Luminescence Decay Characteristics Research Articles

Optically active centers in Eu implanted, Eu in situ doped GaN, and Eu doped GaN quantum dots

A comparison is presented between Eu implanted and Eu in situ doped GaN thin films showing that two predominant Eu sites are optically active around 620 nm in both types of samples with below and above bandgap excitation. One of these sites, identified as a Ga substitutional site, is common to both types of Eu doped GaN samples despite the difference in the GaN film growth method and in the doping technique. High-resolution photoluminescence (PL) spectra under resonant excitation reveal that in all samples these two host-sensitized sites are in small amount compared to the majority of Eu ions which occupy isolated Ga substitutional sites and thus cannot be excited through the GaN host. The relative concentrations of the two predominant host-sensitized Eu sites are strongly affected by the annealing temperature for Eu implanted samples and by the group III element time opening in the molecular beam epitaxy growth. Red luminescence decay characteristics for the two Eu sites reveal different excitation paths. PL dynamics under above bandgap excitation indicate that Eu ions occupying a Ga substitutional site are either excited directly into the D50 level or into higher excited levels such as D51, while Eu ions sitting in the other site are only directly excited into the D50 level. These differences are discussed in terms of the spectral overlap between the emission band of a nearby bound exciton and the absorption bands of Eu ions. The study of Eu doped GaN quantum dots reveals the existence of only one type of Eu site under above bandgap excitation, with Eu PL dynamics features similar to Eu ions in Ga substitutional sites.

Effects of dopants and excitation wavelength on the temperature sensing of Ln 3+-doped 7YSZ

Rare-earth ions doped yttrium-stabilized zirconia (7YSZ) has been proven as a promising candidate in temperature sensing of TBCs. In this work, the influences of different dopants and the excitation wavelength on the temperature sensing of Ln 3+-doped 7YSZ are investigated. The luminescence from 7YSZ doped with trivalent rare-earth ions, Sm 3+, Eu 3+, and Er 3+, was measured both at room temperature and up to 1200 °C. Strong luminescence was observed in all the three materials using either 532 nm or 355 nm excitation, indicating little effect of excitation wavelength on the luminescence temperature sensing. Sm 3+, Eu 3+ doping is found to give rise to superior luminescence decay characteristics to temperatures up to 1000 °C, while the sample doped with Er 3+ shows longer decay time above that temperature and measurement can be made up to 1200 °C. The luminescent lifetime-temperature dependences for all the three materials were also discussed based on the multi-phonon relaxation model.

Luminescence spectra and kinetics of LiF〈U〉-O crystals

The promising phosphor LiF〈U〉-O has been studied by nanosecond-resolution pulsed spectrometry. The uranium and oxygen luminescence buildup and decay characteristics have been measured at 300 K. The uranium luminescence buildup under nanosecond electron excitation has been shown to involve two steps. The effect of pulsed electron irradiation on the activator luminescence efficiency has been assessed.

Redshift of Green Photoluminescence of Ca3Sc2Si3O12 : Ce3 + Phosphor by Charge Compensatory Additives

The luminescence wavelength of , which is a green-emitting phosphor under blue light excitation, was moved toward longer wavelength by addition of Mg. This modification is very valuable for commercial applications because it increases the brightness of the phosphor by increasing the overlap with the visibility curve. Magnesium is likely incorporated into the Sc position of the host crystal because the ionic radius of Mg is close to that of Sc. The substitution probably compensates for the excess positive charge induced by incorporation of Ce into the Ca position. The content of Ce in the host crystal was increased by increasing the amount of Mg in the raw material mixture; the inverse relationship was also observed. The spectral shift was also observed by adding Na or Li, which probably substituted the Ca position and compensated the excess charge induced by Ce in the Ca position. The change in luminescence spectra occurred irrespective of the type of charge compensatory additive used. Therefore, the spectral modification was mainly due to the increase in activator. Modification of the luminescence decay characteristics by the additives was also examined. The decay curve of the phosphor containing Mg showed a nonexponential profile.

Fluorescence Quenching and Luminescence Sensitization in Complexes of Tb3+ and Eu3+ with Humic Substances

Intrinsic fluorescence quenching of humic substances (HS) and the sensitization of Ln3+ luminescence (Ln3+ = Tb3+, Eu3+) in HS complexes were investigated. Both measurements yielded complementary information on the complexation of metals by HS. Large differences between fulvic acids (FA) and humic acids (HA) were found. From time-resolved luminescence measurements it is concluded that a combination of energy transfer and energy back transfer between HS and Ln3+ is responsible for the observed luminescence decay characteristics. In the case of Eu3+, an additional participation of charge-transfer states is suggested. A new concept for the evaluation of the sensitized luminescence decays of Ln3+ was adapted.

Luminescence of trivalent praseodymium in oxides and fluorides

Three types of Pr-activated phosphors are studied: those with predominant 5d-4f transitions, those with transitions from the 3 P 0-level, and those with successive transitions from the 1 S 0− and 3 P 0-levels of 4f-configuration. The theoretical-model analysis of a photon cascade emission (PCE) is carried out. The basic properties of host matrixes necessary for PCE detection are ascertained. The spectral and kinetic (luminescence decay) characteristics of a number of Pr-activated oxides and fluorides are measured. SrAlF3:Pr is found to exhibit the best luminescent properties among the phosphors studied. The host matrixes for the continued search for the PCE effect are suggested.

Spectroscopic Properties of Cr3+-Doped Phosphate Glasses

Emission spectroscopy of Cr 3+ -doped phosphate glasses has been studied. Different site distributions of the chromium ions having an environment with nearly octahedral symmetry could be identified on the basis of the luminescence decay characteristics and the time-resolved emission study. The low quantum yield in luminescence is attributed to an efficient multiphonon relaxation process deexciting rapidly ions submitted to the weaker crystal field.

Exciton luminescence in BaFCl crystal

The luminescence spectra and decay characteristics of both 5.45 and 3.4eV bands in BaFCl oxygen-free crystals were investigated at 5–300K temperature range using vacuum ultraviolet excitation as well as X-ray excitation. The similarities with excitons in alkali halides allow us to consider both excitons in BaFCl as on-center excitons and strong off-center excitons.

An investigation of inhomogeneities in and variations of luminescence and excitation properties of Ru(bpy) 2+3 in solid solutions

Deviations from (global) single exponential luminescence decay characteristics for Ru(bpy) 2+ 3 in frozen 4:1 ethanol-methanol and 9M LiCl solutions, as well as in poly(vinyl-alcohol) films are reported. These deviations vary quite characteristically with detected energy. The luminophore in single crystal environments is demonstrated to have (detected energy) invariant exponential behavior. The global fitted lifetimes in differing environments are remarkably similar ((5.2±0.1) μs) at 77 K, except in 9M LiCl (4.0 μs). A least squared analysis on large data sets is performed to test models of the inhomogeneity. The detected energy dependence of excitation polarization ratio data is also reported. The analysis lends some support to a two-site model, with a weaker, shifted component having a = 2 μs lifetime. The magnetic circular polarization of luminescence in poly(vinyl-alcohol) shows accurate 1 / T behavior in the 80–200 K range whereas measurements in 9M LiCl in the same range indicate a significant change in the luminophore due to the softening of the glass in this range. A discussion of the significance of the MCPL behavior is presented.

Luminescence decay characteristics of Ru(bpy) 2+3 in dilute single crystals and polymer hosts

Luminescence decay characteristics for Ru(bpy) 2+ 3 doped in dilute single crystals and polymer hosts have been measured in the temperature range 1.5 to 77 K using 1 ms square-pulse excitation from an acousto-optically modulated Ar + laser. Previously observed deviations from single-exponential decay behaviour in experiments using higher-power, short (3 ns) pulse excitation, are shown to be mainly due to sample heating effects. The temperature dependence of the decay times is consistent with luminescence from three states of equal degeneracy, separated by (8±0.5) and (60±5) cm -1 respectively.

Recombination kinetics of excitonic molecules and free excitons in intrinsic silicon

The luminescence decay characteristics of the excitonic molecule (EM) - free exciton (FE) system in pure silicon are described and explained in detail by a pair of coupled differential equations governing the time decay of the EM and FE densities. Comparison of the theoretical and experimental decays shows that the EM decay time is 59 ns and the capture coefficient at 1.8° K for the formation of an EM from two FEs is about 2 × 10-9 cm3/sec. These data imply the EMs decay primarily by a nonradiative Auger mechanism, which accounts for the low radiative efficiency.