Efficient Energy Transfer from Trap Levels to Eu3+ Leads to Antithermal Quenching Effect in High-Power White Light-Emitting Diodes

Continuous-flow synthesis of CsPbI3/TiO2 nanocomposites with enhanced water and thermal stability

The inherent poor stability of CsPbI3 nanocrystals hinders the practical application of this material. Therefore, it is still a challenge to improve the stability of CsPbI3 nanocrystals and realize their large-scale continuous preparation. In this work, we report the preparation of CsPbI3/TiO2 nanocomposites with high stability by a microfluidic method. After the combination of CsPbI3 nanorods with TiO2, the PL intensity increased by 1.3 times under excitation at 577 nm due to the passivating effect of TiO2 on the surface of CsPbI3 nanorods and its carrier transport characteristics. Meanwhile, due to the coating of TiO2, the surface exposure area of CsPbI3 nanorods is reduced, which blocks external environmental effects to some extent and effectively improves the stability of CsPbI3 nanorods. Finally, an LED with a color gamut of 142% NTSC and a color temperature (CCT) of 3952 K was obtained by combining CsPbI1.5Br1.5/TiO2 and CsPbBr3/TiO2 nanocomposites with a blue light chip (455 nm). This study shows that the continuous and controllable synthesis of all inorganic halide perovskite nanocrystals by a microfluidic method is of great significance in the fabrication of high-performance optoelectronic materials and display devices.

Development of a novel Eu2+ activated oxonitridosilicate cyan phosphor for enhancing the color quality of a violet-chip-based white LED

Cyan phosphors are urgently needed to fill the cyan gap and improve the spectral continuity of white light-emitting diodes (LEDs) to cater to the high demand for high-quality lighting. Here, a series of new Eu2+-activated La3Si6.5Al1.5N9.5O5.5 (LSANO) cyan phosphors were prepared, and their luminescence properties and color centers were analyzed through fluorescence spectral measurements from 7 K to 475 K. At 300 K, the photoluminescence excitation (PLE) spectrum monitored at 483 nm presents a broadband of 200-460 nm with a peak at 398 nm, matching well with commercial violet LED chips. When excited by 398 nm violet light, the photoluminescence emission (PL) spectrum of LSANO:0.01Eu2+ exhibits a cyan emission band at about 483 nm. At 7 K, the emission spectrum clearly shows an asymmetric emission band and the emission peak wavelength changes from 483 nm (300 K) to 500 nm (7 K), indicating that there are two possible color centers in the LSANO:Eu2+ phosphor. Moreover, the maximum emission value can be adjusted from 480 to 499 nm by adjusting the doping content of Eu2+. Finally, a violet-chip-based white LED with the optimized color quality of Ra = 91.4, Rf = 90.1, and Rg = 93.6 was fabricated by adding the prepared cyan phosphor, verifying the potential application of the prepared cyan phosphor LSANO:Eu2+ in high-quality white LEDs.

Color-tunable emissions realized by Tb3+ to Eu3+ energy transfer in ZnGdB5O10 under near-UV excitation

Photoluminescent (PL) energy transfer (ET) between two typical rare earth activators Tb3+ and Eu3+ is utilized to achieve color-tunable emission and the color range is apparently dependent on the ET efficiency. In the target host ZnGdB5O10 (ZGBO), the relatively low symmetric coordination environment of the rare earth cation not only suppresses the parity-forbidden law of the 4f-4f transitions of Tb3+ in the near-UV region, but also enhances the internal quantum efficiency (IQE), where the optimal IQE is 65.61% for ZGBO:0.8Tb3+. Moreover, its ET to Eu3+ is highly efficient, i.e. 94.71% in ZGBO:0.8Tb3+,0.10Eu3+, which eventually leads to a wide range of color-tunable emissions from green (0.2915, 0.5915) to red (0.6207, 0.3731). The systematic PL spectral study on Tb3+/Eu3+ singly doped and co-doped phosphors suggests that the ET mechanism takes place through the electric dipole-dipole interaction according to the Inokuti-Hirayama (I-H) model. Additionally, the in situ high temperature PL spectra indicate the very high thermal stability of ZnGd0.19Tb0.8Eu0.01B5O10, indicating that it can be a potential candidate for near-UV light emitting diode-pumped phosphors.

Boosting Red Luminescence of Mn4+ in Tantalum Heptafluoride Based on an Ab Initio-Facilitated Sensitizer and Hydrophobic Surface Modification

A narrow-band red-light component is critical to establish high color rendition and a wide color gamut of phosphor-converted white-light-emitting diodes (pc-WLEDs). In this sense, Mn⁴⁺-doped K₂SiF₆ fluoride is the most successful material that has been commercialized. As with K₂SiF₆:Mn⁴⁺ phosphors, Mn⁴⁺-doped tantalum heptafluoride (K₂TaF₇:Mn⁴⁺) fulfills a similar luminescence behavior and has been brought in a promising narrow-band red phosphor. But the limited brightness and low moisture-resistant performances have inevitably blocked its practical application. Herein, we employed the density functional theory (DFT)-based ab initio estimation approach to quickly identify the proper sensitizer by systematically investigating the electronic-band coupling between the several possible sensitizers (Rb, Hf, Zr, Sn, Nb, and Mo) and the luminescent center (Mn). Combined with experimental results, Mo was demonstrated to be the optimal sensitizer, which resulted in a 60% enhancement of the emission. On the side, the moisture sensitivity has been effectively improved via grafting the hydrophobic octadecyltrimethoxysilane (ODTMS) layer on the phosphor surface. Through employing the K₂TaF₇:Mn⁴⁺,Mo⁶⁺@ODTMS composite as a red component, warm WLEDs with good performance were achieved with a correlated color temperature (CCT) of 4352 K, a luminous efficacy (LE) of 90.1 lm/W, and a color rendering index (Ra) of 83.4. In addition, a wide color gamut reaching up to 102.8% of the NTSC 1953 value could be realized. Aging tests at 85 °C and 85% humidity for 120 h on this device manifested that the ODTMS-modified phosphor had much better moisture stability than that of the unmodified one. These studies provided viable tools for optimizing Mn⁴⁺ luminescence in fluoride hosts.

Thermally boosted upconversion and downshifting luminescence in Sc2(MoO4)3:Yb/Er with two-dimensional negative thermal expansion

Rare earth (RE³⁺)-doped phosphors generally suffer from thermal quenching, in which their photoluminescence (PL) intensities decrease at high temperatures. Herein, we report a class of unique two-dimensional negative-thermal-expansion phosphor of Sc₂(MoO₄)₃:Yb/Er. By virtue of the reduced distances between sensitizers and emitters as well as confined energy migration with increasing the temperature, a 45-fold enhancement of green upconversion (UC) luminescence and a 450-fold enhancement of near-infrared downshifting (DS) luminescence of Er³⁺ are achieved upon raising the temperature from 298 to 773 K. The thermally boosted UC and DS luminescence mechanism is systematically investigated through in situ temperature-dependent Raman spectroscopy, synchrotron X-ray diffraction and PL dynamics. Moreover, the luminescence lifetime of ⁴I_13/2 of Er³⁺ in Sc₂(MoO₄)₃:Yb/Er displays a strong temperature dependence, enabling luminescence thermometry with the highest relative sensitivity of 12.3%/K at 298 K and low temperature uncertainty of 0.11 K at 623 K. These findings may gain a vital insight into the design of negative-thermal-expansion RE³⁺-doped phosphors for versatile applications.

Rare-earth doped phosphors with negative thermal expansion (NTE) may display thermally-enhanced emission, but their performance is generally limited. Here the authors report thermally-boosted green upconversion luminescence and near-infrared downshifting luminescence in Sc₂(MoO₄)₃:Yb/Er phosphors with two-dimensional NTE, and their application in temperature sensing.

A novel red-emitting phosphor with an unusual concentration quenching effect for near-UV-based WLEDs

An unusual concentration quenching effect is caused by the anisotropic distribution of quenching centers in GdGaTi2O7:Eu3+.

Variation from Zero to Negative Thermal Quenching of Phosphor with Assistance of Defect States

Proper defect states are demonstrated to be beneficial to overcome thermal quenching of the corresponding phosphors. In this work, a cyan-emitting KGaGeO₄/Bi³⁺ phosphor with abundant defect states is reported, the emission intensity of which exhibits an abnormal thermal quenching performance under excitation with different photon energies. A 100% emission intensity is achieved at 393 K under 325 nm excitation compared with that at room temperature, while significantly enhanced intensities of 207% at 393 K and even 351% at 513 K under 365 nm excitation are recorded. The excellent thermal stability performance is confirmed to be not only related to the direct energy transfer from the defect states but also depended on the efficiency of capturing carriers for the trap centers, which is clarified in this work. In addition, the mechanism of the double tunneling process of carriers from trap centers to luminescence centers and luminescence centers to trap centers is studied. These results are believed to provide new insights into the thermal stability of the corresponding fluorescent materials and could inspire studies to further explore novel fluorescent materials with high thermal stability based on defect state engineering.

Realization of an Optical Thermometer via Structural Confinement and Energy Transfer

The influence of temperature on a variety of physiological or chemical processes has generated considerable interest, and recently noninvasive lanthanide-incorporated optical thermometers have been considered as promising candidates for monitoring its changes at different scales. Herein, a novel Bi³⁺-activated Sr_3-xGd_xGaO_4+xF_1-x phosphor with tunable color has been constructed by a cooperative cation-anion substitution strategy with to the replacement of [Sr²⁺-F^-] by [Gd³⁺-O^2-]. When x = 0, the sample Sr₃GaO₄F/Bi³⁺ possesses a peak wavelength at 438 nm, and this value will shift to 470 nm if x is equal to 1 (Sr₂GdGaO₅/Bi³⁺). In addition, photoluminescence tuning from blue to red has been realized successfully by an efficient Bi³⁺ → Eu³⁺ energy migration model in Sr_2.6Gd_0.4GaO_4.4F_0.6 samples. The specific Bi³⁺ → Eu³⁺ energy transfer has been explained by dipole-dipole interactions derived from a model of the Dexter pathway. Intriguingly, the two dopants (a blue signal from Bi³⁺ and a red signal from Eu³⁺) possess different thermal responses to increasing temperature. Accordingly, the intensity ratio values are sensitive to the temperature changes. The energy level cross relaxation causes the quenching effect of Bi³⁺, and the multi-phonon de-excitation mode leads to the thermal quenching of Eu³⁺. At room temperature (298 K), the determined maximum relative sensitivity (S_r) is 1.27% K^-1. Moreover, the absolute sensitivity (S_a) is 0.067 K^-1 since the temperature is elevated to 523 K. The collected results are superior to most of the reported optical thermometry materials.

SrBPO5: Ce3+, Dy3+ - A cold white-light emitting phosphor

A single-component white-light emitting phosphor SrBPO₅: Ce³⁺, Dy³⁺ with high color purity, good quantum efficiency and high thermal stability was prepared through the conventional high temperature solid state reaction. PXRD studies confirmed its phase purity. The suitability of Strontium borophosphate as a host for phosphor was confirmed through DFT calculations. The presence of Ce³⁺ along with Dy³⁺ in this host resulted in efficient energy transfer from Ce³⁺ to Dy³⁺ through a non-radiative multipole-multipole mechanism leading to the enhancement of Dy³⁺ luminescence towards white light emission. Lifetime decay and time-resolved emission studies confirmed the energy transfer along with multisite occupancy of Ce³⁺. In addition to energy transfer, the thermal stability of the phosphor was confirmed through temperature-dependent photoluminescence studies and the particle shape, size, uniformity of dopants of the phosphor was studied using the SEM and EDX spectroscopy.