Photoluminescence Decay Research Articles

Length-Dependent Photocatalytic Activity of Hybrid Ag-CdS Nanorods

Hybrid metal–semiconductor nanostructures are promising photocatalysts for a wide span of reactions. Determining the relationship of their geometry with catalytic efficiency is critical for optimization of the catalysts. In this work, Ag-CdS nanorods with five different lengths (from 25.0 to 106.5 nm) have been synthesized using a seed-mediated growth method. High-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) studies confirmed that the formation of a metal tip and semiconductor body part and they absorb strongly and broadly in the UV and visible wavelength range. The nanorods have been employed as photocatalysts for methyl orange degradation and their catalytic efficiency exhibited length-dependence. Specifically, the highest efficiency was observed in the rods of intermediate length (72.1 nm). Time-dependent photoluminescence decay revealed that the Ag-CdS rods with high catalytic efficiency have a higher probability to go through charge-separation-recombination pathway than the rods with low catalytic efficiency. Understanding the physical process in these hybrid structures provides insight into fine-tuning their geometry to improve the charge transfer efficiency from the metal to the semiconductor domain. Thus, better hybrid nanomaterials can be designed and fabricated for photocatalytic and other applications.

Optical transitions, exciton radiative decay, and valley coherence in lead chalcogenide quantum dots

We propose the concept of valley coherence and superradiance in the reciprocal space and show that it leads to an $N$-fold decrease of the bright exciton radiative lifetime in quantum dots (QDs) of an $N$-valley semiconductor. Next we explain why, despite this, the exciton radiative lifetimes in PbX (X = S, Se, Te) QDs, measured from the photoluminescence decay, are in the microsecond range. We also address peculiarities of the light-matter interaction in nanostructures made of narrow-gap materials with strong inter-band coupling.

Barrier Molecules Based on Formamidinium for Low-Dimensional Perovskite Solar Cells

Low-dimensional perovskites are attractive materials for solar cells; this mainly includes barrier molecules based on ammonium or diammonium groups. In this work, we used barrier molecules based on a formamidinium (FA) functional group instead of the traditional ammonium functional group. Two FA-based barrier molecules were studied and compared: phenyl acetimidamide chloride (PhACCl) and benzamidine chloride (BzmCl). The main difference between the barrier molecules is the distance of the FA group from the aromatic ring, which results in the formation of two-dimensional (2D) perovskite in the case of PhACCl, whereas BzmCl formed a quasi-1D (one-dimensional) structure based on single-crystal X-ray diffraction (XRD). Absorbance, photoluminescence (PL), and XRD measurements show the existence of a 2D structure for PhACCl. The existence of a 1D phase in perovskite based on BzmCl as the barrier molecule inhibits charge transfer and forms pinholes in the film. Charge extraction and PL decay measurements show a high recombination rate with BzmCl, whereas with PhACCl there is better alignment of the energy levels to the selective contacts; this minimizes the hysteresis and enhances the cycle stability. The photovoltaic performance of low-dimensional n = 7 cells, based on PhACCl, shows 11.5% efficiency, with an open-circuit voltage of 1.08 V. This study investigates the use of FA-based barrier molecules and shows the importance of steric hindrance in the formation of a 2D perovskite structure.

Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties.

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, single-nanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron-hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.

Photoluminescence of Ca0.5Ba0.5Ga2S4:Сe3+, Sm3+ solid solutions over a wide range of temperatures and excitation levels

Сa0.5Ва0.5Ga2S4 solid solutions activated by Ce3+ and Sm3+ ions have been synthesized by solid state reaction and photoluminescence (PL) properties are investigated. PL emission and PL excitation spectra of thiogallates with different concentration of activator ions have been measured. Temperature quenching of PL emission of Ca0.5Ba0.5Ga2S4:Ce3+, Sm3+ solid solutions over wide temperature ranges (10–300 K) are investigated. The influence of optical excitation intensity on position stability and shapes of radiation spectra, as well as the emission efficiency of micropowders under investigation within the range of pulsed laser excitation levels from 10 to 2.2 × 105 W/cm2, has been established. PL decay kinetics of Ca0.5Ba0.5Ga2S4:Ce3+,Sm3+ solid solution micropowders at femtosecond laser excitation are studied in detail.

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Acceleration of the yellow band luminescence in GaN layers via Si and Ge doping

A huge acceleration of the yellow band defect luminescence (YB) with increasing Si and Ge doping concentration in GaN layers has been observed and studied. The donor doping concentrations in the n-type GaN varied from 5 × 10 16 cm −3 to 1.5 × 10 19 cm −3 for undoped, Si-doped, and Ge-doped samples. Consequently, the fastest component of the photoluminescence (PL) decay time curve accelerated from 0.9 ms to 40.3 ns, and the mean decay time from 20 ms to 260 ns with increasing doping concentration. We have proposed an explanation based on a theoretical model of donor-acceptor pair transition (DAP) and electron-acceptor (e-A 0 ) recombination at higher dopant concentrations, which is supported by several measurement techniques as room-temperature radioluminescence (RL), PL measurements or thermally stimulated luminescence (TSL). Last but not least, we have shown a change in the morphology of samples with the increasing dopant concentration, especially 3D columns formation with the high level of Si-doping and its influence on the light extraction from the GaN layer. The paper shows an uncommon way of dealing with slow unwanted YB, which detriment the GaN luminescence properties. • Si and Ge doping of GaN may cause a huge acceleration of yellow band luminescence. • Steady-state photoluminescence intensity remains almost unchanged. • Donor-acceptor and electron-acceptor pair transitions explain the luminescence processes. • Highly Si-doped GaN growths in 3D columns, whereas Ge-doped remains smooth. • The light extraction is influenced by the morphology change.

Synthesis of rare earth doped Si3N4 nanowires with excellent luminescence properties by plasma-assisted direct nitridation method

Si 3 N 4 nanowires doped with rare earth ions (such as Ce 3+ , Tb 3+ , Eu 2+ and Eu 3+ ) were synthesized by plasma assisted direct nitridation method using Si, rare earth oxides and N 2 as raw materials. The prepared doped Si 3 N 4 nanowires were characterized by XRD, EDS, XPS, SEM and TEM. The obtained single-crystal doped Si 3 N 4 nanowires have uniform diameters of about 50–100 nm and lengths of more than 10 µm. The photoluminescence (PL), PL decay curves as well as thermal quenching behaviors of doped Si 3 N 3 nanowires were systematically investigated. This work provides an effective strategy for doping large-size functional atoms in Si 3 N 4 nanowires. • Rare earth doped Si 3 N 4 nanowires were synthesized by a direct nitridation method. • Rare earth doped Si 3 N 4 nanowires have excellent luminescence properties. • The luminescence peaks of doped Si 3 N 4 nanowires are related to the doping type.

Surface-Plasmonic-Coupled Photodetector Based on MAPbI3 Perovskites with Au Nanoparticles: Significantly Enhanced Photoluminescence and Photodetectivity

Organic–inorganic metal halide perovskites (OMHPs) are promising active materials suitable for highly efficient solar cells, photodetectors, light-emitting diodes, and sensors. In this study, methylammonium lead iodide (MAPbI3) thin sheets (TSs) were synthesized as OMHPs using both hybrid vapor-solution method for optical study and anti-solvent solution method for the photodetector. π-Conjugated polyelectrolyte (π-CPE), such as poly(9,9-bis(4′-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was spin-coated on the MAPbI3 TS, and functionalized gold nanoparticles (Au-NPs) were hybridized. The laser confocal microscope photoluminescence (PL) intensity of the MAPbI3 TS was significantly enhanced after hybridization with Au-NPs/FPS-K, owing to the passivating effect of the FPS-K and the generation of extra-photoexcited charges by local surface plasmon resonance (LSPR) coupling with Au-NPs. These results were supported by the variation in the exciton lifetime measured from the time-resolved PL decay curves. The photocurrent of the MAPbI3 photodetector increased up to 1.1 × 104 times, and the photoresponsivity (R) and photodetectivity (D*) increased by 70 and 13 times, respectively, with the hybridization of Au-NPs/FPS-K. The highest D* of the Au-NPs/FPS-K/MAPbI3 photodetector was measured to be 7.5 × 1010 Jones at 735 nm excitation. The power and wavelength dependencies of R and D* for the MAPbI3 photodetector were also significantly improved by the Au-NPs/FPS-K hybrid. These results support the development of high-performance perovskite photodetectors utilizing LSPR coupling with the π-CPE layer.

Low-Temperature, Solution-Based Synthesis of Luminescent Chalcogenide Perovskite BaZrS3 Nanoparticles.

Chalcogenide perovskites constitute a promising earth-abundant, non-toxic, and robust semiconductor family with the potential to compete with hybrid perovskites as high-quality photovoltaic absorbers. However, a low-temperature, solution-based synthesis route has eluded researchers in this area. Here we report the colloidal synthesis of chalcogenide perovskite BaZrS3 nanoparticles at 330 °C in organic solvent. The nanoparticles (10-20 nm) are found to be comprised of smaller (3-5 nm) crystalline domains. Promising optoelectronic properties for the nanoparticles are measured, with photoluminescence decay times as high as 4.7 ns.

Spectroscopic characterizations of Dy3+ ions doped phosphate glasses for epoxy-free white LED applications

The present work illustrates a detailed spectroscopic analysis carried out on dysprosium ions doped Zinc Alumino Sodium Phosphate (ZnAlNaP) glasses with chemical composition of (10-x)ZnO–20Al2O3–10Na2O–60P2O5-xDy2O3 (x = 0.1–2.0 mol%). The XRD spectra verified for an un-doped and Dy3+ ions doped ZnAlNaP glasses demonstrate an enormous hump confirming its amorphous behavior. The FT-IR and Raman spectrum recorded for an un-doped ZnAlNaP and ZnAlNaPDy1.0 glass elucidates various functional groups and vibrational modes involved, respectively. The DSC & TGA studies conducted on an un-doped ZnAlNaP glass revealed its thermal stability and overall weight loss. The absorption spectra were used to calculate the optical bandgap for the as-synthesized glasses and were found to be in the range of 2.9 eV–4.7 eV. Photoluminescence (PL) excitation, emission and decay spectral analysis were done to investigate the suitability of the as-synthesized glasses for epoxy-free solid-state lighting (SSL) devices. The PL emission recorded under 350 nm excitation show two significant bands in blue and yellow regions. Temperature-dependent PL studies conducted on ZnAlNaPDy1.0 glass revealed activation energy of 0.212 eV with a 25.6% loss in PL intensity. The CIE color chromaticity coordinates and correlated color temperature (CCT) were calculated from the PL spectral characteristics. All the spectroscopic investigations conducted on the as-synthesized ZnAlNaPDy glasses finally reveal their superior nature for fabricating epoxy-free white-light emitting diodes (w-LEDs).

Modifier’s influence on spectral properties of dysprosium ions doped lead boro-telluro-phosphate glasses for white light applications

Trivalent dysprosium doped Lead boro-telluro-phosphate glasses were synthesized with varying modifiers such as Li+, Zn2+, Ca2+, Sr2+, and Ba2+ using traditional route of glass preparation (melt quenching). The basic network formation and spectroscopical aspects of the as piped outed glasses was analyzed using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), UV–vis-NIR, photoluminescence (PL) and emission decay measurements. The existence of elementary functional groups pertaining to the present glass network was explored through FTIR spectral analysis. Bonding parameters (δ) were found to analyze the bonding nature of the rare earth ions and the surrounding metal ligand site. The optical bandgap, band tailing parameter and Urbach energy was found using Tauc’s method in order to identify the transparent region of the prepared glass samples. Furthermore, the symmetry of ligands around the Dy3+ ions has been explored with absorption spectral data by inspecting the Judd-Ofelt (JO) parameters and oscillator strengths of the as-synthesized samples. The radiative properties for instance branching ratios (βR), stimulated emission cross-section (σP) and lifetime (τR) of the meta stable states are estimated to realize the novel applications. The CIE colour coordinates and Y/B ratio values were estimated employing the photoluminescence spectra, whereas the colour purity and it’s colour correlated temperature (CCT) were determined following the (x,y) values thus confirms the generation of neutral white light emitting ability of the prepped samples. The decay or lifetime analysis shows the well fitted non-exponential behavior of the present Dy3+ doped Lead boro-telluro-phosphate glasses.

Investigation on luminescence behavior of Mg4Ta2O9 through experimental and first-principles calculation methods

In this work, Mg4Ta2O9 single crystal was synthesized by optical floating zone method in N2 atmosphere. The luminescence behavior of Mg4Ta2O9 single crystal was investigated through experimental and first-principles calculation methods. Its photoluminescence, scintillation and thermal stimulated luminescence properties, including photoluminescence excitation, photoluminescence, photoluminescence decay, thermal stimulated luminescence and X-ray excited luminescence spectra, were measured. In addition, the temperature dependence of photoluminescence, photoluminescence decay and photoluminescence afterglow curves of Mg4Ta2O9 were measured and discussed. Combined with the experimental and density functional theory calculations result, the energy transfer schematic diagrams of the Mg4Ta2O9 were proposed to explain the photoluminescence, scintillation, and thermal stimulated luminescence behaviors.

Characterisation of Sm2+-doped CsYbBr3, CsYbI3 and YbCl2 for near-infrared scintillator application

Fast energy transfer from Yb2+ to Sm2+ is a requirement when using Yb2+ as a sensitiser for Sm2+ emission for near-infrared scintillator applications. This cannot be achieved through dipole-dipole interactions due to the spin-forbidden nature of the involved Yb2+ transition, making the rate of energy transfer too slow for application. This work explores whether exploiting the exchange interaction by increasing the Yb2+ concentration to 99% is an effective way to increase the rate at which energy is transferred from Yb2+ to Sm2+. The scintillation characteristics of CsYbBr3:1%Sm, CsYbI3:1%Sm and YbCl2:1%Sm single crystals were studied through 137Cs excited pulse height spectra, X-ray excited decay and X-ray excited luminescence spectra. An energy resolution of 7% and a light yield of 30,000 ph/MeV was achieved with CsYbI3:1%Sm. Photoluminescence spectroscopy and decay studies were performed to study the band structure and relaxation dynamics.

Near-infrared photoluminescence studies of neodymium ions doped SrO-Al2O3-BaCl2-B2O3-TeO2 glasses for laser and fiber amplifier applications

This paper reports, preparation and characterization of Neodymium (Nd3+) ions doped Oxy Chloro Boro Tellurite (OCBT) glasses. The influence of Nd3+ ions concentration on physical, absorption, emission, and decay properties of OCBT glasses were studied in detailed using spectroscopic techniques. The experimental oscillator strengths measured from the absorption spectral data were subjected for Judd-Oflet (JO) analysis to estimate the best fit JO parametes and the dependent radiative properties of the as prepared glasses. The photoluminescence (PL) spectra recorded under 824 nm laser diode excitation reveal three emission peaks positioned at 887, 1064 and 1340 nm pertaining to 4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions respectively. Correlation of radiative parameters information with PL spectral data allows to estimate various lasing parameters like emission cross section (σsec), gain bandwidth (σsec×ΔλP) and optical gain (σsec×τR). The PL decay spectra recorded for the as prepared glasses for 4F3/2→4I11/2 intense emission transition (observed at 1064 nm) reveal single exponential nature. The experimental lifetimes (τexp) measured from the PL decay profiles are correlated with the radiative lifetimes (τR) measured from the JO analysis to estimate quantum efficiency (η%) of the titled glasses. Among all the as prepared OCBTNd samples, 1 mol% of Nd3+ ions doped OCBTglass sample flourishes highest quantum efficiency. From the productive results obtained, it can be suggested that OCBTNd1.0 glass grabs plenty of applications in solid state lasers technology, fabrication of optical fiber amplifiers and NIR laser applications.

Tunable photoluminescence studies of KZABS: RE3+ (RE3+ = Tm3+, Tb3+ and Sm3+) glasses for w-LEDs based on energy transfer

Here singly and tri-doped (Tm3+, Tb3+, and Sm3+) borosilicate glasses in potassium Zinc alumino borosilicate (KZABS) glasses were prepared by using the melt quenching technique. The tri-doped as-prepared KZABS glasses were characterized through photoluminescence (PL) emission and PL decay to demonstrate red (Sm3+), green (Tb3+) and blue (Tm3+) colors simultaneously and white light realization by varying the Sm3+ ion concentration under 356 nm excitation. The energy transfer mechanism from Tm3+ to Tb3+ (1G4 to 5D4), Tm3+ to Sm3+ (1G4 to 4G7/2), and Tb3+ to Sm3+ (5D4 to 4G7/2) are confirmed by the lifetime values. The I–H model fitted with non-exponential decay curves reveals the dipole-dipole interaction. The temperature-dependent PL (TDPL) measurement and corresponding CIE points demonstrate the thermal stability of the as-prepared glasses. CIE coordinates and CCT values were observed near the standard white light (0.333, 0.333, and 5500 K). The results obtained reveal that Tm3+/Tb3+/Sm3+ tri-doped KZABS glasses could be a potential candidate for the fabrication of visible RGB luminescent materials, display devices and w-LEDs.

A Red-Emitting Hybrid Manganese Halide Perovskite C5H5NOMnCl2·H2O Featuring One-Dimensional Octahedron Chains.

Herein, we successfully synthesized a new organic-inorganic hybrid manganese halide perovskite C5H5NOMnCl2·H2O, in which organic molecules, water molecules (through O atoms), and Cl atoms coordinate with Mn atoms to form deformed [MnO3Cl3] octahedrons. Then, octahedrons form a chain through edge sharing, resulting in a 1D-chain single crystal structure. The high-quality C5H5NOMnCl2·H2O single crystal prepared by a simple solvent evaporation method produced bright red emission at 656 nm attributed to the d-d transition of Mn2+. Also, it has a photoluminescence quantum yield (PLQY) of 24.2%. Photoluminescence excitation and absorption spectra were both featured with multiple bands and were in good agreement with the Mn2+ 3d energy levels. The photoluminescence decay spectrum showed an average lifetime of 0.466 ms, which further proves the d-d transition mechanism. The C5H5NOMnCl2·H2O single crystal had a direct band gap of 1.43 eV. Moreover, a red light LED with a CCT of 1857 K was obtained based on the C5H5NOMnCl2·H2O powder, indicating its promising application in red-emitting LED.

Indent-Free Vapor-Assisted Surface Passivation Strategy toward Tin Halide Perovskite Solar Cells.

Sn halide perovskite solar cells (PKSCs) are the most promising competitors to conventional lead PKSCs. Nevertheless, defects at the surfaces and grain boundaries hinder the improvement of the PKSCs' performance. Liquid surface passivation on the perovskite layer is commonly used to decrease these defects. In the case of tin perovskite solar cells, the liquid passivation improved the open-circuit voltage (Voc). However, this decreased the short-circuit current density (Jsc). We found that this Jsc loss is brought about by the thickness loss after the liquid passivation because tin perovskite layers are partially soluble in common solvents, and the calculated impact pressure was up to 155.4 kPa. Here, we introduce new vapor passivation including solvent and passivation molecules and report efficiency enhancement without decreasing Jsc. The vapor-passivated film showed longer time-resolved photoluminescence decay, smoother morphology, and lower defect densities. Most importantly, the vapor passivation method significantly enhanced the efficiency from 9.41 to 11.29% with Jsc increasing from 22.82 to 24.05 mA·cm-2. On the contrary, the corresponding liquid passivation method gave an efficiency of 10.90% with a decreased Jsc from 22.82 to 22.38 mA·cm-2. A commonly used and simple indent-free surface passivation strategy is proposed to enhance the efficiency and stability of PKSCs.

Excellent long-wavelength-pass filters of CsPbBr3 and CsPb(Cl/Br)3 quantum dot glasses by a Cu2+ quenching strategy

Perovskite C s P b B r 3 quantum dot (QD) glasses doping C u 2 + are reported for excellent long-wavelength-pass filters. Transmittance, optical density (OD), photoluminescence (PL), PL excitation, and PL decay are used to investigate filter properties and the quenching mechanism. At low C u 2 + molar concentrations of 0%, 0.0025%, 0.005%, and 0.01%, the filters exhibit good properties in transmittance and OD. Transition wavelengths are 518, 513, 513, and 512 nm, respectively. The shape and position of transmittance and OD are almost not changed. As C u 2 + concentration increases, PL intensity is monotonously quenched. PL decay is obviously faster. The quenching mechanism is dynamic quenching, which is electron transfer from QDs to C u 2 + . In comparison with the initial intensity, the intensity decreases by factors of 7.6, 12.8, and 18.8. Oxidation and reduction atmospheres are introduced, and the dynamic quenching mechanism is further confirmed. At high C u 2 + concentrations of 0.013%–0.02%, the quenching mechanism is static quenching. Double-anion C s P b ( C l / B r ) 3 QD glasses of different C u 2 + concentrations extend the operation wavelength to the blue light region. For C u 2 + concentrations of 0%, 0.005%, and 0.01%, the transition wavelengths are 470, 472, and 473 nm for C s P b ( C l / B r ) 3 (NaCl as Cl source) QD glasses, and 430, 426, and 445 nm for C s P b ( C l / B r ) 3 ( P b C l 2 as Cl source) QD glasses, respectively.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel.

CdS quantum dots (QDs), synthesized by a sol-gel method, exhibit significantly Stokes shifted bright photoluminescence (PL), predominantly from the trap states. Surprisingly, the PL decay at the emission maximum is single-exponential. This is an unusual observation for as-prepared QDs and indicates a narrow distribution in the nature of trap states. A closer look reveals an additional fast component for the decays at shorter emission wavelengths, presumably due to the band edge emission, which remains elusive in the steady-state spectra. Indeed, a significantly narrower and blue-shifted emission band is observed in the decay-associated spectra. The contribution of this component to the steady-state PL intensity is shown to be overwhelmed by that of the significantly stronger trap emission. Exciton dynamics in the quantum dots is elucidated using transient absorption spectra, in which the stimulated emission is observed even at low pump power.