Visible Light-emitting Diodes Research Articles

Development of the Calibration System and Characterization of the Avalanche Photodiode for Scintillation Detection

Branches of physics and application technologies have widely utilized avalanche photodiodes (APDs) by coupling of scintillators with advantages of multiplication process and effective photosensitivity in a wide range of visible light wavelength. In addition, to meet the requirements for the purpose of each experiment, various types of APDs have been developed. The calibration system and characteristics of APD were constructed and studied by comparing several calibration methods. Electronics calibration is presented with charge injection and quantum efficiency (QE)-calibrated photodiode. By utilizing visible light-emitting diodes (LEDs) and with conventional electronic characteristics and a scintillation crystal with <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na radioactive source, the APD gain was measured. Furthermore, the following were measured from the calibration method: QE, excess noise factor, and noise level. In this article, we compare our test results with one another, which are provided in the datasheet presented by the manufacturer, and discuss the possibility of standardizing the characterization of APDs by this calibration system.

Efficient Advanced Oxidation Process (AOP) for Photocatalytic Contaminant Degradation Using Exfoliated Metal-Free Graphitic Carbon Nitride and Visible Light-Emitting Diodes

The photocatalytic performance of metal-free graphitic carbon nitride (g-C3N4) was examined using visible light-emitting diodes (LEDs). A comparative and parametric study was conducted using the photocatalytic degradation of phenol as a model reaction. The g-C3N4 photocatalyst was synthesized from melamine using thermal condensation, followed by a thermal exfoliation that increases the catalyst surface area from 11 to 170 m2/g. Different characterization techniques, namely X-ray powder diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption using the Brunauer–Emmett–Teller method, ultraviolet-visible (UV–vis) spectroscopy, transmission electron microscopy, photoluminescence spectroscopy (PL), and zeta potential analysis, were used to characterize the photocatalyst. A comparison of the photodegradation experiments conducted with a full-spectrum xenon lamp and a custom-made single-wavelength LED immersion lamp showed that the photocatalyst performance was better with the LED immersion lamp. Furthermore, a comparison of the performance of exfoliated and bulk g-C3N4 revealed that exfoliated g-C3N4 completely degraded the pollutant in 90 min, whereas only 25% was degraded with bulk g-C3N4 in 180 min because the exfoliated g-C3N4 enhances the availability of active sites, which promotes the degradation of phenol. Experiments conducted at different pH have shown that acidic pH favors the degradation process. The exfoliated g-C3N4 has shown high photocatalytic performance in the photodegradation of other phenolic compounds, such as catechol, m-cresol, and xylenol, as well.

One/two-photon sensitive sulfonium salt photoinitiators based on 1,3,5-triphenyl-2-pyrazoline

Three 1,3,5-triphenyl-2-pyrazoline-based sulfonium salt photoinitiators (PIs) were synthesized, and the relationship between their molecular structures and photophysical and photochemical properties under one/two-photon irradiation were presented. The sulfonium salts had absorption peaks from 343 nm to 372 nm that overlapped the emission of 365–425 nm light-emitting diode (LED) light sources. The energy levels of molecular orbitals were closely related to the quantum yield of photoacid generation (ΦH+) as determined by the electrochemical and absorption properties of the PIs, and the introduction of an electron-pushing group could increase ΦH+ due to the enlarged energy gap between the singlet excited states S1 and S2. Interestingly, the wavelength dependence of ΦH+ was due to the nature of the transition. The π–π* transition showed high ΦH+ values (0.56–0.66) under excitation by UV LEDs, while the π–σ* transition showed low efficiency under visible LED irradiation. Intramolecular charge transfer from the pyrazoline ring to sulfonium salts induced efficient photolysis and high ΦH+. The sulfonium salts could efficiently initiate cationic polymerization and free radical polymerization. In addition, their high two-photon absorption cross-sections (δ780 nm = 75–105 GM) reduced the threshold of two-photon microfabrication. These PIs should have potential in LED photocuring and two-photon 3D microfabrication.

Methyl Benzoylformate Derivative Norrish Type I Photoinitiators for Deep-Layer Photocuring under Near-UV or Visible LED

We designed and prepared a series of methyl benzoylformate (MBF) derivatives Norrish type I photoinitiators (MBFs) for light-emitting diode (LED)-induced photopolymerization through computer simula...

Abatement of 1,2,4-Trichlorobencene by Wet Peroxide Oxidation Catalysed by Goethite and Enhanced by Visible LED Light at Neutral pH

There is significant environmental concern about chlorinated organic compounds (COCs) in wastewater, surface water, and groundwater due to their low biodegradability and high persistence. In this work, 1,2,4-trichlorobenzene (124-TCB) was selected as a model compound to study its abatement using wet peroxide oxidation at neutral pH with goethite as a heterogeneous catalyst, which was enhanced with visible monochromatic light-emitting diode (LED) light (470 nm). A systematic study of the main operating variables (oxidant and catalyst concentration and irradiance) was accomplished to investigate their influence in the abatement of 124-TCB in water. The reaction was carried out in a well-mixed reactor of glass irradiated by a visible LED light. The hydrogen peroxide concentration was tested from 0 to 18 mM, the goethite concentration within the range 0.1–1.0 g·L−1 and the irradiance from 0.10 to 0.24 W·cm−2 at neutral pH. It was found that this oxidation method is a very efficient technique to abate 124-TCB, reaching a pollutant conversion of 0.9 when using 0.1 g·L−1 of goethite, 18 mM of H2O2, and 0.24 of W·cm−2. Moreover, the system performance was evaluated using the photonic efficiency (ratio of the moles of 124-TCB abated and the moles of photons arriving at the reactor window). The maximum photonic efficiencies were obtained using the lowest lamp powers and moderate to high catalyst loads.

In situ Observations of Porcine Ocular Surface Cells with Handheld 2K-pixel Microscope

Introduction: To present our findings of the porcine ocular surface that were obtained with an ultra-compact hand-held microscope that weighs less than 500 g, we examined the corneal epithelial cells with this hand-held microscope. Methods: This device is equipped with an automatic focusing mechanism that enabled us to observe living cells in macro to micro magnifications with a series of operations. The focus is semi-automatically adjusted by the infrared and ultrasonic distance sensor. The instrument has a commercially-available microscope objective lens of 20x or 40x magnification and has a high-resolution 2K Complementary Metal-Oxide-Semiconductor (CMOS) camera. The theoretical spatial resolution is around 300 nm with a higher Numerical Aperture (high-NA) lenses. The widefield reflectance-based imaging system is equipped with three-color visible Light-Emitting Diodes (LEDs) for use in bright environments and an infrared LED for dark environments. Ten normal and two injured porcine corneas were examined with this hand-held microscope. Results: Our observations showed that the corneal and conjunctival epithelial cells could be continuously observed. The epithelial cells of the central cornea, limbus, and conjunctiva were clearly seen. The epithelial cells on the injured corneal surface were also easily and clearly observed. Conclusion: This hand-held microscopic imaging device allows medical health care workers such as ophthalmologists and endoscopists to obtain real-time in vivo optical biopsies without collecting tissues and cells. Our system enables us to observe single cells in the superficial layers without any fluorescein or other dyes.

Strategically constructed patterned sapphire with silica array to boost substrate performance in GaN-based flip-chip visible light-emitting diodes

A strategically constructed substrate, patterned sapphire with silica array (PSSA), was developed to boost the efficiency of patterned sapphire substrate (PSS) in GaN-based light-emitting diodes (LEDs) application. The light output power of a flip-chip LED on PSSA improved by 16.5% at 120 mA than that of device grown on PSS. The XRD and STEM measurements revealed that the GaN epilayer grown on PSSA had better crystalline quality compared to the epilayer grown on PSS, which was the result of decreased misfit at coalescence boundary in the PSSA case. Moreover, the light extraction efficiency of the flip-chip LED on PSSA was significantly enhanced, benefiting from the small refractive-index contrast between the patterned silica array and air. This small refractive-index contrast also contributed to a more convergent emission pattern for the flip-chip LED on PSSA, as demonstrated by the far-field radiation pattern measurements. The discovery that PSSA could excel at defect suppression and light extraction revealed a new substrate platform for III-nitride optoelectronic devices.

Facile synthesis of CuCr2O4/BiOBr nanocomposite and its photocatalytic activity towards RhB and tetracycline hydrochloride degradation under household visible LED light irradiation

Here we report the development of CuCr2O4/BiOBr nanocomposite photocatalysts for the first time to serve the purpose of rhodamine B (RhB) and tetracycline hydrochloride (TC-HCl) degradation using household visible light emitting diode (LED) light. In this nanocomposite, sol-gel combustion made spherical shaped CuCr2O4 nanoparticles are decorated on BiOBr plates in single pot via precipitation method using cetyltrimethylammonium bromide (CTAB) as the Br source as well as the template. The present investigation highlights a significant advancement to tackle the challenge of visible light induced photocatalytic degradation of harmful organic dye RhB and antibiotic-pollutant TC-HCl. The 20%CuCr2O4/BiOBr nanocomposite exhibits high degradation activity for both RhB (96.0% in 15 min) and TC-HCl (96.7% in 300 min) in presence of LED light with good recycling characteristics. Detailed structural and optical characterization using XRD, BET, FESEM, TEM, HRTEM, EDX, XPS, UV−Vis DRS, PL spectroscopy and Raman spectroscopy have been carried out prior performing photocatalytic activity tests. The structural as well as optical data have revealed a significant improvement in visible light absorption of BiOBr in presence of CuCr2O4. Based on energy band modification at the heterojunction of nanocomposite materials and their radical trapping ability, a plausible mechanistic path of photocatalytic degradation is presented.

Measurement of nitrogen dioxide and nitric oxide densities by using CEAS (cavity‐enhanced absorption spectroscopy) in nonthermal atmospheric pressure air plasma

AbstractNonthermal atmospheric pressure plasmas are widely used in biomedicine and agriculture. Currently, radical density is used as an important parameter for the development of plasma biomedicine and agriculture. Especially, nitrogen dioxide (NO2) and nitric oxide (NO) radicals are significant species in these applications. These could be measured by using the cavity‐enhanced absorption spectroscopy (CEAS) technique. For the NO2 and NO radical density measurement, we used a visible LED (light‐emitting diode) of 660 nm and a mid‐infrared LD (laser diode) of 5238.6 nm, respectively. Radical densities can be calculated with the transmission ratio using Beer–Lambert law, which is obtained by the absorbed amount of laser intensity passed through the gas from the plasma jet in an optical cavity of CEAS.

Efficient light-emitting diodes from mixed-dimensional perovskites on a fluoride interface

Light-emitting diodes based on halide perovskites have recently reached external quantum efficiencies of over 20%. However, the performance of visible perovskite light-emitting diodes has been hindered by non-radiative recombination losses and limited options for charge-transport materials that are compatible with perovskite deposition. Here, we report efficient, green electroluminescence from mixed-dimensional perovskites deposited on a thin (~1 nm) lithium fluoride layer on an organic semiconductor hole-transport layer. The highly polar dielectric interface acts as an effective template for forming high-quality bromide perovskites on otherwise incompatible hydrophobic charge-transport layers. The control of crystallinity and dimensionality of the perovskite layer is achieved by using tetraphenylphosphonium chloride as an additive, leading to external photoluminescence quantum efficiencies of around 65%. With this approach, we obtain light-emitting diodes with external quantum efficiencies of up to 19.1% at high brightness (>1,500 cd m−2). Green perovskite light-emitting diodes with external quantum efficiencies of up to 19.1% at high brightness can be created by depositing an ultrathin layer of strongly polar lithium fluoride between the perovskite and hole-transport layers.

Benzylidene piperidones as photosensitizers for visible light photopolymerization

Benzylidene piperidones (CZ-PD, TPA-PD, and PTZ-PD) with conjugated structure were synthesized by the aldol condensation of aromatic aldehyde and N-benzyl piperidone. These D-π-A-π-D dyes showed good visible light absorption and photosensitizing properties to iodonium salts in visible light-emitting diode (LED) photopolymerization. The photophysical properties of the synthesized structure were investigated via UV–vis spectroscopy and density functional theory (DFT) calculation. The mechanisms of structure-sensitized iodonium salts were explored through steady-state photolysis experiments. Polymerization experiments proved that these dyes with iodonium salt were an effective photoinitiating system for radical photopolymerization under visible LED light source and showed the ability to initiate deep curing.

Photoacoustic imaging demonstration

Experiments [T. J. Allen and P. C. Beard, “High power visible light emitting diodes as pulsed excitation sources for biomedical photoacoustics,” Biomed. Opt. Express 7(4), 1260–1270 (2016)] describe the use of a visible light-emitting diode (LED) as an alternative to using a Q-switched Nd:YAG laser as a photoacoustic excitation (1 mJ pulse) source for below-the-surface vascular imaging. They describe the feasibility of using a low-cost LED, overdriven by a factor of 10 in current, with short pulses ∼200 ns operating at a 0.01% duty cycle to achieve ∼10 μJ in an imaging experiment involving hemoglobin absorption (wavelength of 623 nm). A version of the experiment is demonstrated where an SST-90R LED (λ = 620 nm, 18 A maximum CW current) is driven from a home-made MOSFET pulsed driver, capable of providing pulses up to 50 A. Our demonstration of photoacoustic tomography involves a ∼1cm diam LED beam illuminating three closely-spaced vertical 1.4 mm tubes filled with blood (35% haematocrit), mounted in an open acrylic water tank. An immersion transducer (3.5 MHz, cylindrically focused PZT with 60 dB gain) detects time-averaged photoacoustic signals versus rotation angle. Then back-projection imaging can construct a two-dimensional image.

Novel III-V Nitride Polymorphs in the P42/mnm and Pbca Phases.

In this work, the elastic anisotropy, mechanical stability, and electronic properties for P42/mnm XN (XN = BN, AlN, GaN, and InN) and Pbca XN are researched based on density functional theory. Here, the XN in the P42/mnm and Pbca phases have a mechanic stability and dynamic stability. Compared with the Pnma phase and Pm-3n phase, the P42/mnm and Pbca phases have greater values of bulk modulus and shear modulus. The ratio of the bulk modulus (B), shear modulus (G), and Poisson’s ratio (v) of XN in the P42/mnm and Pbca phases are smaller than those for Pnma XN and Pm-3n XN, and larger than those for c-XN, indicating that Pnma XN and Pm-3n XN are more ductile than P42/mnm XN and Pbca XN, and that c-XN is more brittle than P42/mnm XN and Pbca XN. In addition, in the Pbca phases, XN can be considered a semiconductor material, while in the P42/mnm phase, GaN and InN have direct band-gap, and BN and AlN are indirect wide band gap materials. The novel III-V nitride polymorphs in the P42/mnm and Pbca phases may have great potential for application in visible light detectors, ultraviolet detectors, infrared detectors, and light-emitting diodes.

The potential of violet, blue, green and red light for the inactivation of P. fluorescens as planktonic cells, individual cells on a surface and biofilms

Microbial contaminations are among the major health concerns worldwide. New innovative non-thermal technologies, such as photodynamic inactivation (PDI), are emerging as mild antimicrobial treatments. This research focussed on the effects of four different colours of visible light-emitting diodes (violet, blue, green and red, at 400, 470, 522 and 644 nm) on three different types of P. fluorescens contaminations: planktonic cells, individual cells on a surface and biofilms. The results demonstrated that only violet light inactivates planktonic P. fluorescens in a suspension (0.5-log reduction). Individual cells on a surface could be inactivated (i) effectively by violet light (3.7-log reduction in 30 min) when the cells were at optimal conditions and (ii) below the detection limit by both violet and blue light when salt stress was added to the environment. Finally, for the hydrated biofilms on polystyrene surfaces, no inactivation was achieved with any of the light sources. This study suggests that PDI is a promising technology for decontamination and preservation applications in the food industry where continuous lighting can be easily applied. It is considered more suitable as a technology to prevent microbial proliferation by intensive implementation than as a method to disinfect developed microbial populations.

Bottom tunnel junction blue light-emitting field-effect transistors

A recent thrust toward efficient modulated light emitters for use in Li-Fi communications has sparked renewed interest in visible III-N InGaN light-emitting diodes (LEDs). With their high external quantum efficiencies, blue InGaN LEDs are ideal components for such devices. We report a method for achieving voltage-controlled gate-modulated light emission using monolithic integration of fin- and nanowire-n–i–n vertical FETs with bottom-tunnel junction planar blue InGaN LEDs. This method takes advantage of the improved performance of bottom-tunnel junction LEDs over their top-tunnel junction counterparts, while allowing for strong gate control on a low-cross-sectional area fin or wire without sacrificing the LED active area as in lateral integration designs. Electrical modulation of five orders and an order of magnitude of optical modulation are achieved in the device.

Shining Light on Poly(ethylene glycol): From Polymer Modification to 3D Laser Printing of Water Erasable Microstructures.

The implementation of stimuli-responsive bonds into 3D network assemblies is a key concept to design adaptive materials that can reshape and degrade. Here, a straightforward but unique photoresist is introduced for the tailored fabrication of poly(ethylene glycol) (PEG) materials that can be readily erased by water, even without the need for acidic or basic additives. Specifically, a new class of photoresist is developed that operates through the backbone crosslinking of PEG when irradiated in the presence of a bivalent triazolinedione. Hence, macroscopic gels are obtained upon visible light-emitting diode irradiation (λ > 515 nm) that are stable in organic media but rapidly degrade upon the addition of water. Photoinduced curing is also applicable to multiphoton laser lithography (λ > 700 nm), hence providing access to 3D printed microstructures that vanish when immersed in water at 37 °C. Materials with varying crosslinking densities are accessed by adapting the applied laser writing power, thereby allowing for tunable hydrolytic erasing timescales. A new platform technology is thus presented that enables the crosslinking and 3D laser printing of PEG-based materials, which can be cleaved and erased in water, and additionally holds potential for the facile modification and backbone degradation of polyether-containing materials in general.

Thioxanthone-based amidine: An efficient nonionic photobase generator for thiol-based click polymerization under visible LED light

With the fast development of the long-wavelength irradiation sources, such as light emitting diodes (LED), which possess advantages regarding irradiation safety and energy utilization efficiency, it is highly desired to develop efficient photobase generators (PBGs) sensitive to visible light. Currently, the reported efficient visible light PBGs mainly undergo photodecarboxylation to generate the super bases, while the released CO2 can potentially cause bubbles and cracks in the cured materials. Moreover, these ionic PBGs generally have poor solubility in organic media and limited thermal stability. In the present work, we have investigated a nonionic amidine PBG TX-DBN, which contains thioxanthone as a chromophore and a reduced form of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) as a latent base species. TX-DBN, with a strong absorption above 380 nm, showed excellent solubility in most organic solvents and commercial monomers/oligomers. Under irradiation with an LED lamp at 405 nm, TX-DBN undergoes a CN bond cleavage followed by a fast hydrogen transfer, leading to the generation of the super base DBN without the generation of CO2. The as-released active species, namely super bases and active free radicals, can trigger thiol-epoxy and thiol-ene polymerization to fabricate hybrid network.

Carbon doped ultra-small TiO2 coated on carbon cloth for efficient photocatalytic toluene degradation under visible LED light irradiation

Photocatalytic activity triggered by visible light from light emitting diode (LED) is highly restricted by limited active sites, poor light absorption, and sluggish photo-generated carrier separation of the photocatalysts. Particularly for continuous degradation reaction of volatile organic compounds (VOCs), confined contact between flowing air pollutant gas and photocatalysts renders the degradation ineffective within a transient retention period. Considering the restriction of photocatalytic reaction occurred on semiconductor surface, ultra-small TiO2 (USTiO2) nanoparticles were fabricated by a facile acid-assisted solvent method, which possess copious active sites. A simple heating process was further applied for carbon doping on USTiO2 (C-USTiO2) surface. As a result, the obtained C-USTiO2 exhibited high visible light absorption, rich photo-generated carrier density, and its rapid carrier separation performance. In this study, optimized C-USTiO2 photocatalyst was deposited on carbon cloth and tested for continuous toluene degradation under LED light irradiation. Results showed that the toluene removal efficiency can reach higher than 80% with high concentration of CO2 generated and an excellent cycle stability of over 180 min.

Bismuth vanadate thin films for efficient photoelectrochemical water splitting

Monoclinic bismuth vanadate has attracted much attention as one of the efficient photoanode materials that can harvest a wide range of visible light to split water efficiently. In this study, we report the preparation of nanoporous BiVO4 via wet chemical approach coupled with dip-coating technique. The monoclinic scheelite phase of BiVO4 is revealed by X-ray diffraction (XRD). Characterisation techniques like Raman spectroscopy, diffuse reflectance spectroscopy (DRS) and scanning electron microscopy (SEM) have been analysed and confirmed the formation of BiVO4 layers on FTO. Photoelectrochemical studies of BiVO4 thin films have been performed in a three-electrode potentiostat system, illuminated under visible light-emitting diode (LED) source of intensity 100 mW/cm2. The optimised 10 layered BiVO4 thin film with average thickness of ~ 300 nm shows an optimum photocurrent density of ~ 0.67 mA/cm2 at 1.23 V vs RHE and photoconversion efficiency of 0.4% at 1.23 V vs RHE as compared with different layered BVO photoanodes. The measurements of flat band potential and confirmation of n-type behaviour of BiVO4 semiconductors were carried out by Mott–Schottky analysis. Electrochemical impedance spectroscopy (EIS) study clearly indicate the less charge transfer resistance and high hole transfer mobility of optimised BiVO4 photoanode films.

High speed transmission of signal level for white light emitting diode (LED) as a transmitter device by using modified phase equalization

Visible light communication (VLC) also known as "Li-Fi", uses standard LEDs to transmit data such as information, images, music, and videos. The first LED was developed in 1927 by Oleg Vladimírovich Lósev (1903-1942), however it was not used in the industry until the 1960s.In this paper, will describe the implementation of Modified Phase Equalization (MPH) on visible white LED lamps signal because it's has slow transition time that severely limits in the communication system data speeds with phase equalization that increases the bandwidth of the LED modulation. Employ two filters with Modified Phase Equalization: first, Complementary (C) filter to combine between all kinds moving signals and second, finite impulse response (FIR) filter to obtain all coefficients that respective. Implementation in two phases: first, frequency 150KHz with number of signals 15000 signals and second, doubling frequency 300KHz with number of signals 30000 signals of LEDs without losing their main functionality as illumination sources.