Emission Decay Time Research Articles

Structures, electronic and luminescent properties of Cu(I)-quinoline complex at different temperatures and its application to red light-emitting diode

A mononuclear Cu(I) complex, Cu(dppb)(2,2′-biquinoline)]BF4(1) (dppb = 1, 2-bis(diphenyl phosphino)benzene), has been synthesized and characterized at 100 K, 150 K, 200 K, 250 K and 298 K. The structural analysis reveals that the rise of temperature from 100 K to 298 K leads to the increasement of molecular size, such as the unit-cell parameters and volume, and the change of bond lengths, bond angles, CH…π and π…π interactions. DFT calculations indicate that the HOMO → LUMO energy gap and Mülliken atomic charges are changed by the increase of temperature from 100 K to 298 K, and the component of the HOMOs and LUMOs are barely changed at 100–298 K, which is in accordance with the variation of DOS and PDOS at different temperatures. Meanwhile, the surprising broad blue-yellow excitation bands are observed at 100–298 K and the maximum emission is increasing with the blue shift from 735 nm at 100 K to 685 nm at 298 K. Furthermore, the emission decay time of complex 1 reaches 3 μs at 298 K. In addition, copper(I) complex 1 is used to fabricate the monochromatic LED, which emits a red light.

Fluorescent 1-hydroxy-10-alkylacridin-9(10H)-one BF2-chelates: Large Stokes shift and long emission decay times

New 1-hydroxy-10-alkylacridin-9(10H)-one BF2-chelates absorb in the blue-green part of the electromagnetic spectrum and emit fluorescence with moderate quantum yields of 8–45% in toluene. The dyes show large Stokes shifts about 4300 cm−1, decay times between 5 ns and 15 ns in toluene and high photostabilities. Introduction of a fluorine atom into the acridone cycle results in an increase of the fluorescence quantum yield and decay time whereas immobilization in a rigid polymer matrix (polystyrene) further extends the lifetime up to 18 ns. Large Stokes shifts and long emission decay time make this dye class an interesting platform for time-resolved imaging and sensing applications.

Cerium-doped gadolinium fine aluminum gallate in scintillation spectrometry

Performance of Mg2+ co-doped GAGG(Ce), hereafter called as cerium-doped gadolinium fine aluminum gallate, GFAG(Ce), in gamma-ray spectrometry has been investigated. The study of two samples of 10 × 10 × 5 mm3 GFAG(Ce) crystals covers the tests of emission spectra, light output, non-proportionality, energy resolution, time resolution, and decay time of light pulses. The results obtained with the GFAG(Ce) were compared to the scintillators, such as GAGG(Ce), LSO(Ce), NaI(Tl), or LuAG(Pr). The measured light yield for tested GFAG(Ce) samples is 25000–27000 ph/MeV, with the energy resolution for 662 keV gamma-rays from 137Cs source of about 7%.

Lithium-aluminum-zinc phosphate glasses activated with Sm3+, Sm3+/Eu3+ and Sm3+/Tb3+ for reddish-orange and white light generation

Spectroscopic evaluation of Sm3+, Sm3+/Eu3+ and Sm3+/Tb3+ doped lithium-aluminum-zinc phosphate glasses, based on excitation and emission spectra, and emission decay time measurements, were particularly focused on reddish-orange and white light emitting diode applications. The Sm3+ doped glass exhibits a reddish-orange emission tonality of 1676 K and a high color purity (CP) of 97.6% upon 408 nm excitation. The Sm3+/Eu3+ co-doped glass displays reddish-orange emission tonality of 1621 K (CP = 98.0%) and 2012 K (CP = 98.7%), upon 346 and 392 nm excitations, respectively. The Sm3+/Tb3+ co-doped glass emits neutral white light of 4946 and 4301 K upon 337 and 377 nm excitations, respectively, as well warm white light of 3504 K and reddish-orange light of 1758 K (CP = 91.5%) upon 370 and 396 nm excitations, respectively. The Sm3+/Tb3+ co-doped glass, excited at 337 and 396 nm, shows the highest values of luminous efficiency of radiation (LER = 444 lm/W) and color rendering index (CRI = 97), respectively. The Tb3+ and Sm3+ emission decay shortening in presence of Sm3+ and Eu3+, respectively, points out to Tb3+→Sm3+ and Sm3+→Eu3+ non-radiative energy transfers, with efficiencies of 8% and 21%, respectively. The Inokuti-Hirayama model suggests that such energy transfer processes might be dominated by an electric dipole-dipole interaction inside Tb3+-Sm3+ and Sm3+-Eu3+ clusters.

Strategy to modify intrinsic luminescence via post-annealing treatment and impurity doping in SbNb3(PO4)6

Self-activated luminescence of a phosphor has unique advantages such as natural homogeneity, wide wavelength, without rare-earth dopant, easy preparation, etc. However, its luminescence efficiency usually is not satisfied. This work reports several strategies to modify the intrinsic luminescence intensity of SbNb3(PO4)6 synthesized via the sol–gel method. XRD measurements were conducted to verify phase formations and structural characteristics. The transition natures and band energy were determined on the basis of the optical absorption. SbNb3(PO4)6 shows a broad self-activated luminescence (400–750 nm) peaked at 485 nm. Impurity ion V5+ doping improves the intrinsic luminescence of SbNb3(PO4)6. The optimal efficiency of 31.63% was observed in 7.0 mol% V5+-doped SbNb3(PO4)6. Post-annealing of as-prepared SbNb3(PO4)6 in the air can enhance the luminescence, while, annealing in CO atmosphere deteriorates the intensity. The luminescence thermal stabilities and activation energies were evaluated via the temperature-dependent emission intensities and decay lifetimes. The intrinsic defects such as oxygen vacancies (VO••) were confirmed via XPS measurements, which exerted great influences on the emission and thermal stabilities of the emission. The abnormal blue-shift of the emission spectra was presented from 10 to 350 K. The luminescence mechanism was proposed for the self-activated emission based on the dynamic emission spectra, decay time, and multiple color centers.

Luminescent properties of titania doped with nanoparticles of gadolinium oxide and europium

In this work we synthetized seven samples of titania doped with nanoparticles of gadolinium oxide which, previously, were doped with europium at 2 mol%. The doped samples of titania were synthetized using the sol gel method and the nanoparticles of gadolinium oxide were synthetized by the precipitation reaction of precursors of gadolinium nitrate hexahydrate and europium nitrate pentahydrate. All the titania samples were submitted to a thermal treatment at 750 °C. Optical properties, such as the absorption, emission, and emission decay times, besides the X-ray diffraction patterns and the Raman spectra were recorded and analysed. The band gap was calculated from the extrapolation of the linear fit in the absorption edge range in the Tauc plot, the electronic transitions of the europium were identified. Finally, due to the high luminescent intensity of the nanoparticles of gadolinium oxide doped with europium, the possible incorporation of these into the matrix of titania, the possibility for exciting these using lower energy (375 nm) and consequently the shift in the band gap; made it possible to propose the material be used for photocatalytic and photonic applications.

Microstructural, crystallographic, and luminescent analysis after grinding high-temperature monoclinic bismuth phosphate

Phase transformation of inorganic materials is usually done via either the re-arrangement of structural units at high pressures and/or temperatures or bottom-up construction from cations and anions using solution chemistry-related approaches. The purpose of this study was to evaluate the microstructural, crystallographic, and luminescent changes after grinding a high-temperature monoclinic phase (H-BPO) of bismuth phosphate. Initially, the pre-ground H-BPO was formed by solid-state reaction of the starting materials. After mechanical grinding, the pre-ground H-BPO gradually transformed to another low-temperature monoclinic phase (L-BPO). Furthermore, a switchable phase transformation from L-BPO to H-BPO was observed for the sample after grinding followed by annealing at high temperature. For the samples obtained by grinding or grinding followed by annealing, the microstructure, crystallography, and luminescence properties were characterized by SEM, XRD with Rietveld analysis, FT-IR, Raman, and photoluminescence (PL) spectra in detail. The switchable phase transformation was elucidated by taking into account the effects of the particle size, and lattice strain as well as other structural factors such as the arrangements of BiO8 polyhedra and PO4 tetrahedra, unit lattice volume (V/Z), and the symmetry of PO4 groups. The phase transition from H-BPO to L-BPO occurred at a relatively low pressure generated under grinding, which is irreversible and 1st order nature. Moreover, with the switchable phase transformation, the luminescence properties including emission intensity, decay time, and quantum efficiency were assessed.

Concentration and temperature dependent luminescence properties of the SrI2-TmI2 system

The concentration dependent luminescence of the SrI2-TmI2 system was investigated. For Tm2+ concentrations up to 5 mol %, the quantum efficiency (QE) of the 2F5/2→2F7/2 emission exhibits a constant value above 50%. The QE drops for higher Tm2+ concentrations, partly due to concentration quenching, as evidenced by a decreasing luminescence lifetime of the 2F5/2→2F7/2 emission, and partly due to the formation of a second crystal phase with CdCl2 structure, in which the 2F5/2→2F7/2 emission is quenched. The temperature and time dependent relaxation dynamics were studied to identify the origin of the limited QE for Tm2+-doping levels below 5 mol %. An anti-correlation between the 5d-4f (3H6,t2g)S=3/2→2F7/2 and 4f-4f 2F5/2→2F7/2 emission intensities was found and rationalised by non-radiative, thermally stimulated, inter-configurational 5d-4f relaxation to the emitting 2F5/2 level of Tm2+. Both, the rise time of the 4f-4f and the decay time of the 5d-4f emission become shorter with increasing temperature. We suggest a similar non-radiative relaxation from the 5d level towards the 2F7/2 ground state to limit the QE below unity. This route becomes more efficient when the 5d (3H6,t2g)S=3/2 state moves closer to the 4f 2F5/2 and 2F7/2 states, which is the case for the CdCl2 phase with a QE close to zero.

Up-conversion luminescence of hafnium, erbium, ytterbium and lithium co-doped yttrium oxide

UP-conversion luminescence of hafnium, erbium, ytterbium and lithium multiple doped yttrium oxide powders are presented. The measured luminescent spectra were obtained using two different lasers with wavelengths of 980 and 1534 nm; emission peaks were observed in the ranges of 530–580 nm (green emission), 630–680 nm (red emission), 960–1100 nm (infrared emission) and 1510 to 1560 (infrared emission) corresponding to the different intra electronic energy levels transitions of the Er3+ and Yb3+ ions. These emission peaks manifest the interactions between these ions that propitiate the phenomenon of up-conversion. The incorporation of lithium in the samples allowed an increase in the luminescent emission of up to 88% for the red emission and up to 12% for the emission in the IR range with respect to the samples that were not doped with lithium. The powders, object of this study, were synthesized by means of the solvent evaporation technique using nitrates and chlorides as precursors. The analysis of XRD showed a cubic crystalline structure of yttrium oxide with the spatial group: Ia-3 (206). The obtained SEM images showed a granular morphology only for samples doped with lithium with dimensions less than a micron. The chemical composition was obtained by measurements of EDS and FT-IR and finally the decay times of the different luminescent emissions are also reported.

Optical, scintillation, and dosimetric properties of Mn-doped MgAl2O4 single crystals

Mn-doped MgAl2O4 single crystals were synthesized by the floating zone method equipped with four xenon arc lamps, and photoluminescence (PL), scintillation, and dosimetric properties were investigated. In PL and scintillation spectra, Mn-doped crystals showed an emission peak at 520 nm. The luminescence was ascribed to the d–d transition of Mn2+ ion judging from the emission wavelength and decay time constants. In addition, Mn-doped crystals showed thermally and optically stimulated luminescence (TSL and OSL) with the emission peak at 520 nm, which was also considered to be due to the d–d transition of Mn2+ ion. In both TSL and OSL dose–response functions, the 0.1% Mn-doped crystal showed linearly response from 0.01 to 1000 mGy.

Effect of low-density polyethylene on the luminescent properties of thulium-doped yttrium oxide obtained by the polyol technique

In the present work polyethylene is used as a support of yttrium oxide particles doped with thulium atoms and its effect on the luminescent properties is studied. Thulium-doped yttrium oxide was prepared through the polyol method, different values of thulium are explored: 0, 0.5, 1, 1.5, 2.5 and 3.5 at%, as well as different calcinating temperatures: 400, 600, 800 and 1000 °C, are explored in order to determine the synthesis conditions needed for the maximum emission intensity, whose values were 1.5 at% of thulium and 1000 °C. The synthesized powder is dispersed in low-density polyethylene and dispersed as a thin film on glass substrates. The luminescent excitation and emission spectra showed important changes attributed to the influence of the -CH2– chain vibration; in addition, the presence of the polymer had an important influence on the emission decay time: while the powder showed a single emission process with a time of 9.293 μs, the film showed a two decay process with 1.156 μs and 4.086 μs, each.

Laser-induced thermal emission of rough carbon surfaces

The decay of thermal emission of rough surface layers of different carbon materials under the pulsed laser excitation is analyzed both theoretically and experimentally. For pulsed laser heating of rough surfaces, computer simulations revealed that laser-induced thermal radiation is mainly emitted by peaks of the surface relief, and the emission decay time depends on the relation between the laser penetration depth and the temperature diffusion length. It is also concluded that the presence of surface roughness can significantly increase the emission decay time. In the experiments, carbon materials with different thermal characteristics were used; however, all of the investigated samples demonstrated close values of the emission decay time. This fact shows that the material's characteristics on the peaks of surface relief are similar for different carbon materials at high temperature.

Optical properties of nanopatch antennas based on plasmonic nanoparticles and ruthenium complex

In this work, a significant reduction (up to 7 ns) of the excited states lifetime of the Ru dye (with a decay time of 850 ns) in an inhomogeneous «aluminium – silver nano-prism system (nano-patch antenna, NPA)», as well as an increase in the photoluminescence emitter to 2-3 times. The increase in the spontaneous emission rate of this substance is caused by the Purcell effect. The Purcell coefficient for the emitter in the cavity with hexagonal silver particles was 120. This decrease in the spontaneous emission decay time in the nano-patch antenna configuration is associated with an increase in the local density of photon states in the plasmon resonator, which increases the probability of a spontaneous transition from the excited state of the emitter. The results obtained by shortening the spontaneous emission time and increasing the intensity of the emitter radiation in a nano-patch antenna show possible prospects for using quantum dots and individual molecules with shorter luminescence times in nano-patch antennas. Such nano-patch antennas can form the basis for creating compact high-speed optical devices. Modeling in the mathematical package Comsol Multiphysics showed that the maximum values of the Purcell factors can exceed 104 in the NPA.

The effect of pH and ionic strength on the fluorescence properties of a red emissive DNA-stabilized silver nanocluster

DNA-stabilized silver nanoclusters (DNA-AgNCs) are a class of promising fluorophores for imaging and sensing applications. All aspects of their spectroscopic properties are not yet fully characterized, leaving this field still with a number of fundamental studies to be addressed. In this work, we studied the spectroscopic properties of red-emitting DNA-AgNCs at different pH (5 to 9) and ionic strength μ (0.005 to 0.5). The photophysical properties of high performance liquid chromatography (HPLC) purified DNA-AgNCs proved to be constant over a large range of pH and μ, with absorption, emission and fluorescence decay times only being affected at very high pH and μ values. Non-purified DNA-AgNCs were also unaffected by pH and/or μ variations, but significant differences can be observed between the rotational correlation times of purified and non-purified DNA-AgNCs.

Tunable lanthanide/transition metal ion-doped novel phosphors for possible application in w-LEDs: a review.

Lanthanide-based phosphors have been extensively investigated for their possible applications in solid-state lighting technologies especially for white-light-emitting diodes. In this review article emphasis has been laid on discussing the recent developments of phosphors for warm white-light production based on various optical characteristics such as quantum efficiency, thermal stability, short emission decay time, long-term stability, facile synthesis, and low cost of production. We have tried to cover the essential and latest discoveries of the lanthanide/rare earth-doped phosphors after 2010. New generations of narrow-band phosphors have also been included. The optical and material properties of several novel phosphors and their luminescence characteristics have been thoroughly discussed.

Unusually Fast Phosphorescence from Ir(III) Complexes via Dinuclear Molecular Design.

The design and detailed photophysical study of two novel Ir(III) complexes featuring mono- and dinuclear design are presented. Emission quantum yield and decay times in solution are ΦPL = 90% and τ(300 K) = 1.16 μs for the mononuclear complex 5, and ΦPL = 95% and τ(300 K) = 0.44 μs for the dinuclear complex 6. These data indicate an almost 3-fold increase in the phosphorescence rate for dinuclear complex 6 compared to 5. Zero-field splitting (ZFS) of the T1 state also increases from ZFS = 65 cm-1 for the mononuclear complex to ZFS = 205 cm-1 for the dinuclear complex and is accompanied by a drastic shortening of the individual decay times of T1 substates. With the help of TD-DFT calculations, we rationalize that the drastic changes in the T1 state properties in the dinuclear complex originate from an increased number of excited states available for direct spin-orbit coupling (SOC) routes as a result of electronic coupling of Ir-Cl antibonding molecular orbitals of the two coordination sites.

Anisotropic Radio-wave Scattering and the Interpretation of Solar Radio Emission Observations

Abstract The observed properties (i.e., source size, source position, time duration, and decay time) of solar radio emission produced through plasma processes near the local plasma frequency, and hence the interpretation of solar radio bursts, are strongly influenced by propagation effects in the inhomogeneous turbulent solar corona. In this work, a 3D stochastic description of the propagation process is presented, based on the Fokker–Planck and Langevin equations of radio-wave transport in a medium containing anisotropic electron density fluctuations. Using a numerical treatment based on this model, we investigate the characteristic source sizes and burst decay times for Type III solar radio bursts. Comparison of the simulations with the observations of solar radio bursts shows that predominantly perpendicular density fluctuations in the solar corona are required, with an anisotropy factor of ∼0.3 for sources observed at around 30 MHz. The simulations also demonstrate that the photons are isotropized near the region of primary emission, but the waves are then focused by large-scale refraction, leading to plasma radio emission directivity that is characterized by a half width at half maximum of about 40° near 30 MHz. The results are applicable to various solar radio bursts produced via plasma emission.

Synthesis, NMR and optical features of intense green color terbium(III) complexes

The paper reports the three highly green luminescent terbium(III) complexes functionalized with 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dione (L) as main ligand, 1,10-phenanthroline (phen) and2,2′-bipyridyl (bipy) as ancillary ligands. These complexes were characterized by means of IR, 1H-NMR, elemental analysis, UV–vis, TG-DTG and photoluminescence (PL) spectroscopy. In 1H-NMR spectra, large highfield and lowfield shifts are noticed in these paramagnetic complexes. The proton resonances of phen and bipy have been shifted to highfield, while, the resonance of methine proton in main ligand has been shifted to lowfield. In this way, all the proton signals are resolved as revealed in NMR spectra. The photoluminescent features clearly indicate that the substitution of solvent molecules by ancillary ligand in the coordination sphere enhance the emission intensity and decay time of the complexes. Furthermore, the intramolecularenergy transfer mechanism shows that an efficient transfer of energy from triplet level of ligands to the emitting level of terbium(III) ion takes place in the complexes. In this manner, main ligand and ancillary ligands sensitize the terbium(III) ion yielding the intense and characteristic green luminescence in the complexes. The CIE color coordinates of Tb(L)3bipy complex exactly matches with standard pure green color (x = 0.21, y = 0.71) of NTSC (National Television Standard Committee) which makes them pure green promising component for potencial application as luminescent materials. The beauty of complexes is that these acts as NMR shift reagent as well as these are of great interest in the field of emitting material and display devices.

Cold bluish white and blue emissions in Cu+-doped zinc phosphate glasses

Reducing agent-free zinc phosphate glasses doped with cooper ions (Cu+) were synthesized by melt-quenching technique. The physical properties of these glasses such as density, molar volume and refractive index were studied. The X-ray diffraction (XRD) patterns for all prepared glasses, confirm their amorphous structure. Raman spectroscopy suggest that the phosphate network is composed by tetrahedral PO4 units, since the vibrations of P–O–P, PO2 and PO, observed in the spectra, belong to the PO4 units. All glasses display absorptions bands in the ultraviolet and near infrared regions, associated with the absorption edge and contribution of the Cu+:1Ag → 1T2g + 3T2g (280 nm) and Cu2+: 2Eg → 2Tg (890 nm) transitions, respectively. The optical band gap values determined from the absorption spectra assuming indirect optical band gap, are reduced from 4.83 to 2.95 eV for Cu+ content in the range of 0–0.3 mol%. For Cu+ concentration higher than 0.3 mol%, band gap values increase up to 3.17 eV. Such last behaviour was attributed to the reduction of the amount of Cu+ species and/or Moss-Burstein effect. The emission spectra of the Cu+-doped zinc phosphate glasses upon 272 and 290 nm excitation, displayed overall bluish white light and blue emission, depending the excitation wavelength and Cu+ contents. A maximum photoluminescence quantum yield value of 12.5% with an inherent uncertainty up to 3.75%, was reached under 290 nm excitation. The emission decay time profiles were well fitted to a bi-exponential function from which the average lifetimes values resulted to be in the interval of 26.8–16.7 μs, which are shortened for Cu+ contents higher than 0.3 mol%. The structural and photoluminescence characteristic makes of the reducing agent-free Cu+-doped zinc phosphate glasses potentially attractive for blue phosphors and WLEDs applications.

Nanodefects in YAG:Ce-Based Phosphor Microcrystals

The present paper focused on the study of spectral–kinetic characteristics of luminescence in two batches of yttrium aluminum garnet (YAG):Ce-based phosphors synthesized in different years by two manufacturers: NPO “Platan” in Russia and “GrandLux” in the People’s Republic of China (PRC). Upon studying the structural characteristics of the phosphors—elemental composition, morphology, and X-ray diffraction (XRD) patterns—it was concluded that both types of YAG:Ce phosphors are highly imperfect. The presence of heterogeneities of different nature was accompanied by the introduction—to compensate for charges and elastic stresses—of intrinsic lattice defects during synthesis. There is a high probability of creating complex defects during phosphor synthesis. Luminescence properties (full width at half maximum (FWHM), spectral position of the emission peaks, excitation spectra of emission, emission decay time) are affected by the nearest environment of the luminescence center; whereas the degree of correlation of defects (distance between the components of the donor–acceptor pair) does not depend on the concentration of impurities, intrinsic defects, and their ratio. The results do not fit into the framework of existing ideas regarding the processes in phosphors as systems with widely distributed luminescence centers. The patterns obtained in the paper are discussed based on a hypothesis according to which a nanodefect phosphorus crystal phase is formed during the synthesis.