Photoluminescence Lifetime Measurements Research Articles

Up-conversion photoluminescence specificity of a hybrid sponge nanostructures

Over the past decades, silicon is proved to be as a promising material for the development of devices in the fields of nanophotonics and optoelectronics. However, the material so popular at the current time did not find its application in nanoscale radiation sources due to the indirect bandgap of the semiconductor, which leads to low quantum efficiency. This work represents experimental results on the features of the silicon up-conversion photoluminescence enhanced by the optical resonances of the plasmonic nanosponge. The internal configuration of the nanostructure was confirmed by scanning transmission electron microscopy. The optical characterisation was provided by the dark-field spectroscopy, up-conversion photoluminescence generation and life-time measurements. The such new nanostructure type is promising for the development of nanoscale sources of broadband radiation and other applications of silicon photonics.

Synthesis and optical properties of π‐conjugated polymers and oligomers bearing hexaphenylbenzene and tetraphenyl ethene units

Abstractπ‐Conjugated polymers (CPs) and oligomers (COs) bearing hexaphenylbenzene (HPB) and tetraphenyl ethene (TPE) units were synthesized by Sonogashira and Suzuki–Miyaura coupling reactions. The optical properties of the CPs and COs were investigated by UV–vis, photoluminescence (PL), and fluorescence lifetime (τ) measurements. The PL intensities of the solutions of the CPs and COs synthesized by Sonogashira coupling were reduced upon addition of poor solvent like water, which is attributed to aggregation‐caused quenching. In contrast, the CP and COs bearing the TPE unit synthesized by Suzuki–Miyaura coupling exhibited aggregation‐induced emission enhancement in solution. This difference in fluorescence behavior is discussed herein in terms of the molecular sizes, conformations in solution, and τ values of the CPs and COs.

Polyfluorenes bearing N1-Alkylcytosine, Alkylphosphoryl, and Alkylammonium side chains: Synthesis, chemical properties, and sensing ability for metal ions

Poly(9,9-bis(6-N1-cytosinehexyl)fluorene-2,7-diyl-alt-9,9-bis(6′-diethoxyphosphorylhexyl)fluorene-2,7-diyl) (polymer-1) and poly(9,9-bis(6-N1-cytosinehexyl)fluorene-2,7-diyl-alt-9,9-b,is(6-N,N,N-trimethylammoniumhexyl)fluorene-2,7-diyl) (polymer-2) were synthesized via Pd-complex-catalyzed condensation reactions. The Mn and Mw of the polymers were determined by GPC. The sensing behavior of the polymers for metal ions was investigated by UV–vis, photoluminescence (PL), and PL lifetime measurements. Polymer PL decreased with the addition of metal ions, such as Cu+, Cu2+, Pd2+, Ag+, and Pt2+, while it remained virtually unchanged with the addition of Na+ and Fe3+. The decrease in PL intensity upon the addition of metal ions is most likely caused by polymer aggregation resulting from metal complexation by the cytosine group. Cyclic voltammetry analysis of the polymers revealed their electrochemically active nature.

Photoluminescent properties of BaF2 scintillator-polystyrene composite films under vacuum ultraviolet radiation

This work reports a new synergistic effect observed in self-sustainable composite films, composed by barium fluoride (BaF2) nanoparticles embedded in polystyrene matrix. Optical absorption measurements were performed in the ultraviolet and visible range. The luminescent properties were investigated by photoluminescence (PL) under excitation in the vacuum ultraviolet region. The PL results showed that self-trapped excitons of BaF2 nanoparticles incorporated into the polymeric matrix are excited at 5.6 eV. This value is 3.4 eV lower than the typical exciton formation energy observed for BaF2 isolated nanoparticles. This phenomenon can be understood as result of a synergistic effect, between polystyrene and BaF2, involving antenna effect and Foster energy transfer. PL lifetime measurements showed that the decay is composed by two main process with short lifetime. From chromatic coordinates of the emitted light it was possible to observe that light emission can be tuned to different shades of colors between blue and green.

Structural and optical properties of Ce-doped strontium borophosphate glasses

In the present study, cerium-doped (0–1 mol%) strontium borophosphate (SBP) glasses were prepared through the melt quenching method. The structural and thermal properties of the glasses were investigated using XRD, FT-IR and TG–DTA. The elemental mapping of Ce-doped glasses was carried out using energy-dispersive X-ray (EDX). The photoluminescence (PL) excitation spectrum of Ce-doped SBP glass exhibited two peaks at 264 and 315 nm, and the emission spectrum exhibited a broad peak around 360 nm. PL lifetime measurements of Ce3+ indicated two lifetimes (< 25 ns) suggesting its presence at two different sites. The PL intensity and lifetime of Ce3+ increased with the increase in Ce-concentration up to 0.3 mol% and beyond that it decreased due to concentration quenching. Thermally stimulated luminescence (TSL) of γ-irradiated (40 Gy) SBP: Ce glasses showed a broad glow peak around 629 K. TSL intensity increased with the increase in Ce-concentration up to 0.5 mol% and beyond that it decreased. TSL trap parameters such as trap depth, frequency, symmetry and the order of kinetics were determined to explore the dosimetric properties of the material.

Optical Absorption and NIR Photoluminescence of Nd3+-Activated Strontium Phosphate Glasses

Strontium phosphate glasses with various concentrations of Nd2O3 have been prepared by melt quenching method. Absorption and photoluminescence spectra and lifetime measurements have been carried out to obtain the optical properties of these glasses. From the absorption spectrum, intensity and radiative parameters have been evaluated using Judd–Ofelt analysis. Near-infrared photoluminescence spectra consist of three bands centered at 875 nm, 1056 nm and 1327 nm, among which the highest intensity has been noticed for the band at 1056 nm that corresponds to the 4F3/2 → 4F11/2 transition of Nd3+ ion. Laser parameters such as branching ratio, band width, stimulated emission cross section and gain bandwidth for the 4F3/2 → 4F11/2 transition are found to be 0.52, 28 nm, 2.31 × 10−20 cm2 and 6.5 × 10−26 cm3, respectively. Decay curves of the 4F3/2 level of Nd3+ ions exhibit a single exponential nature at lower concentrations, while they become non-exponential at higher concentrations (≥ 0.1 mol.%) due to non-radiative energy transfer processes. This feature is also associated with shortening of lifetime from 307 μs to 82 μs when the Nd3+ ion concentration increases from 0.1 mol.% to 4.0 mol.%. The results have been compared with those of barium and magnesium phosphate glasses and commercial glasses. The results suggest that these glasses have potential applications as infrared laser materials at 1.05 μm.

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.

Energy transfer, tunable photoluminescence of Sr3Lu(PO4)3:Tb3+, Eu3+ phosphors

Series of Tb3+ or Eu3+ singly doped and Tb3+/Eu3+ co-doped Sr3Lu(PO4)3 phosphors were prepared by the high temperature solid reaction method. Compared with the excitation spectra of Tb3+ singly doped samples, the effective excitation range of Tb3+/Eu3+ co-doped samples is extended in the near UV region, and the excitation intensity in range 330–380 nm is enhanced. The existence of energy transfer from Tb3+ to Eu3+ ions was confirmed by photoluminescence spectra and lifetime measurements. Under the excitation of 377 nm, the characteristic emission peaks of Tb3+ and Eu3+ ions were both observed, and the ratio of green and red emission components was changed by increasing the concentration of Eu3+. Hence, the emission color of samples Sr3Lu1-x(PO4)3:0.08Tb3+, xEu3+ (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.10) can be turned from green to red region.

The effect of Gd3+ doping on luminescence properties of (Gd, Ce): SrF2 nanopowders and transparent ceramics

The effect of doping of Gd3+ ions into Ce: SrF2 on luminescence properties was investigated in (Gd, Ce): SrF2 nanoparticle powders synthesized via the co-precipitation method, which contained fixed amount of Ce3+ ions and variable amounts of Gd3+ ions. (Gd, Ce): SrF2 nanoparticle powders showed UV range emissions under λex = 273 nm and λex = 298 nm excitation. Photoluminescence emission, excitation, and lifetime measurements revealed the existence of Ce3+ → Gd3+ radiative energy transfer and provided evidence to the prevailing effect of Ce3+ in excitation and emission in the (Gd, Ce) co-doped system due to the existence of fully allowed 4f-5d transitions. The presence of Gd3+ was shown to affect the crystal field and symmetry around Ce3+ ions and enhances the absorption efficiency by two-fold. Furthermore, (Gd, Ce): SrF2 transparent ceramics with 13.2% of in-line optical transmittance in the UV spectral range was obtained via the vacuum hot-pressing method. The increase in the UV luminescence emission intensity of Ce3+ in presence of Gd3+, as well as the ability to process (Gd, Ce): SrF2 as transparent ceramics, are what make (Gd, Ce): SrF2 a promising material for optical applications in the UV region.

Cryogenic Luminescent Ratiometric Thermometers Based on Tetragonal Na[LnSiO4]·xNaOH (Ln = Gd, Tb, Eu; x ≈ 0.2)

The hydrothermal synthesis under mild conditions and structural characterization of Na[LnSiO4]·xNaOH (Ln = Gd, Tb, Eu; x ≈ 0.2) are reported. These materials crystallize in the tetragonal non‐centrosymmetric I4– space group. The crystal phase purity is ascertained by powder X‐ray diffraction and scanning electron microscopy, and further confirmed by photoluminescence spectroscopy and lifetime measurements at 294 and 12 K. Na[(Gd0.82Tb0.14Eu0.04)SiO4]·xNaOH and Na[(Gd0.66Tb0.30Eu0.04)SiO4]·xNaOH perform well as visible ratiometric thermometers, based on the ratio between the Tb3+ and Eu3+ emissions, presenting good relative thermal sensitivities at cryogenic temperature (&lt;100 K) with maxima of 1.9 %K–1 at 14 K and 6.2 %K–1 at 12 K, respectively.

Carrier recombination of organic-inorganic 3D halide perovskite single crystals

Organic-inorganic 3D halide perovskite materials recently have become one of the major players of hybrid semiconductors for photovoltaic and optoelectronic applications. The diffusion length of charge carriers is one of the critical parameters for justifying photovoltaic applications of materials. In this work, we propose a realistic kinetic model in order to fully understand carrier relaxation rate of photoexcited organic perovskites with a negligible exciton formation in photoluminescence lifetime measurements. We find that the extraction of carrier relaxation rate has to be made from multiple fluence-dependent photoluminescence lifetime measurements with global fittings, instead of a traditional single-fluence lifetime measurement. To demonstrate the validity of the model, two kinds of p-doped CH3NH3PbI3 single crystals were grown up by intentionally increasing defects. Global fittings of the kinetic model to the two kinds of single crystals yield doping density, trap density, and recombination constants. Our methodology provides a self-contained approach to determine diffusion lengths of organic 3D halide perovskite materials.

Molecular Junctions on Polymeric Carbon Nitrides with Enhanced Photocatalytic Performance.

Molecular catalysts (MC), namely homogeneous catalysts, have demonstrated great promise for efficient solar-to-chemical energy conversion in the hybrid system. However, the poor interfacial interaction between MC and photosensitizers (PS) impedes the efficient and fast interfacial electron transfer. To promote interfacial communication between PS and MC, a proof-of-concept method was developed for the combination of polymeric carbon nitride (PCN) PS with bipyridine cobalt [Co(bpy)3 2+ ] MC by covalent bonds, creating molecular junctions to promote interfacial electron transfer as confirmed by transient photoluminescence lifetime and electrochemical measurements. As a result, the binary photocatalyst [Co(bpy)3 2+ /BINA2 -CN] showed extensively enhanced photocatalytic activity such as H2 and CO2 reduction in comparison with the physical mixture of Co(bpy)3 2+ and PCN. This observation highlights the importance of construction of surface molecular junctions between PS and MC to accelerate the interfacial charge-carrier mobility and, consequently, improve the photocatalytic activity.

ZnO nanocolumns synthesized by chemical bath process and spray pyrolysis: Ultrasonic and mechanical dispersion of ZnO seeds and their effect on optical and morphological properties

Hexagonal ZnO nanocolumns were synthesized by chemical bath deposition on previously deposited ZnO nanoseeds on Corning glass substrates, at deposition temperatures from 300 to 550 °C in steps of 50 °C, by ultrasonic spray pyrolysis technique. The ZnO nanoseeds were previously suspended in the spraying solution, with and without ultrasonic dispersion. Scanning Electron Microscopy study of surface morphology was carried out revealing the formation of hexagonal ZnO nanocolumns; the smaller diameters were obtained for ZnO seed layers deposited at substrate temperature between 400 and 450 °C. The best vertical alignment was observed on nanocolumns grown from ultrasonic-dispersed ZnO nanoseeds deposited at 400 °C. Raman spectroscopy was used to evaluate the crystal quality of the ZnO nanocolumns, a relationship was determined between nanocolumns stress with a shift to lower wavenumbers of E2H mode, the ZnO seeds deposition temperature and the nanocolumn diameters. Intrinsic defects of ZnO nanocolumns were studied with photo and cathodoluminescence spectroscopy. Also, photoluminescence life time measurements were carried out. Samples grown from ultrasonically dispersed ZnO seeds generated nanocolumns with greater stoichiometry and purity.

Emerging switchable ultraviolet photoluminescence in dehydrated Zn/Al layered double hydroxide nanoplatelets

Layered double hydroxides show intriguing physical and chemical properties arising by their intrinsic self-assembled stacking of molecular-thick 2D nanosheets, enhanced active surface area, hosting of guest species by intercalation and anion exchanging capabilities. Here, we report on the unprecedented emerging intense ultraviolet photoluminescence in Zn/Al layered double hydroxide high-aspect-ratio nanoplatelets, which we discovered to be fully activated by drying under vacuum condition and thermal desorption as well. Photoluminescence and its quenching were reproducibly switched by a dehydration–hydration process. Photoluminescence properties were comprehensively evaluated, such as temperature dependence of photoluminescence features and lifetime measurements. The role of 2D morphology and arrangement of hydroxide layers was demonstrated by evaluating the photoluminescence before and after exfoliation of a bulk phase synthetized by a coprecipitation method.

Probing the optical properties and luminescence mechanism of a UV-emitting SrBPO5 :Ce3+ phosphor.

Ce-doped (1×10-5 to 3.0mol%) SrBPO5 phosphors were synthesized using a conventional solid-state reaction route at 1273K in an air atmosphere. Phase and morphology of the samples were studied from powder X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) micrographs, respectively. The band gap energies of the pure and Ce-doped SrBPO5 phosphors were calculated from the recorded diffuse reflectance spectra. Photoluminescence (PL) and Ce3+ lifetime were recorded at 300 and 77K. Photoluminescence lifetime measurements revealed two-lifetime values for Ce3+ at both 300K (17 and 36nsec) and 77K (12 and 30nsec), suggesting the presence of two different environments around Ce3+ . Time-resolved emission spectroscopy (TRES) studies confirmed the presence of Ce3+ in two different environments. In addition, SrBPO5 :Ce exhibited intense UV emission, signifying its possible use as an efficient sensitizer for solid-state lighting applications. The effect of γ-irradiation on PL was also determined. Thermally stimulated luminescence (TSL) glow curves of the γ-irradiated phosphor, along with trap parameters, dose-response, and the possible TSL mechanism were also investigated. Positron annihilation lifetime spectroscopy was carried out to probe defects present in undoped and Ce-doped SrBPO5 .

Light Emission Enhancement by Tuning the Structural Phase of APbBr3 (A = CH3NH3, Cs) Perovskites.

Lead halide perovskite (APbX3) has recently emerged as a promising active layer in light-emitting diodes (LEDs) as well as an absorber for photovoltaic devices. For better LED properties, it is important to understand the fundamental mechanism of the optoelectronic behaviors, e.g., how the nanostructure of the APbX3 thin film correlates with its emitting properties. We investigated the effect of APbBr3 (A = CH3NH3, Cs) crystallite size on the photophysical properties regarding its crystallographic changes and spin-orbit coupling. Photoluminescence lifetime measurements, X-ray and electron diffraction analyses, and density functional theory calculations were performed. We demonstrate that the emitting properties of mesoscale APbBr3 crystallites are improved due to the formation of a pure cubic phase that leads to the spin- and momentum-allowed carrier recombination. Our findings provide fundamental insights into the emitting behavior of APbBr3, which suggests a control of its optoelectronic properties by means of modulating the crystal morphology and resultant electronic band structures.

Photoluminescence Lifetime Based Investigations of Linker Mediated Electronic Connectivity Between Substrate and Nanoparticle.

The evolution of systems based on nanoparticles as the main component seems to be a self-accelerating process during the last five decades. Hence, an overview across this field gets more and more challenging. It is sometimes rewarding to focus on the fundamental physical phenomenon of the electronic interconnection between the different building blocks of the obtained devices. Therefore, the investigation of charge transport among the utilized particles and their substrate is one of the mandatory steps in the development of semiconductor nanoparticle based devices like e.g., sensors and LEDs. The investigation of the influence of tunneling barriers on the properties of nanoparticle-functionalized surfaces is a challenging task. The different basic influences on the charge transport dynamics are often difficult to separate from each other. Non-invasive and easily viable experiments are still required to resolve the charge distributing mechanisms in the systems. In the presented work, we want to focus on thin and transparent indium tin oxide (ITO) layers covered glass slides since this substrate is frequently utilized in nanoelectronics. CdSe/CdS nanorods (NRs) are applied as an optically addressable probe for the electronic surface states of the conductive glass. The presented experimental design provides the proof of electronic interconnections in ITO coated glass/linker/NR electrodes via easy reproducible functionalization and polishing experiments. UV/Vis absorption and photoluminescence (PL) lifetime measurements revealed changes in the optical properties caused by differences in the charge carrier dynamics between the system. Our work is focused on the modification of charge carrier dynamics due to the application of linker molecules with different functional groups like (3-mercaptopropyl)methoxysilane (MPTMS) and (3-aminopropyl)trimethoxysilane (APTMS). The presented observations are explained with a simple kinetic model.

Photoactivable Polymers Embedded with Cadmium-Free Quantum Dots and Crystal Violet: Efficient Bactericidal Activity against Clinical Strains of Antibiotic-Resistant Bacteria.

The rising incidence of antibiotic-resistant infections from contaminated surfaces in hospitals or implanted medical devices has led to increasing interest in new antibacterial surfaces. Photoactivatable surfaces that can generate cytotoxic reactive oxygen species under exposure to ambient light is a promising approach to inactivation of surface-borne microorganisms. There is growing interest in the use of quantum dots (QDs) as light-harvesting agents for photobactericidal applications, but the cadmium in commonly used QDs will restrict clinical application. Herein, the photobactericidal activity of novel polyurethane substrates containing cadmium-free QDs was tested against clinical multidrug-resistant Gram-positive and Gram-negative bacterial strains: methicillin-resistant Staphylococcus aureus (MRSA) and a carbapenemase-producing strain of Escherichia coli ( E. coli). To enhance the capacity for reactive oxygen species generation, QDs were incorporated into the polymer with a photosensitizing dye, crystal violet. Close proximity between the QD and dye enables electron and energy transfer processes leading to generation of cytotoxic singlet oxygen and superoxide radicals. A QD solution in cyclohexane was premixed with a solution of CV in the more polar solvent, dichloromethane, to promote the formation of QD-CV nanocomposite complexes via CV adsorption. This solution was then used to embed the QDs and crystal violet into medical grade polyurethane via swell-encapsulation. The combination of QD and CV elicited significant synergistic antibacterial activity under visible light against MRSA within 1 h (99.98% reduction) and E. coli within 4 h (99.96% reduction). Photoluminescence lifetime and singlet oxygen phosphorescence measurements demonstrated that interaction between the QDs and the crystal violet occurs within the polymer and leads to enhanced generation of reactive oxygen species. Strong inhibition of kill was observed using the superoxide scavenger, superoxide dismutase. The efficacy of these QD-CV polymer substrates, that can harvest light across the visible spectrum, against multidrug-resistant bacteria demonstrates the feasibility of this approach.

Photostability of quantum dot micelles under ultraviolet irradiation.

Phospholipid quantum dot micelles are useful for bio-applications because of their amphiphilicity and exceptional biocompatibilities. We investigated the uptake of phospholipid [polyethylene glycol (PEG), biotin, and folic acid terminated] modified CdSe/ZnS quantum dot micelles by cancer cells and its photostability under ultrviolet light in the C spectrum (UV-C) (254nm) or UV-A (365nm) light irradiation. The stability of micelles to the exposure of UV-C and UV-A light was assessed. Biotin-modified quantum dot micelles give photoluminescence enhancement under UV-C light irradiation. Folate modified micelle under UV-C and UV-A results show considerable photoluminescence enhancement. Photoluminescence lifetime measurements showed 7.04, 8.11 and 11.42ns for PEG, folate, and biotin terminated phospholipid micelles, respectively. Folate and biotin-modified quantum dot micelles showed excellent uptake by HeLa cells under fluorescence confocal microscopy. Phospholipid CdSe/ZnS quantum dot micelles can be potentially used for diagnosis and treatment of cancer in the future.

Fluorescence Behavior of Bis(cyanostyryl)pyrrole Derivatives Depending on the Substituent Position of Cyano Groups in Solution and in Solid State.

We synthesized a novel fluorophore of distyrylpyrrole derivatives possessing cyano groups at different positions on olefin. Their fluorescence properties in solution and solid state were investigated by photoluminescence quantum yield and lifetime measurements, which provided a radiative decay constant ( kf) and nonradiative decay constant ( knr). The derivative with cyano groups at the inner position of the molecule, inner isomer, shows a high fluorescence quantum yield (Φf = 0.43) in solution, while another derivative with a cyano group at the outer position, outer isomer, hardly shows fluorescence (Φf < 0.01) due to the large nonradiative decay ( knr > 10 ns-1). Upon formation of a single crystal or nanoparticles, these difference were inverted; the quantum yield of the outer and inner isomer was enhanced and diminished, respectively. We explained these differences between in solution and solid state by means of analysis of a single X-ray structure and computation study.