Time-resolved Luminescence Decay Research Articles

Fluorescent silicon nanoparticle-based quantitative hemin assay.

Hemin with functions such as oxygen carrying, oxygen storing, promoting redox, and performing electron transfer is important for the health of organisms. In this paper, green synthetic silicon nanoparticles (Si-NPs) were synthesized and used for free hemin detection in serum (a low limit of detection (LOD), 29.5 nM). The quenching mechanism was investigated by UV-vis absorption spectra, time-resolved luminescence decay curve, and circular dichroism (CD) spectra. It was confirmed that multiple redox centers of hemin led to intensified effective collision and increased the electron transfer rate, therefore enhancing the dynamic quenching, and it was undeniable that the inner filter effect (IFE) also played a role in the quenching.

Na+-functionalized carbon dots with aggregation-induced and enhanced cyan emission

A new type of carbon dots that emit blue emission in aqueous state while cyan emission in solid state was synthesized by a simple hydrothermal method. The photoluminescence quantum yield of the carbon dots in aqueous state and solid state is 7.6% and 29.2%, respectively. The enhanced and red-shifted emission observed in solid state carbon dots is ascribed to surface state change caused by aggregation. The occurrence of surface state change in solid state carbon dots has been evidenced by concentration dependent steady-state photoluminecence spectra and time-resolved luminescence decay. Surface functionalization by Na+ is beneficial for carbon dots to resist luminescence quenching in solid state. A proof-of-concept study was performed to demonstrate the potential application of the obtained carbon dots as inks for anti-counterfeiting and printing high quality fluorescent images.

Ring-Shaped Polyoxometalate Built by {Mn 4 PW 9 } and PO 4 Units for Efficient Visible-Light-Driven Hydrogen Evolution

A novel mixed-valence, ring-shaped multinuclear Mn-containing polyoxometalate, [H2N(CH3)2]15NaH8-[MnIII3MnIV(μ3-O)3(OAc)PO4(B-α-PW9O34)]4·36H2O (1) was made and systematically characterized using v...

Fluorescence lifetime imaging with a single-photon SPAD array using long overlapping gates: an experimental and theoretical study.

Developing large arrays of single-photon avalanche diodes (SPADs) with on-chip time-correlated single-photon counting (TCSPC) capabilities continues to be a difficult task due to stringent silicon real estate constraints, high data rates and system complexity. As an alternative to TCSPC, time-gated architectures have been proposed, where the numbers of photons detected within different time gates are used as a replacement to the usual time-resolved luminescence decay. However, because of technological limitations, the minimum gate length implement is on the order of nanoseconds, longer than most fluorophore lifetimes of interest. However, recent FLIM measurements have shown that it is mainly the gate step and rise/fall time, rather than its length, which determine lifetime resolution. In addition, the large number of photons captured by longer gates results in higher SNR. In this paper, we study the effects of using long, overlapping gates on lifetime extraction by phasor analysis, using a recently developed 512×512 time-gated SPAD array. The experiments used Cy3B, Rhodamine 6G and Atto550 dyes as test samples. The gate window length was varied between 11.3 ns and 23 ns while the gate step was varied between 17.86 ps and 3 ns. We validated the results with a standard TCSPC setup and investigated the case of multi-exponential samples through simulations. Results indicate that lifetime extraction is not degraded by the use of longer gates, nor is the ability to resolve multi-exponential decays.

Updates to OECD guidance document 23: aquatic toxicity testing of difficult substances and mixtures

In this study, tris (8-hydroxyquinoline) aluminum/nano-titanium dioxide (Alq3/nano-TiO2) composites were synthesized using a simply in-situ process with 8-vinyl polyhedral oligomeric silsesquioxane (POSS) as a modifier. The as-prepared Alq3/nano-TiO2 composites were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and ultraviolet visible (UV–vis) absorption spectra. The effect of modification on luminescence properties for the samples was studied by photoluminescence (PL) spectra, electroluminescence (EL) spectra and time-resolved luminescence decay curves. Organic light emitting diodes (OLEDs) with the corresponded emitting layer structure were investigated. The results show that the amphiphilicity of the 8-vinyl POSS leads to well-dispersion state of the nano-TiO2 in the Alq3. Adding a proper weight percentage of 8-vinyl POSS is beneficial for the PL and EL properties enhancement of the composites. OLED using the Alq3/nano-TiO2 with 1 wt% 8-vinyl POSS emitting layer has the low turn-on voltage (4.7 V at 1 cd/m2), high maximum luminance (7463 cd/m2 at 8.75 V), and high luminous efficiency (1.13 cd/A at 100 mA/cm2). Adding 1 wt% 8-vinyl POSS in Alq3/nano-TiO2 can increase the EL intensity by a factor of 37.1 at 8 V. These values are better than those for OLEDs using the Alq3 emitting layer. The increase in luminance and current efficiency stability can be attributed to the energy transfer process between the Alq3 and the nano-TiO2, and the suppression of the self-quenching by caged 8-vinyl POSS molecules.

Anti-Reflection Characteristics of Si Nanowires for Enhanced Photoluminescence from CdTe/CdS Quantum Dots**Supported by the Qing Lan Project of the Higher Education Institutions of Jiangsu Province, Qing Lan Project of Yangzhou Polytechnic Institute, the Natural Science Foundation of Yangzhou City under Grant No YZ2016123, and the National Natural Science Foundation of China under Grant No 61376004.

CdTe/CdS quantum dots (QDs) are fabricated on Si nanowires (NWs) substrates with and without Au nanoparticles (NPs). The formation of Au NPs on Si NWs can be certified as shown in scanning electron microscopy images. The optical properties of samples are also investigated. It is interesting to find that the photoluminescence (PL) intensity of CdTe/CdS QD films on Si nanowire substrates with Au NPs is significantly increased, which can reach 8-fold higher than that of samples on planar Si without Au NPs. The results of finite-difference time-domain simulation indicate that Au NPs induce stronger localization of electric field and then boost the PL intensity of QDs nearby. Furthermore, the time-resolved luminescence decay curve shows the PL lifetime, which is about 5.5 ns at the emission peaks of QD films on planar, increasing from 1.8 ns of QD films on Si NWs to 4.7 ns after introducing Au NPs into Si NWs.

Enhanced PL and EL properties of Alq3/nano-TiO2 with the modification of 8-vinyl POSS

In this study, tris (8-hydroxyquinoline) aluminum/nano-titanium dioxide (Alq3/nano-TiO2) composites were synthesized using a simply in-situ process with 8-vinyl polyhedral oligomeric silsesquioxane (POSS) as a modifier. The as-prepared Alq3/nano-TiO2 composites were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and ultraviolet visible (UV–vis) absorption spectra. The effect of modification on luminescence properties for the samples was studied by photoluminescence (PL) spectra, electroluminescence (EL) spectra and time-resolved luminescence decay curves. Organic light emitting diodes (OLEDs) with the corresponded emitting layer structure were investigated. The results show that the amphiphilicity of the 8-vinyl POSS leads to well-dispersion state of the nano-TiO2 in the Alq3. Adding a proper weight percentage of 8-vinyl POSS is beneficial for the PL and EL properties enhancement of the composites. OLED using the Alq3/nano-TiO2 with 1 wt% 8-vinyl POSS emitting layer has the low turn-on voltage (4.7 V at 1 cd/m2), high maximum luminance (7463 cd/m2 at 8.75 V), and high luminous efficiency (1.13 cd/A at 100 mA/cm2). Adding 1 wt% 8-vinyl POSS in Alq3/nano-TiO2 can increase the EL intensity by a factor of 37.1 at 8 V. These values are better than those for OLEDs using the Alq3 emitting layer. The increase in luminance and current efficiency stability can be attributed to the energy transfer process between the Alq3 and the nano-TiO2, and the suppression of the self-quenching by caged 8-vinyl POSS molecules.

Composed in the f-block: solution structure and function of kinetically inert lanthanide(iii) complexes.

It has been more than 15 years since the last authoritative report on the solution structure of lanthanide complexes made from cylcen derived polydentate ligands. The field has progressed and diversified, and tools have been developed that should enable a step-change in the field in the imminent future. This will only happen if the tools are used, and the results communicated in a form that is consistent within the field and readily accesible to scientists outside the field. In this perspective, the fundamental tools for designing and investigating kinetically inert lanthanide complexes in solution will be covered. The fundamentals of this type of complexes will be laid out. The conformations of lanthanide complexes from cyclen derived ligands and the rate of exchange between conformations will be linked to their 1H NMR and luminescence spectra. The information rich ligand- and metal centred emission spectra will be discussed, and the time-resolved luminescence decay lifetimes are shown to be directly related to the solution structure. The aim is to provide the reader with the information needed to become excited by lanthanide coordination chemistry.

Effect of carbonates on hydroxyapatite self-activated photoluminescence response

The effect of carbonate impurities in the photoluminescence (PL) properties of hydroxyapatite has been studied. Different hydroxyapatite samples were synthesized by chemical precipitation with some differences in the purity of the precursors or the final neutralization treatment. All the materials were calcined in the temperature range of 200–800°C. Calcination treatments in the range of 400–450°C resulted in intense and broad luminescence spectra. Especially high PL intensity was obtained in samples HaB which was neutralized with phosphoric acid and in HaD a carbonated apatite. The band gap of these samples was in the range of 3.1–3.96eV. The materials were characterized by Fourier Transformed Infrared spectroscopy, scanning and transmission electron microscopy, differential scanning calorimetry and x-ray diffraction. The time resolved luminescence decay was in the range from 500 to 980ms in samples HaB, for calcination temperatures from 400 to 500°C. The PL properties of these non-toxic materials made them very promising for new optical applications.

Synthesis of visible-light-driven BiOBrxI1-x solid solution nanoplates by ultrasound-assisted hydrolysis method with tunable bandgap and superior photocatalytic activity

In this study, a series of visible-light-driven BiOBrxI1-x solid solution nanoplates photocatalysts are successfully prepared by an ultrasound-assisted hydrolysis method, which does not use organic reagents, with advantages of cost-effectiveness and non-toxicity. Under visible-light irradiation, all of the as-prepared BiOBrxI1-x nanoplates exhibit superior photocatalytic activities compared to those of pure BiOBr and BiOI for the degradation of methyl orange (MO). BiOBr0.3I0.7 exhibits the highest photocatalytic activity, corresponding to the degradation of 92% MO in 40 min under visible-light irradiation. The structures and elemental composition of the as-prepared BiOBrxI1-x nanoplates samples are characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. From the results obtained from X-ray photoelectron spectroscopy, UV–vis diffuse reflectance spectroscopy, and transient time-resolved luminescence decay, it is suggested that the enhanced photocatalytic activity of BiOBrxI1-x is possibly related to the narrowing of the band gap and high separation of the photo-generated electron–hole pairs. Electron paramagnetic resonance and mechanistic experiments indicated that O2− and h+ are active radicals for photocatalytic degradation. In conclusion, an ultrasound-assisted hydrolysis method which is free of organic reagents is developed for synthesizing BiOBrxI1-x nanoplates photocatalysts with tunable bandgap and enhanced photocatalytic activity.

In Situ Hydrothermal Construction of Direct Solid-State Nano-Z-Scheme BiVO4/Pyridine-Doped g-C3N4 Photocatalyst with Efficient Visible-Light-Induced Photocatalytic Degradation of Phenol and Dyes.

In the current study, a mediator-free solid-state BiVO4/pyridine-doped g-C3N4 nano-Z-scheme photocatalytic system (BDCN) with superior visible-light absorption and optimized photocatalytic activity was constructed via an in situ hydrothermal method for the first time. The pyridine-doped g-C3N4 (DCN) nanosheets show strong absorbance in the visible-light region by pyridine doping, and the BiVO4 (∼10 nm) nanoparticles are successfully in situ grown on the surface of DCN nanosheets by the controlled hydrothermal method. Under the irradiation of visible light (λ > 420 nm), the BiVO4/DCN nanocomposite photocatalysts efficiently degrade phenol and methyl orange (MO) and display much higher photocatalytic activity than the individual DCN, bulk BiVO4, or the simple physical mixture of DCN and BiVO4. The greatly improved photocatalytic ability is attributed to the construction of the direct Z-scheme system in the BiVO4/DCN nanocomposite free from any mediator, which leads to enhanced separation of photogenerated electron–hole pairs, as confirmed by the photocurrent analysis. The possible Z-scheme mechanism of the BiVO4/DCN nanocomposite photocatalyst was investigated by transient time-resolved luminescence decay spectrum, active species trapping experiments, electron paramagnetic resonance (EPR) technology, and hydrogen evolution test.

Theoretical study of time-resolved luminescence in semiconductors. IV. Lateral inhomogeneities

In the fourth part of this series, we study the impact of lateral inhomogeneities on the time-resolved luminescence decay (TRL) after a pulsed excitation by means of simulation with Synopsys® TCAD and analytical approximation. This work consists of two parts: In the first part, the effect of excitations being inhomogeneous on a lateral scale is investigated. It turns out that for localized excitations there may be a strong lateral diffusion of charge carriers, thereby limiting the resolution of a micro-TRL experiment. In this case, a replacement of the inhomogeneous excitation in the simulation by a homogeneous excitation and an average photon density is not possible, especially due to defect saturation depending non-linearly on the excitation. In the second part, we consider a homogeneous excitation and study inhomogeneous material parameters, namely, inhomogeneous charge carrier lifetimes, band gaps, and doping densities. We find that their effects strongly depend on their characteristic lengths of variation. For length scales smaller than the diffusion length, inhomogeneous material parameters can lead to curved luminescence decays.

Plasmon resonance-induced photoluminescence enhancement of CdTe/Cds quantum dots thin films

CdTe/CdS quantum dots/Au nano-rods nano-composite films were fabricated on planar Si substrates. The optical properties of all samples were investigated and the corresponding simulations were studied. It was found that the photoluminescence intensity of the CdTe/CdS quantum dots films was enhanced about 9-fold after the incorporation of Au nano-rods, the internal quantum efficiency increased from 24.3% to 35.2% due to the localized surface plasmon resonance. The time-resolved luminescence decay curves showed that the lifetimes of CdTe/CdS quantum dots films decreased to 2.8ns after interaction with Au nano-rods. The results of finite-difference time-domain simulation indicated that Au nano-rods induced the localization of electric field, which enhanced the PL intensity of quantum dots films in the vicinity of Au nano-rods.

Theoretical study of time-resolved luminescence in semiconductors. III. Trap states in the band gap

In the third part of this series, we study the influence of trap states in the band gap of semiconductors on the time-resolved luminescence decay (TRL) after a pulsed excitation. The results based on simulations with Synopsys TCAD® and analytical approximations are given for p-doped Cu(In,Ga)Se2 as a working example. We show that a single trap can be mostly described by two parameters which are assigned to minority carrier capture and emission. We analyze their influence on the luminescence decay and study the difference between a single trap and an energetic Gaussian trap distribution. It is found that trap states artificially increase the TRL decay and obscure the recombination dynamics. Thus, there is a demand for experimental methods which can reveal the recombination of minority carriers in a TRL experiment without trapping effect. In this regard, a variation of the device temperature, the excitation frequency, the injection level, as well as a bias illumination may be promising approaches. We study these methods, discuss advantages and disadvantages, and show experimental TRL for prove of concept. At the end, we validate our approach of simulating only band-to-band radiative recombination although photoluminescence spectra often exhibit free-to-bound radiative recombination of charge carriers.

DC/RF magnetron sputter deposition and characterisation of Ca3Si2N2O4:Eu2+ luminescent thin films

Luminescent thin films were deposited using reactive magnetron sputtering of Ca, Si (RF) and Eu (DC) in an Ar/N2 atmosphere. A crystalline Ca3N2Si2O4 phase was detected using XRD analysis. The molar ratio of Ca to Eu in the film was found to be 3:0.3. Luminescence excitation, emission and decay measurements reveal two broad bands of Eu2+ emission with local maxima around 550nm and 630nm under UV excitation. The measured luminescence is characteristic for Eu2+ emission in reported Ca3Si2N2O4 phosphors. This was confirmed with time-resolved luminescence decay measurements that show a two component luminescence decay spectrum characterised by time constants of 0.13μs and 0.56μs. EDS measurements showed a metal-to-Si ratio of 3:2.4, metal-to-N ratio of 3:1.5 and a metal-to-O ratio of 3:5.1, where (Ca+Eu) is the metal fraction. The overestimation of Si and O and the underestimation of N is caused by the presence of a crystalline, non-luminescent oxide phase. This was confirmed with XRD, where a weak Ca2SiO4 signal was observed.

Blue luminescence and structural properties of Ce3+-activated phosphosilicate apatite Sr5(PO4)2(SiO4)

Blue-emitting phosphors of the Ce(3+)-activated phosphosilicate Sr(5)(PO(4))(2)SiO(4) with an apatite structure were prepared via a conventional solid-state reaction method. The X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectra, time-resolved luminescence, thermal quenching and luminescence decay curves (lifetimes) were applied to characterize the phosphors. The effects of the Ce(3+) activator concentration on the phase evolution and luminescence properties were investigated. Sr(5-5x)Ce(5x)(PO(4))(2)SiO(4) (x = 0.03) exhibits the brightest blue emission with CIE coordinates of (x = 0.185, y = 0.165) and a quantum efficiency of 46%. The crystallographic site of the Ce(3+) ions in the Sr(5)(PO(4))(2)SiO(4) lattice was identified and discussed. There is only one distinguishable crystallographic Ce(3+) site occupied on a Sr(ii) site in the Sr(5)(PO(4))(2)SiO(4) lattices, which results in a typical doublet emission band. The wavelengths of the excitation and emission bands show a red shift with increasing Ce(3+) doping concentration. The luminescence features of the Ce(3+) ions were discussed on the base of the crystal structure. The thermal stability of the blue-emitting phosphors was evaluated by the temperature-dependent luminescence and the activation energy of the thermal quenching.

The Effect of Polyvinylpyrrolidone on the Optical Properties of the Ni-Doped ZnS Nanocrystalline Thin Films Synthesized by Chemical Method

We report the optical properties of polyvinyl-pyrrolidone (PVP) and the influence of PVP concentration on the photoluminescence spectra of the PVP (PL) coated ZnS : Ni nanocrystalline thin films synthesized by the wet chemical method and spin-coating. PL spectra of samples were clearly showed that the 520 nm luminescence peak position of samples remains unchanged, but their peak intensity changes with PVP concentration. The PVP polymer is emissive with peak maximum at 394 nm with the exciting wavelength of 325 nm. The photoluminescence exciting (PLE) spectrum of PVP recorded at 394 nm emission shows peak maximum at 332 nm. This excitation band is attributed to the electronic transitions in PVP molecular orbitals. The absorption edges of the PVP-coated ZnS : Ni0.3% samples that were shifted towards shorter wavelength with increasing of PVP concentration can be explained by the absorption of PVP in range of 350 nm to 400 nm. While the PVP coating does not affect the microstructure of ZnS : Ni nanomaterial, the analyzed results of the PL, PLE, and time-resolved PL spectra and luminescence decay curves of the PVP and PVP-coated ZnS : Ni samples allow to explain the energy transition process from surface PVP molecules to the Ni2+centers that occurs via hot ZnS.

Optical properties of Yb 3+-doped phosphate laser glasses

Ytterbium-doped phosphate glasses have been prepared and studied their spectroscopic properties through absorption, emission and Fourier transform infrared (FTIR) spectral studies and time-resolved luminescence decay curves. The absorption cross-section has been found to vary with the variation of Yb 2O 3 concentration. The results of the FTIR spectra show that the OH − content is increasing with increase of the Yb 2O 3 concentration in these glasses. The decay curves of the 2F 5/2 level of Yb 3+ ions exhibit a single exponential nature for all the concentrations. The lifetimes of the 2F 5/2 level of Yb 3+ ions decreases from 1.04 to 0.27 ms when the Yb 2O 3 concentration is increased from 0.1 to 6.0 mol%. The quenching of lifetimes has been found to vary directly with the inter-ionic distance between the Yb 3+ ions. The concentration quenching of the lifetime has been analyzed using different energy transfer processes and no evidence of cooperative luminescence of Yb 3+ ions has been found in these glasses, which reveals that the present glasses are useful for photonic device applications. The laser performance properties have also been evaluated for these glasses and compared with those of other reported Yb 3+-doped glass systems.

Luminescence of La 3+ and Sc 3+ impurity centers in YAlO 3 single-crystalline films

The luminescence of La Y 3+ and Sc Y 3+ and Sc Al 3+ centers created by lanthanum and scandium ions at Y 3+ and Al 3+ cation sites of YAlO 3 perovskite lattice was investigated. The features of emission of excitons localized at the mentioned centers in YAlO 3:La and YAlO 3:Sc single-crystalline films were analyzed by means of time-resolved emission spectroscopy and luminescence decay kinetics measurements under excitation by synchrotron radiation at 9 and 300 K.

Mycosporine Glycine Protects Biological Systems Against Photodynamic Damage by Quenching Singlet Oxygen with a High Efficiency¶

This report concerns physiological function of mycosporinelike amino acids (MAA) as an active defense against the photooxidative effects of sunlight in marine organisms. Mycosporine glycine (MG) is a representative member of MAA family and was found to effectively suppress various detrimental effects of the Type-II photosensitization in biological systems, such as inactivation of mitochondrial electron transport, lipid peroxidation of microsomes, hemolysis of erythrocytes and growth inhibition of Escherichia coli. The presence of MG in solutions of eosin Y or methylene blue resulted in a marked decrease in the level of singlet oxygen (1O2) produced by the sensitizers under illumination. The rate constant of 1O2 quenching by MG was determined to be 5.6 × 107M−1s−1 by the time-resolved 1O2 luminescence decay method, which is higher than, or at least comparable to, the values for 1O2 reaction of well-known quenchers such as 1,4-diazabicyclo[2,2,2]octane and furfuryl alcohol. The results suggest that MG probably together with some other active MAA may play an important role in protecting marine organisms against sunlight damage by eliminating 1O2 generated from certain endogenous photosensitizers.