Published papers

Low-toxic and efficient fluorescent core-shell CuInS2/ZnS (CIS/ZnS) quantum dots (QDs) are good candidates for optoelectronic device applications. They are synthesized in a hydrophobic environment, while large amounts of organic solvents used in the preparation of fluorescent films have significant problems on environmental load and human health. CIS/ZnS QDs hydrophilized by adsorbing 3-mercaptopropionic acid on their surfaces can be used in the aqueous film fabrication process. In this work, the aqueous electrophoretic deposition (EPD) of the hydrophilized QDs with silicone-modified acrylic resin nanoparticles was performed to fabricate fluorescent nanocomposite films. The hydrophilized QDs and resin nanoparticles were simultaneously dispersed in basic aqueous solutions due to electrostatic repulsion resulting from their negatively charged surfaces. Transparent films were obtained on a transparent conductive substrate at the anode side by the EPD. They showed yellow fluorescence of the QDs. The thickness increased with increasing the deposition time; however, hemispherical holes attributed to oxygen gas generated by water electrolysis were observed at the longer time. The electron microscopy revealed that the films were densely and homogeneously deposited. The QDs were dispersed around the resin nanoparticles without aggregation. The fluorescence (FL) quantum yield was 43%. The optical absorption peak and FL intensity of the QDs increased accompanied by the film growth. The nanocomposite film showed good heat resistance at 80-120 °C for 5 h; therefore, the prepared films have feasibility in white light-emitting diode (LED) applications. A lightening device structured with the obtained EPD film placed on a blue LED successfully emitted white light. In addition, the flexibility of the nanocomposite film was demonstrated. The aqueous EPD method would be one of the suitable methods for the industrial production of fluorescent QD films. This technique can be applied to other hydrophilic fluorescent QDs with charged surfaces. Realization of various fluorescent QD films would expand the application possibilities.

Graphene quantum dots (GQDs) are environmentally friendly fluorescent carbon-based nanomaterials. However, there is no report on the massive synthesis of GQDs with narrow-band fluorescence and a high photoluminescence quantum yield (PLQY) using a simple liquid-phase method under atmospheric conditions. In this study, GQDs were successfully synthesized in ∼100% product yield by heating phloroglucinol (PG) with Na3PO4·12H2O in 1,2-pentanediol at 180 °C for 6 h in an open system with air flow, followed by dialysis purification. The high product yield was attributed to the addition of Na3PO4·12H2O as a base catalyst, which promoted the dehydration-condensation reaction between PG molecules. The dispersion of PG derived GQDs (PG-GQDs) in ethanol resulted in blue fluorescence with a full width at half maximum of 32 nm and a PLQY of 54%. Further purification of PG-GQDs by silica gel column chromatography improved the PLQY to 75%. Fourier-transform infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, and X-ray photoelectron spectroscopy confirmed that dehydration-condensation reactions occurred not only between PGs but also between GQDs and 1,2-pentanediol. The binding of 1,2-pentanediol to the edges of GQDs suppressed the stacking of GQDs and prevented concentration quenching, resulting in a high PLQY. PG-GQDs exhibited negative fluorescence solvatochromism, i.e., the fluorescence wavelength blue-shifted with increasing solvent polarity. Dispersion of PG-GQDs in N-methyl-2-pyrrolidone (MP) resulted in green fluorescence with a PLQY of 96%. Dispersion of PG-GQDs in water resulted in blue fluorescence and a low PLQY of 6% at pH 7, while the PLQY was more than 50% at pH ≥ 11. Using these properties, the sensing of water (pH 13) in MP was investigated. The results showed that as the water content was increased from 0% to 100%, the fluorescence color gradually changed from green to blue and the fluorescence wavelength continuously shifted from 514 nm to 466 nm, indicating their applicability in water sensing.

CsPbBr3 nanocrystals (NCs) synthesized by the conventional hot-injection method are photochromatic luminescent nanomaterials due to the photoinduced desorption and re-adsorption of the surface ligands. The apparent color and the luminescence intensity were changed significantly during excitation light irradiation and following dark storage; however, the emission wavelength was almost retained. This work investigates the change in emission color of light-emitting diode lighting using the CsPbBr3 NCs to realize photochromatic luminescence. The results showed definite shifts in emission color caused by changes in optical absorption and green luminescence intensity of the NCs, potentially broadening the application feasibility of CsPbBr3 NCs as photochromatic luminescent nanomaterials.

The power generation performance of luminescent solar concentrators (LSCs) is decreased by unignorable optical losses including self-absorption of luminescence by the phosphor and optical absorption by the light guide plate. To address the optical loss due to self-absorption, this research focused on the use of core/shell CuInS2 (CIS)/ZnS quantum dots (QDs) with a large Stokes shift. The QDs were dispersed in ethylene-vinyl acetate (EVA) copolymer to fabricate transparent fluorescent nanocomposite films. LSCs were prepared with the nanocomposite films and soda glass plate, a common building material to investigate the effect of optical absorption by the light guide plate on the power generation performance. Synthesized QDs showed yellow emission at 587 nm with a high photoluminescence quantum yield (PLQY) of 61.1%, and their large Stokes shift of 0.82 eV reduced self-absorption, making them suitable for LSC applications. We evaluated the effects of QD concentration, film thickness, light guide plate material, and film position on the performance of the LSCs. Optimized performance was observed at a QD concentration of 6.2 wt% and film thickness of 324 μm, showing 1.71-fold higher photocurrent and 3.58-fold larger area of incident photon-to-current efficiency spectrum compared to without film. Replacing soda glass with low-absorption white glass increased power generation by fourfold. This reveals the significant absorption losses of the PL and scattered incident light by soda glass. Despite the considerable optical loss, soda glass is a necessary component in a light guide plate to promote the use of LSCs as building materials. Our findings indicate that QDs with a narrow emission peak at ∼520 nm are required to minimize the absorption loss by soda glass.

Small-molecule dyes with longer wavelength and higher photoluminescence quantum yield (PLQY) in water have attracted increasing attention due to their bright potential in fluorescence analysis technologies. Here, a red-emitting fluorescent dye of 1,2,4-triaminobenzene (TriAB) was modified with 3-aminopropyltriethoxysilane (APTES) to avoid contact with water molecules and to suppress fluorescence quenching due to O-H vibrations in water. The formation of a silica-like structure from hydrolytically polycondensed APTES was confirmed by X-ray diffractometry and Fourier-transform infrared absorption spectroscopy. Modification of TriAB with hydrolytically polycondensed APTES induced orange emission in the powder state, although TriAB powder did not exhibit any fluorescence before modification. The PLQY of APTES-modified TriAB in water was 11.1 %, which was higher than that of TriAB in water, 4.5 %. These results confirmed that the APTES derived silica-like matrix shielded TriAB from both water and intermolecular interactions with other TriAB molecules, overcoming fluorescence quenching.

Currently, most carbon dots (CDs) are synthesized from carbon-based materials in autoclaves with high pressure, and then applied in various fields. This work explored the facile synthesis of CDs at atmospheric pressure, which were then purified and embedded in the ethylene-vinyl acetate (EVA) copolymer to form CDs@EVA films and applied in luminescent solar concentrator (LSC) devices. First, crude CDs were synthesized by refluxing p-phenylenediamine (p-PD) in diphenyl ether at 250 °C for 2 h under ambient air conditions, followed by purification using silica gel column chromatography to obtain purified CDs with a photoluminescence (PL) quantum yield of 56% and an average particle size of 15.1 ± 4.4 nm. The CDs@EVA films were fabricated at CD concentrations of 0.05 wt%, 0.10 wt%, and 0.15 wt%, and their thickness was varied from ∼100 μm to ∼500 μm. The yellowish-transparent CDs@EVA films exhibited greenish-yellow emission with absorption peaks at 350 nm and 470 nm, corresponding to the n-π* transition of C=N bonds and the π-π* transition between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of π-conjugated electrons. The PL intensity reached its maximum at 0.05 wt% CD concentration and 408 μm thickness due to self-absorption of emission from CDs. According to the current-voltage curve measurements under AM1.5G simulated sunlight, this film exhibited the maximum short-circuit current (Isc) and power conversion efficiency (η) of the LSC device. Additionally, incident photon-to-current efficiency spectrum measurements revealed that the enhanced Isc and η in CDs@EVA films mainly resulted from emission by absorption through the n-π* and π-π* transitions.

A repeatable enhancement of the photoluminescence (PL) from CsPbBr3 nanocubes (NCs) is promoted by the assembling of NCs. The PL quantum yield (QY) of ordered NC arrays increases with photoirradiation and decreases in the dark. The repeatable enhancement of the PLQY cannot be observed from isolated NCs. The nanospaces between NCs in the ordered arrays allow a reversible change in thermally stimulated desorption and photo-induced adsorption of surface ligands that affect the PLQY.

© 2021 The Authors. Published by American Chemical Society. Fluorescent carbon dots (CDs) represent a promising eco-friendly next-generation phosphor. However, most CDs exhibit broad photoluminescence (PL) spectra [full width at half-maximum (fwhm) over 60 nm]; few works on CDs with sharp PL spectra (fwhm less than 40 nm) have been reported. In addition, their syntheses and color tuning require harsh conditions of high temperatures, long reaction times, and high pressures with catalysts. Here, we successfully prepared narrow-bandwidth emissive CDs (fwhm of 27-40 nm) from phloroglucinol in a glycol solvent of 1,2-pentanediol at temperatures as low as 180 °C for a reaction duration of as short as 6 h under ambient conditions without any catalysts via an open reaction system in which dehydration and condensation reactions among phloroglucinol molecules were enhanced. We shifted the emission peak from 463 to 511 nm by selecting seven kinds of solvents with different polarities, that is, emission colors could be tuned from blue to green by taking advantage of fluorescence solvatochromism. The CD-dispersed polymer films showed a similar solvatochromic behavior and sharp PL spectra, verifying the feasibility of applying the CDs to displays with a wide color gamut.

Changes in photoluminescence (PL) peak width and quantum yield (PLQY) with halide composition were compared for CsPb(Br1-x I x )3 nanocrystals (NCs) prepared by hot-injection and ion-exchange methods. The hot-injection method produced single-phase cubic CsPb(Br1-x I x )3 NCs for each iodide ratio. In contrast, a small amount of orthorhombic phase was observed at higher x for NCs prepared by the ion-exchange methods. The PL peak width of mixed-halide NCs at x = 0.55 was broader than those of CsPbBr3 and CsPbI3, possibly as a result of compositional inhomogeneity among the individual NCs. However, the maximum PL peak width occurred at larger x for the ion-exchange methods, possibly because of partial transition to orthorhombic phase from cubic phase for CsPb(Br1-x I x )3 NCs. The minimum PLQY for the hot-injection method was 25.0% at x = 0.50, which was lower than the PLQYs of CsPbBr3 and CsPbI3 NCs (∼70%). In contrast, the minimum PLQY for the ion-exchange methods occurred at x = ∼0.3. Furthermore, the minimum PLQY was ∼10%, which was lower than 25.0% for the hot-injection method. The addition of halide solution for ion exchange might affect the adsorption state of surface modifiers, leading to the decreased PLQY.

We studied the formation of Y2O3:Bi3+,Eu3+ nanosheets from different Y2O3 precursors to propose a mechanism for their formation. One limitation of solar cells is their low sensitivity to near-ultraviolet light, which may be improved by including a phosphor to convert near-ultraviolet light to visible light, which can be readily absorbed. An attractive phosphor for this role is Y2O3:Bi3+,Eu3+; however, it remains unclear how Y2O3:Bi3+,Eu3+ forms from Y2O3 precursor nanosheets. Therefore, we studied the formation of Y2O3:Bi3+,Eu3+ nanosheets from Y2O3 precursors fabricated under various conditions including different pH, base type, and autoclave duration. We found that single-crystalline Y2O3:Bi3+,Eu3+ nanosheets formed from Y2O3 precursor nanosheets produced under suitable conditions. The Y2O3:Bi3+,Eu3+ nanosheets retained the crystallographic orientation of the precursor Y2O3 nanosheets. The formation of these nanosheets did not require the presence of organic ligands. Our findings indicate that Y2O3:Bi3+,Eu3+ nanosheet formation involved anisotropic assembly of crystallized nanoparticles and a subsequent dissolution-recrystallization process. The Y2O3:Bi3+,Eu3+ nanosheets fabricated under certain conditions displayed absorption and emission properties suitable for use as solar converters.

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.

Fluorescent CsPbBr3perovskite nanocrystals (NCs) degraded by photoinduced desorption of surface ligands can exhibit self-recovery; however, irreversible oxidative damage from oxygen in the atmosphere must be avoided. In this work, gas-barrier polymer films made of ethylene vinyl alcohol (EVOH) and ethylene vinyl acetate (EVA) copolymers were investigated as sealing materials for CsPbBr3NC films. The NC film covered with EVOH showed photodegradation with blackening and photoluminescence quenching under light excitation, while self-recovery of the optical properties was observed during dark storage. These phenomena were attributed to the photoinduced desorption of surface ligands (and thus the formation of surface defects) and their re-adsorption, respectively. In contrast, self-recovery did not occur when an EVA film was used. X-ray photoelectron spectroscopy indicated the formation of Pb-O bonds for the EVA-coated film. Thus, EVOH, which has a lower oxygen permeability than EVA, successfully prevented irreversible damage by permeated oxygen. This is an important finding for the reuse or lifetime-extension of CsPbBr3NCs.

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.

This work investigated CuGaS2 (CGS)/ZnS quantum dots (QDs) as fluorescent materials for a luminescent downshifting (LDS) layer and a luminescent solar concentrator (LSC) with a single-crystalline silicon (sc-Si) solar module. The bandgap of the CGS/ZnS QDs was adjusted to ∼3.1 eV by changing the molar ratio of Cu/Ga, and in so doing a transparent and colorless LDS layer that converted near-ultraviolet (UV) light to visible light was fabricated. The resulting QDs, which exhibited a high photoluminescence quantum yield of 73%, were embedded in ethylene-vinyl acetate (EVA) copolymer resin to fabricate a QDs@EVA film. The short-circuit current (ISC) of the sc-Si solar module under simulated solar light decreased from 39.35 mA to 38.30 mA (−2.7%) by using the QDs@EVA film instead of an EVA film without QDs. Correspondingly, the photoelectronic conversion efficiency (η) changed from 18.07% to 17.56% (−2.8%). The decreased photocurrent was because of light scattering by aggregated QDs, and overcame the wavelength-conversion effect of the LDS film. In contrast, the QDs@EVA film was effective for LSC because the ISC and η relatively increased by +77.7% and +96.2%, respectively. The visible light emitted by the UV-excited QDs and the visible light scattered by the QDs contributed to the photoelectric conversion.

Surface modification of p-phenylenediamine-derived carbon dots using 4-morpholinoaniline with an electron-donating and rigid structure followed by purification using silica gel column chromatography increased the photoluminescence quantum yield from 28.3 to 41.8% in chloroform. The improved photoluminescence quantum yield was observed for the surface modification by postaddition of the surface modifier after the formation of the π-conjugated structure of the carbon dot core but was not observed for the carbon dots prepared from a mixture of p-phenylenediamine and 4-morpholinoaniline. The finding of this work gives us an idea to design carbon dots with high photoluminescence quantum yield for application of color conversion in white-light-emitting diodes.