, Y2O3:Bi3+,Yb3+ nanosheets were prepared by calcination of hydrothermally-synthesized precursors. The effects of the calcination temperature on their properties were investigated. The nanosheets calcined at ≤ 800 °C had sheet-like, square morphologies with average lateral sizes of 210–240 nm, which were smaller than precursor nanosheet with 257 nm attributed to shrinkage through the calcination. The thickness of the nanosheet before and after calcination at 800 °C was ~20 nm. Moreover, the nanosheet had a single-crystal nature. However, at calcination temperatures ≥ 900 °C, the nanosheets lost their morphologies, and transformed into irregular particles. The Y2O3:Bi3+,Yb3+ nanosheets showed visible emission from Bi3+ and NIR emission from Yb3+ under near-UV excitation. The PL intensity and PL decay time of the NIR emission monotonically increased with increasing calcination temperature. Both increases can be explained by the following effects: (i) removal of H2O molecules adsorbed on the nanosheet surface, (ii) improvement of crystallinity and decrease of specific surface area, (iii) increase in probability of energy transfer from Bi3+ to Yb3+, and (iv) oxidation of reduced bismuth during calcination. Furthermore, the photostability of the nanosheets under near-UV excitation was improved by increasing calcination temperature.

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Electrophoretically Deposited Y<inf>2</inf>O<inf>3</inf>:Bi3+,Eu3+ Nanosheet Films with High Transparency for Near-Ultraviolet to Red Light Conversion

Kosuge Y., Iso Y., Isobe T.

Langmuir （Langmuir） 34 （ 13 ） 3813 - 3820 2018年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 07437463

© 2018 American Chemical Society. Fluorescent films were fabricated by depositing Y2O3:Bi3+,Eu3+ nanosheets, which emit red light under near-UV irradiation. The Y2O3:Bi3+,Eu3+ nanosheets were obtained by calcining hydroxide precursor nanosheets synthesized through a hydrothermal method. An aqueous dispersion of positively charged Y2O3:Bi3+,Eu3+ nanosheets with polyethyleneimine adsorbed to the surface was prepared for their deposition. Fluorescent nanosheets were electrophoretically deposited on a transparent conductive substrate under a constant voltage. The obtained nanosheet films were dense and uniform and showed excellent photostability against the excitation light. Growth of the nanosheet film caused a decrease in transmittance and an increase in the photoluminescence intensity. The former effect was attributed to light scattering from inner voids and the rough surface of the film. A polyvinylpyrrolidone (PVP) coating on the film improved the transmittance to be greater than 70% over the visible region. These effects were attributed to antireflection effects at the film surface owing to the low refractive index of PVP. Furthermore, suppression of light scattering by coating the rough surface with a smooth PVP film and filling of voids in the nanosheet film with PVP also improved the transmittance.

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Fluorescent Ag+-exchanged zeolite nanoparticles with improved photoluminescence properties via X-ray irradiation

Taiji K., Iso Y., Isobe T.

Journal of Luminescence （Journal of Luminescence） 196 214 - 220 2018年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00222313

© 2017 Elsevier B.V. Fluorescent Faujasite (FAU)-Y Ag+-exchanged zeolite is a lanthanide-free phosphor with a high photoluminescence (PL) quantum yield to convert near-ultraviolet (near-UV) to visible light. Here, we focused on nanometer-sized fluorescent FAU Ag+-exchanged zeolite as a wavelength converter for optoelectronic devices. The prepared Ag+-exchanged zeolite nanoparticles were irradiated with X-rays to improve their PL properties because X-ray irradiation does not possess the disadvantages of other treatments, such as aggregation and sintering of nanoparticles exposed to thermal treatment, excessive growth of silver particles with visible light absorption following chemical reduction, and amorphization of zeolite by high-energy electron beam irradiation. FAU-Y zeolite nanoparticles were prepared by template-free hydrothermal synthesis and then fluorescent Ag+-exchanged zeolite nanoparticles were obtained through the ion exchange of Na+ with Ag+. The prepared Ag+-exchanged zeolite nanoparticles were irradiated by X-rays from a commercial tabletop device. The average size of the as-prepared zeolite nanoparticles was 35.7 ± 12.6 nm according to transmission electron microscopy (TEM) images. TEM also confirmed that Ag-related particles such as silver clusters and/or silver nanoparticles were generated and grew from Ag+, possibly through reduction accompanied with oxidation of oxide ions of the zeolite framework during X-ray irradiation. PL quantum efficiency was enhanced by X-ray irradiation by up to ~3 times. Analysis of PL properties and electron spin resonance spectra implied that fluorescent Ag42+ clusters were formed under X-ray irradiation, resulting in the enhancement of PL quantum efficiencies. Similar behavior was also observed for micrometer-sized Ag+-exchanged zeolites. X-ray irradiation of fluorescent Ag+-exchanged zeolites is therefore a useful method to improve their PL properties.

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Particulate, structural, and optical properties of D-glucose-derived carbon dots synthesized by microwave-assisted hydrothermal treatment

Yoshinaga T., Iso Y., Isobe T.

ECS Journal of Solid State Science and Technology （ECS Journal of Solid State Science and Technology） 7 （ 1 ） R3034 - R3039 2018年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 21628769

© The Author(s) 2017. Published by ECS. Environmentally friendly and visible fluorescent carbon dots (CDs) have received attention as alternatives to compound semiconductor quantum dots containing toxic elements. Hydrothermal reaction of water-soluble carbohydrates can provide an eco-friendly and facile synthesis approach to CDs. In this study, we have attempted to efficiently produce uniform CDs using a microwave heating system that can maintain the reaction solution at a constant temperature.We synthesized CDs from D-glucose through hydrothermal treatment at 200°C for 5-60 min. From transmission electron microscopy, the particle size of the CDs increased in proportion to the hydrothermal duration. Furthermore, the absorbance and maximum photoluminescence intensity of the CDs aqueous dispersions (10 mg L-1) increased with an increase in hydrothermal duration. The maximum photoluminescence intensity of the dispersions decreased at pH < 4 and pH > 10; the absorbance at the excitation wavelength remained unchanged. These results indicate that the number of emission sites and absorption sites increased with an increase in the particle size and that emission sites were located near the surface of the obtained CDs.

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