Published papers

CsPb(Br,I) 3 quantum dots (QDs) show application potential for optoelectronic devices. However, their thermal degradation is a significant problem. In this work, the effects of perfluorodecanoic acid (PFDA) modification on the photoluminescence (PL) and thermal resistance of CsPb(Br,I) 3 QDs were evaluated. The PL intensity of oleic-acid-modified quantum dots (OA-QDs) in toluene decreased drastically upon heating at 100°C. The PL quantum yield of the QDs increased from 69.6% to 77.4% upon modification with PFDA. Furthermore, the PL intensity of the QDs modified with PFDA (PFDA-QDs) increased to 140.6% upon heating, because of the reduction of surface defects upon adsorption of PFDA and its optimized adsorption state. A solid-film PFDA-QDs sample heated at 80°C for 4 h showed temporary PL enhancements for the OA-QDs and PFDA-QDs films to 445% and 557% of their initial values, respectively, upon heating for 0.25 h. This was attributed to the optimized adsorption states of the surface ligands. PFDA-QDs film maintained 354% after 4 h of heating, whereas that of OA-QDs film was 104%. Thus, PFDA modification enhances PL intensity and suppresses PL degradation under heating, which is important for wavelength converters for optoelectronic device applications.

Quantum dots (QDs) dispersed in UV-curable resin are used for patterning in photolithography and inkjet printing. However, low affinity between the main component of UV-curable resins known as celloxide, an alicyclic diepoxy compound, and QD surface ligands with alkyl chains causes significant aggregation of QDs. In this study, the dispersibility of core/shell CuInS2/ZnS QDs with adsorbed 1-dodecanethiol and oleic acid in celloxide was improved using the ligand exchange method to prepare transparent fluorescent nanocomposites. Cyclohexyl 3-mercaptopropionate (MPACH) and 3-mercaptopropionic acid (MPA) were successfully adsorbed onto the QDs. MPACH-modified QDs (QD-MPACH) were well dispersed in the UV-curable resin, whereas MPA-modified QDs (QD-MPA) exhibited significant aggregation. Nanocomposite plates containing dispersed QDs were prepared by UV irradiation. The QD-MPACH nanocomposite plate was transparent, while the QD-MPA nanocomposite plate was turbid. The homogeneous dispersion of QD-MPACH was attributed to the similarity in the molecular structure between MPACH and celloxide. The photoluminescence (PL) peak of the QD-MPA nanocomposite occurred at a longer wavelength than that of the QD-MPACH nanocomposite. Furthermore, compared with the absolute photoluminescence quantum yield (PLQY) of the as-prepared QDs in toluene (55%), that of the QD-MPA nanocomposite was smaller (46%), and that of the QD-MPACH nanocomposite was higher (61%). An enhanced self-absorption effect was observed for the QD-MPA nanocomposite because of significant light scattering by the aggregates and concentration quenching, resulting in the PL redshift and decreased PLQY. Moreover, the PL intensity of the QD-MPACH nanocomposite was maintained at 98% of the initial value after continuous excitation-light irradiation for 5 h. The high PLQY and photostability of the QD-MPACH nanocomposite are beneficial in practical applications.

Carbon quantum dots (CQDs) are environmentally friendly phosphors. However, there have been few reports on red-emitting CQDs with narrow-bandwidths, all of which were synthesized via solvothermal methods using autoclaves. Here, red-emitting CQDs were synthesized from 4,6-diaminoresorcinol dihydrochloride (DAR), which is an aromatic compound with electron-donating hydroxy and amino groups. The synthesis involved heating at 180 °C for 6 h in a high-boiling-point 1,2-pentanediol solvent in an open system without using autoclaves and catalysts. The DAR-derived-CQDs in ethanol exhibited red photoluminescence with a 0.14 eV full width at half-maximum that was comparable to that of narrow-bandwidth blue-fluorescent phloroglucinol (PG)-derived-CQDs. The red emission was attributed to a weakened quantum-size effect and electron-donating NH2 groups. The hydroxy groups of PG-derived-CQDs interacted with Al3+ and Fe3+ ions, resulting in fluorescence quenching. DAR-derived-CQDs have hydroxy and amino groups, which facilitate the fluorescence quenching for Al3+, Fe3+, Cu2+, and Pb2+ ions.

This work clarified the effects of self-assembly of perovskite cesium lead bromide (CsPbBr3) nanocubes (NCs) covered with didodecyldimethyl ammonium bromide (DDAB) on photoluminescence (PL) properties. Although the PL intensity of isolated NCs was weakened in the solid state even under inert conditions, the quantum yield of PL (PLQY) and the photostability of DDAB-covered NCs were drastically improved by the formation of two-dimensional (2D) ordered arrays on a substrate. The PLQY of the 2D arrays increased to ca. 60% by initial excitation illumination at 468 nm and was maintained for over 4000 h. The improved PL properties are attributable to the fixation of the surface ligand around the NCs in the specific ordered arrays.

Photoinduced desorption of surface ligands causes photodegradation of nanophosphors due to enhanced non-radiative relaxation, which is attributed to generated surface defects. We previously reported the self-recovery of photodegraded CsPbBr3 nanocrystals (NCs) and revealed that its mechanism can be attributed to readsorption of the desorbed surface ligands. Thus, using different surface ligands would change the photodegradation and subsequent self-recovery behaviors. Accordingly, in the present work, perfluorodecanoic acid (PFDA) was added to pre-synthesized CsPbBr3 NCs modified with oleic acid and oleylamine to evaluate its effects on photodegradation and subsequent self-recovery. The as-prepared NCs (OA-NCs) and PFDA-modified NCs (PFDA-NCs) were dispersed in nanocomposite ethylene-vinyl acetate resin films to suppress concentration quenching. OA-NCs showed monotonical photoluminescence (PL) decrease under blue excitation light and self-recovery during subsequent dark storage. In contrast, the PFDA-NCs exhibited temporary PL enhancement under excitation irradiation. The photo-activation is attributed to optimization of the adsorption states of the surface ligands in the early stage, while the influence of photodegradation due to ligand desorption becomes dominant later. However, increased PFDA modification prevented self-recovery, indicating that desorbed PFDA may be resistant to readsorption on the NC surface.

Carbon dots (CDs) were prepared from p-phenylenediamine, modified with trifluoroacetic acid, and dispersed in a fluoropolymer (FP) film. Unmodified CDs did not disperse well in the FP film, resulting in concentration quenching; whereas surface modification improved the affinity between the CDs and the FP, resulting in a homogeneous dispersion and an improved photoluminescence quantum yield.

Higher photoluminescence quantum yield (PLQY) is always a required property for fluorescent molecule dyes used as probes in aqueous environment because of non-radiative relaxation via O–H vibrations. 1,2,4-triaminobenzene (TriAB) dihydrochloride has been reported as a red-emitting fluorescent dye, although its photoluminescence (PL) intensity is very low. Here, a bright orange-emitting fluorophore was prepared by simply mixing TriAB and polyethyleneimine (PEI) in water at pH 7 and applied to metal ion sensing. The PLQY was increased from 3.1 % to 10.0 % by the addition of PEI into an aqueous TriAB solution. Our experiments by exchanging H2O for D2O and changing the pH from 7 to 1 revealed that the role of PEI in enhancing the PL intensity is attributed to the interaction between the amino groups of TriAB and PEI via hydrogen bonding, the prevention of contact between TriAB and water molecules, and thus the suppression of the non-radiative relaxation via O–H vibrations of H2O. We further demonstrated this role for 1,4-diaminobenzene and 1,2,4,5-tetraaminobenzene.

© 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.

1-Dodecanethiol on CuInS2/ZnS quantum dots (QDs) was exchanged with cyclohexanethiol. The obtained QDs were transparently dispersed in alicyclic diepoxy compound to yield the QD ink for ink-jet printing. This homogeneous dispersion is attributed to similarity in molecular structure between the ligand and the ink component.

© 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.

© 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.

© 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.

© 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.

© The Author(s) 2017. Published by ECS. All-inorganic cesium lead halide perovskite CsPbX3(X = Cl, Br, I) nanocrystals have emerged as promising luminescent quantum dots. In this review, we summarize recent work on their synthesis, luminescent properties, and stabilities. Through controlling the composition of the alloyed halides and quantum size effects, the band gaps and emission wavelengths of CsPbX3nanocrystals are readily tunable over the entire visible range. Their sharp emission line-widths extend the color gamut of liquid crystal displays. They also exhibit high photoluminescence (PL) quantum yields and fast PL lifetimes. CsPbX3nanocrystals with both cubic and plate shapes have been successfully synthesized through several liquid-phase methods, such as hot-injection, room-temperature, and amine-free methods. The sizes and shapes of CsPbX3nanocrystals can be controlled via the reaction temperature and the length of the n-alkyl carboxylic acids and n-alkylamines used in their synthesis. Several attempts have been made to improve their stabilities during storage, at high temperature, and under light irradiation, for example by incorporation of CsPbX3nanocrystals into silica or polymer matrixes.

© 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.

© 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.

© 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.

© 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.

© 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.

© 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%.