Broad Emission Spectrum Research Articles

Synthesis, electrochemical and photochemical properties of neutral Ir(III) complex based on 3-Iodopyridine-2-carboxylate ligand

In this work, we report the synthesis and characterization of mer-[Ir(ppy)2(Ipic)], where ppy = 2-phenylpyridine and Ipic = 3-iodopyridine-2-carboxylate, as well as, the electrochemical and photophysical properties in solution. The electronic absorption spectra profile can be divided into two main regions: intense bands in the high energy UV-region (200–300 nm) assigned to ligand centered (IL) transition on both the cyclometalated (ppy) and the ancillary (Ipic) ligands, and lower intensity absorption bands located in the less energetic region (320–460 nm) can be experimentally assigned to the spin-allowed metal-to-ligand charge transfer (1MLCT) transition and the spin-forbidden 3MLCT. From the electrochemical results, were obtained the oxidation peak (Ir(III)→Ir(IV) couple) at +0.59 V resulting in a HOMO of −5.09 eV, and the reduction potentials −1.65 and −2.00 V, which can be attributed to the Ipic and ppy, respectively. Additionally, it is possible to observe that the introduction of the halogen atom promotes a significant variation in LUMO energy (2.75 eV), while the HOMO level is almost unchanged. The complex displayed broad and structureless emission spectra in acetonitrile (λmax = 510 nm, φ = 0.0032 and τ = 160 ns) and dichloromethane (λmax = 510 nm, 525 nm) solution at room temperature but becomes structured (λmax = 490, 522 nm) in propionitrile:butyronitrile at 77 K, indicating that the emitting state at room temperature has a mixed 3MLCT-3LC character but at 77 K the emission has a strong 3LC character.

Three‐ and Four‐Coordinate, Boron‐Based, Thermally Activated Delayed Fluorescent Emitters

AbstractRecent developments in purely organic‐material‐based thermally activated delayed fluorescence (TADF) emitters are helping to make them suitable for commercial applications in the near future. In spite of their high external quantum efficiencies, the broad emission spectra and short device lifetimes are the main barriers to using TADF emitters. Among the classes of materials, boron‐embedded polycyclic aromatic compounds have shown potential as TADF emitters that overcome the technical issues. In particular, highly efficient boron‐based TADF emitters with the advantage of a narrowband emission spectrum have been reported recently relying on the concept of a multi‐resonance effect. Hence, boron TADF emitters have the potential to become next‐generation emitters. Based on the future scope of the boron‐based TADF emitters (especially in development of blue TADF emitters), three‐ and four‐coordinate boron TADF emitters are reviewed. Additionally, future prospects of boron‐based TADF emitters are discussed to resolve the issues of low efficiency and short lifetimes of blue emitters.

Light-Emitting Diode Excitation for Upconversion Microscopy: A Quantitative Assessment

Lanthanide-based upconversion nanoparticles (UCNPs) generally require high power laser excitation. Here, we report wide-field upconversion microscopy at single-nanoparticle sensitivity using incoherent excitation of a 970 nm light-emitting diode (LED). We show that due to its broad emission spectrum, LED excitation is about 3 times less effective for UCNPs and generates high background compared to laser illumination. To counter this, we use time-gated luminescence detection to eliminate the residual background from the LED source, so that individual UCNPs with high sensitizer (Yb3+) doping and inert shell protection become clearly identified under LED excitation at 1.18 W cm-2, as confirmed by correlated electron microscopy images. Hydrophilic UCNPs are obtained by polysaccharide coating via a facile ligand exchange protocol to demonstrate imaging of cellular uptake using LED excitation. These results suggest a viable approach to bypassing the limitations associated with high-power lasers when applying UCNPs and upconversion microscopy to life science research.

Highly efficient yellow‐orange emission and superior thermal stability of Ba2YAl3Si2O12:Ce3+ for high‐power solid lighting

AbstractWith great economic benefits, white LEDs (w‐LEDs) have aroused worldwide attention. For phosphor‐converted w‐LED, highly efficient emission and good thermal stability of phosphor are significant parameters in practical application. Here, a yellow‐orange garnet‐structural phosphor, Ba2YAl3Si2O12:xCe3+ (x = 0‐0.1) (BYAS:xCe3+) was developed by solid solution design. The broad emission spectrum of the as‐synthesized phosphor could guarantee the effective increase of the color rendering index when it is combined with the InGaN blue chip. Benefiting from the garnet‐type highly rigid framework, BYAS:xCe3+ exhibits an excellent thermal stability (50%@673K of the initial integrated intensity at 280 K) as well as high absolute quantum efficiency (80.4%@460 nm excitation light). Utilizing the approach of “phosphor‐in‐glass” (PiG), a high‐power warm w‐LED is achieved based on a blue LED plus PiG and the illuminance of this w‐LED device can be as high as 4227 lx.

Exciton Relaxation Dynamics in Perovskite Cs4PbBr6 Nanocrystals.

Lower-dimensional metal halide perovskites have been recognized as an efficient white light emitter. The broad band emission spectrum originates from the recombination of excited charge carriers through free excitons (FEs), self-trapped excitons (STEs), and defect-trapped excitons. However, the emission properties of zero-dimensional (0-D) perovskites have not been explored extensively. Here, in this work, we have performed low-temperature absorbance, photoluminescence (PL), PL excitation (PLE), PL lifetime, and Raman measurements to understand the exciton relaxation processes in Cs4PbBr6 NCs. Our experimental observations indicate that two distinct UV light spectra evolved from the photoexcited carrier recombination through FE and STE states. We emphasize that such UV light sources can be beneficial for various applications, like curing of materials, disinfection of viruses, hygiene control, etc.

Structural and dielectric properties of Cu-doped α-ZnMoO4 ceramic system for enhanced green light emission and potential microwave applications

In this article, the synthesis of the Zn1-xCuxMoO4 ceramic system was carried out by a solid-state reaction route, and their structural, morphological, and optical properties were investigated. The samples were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, FT-IR spectroscopy, and UV–Vis spectrophotometry. Structural analysis confirms the formation of a triclinic structure with space group P $$\stackrel{-}{1}$$ and point group symmetry C1 without any secondary phase formation. The decrease in effective nuclear charge with the introduction of Cu2+ ion decreases the bandgap from 3.57 to 2.78 eV. The potential application in the visible (green) region of the electromagnetic spectrum is supported by the reduction of the bandgap. The increasing Cu2+ concentration is accompanied by Photoluminescence peaks being shifted toward larger wavelength side, covering a broad range of visible region from 300 to 600 nm. The broad range luminescence emission spectra that happened with ZnMoO4 occurred due to the electronic transition between (MoO4)2− complexes or between Mo4d and O2p states. Hakki–Coleman method was applied for the study of microwave dielectric parameters and shows increase of eobs from 8.35 to 12.52 with a simultaneous decrease in dielectric loss (tan δ) from 0.01 to 0.005, respectively. The corresponding quality factor (Q $$\times$$ f) was calculated. The observed high-quality factor could be used as high signal speed microwave materials.

Time-Gated FRET Nanoprobes for Autofluorescence-Free Long-Term In Vivo Imaging of Developing Zebrafish.

The zebrafish is an important vertebrate model for disease, drug discovery, toxicity, embryogenesis, and neuroscience. In vivo fluorescence microscopy can reveal cellular and subcellular details down to the molecular level with fluorescent proteins (FPs) currently the main tool for zebrafish imaging. However, long maturation times, low brightness, photobleaching, broad emission spectra, and sample autofluorescence are disadvantages that cannot be easily overcome by FPs. Here, a bright and photostable terbium-to-quantum dot (QD) Förster resonance energy transfer (FRET) nanoprobe with narrow and tunable emission bands for intracellular in vivo imaging is presented. The long photoluminescence (PL) lifetime enables time-gated (TG) detection without autofluorescence background. Intracellular four-color multiplexing with a single excitation wavelength and in situ assembly and FRET to mCherry demonstrate the versatility of the TG-FRET nanoprobes and the possibility of in vivo bioconjugation to FPs and combined nanoprobe-FP FRET sensing. Upon injection at the one-cell stage, FRET nanoprobes can be imaged in developing zebrafish embryos over seven days with toxicity similar to injected RNA and strongly improved signal-to-background ratios compared to non-TG imaging. This work provides a strategy for advancing in vivo fluorescence imaging applications beyond the capabilities of FPs.

Progress and perspective on CsPbX3 nanocrystals for light emitting diodes and solar cells

All-inorganic perovskite CsPbX3 nanocrystals (NCs) have made remarkable achievements in optoelectronic applications due to their enhanced stability, low-cost, easy-to-perform synthetic routes, broad emission spectra tunability, and high photoluminescence quantum yields, especially for light emitting diodes (LEDs) and solar cells (SCs). In this perspective, the structure and optoelectronic properties of CsPbX3 NCs are reviewed, and the recent research progresses in LEDs and SCs are presented. Finally, a brief outlook of this field is proposed to point out some important challenges and possible solutions.

Compositional study of mixed halide perovskite films CH3NH3Pb(I1-xBrx)3 and CH3NH3Pb(I1-xClx)3 prepared by close space sublimation

The preparation and characterization of homogeneous and crystalline films of the perovskites CH3NH3Pb(I1-xBrx)3 and CH3NH3Pb(I1-xClx)3 obtained by the close space sublimation method are reported. Perovskites with a mixture of halides were prepared by the sublimation of the methylamine halide CH3NH3Br or CH3NH3Cl over predeposited lead iodide films at different sublimation times to modify the I:Br or I:Cl ratio. The perovskite films CH3NH3Pb(I1-xBrx)3 showed a bandgap shift. For CH3NH3Pb(I1-xClx)3 films with sublimation times of 2.5−40 min, no changes were observed; however, for longer times, the absorption spectrum became similar to that of the CH3NH3PbCl3 material. In this study, the mixed halide perovskites CH3NH3Pb(I1-xBrx)3 and CH3NH3Pb(I1-xClx)3 were prepared with just two precursors, instead of the common approaches that use PbI2, PbBr2, MAI and MABr, as well as PbI2, PbCl2, MAI and MACl. Perovskite layers were applied for the fabrication of solar cell devices, and efficiencies of 9.9 % and 10.2 % were achieved for CH3NH3Pb(I1-xBrx)3 and CH3NH3Pb(I1-xClx)3, respectively. In addition, perovskite LEDs were also prepared. While the well-known green emission at 533 nm was observed for CH3NH3Pb(I1-xBrx)3, for the CH3NH3Pb(I1-xClx)3 based-devices, a broad emission spectrum similar to a warm white emission was obtained. Therefore, a new approach through the sublimation method is reported to fabricate solar cells and LEDs where the main parameter to regard is the sublimation time.

Pyrrolopyrazine-based triads decorated with donor-acceptor groups: pH and polarity induced visible color switching sensors

Three series of pyrrolopyrazine-based triads functionalised with strong donor-acceptor groups were obtained by twofold Suzuki cross-coupling reactions under microwave irradiation. The resulting push-pull chromophores were intensively colored. Their properties can be tuned by proper chemical engineering. The photophysical, thermal, and electrochemical properties were thoroughly investigated. The coupled computational and classic synthetic strategies show that subtle changes in the terminal group, modulates the optical and electrochemical properties, resulting in broad absorption and emission spectra. The prepared compounds revealed large Stokes shift, acidochromism, positive solvatochromism and color tunability in various solvents.

TSL and OSL properties of SPS-derived CsBr transparent ceramics doped with various concentrations of Eu

We have investigated doping concentration dependence of optically-stimulated luminescence (OSL), as well as the fundamental optical and thermally-stimulated luminescence (TSL), in Eu-doped CsBr transparent ceramics. The photoluminescence, TSL and OSL showed a broad emission spectrum peaking around 440 nm, and the emission origin was the 5d-4f transitions of Eu2+. The emission peak position shifted to longer wavelength with increasing the concentrations of Eu. For OSL, the dose measurement sensitivity was confirmed to be as low as 0.01 mGy when the doping concentration of Eu was 0.3–1.0%; therefore, the optimal concentration of Eu was determined to be 0.3–1.0%.

Field emission and electron energy distributions from point-type triangular-shaped emitters made of thin graphene films

We report on fabrication and detailed characterization of point-type graphene emitters, which can be further used as electron sources in modern vacuum devices. Two-dimensional free-standing and triangular-shaped field emitters with the tip angles of 30°, 60°, and 90° and a height of ∼1 mm were made of thin graphene films of varying thicknesses (2.5-μm and 8-μm). The field emission properties of these emitters were systematically investigated by different measurement techniques. Wider and thicker graphene film emitters exhibited better stability and provided higher emission currents (up to ∼100–420 μA). The short-term current fluctuations stayed within 8%–14%. The graphene film emitter with a tip angle of 90° and 8-μm thick yielded a high field emission current of up to 2.2 mA at 9 V/μm. All emitters reproducible showed a non-linear Fowler–Nordheim behavior, which was correlated with the electron energy spectroscopy results. High-resolved energy spectra showed that up to three discrete peaks can be observed from the graphene edges at currents of <1 μA. The integral energy distributions of electrons at higher currents showed single broad emission spectra with a width of up to 1–2 eV. Additionally, graphene point emitters were characterized in various vacuum environments (Xe, Ar, N2, H2, O2, and air), different pressure levels (2 × 10−4 Pa, 2 × 10−3 Pa, and 2 × 10−2 Pa), and at various distances between the anode and the graphene emitter tip. The results showed that graphene emitters can operate in non-ultrahigh vacuum conditions, and further optimization of the vacuum gap could result in a lower turn-on voltage.

Broadband near-infrared emitting from Li1.6Zn1.6Sn2.8O8:Cr3+ phosphor by two-site occupation and Al3+ cationic regulation

Broadband sources of near-infrared (NIR) radiation have garnered considerable attention in medical applications and non-destructive analysis. In this work, we have synthesized a lithium zinc stannate phosphor Li1.6Zn1.6Sn2.8O8:Cr3+ (LZSO:Cr) for broadband NIR light sources. The LZSO:Cr phosphor exhibited broadband excitation band matches exceptionally well with the emission of commercial 450-nm blue light-emitting diode (LED) chips. As expected, the broadband emission of infrared radiation from 650 nm to 1200 nm was realized due to the unique crystal field environment, where Cr3+ ions were substituted in two distorted octahedral sites. The broad emission spectrum exhibited peaks around 830 nm and 930 nm with a full width at half maximum (FWHM) of ~190 nm and internal quantum efficiency of 53%. The FWHM could be tuned to ~220 nm by co-doping with Al3+ ions because of the crystal splitting. The luminescence properties including excitation, emission, decay time, and their relationship between the lattice crystal properties have also been investigated. A phosphor-converted LED (pc-LED) was packaged by combining the phosphor with a 450-nm blue LED chip, which proves the feasibility of our design strategy for broadband NIR light sources.

Highly luminescent and stable lead-free cesium copper halide perovskite powders for UV-pumped phosphor-converted light-emitting diodes

Lead halide perovskites have drawn extensive attention over recent decades owing to their outstanding photoelectric performances. However, their toxicity and instability are big issues that need to be solved for further commercialization. Herein, we adopt a facile dry ball milling method to synthesize lead-free Cs 3 Cu 2 X 5 ( X = I , Cl) perovskites with photoluminescence (PL) quantum yield up to 60%. The optical features including broad emission spectrum, large Stokes shift, and long PL lifetime can be attributed to self-trapped exciton recombination. The as-synthesized blue emissive Cs 3 Cu 2 I 5 and green emissive Cs 3 Cu 2 Cl 5 lead-free perovskite powders have good thermal stability and photostability. Furthermore, UV-pumped phosphor-converted light-emitting diodes were obtained by using Cs 3 Cu 2 I 5 and Cs 3 Cu 2 Cl 5 as phosphors.

Low cost UV sensor using ZnO nanorods on ITO electrodes

Ultraviolet (UV) sensors have gained a lot of importance in the field of environmental sciences, due to their pollution monitoring applications. The current limitations of the UV sensors are their higher temperature operability resulting in high power consumption, degradation and low efficiency. Hence an attempt was made to fabricate room temperature (RT) operable, UV index sensors, using ZnO nanorods on Indium Tin Oxide (ITO) electrodes. The nano rods were grown using hydrothermal method with an optimized concentration and growth duration of 5 mM and 3 h respectively. The vertical rods with a hexagonal face, showed an aspect ratio of six, with weak quantum confinement and a strain of the order of ∼0.002. Nanorods were polycrystalline with an average crystallite size of 50 nm. The broad emission spectra were mostly seen in blue region, with transitions due to singly and doubly ionized zinc interstitials, whose concentration increased with increase in growth duration. The UV response in direct sunlight gave a sensitivity of 118, with rise time and fall time of 35 s and 65 s respectively. The experiments not only yielded reproducible results but the UV index profile for the sensor in response to sunlight, over a full day was in accordance with the profile of the standard WHO sensor. This is of great significance especially from the point of view of ozone depletion, health concerns like skin cancer and environment pollution.

Synthesis of novel Eu2+ activated K3Ca2(SO4)3F down-conversion phosphor for near UV excited white light emitting diode

In the present paper, we have successfully synthesized Eu2+ activated K3Ca2(SO4)3F phosphor by two-step solid-state reaction method at high temperature (800 °C). Pure crystalline formation, morphological behavior, and their vibration bonds of synthesized Eu2+ activated K3Ca2(SO4)3F phosphor were verified by the X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Fourier transform Infrared Spectrum (FT-IR). The photoluminescence (PL) spectrum and CIE coordinate with color purity also characterized. In PL emission spectra clearly represented that broad excitation band from 250 nm to 400 nm ranged. The center of this PL excitation spectra observe at 326 nm, but some parts of these spectra lies to 350 nm–400 nm. This excitation wavelength is very useful for the generation of white light because it may be used as near UV light. PL emission represents broad emission spectra from 400 nm to 550 nm centred at 440 nm. Most part of these PL emission spectra lies in the blue region of the spectrum. PL spectra arise from the transitions between the 5d and 4f orbital’s transition of Eu2+ ions. EVI parameters such as Stokes Shift (ΔEs), the Huang-Rhys factor (S),effective phonon energy (ħω) and the Zero-phonon line (ZPL) are calculated. In our present work, we are calculated that the value of Huang–Rhys factor constant (S) is 3.74 that means coupling is intermediately strong. CIE chromaticity coordinate of K3Ca2(SO4)3F:xmol%Eu2+[x = 0.5, 2.0, 3.0, 5.0, 7.0, 10mol%] phosphor were also calculated by using OSRAM SYLVANIA color calculator 1931. By using these CIE chromaticity coordinate was calculated the value of color purity for each concentration of Eu2+ ions. After seeing all results of synthesized phosphor material we are concluded that it may be a better option used as commercial phosphor for obtaining white light. So it may be a promising candidate for NUV WLEDs.

Properties of Random Lasers in a hollow core photonic crystal fiber

We investigate the properties of random lasers based on light guiding and scattering in a hollow core photonic crystal fiber. We develop air-filled and filled cladding for different fiber core diameters of photonic crystal fibers. The fiber cores are filled with various concentrations of alumina nanoparticles (~50 nm) and Rhodamine 6G in ethylene glycol solution. The effects of light guiding and scattering were studied in terms of filled and air-filled cladding, different fiber core diameters and scattering regimes. Fiber with air-filled cladding provides guiding to the sample leading to random lasing, whereas the sample without scattering does not produce any narrow emission spectra. Less scattering is needed for random lasers in a fiber because too much scattering may hinder guiding in the fiber core leading to high lasing threshold. Both guiding and scattering are needed in random lasers in a fiber as the absence of one of these light phenomena results in broad emission spectra with no lasing threshold. The experimental results are supported with the numerical modeling. We observe that random lasers perform better in the weakly scattering regime than the diffusive regime, which is well predicted by the modeling.

Do dense molecular cores with broad emission spectra at |l| ≈ 5.4°, |b| ≈ 0.4° trace the Galactic bar? A multimolecular line study from HOPS

ABSTRACT We used NH$\rm _3$(1,1) data from the H$\rm _2$O Southern Galactic Plane Survey (HOPS) between −60° < l < −2.7° and 3.9° < l < 30° and |b| < 0.5° to identify dense molecular regions/clumps of the Milky Way Galaxy that are likely to form high-mass stars. We identified ∼500 such clumps and in this paper we report on 14 of these clumps near the Central Molecular Zone that show broad emission spectral lines (with observed velocity widths between 19.8 and 47.6 km s−1 corresponding to intrinsic velocity widths between 7.1 and 25.2 km s−1). We find that these clumps are grouped into three clusters of dense molecular cores centred at l ≈ 5.4°, −5.4°, and −10°. We name them ‘Cluster-1’, ‘Cluster-2’ (also known as ‘Bania’s Clump 1’), and ‘Cluster-3’. We find that the same clumps exhibit broad emission spectra for other molecular lines – NH$\rm _3$(2,2), (3,3), and HC3N(3 − 2). The antisymmetry of Cluster-1 and Cluster-2 in the l − b plane, and the large velocity dispersion of each clump in these two clusters suggest that clusters 1 and 2 may be associated with the Galactic bar potential x−1 orbits. Our assessments show that the clumps of these three clusters host hot gases and their emission-line broadening are associated with shock heating. We find that Cluster-3 is likely to be undergoing high-mass star formation, as suggested by the presence of emission from H$\rm _2$O maser and radio recombination line (H69α) from HOPS.

Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?

Yb:YAG thin-disk (TD) technology has enabled construction of laser/amplifier systems with unprecedented average/peak power levels, and has become the workhorse of many scientific investigations. On the other hand, for some applications, the narrow emission bandwidth of Yb:YAG limits its potential, and the search for alternative broadband TD gain media with suitable thermo-optomechanical parameters is ongoing. The alexandrite gain medium has a broad emission spectrum centered around 750 nm, possesses thermomechanical strength that even outperforms Yb:YAG, and has unique spectroscopic properties enabling efficient laser operation even at elevated temperatures. In this work, we have numerically investigated the power scaling potential of continuous-wave (cw) alexandrite lasers in TD geometry for the first time. Using a detailed laser model, we have compared the potential cw laser performance of Yb:YAG, Ti:Sapphire, Cr:LiSAF, Cr:LiCAF, and alexandrite thin-disk lasers under similar conditions and show that among the investigated transition metal-doped gain media, alexandrite is the best alternative to Yb:YAG in power scaling studies at room temperature. Our analysis further demonstrates that potentially Ti:Sapphire is also a good alternative TD material, but only at cryogenic temperatures. However, in comparison with Yb:YAG, the achievable laser gain is relatively low for both alexandrite and Ti:Sapphire, which then requires usage of low-loss cavities with small output coupling for efficient cw operation.

Size-dependent optical properties of MoS2 nanoparticles and their photo-catalytic applications

While two-dimensional (2D) layered MoS2 nanosheets have been extensively studied owing to their fascinating optoelectronic properties, less attention has been paid to the corresponding zero-dimensional nano-crystals. In this contribution, we report the efficacy of MoS2 nanocrystals for their size tunable properties for optical and photocatalytic applications. We have synthesized differently sized (10–70 nm) crystalline, hexagonal 2H-MoS2 nanoparticles (NPs) dispersed in DMF solvent using a simple exfoliation technique. Synthesized NPs are found to exhibit size-dependent optical properties and excitation-dependent fluorescence characteristics in the visible region, which are otherwise not observed in bulk or 2D MoS2 layers. Size tunable bandgap and broad absorbance and emission spectrum covering the visible range could be exploited in the fabrication of various opto-electronic devices. Charge carrier emission dynamics of differently sized MoS2 NPs are investigated using time correlated single photon counting (TCSPC) spectroscopic technique. We found two time components, one in the order of several hundreds of ps, which arises due to the radiative recombination of charge carriers, while the other one is of the order of a few ns, which emanates from the defect states of MoS2 NPs. The average time constants are found to decrease with increase in particle size. A noticeable photocatalytic activity of the synthesized MoS2 NPs under visible light illumination for the degradation of brilliant green dye is also demonstrated for the first time and the effect of size variation of NPs in the dye degradation process is reported.