Suppression of Thermal Quenching for CsPbX₃ (X = Cl, Br, and I) Quantum Dots via the Hollow Structure of SrTiO₃ and Light-Emitting Diode Applications

All-inorganic perovskite quantum dots (PQDs, CsPbX3, X = Cl, Br, and I) show outstanding application prospects in the field of photoelectric devices. In recent years, the development of PQDs has greatly improved their stability to water, oxygen, and light. However, thermal quenching of PQDs greatly limits their practical application. Herein, we embed PQDs into ATiO3 (A = Ca, Ba, and Sr) of three different mesoporous spherical structures to explore the effect on thermal quenching of PQDs. Because of the unique mesoporous hollow microsphere structure and low thermal conductivity of SrTiO3, it can effectively block the heat transfer and improve the thermal quenching of PQDs. The photoluminescence (PL) intensity of CsPbBr3@SrTiO3 composites is 72.6% of the initial intensity after heating to 120 °C. Moreover, the PL intensity of CsPbBr3@SrTiO3 composites remains about 80% of the initial value even when stored in air for 20 days or irradiated by 365 nm UV light for 48 h. A neutral white light-emitting diode is assembled by a blue chip, CsPbBr3@SrTiO3 composites, and red phosphor of K2SiF6:Mn4+, which has a color temperature of 5389 K and a color gamut covered 133% of National Television Standards Committee (NTSC)

Green and High-Efficiency Production of Graphene by Tannic Acid-Assisted Exfoliation of Graphite in Water

A green and high-efficiency method was developed to prepare low-cost and high-quality graphene on a large scale through direct exfoliation of graphite in aqueous media using tannic acid (TA) as the stabilizer. The influence of preparation parameters on graphene concentration (C_G) and graphite exfoliation efficiency (C_G/C_G,i), including TA concentration (C_TA), initial graphite concentration (C_G,i), pH, ionic strength, sonication time, and cycles, was systematically investigated. Under the optimum conditions, the highest C_G that can be attained is 1.25 mg·mL^–1 with C_G/C_G,i equal to 2.5%, and 92% of the as-formed graphene are few-layer graphene (below 5 layers) with the electrical conductivity as high as 488 S·cm^–1. Due to TA on the graphene surface acting as the dual roles of dispersant and interfacial regulator, the high-quality graphene can be uniformly dispersed and tightly integrated into polymer matrices for high-performance and multifunctional polymer nanocomposites. In a word, this contribution provides a simple, green, high-efficiency, and scalable avenue for mass production and utilization of high-quality graphene

Redistribution of Activator Tuning of Photoluminescence by Isovalent and Aliovalent Cation Substitutions in Whitlockite Phosphors

Many strategies, including double substitution, addition of charge compensation, cation-size-mismatch and neighboring-cation substitution, have contributed to tuning photoluminescence of phosphors for white light-emitting diodes. These strategies generally involve modification of a certain special site where the activator occupies; tuning strategy based on multiple cation sites is very rare and desirable. Here we report that isovalent (Sr²⁺) and aliovalent (Gd³⁺) substitutions for Ca²⁺ tune the photoluminescence from one band to multiple bands in whitlockite β-Ca_3–<i>x</i>Sr_<i>x</i>(PO₄)₂:Eu²⁺ and β-Ca_{3–3<i>y</i>/7}Gd_2<i>y</i>/7(PO₄)₂:Eu²⁺ phosphors. The saltatory variation of the emission spectra is caused by the removal of Eu²⁺ from the site M(4) to other sites. Moreover, we found the mechanisms of dopant redistribution tuning the luminescence are different. The incorporation of Gd³⁺ makes the site M(4) empty according to the scheme 3Ca²⁺ = 2Gd³⁺ + □, while Sr²⁺ substitution causes the cation sites to be enlarged due to cation size mismatch. Additionally, the influence of the cation substitutions on the photoluminescence thermal stability of phosphors is researched. The strategies, emptying and enlarging sites, developed herein are expected to provide a general route for tuning luminescence of phosphors with multiple sites in the future

Changing Ce³⁺ Content and Codoping Mn²⁺ Induced Tunable Emission and Energy Transfer in Ca_2.5Sr_0.5Al₂O₆:Ce³⁺,Mn²⁺

A series of color-tunable Ce³⁺ single-doped and Ce³⁺, Mn²⁺ codoped Ca_2.5Sr_0.5Al₂O₆ phosphors were synthesized by a high-temperature solid-state reaction. The crystal structure, luminescent properties, and energy transfer were studied. For Ca_2.5Sr_0.5Al₂O₆:Ce³⁺ phosphors obtained with Al­(OH)₃ as the raw material, three emission profiles were observed. The peak of photoluminescence (PL) spectra excited at ∼360 nm shifts from 470 to 420 nm, while that of the PL spectra excited at 305 nm stays unchanged at 470 nm with the increase of Ce³⁺ content. Furthermore, the peak of PL spectra is situated at 500 nm under excitation at ∼400 nm. The relationship between the luminescent properties and crystal structure was studied in detail. Ce³⁺, Mn²⁺ codoped Ca_2.5Sr_0.5Al₂O₆ phosphors also showed interesting luminescent properties when focused on the PL spectra excited at 365 nm. Obvious different decreasing trends of blue and cyan emission components were observed in Ca_2.5Sr_0.5Al₂O₆:0.11Ce³⁺,<i>x</i>Mn²⁺ phosphors with the increase in Mn²⁺ content, suggesting different energy transfer efficiencies from blue- and cyan-emitting Ce³⁺ to Mn²⁺. Phosphors with high color-rendering index (CRI) values are realized by adjusting the doping content of both Ce³⁺ and Mn²⁺. Studies suggest that the Ca_2.5Sr_0.5Al₂O₆:Ce³⁺,Mn²⁺ phosphor is a promising candidate for near UV-excited w-LEDs

Composition Screening in Blue-Emitting Li₄Sr_1+<i>x</i>Ca_{0.97–<i>x</i>}(SiO₄)₂:Ce³⁺ Phosphors for High Quantum Efficiency and Thermally Stable Photoluminescence

Photoluminescence quantum efficiency (QE) and thermal stability are important for phosphors used in phosphor-converted light-emitting diodes (pc-LEDs). Li₄Sr_1+<i>x</i>­Ca_{0.97–<i>x</i>}­(SiO₄)₂:​0.03Ce³⁺ (−0.7 ≤ <i>x</i> ≤ 1.0) phosphors were designed from the initial model of Li₄SrCa­(SiO₄)₂:​Ce³⁺, and their single-phased crystal structures were found to be located in the composition range of −0.4 ≤ <i>x</i> ≤ 0.7. Depending on the substitution of Sr²⁺ for Ca²⁺ ions, the absolute QE value of blue-emitting composition-optimized Li₄Sr_1.4­Ca_0.57­(SiO₄)₂:​0.03Ce³⁺ reaches ∼94%, and the emission intensity at 200 °C remains 95% of that at room temperature. Rietveld refinements and Raman spectral analyses suggest the increase of crystal rigidity, increase of force constant in CeO₆, and decrease of vibrational frequency by increasing Sr²⁺ content, which are responsible for the enhanced quantum efficiency and thermal stability. The present study points to a new strategy for future development of the pc-LEDs phosphors based on local structures correlation via composition screening