Light-scattering Centers Research Articles

Highly efficient and thermally robust phosphor-in-glass ceramic (PiGc) for high-brightness laser-driven lighting

How to improve the luminous efficiency (LE) and thermal stability of phosphor conversion materials have always been the focus of high-brightness laser-driven lighting. In this study, we successfully prepared a series of phosphor-in-glass ceramics (PiGc) by conducting a crystallization heat treatment on phosphor-in-glass (PiG) made from LuAG:Ce3+ phosphor and Li2O–Al2O3–SiO2 (LAS) glass via a spark plasma sintering (SPS). Comparing to PiG, the optical and thermal performances of PiGc have been enhanced significantly, including a higher internal quantum effect of 81.2% and lower thermal quenching (it remains 86% at 473k). By precipitating β-spodumene crystals, the coefficient of thermal expansion (CTE) of the PiGc decreased by 15% at 700 °C. Meanwhile, the use of precipitated nanocrystals as 'light-scattering centers' improved luminous efficiency remarkably. Under excitation of 450 nm blue laser with 7.04W/mm2, the LE of the most optimal sample was improved from 209.5 lm/W to 232.2 lm/W. These results indicate that crystallization heat treatment is an effective method for improving the LE and thermal stability of PiG. The PiGc is a highly efficient and thermally robust phosphor conversion materials for high-brightness laser-driven lighting.

Thermoresponsive Hydrogel-Enabled Thermostatic Photothermal Therapy for Enhanced Healing of Bacteria-Infected Wounds.

Photothermal therapy (PTT) has emerged as an attractive technique for the treatment of bacterial infections. However, the uncontrolled heat generation in conventional PTT inevitably causes thermal damages to healthy tissues and/or organs. It is thus essential to develop a smart and universal strategy to regulate the photothermal equilibrium temperature to a preset safe threshold. Herein, a thermoresponsive hydrogel-enabled thermostatic PTT system for enhanced healing of bacteria-infected wounds is reported. In this system, the near-infrared (NIR)-triggered heat generation by photothermal nanomaterials is spontaneously transferred to a thermoresponsive hydrogel with a lower critical solution temperature (LCST), leading to its rapid phase transition by forming considerable light-scattering centers to block NIR penetration. Such a dynamic and reversible process automatically regulates the photothermal equilibrium temperature to the phase-transition point of the LCST-type hydrogel. In contrast to temperature-uncontrolled conventional PTT with severe thermal damages, the thermoresponsive hydrogel-enabled thermostatic PTT provides effective protection on healthy tissues and/or organs, which remarkably accelerates wound healing by efficient bacterial eradication. This study establishes a smart, simple and universal PTT platform, holding great promise in the safe and efficient treatment of bacterial skin infections.

Size-Controllable Metal Chelates as Both Light Scattering Centers and Electron Collection Layer for High-Performance Polymer Solar Cells

An electron collection layer (ECL) between a photoactive overlay and an electrode plays a crucial role in optimizing the light field and charge extraction in bulk-heterojunction (BHJ) polymer solar...

Size-Controllable Metal Chelates as Both Light Scattering Centers and Electron Collection Layer for High-Performance Polymer Solar Cells

An electron collection layer (ECL) between a photoactive overlay and an electrode plays a crucial role in optimizing the light field and charge extraction in bulk-heterojunction (BHJ) polymer solar...

Stimuli responsive optical polymers through omnidirectional and reconfigurable porosity

Manufacturing of porous materials with reconfigurable porosity is beneficial to systems with tailored material properties. This study describes the design and characterization of dynamic omnidirectional porosity in thermoplastic porous networks and their associated applications as novel indicators of temperature and chemical vapors. Herein we report on porous thermo-responsive films that undergo considerable pore size reduction (~40%) and pore density reduction (~86%) when heated to an activation temperature (Ta = 100 °C). These films are synthesized using a solid-state, high-pressure gas saturation and phase separation process on physically crosslinked block copolymers such as poly (styrene-butadiene-styrene) (SBS), and poly (styrene-ethylene/butylene-styrene) (SEBS). The pores act as light-scattering centers, leading to optical opacity of the film. Upon activation, there is a noticeable volume change (~18%) and a concomitant optical opaque-to-transparent transition (OTT) by virtue of the omnidirectional shrinkage of pores. The transparency of these films changes from 45% to 95%, with modulation occurring indiscriminately across a broad spectrum (e.g., UV, visible, and IR). We detail the underlying mechanism for the thermo-responsive dynamic porosity and its interlinked relationship with OTT, and control of OTT temperature onset and its associated shape recovery time. Furthermore, we demonstrate the utility of dynamic porosity for actuation and chemical vapor sensing purposes.

Optimization of the First Step of Enoxaparin Synthesis by Hydrolytic Depolymerization of Unfractionated Heparin

The first step of low-molecular-mass heparin (enoxaparin) preparation by hydrolytic depolymerization of unfractionated heparin was investigated. The step involved chemical reaction of starting heparin and benzethonium chloride in aqueous saline solutions. The number of acid equivalents in unfractionated heparin was shown to increase as the NaCl concentration increased. This was probably related to effects of the solution ionic strength on the heparin macromolecular conformation, which was confirmed using dynamic light scattering. The average size of the observed light-scattering centers in aqueous heparin solutions decreased with increasing NaCl concentration. Rinsing with H2O and not NaCl solutions and application of ultrasonication were recommended to accelerate purification of benzethonium heparinate from starting materials and to reduce the amount of rinse water. The compositions of the produced and purified benzethonium heparinate samples were confirmed using PMR spectroscopy.

Phase composition, microstructure and luminescent property evolutions in “light-scattering enhanced” Al2O3-Y3Al5O12: Ce3+ ceramic phosphors

Classic transparent ceramics for laser gain medium and window materials may benefit from their distinctive features of structural homogeneity and high transparency at large scales. However, this has restricted the ability of the ceramics for local light management. Herein, a strategy for ceramic-phosphor design was conducted in the present work via introducing light-scattering centers into single phased YAG: Ce3+ transparent ceramics, and an enhancement of light extraction was experimentally realized through the control of Al2O3 grain size. UV–vis-NIR diffuse reflectance spectra further confirmed the important roles of the excrescent Al2O3 grains. More importantly, PL characterization shows orange-white light emission with high brightness at high temperature. The results highlight that the unique configuration enables simultaneous control of light propagation, luminous efficiency and thermal stability of luminescence, and the design strategy may create great opportunities for laser lighting and displays with high laser power density.

Luminescent oxygen-sensing films with improved sensitivity based on light scattering by TiO2 particles

We investigated the effects of embedded titanium dioxide (TiO2) nanoparticles on the photocurrent and sensitivity of luminescent oxygen-sensing films based on platinum octaethylporphyrin (PtOEP), which is an indicator dye. The TiO2 nanoparticles acted as light-scattering centers in the sensing films, and the light-scattering performance was evaluated in relation to TiO2 nanoparticle diameter and crystal structure. In the presence of embedded light-scattering particles, the probabilities of PtOEP molecules encountering and being excited by light were expected to increase, resulting in improvement of the photoluminescence and sensitivity of the sensing films to oxygen gas. The diameter of the TiO2 nanoparticles was changed by laser irradiation while they were dispersed in acetone. Through the laser irradiation, the particle size of the TiO2 nanoparticles grew, and their morphology became spherical. The X-ray diffraction (XRD) analysis showed that the crystalline structure of the TiO2 nanoparticles changed in relation to its calcination temperature. The photocurrent and sensitivity of the sensing films with embedded TiO2 nanoparticles in relation to the oxygen gas concentration were measured using a green light-emitting diode (LED) as the light source and an Si photodiode as the photocurrent measuring device. Based on the results, it was confirmed that the photocurrent and sensitivity of the oxygen-sensing films were greatly improved with increases in the size and refractive index (n) of the embedded TiO2 nanoparticles. Furthermore, the improvements in the photocurrent and sensitivity of the oxygen-sensing films allowed the fabrication of a high-resolution oxygen-sensing film that can detect the oxygen distribution over large areas.

Light losses from scattering in luminescent solar concentrator waveguides.

The reductions in the transmission of emission originating from a fluorophore dissolved in a polymer matrix due to light scattering were compared in two forms of planar waveguides used as luminescent solar concentrators: a thin film of poly(methylmethacrylate) (PMMA) spin-coated on a glass plate and a solid PMMA plate of the same dimensions. The losses attributable to light scattering encountered in the waveguide consisting of the thin film of polymer coated on a glass plate were not detectable within experimental uncertainty, whereas the losses in the solid polymer plate were significant. The losses in the solid plate are interpreted as arising from light-scattering centers comprising minute bubbles of vapor/gas, incomplete polymerization or water clusters that are introduced during or after the thermally induced polymerization process.

40% enhanced photocurrent of dye sensitized solar cells using lotus-shaped H2-treated anatase TiO2 with {0 0 1} dominated facets

After hydrogen treatment, lotus-shaped nitrogen and fluorine co-modified anatase TiO2 (NF-TiO2) with {001} facets have been synthesized and employed as photoanode materials in dye sensitized solar cells (DSSCs). In comparison with non-modified TiO2 based DSSC, we obtain a 40% enhancement of photocurrent and a 38% increase of photoelectric conversion efficiency. UV–visible absorption spectroscopy (UV–vis) and Mott-Schottky plots combined with valence band analysis are used to characterize the electronic structure variation and study the synergistic effect of hydrogen treatment. The result shows an enhanced light absorbing ability, is a positively shifted flat band potential and an increased donor density. These improvements can be mainly ascribed to the appropriate electronic structure and the effective light-scattering centers. This work provides systematic and comprehensive research on the application of hydrogenation and {001} facets dominated nitrogen and fluorine co-modified TiO2 in DSSC, and the hydrogenation treatment can lead to improved charge transport and recombination, enhanced light absorbing ability as well as elevated donor density of semiconductor. With these merits, {001} facets dominated HNF-TiO2 nanopowders in this work may find its way in enormous energy and photoelectric fields.

Enhancement of Dye-Sensitized Solar Cell Efficiency by Spherical Voids in Nanocrystalline ZnO Electrodes

Light scattering enhancement is widely used to enhance the optical absorption efficiency of dye-sensitized solar cells. In this work, we systematically analyzed the effects of spherical voids distributed as light-scattering centers in photoanode films made of an assembly of zinc oxide nanoparticles. Spherical voids in electrode films were formed using a sacrificial template of polystyrene (PS) spheres. The diameter and volume concentration of these spheres was varied to optimize the efficiency of dye-sensitized solar cells. The effects of film thickness on this efficiency was also examined. Electrochemical impedance spectroscopy was performed to study electron transport in the electrodes. The highest power conversion efficiency of 4.07 % was observed with film thickness. This relatively low optimum thickness of the electrode film is due to the enhanced light absorption caused by the light scattering centers of voids distributed in the film.

Functional patterns for thin-film-type amorphous silicon solar cells

Three types of patterns acting as light-scattering centers were constructed on bulk glass surfaces and applied to solar cell fabrication. In order to fabricate a scattering center in the transparent conducted oxide (TCO) layer, the SiO2-based solution hydrogen silsesquioxane (HSQ; Dow Corning FOx-16) was used for direct nano-patterning. Direct nano-patterning is a simple and fast process that exploits the solvent permeability of poly(dimethylsiloxane) (PDMS). The nano- and micro-structured SiO2 layer fabricated directs diffused light into the active layer of the solar cell, enabling effective use of this layer. This increases the external quantum efficiency and conversion efficiency of the cell. Additionally, a thinner Si junction was fabricated to ensure the effect of each pattern.

Rapid Electron Injection in Nitrogen- and Fluorine-Doped Flower-Like Anatase TiO2 with {001} Dominated Facets and Dye-Sensitized Solar Cells with a 52% Increase in Photocurrent

Nitrogen- and fluorine-doped flower-like anatase TiO2 (NF-TiO2) with {001} dominated facets was fabricated by a facile hydrothermal method. We investigated its electrical and optical properties, applied it in dye-sensitized solar cells as an electrode material, and achieved an improved conversion efficiency of 8.20%. There was a 52% increase in the photocurrent and 22% improvement in the conversion efficiency. Such improvements mainly arise from the positive-shift flat band, increasing the driving force of electrons and then improving the electron injection efficiency from the lowest unoccupied molecular orbital (LUMO) of the dye and the conduction band of TiO2; from the incorporation of the flower-like anatase NF-TiO2 crystals with exposed {001} facets, serving as effective light-scattering centers and improving the reactivity, both important attributes for achieving a higher light harvesting and thus an improved Jsc and conversion efficiency; and from the narrowed band gap, expanding the response in the...

11th International Conference "Correlation Optics": Propolis films for hybrid biomaterial-inorganic electronics and optoelectronics.

We report on the analysis of optical, polarimetric, and electrical properties of propolis films and hybrid biomaterial-inorganic heterojunctions based on them. It was shown that the material of the propolis films belongs to wide-bandgap optically active substances with the light-scattering centers, which possess complex optical properties. The values of the specific resistance ρ(P)=1.9·10⁷ Ω·cm and dielectric constant ε(P)=19.5 of the propolis film were determined from the spectral distribution of the real and imaginary components of its impedance at room temperature, respectively. The dominating current transport mechanisms through the hybrid bioinorganic heterojunction propolis/p-CdTe were established to be the interface-states-assisted generation-recombination within the depletion region via deep energy levels at forward bias as well as the leakage current through the shunt resistance at reverse bias.

Ultrathin SnO2 Scaffolds for TiO2‐Based Heterojunction Photoanodes in Dye‐Sensitized Solar Cells: Oriented Charge Transport and Improved Light Scattering

In this paper, band-structure matching strategy of a TiO2-based heterojunction within which electrons can be collected from TiO2 nanoparticles and transported rapidly in the bulk structure is reported. On the basis of the band-structure analysis of different TiO2-based heterostructures, focus was directed to the SnO2 nanosheet because of its appropriate band position and high electrical conductivity. Through a systematic investigation of the incorporation of ultrathin SnO2 nanosheet scaffolds for TiO2-based photoanodes in dye-sensitized solar cells (DSCs), we propose an anisotropy "constrained random walk" model to describe the controlled electron transit process. In this system, electrons are transferred orientedly overall, as well as randomly locally, leading to a significant reduction in the charge diffusion route compared to the conventional isotropic "random walk" model. In brief, the 2D ultrathin nanosheets provide rapid transit pathways and improved light-scattering centers, which can ensure a sufficient amount of dye loading and slow recombination. An overall light-to-electricity conversion efficiency as high as 8.25% is achieved by embedding the appropriate amount of SnO2 scaffold in a TiO2-based photoanode.

Effect of photoelectrode morphology of single-crystalline anatase nanorods on the performance of dye-sensitized solar cells

TiO2 terpineol-based pastes with nanorods (NRs) of over 25μm thickness have been prepared for the photoactive electrodes of the dye-sensitized solar cells (DSSCs). The NRs, with a length of approximately 80nm and an aspect ratio of about 3, are made by a two-step hydrothermal process. They have the single crystalline anatase structure and can be dispersed well in water and ethanol. With a high thermal stability and larger surface area (47.2m2g−1) than commercial TiO2 particles (P25, 39.1m2g−1), the well-dispersed anatase NR films with aggregate-free morphology are transparent. For the photocurrent–voltage measurements, the NR cell exhibits high short-circuit photocurrent (JSC) under 1 Sun AM 1.5 simulated sunlight due to the higher surface area and transmittance. The open-circuit voltage (VOC) of NR films is not obviously reduced with incremental thickness, which results from the one-dimensional single crystalline structure of NR due to less surface defects. As compared with the P25 cell, DSSCs made with NRs have a higher fill factor (FF) because of the uniform void spaces. An enhancement of conversion efficiency from 4.88% for P25 to 5.67% for NR is achieved. The P25 particles are incorporated in NR films as light-scattering centers, while the R1P1 containing 50wt.% of P25 has a high VOC and FF as compared with P25, but the JSC is still lower than that of the NR.

Characterization of a Transient Unfolding Intermediate in a Core Mutant of γS-Crystallin

In many age-related and neurological diseases, formerly native proteins aggregate via formation of a partially unfolded intermediate. γS-Crystallin is a highly stable structural protein of the eye lens. In the mouse Opj cataract, a non-conservative F9S mutation in the N-terminal domain core of γS allows the adoption of a native fold but renders the protein susceptible to temperature- and concentration-dependent aggregation, including fibril formation. Hydrogen/deuterium exchange and denaturant unfolding studies of this mutant protein (Opj) have suggested the existence of a partially unfolded intermediate in its aggregation pathway. Here, we used NMR and fluorescence spectroscopy to obtain evidence for this intermediate. In 3.5 M urea, Opj forms a stable and partially unfolded entity that is characterized by an unstructured N-terminal domain and a largely intact C-terminal domain. Under physiologically relevant conditions, Carr–Purcell–Meiboom–Gill T2-relaxation dispersion experiments showed that the N-terminal domain residues were in conformational exchange with a loosely structured intermediate with a population of 1–2%, which increased with temperature. This provides direct evidence for a model in which proteins of native fold can explore an intermediate state with an increased propensity for formation of aggregates, such as fibrils. For the crystallins, this shows how inherited sequence variants or environmentally induced modifications can destabilize a well-folded protein, allowing the formation of intermediates able to act as nucleation sites for aggregation and the accumulation of light-scattering centers in the cataractous lens.

Advantageous Effect of Macroporous TiO<sub>2</sub> Particles on Dye-Sensitized Solar Cells

In dye-sensitized solar cells (DSC), large-size particles in photoanodes can act as light-scattering center to enhance light harvesting efficiency. In this study, macroporous secondary TiO2 particles were prepared by a spray drying method and were introduced to DSC. A few kinds of layered structures were fabricated as photoanodes to evaluate effects of macroporous particles. It was found that open circuit voltage increased due to the presence of macroporous particles. A photoanode composed of stacked nanoparticles and macroporous particles showed better cell performance.

Preparation of sub-micron size anatase TiO2 particles for use as light-scattering centers in dye-sensitized solar cell

The sub-micron size anatase TiO2 particles with size about 0.2-0.3 μm were synthesized with basic peptizing agent and hydrothermal method and added into TiO2 film as light scattering center. The addition of the sub-micron size anatase TiO2 particles enhanced light scattering and dye adsorption abilities of the TiO2 film. When the weight proportion of the sub-micron size/nano-size TiO2 particles in the TiO2 film attained to 1.25/10, the highest energy conversion efficiency about 7.41% was obtained, which was 23% enhancement comparing with the TiO2 film containing pure nano-size TiO2 particles. It presented an efficient way for improving the photovoltaic performance of dye-sensitized solar cell.

Structural Phase Transition of AgOx Sandwiched Between ZnS–SiO2 Protective Layers Under Thermal and Laser Pulse Annealing for Super-Resolution Near-Field Recording

The as-deposited AgOx prepared at an oxygen flow ratio of 0.5 consisted of Ag2O and AgO phases. During thermal annealing, the reduction of AgO into Ag2O, decomposition of Ag2O into Ag and O2, and out diffusion and aggregation of decomposed Ag would take place successively. The chemical decomposition of AgOx film sandwiched between two ZnS–SiO2 protective layers was confirmed to be an irreversible process. As being irradiated by a high power laser pulse similar to the recording process, a hollow Ag cylinder, or ring, serving as an aperture, was formed in the AgOx mask layer, and small Ag particles, serving as light-scattering centers, were precipitated in the center region. During the readout process, the small aperture can significantly reduce the readout laser spot size, while the strong near-field interaction between precipitated Ag particles and sub-wavelength marks can effectively enhance the readout signal. That elucidates the recording and readout mechanisms of super-resolution near-field structure disk with an AgOx mask layer.