Temperature-dependent Luminescence Spectra Research Articles

White emitting Sr3Ga2Ge4O14: Dy3+ phosphors with high purity and good thermal stability

A series of white emitting phosphors Sr3Ga2Ge4O14: xDy3+ (x = 1, 3, 5, 7, 9 and 11 %) were synthesized by a high-temperature solid-phase method. All samples exhibit two emission peaks at 478 nm and 571 nm, corresponding to the 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+ ions, respectively. The optimal doping concentration of Sr3Ga2Ge4O14: xDy3+ phosphors was determined as 7 % mol, and the concentration quenching mechanism is confirmed as a dipole–dipole interaction. Finally, the temperature dependent luminescence spectra indicate that the synthesized Sr3Ga2Ge4O14: 7 % Dy3+ phosphors had good thermal stability.

Defect level induced negative thermal quenching of β-Ba3LuAl2O7.5: Yb3+, Er3+ phosphor

Temperature-dependent behavior of phosphors has recently attracted significant research attention. This is due to the widespread applications of phosphors. Unfortunately, the thermal quenching effect in many phosphors greatly limits their performance at high temperatures. Herein, negative thermal quenching phosphor β-Ba3LuAl2O7.5: 0.3Yb3+, 0.03Er3+ is synthesized using the high-temperature solid-state method. X-ray Rietveld refinement and EPR results indicate the defect state of oxygen vacancy. Temperature-dependent upconversion luminescence spectra demonstrate that the green and red emission intensities increase with temperature to reach maximums at 398 K and 348 K respectively. Diffuse reflectance spectrum and downshifting luminescence spectrum prove that the defect state is near the 4F7/2 state of Er3+ ions. The defect state traps electrons at room temperature and releases them to populate the 4F7/2 state of Er3+ ion at elevated temperatures to cause the observed thermal enhancement of upconversion luminescence. This material's negative thermal quenching effect makes it a suitable candidate for applications in displays and anti-counterfeiting.

Interrupting the long-range energy migration among Eu3+ by the introduction of unequivalent [NaO8] units to achieve both high quenching concentration and quantum yield in NaY2Ga2InGe2O12

In the pursuit of optimizing white light-emitting diodes (WLEDs), Eu3+-doped phosphors have emerged as a key component in strategies that combine ultraviolet (UV) LED chips with red, green, and blue (RGB) tricolor phosphors. However, their application has been notably constrained by concentration quenching and thermal quenching. To address this challenge, we have developed a series of novel red phosphors, NaY2-xGa2InGe2O12:xEu3+. The powder X-ray diffraction (XRD) indicates that the NaY2Ga2InGe2O12(NYGIG) belongs to the garnet family. The [NaO8] unit replaces one-third of the eight-coordinated sites of garnet and leaves the other two-thirds for the [YO8], which helps to prevent long-range energy migration among Eu3+ especially when the eight-coordinated sites were highly substituted. Such a structure feature allowed the NaY2-xGa2InGe2O12:xEu3+ phosphor to achieve an optimal quantum yield of 95.6% at an extremely high Eu3+ doping x value up to 1.2. Moreover, further studies on the temperature-dependent luminescence spectra confirm that an anomalous thermal quenching phenomenon will occur when the NaY2-xGa2InGe2O12:1.2Eu3+ phosphor is properly excited. In application, the combination of the red-emitting NaY2-xGa2InGe2O12:1.2Eu3+, blue-emitting BaMgAl10O17:Eu2+, and green-emitting Zn2SiO4:Mn2+ phosphors can realize high color rendering (CRI∼93) white light with the with CIE coordinates of (0.353,0.350), indicating that the NYGIG:Eu3+ phosphor own a promising potential for WLEDs.

The Effect of Lu3+ Doping on the Structural Stability and Luminescence Performances of Gd3Al5O12:Dy Phosphors

Transparent ceramics (Gd0.975−xLuxDy0.025)3Al5O12 (x = 0.2, 0.5, 0.975) are fabricated by vacuum sintering with the transmittance of ~60% at 583 nm. The doping of Lu3+ was beneficial to stabilize the garnet phase, and the best doping concentration was 20 at.%. All three (Gd, Lu)3Al5O12:Dy ceramics display typical Dy3+ emission, originating from 4F9/2 → 6HJ (J = 15/2, 13/2, 11/2) transition. Temperature-dependent luminescence spectra from 303.15 K to 603.15 K prove that the (Gd, Lu)3Al5O12:Dy ceramics had good thermal stability and could be an ideal yellow-emitting candidate in light-emitting diodes. The luminous efficiency of the encapsulated white LED lamp was found to be ~87.9 lm/W when the injection current was 600 mA. The transparent (Gd, Lu)3Al5O12:Dy ceramics show great application potential in various optical systems especially in the fields of luminescence and display.

Spectroscopic and temperature sensing properties of Sm3+ in self-activated CsLu(WO4)2 phosphors

In this research, a series of Sm3+ doped CsLu(WO4)2 phosphors was prepared via high temperature solid phase technique to design new red phosphors and optical thermometric materials. Their structures, morphology, band gap and luminescence properties were characterized by X-ray diffraction, scanning electron microscopy, diffuse reflection and luminescence spectra, respectively. Under UV excitation, CsLu(WO4)2 gives rise to a blue broad emission band between 350 and 700 nm, which stems from the 3T1u → 1A1g transition of WO66− groups. When Sm3+ is introduced into CsLu(WO4)2, energy transfer between WO66− groups and Sm3+ ions takes place in CsLu(WO4)2:Sm3+ phosphors, and color-tunable luminescence from blue to red is realized by controlling the Sm3+ doping concentration. The energy transfer efficiency between WO66− groups and Sm3+ ions was analyzed, and the energy transfer mechanism was determined to be dipole–dipole interactions. According to the temperature-dependent luminescence spectra, WO66− groups and Sm3+ ions exhibit large discrepancy in thermal quenching rates, and thus the temperature sensing properties of CsLu(WO4)2:Sm3+ in the temperature range of 283–403 K were analyzed. Based on the framework of fluorescence intensity ratio theory, the basic optical thermometry parameters including absolute and relative sensitivity of CsLu(WO4)2:Sm3+ were calculated and the results show that it has great potential for application in optical thermometry.

High-Efficiency Ce3+ Activated Orthorhombic Lanthanide Silicate Blue Phosphors for Plant Growth Lighting.

Plant growth can be controlled and freed from natural environmental interference through indoor plant cultivation. Artificial light sources with better quality are required to promote indoor plant growth. In this study, we used a simple high-temperature solid-state reaction to synthesize high-efficiency Ce3+-activated NaGdSiO4 (NGSO) phosphors. X-ray diffraction and Rietveld refinement were performed to determine the detailed crystal structure of the NGSO:Ce3+ phosphors. The morphology of NGSO:Ce3+ and the elemental state of Ce3+ were measured and analyzed. Under near-ultraviolet (n-UV) light excitation, the Ce3+-activated NGSO phosphors exhibit a broad emission band from 375 to 500 nm, and their emission peaks are at approximately 401 nm. This asymmetrical blue emission band is caused by the spin-allowed 5d → 4f transition of Ce3+ and overlaps well with the blue absorption region of carotenoids and chlorophyll. The temperature-dependent luminescence spectra were utilized to assess the thermal stability of NGSO:Ce3+. The external quantum efficiency (EQE) was measured to be 60.91%, and the internal quantum efficiency (IQE) was measured to be 73.39%. A blue LED device assembled from the NGSO:Ce3+ phosphor has demonstrated the application potential in accelerating plant growth.

High-sensitivity NaYF4:Yb3+/Ho3+/Tm3+ phosphors for optical temperature sensing based on thermally coupled and non-thermally coupled energy levels.

Non-contact optical temperature sensors are highly sought after by researchers due to their satisfactory temperature resolution (δ(T) < 0.1 °C), high relative thermal sensitivity (Sr > 1% °C-1), fast temporal response (t < 0.1 s), and long-term optical stability. In this study, NaYF4:Yb3+/Ho3+/Tm3+ upconversion nanoparticles were prepared by a solvothermal method, and their crystal structure, microscopic morphology, and luminescence mechanism, together with the temperature sensing properties of the specimens, were investigated. Under 980 nm laser excitation, the specimens exhibited strong upconversion luminescence, and the emission peaks corresponded to the characteristic energy level jumps of Ho3+ and Tm3+, respectively. The temperature-dependent luminescence spectra of the samples were investigated based on the fluorescence intensity ratio (FIR) technique over a temperature gradient of 295-495 K. The samples are based on thermally coupled energy levels (TCLs: 1G4(1,2) → 3H6(Tm3+)) and non-thermally coupled energy levels (NTCLs: 3F3 → 3H6(Tm3+) and 5F3 → 5I8(Ho3+), 3F3 → 3H6(Tm3+) and 1G4 → 3H6(Tm3+), 3F3 → 3H6(Tm3+) and 5F5 → 5I8(Ho3+), 3F3 → 3H6(Tm3+) and 5F4 → 5I8(Ho3+)) for temperature sensing performance. The maximum absolute sensitivity (Sa), relative sensitivity (Sr), and minimum temperature resolution δ(T) were found to be 0.0126 K-1 (495 K), 1.7966% K-1 (345 K), and 0.0167 K, respectively, which are better than those of most sensing materials, and the simultaneous action of multiple coupling energy levels can further improve the temperature precision. This study indicates that the sample has a good value for optical temperature measurement and also provides new ideas for the exploration of other high-quality optical temperature sensing materials.

Optical, emission, and excitation dynamics of Eu3+ -doped bismuth-based phosphate glass for visible display laser applications.

Eu3+ -doped-bismuth-based phosphate glasses with chemical equation (60 - x)P2 O5 -20Bi2 O3 -10Na2 CO3 -10SrF2 -xEu2 O3 (PBNSEu), (where x= 0, 0.1, 0.5, 1.0, 1.5 and 2mol%) were fabricated using the melt-quenching method. Obtain X-ray diffraction (XRD), energy-dispersive X-ray (EDAX), and Fourier transform infrared (FTIR) spectra were used to characterize the structure of the prepared PBNSEu glass. The J-O (Judd-Ofelt) intensity parameters (Ω2 , Ω4 ) were estimated using photoluminescence emission spectra. When excited with a xenon lamp at λexc = 394 nm, the most intense red-emission transition occurred at ~612 nm (5 D0 →7 F2 ). J-O intensity parameters were used to calculate radiative properties, whereas the radiative branching ratio (βR ), radiative transition probability (AR ), radiative lifetime (τR ), and total radiative transition rate (Aτ ) were calculated for the transitions 5 D0 →7 FJ (where J = 0-4) and were obtained in the emission spectra for europium ion-doped in the current glass. Using the CIE1931 chromaticity coordinates axes, the colours of various concentrations of Eu3+ ion-doped PBNS glass were evaluated using the emission spectra. Temperature-dependent luminescence spectra were recorded for the optimized PBNSEu20 glass to calculate the activation energy. These results strongly suggested red components in w-LEDs and visible display laser applications.

Novel Sm3+-activated YCa4O(BO3)3 phosphors with high quantum efficiency and thermal stability for red-backlight display and indoor illumination

Novel Sm3+-activated YCa4O(BO3)3 (YCOB) phosphors with high color purity, excellent thermal stability and high luminescence quantum efficiency were developed. Upon 405 nm excitation, all samples displayed strong orange-red emissions. The crystal structure and luminescence characteristics of YCOB in relation to Sm3+ concentrations were explored. The optimal doping concentration was found to be 5% mol with remarkable color purity of 97.73%. The concentration quenching mechanism in YCOB host lattices was dominated by dipole-dipole interaction. Meanwhile, temperature-dependent luminescence spectra revealed that the investigated phosphors have excellent thermal stability. Impressively, it was calculated that the internal quantum efficiency of synthesized phosphors was as high as 76.54%. To further verify the practicability of synthesized phosphors, both red light-emitting diodes and white light-emitting diodes were fabricated using YCOB: 5%Sm3+ phosphors, and they exhibited bright red and white light, respectively. These results suggest that the novel YCOB: Sm3+ phosphors with high quantum efficiency and luminescent stability were promising for red-backlight display and indoor illumination.

Carrier thermalization and zero-point bandgap renormalization in halide perovskites from the Urbach tails of the emission spectrum

We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred to be about 33 meV. This small value of the bandgap renormalization rules out the formation of small polarons and points to weak electron–phonon coupling. The experiments are performed on the nanosheets of the archetypal halide perovskite, CsPbBr3.

Luminescence properties and phase transformation of broadband NIR emitting A2(WO4)3:Cr3+ (A=Al3+, Sc3+) phosphors toward NIR spectroscopy applications

The synthesis, structural, and luminescence properties have been carried out for Cr3+-activated Al2(WO4)3 (AWO) and Sc2(WO4)3 (SWO) phosphors for application in pc-NIR LED. Upon blue excitation, these compounds are capable of exhibiting broadband NIR emission stems primarily from 4T2→4A2 transition in the range of 670–1200 nm (maxima ∼808 nm, FWHM ∼140 nm) for AWO:Cr and of 700–1300 nm (maxima ∼870 nm, FWHM ∼164 nm) for SWO:Cr. The significant shift of NIR emission is attributed to the substitution of AlO6 with larger ScO6 octahedrons. To gain insight into the luminescence the crystal field strength, Racah parameters, nephelauxetic effect, and electron-phonon coupling have been analyzed based on spectroscopic results. The electron-phonon coupling parameter S for SWO:Cr was determined to be 11.5, twice as large as that for AWO:Cr, which is in accordance with its strong thermal quenching. The abrupt changes occurring at 275 K in temperature-dependent luminescence spectra and decay lifetime of AWO:Cr is associated with temperature-driven phase transformation from low-temperature monoclinic to high-temperature orthorhombic phase. Pressure induced amorphization of AWO:Cr at pressures higher than 25 kbar was confirmed by employing high pressure evolution of Raman spectra. A high-power NIR pc-LED, fabricated by coating AWO:0.04Cr on a commercial 470 nm LED chip, shows good performance with an output power of 17.1 mW driven by a current of 320 mA, revealing potential application of studied materials for NIR light source.

Giant Ln 30 -Cluster-Embedded Polyoxotungstate Nanoclusters with Exceptional Proton-Conducting and Luminescent Properties

Giant Ln ₃₀ -Cluster-Embedded Polyoxotungstate Nanoclusters with Exceptional Proton-Conducting and Luminescent Properties

Электрон-фононное взаимодействие в композитах с колллоидными квантовыми точками: исследование методами люминесцентной спектроскопии и комбинационного рассеяния света

The temperature-dependent luminescence spectra were analyzed to determine the parameters of the electron-phonon interaction (Huang-Rhys factor and the average phonon energy) for nanocomposites with colloidal CdSe/CdS/ZnS quantum dots (deposited on the surface of a glass substrate and embedded in a thin polymer film of polyisobutylene, and in a frozen colloidal solution in toluene). The measured values of the parameters are analyzed in comparison with model calculations and data obtained using the low-frequency Raman spectroscopy. It is found that in the case of a vitrified colloidal solution of quantum dots in toluene, the matrix effect leads to a noticeable change in the parameters of the electron-phonon interaction.

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The temperature-dependent luminescence spectra were analyzed to determine the parameters of the electron-phonon interaction (Huang-Rhys factor and the average phonon energy) for nanocomposites with colloidal CdSe/CdS/ZnS quantum dots (deposited on the surface of a glass substrate and embedded in a thin polymer film of polyisobutylene, and in a frozen colloidal solution in toluene). The measured values of the parameters are analyzed in comparison with model calculations and data obtained using the low-frequency Raman spectroscopy. It is found that in the case of a vitrified colloidal solution of quantum dots in toluene, the matrix effect leads to a noticeable change in the parameters of the electron-phonon interaction.

Luminescent Nd3+, Cr3+ codoped YAG nanocrystals for thermal sensing: Influence of the excitation wavelength

Y3Al5O12 (YAG) nanocrystals codoped with Nd3+ and Cr3+ ions have been produced by a modified solvothermal method. The incorporation of both Nd3+ and Cr3+ inside the YAG network has been attested by X-ray diffraction and photoluminescence measurements. Modification of the excitation wavelength leads to ratiometric intensity changes between the luminescence attributed to Nd3+ and that of Cr3+. Temperature-dependent luminescence spectra have been analyzed according to the different excitation wavelengths. The highest relative thermal intensity (Sr = 2.5%.K−1) is obtained under a 532 nm excitation, which favors the direct and independent excitation of Cr3+ and Nd3+.

Systematic optical study of high-x InxGa1-xAs/InP structures for infrared photodetector applications

Optical and structural properties in high-x InxGa1-xAs (x > 0.65) samples with varying indium concentration grown on InP (100) substrate are reported. By increasing the indium fraction, it was found by the high-resolution X-ray diffraction (HR-XRD) study that the dislocation density in the InxGa1-xAs epitaxial layer significantly increased, and the surface quality deteriorated remarkably. Photoreflectance (PR) spectra show the presence of Franz-Keldysh Oscillations (FKOs) features above the InxGa1-xAs energy bandgap. The strain-induced electric field is then estimated directly from the FKOs periods. Temperature-dependent photoluminescence (TDPL) measurements from 10 K to 300 K showed carrier locations (S-shape). This abnormal behavior is due to the dislocation density associated with fluctuations in the indium concentration. A quasi-stationary rate equation model for the temperature-dependent luminescence spectra of the localized state material system is proposed to interpret the band gap emission process quantitatively. Low-temperature (10 K) time-resolved PL measurements show the increase of lifetime with increasing the indium concentration. Yet, the addition of only 1.7% of indium concentration results in a strong enhancement of PL lifetime by ∼ 80%.All these results reveal a more precise picture of the localization and recombination mechanisms of photogenerated carriers in the InGaAs layer, which could be the crucial factors in controlling the performance of high indium content InGaAs SWIR detector.

Abnormal reduction and luminescence properties of Sm2+-doped Sr5(PO4)3Cl prepared by solution combustion synthesis

Sm-doped Sr5(PO4)3Cl was prepared by a solution combustion synthesis in the air atmosphere. The pure hexagonal phases of the samples were confirmed via the XRD, Raman, and FT-IR measurements. The typical microrod shape of the products was also demonstrated by SEM, TEM, and HRTEM images. The spectral properties and lifetimes were applied to characterize the luminescence of the samples as-prepared and subsequently heated in the air atmosphere. The luminescence of as-prepared Sm-doped Sr5(PO4)3Cl contains a broad emission band and a group of narrow lines, which are ascribed to the radiative transitions of 4f5d→4f and 5D0→7FJ (J = 0, 1, 2) of Sm2+ ions, respectively. It indicates that the Sm3+ can be successfully reduced to Sm2+ ions in hexagonal Sr5(PO4)3Cl prepared by combustion synthesis. The temperature dependent luminescence spectra and decay curves of Sm2+ were measured in a very wide temperature region from 10 K to 300 K. The thermal quenching and activation energy of Sm2+ doped Sr5(PO4)3Cl were reported. The crystal field energy level diagram and excitation mechanism of Sm2+ ions-doped Sr5(PO4)3Cl were characterized by the high resolution luminescence at low temperature. The luminescence of Sm2+ in Sr5(PO4)3Cl shows a great dependence on heating temperature in the air atmosphere (50–700 °C). The heating treatments at the elevated temperature depress Sm2+ emission along with the appearance of Sm3+ ions. The temperature-induced changes in the modification of the spectral components could be applied as potential luminescence thermometry.

An insight into the luminescent properties and Judd–Ofelt analysis of Eu3+ doped CaZn2(PO4)2 phosphors

A batch of Eu3+ doped CaZn2(PO4)2 orange–red emitting phosphors were synthesized by the conventional high temperature solid state reaction method. The structural, morphological and spectroscopic properties of the prepared samples were investigated by experimental and theoretical studies. Powder XRD and FTIR results were used to evaluate the structural aspects of the prepared phosphors and morphology was investigated by TEM analysis. The elemental composition analysis of the representative sample was made using energy dispersive x-ray spectroscopy (EDS). The optical properties were analyzed with the aid of UV–Vis-NIR spectroscopy and photoluminescence (PL) spectroscopy. The PL excitation spectra revealed that the prepared phosphors can be efficiently excited under 393 nm that matches well with the dominant emission band of near UV- LED chip. The PL emission studies demonstrate the emission of an orange–red light with CIE coordinates (0.6075, 0.3920) with color purity 100.0% under an excitation of 393 nm. The maximum emission intensity was observed at Eu3+ concentration of 4 mol%, from which the critical distance (Rc = 1.98 nm) of quenching was calculated. The emission spectra were further used to interpret the site symmetry of Eu3+ ions. Judd–Ofelt theory was employed to predict various radiative properties such as branching ratio (βr), total transition probability (AT), radiative life time (τcalc) and stimulated emission cross section (σe). The thermal stability of the prepared phosphor was investigated using temperature dependent luminescence spectra and the emission intensity was observed to remain at 70.83% of its initial value when the temperature is raised from 310 K to 430 K. The decay kinetics analysis confirms the lifetime of the activator ions to be in the millisecond (ms) range. The better stimulated emission cross section and strong orange–red emission suggest the Eu3+ doped CaZn2(PO4)2 system as a promising candidate in the design of various optoelectronic devices such as phosphor converted LEDs (pc- LEDs) and display panels.

Dual-mode optical temperature sensing behavior of double-perovskite CaGdMgSbO6:Mn4+/Sm3+ phosphors

Mn4+ and Sm3+ singly doped and Mn4+/Sm3+ codoped CaGdMgSbO6 (abbreviated as CGMS) phosphors with double perovskite structure have been successfully prepared by the solid-state reaction. X-ray powder diffraction and luminescence spectra were used to analysis their crystal phases and optical properties. Especially, temperature-dependent luminescence spectra and luminescence decay curves of the CGMS:Mn4+/Sm3+ phosphor were measured at a large temperature range of 298–573 K Mn4+ and Sm3+ possess different trends of luminescence change where the luminescence intensity of Mn4+ ions decreases much more rapidly compared to that of Sm3+ ions as the temperature increases. Accordingly, based on the fluorescence intensity ratio (FIR) between the two activator Mn4+ and Sm3+ as well as the luminescence lifetime of Mn4+, the optimal relative sensitivity of temperature sensing in the CGMS:Mn4+/Sm3+ sample could reach 1.54% K−1 at 473 K and 1.23% K−1 at 523 K, respectively. All these indicate that this CGMS:Mn4+/Sm3+ material is a promising candidate for dual-mode high-sensitivity optical temperature sensing.

Ratiometric optical thermometer with high-sensitive temperature sensing based on synergetic luminescence of Ce3+-Eu2+ in LiSr4(BO3)3 phosphors

Abstract Single-phase Ce3+, Eu2+ co-doped LiSr4(BO3)3 phosphors are successfully prepared via solid reaction process. The crystal structure, luminescence characteristics and thermal quenching performance of LiSr4(BO3)3:Ce3+, Eu2+ samples are systematically studied. The presence of two crystallographic positions of Sr2+ ions in LiSr4(BO3)3 host makes the emission bands of Ce3+ ion and Eu2+ ion exhibit asymmetry. Based on the configuration coordinate model, the thermal quenching of luminescence from Ce3+ ion and Eu2+ ion is investigated in detail. The temperature-sensitive performance is further studied by analyzing the temperature-dependent luminescence spectra of LiSr4(BO3)3:0.06Ce3+, 0.04Eu2+ phosphor. The maximum absolute and relative sensitivities are 0.057 K-1 and 4.017% K−1, respectively, which are better than many previously reported inorganic optical thermometry materials. Besides, the emissions of Ce3+ ion and Eu2+ ion are clearly separated, providing good signal discrimination ability. The results demonstrate that LiSr4(BO3)3:Ce3+, Eu2+ phosphors have outstanding temperature-sensitive properties and can be the excellent optical thermometric materials.