Blue Wavelength Region Research Articles

Luminescence enhancement of La4Ti9O24:Eu3+ red-emitting phosphors via cation-substitution of M (Ga, Nb, Ta, Zr, Al) ions

Eu3+- and M (Ga3+, Nb5+, Ta5+, Zr3+, Al3+)-co-doped La4Ti9O24 phosphors were prepared by the solid-state reaction method. The structural phase formation, luminescence properties, and possible LED-lighting application were investigated. The undoped La4Ti9O24 shows an indirect transition with a band-energy of 2.88 ​eV La4Ti9O24 has a broad intrinsic emission band (350–700 ​nm) peaked at 530 ​nm (2.34 ​eV) at 300K. Eu3+-doped La4Ti9O24 presents red luminescence from 5D0→7FJ (J ​= ​0, 1, 2, 3, 4) transitions, which was obviously enhanced with the cation-substitution of M (Ga3+, Nb5+, Ta5+, Zr3+, Al3+) on Ti4+ sites in the lattices. The typical thermal quenching of La3.72Eu0.28Ti8.55M0.45O24 (M ​= ​Al) phosphor is ΔE ​= ​0.342 ​eV. Importantly, the optical excitation of the samples fit well with the commercial LED-chip because of its efficient absorption in near-UV region (395 ​nm) and blue (465 ​nm) wavelength region. The red-LED device packaging with the optimal phosphor was achieved, which has a luminous efficiency of 42 lm/W with a working current of 200 ​mA and a testing voltage of 3.2 ​V. The experiments suggest that the phosphor could be potential for solid-state LED-lighting and display.

Synthesis and Photophysical Properties of Light-Harvesting Gold Nanoclusters Fully Functionalized with Antenna Chromophores.

The development of efficient light-harvesting systems is important to understand the key aspects of solar-energy conversion processes and to utilize them in various photonic applications. Here, atomically well-defined gold nanoclusters are reported as a new platform to fabricate artificial light-harvesting systems. An efficient amide coupling method is developed to synthesize water-soluble Au22 clusters fully protected with pyrene chromophores by taking advantage of their facile phase-transfer reaction. The synthesized Au22 clusters with densely packed 18 pyrene chromophores (Au22 -PyB18 ) exhibit triple-emission in blue, green, and red wavelength regions arising respectively from pyrene monomer, pyrene excimer, and Au22 emission, producing bright white light emission together. The photoluminescence of Au22 is enhanced by more than tenfold, demonstrating that pyrenes at the periphery efficiently channel the absorbed energy to the luminescent Au22 at the center. A combination of femtosecond transient absorption and anisotropy measurements of Au22 -PyB18 explicitly reveals three main decay components of 220 fs, 3.5ps, and 160ps that can be assigned to energy migration between pyrenes and energy transfer processes from pyrene monomer and excimer to the central Au22 , respectively.

Effect of annealing on metal nanoclusters tuned luminescence shift in Ag and Cu embedded nanocomposite glasses

Nanoscale silver and copper clusters are prepared in the soda-lime glass by sequential ion-exchange technique and thermal annealing process. The observed peak values of optical absorption spectroscopy result in the confirmation of embedded Ag and Cu nanoclusters in soda-lime glass. The effect of temperature on the plasmonic response has been investigated for various temperatures. The temperature dependence of surface plasmon resonance (SPR) shifted towards the blue wavelength region, due to growth of silver and copper nanoclusters. Similar to the absorption peak shift, the blue shift in emission spectra has been observed in the photoluminescence spectra of embedded Ag and Cu nanoclusters. The optical bandgap values of Ag and Cu nanoclusters are determined from Tauc's plot. The bandgap variations with luminescence spectra and their emission peak shift with the temperature make them as a promising material for temperature sensors. The FESEM images confirm the cluster distribution of these embedded metal nanoparticles.

The limitations of extending nature’s color palette in correlated, disordered systems

Living organisms have developed a wide range of appearances from iridescent to matte textures. Interestingly, angular-independent structural colors, where isotropy in the scattering structure is present, only produce coloration in the blue wavelength region of the visible spectrum. One might, therefore, wonder if such observation is a limitation of the architecture of the palette of materials available in nature. Here, by exploiting numerical modeling, we discuss the origin of isotropic structural colors without restriction to a specific light scattering regime. We show that high color purity and color saturation cannot be reached in isotropic short-range order structures for red hues. This conclusion holds even in the case of advanced scatterer morphologies, such as core-shell particles or inverse photonic glasses-explaining recent experimental findings reporting very poor performances of visual appearance for such systems.

Lipofuscin in keratinocytes: Production, properties, and consequences of the photosensitization with visible light

A dysfunction in the mitochondrial-lysosomal axis of cellular homeostasis is proposed to cause cells to age quicker and to accumulate lipofuscin. Typical protocols to mediate lipofuscinogenesis are based on the induction of the senescent phenotype either by allowing many consecutive cycles of cell division or by treating cells with physical/chemical agents such as ultraviolet (UV) light or hydrogen peroxide. Due to a direct connection with the physiopathology of age-related macular degeneration, lipofuscin that accumulates in retinal pigment epithelium (RPE) cells have been extensively studied, and the photochemical properties of RPE lipofuscin are considered as standard for this pigment. Yet, many other tissues such as the brain and the skin may prompt lipofuscinogenesis, and the properties of lipofuscin granules accumulated in these tissues are not necessarily the same as those of RPE lipofuscin. Here, we present a light-induced protocol that accelerates cell aging as judged by the maximization of lipofuscinogenesis. Photosensitization of cells previously incubated with nanomolar concentrations of 1,9-dimethyl methylene blue (DMMB), severely and specifically damages mitochondria and lysosomes, leading to a lipofuscin-related senescent phenotype. By applying this protocol in human immortalized non-malignant keratinocytes (HaCaT) cells, we observed a 2.5-fold higher level of lipofuscin accumulation compared to the level of lipofuscin accumulation in cells treated with a typical UV protocol. Lipofuscin accumulated in keratinocytes exhibited the typical red light emission, with excitation maximum in the blue wavelength region (~450 nm). Fluorescence lifetime image microscopy data showed that the keratinocyte lipofuscin has an emission lifetime of ~1.7 ns. Lipofuscin-loaded cells (but not control cells) generated a substantial amount of singlet oxygen (1O2) when irradiated with blue light (420 nm), but there was no 1O2 generation when excitation was performed with a green light (532 nm). These characteristics were compared with those of RPE cells, considering that keratinocyte lipofuscin lacks the bisretinoids derivatives present in RPE lipofuscin. Additionally, we showed that lipofuscin-loaded keratinocytes irradiated with visible light presented critical DNA damages, such as double-strand breaks and Fpg-sensitive sites. We propose that the DMMB protocol is an efficient way to disturb the mitochondrial-lysosomal axis of cellular homeostasis, and consequently, it can be used to accelerate aging and to induce lipofuscinogenesis. We also discuss the consequences of the lipofuscin-induced genotoxicity of visible light in keratinocytes.

Use of poly(ethylene naphthalate) as a self-vetoing structural material

Poly(ethylene naphthalate), PEN, is an industrial polyester which has been shown to scintillate in the blue wavelength region. Combined with measurements of a high intrinsic radiopurity, this has sparked interest in the material for use in low-background experiments.

Structural characterization of porous GaN distributed Bragg reflectors using x-ray diffraction

Porous GaN distributed Bragg reflectors (DBRs) provide strain-free, high-reflectivity structures with a wide range of applications across nitride optoelectronics. Structural characterization of porous DBRs is currently predominantly achieved by cross-sectional scanning electron microscopy (SEM), which is a destructive process that produces local data and has accuracy limited to around 3% by instrument calibration uncertainty. Here, we show that high-resolution x-ray diffraction (XRD) offers an alternative, nondestructive method for characterizing porous nitride structures. XRD scans of porous GaN DBRs show that despite the constant lattice parameter across the DBR layers, characteristic satellite peaks still arise, which are due to the interference between x-rays reflected from the porous and nonporous layers. By comparing the intensities and positions of the satellite peaks through diffraction patterns simulated from a kinematic model, the structural properties of the porous GaN DBRs can be analyzed. Using our method, we have measured a series of DBRs with stop bands from the blue wavelength region to the IR and compared their structural values with those from SEM data. Our results show that the XRD method offers improvements in the accuracy of determining layer thickness, although uncertainty for the value of porosity remains high. To verify the results gained from the XRD and SEM analysis, we modeled the optical reflectivity using the structural values of both methods. We found that the XRD method offered a better fit to the optical data. XRD, therefore, offers accurate, nondestructive characterization of porous DBR structures based on macroscale measurements and is suitable for full wafer analysis.

Novel orange–red emitting phosphor Ba2ScNbO6:Eu3+ for WLEDs: synthesis and luminescence properties

A novel perovskite type orange–red emitting phosphor Ba2ScNbO6:xEu3+ (BSN:xEu3+) was designed and synthesized by solid-state route. The X-ray diffraction, luminescent properties, fluorescent decay times and thermal stability were utilized to characterize the as-prepared samples. The X-ray diffraction confirmed BSN:xEu3+ had a cubic structure with the space group of Pm-3m (221). Photoluminescence (PL) excitation spectra consisted of a broad charge-transfer band and the f–f transition of Eu3+ ions in the violet and blue wavelength regions. The PL spectra displayed orange–red emitting luminescence with the strongest 5D0 → 7F1 transition at 595 nm. The mechanism of energy transfer between Eu3+ ions in BSN has been confirmed to be resonant type via dipole–dipole interaction. Besides, BSN:xEu3+ phosphors exhibit distinct thermal quenching properties, whose emission intensity at 423 K still exceed 71% of the initial value at room temperature. And the CIE coordinate of BSN:0.18Eu3+ phosphor is closed to the Nichia Corporation developed Amber LED NSPAR70BS. All results indicate that BSN:xEu3+ could be a promising phosphors for near-ultraviolet-based white light-emitting diodes.

Quartz thermoluminescence spectra in the high-dose range

The red thermoluminescence (RTL) emission of quartz is associated with advantageous features such as high saturation dose and good reproducibility. Previous studies, however, noted inexplicable RTL glow curve shapes with new peaks at large doses (kGy range). Here we present TL spectra of two granitic quartz samples over the additive γ-dose range 0.1–47.9 kGy. While for doses between 0.4 and 1 kGy the TL spectra are dominated by the red emission at 1.95 eV (630 nm), a blue emission at 2.67 eV (465 nm) becomes prominent for higher doses. For one sample, this blue component completely dominates the spectrum for doses > 12.2 kGy with intensity maxima around 200 °C and > 350 °C (heating rate 2 K s−1). The other sample still contains well resolvable red and blue emissions at the largest dose with similar TL peak positions. Signal saturation for the blue emission in the glow curve range 260–300 °C is not yet reached following an additive γ-dose of 47.9 kGy, whereas the red emission generally shows a more subdued signal response for doses > 5–12 kGy. These findings agree qualitatively with additional monochromatic blue and red TL measurements on the same samples. The evolution of supplementary radiofluorescence spectra over the entire γ-dose range is more complex, but suggests that the sensitisation of the blue wavelength region occurs during heating and not during irradiation and through creation of electron traps rather than recombination centres (most likely [AlO4]0 sites). The sharp sensitivity increase at 1 kGy might likewise be related to alkali ion redistribution and/or the removal of non-radiative competitive recombination pathways. While the blue emission still requires thorough investigation, care should be taken when recording RTL using optical filters since significant portions of the registered TL could originate from the blue component entering the RTL transmission window. In practical terms, the dose-dependent change in relative intensities of blue and red TL emissions might help in detecting exposure to high doses.

Luminous Efficiency of Pd-Doped Ag-Alloy Wire Bonded LED Package after Reliability Tests

In this study, binary Ag-alloy wires were doped with different Pd concentrations, and each wire was encapsulated in an LED package. The initial optical characteristics were tested, and reliability was tested with the high temperature storage life (HTSL), high temperature operating life (HTOL) and wet high temperature operating life (WHTOL). The luminous efficiency of the Ag-alloy wire LED package was about 2% higher than that of the Au wire package, but the addition of 6% Pd to the Ag-alloy wire decreased the luminous efficiency to close to that of the Au-wire LED package. This was due to the high reflectivity of silver in the blue wavelength region, as compared to the low reflectivity of palladium. After 1,000 hours of HTOL and WHTOL, the results showed that the performance of luminous flux maintenance increased with increasing Pd content, indicating that Ag-alloy wires doped with a sufficient amount of Pd can inhibit degradation due to oxidation reaction and thermal and humidity aging. Therefore, binary Ag-Pd alloy wires produced with specific drawing and annealing processes are suitable for mid-power white light LEDs in lighting applications.

A Divide-and-Conquer Strategy for Quantification of Light Absorption, Scattering, and Emission Properties of Fluorescent Nanomaterials in Solutions.

Optical properties of fluorescent materials including their UV-vis absorption, scattering, and on-resonance fluorescence activities are strongly wavelength-dependent. Reported herein is a divide-and-conquer strategy for experimental quantification of fundamental optical constants of fluorescent nanomaterials including their UV-vis absorption, scattering, and on-resonance-fluorescence (ORF) cross-section spectra and ORF fluorescence and light scattering depolarization spectra. The fluorophore UV-vis extinction spectrum is first divided into a blue and a red wavelength region. The UV-vis extinction cross-section spectrum in the blue wavelength region is decomposed into its absorption and scattering extinction spectra straightforwardly using the established polarized resonance synchronous spectroscopic technique. In its red wavelength region, however, the fluorophores can be simultaneous photon absorbers, scatterers, and anti-Stokes-shifted, on-resonance, and Stokes-shifted fluorescence emitters under the resonance excitation and detection conditions. A polarized anti-Stokes'-shifted, on-resonance, and Stokes'-shifted spectroscopic method is developed for quantifying fluorophore absorption, scattering, one-resonance fluorescence (ORF) cross-section spectra, and scattering and ORF fluorescence depolarization spectra in this wavelength region. Example applications of the presented techniques were demonstrated with fluorescent polystyrene nanoparticles, fluorescent quantum dots, and molecular fluorophores Rhodamine 6G and Eosin Y.

Fabrication of Fiber Optic Based Temperature Sensor

The metal oxide semiconductors (ZnO, SnO2, Al2O3 and TiO2) were synthesized by co-precipitation method. The synthesized nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and UV-Visible spectrometer in diffused reflectance (DR) mode. The XRD results stipulated that the ZnO nanoparticle is crystallized in hexagonal wurtzite structure, SnO2 nanoparticles in rutile tetragonal structure, Al2O3 nanoparticle in rombohedral structure and TiO2 nanoparticle in rutile anatase structure. The SEM investigation affirms that all the synthesized nanopowders are composed of uniformly distributed grains. The UV-Vis spectrum proclaimed that the synthesized nanoparticles having the band gap of 3.2 eV (ZnO), 3.3 eV (SnO2) and 3.5 eV (TiO2) respectively. The synthesized nanoparticles were replaced with small cladding region of the optical fiber and act as a temperature sensing materials. The temperature sensing characteristics of the synthesized nanoparticles were investigated for broad wavelength range (200-1000 nm). It reveals that the synthesized Al2O3 nanoparticles were given linear and high sensitivity (∼27) at 697 nm compared with other sensing materials. Further, we have studied the wavelength dependent temperature sensing characteristics of Al2O3nanopowders and it show better sensitivity (∼34) in blue wavelength region (450 nm-495 nm).

Fabrication of alkali metal sulfate–doped zinc tungstate and their photoluminescence

The enhancement of photoluminescence of alkali metal sulfate-doped zinc tungstate was investigated. All samples were prepared by solid-state reaction at 800 °C using ZnO, WO3, and alkali metal sulfates as starting powders. The zinc tungstate phase was confirmed from the samples by X-ray diffraction. The emission peak of the samples excited at 275 nm was observed at 465 nm. Rb2SO4 > K2SO4 > Cs2SO4 > undoped > Na2SO4 > Li2SO4 was the order of peak emission intensity of alkali metal sulfate-doped samples. Alkali metals with a large ionic radius (K, Rb, Cs) enhanced the emission of intrinsic luminescence in the blue wavelength region. Moreover, sulfates had the potential to change the luminescence activities of the samples.

Influence of compositional variation on optical and structural properties of Tb3+ and Dy3+ doped phosphosilicate glasses

The Tb3+ and Dy3+ doped phosphosilicate glasses prepared by melt quenching method are studied for light emitting diode applications. For certain base glass compositions, the crystals Ca3(PO4)2 and CaZn2(PO4)2 are precipitated during the melt quenching process, which induce the narrowness of the full width at half maximum (FWHM) of the emission bands peaked at 574 nm of Dy3+ ions. The intensity of emission bands of Tb3+ ions at 489 and 544 nm is significantly enhanced with the increase of P2O5 content. The ratio of the emission located at the yellow wavelength portion to that located at the blue wavelength region of the Dy3+ ions can be apparently tuned by changing the P2O5 content as well. In addition, the energy transfer between Tb3+ and Dy3+ in the as-prepared glasses is also discussed. The Commission International de I'Eclairage (CIE) chromaticity diagram shows that the emission hue is dependent on the glass composition. The Tb3+ and Dy3+ doped phosphosilicate glasses have promising prospects in light emitting diode applications.

High Speed and High Responsivity Avalanche Photodiode Fabricated by Standard CMOS Process in Blue Wavelength Region

High Speed and High Responsivity Avalanche Photodiode Fabricated by Standard CMOS Process in Blue Wavelength Region

Dy and Eu activated Ca3B2O6 phosphors for near ultraviolet‐based light‐emitting diodes

AbstractThe Dy‐ and Eu‐activated Ca3B2O6 phosphors were synthesized by a high‐temperature solid‐state reaction technique and their structural and luminescent properties were investigated. The phosphors are characterized by X‐ray diffraction, photoluminescence spectra, and Commission International de I'Eclairage (CIE) chromaticity coordinates. It is found that the charge compensator Na+ plays an important role in modifying the emission spectral profiles of Dy and Eu ions in the phosphors. The ratio of the emission located at the yellow wavelength portion to that located at the blue wavelength region of the Dy3+ ions can be apparently tuned by changing the Na+ content. The luminescence intensity of the phosphors can be enhanced with introducing Na+ ions as well. The emission colors of Dy/Eu codoped phosphors change from blue to white and successfully acquire the superior white light emission (x = 0.330, y = 0.329) by appropriately tuning the Na+/Dy3+ content and the excitation wavelength. The energy transfer process from Eu2+ to Dy3+ and Eu3+ occurs in the Dy/Eu codoped phosphors, providing a further approach to modify the emission spectral profile of the examined phosphors. The phosphors presented here have promising applications in the fields of light‐emitting diodes.

The giant butterfly-moth Paysandisia archon has spectrally rich apposition eyes with unique light-dependent photoreceptor dynamics

The palm borer moth Paysandisia archon (Burmeister, 1880) (fam. Castniidae) is a large, diurnally active palm pest. Its compound eyes consist of ~ 20,000 ommatidia and have apposition optics with interommatidial angles below 1°. The ommatidia contain nine photoreceptor cells and appear structurally similar to those in nymphalid butterflies. Two morphological ommatidial types were identified. Using the butterfly numbering scheme, in type I ommatidia, the distal rhabdom consists exclusively of the rhabdomeres of photoreceptors R1–2; the medial rhabdom has contributions from R1–8. The rhabdom in type II ommatidia is distally split into two sub-rhabdoms, with contributions from photoreceptors R2, R3, R5, R6 and R1, R4, R7, R8, respectively; medially, only R3–8 and not R1–2 contribute to the fused rhabdom. In both types, the pigmented bilobed photoreceptors R9 contribute to the rhabdom basally. Their nuclei reside in one of the lobes. Upon light adaptation, in both ommatidial types, the rhabdoms secede from the crystalline cones and pigment granules invade the gap. Intracellular recordings identified four photoreceptor classes with peak sensitivities in the ultraviolet, blue, green and orange wavelength regions (at 360, 465, 550, 580 nm, respectively). We discuss the eye morphology and optics, the photoreceptor spectral sensitivities, and the adaptation to daytime activity from a phylogenetic perspective.

Fabrication of potassium salts doped zinc tungstates prepared with nitrate, sulfate, chloride and their photoluminescence properties

In this work, we found the significant improvement of photoluminescence of zinc tungstate doped with potassium salts. The samples were prepared by solid-state reaction using ZnO, WO3, KNO3, K2SO4, and KCl as starting powders. The powders were mixed with the molar ratios of ZnO: WO3: potassium salts = 1: 1: x and sintered at 800°C for 3h in air. The emission peak of photoluminescence excited at 275nm was observed at 465nm. With increasing potassium salts content, the intensity increased reached its maximum at x = 0.02 and decreased. The tendency of the luminescence intensity for KCl doping was almost the same as KNO3 doping, whereas, for K2SO4 doping, the tendency of the decrease was less steep than other doping. The order of each maximum intensity was KNO3>K2SO4>KCl. It was considered that K replacing Zn in the crystal lattice enhanced the intrinsic luminescence in the blue wavelength region. Moreover, different anions doping had the different effect of the enhancement of the luminescence.

Spectroscopic Influence of Virtual Reality and Augmented Reality Display Devices on the Human Non-Visual Characteristics and Melatonin Suppression Response

It has been elucidated that the responses to light by the human eye influence not only color and brightness recognition but also physiological aspects such as hormones. Melatonin, which affects human circadian rhythms, is sensitive to the blue wavelength region of light. We have investigated the spectroscopic effects of light from virtual reality (VR) and augmented reality (AR) display devices on nonvisual characteristics by means of an analysis of the spectral power distribution (SPD). The circadian illuminance and melatonin suppression value were introduced as representative figures of merits for spectroscopically evaluating nonvisual characteristics. A VR/AR-like custom-made instrument was used to study how the SPD of the display light source affects nonvisual characteristics. Moreover, using multilayer thin-film filters, optimal conditions for minimizing the change of the visual color characteristics of the display while reducing the influence on the non-visual characteristics are also discussed. For the AR device, we found that its spectrum was mostly affected by external light rather than by the spectrum of the AR display itself.

Structural and Optoelectronic Properties of β‐In2S3 Thin Films to be Applied on Cadmium Reduced Solar Cells

In2S3 thin films are prepared by chemical bath deposition (CBD) technique to be applied as buffer layer in CdTe solar cells. CdTe photovoltaic devices are developed using In2S3 as “standard buffer layer” in order to reduce the CdS thickness used as window material. It is important to examine potential thin films in a prospective life cycle study, focusing on direct costs, resource availability, and environmental impacts. Open and closed CBD system influence on the In2S3 physical properties is analyzed. Stable tetragonal β‐In2S3 phase was confirmed by X‐ray diffraction. Electrical properties were determined by four‐point probe technique obtaining a resistivity value of 102 Ω cm. CdTe solar cells performance was studied by measuring J–V characteristics and spectral quantum efficiencies. These results reveal In2S3 thin films as buffer layer reduce the cadmium quantity used in solar cells manufacture and improve their current collection in blue wavelength region (300–500 nm).