Yoshiki Iso

Electrophoretic deposition and characterization of transparent nanocomposite films of YVO4:Bi3+,Eu3+ nanophosphor and silicone-modified acrylic resin.

We fabricated nanocomposite films from an aqueous suspension of red-emitting YVO4:Bi(3+),Eu(3+) nanoparticles (hydrodynamic size: 22 ± 6 nm) and silicone-modified acrylic resin nanoparticles of (60 ± 15 nm) by electrophoretic deposition under application of a constant voltage. The nanocomposite films were formed from these negatively charged nanoparticles on ITO-coated glass substrates on the anodic side at the volume ratio of nanophosphor:resin ∼ 40:60. According to transmission electron microscopy observations, the YVO4:Bi(3+),Eu(3+) nanoparticles are well-dispersed around the resin nanoparticles. The fabricated films are transparent to the naked eye under white light because both nanoparticles show no absorption and low light scattering in the visible region. A silicone-modified acrylic resin film without the nanophosphor exhibits no absorption in the UV region (>300.0 nm). However, the fabricated nanocomposite films show near-UV absorption owing to the interband transition between the valence band and the conduction band of the YVO4:Bi(3+),Eu(3+) nanoparticles. A sharp emission peak corresponding to the (5)D0 → (7)F2 transition of Eu(3+) is observed at 619.5 nm, under 365.0 nm excitation, for each nanocomposite film. The photoluminescence intensity at 619.5 nm under 365.0 nm excitation is proportional to 1-10(-OD) (OD: optical density at 365.0 nm) for film thicknesses ≤6 μm. This is attributed to the low light scattering from both nanoparticles in the nanocomposite film. Conversely, the observed photoluminescence intensity for film thicknesses >6 μm is higher than the value expected from the proportional relationship. This suggests that the excitation of the nanophosphors efficiently occurs due to multiple scattering of excitation light.

Synthesis of Y2O3:Bi3+,Eu3+ nanosheets from layered yttrium hydroxide precursor and their photoluminescence properties

We produced Y2O3:Bi3+,Eu3+ nanosheets by calcining layered yttrium hydroxide (LYH) precursor nanosheets at 800 °C for 2 h. The precursor nanosheets were synthesized from metallic chlorides dissolved in methanol, via a solvothermal reaction at 200 °C for 2.5 h. X-ray diffraction and transmission electron microscopy revealed that the LYH nanosheets were composed of crystallites with a uniform crystallographic orientation. Their sheet-like morphology and single-crystal nature remained after calcination, while the thickness of the nanosheets decreased. Their excitation spectrum was monitored at the 612 nm emission wavelength, corresponding to the 5D0 → 7F2 transition of Eu3+, and featured a broad band at 332 nm that was attributed to the 6s2 → 6s6p transition of Bi3+. The Y2O3:Bi3+,Eu3+ nanosheets therefore exhibited red emission from Eu3+ via energy transfer from Bi3+ to Eu3+ following near-UV excitation of Bi3+. The photoluminescence (PL) properties of the calcined samples were investigated with various concentrations of Bi3+ in Y2O3 nanosheets codoped with 2 at% Eu3+. The highest PL quantum yield was 23% at a Bi3+ concentration of 0.2 at%. The PL lifetimes of the calcined samples decreased with increasing Bi3+ concentration due to concentration quenching. The PL intensity increased over time under continuous excitation, which might be attributable to the photooxidation of Bi following its reduction by polyethyleneimine or methanol during the LYH synthesis.

Combinatorial optimization of the atomic compositions for green-emitting YBO3:Ce3+,Tb3+ and red-emitting YBO3:Ce3+,Tb3+,Eu3+ phosphors using a microplate reader

Microplate readers are versatile devices that can rapidly measure the photoluminescence intensities of multiple samples, and are widely used in biological chemistry. In this work, using a commercial microplate reader, we attempted to optimize the atomic compositions of green-emitting phosphor Y1−x−yCexTbyBO3 and red-emitting phosphor Y1−x−y−zCexTbyEuzBO3. We filled 48 individual wells of an alumina microplate with aqueous solutions of nitrates of Y3+, Ce3+, Tb3+, and Eu3+ with different compositions, and then added an aqueous solution of boric acid to each well. After drying, the microplate was heated at 550 °C for 2 h in air, and then at 1100 °C for 3 h in a reducing atmosphere. Y1−x−yCexTbyBO3 absorbed near ultraviolet light through 4f → 5d transitions of Ce3+ and emitted green fluorescence corresponding to 4f → 4f transitions of Tb3+ through Ce3+ → Tb3+ energy transfer. Moreover, Y1−x−y−zCexTbyEuzBO3 emitted red fluorescence corresponding to 4f → 4f transitions of Eu3+ through Ce3+ → Tb3+ → Eu3+ energy transfer under near-ultraviolet light. Measurement of the photoluminescence intensity of each well by a microplate reader revealed that the optimized green and red phosphors were Y0.835Ce0.025Tb0.14BO3 and Y0.535Ce0.005Tb0.45Eu0.01BO3, respectively.

Preparation and photoluminescence properties of yellow-emitting CuInS2/ZnS quantum dots embedded in TMAS-derived silica

The degradation of CuInS2 (CIS) quantum dots (QDs) under excitation light due to photo-oxidization by O2 has been a significant problem. Embedding QDs into a matrix to protect them against O2 would improve their photostability. In this paper, hydrophilized CIS/ZnS/ZnS QDs prepared by a ligand exchange method were embedded in silica through a sol–gel method using tetramethylammonium silicate (TMAS) aqueous solution, in which negatively-charged nanoparticles can be well dispersed. QDs modified with 3-mercaptopropionic acid (MPA) were well dispersed into TMAS-derived silica. The obtained monolithic TMAS-derived silica composites containing embedded MPA-modified CIS/ZnS/ZnS QDs exhibited high photoluminescence (PL) quantum yields (43–47%). Changes in PL intensity under continuous excitation were measured to evaluate the photostability of the QDs. The PL intensity of the composite was 105% that of the initial value after 5 h irradiation, while the PL intensities of as-prepared QDs and a PMMA composite decreased to 88% and 92%, respectively. The good gas barrier properties of TMAS-derived silica likely caused the high photostability by preventing O2 from reaching the surface of the embedded QDs.

Electrophoretically Deposited Y2O3:Bi3+,Eu3+ Nanosheet Films with High Transparency for Near-Ultraviolet to Red Light Conversion

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.