Slow Luminescence Research Articles

Photoluminescence and Electron Spin Resonance of Silicon Dioxide Crystal with Rutile Structure (Stishovite)

An electron spin resonance (ESR) and photoluminescence signal is observed in the as grown single crystal of stishovite indicating the presence of defects in the non‐irradiated sample. The photoluminescence of the as received stishovite single crystals exhibits two main bands – a blue at 3 eV and an UV at 4.75 eV. Luminescence is excited in the range of optical transparency of stishovite (below 8.75 eV) and, therefore, is ascribed to defects. A wide range of decay kinetics under a pulsed excitation is observed. For the blue band besides the exponential decay with a time constant of about 18 μs an additional ms component is revealed. For the UV band besides the fast component with a time constant of 1–3 ns a component with a decay in tens μs is obtained. The main components (18 μs and 1–3 ns) possess a typical intra‐center transition intensity thermal quenching. The effect of the additional slow component is related to the presence of OH groups and/or carbon molecular defects modifying the luminescence center. The additional slow components exhibit wave‐like thermal dependences. Photo‐thermally stimulated creation–destruction of the complex comprising host defect and interstitial modifiers explains the slow luminescence wave‐like thermal dependences.

Slow luminescence kinetics of semi-synthetic aequorin: expression, purification and structure determination of cf3-aequorin.

cf3-Aequorin is one of the semi-synthetic aequorins that was produced by replacing 2-peroxycoelenterazine (CTZ-OOH) in native aequorin with a 2-peroxycoelenterazine analog, and it was prepared using the C2-modified trifluoromethyl analog of coelenterazine (cf3-CTZ) and the histidine-tagged apoaequorin expressed in Escherichia coli cells. The purified cf3-aequorin showed a slow luminescence pattern with half-decay time of maximum intensities of luminescence of 5.0 s. This is much longer than that of 0.9 s for native aequorin, and its luminescence capacity was estimated to be 72.8% of that of native aequorin. The crystal structure of cf3-aequorin was determined at 2.15 Å resolution. The light source of 2-peroxytrifluoromethylcoelenterazine (cf3-CTZ-OOH) was stabilized by the hydrogen-bonding interactions at the C2-peroxy moiety and the p-hydroxy moiety at the C6-phenyl group. In native aequorin, three water molecules contribute to stabilizing CTZ-OOH through hydrogen bonds. However, cf3-aequorin only contained one water molecule, and the trifluoromethyl moiety at the C2-benzyl group of cf3-CTZ-OOH interacted with the protein by van der Waals interactions. The slow luminescence kinetics of cf3-aequorin could be explained by slow conformational changes due to the bulkiness of the trifluoromethyl group, which might hinder the smooth cleavage of hydrogen bonds at the C2-peroxy moiety after the binding of Ca2+ to cf3-aequorin.

The Effect of Aggregation of Impurity Nitrogen on Diamond X-Ray Luminescence

Two samplings of 65 diamond crystals divided by the intensity of a slow X-ray luminescence component are studied from the Arkhangel’skaya and Karpinskogo-1 pipes. IR and EPR spectroscopies revealed a relationship between the nitrogen A and P2 centers of the diamonds and the presence of a slow X-ray luminescence component. Its absence in most diamonds with high content of P1 (C) centers is explained by the low number of A and P2 centers.

Concentration self-quenching of luminescence in crystal matrices activated by Nd3+ ions: Theory and experiment

Resonance migration and cross-relaxation processes leading to concentration self-quenching of fluorescence have been studied theoretically and experimentally with Nd3+: LaF3 bulk crystal as an example. The time range of theoretical description of self-quenching kinetics has been expanded by including into consideration the long-range fluctuation stage that determines the slow luminescence quenching of donor centers isolated due to fluctuations of donor surroundings. New experimental data on fluorescence kinetics concentration dependence in a Nd3+:LaF3 bulk crystal are consistently described with single set of energy migration CDD and cross-relaxation CDA microparameters. The obtained results is the first step to solve the problem of quenching in RE doped nanocrystals where, along with self-quenching, the quenching by additional acceptors namely, various molecular groups, residing in the volume of the nanoparticles have to be accounted.

A Chinese Pane-Like 2D Metal-Organic Framework Showing Magnetic Relaxation and Luminescence Dual-Functions

The discovery of graphene kicked off the curtain of atom-type two-dimensional (2D) materials. Layered metal-organic frameworks (MOFs) as parallel molecule-based 2D materials are more designable and more diverse, and magnetism may be induced by their metal ion nodes. However, the multifunctional 2D plane-like MOFs are very difficult to obtain. Here we describe a Chinese pane-like 2D MOF constructed from the Ln3+ cation and the nanoscale luminescent tritopic ligand tris(4′-carboxybiphenyl)-amine, responding to the slow magnetic relaxation and luminescence properties, respectively. Notably, the Dy-Dy distances separated by the tritopic ligand are up to 2 nm. Such a 2D molecular material is expected to have potential applications in optoelectronics and multimodal sensing.

2D carboxylate-bridged LnIII coordination polymers: displaying slow magnetic relaxation and luminescence properties in the detection of Fe3+, Cr2O72- and nitrobenzene.

Eight new 2D isostructural lanthanide coordination polymers (Ln-CPs), [Ln(HL)(H2O)2(NO3)]·NO3 (1-Ln), Ln = NdIII, SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, and YbIII ions, H2L = 4-(3,5-dicarboxylphenyl)-2-methylpyridine, were synthesized by using solvothermal methods and studied by structural analyses, magnetic analyses and luminescent probes. Crystallographic studies revealed that these compounds are 2D frameworks in which dinuclear units with double μ-syn,syn-carboxylate bridges are interlinked by single μ-trans,trans-carboxylate bridges from organic spacers. The layers are further stabilized and combined into 3D architectures through intra- and interlayer ππ stacking interactions and hydrogen bonding, respectively. Magnetic investigations indicated that the carboxylate bridges transmit intralayer antiferromagnetic coupling in 1-Nd, 1-Gd and 1-Ho but transmit intralayer ferromagnetic coupling in 1-Dy. Furthermore, 1-Dy also displays slow magnetic relaxation behavior with a high relaxation energy barrier (ΔUeff) of 100.7 K and a pre-exponential factor (τ0) of 1.4 × 10-8 s under zero dc field. The luminescence investigations showed that CPs 1-Eu and 1-Tb can serve as highly selective and recyclable sensing materials for Fe3+, Cr2O72- and nitrobenzene. Thus, both 1-Eu and 1-Tb should be excellent candidates for multifunctional sensors.

Note: On the choice of the appropriate excitation-pulse-length for assessment of slow luminescence decays.

The decay-time distribution deduced from luminescence kinetics experiments is, in general, dependent on the excitation pulse length as a direct consequence of different onset dynamics. We demonstrate this effect for the case of square excitation pulses applied to study the luminescence kinetics in Si nanocrystals. The short- and long-pulse limits are defined as 0.1 times the shortest lifetime in the distribution and 3 times the longest time, respectively. Outside these limits the decay-time distribution is independent on the pulse duration. In addition, we describe experimental conditions required to obtain a correct depiction of slow luminescence decay in the μs to ms time range.

X-ray excited luminescence of polystyrene composites loaded with SrF2 nanoparticles

The polystyrene film nanocomposites of 0.3mm thickness with embedded SrF2 nanoparticles up to 40wt% have been synthesized. The luminescent and kinetic properties of the polystyrene composites with embedded SrF2 nanoparticles upon the pulse X-ray excitation have been investigated. The luminescence intensity of the pure polystyrene scintillator film significantly increases when it is loaded with the inorganic SrF2 nanoparticles. The film nanocomposites show fast (∼2.8ns) and slow (∼700ns) luminescence decay components typical for a luminescence of polystyrene activators (p-Terphenyl and POPOP) and SrF2 nanoparticles, respectively. It is revealed that the fast decay luminescence component of the polystyrene composites is caused by the excitation of polystyrene by the photoelectrons escaped from the nanoparticles due to photoeffect, and the slow component is caused by reabsorption of the self-trapped exciton luminescence of SrF2 nanoparticles by polystyrene.

Long-lived emission in Mn doped CdS, ZnS, and ZnSe diluted magnetic semiconductor quantum dots

Slow luminescence is studied in Mn doped CdS, ZnS, and ZnSe quantum dots. Because of the high degeneracy of Mn d-orbitals, we employ the multi-determinant SAC–CI computational method to determine the spin-forbidden transition from the 4T1 first excited to 6A1 ground state. We find that the transition energies for each material are in the excellent agreement with the experimental data. The computations reveal that the absorption spectra are independent of the presence of Mn impurities in quantum dots. The calculations show that the Mn impurity levels are located inside the QD gaps and the slow emission energies are independent of QD sizes. These features allow us to conclude that there are two luminescence peaks in the spectrum with fast (the higher energy) and slow (the lower energy) relaxations. In experiments sometimes the fast luminescence band disappears. This effect depends on Mn concentrations and a doping method. For different QD crystal structures the Mn–S (Se) bond lengths can vary. Therefore we find that the slow luminescence energy is very sensitive to a bond length. Indeed if we change the Mn–S bond length by 0.1Å, the energy increases by 0.2eV within the calculated range of bond lengths.

Pixel Bleeding Correction in Laser Scanning Luminescence Imaging Demonstrated Using Optically Stimulated Luminescence.

This paper describes and investigates the performance of an algorithm to correct for "pixel bleeding" caused by slow luminescence centers in laser scanning imaging (e.g., X-ray imaging using photostimulable phosphors and 2D dosimetry using optically stimulated luminescence). The algorithm is based on a deconvolution procedure that takes into account the lifetime of the slow luminescence center and is further constrained by the detection of fast and slow luminescence centers and combining rows scanned in opposite directions. The algorithm was tested using simulated data and demonstrated experimentally by applying it to image reconstruction of two types of Al2O3 X-ray detector films ( Al2O3:C and Al2O3 :C,Mg), whose use in 2D dosimetry in conjunction with laser-scanning readout has so far been prevented by slow luminescence centers (F-centers, 35 ms lifetime). We show that the algorithm allows the readout of Al2O3 film detectors 300-500 times faster than generally allowed considering the lifetime of the main luminescence centers. By relaxing the stringent requirements on the detector's luminescence lifetime, the algorithm opens the possibility of using new materials in 2D dosimetry as well as other laser scanning applications, such as X-ray imaging using storage phosphors and scanning confocal microscopy, although the effect of the noise introduced must be investigated for each specific application.

Tunable Luminescence in CdSe Quantum Dots Doped by Mn Impurities

In this work, we study the effect of a dual luminescence for CdSe quantum dots (QDs) doped by a manganese impurity. In this effect, one line has fast relaxation and corresponds to light emission from CdSe conduction band, whereas the second is very slow in relaxation with the relaxation time of 0.1–1.0 ms. The second line disappears for quantum dots (QDs) with diameters D ≥ 3.3 nm, and therefore, the luminescence becomes tunable by a QD size. The problem is computationally challenging because of large size QDs and a high degeneracy of the energy states. To overcome this problem, we make four assumptions. These assumptions are the following: (1) a QD optical gap is independent of an Mn impurity for small concentrations; (2) we combine electronic structure calculations for medium size CdSe QDs with the effective mass calculations for medium and large QD sizes and match them in the medium size region, assuming that based on assumption 1, the optical spectrum is independent of Mn even for larger QDs; (3) we cut an MnSe4 fragment out of the middle of Cd23Mn1Se24, Cd31Mn1Se32, and Cd81Mn1Se82 QDs and check whether we can quantitatively explain the mechanism of the second, slow relaxation emission in these experiments using the ab initio SAC–CI multiconfiguration method; and (4) the slow luminescence line is independent of a QD size. We have proved theoretically all these assumptions and found that the critical size of a quantum dot, the size when the second luminescence line disappears, is 3.2 nm, and the slow luminescence energy is 2.3 eV in a tetrahedral ligand field. We also study the case for an Mn impurity placed at a QD surface. Then the symmetry of a fragment is C3v, and the results of the calculations reveal that the slow luminescence energy is 2.47 eV with the critical size D = 2.7 nm, instead of 3.2 nm for a tetrahedral ligand field. We also predict how this energy depends on the length of an Mn–Se bond. The dependencies appear to be the opposite in these two cases. For the tetrahedral symmetry, the luminescence energy grows with the bond length, whereas for the pyramid, C3v, symmetry (an Mn is at the surface), it goes down. Furthermore, we study a luminescence energy dependence on a Se–Mn–Se pyramid angle. We find that it is angle independent. These results could be useful for CdSe nanocrystal structures different than Würtzite.

Origin of slow low-temperature luminescence in undoped and Ce-doped Y2SiO5and Lu2SiO5single crystals

At 4.2–300 K, the steady-state and time-resolved emission and excitation spectra as well as the luminescence decay kinetics in the 10 μs–10 s time range are studied for the undoped and Ce3+-doped single crystals of Y2SiO5 and Lu2SiO5. At low temperatures, a broad intrinsic emission band located at 2.55 eV in Y2SiO5 and 2.58 eV in Lu2SiO5 is observed in the luminescence spectra of all the crystals studied under excitation in the charge-transfer absorption region (with Eexc > 4.2 eV). This emission reveals the slow non-exponential decay kinetics characteristic for tunneling recombination processes. In the slow decay kinetics of the low-temperature luminescence of Ce3+-doped crystals, both the multi-exponential and the non-exponential decay stages are detected. The origin of the defects, responsible for the undesirable slow low-temperature luminescence of the undoped and Ce3+-doped Y2SiO5 and Lu2SiO5 crystals is considered. A new mechanism of the processes, resulting in the appearance of different luminescence decay stages, is proposed.

Study on fast luminescence component induced by gamma-rays in Ce doped LiCaAlF6 scintillators

We discuss the origin of the fast luminescence component induced by fast electrons generated in gamma-ray interactions in Ce doped LiCaAlF6 scintillators. Although the slow luminescence component induced by Ce3+ emissions depends on the Ce concentration in the LiCaAlF6 scintillator, the fast component is independent. The fast component is suggested to be generated in the host matrix of the LiCaAlF6 crystal. From quantitative considerations based on Frank–Tamm equation, which shows the light yield of the Cherenkov radiation, the Cherenkov radiation was determined as the origin of the fast component. We, additionally, found that the slow rise time of main Ce3+ emissions in the Ce:LiCaAlF6 scintillator plays an important role to perform the pulse shape discrimination.

Development of a 2D dosimetry system based on the optically stimulated luminescence of Al2O3

A laser-scanning 2D dosimetry system based on the Optically Stimulated Luminescence (OSL) signal from Al2O3 films was built and demonstrated. The main challenge of using the OSL from Al2O3 for 2D dosimetry by laser scanning is the long lifetime (∼35 ms) of the main luminescence centers in this material (F-centers). In this work, we demonstrated the possibility of performing 2D dosimetry by laser scanning using a combination of the fast UV emission of F+-centers (lifetime <7 ns) and the slow F-center emission of Al2O3:C, and an algorithm to correct for the slow F-center luminescence lifetime. We also investigated the possibility of using Al2O3:C,Mg, to take advantage of its greater F+-center emission compared to Al2O3:C. Results from 6 MV photon beam irradiations from a clinical linear accelerator were compared to radiographic and radiochromic film profiles showing a good qualitative agreement.

Ultraviolet luminescence of ScPO4, AlPO4 and GaPO4 crystals

The luminescence of self-trapped excitons (STEs) was previously observed and described for the case of tetragonal-symmetry ScPO4 single crystals. The subject band in this material is situated in the UV spectral range of ∼210 nm or ∼5.8 eV. In the present work, we are both expanding this earlier luminescence study and seeking to identify similar luminescence phenomena in other orthophosphate crystals, i.e., AlPO4 and GaPO4. These efforts have proven to be successful—in spite of the structural differences between these materials and ScPO4. Specifically we have found that for AlPO4 and GaPO4, in addition to an α-quartz-like STE, there is a UV luminescence band that is similar in position and decay properties to that of ScPO4 crystals. Potentially this represents an STE in AlPO4 and GaPO4 crystals that is analogous to the STE of ScPO4 and other orthophosphates. The decay kinetics of the UV luminescence of ScPO4 was studied over a wide temperature range from 8 to 300 K, and they exhibited some unusual decay characteristics when subjected to pulses from an F2 excimer laser (157 nm). These features could be ascribed to a triplet state of the STE that is split in a zero magnetic field. A fast decay of the STE was detected as well, and therefore, we conclude that, in addition to the slow luminescence corresponding to a transition from the triplet state, there are singlet–singlet transitions of the STE. Time-resolved spectra of the slow and fast decay exhibit a small shift (∼0.15 eV) indicating that the singlet–triplet splitting is small and the corresponding wavefunction of the STE is widely distributed over the atoms of the ScPO4 crystal where the STE is created.

Energy Transfer from CdSe Quantum Dots to Porphyrin via Two-Photon Excitation

CdSe quantum dots have been used as energy donors to activate meso-tetra (4-sulfonatophenyl) porphine dihydrochloride. Pulses of 130 fs duration at a wavelength of 800 nm are used as the two-photon excitation light source. After excitation, steady-state and time-resolved fluorescence spectra are collected for samples with different ratio between the amount of porphyrins and quantum dots. Decay kinetic curves of CdSe quantum dots with and without porphyrins are well fitted by the biexponential decay curve, which indicates a combination of two components (excitonic and trapping state) in the luminescence behavior of CdSe quantum dots. Relative intensity weights of the excitonic and trapping state, total and nonradiative energy transfer efficiency, average luminescence lifetimes of donors are calculated. The nonradiative transfer mechanism becomes the leading factor as the concentration of acceptors gets higher. It is considered to take place through the channel of trapping states of CdSe quantum dots, which is expressed by the weight change between the fast and slow luminescence components. This deduction presents a new way of raising the energy transfer efficiency by increasing the trapping state proportion of the quantum dots, which can be easily realized by surface modification.

LPE Growth and Scintillation Properties of (Zn,Mg)O Single Crystalline Film

Among the direct wide band-gap semiconductors, ZnO is an attractive scintillator for alpha particle monitoring. However, the undoped ZnO has a dominant slow luminescence around 500-600 nm due to lattice defects. In this study, the Mg-substituted ZnO ((Zn,Mg)O) single crystalline films with high crystallinity are investigated. Mg doping is found to improve the lattice order and suppress slow luminescent component around 500-600 nm. (Zn,Mg)O single crystalline films were grown by the Liquid Phase Epitaxy (LPE) method. Alpha-ray excited radioluminescence spectra of (Zn,Mg)O film show only one emission peak around 400 nm and decay time of a few nanoseconds. This emission is assigned to free exciton. Light yield of LPE grown (Zn,Mg)O film is evaluated of about 90% of BGO.

Novel Scintillation Material—ZnO Transparent Ceramics

ZnO-based scintillation ceramics for application in HENPA LENPA analyzers have been investigated. The following ceramic samples have been prepared: undoped ones (ZnO), an excess of zinc in stoichiometry (ZnO:Zn), doped with gallium (ZnO:Ga) and lithium (ZnO:Li). Optical transmission, x-ray excited emission, scintillation decay and pulse height spectra were measured and analyzed. Ceramics have reasonable transparency in visible range (up to 60% for 0.4 mm thickness) and energy resolution (14.9% at 662 keV Cs137 gamma excitation). Undoped ZnO shows slow (1.6 {\mu}s) luminescence with maximum at 2.37 eV and light yield about 57% of CsI:Tl. ZnO:Ga ceramics show relatively low light yield with ultra fast decay time (1 ns). Lithium doped ceramics ZnO:Li have better decay time than undoped ZnO with fair light yield. ZnO:Li ceramics show good characteristics under alpha-particle excitation and can be applied for the neutral particle analyzers.

Luciferase from Fulgeochlizus bruchi (Coleoptera:Elateridae), a Brazilian click-beetle with a single abdominal lantern: molecular evolution, biological function and comparison with other click-beetle luciferases

Bioluminescent click-beetles emit a wide range of bioluminescence colors (λ(Max) = 534-594 nm) from thoracic and abdominal lanterns, which are used for courtship. Only the luciferases from Pyrophorus and Pyrearinus species were cloned and sequenced. The Brazilian Fulgeochlizus bruchi click-beetle, which inhabits the Central-west Cerrado (Savannas), is noteworthy because, differently from other click-beetles, the adult stage displays only a functional abdominal lantern, which produces a bright green bioluminescence for sexual attraction purposes, and lacks functional thoracic lanterns. We cloned the cDNA for the abdominal lantern luciferase of this species. Notably, the primary sequence of this luciferase showed slightly higher identity with the green emitting dorsal lantern luciferases of the Pyrophorus genus instead of the abdominal lanterns luciferases. This luciferase displays a blue-shifted spectrum (λ(Max) = 540 nm), which is pH-insensitive from pH 7.5 to 9.5 and undergoes a slight red shift and broadening above this pH; the lowest K(M) for luciferin among studied click-beetle luciferases, and the highest optimum pH (9.0) ever reported for a beetle luciferase. At pH 9.0, the K(M) for luciferin increases, showing a decrease of affinity for this substrate, despite the higher activity. The slow luminescence decay rate of F. bruchi luciferase in vitro reaction could be an adaptation of this luciferase for the long and sustained in vivo luminescence display of the click-beetle during the courtship, and could be useful for in vivo intracellular imaging.

Luminescence decay in hydrogenated amorphous silicon and silicon nanostructures

The stretched exponential luminescence decay observed at temperatures lower than 20K transits to the power law decay due to the electron-hopping at localized band tail states near 60K in the hydrogenated amorphous silicon (a-Si:H). The luminescence decay at 4.2K in a-Si:H is quite similar to that of Si-nanoparticles in the porous Si (p-Si). It is explained from the comparison with p-Si that the slow luminescence of the life time of ~1ms is due to the recombination of excitonic electron–hole pairs at the spin triplet state quantum-confined in the hydrogen-free Si nanostructure in a-Si:H. The fast luminescence of the life time of ~1μs is due to the recombination of the pairs at the spin-singlet state and the life time is explained as due to the indirect optical transition.