Luminescence Decay Rate Research Articles

Further investigation of spatially resolved single grain quartz OSL and TL signals

The use of luminescence signals from single mineral grains for optical dating has become a valuable and frequently utilised tool in Quaternary Geochronology. Single grain luminescence dating is particularly beneficial in complex depositional settings, however the ability to measure single grain signals also offers the opportunity to assess intrinsic luminescence properties of individual mineral grains. The use of spatially resolved luminescence technologies such as an electron multiplier charge coupled device is of benefit when making luminescence measurements at single grain scales because they allow stimulation with light emitting diodes, and this offers a number of key benefits related to stimulation power when it comes to the assessment of characteristics such as optically stimulated luminescence (OSL) decay rate and the calculation of parameters such as the fast ratio and photo ionisation cross-sections. In this paper, the intra- and inter-sample variability of sensitised single grain thermoluminescence (TL) and OSL signals is considered. A comparison between TL and OSL signals is undertaken, as well as calculation of the fast ratio, OSL component photo ionisation cross-sections, thermal stability, and characteristic dose for a suite of quartz samples from a range of geographic locations and depositional settings. For these heated signals, key findings include the lack of relationship between OSL signal intensity and dominance of the fast component, the fitting of two components (a fast and slow component) is the most common fit for single grain OSL signals, characteristic doses from fast dominated signals suggesting saturation at c. 150 Gy, and the identification of the ultrafast OSL component. Intra-sample variability across all measured parameters is observed, suggesting that for this suite of samples, variability is the norm rather than the exception, and that the intrinsic luminescence characteristics of quartz are variable and diverse.

Photoluminescence study of anatase TiO2 photocatalysts at the pico- and nanosecond timescales.

We studied the photoluminescence decay kinetics of three nanosized anatase TiO2 photocatalysts (particle diameter: 7, 25, or 200nm) at the pico- and nanosecond timescales for elucidating the origin of the luminescence. Luminescence spectra from these photocatalysts obtained under steady-state excitation conditions comprised green luminescence that decayed on the picosecond timescale and red luminescence that persisted at the nanosecond timescale. Among the photocatalysts with different sizes, there were marked differences in the rate of luminescence decay at the picosecond timescale (<600ps), although the spectral shapes were comparable. The similarity in the spectral shape indicated that self-trapped excitons (STEs) directly populated in the bulk of the particle by light excitation emit the luminescence in a picosecond timescale, and the difference in the rate of luminescence decay originated from the quenching at the particle surface. Furthermore, we theoretically considered excitation light intensity dependence on the quantum yield of the luminescence and found that the quenching reaction was not limited by the diffusion of the STEs but by the reaction at the particle surface. Both the spectral shape and time-evolution of the red luminescence from the deep trapped excitons in the nanosecond timescale varied among the photocatalysts, suggesting that the trap sites in different photocatalysts have different characteristics with respect to luminescence. Therefore, the relation between trap states and photocatalytic activity will be elucidated from the red luminescence study.

Reversible Multimode Luminescence Modulations in Photochromic-Translucent Yb3+/Eu3+ Codoped K0.5Na0.5NbO3 Ceramics.

Luminescent tunable materials have promising application potential in optical switches, optical information storage, and so on. Although europium (Eu) is a good downconversion red luminescent rare earth element, there are few studies on the upconversion luminescence and photochromism of Eu-doped potassium sodium niobate (KNN) ferroelectrics. In this paper, Eu3+ and Yb3+ codoped KNN translucent ferroelectric ceramics were synthesized and the effect of Yb3+ content on the luminescence and photochromism is studied. Both the up- and downconversion luminescence intensity and decay rate before and after photochromism can be well controlled by Yb3+ content. That is, an upconversion luminescent translucent ceramic that can be completely discolored by 405 nm light illumination for 10 s was obtained. The luminescence modulations are closely related to the evolution of oxygen vacancy and crystal field around the luminescence center, which can be verified by the illumination-induced electron paramagnetic resonance (EPR) signal and local piezoresponse switching behavior variation as well as the discovery of energy level splitting and spectral line shift. We believe that this work shows a paradigm for designing high-performance reversible multimode luminescence modulation ferroelectric ceramics.

High-temperature dynamic luminescence of MgGa2O4: Tb3+, Er3+ phosphors for advanced anti-counterfeiting and information encryption

Long persistent luminescence (LPL) anti-counterfeiting technology is widely used in optical information storage, information security, and anti-counterfeiting fields due to the advantages of non-interference with background light and high imaging resolution. However, the performance of most LPL materials is greatly compromised in harsh environments such as high temperature, which seriously affects the effectiveness of anti-counterfeiting measures due to the accelerated decay rate of LPL luminescence. In this work, we synthesized a series of MgGa2O4: Tb3+ (MGO: Tb3+), which possess stable crystal structure with obvious LPL properties above room temperature, and the optimum temperature is determined to be 423 K. Moreover, the additional introduction of Er3+ ions enhances the security of anti-counterfeiting, and the combination of photo-stimulated luminescence (PSL) and upconversion (UC) phenomena enables color tuning from red to orange to green by extending the 980 nm laser irradiation time. Finally, the anti-counterfeiting strategy utilizing MGO: Tb3+ and MGO: Tb3+, Er3+ possesses an extremely high level of security, making it an attractive performance for designing advanced anti-counterfeiting technology.

Pr3+-doped CeF3 crystals: Analysis of optical traits and fluorescence from Pr3+: 1D2 level for visible and near-infrared lasers

In this work, four Pr3+ -doped CeF3 crystals with 0.2, 0.4, 0.6, and 0.8 at.% Pr3+ ion doping levels nominally in the melt have been successfully grown, and their absorption and emission spectra, including fluorescence decay times were analyzed. Especially, yellow and near-infrared (NIR) emissions originating from Pr3+: 1D2 level were explored in detail. In comparison, 0.2 at.% Pr3+-doped sample showed better spectral parameters in all studied crystals. In such a crystal, at 443 nm wavelength, the derived absorption cross-section (σabs) is 0.88 × 10-20 cm2 with full width at half maximum (FWHM) ∼ 10 nm in π polarization direction, whereas σabs is 1.32 × 10-20 cm2 with FWHM ∼ 6.7 nm in σ polarization direction at the same wavelength. Likewise, the calculated emission cross-section (σem) for 594 nm wavelength is 0.69 × 10-20 cm2 in π direction and 0.46 × 10-20 cm2 in σ direction. Also, acquired σem at 790, 755, 1014, and 1432 nm wavelengths is 6.15 × 10-21 cm2, 7.33 × 10-21 cm2, 7.66 × 10-21 cm2, and 6.56 × 10-21 cm2 individually. Here evaluated fluorescence decay time of 1D2 level is ∼ 200.6 μs. Obtained higher σabs, larger σem, and higher luminescence decay rates of 0.2 at.% Pr: CeF3 crystal specify its potential as a gain medium for orange and NIR lasers.

Isothermal decay of thermoluminescence and energy distribution of traps in Al2O3–BeO ceramics

We studied features of isothermal decay curves of thermoluminescence (TL) in the dosimetric peak at 525 K of Al2O3–BeO composite ceramics. The temperature ranges were revealed in which an anomalous drop in the TL decay rate was observed with increasing temperature. By using the data obtained with the Tm-Tstop and initial rise methods, we proved that there was a complex activation energy distribution of traps with the energies varying from 0.85 to 1.55 eV. We found that the anomalous decrease with temperature in the isothermal luminescence decay rate can be associated with the superposition of the contributions to the TL from several traps with different energy depth values. It was shown that the analysis of the TL isothermal decay rate change can be used as one of the methods for revealing the complex energy structure of traps in dosimetric phosphors.

Luminescence lifetime imaging of ultra-long room temperature phosphorescence on a smartphone.

Luminescence lifetime imaging plays an important role in distinguishing the luminescence decay rates in time-resolved luminescence imaging. However, traditional imaging instruments used for detecting lifetimes within milliseconds would be time-consuming when imaging ultra-long luminescence lifetimes over subseconds. Herein, we present an accessible and simple optical system for detecting lifetimes of persistent luminescence. A smartphone integrated with a UV LED, a dichroic mirror, and a lens was used for recording the persistent luminescence. With only a few seconds of data acquisition, a luminescence lifetime image could be processed from the video by exponential fitting of the gray level of each pixel to the delay time. Since this approach only requires single excitation, no synchronous control is needed, greatly simplifying the apparatus and saving the cost. The apparatus was successfully used for ultra-long luminescence lifetime imaging of mouse tissue dyed with a persistent luminescence molecule. This miniaturized apparatus exhibits huge potentiality in time-resolved luminescence imaging for luminescence study and biological detection.

Wurtzite quantum well structures under high pressure

Quantum well systems based on semiconductors with the wurtzite crystalline structure have found widespread applications in photonics and optoelectronic devices, such as light-emitting diodes, laser diodes, or single-photon emitters. In these structures, the radiative recombination processes can be affected by (i) the presence of strain and polarization-induced electric fields, (ii) quantum well thickness fluctuations and blurring of a well–barrier interface, and (iii) the presence of dislocations and native point defects (intentional and unintentional impurities). A separate investigation of these phenomena is not straightforward since they give rise to similar effects, such as a decrease of luminescence efficiency and decay rate, enhancement of the Stokes shift, and strong blueshift of the emission with increasing pump intensity. In this Perspective article, we review the usefulness of measurements of the quantum well luminescence as a function of the hydrostatic pressure for both scientific research and the development of light-emitting technologies. The results presented here show that high-pressure investigations combined with ab initio calculations can identify the nature of optical transitions and the main physical factors affecting the radiative efficiency in quantum well systems. Finally, we will discuss an outlook to the further possibilities to gain new knowledge about the nature of recombination processes in quantum wells using high-pressure spectroscopy.

Optimal parameters in variable-velocity scanning luminescence lifetime microscopy.

We determine the optimal parameters (scan velocities) for measuring the luminescence lifetime on the microsecond scale using the recently introduced method based on scanning the excitation beam across the sample. Using simulations, we evaluate the standard deviation and bias of the luminescence decay rate determined by scanning with two different velocities. The analysis is performed for Poisson‐ and normal‐distributed signals, representing different types of detection techniques. We also show that a weak uncorrected background induces a bias in the obtained decay rate, and take this effect into account when choosing optimal measurement parameters. For comparison, the analysis is additionally performed for two conventional gating schemes for lifetime measurement. The variable‐velocity scanning method is found to be more robust to the effect of the background signal than the gating schemes.

Properties of a Tamm-Plasmon-Based Microcavity with Metal Intracavity Layers and an Organic Active Region

The properties of a microcavity structure with metal intracavity layers and an organic active region (based on the low-molecular-weight compound 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP)) are analyzed theoretically. The structure under study consists of a CBP layer of a given thickness sandwiched between two silver layers, two thin phase-matching CBP layers (dph = 26 nm), and two quarter-wave Bragg reflectors. It is shown that hybrid modes localized in the central layer of the structure can significantly increase the luminescence decay rate of an organic emitter due to the Purcell effect. It is found that the spectral positions of the hybrid modes are shifted because of their interaction with exciton resonance in CBP. This may indicate the emergence of a strong coupling regime.

Donor–Acceptor Pair Recombination in Size-Purified Silicon Quantum Dots

Shallow impurity doping is an efficient route to tailor optical and electronic features of semiconductor quantum dots (QDs). However, the effect of doping is often smeared by the size, shape, and composition inhomogeneities. In this paper, we study optical properties of almost monodispersed spherical silicon (Si) QDs that are heavily doped with boron (B) and phosphorus (P). The narrow size distribution achieved by a size-separation process enables us to extract doping-induced phenomena clearly. The degree of doping-induced shrinkage of the optical band gap is obtained in a wide size range. Comparison of the optical band gap with theoretical calculations allow us to estimate the number of active donor-acceptor pairs in a QD. Furthermore, we found that the size and detection energy dependence of the luminescence decay rate is significantly modified below a critical diameter, that is ∼5.5 nm. In the diameter range above 5.5 nm, the luminescence decay rate is distributed in a wide range depending on the detection energy even in size-purified Si QDs. The distribution may arise from that of donor-acceptor distances. On the other hand, in the diameter range below 5.5 nm the detection energy dependence of the decay rate almost disappears. In this size range, which is smaller than twice of the effective Bohr radius of B and P in bulk Si crystal, the donor-acceptor distance is not a crucial factor to determine the recombination rate.

Structural and functional consequences of EF-hand I recovery in mnemiopsin 2

Mnemiopsin 2 from Mnemiopsis leidyi is a calcium-regulated photoprotein which has luminescence properties in the presence of calcium and coelenterazine. All calcium-regulated photoproteins contain EF-hand loops consisting of 12 individual residues in which the 6th position is occupied by Gly. However, the 6th residue in mneniopsin 2 is Glu rather than Gly. Here, we investigated the structural and functional consequences of substitution of Glu by Gly (E50G variant) using site-directed mutagenesis and spectroscopic procedures. It was revealed that the luminescence activity of the variant was about 17 times greater than that of wild-type (WT) photoprotein. In comparison with WT protein, our variant showed higher optimum temperature and calcium sensitivity as well as slower rate of luminescence decay. Homology modeling and sequence analysis with other known photoproteins showed that EF-hand I loop can affect the luminescence activity of E50G variant. Structural studies using circular dichroism and fluorescence spectroscopy revealed that mutation leads to the reduction in secondary structural content and local structural alterations. Finally, it can be concluded that the activity of E50G variant increases as a result of more flexibility that brought about by Gly essential for adopting the correct conformation for functional activity.

Influence of fluorinated chain length on luminescent properties of [formula omitted][formula omitted]-diketonate complexes

Five novel europium β-diketonate complexes containing a pyrazole fragment were investigated. The influence of the number of fluorine atoms in diketonate ligands on luminescent properties was analyzed. Luminescence intensity of Eu3+ complexes increases significantly when CH bonds with high oscillation energy are replaced with CF bonds with low oscillation energy as the efficiency of energy transfer is improved. Further extension of the fluorinated chain length results in a substantial decrease in non-radiative relaxation rates while the increase in luminescence decay rates is observed. The theoretical results elaborated in terms of Judd-Ofelt theory are in good agreement with the experimental data obtained by time-resolved luminescence spectroscopy.

Synthesis of Firefly Luciferin Analogues and Evaluation of the Luminescent Properties.

Five new firefly luciferin (1) analogues were synthesized and their light emission properties were examined. Modifications of the thiazoline moiety in 1 were employed to produce analogues containing acyclic amino acid side chains (2-4) and heterocyclic rings derived from amino acids (5 and 6) linked to the benzothiazole moiety. Although methyl esters of all of the synthetic derivatives exhibited chemiluminescence activity, only carboluciferin (6), possessing a pyrroline-substituted benzothiazole structure, had bioluminescence (BL) activity (λmax =547 nm). Results of bioluminescence studies with AMP-carboluciferin (AMP=adenosine monophosphate) and AMP-firefly luciferin showed that the nature of the thiazoline mimicking moiety affected the adenylation step of the luciferin-luciferase reaction required for production of potent BL. In addition, BL of 6 in living mice differed from that of 1 in that its luminescence decay rate was slower.

Synthesis, optical spectroscopy and Judd–Ofelt analysis of Eu3+doped Li2BaP2O7 phosphors

A series of Li2BaP2O7: xEu3+ (x=0.1%, 1%, 3%, 5%) compound have been synthesized by the classic ceramic method. The obtained powders crystallize in the monoclinic system with C2/c space group. Excitation and emission spectra as well as luminescence decays were recorded. Five emission bands have been observed from the 5D0 excited state to the lower lying states 7F0, 7F1, 7F2, 7F3 and 7F4 at 578, 590, 615, 653 and 687nm respectively upon exciting the sample with 375nm wavelength. The Eu3+ Stark energy levels, the strength crystal field and the Calorimetric parameters of the Eu3+ ions in Li2BaP2O7 are calculated. A comparative study of the optical results obtained for the different Eu3+ concentrations was carried out by calculating the asymmetry ratio (1). The symmetry of the local environment of Eu3+ activators has been described in the frame of Judd–Ofelt (J–O) theory, using Ω2 and Ω4 intensity parameters derived from emission spectra. A comparison is made between these parameters in order to determine the type of connection between the Eu3+ activator ions and the Li2BaP2O7 host. The Judd–Ofelt parameters are used to calculate the relative transition probability, branching ratio and relative lifetime. The luminescence decay rates of 5D0 excited state is measured and the correspondent emission quantum efficiency is calculated. The effective bandwidth of the Eu3+ (∆λeff) and stimulated emission cross-section (σp) are also investigated.

Optical and luminescence properties of Li2O[sbnd]Gd2O3[sbnd]MO[sbnd]B2O3[sbnd]Sm2O3 (MO[dbnd]Bi2O3, BaO) glasses

Spectroscopic properties of Sm3+ ion in borate based glasses (Li2OGd2O3-MO-B2O3 where MOBi2O3, BaO) have been investigated. Glasses are prepared by melt quenching technique and their properties are investigated using different techniques which include absorption, photoluminescence (PL), X-ray excited luminescence (XEL) spectra and decay rate analysis. Furthermore, other related physical and optical properties have been calculated. From absorption spectra the most intense and hyper sensitive transition is 6H5/2 → 6P3/2 at 403 nm. Under 403 nm excitation wavelength, four prominent emission peaks are observed at 563, 599, 645 and 707 nm. For Sm3+ ion the quenching points are observed at 2.0 and 1.5 mol% of LGBiBSm and LGBaBSm, respectively. Measured XEL spectra of the glass samples are found in agreement with that of PL emission spectra. The experimental decay rate follows non exponential behavior at higher concentrations and well fitted to the IH model for S = 6. The obtained color coordinates of the prepared glass matrix fall in the orange region of the CIE diagram. Comparing with LGBiBSm glass, LGBaBSm is transparent and exhibits long decay time value. Moreover, LGBaBSm shows high luminescence properties with excellent energy transfer from the host matrix to the luminescence center. On the basis of the observed properties of the developed glasses, it is expected that they will be best for the orange LEDs.

Chemical Partition of the Radiative Decay Rate of Luminescence of Europium Complexes

The spontaneous emission coefficient, Arad, a global molecular property, is one of the most important quantities related to the luminescence of complexes of lanthanide ions. In this work, by suitable algebraic transformations of the matrices involved, we introduce a partition that allows us to compute, for the first time, the individual effects of each ligand on Arad, a property of the molecule as a whole. Such a chemical partition thus opens possibilities for the comprehension of the role of each of the ligands and their interactions on the luminescence of europium coordination compounds. As an example, we applied the chemical partition to the case of repeating non-ionic ligand ternary complexes of europium(III) with DBM, TTA, and BTFA, showing that it allowed us to correctly order, in an a priori manner, the non-obvious pair combinations of non-ionic ligands that led to mixed-ligand compounds with larger values of Arad.

Spectroscopic properties and Judd–Ofelt analysis of Eu3+ doped GdPO4 nanoparticles and nanowires

Eu3+ doped GdPO4 orthophosphates with hexagonal and monoclinic crystal structures were prepared by a solvothermal process at 160°C. Nanoparticles (NPs) and nanowires (NWs) were obtained using a mixture of water and glycerol as solvents. Luminescence properties of 5% Eu3+ doped GdPO4 NPs and NWs were investigated. As-obtained luminescent NWs:Eu and NPs:Eu powders were post-annealed at 750°C since this temperature did not change neither their initial morphology nor their crystalline structure and led to luminescence efficiency intense enough to suit optical applications. A post-heating treatment of the samples at higher temperature (1000°C) resulted in a change in both the crystalline structure from hexagonal to monoclinic form and the morphology. Excitation and emission spectra as well as luminescence decays were recorded on both post-annealed luminescent NW:Eu and NP:Eu powders. A comparative study of the optical results obtained for the different morphologies and structures was carried out by calculating the asymmetry ratio (A21) of Eu3+ from emission spectra. The symmetry of the local environment of Eu3+ activators has been described in the frame of Judd–Ofelt theory, using Ω2 and Ω4 intensity parameters derived from emission spectra data analysis, adopting a standard procedure. It was observed that Ω2 value for the NPs is different from that of the NWs indicating that the environment surrounding the active ion changes depending on the sample morphologies. The radiative lifetimes of the 5D0 excited state and the corresponding branching ratios for different emission transitions were also estimated and compared to the recorded luminescence decay rates at room temperature.

Luminescence Enhancement Mechanism of Lanthanide-Doped Hybrid Nanostructures Decorated by Silver Nanocrystals

Hybrid nanostructures composed of rare earth ion-doped lanthanum trifluoride nanocrystals deposited on silica nanospheres (LaF3:Yb3+, Er3+@SiO2) and decorated with varying quantities of silver nanoparticles (Ag NPs) were synthesized using a simple strategy. Down and upconversion luminescence spectra were recorded. The luminescence dynamics were also recorded following excitation with a 532-nm pulse. Silver loading was found to have a significant effect both on the luminescence intensity and the luminescence decay rate, with the samples with the lowest silver content showing reduced luminescence intensity over silver-free samples, while the samples with large levels of silver loading showed significant luminescence enhancement. The results were successfully (and quantitatively) interpreted in terms of the competition between surface plasmon-induced field enhancement mediated by the Ag nanoparticles and nonradiative energy transfer from the luminescent ions to Ag nanoparticles. By combining the measured luminescence intensity with the luminescence decay rate determined from the dynamics measurements, and with the measured surface plasmon absorption spectra, one could obtain a quantitative and self-consistent understanding of the observed dependence of the green 4S3/2→4I15/2 (~540 nm) and red 4F9/2→4I15/2 (~650) emission bands on the Ag NP metal loading.

Excitation energy transfer in porous silicon/laser dye composites

Excited state relaxation and energy transfer in porous silicon (PS)/laser dye [oxazine 1 (Ox1), rhodamine 6G (Rh6G)] composites have been studied by means of steady-state and time-resolved fluorescence. Fluorescence decay kinetics reveals the nonradiative energy transfer from PS to the dye. Increased decay rate of the dye luminescence in the composite indicates that opposite energy transfer is also likely. Analysis of the time-resolved fluorescence of the PS/R6G composite also shows that there is no energy transfer from silicon oxide responsible for the “blue” fluorescence band to silicon nanocrystalites, and that interaction between Si nanocrystals responsible for the “red” PS fluorescence is absent or weak.