Temperature Dependence Of Decay Time Research Articles

Emission enhancement in the luminescence performance of warm red light-emitting LiSrVO4 :Pr3+ phosphors.

Warm red-emitting praseodymium-doped LiSrVO4 phosphors were synthesized via solid-state reaction. The phase formation was verified using an X-ray diffraction study and the morphology was investigated using a scanning electron microscope study. The LiSrVO4 :Pr3+ phosphors emitted red light when exposed to ultraviolet light, indicating their possibility for use in warm white light-emitting diodes (WLEDs). Furthermore, the effect of charge compensators on the luminescence characteristics was addressed. The decay time was investigated using time-resolved photoluminescence. Furthermore, thermal quenching was analyzed through temperature-dependent photoluminescence spectra. Their sensitivity was calculated using temperature-dependent decay time analysis. The colour purity of the emitted light could be measured by photometric analysis. This comprehensive investigation provides a thorough understanding of the luminescence properties of phosphors for WLED applications.

High-performance solar-blind photodetector based on amorphous BN in harsh environment operations

High-performance solar-blind photodetectors capable of operating in extreme environments are desirable for a wide range of applications, such as engine control, down hole drilling, space exploration, and environmental monitoring. Boron nitride (BN), with an ultrawide bandgap and high band edge absorption coefficient, is especially suitable for such application scenarios owing to its chemical and structural stability at high temperatures and radiative conditions, which, however, in turn, brings difficulties in synthesis of large-area continuous single crystalline BN. Here, we report highly robust solar-blind photodetectors based on amorphous BN (a-BN) films that can operate at high temperatures and high electric fields. The a-BN films are dense and uniform, grown at 500 °C using dual beam-assisted deposition method. The a-BN-based photodetectors exhibit high performance with a responsivity of 0.56 mA/W at 20 V under UV illumination of 222 nm and a high rejection ratio (R222 nm/R295 nm > 200 and R222 nm/R315 nm > 500). More importantly, these photodetectors demonstrate excellent responsivity and stability at high temperatures up to 500 K and high bias of 200 V without breakdown. The photocurrent mechanism at elevated temperatures is analyzed by temperature-dependent decay time of the temporal response, showing the electron–phonon interaction and self-trapped holes are dominant. Our work suggests that the deposition of such a-BN films offers a promising strategy toward highly environment-resistant solar-blind photodetectors.

Ho3+ doped low-phonon single crystals and chalcogenide glasses for mid-IR source application

A comparative study was conducted to investigate the 3.9 µm mid-IR emission properties of Ho3+ doped NaYF4 and CsCdCl3 crystals as well as Ho3+ doped Ga2Ge5S13 glass. Following optical excitation at ∼890 nm, all the studied materials exhibited broad mid-IR emissions centered at ∼3.9 µm at room temperature. The mid-IR emission at 3.9 µm, originating from the 5I5 → 5I6 transition, showed long emission lifetime values of ∼16.5 ms and ∼1.61 ms for Ho3+ doped CsCdCl3 crystal and Ga2Ge5S13 glass, respectively. Conversely, the Ho3+ doped NaYF4 crystal exhibited a relatively short lifetime of ∼120 µs. Temperature dependent decay time measurements were performed for the 5I5 excited state for all three samples. The results showed that the emission lifetimes of Ho3+:CsCdCl3 and Ho3+:Ga2Ge5S13 were nearly temperature independent over the range studied, while significant emission quenching of the 5I5 level was observed in Ho3+:NaYF4. The temperature dependence of the multi-phonon relaxation rate for 3.9 µm mid-IR emission in Ho3+:NaYF4 crystal was determined. The room temperature stimulated emission cross-sections for all three samples were calculated using the Füchtbauer-Landenburg equation. Furthermore, the results of Judd-Ofelt analysis are presented and discussed.

Cryogenic Scintillation Properties of Lead-Free Cs3Cu2I5 Single Crystals.

Perovskite scintillators have become increasingly popular in recent years because of their simple production and high sensitivity to X-ray. Due to large Stokes shifts, high light yield, eco-friendly fabrication, and good stability, the lead-free Cu-based perovskites have gained much attention. In this paper, we prepared the Cs3Cu2I5 single crystals (SCs) by the solution-processed method. At room temperature, we measured the emission band at 440 nm with an average decay time of 595 ns under X-ray excitation. Under 137Cs γ-ray excitation, we determined that the light yield of Cs3Cu2I5 SCs was 23 000 photons/MeV. Notably, under alpha particle excitation by 241Am, the light yield of Cs3Cu2I5 SCs is approximately 3.2 times higher than that of the commercial scintillator LYSO(Ce). In addition, we systematically investigated the cryogenic scintillation properties of Cs3Cu2I5 SCs at the temperature range of 60-300 K. With decreasing temperature, the intensity of the emission band at 440 nm significantly increases, and an additional emission band at 336 nm emerges below 100 K. Meanwhile, the temperature-dependent decay times were determined. The fast and slow decay time of Cs3Cu2I5 SCs are estimated to be 221 and 1193 ns, respectively, at 60 K. Our findings highlight the great potential for Cs3Cu2I5 SCs to be a cryogenic scintillator.

Ethylene glycol associated facile preparation and luminescent behaviors of RE (RE = Sm3+, Dy3+) ions activated NaLuF4 nanoparticles

To achieve high quality luminescent nanomaterials, we synthesized series of rare-earth (RE; RE = Sm3+, Dy3+) ions activated NaLuF4 nanoparticles via a facile synthetic technique at room temperature. The phase category, microstructure and luminescent behaviors of the resultant samples were detailedly investigated. Under a certain excitation wavelength, these prepared nanoparticles can emit glaring visible light and the emission intensities are determined to be sensitive to the doping content. The optimal doping concentration for Sm3+ and Dy3+ ions in the NaLuF4 host lattices are 1 and 3 mol%, respectively, and the involved concentration quenching mechanisms are all contributed by the electric dipole-dipole interaction. The synthesized nanoparticles exhibit splendid thermal stability and the activation energies of the NaLuF:0.01Sm3+ and NaLuF4:0.03Dy3+ nanoparticles are 0.16 and 0.17 eV, respectively. Furthermore, the temperature-dependent decay time reveals that the thermal quenching mechanism of the designed nanoparticles is responsible by the crossover process between the excited level and ground state. These results indicate that the resultant nanoparticles with good luminescent materials and high thermal stability are potential candidates for white light-emitting diode.

Luminescence and scintillation properties of Gd3Sc2(Al3-xGax)O12:Ce (x = 1, 2, 3) garnet crystals

The luminescence and scintillation characteristics of Ce3+ - doped Gd3Sc2(Al3-xGax)O12 (x = 1, 2, 3) garnet crystals were investigated. The Ce3+ luminescence was blue-shifted with increasing Ga content due to the decrease of the crystal field strength. A decrease of Ce3+ luminescence intensity and luminescence decay time at high temperature was observed for all samples due to thermal ionization of the Ce3+ 5d1 excited levels. With increasing Ga content the decrease of thermal activation energies, determined from temperature dependence of decay times, was obtained. Both Gd3Sc2Al2GaO12:Ce and Gd3Sc2AlGa2O12:Ce samples exhibited a comparable light yield (LY) value of 24,000 photons/MeV while the latter sample showed faster scintillation decay time of 12 ns (11%) + 56 ns (89%) with respect to those of 55 ns (28%) + 105 ns (72%) for the former sample. An acceleration of scintillation decay time with an expense of LY value was also obtained for the Mg2+-codoped sample. At low temperature a decrease of scintillation yield was observed in correlation with the intense thermoluminescence peaks, attributing to the localization of electrons at intrinsic shallow traps.

Designing multi-mode optical thermometers via the thermochromic LaNbO4:Bi3+/Ln3+ (Ln = Eu, Tb, Dy, Sm) phosphors

LaNbO4:1.0%Bi3+/x%Ln3+ (Ln = Eu, Tb, Dy, Sm) phosphors, which can emit both emissions of Bi3+ and Ln3+ ions, were prepared by a solid-state reaction technology. At 306 nm excitation, the prepared phosphors present multi-color emissions due to the energy transfer from Bi3+ to Ln3+ ions. Taking advantage of the various thermal quenching properties between Bi3+ and Ln3+ ions, the optical thermometric characteristics of the resultant compounds were evaluated. The maximum Sa values of the prepared phosphors with the dopants of Bi3+/Eu3+, Bi3+/Tb3+, Bi3+/Dy3+ and Bi3+/Sm3+ are 0.044 K−1, 0.041 K−1, 0.008 K−1 and 0.031 K−1, respectively, while their maximum Sr values are 1.89% K−1, 2.36% K−1, 1.26% K−1 and 1.36% K−1, respectively. Moreover, based on the thermochromic behaviors of the studied samples, their temperature sensing abilities are also explored. Furthermore, through monitoring the temperature-dependent decay time of Bi3+ ions, the maximum relative temperature sensitivity values of the designed phosphors with the dopants of Bi3+/Eu3+, Bi3+/Tb3+, Bi3+/Dy3+ and Bi3+/Sm3+ are 0.10%, 0.032%, 0.037% and 0.145% K−1, respectively. Additionally, the thermometric properties of the compounds can be facilely manipulated by adopting diverse sensing technology. These results indicate that LaNbO4:1.0%Bi3+/x%Ln3+ (Ln = Eu, Tb, Dy, Sm) phosphors are promising multi-mode optical thermometers.

Structure identification and strongly enhanced luminescence of Sr9Y2(WO6)4: Mn4+ phosphors by co-doping Mg2+ ions for plant growth LEDs

In this paper, we reported a novel far-red emitting phosphor Sr9Y2(WO6)4: Mn4+ and alkaline earth metal Mg2+ ion co-doped phosphor Sr9Y2(WO6)4: Mn4+, Mg2+. Systematical studies on its crystal structure and luminescence properties are carried out. X-ray powder diffraction confirmed that the pure phase Sr9Y2(WO6)4: Mn4+ and Sr9Y2(WO6)4: Mn4+, Mg2+ phosphors show a single phase. Upon 350 nm excitation, the phosphors could emit far-red emission in the range of 600–800 nm, peaking at 680 nm (14,706 cm−1) due to the 2Eg→4A2g transition, with broad spectra overlap with the absorption band of phytochrome PFR. The optimal doping concentration, concentration quenching mechanism, as well as the content and temperature dependent decay times of Sr9Y2(WO6)4: Mn4+ phosphors are studied in detail. Impressively, Mg2+ dopant is found to strongly improve the luminescence of Mn4+ and the related mechanisms are deeply studied. Ultimately, a far-red light emitting diode lamp is fabricated by exciting a far red phosphor Sr9Y2(WO6)4: 0.0025Mn4+, 0.01Mg2+ using a 365 nm n-UV illuminating LED chip. Our studies have shown that Sr9Y2(WO6)4: Mn4+, Mg2+ phosphors have potential application in controlling or adjusting plant growth in plant-lighting.

Investigation of the dimensionality using temperature-dependent decay times in InGaAs-coupled quantum well-quantum dots structures

The radiative temperature-dependent decay times were exploited to understand the confinement size analyzing the relaxation from thermally-excited states in InGaAs coupled quantum well (QW)–quantum dots (QDs) structures. We investigated the confinement size from coupled QW-QDs structures by measuring the decay time of radiative process from exciton states when the temperature is gradually increased. The photoluminescence (PL) decay time from the exciton states is amended with the PL intensity at low temperature (~5 K) to distinguish the radiative decay time. The zero-dimensional and the two-dimensional confinement size are obtained from the isolated QDs and QW. However, this assumption is no longer correct for optical coupling because of the direct tunneling when the distance between the QW and the QDs becomes less than 10 nm in the coupled QW-QDs structures. Below 6 nm between the QW and the QDs in the coupled QW-QDs structures, the power factor from the temperature variations increases from 0 to 0.3 (for QDs) and from 1 to 1.37 (for QW), which accords with a quasi-zero-dimensional density of states (~Tα=0.3) and a quasi-two-dimensional density of states (~Tα=1.37), respectively, because of the extended wave functions in the optical couplings and the existence of dark states in the coupled QW-QDs structures.

Clarifying photoluminescence decay dynamics of self-assembled quantum dots

We studied the temperature-dependent photoluminescence (PL) and time-resolved PL spectra of multilayer CdTe/ZnTe quantum dots (QDs) to understand their carrier dynamics. We demonstrated a method of enhancing the confinement of carriers in CdTe QDs by modulating the number of stacked layers, leading to enhanced acoustic phonons up to 67 μeV and reducing the optical phonon coupling to 20 meV with an average phonon energy of 20 meV. The temperature-dependent decay time could be explained using a simple model of the thermal redistribution of carrier states. Thermal escape from hole states during multiphonon scattering occurred only at high temperatures, whereas blue shifts and enhanced PL intensity were expected to enhance the electron–phonon coupling and confinement-induced mixing among discrete state and continuum states with separation energies of 3.5–7.4 meV. Time-resolved PL measurements probed the electric field screening effect as a function of the strain distribution in QDs and was established to be 2.5 ± 0.2 MV/cm.

Non-contact luminescence lifetime cryothermometry for macromolecular crystallography.

Temperature is a very important parameter when aiming to minimize radiation damage to biological samples during experiments that utilize intense ionizing radiation. A novel technique for remote, non-contact, in situ monitoring of the protein crystal temperature has been developed for the new I23 beamline at the Diamond Light Source, a facility dedicated to macromolecular crystallography (MX) with long-wavelength X-rays. The temperature is derived from the temperature-dependent decay time constant of luminescence from a minuscule scintillation sensor (<0.05 mm3) located in very close proximity to the sample under test. In this work the underlying principle of cryogenic luminescence lifetime thermometry is presented, the features of the detection method and the choice of temperature sensor are discussed, and it is demonstrated how the temperature monitoring system was integrated within the viewing system of the endstation used for the visualization of protein crystals. The thermometry system was characterized using a Bi4Ge3O12 crystal scintillator that exhibits good responsivity of the decay time constant as a function of temperature over a wide range (8-270 K). The scintillation sensor was calibrated and the uncertainty of the temperature measurements over the primary operation temperature range of the beamline (30-150 K) was assessed to be ±1.6 K. It has been shown that the temperature of the sample holder, measured using the luminescence sensor, agrees well with the expected value. The technique was applied to characterize the thermal performance of different sample mounts that have been used in MX experiments at the I23 beamline. The thickness of the mount is shown to have the greatest impact upon the temperature distribution across the sample mount. Altogether, these tests and findings demonstrate the usefulness of the thermometry system in highlighting the challenges that remain to be addressed for the in-vacuum MX experiment to become a reliable and indispensable tool for structural biology.

Influence of codoping on the luminescence properties of YAG:Dy for high temperature phosphor thermometry

The effects of codoping on the temperature-dependent luminescence of YAG:Dy were investigated for application in high temperature thermometry. YAG:Dy is a well-known thermographic phosphor suitable for high temperature measurements. However, its decay time is too long for studying temperatures in fast transient technical processes. Codoping is a known method for increasing the signal intensity and for decreasing luminescence decay times. Five co-doped samples with three different sensitizers, namely Tm, Tb and Pr were synthesized by solid-state method. Changes in spectral emission behavior and temperature dependent decay time are presented up to 1600K. The intensity ratio used for temperature measurements was similar for all samples, and decay time decreased for all codopings. Codoping by Tm decreased signal intensity considerably, thus YAG:Dy,Tm is not suitable for high temperature measurements. However, codoping YAG:Dy with Tb and especially Pr resulted in improved luminescent characteristics, and these phosphors are promising for high temperature phosphor thermometry.

Radioluminescence and photoluminescence of Th:CaF2 crystals.

We study thorium-doped CaF2 crystals as a possible platform for optical spectroscopy of the 229Th nuclear isomer transition. We anticipate two major sources of background signal that might cover the nuclear spectroscopy signal: VUV-photoluminescence, caused by the probe light, and radioluminescence, caused by the radioactive decay of 229Th and its daughters. We find a rich photoluminescence spectrum at wavelengths above 260 nm, and radioluminescence emission above 220 nm. This is very promising, as fluorescence originating from the isomer transition, predicted at a wavelength shorter than 200 nm, could be filtered spectrally from the crystal luminescence. Furthermore, we investigate the temperature-dependent decay time of the luminescence, as well as thermoluminescence properties. Our findings allow for an immediate optimization of spectroscopy protocols for both the initial search for the nuclear transition using synchrotron radiation, as well as future optical clock operation with narrow-linewidth lasers.

Size-dependent persistent photocurrent and surface band bending inm-axial GaN nanowires

The size-dependent persistent photocurrent (PPC), which refers to a photocurrent persisting for a long time after the excitation light source is terminated, has been investigated in $m$-axial GaN nanowires (NWs) with diameters of 20--130 nm. These NWs possess polar side surfaces and thus exhibit strong surface band bending (SBB). With different diameters, a different rise time in photoconductivity (PC) upon excitation light illumination and a different decay time in the PPC are observed; the latter is attributed to a long carrier lifetime caused by a frustrated recombination process. The intensity ($I$)-dependent photocurrent--gain (\ensuremath{\Gamma}) measurement displays a \ensuremath{\Gamma}-$I$ dependence that follows a power-law relationship with fitting indices of \ensuremath{\sim}0.85--0.89, indicating that the long carrier lifetime--induced PPC of GaN NWs is caused by an SBB effect instead of a bulk trap effect. In addition, size-dependent decay times reveal two regimes for the different sizes of NWs. The decay time of the NW above critical diameter (${d}_{\mathrm{crt}}$, 30--40 nm) is found to be \ensuremath{\sim}13 000 s, while the smaller NW (${d}_{\mathrm{crt}}$) is 800 s. Herein, we propose that the surface-induced effective barrier height for different sizes of GaN NWs is the dominant factor that explains the apparent size dependence. The temperature-dependent decay time measurements determine an effective barrier height of 226 meV for a 65-nm NW, whereas the 20-nm NW has an effective barrier height of 32 meV, confirming that SBB effects of different sizes are responsible for the size-dependent PPC in $m$-axial GaN NWs.

Neutron Spin-Echo Studies of Hemoglobin and Myoglobin: Multiscale Internal Dynamics

Neutron spin-echo spectroscopy was used to study structural fluctuations that occur in hemoglobin (Hb) and myoglobin (Mb) in solution. Using neutron spin-echo data up to a very high momentum transfer q (∼0.62 Å−1), we characterized the internal dynamics of these proteins at the levels of dynamic pair correlation function and self-correlation function in the time range of several picoseconds to a few nanoseconds. In the same protein solution, data transition from pair correlation motion to self-correlation motion as the momentum transfer q increases. At low q, coherent scattering dominates; at high q, observations are largely due to incoherent scattering. The low q data were interpreted in terms of an effective diffusion coefficient; on the other hand, the high q data were interpreted in terms of mean square displacements. Comparison of data from the two homologous proteins collected at different temperatures and protein concentrations was used to assess the contributions made by translational and rotational diffusion and internal modes of motion to the data. The temperature dependence of decay times can be attributed to changes in the viscosity and temperature of the solvent, as predicted by the Stokes–Einstein relationship. This is true for contributions from both diffusive and internal modes of motion, indicating an intimate relationship between the internal dynamics of the proteins and the viscosity of the solvent. Viscosity change associated with protein concentration can account for changes in diffusion observed at different concentrations, but is apparently not the only factor involved in the changes in internal dynamics observed with change in protein concentration. Data collected at high q indicate that internal modes in Mb are generally faster than those in Hb, perhaps due to the greater surface-to-volume ratio of Mb and the fact that surface groups tend to exhibit faster motion than buried groups. Comparison of data from Hb and data from Mb at low q indicates an unexpectedly rapid motion of Hb αβ dimers relative to one another. Dynamic motion of subunits is increasingly perceived as important to the allosteric behavior of Hb. Our data demonstrate that this motion is highly sensitive to protein concentration, temperature, and solvent viscosity, indicating that great care needs to be exercised in interpreting its effect on protein function.

Photoluminescence of Pb2+-doped SrHfO3

Pb2+ excitation and emission spectra and decay kinetics in SrHfO3 host are measured within 10–350 K temperature interval. Temperature dependence of decay times is fit by the phenomenological model describing the dynamics of the lowest excited state of Pb2+ ion. The best fit provided corresponding quantitative parameters of the model. Suitability of the Pb-doped SrHfO3 for phosphor and scintillator application is discussed.

Recombination lifetimes in InN films studied by time-resolved excitation-correlation spectroscopy

Recombination dynamics in degenerate InN were investigated by means of time-resolved excitation-correlation spectroscopy. The photoluminescence decay times are determined beyond the spectral response and temporal resolution limits of conventional photon-counting detectors. Spectral and temperature dependence of decay times reveal the effects of hole localizations on the recombination mechanisms. At low temperatures, the radiative lifetime ${\ensuremath{\tau}}_{r}$ is insensitive to temperature and significantly longer than that predicted for the radiative band-to-band recombination, indicative of a transition dominated by the free-to-bound recombination without $k$ conservation. Above a certain temperature determined by the electron concentration, we find ${\ensuremath{\tau}}_{r}\ensuremath{\sim}{T}^{3/2}$, as expected for the band-to-band transition when the $k$-selection rule holds. We determine a lower limit for the bimolecular recombination coefficient $B$ in InN at 300 K as $5.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }{\text{cm}}^{3}/\text{s}$.

Temperature dependent characteristics of La 2O 2S: Ln [Ln=Eu, Tb] with various Ln concentrations over 5–60 °C

This research is aimed at developing an optical sensor for remotely measuring human skin temperature in electromagnetically hostile environments, such as within a magnetic resonance imaging (MRI) scanner. In this feasibility study, various concentrations of europium-doped lanthanum oxysulphide (La 2O 2S: Eu—0.1–15 mol% (m/o)) and terbium-doped lanthanum oxysulphide (La 2O 2S: Tb—0.005–50 m/o) have been investigated in terms of crystallinity, photoluminescent (PL) spectral and decay time characteristics. For both phosphors, X-ray diffraction (XRD) has shown that as dopancy increases, the (1 0 0) and (0 0 2) reflections merge and there is a reduction in the c-axis parameter as well as the crystallite size. Photoluminescent characterisation (337 nm excitation) has also shown a dependency to dopant concentration through variance of peak intensity. Temperature dependent decay time measurements were carried out over a low temperature range 5–60 °C. Optimum brightness of these temperature dependent lines is achieved at concentrations of 1 and 10 m/o for La 2O 2S: Eu and La 2O 2S: Tb respectively. However, optimum temperature dependency is achieved at lower concentration for La 2O 2S: Eu, specifically at 0.1 m/o. In comparison to conventional phosphor temperature dependent characteristic, La 2O 2S: Tb showed an increase in decay time with respect to temperature for concentrations above 2 m/o.

Growth and characterization of ZnO nanowires on p-type GaN

ZnO nanowires were grown by catalyst-free metal-organic vapour-phase epitaxy on top of a p-type GaN buffer. The optical properties of the ZnO nanowires were investigated by temperature-dependent and time-resolved photoluminescence and compared to those of ZnO nanowires directly grown on sapphire. The luminescence intensity decrease with temperature of the nanowires grown on GaN reveals an original behavior since it is constant over 120K, showing the existence of strong localization centers. In addition, the temperature-dependent decay time measurements indicate a lower density of non-radiative channels for the nanowires grown on GaN.

Study of Photomodulation Dynamics of Excitonic Reflectivity in GaAs/AlAs Quantum Well Heterostructures

The spectral, temperature and time dependences of photoinduced changes of type-I excitonic transitions in GaAs/AlAs single QW structures were studied using wavelength-modulated reflectance spectroscopy. It was revealed that photoinduced changes of exciton spectra are closely related to the photoexcitation and redistribution of photocarriers in the QW region. It was obtained that the decay of photoinduced changes occurs in the millisecond range at 300 K and in the second range at 90 K. The temperature dependence of decay time is discussed in terms of both tunneling and thermal emission of photo-excited carriers from the QW.