Temperature-dependent Decay Research Articles

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.

Dual-Mode Light-Emitting Lanthanide Metal-Organic Frameworks with High Water and Thermal Stability and Their Application in White LEDs.

It is well known that the upconversion luminescence from lanthanide metal-organic frameworks (Ln-MOFs) is difficult to achieve, and thus, there are few reports on dual luminescence-based MOFs. Here, dual-mode light-emitting Ln-MOFs are synthesized using a low-cost hydrothermal method. Our results show that the obtained Ln-MOFs not only have high thermal stability (up to 420°) but also are stable in deionized water. The dual-mode up- and downconversion luminescence is simultaneously observed from Er-Eu-MOFs. The temperature-dependent fluorescence decay time is calculated to be ranging from 0.46 to 0.36 ms for temperatures from 100 to 300 K. We suggested that this phenomenon was because the number of phonons participating in the MOF matrix increases with temperature during the luminescence process, and the phonons interact with the electrons in the material. The values of the J-O parameters calculated from the emission spectra indicated that the symmetry around Eu3+ ions in Eu-MOF is the highest, which was also higher than that of Er-Eu-MOF. To explore the potential applications of Eu-MOFs in white light-emitting diodes (LEDs), red emission from Eu-MOFs was combined with blue, green, and yellow emissions from metal halide perovskites to achieve white light emission. White light with excellent color quality and vision performance was obtained. These findings demonstrate that Ln-MOFs are potentially successful materials for applications in white LEDs.

Composition-, Size-, and Surface Functionalization-Dependent Optical Properties of Lead Bromide Perovskite Nanocrystals.

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on surface passivation. We present a study on the temperature-dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by surface passivation, and a postsynthesis ligand exchange with a quaternary ammonium bromide (QAB) results in more stable passivation over a larger temperature range. The PL intensity is highest from 50 to 250 K, which indicates that ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), surface passivation still needs to be improved to achieve maximum emission intensity in nanocrystal films.

Optical and mechanical properties of Li2Mg2(MoO4)3 crystal grown by Czochralski method

A Li2Mg2(MoO4)3 crystal was grown by self-flux Czochralski method in a platinum crucible. The polycrystalline compound was synthesised using standard solid-state reaction method. The purity of the synthesised compound was analysed through powder X-ray diffraction method. Single crystal growth was carried out using a Czochralski chamber in argon atmosphere and the pulling rate was maintained 0.5 mm/h with a rotation rate of 8−10 rpm. The temperature dependent luminescence and decay time measurements were carried out in a temperature range of 10–300 K. The grown crystal sample exhibits an intrinsic emission band in a wavelength range from 320 to 800 nm with the maximum peak at 590 nm. At low temperatures, a significant increase in emission intensity is observed as well as the red shift in the peak position. The luminescence decay time was fitted with three decay components. Thermally stimulated luminescence glow curve was recorded to study the trap centers and found a trap center at 25 K with 3 meV trap depth. Mechanical behaviour of the grown crystal was analysed by using Vicker’s micro hardness testing. The results suggest that Li2Mg2(MoO4)3 crystal can be used for luminescence applications.

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.

Cr3+ based nanocrystalline luminescent thermometers operating in a temporal domain.

Cr3+ doped nanocrystals were examined as a noncontact temperature sensor in a lifetime-based approach. The impact of both the analysis protocols and host materials on the lifetime-based approach was systematically investigated. Temperature-dependent luminescence decay curves were analyzed according to three different procedures (average lifetime approach, double exponential fit and time-gated ratiometric approach). The advantages and drawbacks of each method are discussed. Additionally, the thermal sensitivities derived from the average lifetime approach and the double exponential fit revealed a strong dependence of the thermal sensitivity of the Cr3+ doped nanocrystals on the crystal field strength. In these cases, it was found that the long metal-oxygen distances in the host materials improve the thermal sensitivity of the system. This work reveals the importance of both host materials and analysis procedures in the lifetime thermal sensitivity of Cr3+ doped nanocrystals and opens up an avenue towards their future optimization.

Light storage and thermal-assisted switching of SrAl2O4:Eu2+, Dy3+

In this work, we demonstrate and evaluate the viability of SrAl2O4:Eu[Formula: see text], Dy[Formula: see text] crystals for long-term light storage applications at low temperatures that can be activated at higher temperatures for on-demand lighting applications. The switching mechanism is discussed from the point of view of the possible interplay between charge transport and the persistent luminescence in this system. Step-wise temperature-dependent luminescence decay time measurement is carried out to determine the optimal temperature for photo-charging and the operational temperature limit of the crystal-under-test as light sources.

Comparative study of Mn4+ 2Eg→4A2g luminescence in isostructural A2CaWO6 (A=Ca, Sr, Ba) with double perovskite structure

Red-emitting Mn4+-doped A2CaWO6 (A = Ca, Sr, Ba) phosphors have been synthesized by solid-state reaction. The crystallinities and structural properties were examined by X-ray diffraction (XRD). The luminescence properties were studied by luminescence excitation, emission spectra and decay curves. All the synthesized materials emit red light peaking around 680 nm which can be assigned to the 2Eg → 4A2g spin-forbidden transition of Mn4+ ions under the excitation of ultraviolet to blue light. The critical quenching concentrations of Mn4+ in A2CaWO6 (A = Ca, Sr, Ba) system are evaluated. The temperature dependent emission spectra and decay curves (10–520 K) were introduced to evaluate the thermal stability. Surprisingly, the proposed three kinds of red phosphors show quite different thermal behavior, which can be ascribed to the different crystal structure environment of Mn4+ ions in A2CaWO6 (A = Ca, Sr, Ba) system.

Anisotropic escape mechanism and elliptic flow of bottomonia

We study the role of anisotropic escape in generating the elliptic flow of bottomonia produced in ultrarelativistic heavy-ion collisions. We implement temperature-dependent decay widths for the various bottomonium states, to calculate their survival probability when traversing through the anisotropic hot medium formed in non-central collisions. We employ the recently developed 3+1d quasiparticle anisotropic hydrodynamic simulation to model the space-time evolution of the quark-gluon plasma. We provide a quantitative prediction for transverse momentum dependence of bottomonium elliptic flow and nuclear modification factor for Pb$\,+\,$Pb collisions in $\sqrt{s_{\rm NN}}=2.76$ TeV at the Large Hadron Collider.

A new generation of dual-mode optical thermometry based on ZrO2:Eu3+ nanocrystals

Abstract For achieving well-performing optical thermometry, a new type of dual-mode optical thermometer is explored based on the valley-to-peak ratio (VPR) and fluorescence lifetime of Eu3+ emissions in the ZrO2:Eu3+ nanocrystals with sizes down to 10 nm. In the VPR strategy, the intensity ratio between the valley (600 nm) generated by the emission band overlap and the 606 nm peak (5D0→7F2), which is highly temperature sensitive, is employed, giving the maximum relative sensing sensitivity (S r ) of 1.8% K−1 at 293 K and good anti-interference performance. Meanwhile, the 606 nm emission exhibits a temperature-dependent decay lifetime with the highest S r of 0.33% K−1 at 573 K, which is due to the promoted nonradiative relaxation with temperature. These results provide useful information for constructing high-performance dual-mode optical thermometers, which may further stimulate the development of photosensitive nanomaterials for frontier applications.

A facile synthesis of high-efficient N,S co-doped carbon dots for temperature sensing application

A high-efficient nitrogen and sulfur-doped carbon dots (CDs) were prepared from l-Cysteine (Cys) and trisodium citrate dihydrate using a hydrothermal method. The Cys-CDs were completely water-soluble and remarkably stable under various pH and ionic strength conditions. Cys-CDs show a absorption maximum at 350 nm and PL maximum at 450 nm with high fluorescence quantum yield of 68%. It is found that Cys-CDs exhibit linear temperature-dependent emission intensity responses at in the 283–343 K range, along with strongly temperature-dependent monoexponential decay. The mechanism of temperature-dependent fluorescence is confirmed as temperature enhanced population of non-radiative channels by comparing radiative and nonradiative recombination rates at different temperature. All results indicate that Cys-CDs could be a promising material for fluorescent temperature-sensing application.

A theoretical study on spectroscopic properties and quantum yields of chiral-at-metal cyclometalated Pt(II) complexes

A new kind of chiral-at-metal Pt(II) complexes have been explored with theoretical methods. These chiral complexes are synthesized with achiral chelate ligands and the platinum(II) ions serve as the stereo center. To explore the intersystem crossing (ISC) and radiative decay process, we investigated the absorption and phosphorescence property, spin-orbit coupling (SOC) matrix element, major intersystem crossing channel and phosphorescent transition rate. For the temperature-dependent nonradiative decay process, the metal-centered excited state (3MC), the transition state (TS), and the minimum energy crossing point (MECP) were investigated with theoretical calculation. The way of temperature-independent nonradiative decay which has weak influence on nonradiative decay rate was also compared through the calculation of reorganization energy (λ) between the ground state (S0) and the lowest triplet excited state(T1).

Synthesis and Photoluminescence Properties of Eu3+-Activated Double Perovskite Ba2YTaO6 Red Phosphor

In this study, Ba2Y1−xTaO6:xEu3+ (BY1−xT:xEu3+; x = 0.03–0.20), as a double perovskite orange-red-emitting phosphor, was synthesized by solid-state reaction. The prepared phosphor samples were characterized by XRD, SEM, photoluminescence spectra, temperature-dependent photoluminescence spectra and decay curve measurements. The X-ray diffraction patterns indicated that the phosphor is a pure phase of a double perovskite structure [space group Fm-3m (225)]. Upon near-ultraviolet excitation, the sample exhibited a strong orange-red emission at 592 nm due to the 5D0 → 7F1 transition, which revealed that the Eu3+ ions were located at the central symmetric lattice position. The critical concentration of Eu3+ ions in Ba2YTaO6 (BYT) was determined to be 12 mol.%. The as-prepared phosphors showed excellent thermal stability according to temperature-dependent emission spectroscopy. The CIE values of BY1−xT:xEu3+ phosphors were found to be located in the red region. These results suggested that the prepared BY1−xT:xEu3+ phosphors will have a potential application in white light emitting diodes (WLEDs).

Tunable white-light emission and energy transfer in single-phase Bi3+,Eu3+ co-doped Ba9Y2Si6O24 phosphors for UV w-LEDs

In this paper, a series of Bi3+ and Eu3+ single-doped as well as Bi3+,Eu3+ co-doped Ba9Y2Si6O24 samples were synthesized through a simple high-temperature solid-state reaction method. Their crystal structures, morphologies, luminescent properties, temperature-dependent emission spectra, decay curves and quantum efficiencies were investigated in detail. Upon ultraviolet light excitation, Bi3+ single-doped samples exhibited blue and green emissions peaking at 408 and 501 nm, corresponding to the 3P1–1S0 transition of Bi3+ ions. The spectral analysis indicated that there were two different luminescent centers of Bi3+ ions due to different occupancies in Ba9Y2Si6O24 host lattice. And the highest quantum efficiency can reach 64.5% in Ba9Y2Si6O24:0.024Bi3+ phosphor. The remarkable spectral overlap between the emission spectra of Bi3+ ions and the excitation spectra of Eu3+ ions indicated the exist of energy transfer from Bi3+ to Eu3+. The efficient energy transfer from Bi3+ to Eu3+ has been investigated in detail and resulted in that tunable and white-light emission can be obtained by modulating Eu3+ doping concentration. White emission can be realized with the CIE coordinates of (0.357, 0.345) and quantum efficiency of 57.6% for Ba9Y2Si6O24:0.024Bi3+,0.02Eu3+. The above results demonstrate that Ba9Y2Si6O24:Bi3+,Eu3+ may be a potential candidate for ultraviolet (UV) driven white light-emitting diodes (w-LEDs).

A theoretical research on intersystem crossing, radiative and nonradiative rates of cyclometalated platinum(II) complexes

A series of cyclometalated platinum(II) complexes with similar molecular structures but distinct phosphorescence quantum yields were investigated. To explore the intersystem crossing (ISC) and radiative decay processes, we investigated the absorption and phosphorescence properties, spin–orbit coupling matrix elements, major ISC channels and phosphorescent transition rates. For the temperature-dependent nonradiative decay processes, the metal-centered excited states (3MC), the transition states, and the minimum energy crossing points were investigated with theoretical calculations. The way of temperature-independent nonradiative decay which has weak influence on nonradiative decay rate was also compared through the calculation of reorganization energy (λ).

Antimony doped Cs2SnCl6 with bright and stable emission

Lead halide perovskites, with high photoluminescence efficiency and narrow-band emission, are promising materials for display and lighting. However, the lead toxicity and environmental sensitivity hinder their potential applications. Herein, a new antimony-doped lead-free inorganic perovskites variant Cs2SnCl6:xSb is designed and synthesized. The perovskite variant Cs2SnCl6:xSb exhibits a broadband orange-red emission, with a photoluminescence quantum yield (PLQY) of 37%. The photoluminescence of Cs2SnCl6:xSb is caused by the ionoluminescence of Sb3+ within Cs2SnCl6 matrix, which is verified by temperature dependent photoluminescence (PL) and PL decay measurements. In addition, the all inorganic structure renders Cs2SnCl6:xSb with excellent thermal and water stability. Finally, a white light-emitting diode (white-LED) is fabricated by assembling Cs2SnCl6:0.59%Sb, Cs2SnCl6:2.75%Bi and Ba2Sr2SiO4: Eu2+ onto the commercial UV LED chips, and the color rendering index (CRI) reaches 81.

Structural Evolution and Effect of the Neighboring Cation on the Photoluminescence of Sr(LiAl3 )1-x (SiMg3 )x N4 :Eu2+ Phosphors.

In this study, a series of Sr(LiAl3 )1-x (SiMg3 )x N4 :Eu2+ (SLA-SSM) phosphors were synthesized by a solid-solution process. The emission peak maxima of SLA-SSM range from 615 nm to 680 nm, which indicates structural differences in these materials. 7 Li solid-state NMR spectroscopy was utilized to distinguish between the Li(1)N4 and Li(2)N4 tetrahedra in SLA-SSM. Differences in the coordination environments of the two Sr sites were found which explain the unexpected luminescent properties. Three discernible morphologies were detected by scanning electron microscopy. Temperature-dependent photoluminescence and decay times were used to understand the diverse environments of europium ions in the two strontium sites Sr1 and Sr2, which also support the NMR analysis. Moreover, X-ray absorption near-edge structure studies reveal that the Eu2+ concentration in SLA-SSM is much higher than that in in SrLiAl3 N4 :Eu2+ and SrSiMg3 N4 :Eu2+ phosphors. Finally, an overall mechanism was proposed to explain the how the change in photoluminescence is controlled by the size of the coordinated cation.

Site occupancy and luminescence of Ce3+ ions in whitlockite-related strontium lutetium phosphate

Ce3+-doped Sr9Lu(PO4)7 polycrystalline material has been synthesized using a solid state reaction at high temperature. Luminescence spectroscopic properties and emission decay kinetics have been studied upon UV-VUV excitation. Emission from Ce3+ ions located in two types of crystallographic sites has been identified in low-temperature spectra. Ce3+ ions have been shown to prefer occupying Sr2+ sites rather than Lu3+ sites. The temperature-dependent decay curves of Ce3+ 5d→4f emission demonstrate an excellent luminescence thermal stability.

High-temperature photocurrent mechanism of β-Ga2O3 based metal-semiconductor-metal solar-blind photodetectors

High-temperature operation of metal–semiconductor–metal (MSM) UV photodetectors fabricated on pulsed laser deposited β-Ga2O3 thin films has been investigated. These photodetectors were operated up to 250 °C temperature under 255 nm illumination. The photo to dark current ratio of about 7100 was observed at room temperature and 2.3 at a high temperature of 250 °C with 10 V applied bias. A decline in photocurrent was observed until a temperature of 150 °C beyond which it increased with temperature up to 250 °C. The suppression of the UV and blue band was also observed in the normalized spectral response curve above 150 °C temperature. Temperature-dependent rise and decay times of temporal response were analyzed to understand the associated photocurrent mechanism at high temperatures. Electron–phonon interaction and self-trapped holes were found to influence the photoresponse in the devices. The obtained results are encouraging and significant for high-temperature applications of β-Ga2O3 MSM deep UV photodetectors.

Red-emitting phosphor series: Ca9Y(PO4)7(1-x)(VO4)7x:Eu3+ (x = 0 − 1) with improved luminescence thermal stability by anionic polyhedron substitution

Superior thermal stability of the phosphors is crucial to retain the light balance of phosphor-converted WLEDs. At present, the fundamental research on the development of the phosphors with high thermal stability remains a challenging task. In this work, a series of red-emitting phosphors Ca9Y(PO4)7(1−x)(VO4)7x:Eu3+ (x = 0 − 1), in which phosphate groups were gradually replaced by vanadate groups, were synthesized via a high temperature solid-state reaction method. Their XRD patterns, Raman spectra, photoluminescent excitation and emission spectra, as well as temperature dependent emission properties and decay curves were investigated. The results of XRD pattern and Raman spectra indicated that a solid solution formed in Ca9Y(PO4)7(1−x)(VO4)7x:Eu3+ from x = 0 to x = 1. The photoluminescent properties study revealed that emission intensity upon the excitation 393 nm decreased while that under the irradiation of 464 nm increased with the improvement of vanadate groups. Moreover, the substitution of vanadium ions can increase the red to orange intensity ratio values of Eu3+. Especially, a limited amount of vanadium in Ca9Y(PO4)7:Eu3+ will improve the luminescence thermal stability. This was confirmed by temperature dependent lifetime values for the phosphors with and without vanadium. The possible reason affecting the above properties have been considered in details.