Temperature Dependence Of Luminescence Intensity Research Articles

Triple-Decker Hexaazamacrocyclic Lanthanide(III) Complexes: Structure, Magnetic Properties, and Temperature-Dependent Luminescence.

The reaction of fluoride anions with mononuclear rare-earth(III) complexes of the hexaazamacrocycle derived from 2,6-diformylpyridine and ethylenediamine affords trinuclear coordination compounds [Ln3L3(μ2-F)4(NO3)2](NO3)3. The X-ray crystal structures of these complexes show triplex cationic complexes where the three roughly parallel macrocyclic lanthanide(III) units are linked by bis-μ2-F bridges. The detailed analysis of the photophysical properties of the [Eu3L3(μ2-F)4(NO3)2](NO3)3·2H2O and [Tb3L3(μ2-F)4(NO3)2](NO3)3·3H2O complexes reveals different temperature dependence of luminescence intensity and luminescence decay time of the Eu(III) and Tb(III) derivatives. The spectra of mixed species of average composition [Eu1.5Tb1.5L3(μ2-F)4(NO3)2](NO3)3·3H2O are in accordance with the ratiometric luminescent thermometer behavior. Measurements of the direct-current (dc) magnetic susceptibility of the [Dy3L3(μ2-F)4(NO3)2](NO3)3·2H2O complex indicate possible ferromagnetic interactions between the Dy(III) ions. Alternating current (ac) susceptibility measurements of this complex indicate single-molecule magnet behavior in zero dc field with magnetic relaxation dominated by Orbach mechanism and an effective energy barrier Ueff = 12.3 cm-1 (17.7 K) with a pre-exponential relaxation time, τ0 of 7.3 × 10-6 s. A similar reaction of mononuclear macrocyclic complexes with a higher number of fluoride equivalents results in polymeric {[Ln3L3(μ2-F)5](NO3)4}n complexes. The X-ray crystal structure of the Nd(III) derivative of this type shows trinuclear units that are additionally linked by single fluoride bridges to form a linear coordination polymer.

Modulated up-conversion luminescence and low-temperature sensing of Gd3Ga5O12:Yb3+/Er3+ by incorporation of Fe3+ ions

Yb3+/Er3+ co-doped crystals show strong up-conversion luminescence and their temperature dependence of luminescence intensity enable the application of temperature sensors. Such sensors usually work at hundreds Kelvin with high sensitivity, while the sensors appropriate to very low temperatures are rarely reported. In this study, we report the low temperature sensing of Gd3Ga5O12:Yb3+/Er3+ nanocrystals by partial or total substitution of Ga3+ by Fe3+. The up-conversion photoluminescence of the nanocrystals is measured on cooling from 300 to 4.2 K. The low-temperature sensing performance is improved, and the structure, magnetic and optical properties are modulated by the substitution of Fe3+ ions, rendering the nanocrystal useful for multi-functional sensing application.

Temperature dependence of luminescence intensity in ZnMoO4 crystals

Luminescence of ZnMoO4 crystals (of stoichiometric composition, with excess of molybdenum and doped with tungsten) under X-ray irradiation in the temperature interval from 4 to 400 K was studied. The difference in the composition has little influence on the luminescence properties of the samples. An increase of the 1.93eV emission band intensity was observed in the temperature interval from 8K to 90K. Two mechanisms, concurrence with excitonic emission and concurrence with non-radiative recombination channel, were considered to explain the observation. Theoretical formulas were obtained to describe the temperature dependence of luminescence intensity. The experimental data are well described by the developed theoretical model which assumes recombination of free charge carriers with carriers of opposite sign localized on shallow traps. The high temperature quenching of luminescence is well described by the Mott formula.

Preparation and photoluminescent properties of near-UV broadband-excited red phosphor (Gd1-xEux)6(Te1-yMoy)O12 for white-LEDs

Generally, the Eu3+-activated red phosphors suffer narrow 4f-4f excitation lines ranging from near-UV to blue part of the spectrum, resulting in poor spectral overlapping with the emission spectrum of the pumping LED and low energy conversion efficiency. In this paper, the strategy of Te6+/Mo6+ mixing is adopted to enhance the excitation bandwidth of Eu3+ via the energy transfer from Mo6+-O2- charge transfer state to Eu3+, which is crucial for LED applications. A series of (Gd1-xEux)6(Te1-yMoy)O12 red phosphors are synthesized by the solid state method at 1200 ℃. The crystal structure, morphology and luminescent properties are investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescent spectrum. The XRD patterns of (Gd1-xEux)6(Te1-yMoy) O12 (x = 0.2, y = 0, 0.4) match well with that of Gd6TeO12 (JCPDS No. 50-0269), but differ from that of Gd6MoO12 (JCPDS No. 24-1085). The phosphor consists of irregular particles with an average size of 10 m. Upon excitation at 393 nm, the (Gd1-xEux)6TeO12 phosphors emit red light corresponding to the intraconfigurational 4f-4f transitions of Eu3+, and the color coordinates are calculated to be (0.647, 0.353). The 5D07F2 electron-dipole transition dominates the emission spectrum, which reveals that Eu3+ occupies a crystallographic site without an inversion center. Moreover, this transition gives rise to three distinguishable emission lines situated at 605, 618, and 632 nm, respectively. This unusual spectral splitting is supposed to originate from the strong interaction exerted by the crystal field of host on the 4f electrons. The optimum doping content of Eu3+ in (Gd1-xEux)6TeO12 phosphor is 20% (mole fraction), the critical distance for energy transfer is 0.75 nm, and the concentration quenching is confirmed to be induced by the dipole-dipole interaction from the linear relationship between lg(I/x) and lg x (I represents the luminescence intensity, and x represents the doping concentration of Eu3+). As the temperature increases, the emission intensity decreases gradually due to thermal quenching. The integrated emission intensity at 423 K is 70% of the initial value at ambient temperature. The thermal activation energy is determined to be 0.1796 eV from the temperature dependence of luminescence intensities. The partial substitution of Te6+ by Mo6+ does not change the emission position nor intensity significantly, but promotes the excitation bandwidth and conversion efficiency remarkably. Compared with (Gd0.8Eu0.2)6TeO12, the compositionoptimized (Gd0.8Eu0.2)6(Te0.6Mo0.4)O12 presents a relatively flat excitation spectrum in the near-UV region. It also provides more intense emission since (Gd0.8Eu0.2)6MoO12 undergoes the strong concentration quenching arising from the high density of [MoO6] groups. In conclusion, the results indicate that (Gd0.8Eu0.2)6(Te0.6Mo0.4)O12 can serve as a broadband-excited red phosphor for near-UV-based white LEDs.

Distribution of D1 dislocation luminescence centers in Si+-implanted silicon and the photoluminescence model

Using step-by-step removal of silicon layers, in which dislocation-related photoluminescence is observed after Si+ (100keV, 1·1015cm−2) ion implantation followed by high-temperature annealing in a chlorine containing atmosphere, it has been found that a majority of dislocation-related centers of luminescence at ~1.5μm (D1 line) is localized at the depths of Si+ ion ranges. Cross-sectional electron microscopy shows that the dislocations introduced by the implantation treatment (implantation plus annealing) penetrate to depths of ~1μm. A phenomenological model of the D1-line dislocation-related luminescence is developed based on the assumption that the K-centers and modified A-centers located in the atmospheres of dislocations are responsible for this luminescence line. The temperature dependence of luminescence intensity calculated on the basis of the model fits well the experimental data for the D1 line.

Luminescence diagnostics of conformational changes of the Hsp70 protein in the course of thermal denaturation

The spectral luminescence properties of aqueous solutions of the Hsp70 protein are studied, the dependence of the luminescence spectrum on the excitation wavelength is revealed, and the temperature dependence of luminescence intensity of tyrosine and tryptophan residues in the temperature interval of 20–80° C is analyzed. The luminescence method is used to determine temperature interval (42–57° C) in which protein melting takes place. An increase in the fluorescence quantum yield of tryptophan and the bathochromic shift of the emission spectrum of denatured Hsp70 prove that transition takes place of tryptophanyls to the surface of the protein molecule.

Distribution of D1 Dislocation Luminescence Centers in Si+–Implanted Silicon and the Photoluminescence Model

Using step − by − step removal of silicon layers, in which dislocation − related photoluminescence is observed after Si + (100 keV, 1 ·10 15 cm − 2 ) ion implantation followed by high − temperature annealing in a chlorine − containing atmosphere, it has been found that a majority of dislocation − related centers of luminescence at ~ 1,5 μm (D1 line) is localized at the depths of Si + ion ranges. Cross − sectional electron microscopy shows that the dislocations introduced by the implantation treatment (implantation plus annealing) penetrate to depths of ~ 1μm. A phenomenological model of the D1 − line dislocation − related luminescence is developed based on the assumption that the K − centers and modified A − centers located in the atmospheres of dislocations are responsible for this luminescence line. The temperature dependence of luminescence intensity calculated on the basis of the model fits well the experimental data for the D1 line.

Luminescence of stable stacking aggregates of adenine and uracil in water

Luminescence and excitation spectra of the highly luminescent stacking dimers of adenine and uracil in water solutions are studied. By the luminescence excitation spectra method it is shown that the stacking aggregates of adenine and uracil are formed with participation of rare forms of monomer molecules: N7H tautomers of adenine and the uracil molecules in rare forms of hydratation, for example molecules without H-bonds with water. The study of temperature dependence of luminescence intensity of monomers and stacking dimers of uracil has shown that stacking dimers do not dissociate even at 85 degrees C similarly as described earlier for adenine and adenosine. Stable stacking aggregates of nucleic bases are most likely to be the precursors of RNA molecules in chemical evolution. This hypothesis is supported by new data on their stability.

The red luminescence characteristics of fully concentrated Eu‐based phosphor EuBaB9O16

AbstractThe red phosphor EuBaB9O16 was prepared by the conventional solid‐state reaction method. The crystallite phase of the phosphor was observed by X‐ray diffraction (XRD) patterns. Scanning electron micrographs were obtained to study the morphologic features and the particle size of the phosphor. The photoluminescence (PL) excitation and emission spectra, temperature dependence of luminescence intensity and the decay curves were investigated. The phosphor, when excited with near‐UV wavelength radiation, exhibited an intense red luminescence. The characteristic emission of Eu3+ ions corresponding to the electric dipole transition 5D0 → 7F2 was located at 613 nm under 254‐ and 365‐nm excitation. Its room temperature fluorescent decay profile exhibits a single‐exponent shape with a decay time of 2.03 ms. The color coordinates were also discussed. In addition, the PL properties of EuBaB9O16 were compared with those of La0.9Eu0.1BaB9O16 and Gd0.9Eu0.1BaB9O16.

Thermal stability of luminescence of NaCaPO4 : Eu2+ phosphor for white-light-emitting diodes

A green-emitting phosphor, Eu2+-activated NaCaPO4, was synthesized by the conventional solid-state reaction. The photoluminescence excitation and emission spectra, and the temperature dependence of luminescence intensity of the phosphor were investigated. The results showed that NaCaPO4 : Eu2+ could be efficiently excited by incident light of 280–420 nm, which matched well with the emission wavelengths of near-UV LED chips. NaCaPO4 : Eu2+ has a higher quenching temperature. With an increase in the temperature, the emission bands of NaCaPO4 : Eu2+ show an abnormal blue-shift with broadening bandwidth and decreasing emission intensity. The phosphor has a promising potential for white-light-emitting diodes pumped by ultraviolet chips. The CIE coordinates and luminescence decay (lifetimes) of Eu2+ ions in this monophosphate were discussed in order to further investigate the potential applications.

Synthesis of phosphate phosphor for a white LED

The Eu2+-activated Sr2Mg3P4O15 (Sr2Mg3P4O15:Eu2+) phosphor is prepared using solid-state reactions of SrCO3, MgO, (NH4)2HPO4, and Eu2O3. The phosphor is excited efficiently by near-ultraviolet (n-UV) light irradiation, showing bright blue emission spectra at wavelengths of 400–550 nm with a maximum peak of 447 nm. The temperature dependence of luminescence intensity for (Sr0.94Eu0.06)2Mg3P4O15 is higher than that of commercial (Y,Gd)3Al5O12:Ce3+. In fact, Sr2Mg3P4O15:Eu2+ has high potential for application as a blue phosphor for a white LED under excitation of n-UV light.

Extrinsic optical recombination in pentacene single crystals: Evidence of gap states

Two luminescence bands observed in pentacene single crystals with different degrees of purity are identified as due to extrinsic optical emissions. A band at 1.49 eV remains in the crystal with the highest purity. Its redshift of about 0.3 eV from the free exciton optical recombination suggests that the extrinsic transitions could involve gap states recently discovered in pentacene transistors. Absence of resonance Raman scattering when photon energies overlap the extrinsic recombination suggests that the gap states are likely due to impurities. The temperature dependence of luminescence intensities is interpreted by activated decay of excitons to radiative and nonradiative states.

Temperature dependence of luminescence intensity under Bose condensation of interwell excitons

Photoluminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructure) containing large-scale random potential fluctuations in the planes of heteroboundaries is studied. The properties of excitons, in which a photoexcited electron and a hole are spatially separated in neighboring quantum wells, were investigated upon variation of the power density of off-resonance laser excitation and temperature (1.5–4.2 K), both under lateral (in the heteroboundary plane) confinement of the excitation region to a few micrometers and without such a limitation (directly from the region of laser-induced photoexcitation focused to a spot not exceeding 30 μ. Under low pumping (with a power smaller than a microwatt), interwell excitons are strongly localized due to small-scale random potential fluctuations and the corresponding photoluminescence line is nonhomogeneously broadened to 2.5–3.0 meV. With increasing pumping power, the narrow line of delocalized excitons with a width of approximately 1 meV emerges in a threshold manner (the intensity of this line increases superlinearly near the threshold with increasing pumping). For a fixed pumping, the intensity of this line decreases linearly upon heating until it completely vanishes from the spectrum. The observed effect is attributed to Bose condensation in a quasi-two-dimensional system of interwell excitons. Within the proposed model, we show that the linear mode in the behavior of the luminescence intensity until its disappearance in the continuum of the photoluminescence spectrum upon a change in temperature is observed only for the condensed part of interwell excitons. At the same time, the luminescence of the above-the-condensate part of excitons is almost insensitive to temperature variations in the temperature range studied.

Temperature dependence of trap luminescence of CdS doped glasses

The temperature dependence of the optical absorption edge and integrated luminescence of CdS semiconductor doped glasses, having similar characteristics but manufactured by different manufactures like Corning 373 and Schott 1743 has been investigated. Transmission Electron Microscope (TEM) analysis is presented for these samples to understand the contribution of the inhomogeneous distribution of nanocrystal size. The energy gap shrinkage coefficient ( α ) and effective phonon energy ( θ ) have been derived from the temperature dependence of the energy gap. The activation energy of thermal photoluminescence (PL) quenching has been derived from the temperature dependence of luminescence intensity. An anomalous behaviour of the luminescence intensity and of bandwidth in the temperature range 50–10 K has been discussed using configuration co-ordinate model. The difference in the behaviour of different samples in the range 50–10 K has been suggested to arise from the inhomogeneous size distribution, which is supported by TEM analysis.

Study of the germanium luminescence in silica: from non-controlled impurity to germano-silicate core of telecommunication fiber preforms

We have studed luminescence properties of doped silica with different concentrations of germanium. The basic luminescence parameters such as spectral dependencies, decay kinetics and polarization at different temperatures were measured. Three spectral ranges 3.5–5.5 eV(I), 5.5–7 eV(II), 7–8 eV(III) in the optical transparency range of silica could be chosen from these data. Range I possesses a weak variation of basic parameters of luminescence of the germanium related oxygen deficient center with the change of luminescence center concentration from extremely low in pure silica to the germano-silica core of optical telecommunication fiber preforms. The temperature dependence of luminescence intensity and spectral content including excitation band are more affected by a change of concentration. The deviation of those parameters could be explained mainly within the framework of inhomogeneous broadening and center interaction with varying environment. The changes of decay kinetics and polarization excited in Ranges II and III are also insensitive to the change of concentration but the spectral content is more sensitive to the history of sample preparation, providing bigger changes in spectral bands that could be explained as a result of different surroundings leading to inhomogeneous broadening of the basic parameters. Range III is more affected by a host defect (ODC(I)), manifesting itself through an absorption band at 7.6 eV, by providing recombination mechanisms of luminescence excitation.

Temperature dependence of luminescence from ZnS 0.11Se 0.89 random and ordered alloys

Temperature dependence of the luminescence spectra was investigated for a ZnS 0.11Se 0.89 random alloy and a (ZnSe) 1/(ZnS 0.22Se 0.78) 1 ordered alloy with equivalent mean sulfur compositions. The luminescence peak for the random alloy as a function of temperature shows anomalous behavior, while that for the ordered alloy exhibits normal behavior. From the temperature dependence of luminescence intensity for the random alloy, the quenching process can be understood with additional thermal activation energy besides exciton dissociation energy. Through the analysis of the temperature variation of the luminescence line width, the exciton–optical phonon coupling constant is found to be reduced in the random alloy. The results of this study demonstrate that exciton is localized by the potential fluctuation in the random alloy and that the potential fluctuation is significantly constrained in the ordered alloy.

Ideality of pressure-sensitive paint. II. Effect of annealing on the temperature dependence of the luminescence

This article reports the effect of annealing on a pressure-sensitive paint (PSP) consisting of platinum tetra(pentafluorophenyl)porphine (PtTFPP) in a fluoroacrylic polymer called FIB (Puklin, E.; Carlson, W. B.; Gouin, S.; Costin, C.; Green, E.; Ponomarev, S.; Tanji, H.; Gouterman, M. J Appl Polym Sci 2000, 77, 2795). Samples annealed at 150°C, 75°C, annealed by a heat gun, and dried at room temperature are compared to nonannealed samples. Temperature dependences of luminescence intensity and lifetime are studied as a function and pressure and temperature and fit with Arrhenius and Stern–Volmer equations. We find that heating above Tg is more important than drying at room temperature in lowering the temperature dependence and obtaining ideal PSP, ideal meaning independent effect of pressure and temperature on luminescence properties of the paint. Ideality is achieved by lowering the activation energy for oxygen diffusion, presumably by relaxation of the polymer network. It is shown that ideal behavior occurs only over a limited temperature region. This range is more useful for PtTFPP in FIB than for ruthenium bathophenanthroline in polydimethylsiloxane (PDMS), another common PSP formulation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2805–2814, 2000

Thermal quenching of luminescence in erbium doped semiconductors

The nature of the temperature dependence of luminescence intensity from Er+ ions in GaInAsP, Si, InP, GaAs, AlGaAs, ZnTe, as observed by Favennecet al [1] has been examined in terms of a double exponential model. The smaller activation energy is found to be 58–100 meV, characteristic of a localized energy barrier at the Er+ centre while the higher activation energy is approximately 0.8Eg attributed to an Auger non-radiative process of carrier excitation into bands. This model has been found to describe the observed temperature dependences with reasonably good agreement.

Model for the photoluminescence behavior of porous silicon.

We outline a simple framework to explain the vast amount of data on the photoluminescence (PL) behavior of porous silicon. Our model involves the competition between an activated radiative process and a Berthelot-type nonradiative process. Our framework successfully explains a wide range of observations by various groups on the temperature, pressure and emission energy dependence of PL. The temperature dependence of luminescence intensity and decay time as predicted by this model is observed for a variety of materials such as amorphous silicon and chalcogenide glasses. The model is transparent, analytic, and does not take recourse to computer modeling or simulation. \textcopyright{} 1996 The American Physical Society.

Temperature dependence of luminescence intensity and decay times in C 60 single crystal

Luminescence properties for C 60 single crystal, the temperature dependence of the luminescence spectra and decay kinetics were measured for a wide range of excitation energies. At low temperatures the emission spectra with well resolved structure were detected. There were not observed any changes in the luminescence spectra under different excitation energy. With careful measurements of the luminescence intensity as function of temperature we were able to detect the particularity on the curve at T ∼ 250 K which corresponds to the phase transition in this material. Opposite to the case of C 60 films, the shape of the single crystal luminescence spectra also changes with temperature. The luminescence kinetic as function of temperature was investigated in nanosecond range. At temperatures T> 300 K the decay curve can be fitted by one exponent with t∼ 1 ns. When temperature decreased this component becomes non exponential and more slower. Thermostimulated current spectroscopy reveals the availability at least two traps for charged carriers.