Luminescence Decay Characteristics Research Articles

Donor-Acceptor Pairs Recombination as the Origin of the Emission Shift In InGaN/GaN Scintillator Heterostructures Doped with Zn

We report luminescence decay characteristics of the InGaN/GaN scintillator heterostructures doped with Zn. Unusually large shifting of luminescence band caused by Zn acceptors incorporated in InGaN is observed both in time-resolved and excitation-dependent spectra. Origins of the shifts are discussed, and model based on donor-acceptor pair recombination is introduced. The results imply a shrinkage of donor Bohr radius compared to the bulk material caused by quantum confinement effect. The slow decay of Zn band points out to the necessity of Zn impurity elimination in applications requiring fast timing characteristics of a scintillator.

Energy transfer dynamics and photoluminescence properties of sol-gel synthesized dense-packed Ca3−(x+y)TbxSmyMgSi2O8 phosphor

The Ca3−(x+y)TbxSmyMgSi2O8 (x = 0–0.15, y = 0–0.04 mol), dense packed merwinite type crystalline phosphors were studied for color tunability via Tb+3 → Sm+3 energy transfer. All the phosphor samples were prepared by acid catalyzed sol-gel reaction route followed by high temperature annealing. The structural and optical characteristics of the phosphors were ascertained from X-ray diffraction, scanning electron microscopy, Fourier transforms infrared and photoluminescence spectroscopy. The synthesized phosphors were devoid of any detectable impurities or phase separation and belonged to phase pure monoclinic crystal system in the P121/a1 space group. Tb+3 doped samples demonstrated intense green emission from 5D4 → 7FS (S = 6, 5) upon f-d (243 nm) and f-f excitation (378 nm). Sm+3 doped samples did not exhibit charge transfer excitation and however, strong orange-red emission was realized due to the 4G5∕2 → 6HS emission upon 404 nm (f-f) excitation. The onset of concentration quenching occurred at 0.07 for Tb+3 and 0.02 mol for Sm+3 corresponding to the Tb+3-Tb+3 and Sm+3-Sm+3 atomic distance of 11.4 and 17.4 Å respectively. Both Tb+3 and Sm+3 exhibited biexponential decay with τavg of 2.23 (Tb+3) and 2.16 ms (Sm+3), which is explained on the basis of their two different oxygen coordination environment in the lattice. The occurrence of Tb+3 → Sm+3 energy transfer (ET) was established from the analysis of excitation and emission spectra, and luminescence decay characteristics of Tb+3 and Tb+3-Sm+3 doped phosphors. Tb+3 → Sm+3 ET mechanism was further illustrated from the energy level diagram, and emission decay curves. The Tb+3 → Sm+3 ET efficiency and ET probability were observed to be maximum for the Tb0.07Sm0.02 codoped phosphor and determined to be 52.9% and 0.504 × 10−3·s−1, respectively. The Dexter hypothesis explicated the dominant energy transfer mechanism to be Tb-Sm dipole-dipole interaction. The Tb+3 → Sm+3 ET lead to the emission colour tunability with discrete CIE coordinates.

Luminescence properties of large-size Li2MoO4 single crystal grown by Czochralski method

Large-size Li2MoO4 single crystal was successfully grown by Czochralski method using the polycrystalline Li2MoO4 bulks synthesized via the conventional solid-state reaction. The phase identification, thermal behavior and optical transmission spectra of Li2MoO4 single crystal were studied systematically. The band gap of Li2MoO4 was estimated from the transmission spectra according to Tauc and Menth’s law. The luminescence spectra and decay characteristics under 355 nm laser excitation were investigated in detail. Li2MoO4 single crystal had a wide luminescence peak around 540 nm as a result of intrinsic luminescence emission of tetrahedral MoO42-. The maximal luminescence intensity increased with the ambient temperature cooling down. Meanwhile, the luminescence decay times decreased monotonically with the temperature increasing from 42 K to 298 K.

Luminescence decay characteristics of CdS quantum dots doped with europium ions

In the present paper, we present an analysis of the luminescence decays for colloidal quantum dots (QDs) with interfaces doped with Eu3+ ions of the corresponding thenoyltrifluoroacetone complexes. The luminescence decay features related to concentration and temperature effects are considered. The Stern-Volmer plots obtained at 300 and 77 K for the trap state luminescence of CdS:Eu3+ QDs show that the QD luminescence quenching is of a dynamic character with the Stern-Volmer constant being Kdyn=3.7⋅105 M−1. Non-radiative excitation energy transfer from the trap state luminescence centers in CdS quantum dots to Eu3+ ions is observed. In addition, the value of the quenching rate kdyn=2.37⋅1013s−1⋅M−1 indicates that the energy transfer process has a non-diffusive nature. This conclusion is supported by the effective quenching in a frozen solution at 77 K where there is no diffusion. For CdS/TGA QDs doped with europium ions, we observe a significant increase in the luminescence lifetime (from 190 μs to 346 μs) for the band peaked at about 615 nm as compared to the free Eu:TTA complex. We conclude that Eu3+ ions are incorporated into the QD interfaces.

Study of absorption and IR-emission of Er3+, Dy3+, Tm3+ doped high-purity tellurite glasses

A study of high-purity TeO2-ZnO based tellurite glasses doped with Er3+, Dy3+ or Tm3+ that could be used as laser media in the 2-3 μm spectral range is presented. The glasses are prepared by melting the oxides mixture inside a silica glass reactor in an atmosphere of purified oxygen. The low level of hydroxyl groups absorption allowed to measure correctly the luminescence decay characteristics of the dopants. The rare-earth ions absorption bands, the luminescence spectra and kinetic characteristics of emission from the levels 4I11/2, 4I13/2 of Er3+, 6H13/2 of Dy3+ and 3H4, 3H5, 3F4 of Tm3+ ions are investigated. The results confirm the high potential of tellurite glasses as an active media for bulk, planar waveguide and fiber lasers.

Investigation of spectral properties of Eu3+ and Tb3+ doped strontium zirconium trioxide orthorhombic perovskite for optical and sensing applications

In this paper, the structural, morphological and spectral properties of Eu3+ and Tb3+ doped strontium zirconium trioxide perovskite phosphors are reported. The samples were synthesized by solid state reaction route with different doping concentrations of Eu3+ and Tb3+ ions. These synthesized phosphors were characterized by PXRD for structural analysis. The phosphors report orthorhombic structure with average crystallite size of 48 nm. FESEM and HRTEM analysis were done here for topographical and morphological studies. Also, the FTIR spectra of synthesized samples were investigated for functional group analysis. Photoluminescence and thermoluminescence spectra of synthesized samples were studied. On subjecting to 230 nm excitation, the phosphors give three distinct emissions of 596, 610 and 690 nm in the visible region corresponding to 5D0 → 7F1, 5D0 → 7F2 and 5D0 → 7F4 of Eu3+ ions. The synthesized samples were also subjected to CIE and Afterglow decay analysis. The average decay lifetime is recorded as 56.24 ns confirming the luminescence decay characteristics of short duration. In TL analysis of these phosphors, second-order kinetics with low activation energy varying from 0.50002 to 0.65668 eV is reported. The enhanced optical characteristics of prepared perovskite phosphor substantiate it as a proficient alternative for photovoltaic, optical and sensing applications.

Temperature dependence of CIE-x,y color coordinates in YAG:Ce single crystal phosphor

Using single-crystal YAG:Ce plates with different dopant concentration under different sample temperatures (77-500(800)K), absorption, emission and luminescence decay characteristics were measured. Simulation is used to explain the luminescence decay time increase with growing temperature. For selected incident blue excitation band and given sample thickness the composed blue-yellow spectrum is calculated after passing through the sample. Temperature dependence of the color coordinates CIE-xy is calculated, arising mostly due to specific temperature dependence of the 4f-5d1 absorption transition of Ce3+ center in the garnet host.

Anomalous carrier life-time relaxation mediated by head group interaction in surface anchored MnSe quantum dots conjugated with albumin proteins

We report on the radiative emission decay dynamics of a less known, γ-phase manganese selenide quantum dot system (MnSe QDs) subjected to bio-functionalization. A short-ligand thioglycolic acid (TGA), and a long-chain sodium dodecyl sulfate (SDS) surfactants were used as surface anchors prior bioconjugation with albumin proteins (BSA). Time resolved photoluminescence (TR-PL) spectra of the QDs have revealed bi-exponential decay trends with the fast (τ1) and slow (τ2) decay parameters assigned to the core state recombination and surface trapped excitons; respectively. The average lifetime (τavg) was found to get shortened from a value of ∼0.87 ns–0.72 ns in unconjugated and BSA conjugated MnSe-TGA QDs; respectively. Conversely, MnSe-SDS QDs with BSA conjugation exhibited nearly four-fold enhancement of τavg with respect to its unconjugated counterpart. Moreover, a considerable amount of Förster resonance energy transfer (FRET) was found to occur from the TGA coated MnSe QDs to BSA and with an ensuing efficiency of ∼61%. The origin of anomalous carrier life-time relaxation features has also been encountered through a simplified model as regards head group interaction experienced by the MnSe QDs with different surfactant types. Exploiting luminescence decay characteristics of a magneto-fluorescent candidate could find immense scope in diverse biological applications including assays, labeling and imaging.

Preparation of Biocompatible, Luminescent-Plasmonic Core/Shell Nanomaterials Based on Lanthanide and Gold Nanoparticles Exhibiting SERS Effects

Multifunctional core/shell type nanomaterials composed of nanocrystalline, lanthanide doped fluorides and gold nanoparticles (Au NPs) were successfully prepared. The products were synthesized to combine luminescence properties of the core NPs, i.e., LnF3/SiO2–NH2 and KLn3F10/SiO2–NH2, and plasmonic activity of the shell Au NPs within a single nanomaterial. The luminescent lanthanide NPs (10 or 150–200 nm) were separated from the gold NPs (6–30 nm) using an amine modified silica shell (thickness ≈30 nm). The synthesized products exhibited bright green (Tb3+) and red (Eu3+) emission under UV light irradiation. Surface modification with Au NPs influenced the product emission and luminescence decay characteristics. The luminescent-plasmonic nanomaterials were used as platforms for surface enhanced Raman scattering (SERS) measurements. 4-Mercaptobenzoic acid, choline, and T4 bacteriophages were utilized as SERS probes. For all synthesized nanomaterials, the SERS spectra for all probes studied exhibited higher ...

Editors' Choice—Rb2SiF6:Mn4+ and Rb2TiF6:Mn4+ Red-Emitting Phosphors

Rb-based hexafluoride red-emitting phosphors, Rb2SiF6:Mn4+ and Rb2TiF6:Mn4+, were synthesized by the coprecipitation method. Optical microscopy observation, X-ray diffraction (XRD) measurement, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and luminescence decay characteristics measurements were used to study the structural and optical properties of the phosphors. The photographs of the bulk samples showed clear crystallographic habits originating from the cubic and trigonal symmetries of the Rb2SiF6 and Rb2TiF6 hosts, respectively, in agreement with the XRD results. The phosphors exhibited an intense narrow-band Mn4+ (2Eg → 4A2g) red emission with internal quantum efficiencies higher than 90% upon blue light excitation. The Franck−Condon analysis of the PLE data yielded the Mn4+ intra-d-shell transitions to occur at ∼2.47 eV (∼2.34 eV) for the 4A2g → 4T2g transition and at ∼2.86 eV (∼2.83 eV) for the 4A2g → 4T1g transition in the Rb2SiF6:Mn4+ (Rb2TiF6:Mn4+) phosphor. Temperature dependence of the PL spectra from T = 20 to 500 K gave the quenching temperature values (Tq’s) at which the PL intensity has fallen to half its maximum value to be Tq ∼ 490 and ∼450 K for Rb2SiF6:Mn4+ and Rb2TiF6:Mn4+, respectively, promising for use as high-temperature stable phosphors in solid-state lighting applications.

Low temperature luminescence and charge carrier trapping in a cryogenic scintillator Li2MoO4

The luminescence and optical properties of promising cryogenic scintillator Li2MoO4 were studied in the temperature region of 2–300K. The data on luminescence spectra and decay characteristics, excitation spectra, thermostimulated luminescence curves and spectra as well as transmission and reflectivity spectra are presented for the single crystals grown by two different procedures, the conventional Czochralski method and the low-temperature gradient Czochralski technique. The bandgap of Li2MoO4 is estimated from the analysis of transmission, luminescence excitation and reflectivity spectra. Up to three luminescence bands with the maxima at 1.98, 2.08 and 2.25eV are detected in the emission spectra of crystals and their origin is discussed. In the thermoluminescence curves of both studied crystals, two high-intensity peaks were observed at 22 and 42K, which are ascribed to the thermal release of self-trapped charge carriers. The coexistence of self-trapped electrons and holes allows one to explain the poor scintillation light yield of Li2MoO4 at low temperatures.

Investigation on luminescence enhancement and decay characteristics of long afterglow nanophosphors for dark-vision display applications

Long afterglow SrAl2O4:Eu2+,Dy3+ nanophosphors were synthesized via a facile but effectual auto-combustion technique followed by post-annealing treatment at elevated temperatures. The influence of various fuels during synthesis and thereafter improvement in the luminescence decay characteristics under various illuminant irradiations of long afterglow nanophosphors have been reported. Extensive studies on structural, morphological and luminescent properties of the as-synthesized afterglow nanophosphors have been presented. Powder X-ray diffraction studies confirm the presence of high-purity, single-phase monoclinic nanophosphors. HRTEM investigations confirm the formation of nanophosphors of particle size less than 50nm. Photoluminescence emission is attributed to the characteristic d–f transition (4f65d1→4f7) of Eu2+ ions and was positioned at 512nm. As-synthesized nanophosphors exhibit considerable confinement effects resulting into blue shift in emission maxima as compared to their bulk counterparts. The mechanism underlined for long afterglow has been discussed using trapping–detrapping model. The nanophosphor being multifunctional finds many interesting applications including dark-vision display, energy storage, fingerprint detection, in vivo and in vitro biological staining, etc.

Structural and Luminescence Properties of EuAlO3 Ternary Compound and Al2O3−Eu2O3 Compositions

EuAlO3 ternary compound and Al2O3−Eu2O3 compositions are synthesized by metal-organic decomposition method and their structural and luminescence properties are investigated using X-ray diffraction analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and luminescence lifetime measurements. The effects of calcination temperature on the structural and Eu3+ emission properties are studied in detail. Temperature dependence of the PL intensity at T = 20–450 K exhibits clear thermal quenching behavior, yielding quenching energies of 14 meV and 0.22 eV at low and high temperature regions, respectively. The stable phase of orthorhombic EuAlO3 is formed at activation energy of ∼0.8 eV. The schematic energy-level diagram for Eu3+ in EuAlO3 is also proposed for the sake of a better understanding of the PL and PLE processes in this host material. The luminescence decay characteristics indicate that the decay times are given by the concentration quenching-limited values in the Eu2O3-rich materials. The measured Eu3+-emission decay times are τav ∼ 20 μs in the (1 − x)Al2O3 : xEu2O3 compounds with x = 0.5 (EuAlO3) and 1.0 (Eu2O3). These values are much smaller than τav ∼ 500 μs in Al2O3:Eu3+ (x = 0.05).

Thermal and luminescent properties of M2Zn(VO3)4 (M = Rb, Cs)

We have developed processes for the synthesis of the Rb2Zn(VO3)4 and Cs2Zn(VO3)4 tetrametavanadates. Rb2Zn(VO3)4 has been prepared by solid-state reaction (350°C) between presynthesized RbVO3 and ZnV2O6 powders, and Cs2Zn(VO3)4 has been prepared by the Pechini method (sol-gel process). Both metavanadates crystallize in monoclinic symmetry (sp. gr. P21/m). Thermochemical characterization results demonstrate that the vanadates undergo complex transformations during heating to 450°C and subsequent cooling. As a result, the materials are in a nonequilibrium state at room temperature and consist of both the parent double metavanadates and their peritectic decomposition products. We believe that the formation of the structure of the M2Zn(VO3)4 compounds from their melts is a kinetically hindered process. These compounds are structurally stable only at temperatures below 369 (Rb2Zn(VO3)4) or 420°C (Cs2Zn(VO3)4). We have measured for the first time the diffuse reflectance and photoluminescence excitation spectra of the two tetrametavanadates in their emission range and their photoluminescence spectra at various excitation wavelengths and determined their chromaticity coordinates. Their X-ray luminescence and scintillation decay characteristics have been determined for the first time under pulsed electron beam excitation. The electron excitation dissipation processes in the cesium and rubidium compounds are shown to be similar. We discuss the origin of the emission bands in the mixed vanadates and their potential application areas.

Dose rate measurements with a ruby-based fiber optic radioluminescent probe

Fiber optic radioluminescence dosimetry allows real-time dose rate measurements in complex, narrow geometries and at places of high dose rates, without exposing the operator or the susceptible electronics. The keys are the spatial separation of radiation sensitive probe and electronic processing system and their optical connection by a flexible light guide. The small probes are capable of measuring fields of high dose rate gradients and the sealed probe-tip qualifies for applications in the fluid milieu and even for in-vivo-dosimetry. One problem of fiber optic dosimetry is the generation of Cherenkov radiation and fiber luminescence in the irradiated light guide, the so called stem effect. Ruby (Al2O3:Cr) has a narrow radioluminescent emission at 694 nm and is a potential luminophor for fiber optic radioluminescence dosimetry. In this work the influence of the stem effect on our ruby-based fiber optic dosimetry system is examined. The behavior of ruby probes under irradiation up to 0.5 kGy, as well as their luminescence decay characteristics and the applicability for measurements in radiotherapeutic fields are investigated.

X-ray detection capability of a BaCl2 single crystal scintillator

The x-ray detection capability of a scintillation detector equipped with a BaCl2 single crystal was evaluated. The scintillation decay kinetics can be expressed by a sum of two exponential decay components. The fast and slow components have lifetimes of 1.5 and 85 ns, respectively. The total light output is 5% that of YAP:Ce. A subnanosecond timing resolution was obtained. The detection efficiency of a 67.41 keV x-ray is 87% for a detector equipped with a BaCl2 crystal 6-mm thick. Thus, excellent timing resolution and high detection efficiency can be simultaneously achieved. Additionally, luminescence decay characteristics under vacuum ultraviolet excitation have been investigated. Radiative decay of self-trapped excitons is thought to be responsible for the fast scintillation component.

In situ ligand exchange of thiol-capped CuInS2/ZnS quantum dots at growth stage without affecting luminescent characteristics

An aliphatic thiol ligand of CuInS2/ZnS core/shell quantum dots is replaced with a hydroxyl-terminated thiol ligand by utilizing ‘on–off state’ of ligands during growth stage of the quantum dots. After the ligand-exchange, negligible differences were observed on both photoluminescence spectrum and luminescent quantum efficiency. The reason for the high retention of luminescent efficiency comes from no local agglomeration and no surface deterioration of QDs. It is also observed that 70% of initial ligands are exchanged by the replacing ligand, determined by FT-IR and 1H NMR. The proposed method provides the quantum dots with an excellent dispersibility in polar solvents, supported by identical luminescence decay characteristics of the QDs.

VUV-UV-visible luminescence of Nd3+, Er3+ and Tm3+ and energy distribution in LiLuF4 single crystal host

It is the aim of this contribution to provide an overview of luminescence spectra and decay characteristics of Nd3+, Er3+ and Tm3+ centers in LiLuF4 single crystal including possible energy transfer mechanisms towards and away from the Nd3+ 5d excited state. Single crystal doped with the mentioned RE ions were prepared by micro-pulling-down technique. Excitation and emission spectra and fast decay kinetics in VUV spectral region are completed with radio-and photoluminescence spectra and decay kinetics in UV-visible region.

Luminescence of YAl3 (BO3)4: Eu2+, Dy3+ phosphor and its luminescence decay characteristics

Eu2+, Dy3+ co-doped YAl3 (BO3)4 phosphors are synthesized by sol-gel method. The phosphors show prominent blue luminescence due to the 4f7–4f65d transition of Eu2+. The emission intensity is greatly improved when Dy3+ is doped into the YAl3 (BO3)4:Eu2+ system. The 1% Dy3+ in Eu2+, Dy3+ co-doped YAl3 (BO3)4 phosphors is the optimum doping concentration. The luminescence decay characteristics of the samples have also been investigated, exhibiting the decay times of approximately 0.1 μs, which is much shorter compare to other Eu2+ doped phosphors.

Time-resolved photoluminescence decay characteristics of bovine serum albumin-conjugated semiconductor nanocrystallites

We report processing and luminescence decay characteristics of Cd1− x Zn x S composite nanocrystals (NCs) conjugated with bovine serum albumin (BSA) proteins. Time-resolved study on unconjugate NCs (with dimensions less than the bulk exciton Bohr radius) suggests that in the radiative emission, the fast (τ 1) and the slow (τ 2) carrier components are equally competitive for a given stoichiometry. Conversely, bioconjugate NCs advocate that the decay component due to the free exciton recombination is ∼9 times faster than the component due to the surface recombination emission. The observation of two distinct decay parameters is due to the fact that the NCs have experienced photostability by way of binding and protecting NC surface with biomolecules (BSA) as binding agents. The occurrence of two decay constants would help in extracting information with regard to the nature of surface recombination, free-exciton relaxation along with the strength of emission. Furthermore, with the increase in % Zn, slow carrier component gets slower owing to the incorporation of extra surface traps due to Zn/Cd incompatibility while making perfect lattice sites in the NCs. As a result, surface emission intensity gets improved compared to the radiative intensity due to core-state direct transitions. Understanding photoluminescence decay of bioconjugated NCs, on a comparative basis, would find scope for biomolecular labelling, sensing and electrophysiology applications.