Luminescence Lifetime Measurements Research Articles

Fe2+-doped CdSe single crystal: growth, spectroscopic and laser properties, potential use as a 6 µm broadband amplifier

We report on the successful growth of single crystals of Fe2+:CdSe by seeded physical vapor transport (SPVT) technique with doping within the growing process and subsequent annealing in Se vapor. Luminescence lifetime measurements, spectroscopic studies of 5E–5T2 transition of Fe2+ in CdSe, and laser experiments were performed. The lifetime of the 5T2 energy level was measured to be 20 ± 5 ns at a room temperature (RT) of 290 K. At liquid nitrogen (LN) temperature, luminescence kinetics displayed a non-exponential decay, which can be fitted to a bi-exponential function with time constants τ1 = 6 µs and τ2 = 29 µs. As much as 3.2 mJ of output energy at 5.2 µm with 27% absorbed pump energy slope efficiency of an Fe2+:CdSe laser was achieved at RT under 2.94 µm nanosecond Er:YAG laser pumping. The Fe2+:CdSe laser was tuned from 4.63 to 6.10 µm. Obtained characteristics of Fe2+:CdSe indicate that the crystal can be considered a promising medium for amplification of femtosecond pulses in the middle infrared range up to 6 µm.

Polymorphism-based luminescence of lanthanide complexes with a deuterated 1,10-phenanthroline

Abstract Deuteration and/or polymorphism effects on the ff emission of lanthanide complexes were discussed by using phenanthroline (phen) and deuterated phenanthroline (phen - d 8 ) complexes with Eu, Tb, Gd and Tm ion as a center metal. Lanthanide complexes with phen or phen-d 8 form polymorphism such as phase IIa and I . The ratio of polymorphism IIa to I of Gd or Tm complexes with phen was newly estimated by a synchrotron XRPD. From the measurement of luminescence spectra, lifetimes and absolute quantum yields at room temperature and 77 K, it was found that the ff emissions of above lanthanide complexes were explained by their energy relaxation process through intermolecular energy transfer and the polymorphism effects. Luminescence of Eu was intensified by the deuteration of phen, due to deactivation with the overtones of C-H/D vibrations, and each luminescence quantum yield of two phase was theoretically estimated. Tb had no influence clearly by the deuteration in phen, but did by the polymorphism formation. Blue luminescence of Tm was observed in both phen/phen-d 8 complexes. Furthermore, the luminescence component ratios of lanthanide complexes with phen corresponded to the ratios from the observation of XRD.

Coordination Chemistry and f-Element Complexation by Diethylenetriamine-N,N″-bis(acetylglycine)-N,N',N″-triacetic Acid.

Potentiometric and spectroscopic techniques were used to evaluate the coordination behavior and thermodynamic features of trivalent f-element complexation by diethylenetriamine-N,N″-bis(acetylglycine)-N,N',N″-triacetic acid (DTTA-DAG) and its di(acetylglycine ethyl ester) analogue [diethylenetriamine-N,N″-bis(acetylglycine ethyl ester)-N,N',N″-triacetic acid (DTTA-DAGEE)]. Protonation constants and stability constants of trivalent lanthanide complexes (except Pm3+) were determined by potentiometry. Six protonation sites and three metal-ligand complexes [ML2-, MHL-, and MH2L(aq)] were quantified for DTTA-DAG. Four protonation sites and one metal-ligand complex [ML(aq)] were observed for DTTA-DAGEE, consistent with the presence of two ester groups. Absorption spectroscopy was utilized to measure the stability constants for complexation of trivalent neodymium and americium by DTTA-DAG and trivalent neodymium by DTTA-DAGEE. The coordination environment of trivalent europium in the presence of DTTA-DAG was investigated at various acidities by luminescence lifetime measurements. Decay constants indicate one water molecule in the inner coordination sphere across the 1.0 < pH < 5.5 range, presumably due to octadentate coordination by DTTA-DAG. A trans-lanthanide pattern of complex stabilities for DTTA-DAG was found to be analogous to that observed for DTPA, with a ∼106 reduction of the complex stability. The lessened strength of complexation, relative to DTPA, was attributed to significant reduction of the total ligand basicity for DTTA-DAG due to the electronic influence of amide functionalization. When DTTA-DAG is used as an aqueous holdback complexant in liquid-liquid distribution experiments, the preferential coordination of Am3+ in the aqueous environment offers efficient An/Ln differentiation. The separation extends to pH 2 conditions, where the kinetics of phase transfer in such liquid-liquid systems are aided by the acid-catalyzed dissociation of a metal/aminopolycarboxylate complex.

Confocal Luminescence Lifetime Imaging with Variable Scan Velocity and Its Application to Oxygen Sensing.

The dependence of the luminescence lifetime on the probe environment is the basis of a range of sensing techniques. The major advantage of using the lifetime as the sensitive parameter is its independence on the probe concentration. However, the instrumentation for lifetime measurements is complex, generally requiring time-resolved excitation and detection. Here, we present a simple method for the measurement of luminescence lifetimes on the microsecond scale based on variable excitation time determined by the scanning velocity. The technique is implemented in a confocal laser scanning microscope (CLSM), thus allowing not only simple lifetime measurement but also phosphorescence lifetime imaging. Since the method exploits the spatiotemporal dependence of sample excitation in a CLSM, there is no need for a pulsed or modulated light source or for additional time-resolved detection. The method can be realized in a standard CLSM without any modifications. The principle is demonstrated on oxygen sensing by collisional quenching of an oxygen-sensitive ruthenium(II) complex.

Tb3+ ion doping into Al2O3: Solubility limit and luminescence properties

Tb3+-activated Al2O3 phosphors with a molar ratio of are synthesized by metal organic decomposition (x = 0–0.15) and subsequent calcination at Tc = 200–1200 °C for 1 h in air. The material properties of the synthesized phosphors are investigated by X-ray diffraction (XRD), photoluminescence (PL) analyses, PL excitation spectroscopy, and luminescence lifetime measurements. At x = 0.015, the metastable phase of γ-Al2O3 is obtained by calcination at Tc ∼ 300–1050 °C and a mixture of γ, θ, and α phases at Tc ∼ 1050–1150 °C. The high-temperature stable phase of α-Al2O3 is obtained only at Tc ≥ 1150 °C. Below Tc ∼ 300 °C, the XRD data suggest the formation of boehmite (AlOOH). The solubility limit of Tb3+ in α-Al2O3 is also clearly determined to be x ∼ 0.015 (1.5%). The PL decay time of the Tb3+ green emission in α-Al2O3 is ∼1.1 ms for x < 0.015 and slowly decreases with further increase in x (Tb3+). The schematic energy-level diagram of Tb3+ in α-Al2O3 is proposed for a better understanding of the present phosphor system. Finally, the temperature dependence of the PL intensity is examined between T = 20 and 450 K, yielding quenching energies of Eq ∼ 0.28 eV (α-Al2O3 and γ-Al2O3).

Synthesis and characterization of BaLuF5: Tb3+ oxyfluoride glass ceramics as nanocomposite scintillator for X-ray imaging

Tb3+-dopedoxy-fluoride glass ceramics (GC) containing BaLuF5 nanocrystals were manufactured successfully by melt-quenching method with further thermal treatment. Their micro-structural and optical properties were investigated systemically by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) techniques, transmittance spectra, photoluminescence (PL), luminescence lifetime measurements and X-ray excited luminescence (XEL). PL and XEL from BaLuF5:Tb3+ GC are both highly enhanced after further thermal treatment benefiting from the preferential incorporation of Tb3+ ions into the crystallized BaLuF5 nanocrystals with low phonon energy. Impressively, the integrated XEL intensity for GC640 is about 26% of the commercial BGO scintillator. Our investigation indicates that such Tb3+-doped transparent BaLuF5 GC are potential candidates of X-ray scintillators for slow event detection.

Luminescence and luminescence quenching of Eu2Mo4O15

A polycrystalline Eu2Mo4O15 phosphor sample was prepared by high temperature solid state reaction. Phase purity and morphological features of the phosphor were investigated by X-ray diffraction and scanning electron microscopy, respectively. Reflectance spectra showed that the optical band gap of Eu2Mo4O15 is 2.95eV. Phosphor emits intensive red light when excited with 394 and 465nm radiation. Temperature dependent emission and luminescence lifetime measurements revealed that external and internal quantum yields decrease at the same rate and that luminescence quenches due to photoionization. The calculated external quantum yields for 394 and 465nm excitation were 7.8% and 53.5%, respectively.

Laser site-selective spectroscopy of Eu3+ ions doped Y4Al2O9

Eu3+ doped Y4Al2O9 (YAM) crystals were prepared by the micro-pulling down method. Optical-absorption and laser-selective-excitation techniques along with the luminescence decays have been used to reveal that Eu3+ ions in YAM occupy three distinct sites, which were characterized and discussed. The Stark energy levels of Eu3+ at three different sites in YAM were assigned from selectively excited emission spectra at 10 K. The intensity ratio of forced electric dipole (5D0 → 7F2) and magnetic dipole (5D0 → 7F1) transitions was discussed in order to obtain information about the degree of asymmetry of the luminescent centers. These results were confirmed by the luminescence lifetime measurements. The temperature dependent photo-luminescence spectra indicated that there is no energy transfer between different sites in the 10–300 K range.

Luminescence investigations of rare earth doped lead-free borate glasses modified by MO (M = Ca, Sr, Ba)

Series of lead-free borate glasses with different oxide modifiers and lanthanide ions were prepared. The effect of oxide modifiers MO (M = Ca, Sr, Ba) on spectroscopic properties of trivalent Ln3+ (Ln = Eu, Er, Pr) were systematically investigated. Especially, the luminescence spectra of Ln3+-doped lead-free borate glasses are presented and discussed in relation to the impact of selective components (CaO, SrO and BaO). Several spectroscopic parameters, such as the fluorescence intensity ratio R/O (Eu3+) and measured luminescence lifetimes for the 5D0 (Eu3+), 4I13/2 (Er3+) and 1D2 (Pr3+) excited states of lanthanide ions were analyzed in details. The research proved that spectroscopic properties of trivalent Ln3+ depend significantly on kind of presence oxide modifiers MO (M = Ca, Sr, Ba) in glass host matrices.

Enhanced emissions in self-crystallized oxyfluoride scintillating glass ceramics containing KTb_2F_7 nanocrystals

Self-crystallized KTb2F7 oxy-fluoride glass ceramics (GC) were successfully manufactured via the traditional melt-quenching method. KTb2F7 nanocrystals were already formed after melt-quenching, which is beneficial to the realization of controllable glass crystallization to some degree for affording desirable nano-crystal size and activator partition. Their microstructural and optical properties were systemically investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), absorption spectra, photoluminescence (PL), luminescence lifetime measurements and X-ray excited luminescence (XEL). Both PL and XEL of GC samples are highly enhanced because more nanocrystals formed and grew up after heat-treatment. Our investigation suggests that transparent KTb2F7 glass ceramics may present potential application in X-ray scintillator for X-ray imaging. And our strategy that takes active ions as host may contribute to designing other oxy-fluoride GC by using active ions as host.

Solubility limit and luminescence properties of Eu3+ ions in Al2O3 powder

Al–Eu–O compounds are synthesized from Al2O3:Eu2O3=(1–x):x mixtures (x=0–0.15) by the metal organic decomposition method and subsequently calcined at various temperatures from Tc=750 to 1200°C in dry O2 atmosphere. The structural and luminescence properties of these compounds are investigated using X-ray diffraction analysis, photoluminescence (PL) analysis, PL excitation spectroscopy, and luminescence lifetime measurements. The present study focuses on the effects of the Eu2O3 addition (x) on the material and phosphor properties of Al2O3:Eu3+. The stable phase of α-Al2O3 is synthesized at Tc>1100°C and cubic γ-Al2O3 phase at Tc≤1100°C. The calcination temperature dependence of the PL intensity yields an activation of Ea~0.8eV for Eu3+ ions in the Al2O3 host. The luminescence decay time is determined to be ~0.8ms, independent of x. Temperature dependence of the PL intensity at T=20–450K exhibits thermal quenching behavior with energies of 17meV and 0.28eV at low (<200K) and high temperatures (>200K), respectively. The solubility limit of Eu3+ ions in α-Al2O3 is determined to be ~1%. The schematic energy-level diagram of Eu3+ in α-Al2O3 is also proposed for the sake of a better understanding of the luminescence process of this phosphor system.

Gd3TCAS2: An Aquated Gd(3+)-Thiacalix[4]arene Sandwich Cluster with Extremely Slow Ligand Substitution Kinetics.

In aqueous solution, Gd3+ and thiacalix[4]arene-p-tetrasulfonate (TCAS) form the complex [Gd3TCAS2]7–, in which a trinuclear Gd3+ core is sandwiched by two TCAS ligands. Acid-catalyzed dissociation reactions, as well as transmetalation and ligand exchange with physiological concentrations of Zn2+ and phosphate, showed [Gd3TCAS2]7– to be extremely inert compared to other Gd complexes. Luminescence lifetime measurements of the Tb analogue Tb3TCAS2 allowed estimation of the mean hydration number q to be 2.4 per Tb ion. The longitudinal relaxivity of [Gd3TCAS2]7– (per Gd3+) was r1 = 5.83 mM–1 s–1 at 20 Hz (37 °C, pH 7.4); however, this relaxivity was limited by an extremely slow water exchange rate that was 5 orders of magnitude slower than the Gd3+ aqua ion. Binding to serum albumin resulted in no relaxivity increase owing to the extremely slow water exchange kinetics. The slow dissociation and water exchange kinetics of [Gd3TCAS2]7– can be attributed to the very rigid coordination geometry.

Formation kinetics of europium(III) complexes of DOTA and its bis(phosphonate) bearing analogs

Complexation of Eu(III) ions with H4dota and its bis(phosphonate)-bearing analogs, H8do3aPBP with phosphinate spacer and H7dotamBP with carboxamide spacer, was studied by UV–Vis spectrophotometry and luminescence life-time measurements. The results show formation of two types of out-of-cage intermediates. The [EuL]oc intermediate is formed under metal ion excess, whereas [EuL2]oc intermediate is formed under ligand excess. At room temperature, their conversion to in-cage complex is comparable but the [EuL2]oc transformation into the in-cage complex is significantly faster at elevated temperatures. Both bis(phosphonate)-bearing ligands show slower complexation rate than H4dota. Comparison of reactivity of H8do3aPBP and H7dotamBP show important role of the spacer connecting the bis(phosphonate) group with the macrocycle. Presence of phosphinate in H8do3aPBP leads to significantly faster (3–4 orders of magnitude) complexation than in case of the amide spacer in H7dotamBP. Both bis(phosphonate)-bearing ligands show nonlinear dependence of the complexation rate on concentration of hydroxide anions due to changes in protonation state of the bis(phosphonate) moiety.

Online analysis of oxygen inside silicon-glass microreactors with integrated optical sensors

A powerful online analysis set-up for oxygen measurements within microfluidic devices is presented. It features integration of optical oxygen sensors into microreactors, which enables contactless, accurate and inexpensive readout using commercially available oxygen meters via luminescent lifetime measurements in the frequency domain (phase shifts). The fabrication and patterning of sensor layers down to a size of 100μm in diameter is performed via automated airbrush spraying and was used for the integration into silicon-glass microreactors. A novel and easily processable sensor material is also presented and consists of a polystyrene- silicone rubber composite matrix with embedded palladium(II) or platinum(II) meso-tetra(4-fluorophenyl) tetrabenzoporphyrin (PdTPTBPF and PtTPTBPF) as oxygen sensitive dye. The resulting sensor layers have several advantages such as being excitable with red light, emitting in the near-infrared spectral region, being photostable and covering a wide oxygen concentration range. The trace oxygen sensor (PdTPTBPF) in particular shows a resolution of 0.06–0.22hPa at oxygen concentrations lower than 20hPa (<2% oxygen) and the normal range oxygen sensor (PtTPTBPF) shows a resolution of 0.2–0.6hPa at low oxygen concentrations (<50hPa) and 1–2hPa at ambient air oxygen concentrations. The sensors were integrated into different silicon-glass microreactors which were manufactured using mass production compatible processes. The obtained microreactors were applied for online monitoring of enzyme transformations, including d-alanine or d-phenylalanine oxidation by d-amino acid oxidase, and glucose oxidation by glucose oxidase.

Room‐Temperature Phosphorescence of Crystalline 1,4‐Bis(aroyl)‐2,5‐dibromobenzenes

AbstractThere is a growing interest in metal‐free organic luminophores that exhibit phosphorescence at room temperature. We report herein that 1,4‐bis(aroyl)‐2,5‐dibromobenzenes serve as a new class of such luminophores. The bis(aroyl)benzene derivatives were not emissive either in solution or in a doped polymer thin film. In contrast, crystals of bis(aroyl)benzenes showed photoluminescence under ambient conditions, which was subsequently confirmed as phosphorescence with a luminescence lifetime measurement. The emission color ranged from blue to green, depending on the nature of the aroyl groups, and the luminescent quantum yields were 0.05–0.18 at room temperature and 0.38–0.67 at 77 K. In each crystal, several intermolecular interactions such as C=O···H, Br···Br, C=O···Br, F···F, S···H, and MeO···H were observed, which contributed to the restriction of intramolecular motions, thereby diminishing the nonradiative decay process from the triplet excited states.

Transformation of LaOF into LaF3owing to Al3+in luminescent Eu3+-doped crystalline powders

Conversion of LaF3 into LaOF has been demonstrated in the literature through heat treatment of the samples. In this work, we investigated the possibility of converting LaOF into LaF3 using Al3+ in a combustion synthesis technique. The conversion was proposed by taking advantage of the affinity that aluminium has for oxygen. The morphological study via scanning electron microscopy has shown an increase in the porosity of the samples with the inclusion of aluminium. X-ray powder diffraction data analysis demonstrated complete crystallization and conversion of LaOF into LaF3 due to the presence of aluminium. A small fraction of LaAlO3 has been detected and it confirms that aluminium bonds with oxygen, as expected. The samples were also doped with the rare-earth ion europium as a luminescence probe. Stokes photoluminescence was produced using UV light (λ = 355 nm) and the spectral analysis of Eu3+ 4f–4f electronic transitions has confirmed that LaOF is converted into LaF3. The measured luminescence lifetimes of reactant and product are characteristic of Eu3+ in the compounds LaOF and LaF3.

Photoluminescence of Pr3+-doped calcium and strontium stannates

A series of Pr3+-doped CaSnO3, SrSnO3 and Ca2SnO4 samples were prepared by a conventional high temperature solid-state reaction route. All samples were characterized by powder X-ray diffraction (XRD) analysis, photoluminescence (PL), photoluminescence thermal quenching (TQ) and fluorescence lifetime (FL) measurements. Moreover, luminous efficacies (LE) and color points according to the CIE 1931 color space diagram were calculated and discussed. The incorporation of Pr3+ ions into CaSnO3 and SrSnO3 host materials results in intense white luminescence, which originates from the mixture of green, orange and red emissions. Pr3+-doped Ca2SnO4 phosphor emits intense red light upon excitation. Temperature dependent luminescence lifetime measurements of SrSnO3:Pr3+ phosphor showed abnormalities near 180 and 260K, which confirm the existence of a low temperature structural phase transitions in SrSnO3.

Enhanced emissions in Tb3+-doped oxyfluoride scintillating glass ceramics containing KLu2F7 nano-crystals

KLu2F7: Tb3+ oxyfluoride glass ceramics (GC) were successfully manufactured via traditional melt-quenching route with further heat-treatment. Their micro-structural and optical properties were systemically investigated by XRD, TEM, HRTEM techniques, transmittance spectra, excitation spectra, emission spectra, luminescence lifetime measurements and X-ray excited luminescence (XEL). Both photoluminescence and XEL of KLu2F7 GC doped with Tb3+ are highly enhanced after further heat-treatment resulting from the partition of Tb3+ ions into the KLu2F7 crystalline lattice with low phonon energy. Our investigation suggests that such Tb3+-doped transparent KLu2F7 glass ceramics may present potential application in X-ray scintillator for slow event detection.

PH and copper ion luminescence on/off sensing by a dipyrazinylpyridine-appended ruthenium complex

A new ruthenium(II) complex of [Ru(bpy)2(HL1)](ClO4)2·4H2O (1·4H2O) {bpy=2,2′-bipyridine and HL1=2-(4-(2,6-di(pyrazin-2-yl)pyridin-4-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline} was synthesized and characterized by elementary analysis, proton nuclear magnetic resonance spectroscopy and mass spectrometry. The ground- and excited-state acid–base properties of 1 were studied by means of UV–vis absorption spectrophotometric and spectrofluorimetric titrations in 100:1 (v/v) Britton–Robinson buffer/CH3CN solution in combination with luminescence lifetime measurements. The complex exhibited two-step separate protonation/deprotonation processes in both the ground and excited states, acting as pH-induced “off–on–off” luminescence switches (Ion/Ioff=100.0 and 3.96). Importantly, 1 also acted as a highly selective luminescent chemosensor toward Cu2+ over other metal cations tested (Na+, Mg2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Ag+).

Macrocyclic Gd(3+) complexes with pendant crown ethers designed for binding zwitterionic neurotransmitters.

A series of Gd(3+) complexes exhibiting a relaxometric response to zwitterionic amino acid neurotransmitters was synthesized. The design concept involves ditopic interactions 1) between a positively charged and coordinatively unsaturated Gd(3+) chelate and the carboxylate group of the neurotransmitters and 2) between an azacrown ether appended to the chelate and the amino group of the neurotransmitters. The chelates differ in the nature and length of the linker connecting the cyclen-type macrocycle that binds the Ln(3+) ion and the crown ether. The complexes are monohydrated, but they exhibit high proton relaxivities (up to 7.7 mM(-1) s(-1) at 60 MHz, 310 K) due to slow molecular tumbling. The formation of ternary complexes with neurotransmitters was monitored by (1) H relaxometric titrations of the Gd(3+) complexes and by luminescence measurements on the Eu(3+) and Tb(3+) analogues at pH 7.4. The remarkable relaxivity decrease (≈80 %) observed on neurotransmitter binding is related to the decrease in the hydration number, as evidenced by luminescence lifetime measurements on the Eu(3+) complexes. These complexes show affinity for amino acid neurotransmitters in the millimolar range, which can be suited to imaging concentrations of synaptically released neurotransmitters. They display good selectivity over non-amino acid neurotransmitters (acetylcholine, serotonin, and noradrenaline) and hydrogenphosphate, but selectivity over hydrogencarbonate was not achieved.