Daqin Chen

Dual-Phase Glass Ceramic: Structure, Dual-Modal Luminescence, and Temperature Sensing Behaviors

Yb(3+)/Er(3+)/Cr(3+) triply doped transparent bulk glass ceramic containing orthorhombic YF3 and cubic Ga2O3 nanocrystals was fabricated by a melt-quenching route to explore its possible application in optical thermometry with high spatial and temperature resolution. It was experimentally observed that Yb(3+)/Er(3+) ions incorporated into the precipitated YF3 nanophase, while Cr(3+) ions partitioned into the crystallized Ga2O3 nanophase after glass crystallization. Importantly, such spatial isolation strategy efficiently suppressed adverse energy transfer among different active ions. As a consequence, intense green anti-Stokes luminescence originated from Er(3+): (2)H11/2,(4)S3/2 → (4)I15/2 transitions, and deep-red Stokes luminescence transitions assigned to Cr(3+): (2)E → (4)A2 radiation were simultaneously realized. Impressively, the intermediate crystal-field environment for Cr(3+) in Ga2O3 made it possible for lifetime-based temperature sensing owing to the competition of radiation transitions from the thermally coupled Cr(3+) (2)E and (4)T2 excited states. In the meantime, the low-phonon-energy environment for Er(3+) in YF3 was beneficial for upconversion fluorescence intensity ratio-based temperature sensing via thermal population between the (2)H11/2 state and (4)S3/2 state. The Boltzmann distribution theory and the two-level kinetic model were adopted to interpret these temperature-dependent luminescence of Er(3+) and Cr(3+), respectively, which gave the highest temperature sensitivities of 0.25% K(-1) at 514 K for Er(3+) and 0.59% K(-1) at 386 K for Cr(3+).

Silica-Coated Mn-Doped CsPb(Cl/Br)3 Inorganic Perovskite Quantum Dots: Exciton-to-Mn Energy Transfer and Blue-Excitable Solid-State Lighting

Tunability of emitting colors of perovskite quantum dots (PQDs) was generally realized via composition/size modulation. Due to their bandgap absorption and ionic crystal features, the mixing of multiple PQDs inevitably suffers from reabsorption and anion-exchange effects. Herein, we address these issues with high-content Mn2+-doped CsPbCl3 PQDs that can yield blue-excitable orange Mn2+ emission benefited from exciton-to-Mn energy transfer and Cl-to-Br anion exchange. Silica-coating was applied to improve air stability of PQDs, suppress the loss of Mn2+, and avoid anion-exchange between different PQDs. As a direct benefit of intense multicolor emissions from Mn2+-doped PQD@SiO2 solid phosphors, a prototype white light-emitting diode with excellent optical performance and superior light stability was constructed using green CsPbBr3@SiO2 and orange Mn: CsPb(Cl/Br)3@SiO2 composites as color converters, verifying their potential applications in the field of optoelectronics.

Full-Spectral Fine-Tuning Visible Emissions from Cation Hybrid Cs1–mFAmPbX3 (X = Cl, Br, and I, 0 ≤ m ≤ 1) Quantum Dots

Full-color visible emissions are particularly crucial for applications in displays and lightings. In this work, we developed a facile room-temperature ligand-assisted supersaturated recrystallization synthesis of monodisperse, cubic structure Cs1-mFAmPbX3 (X = Cl, Br, and I or their mixtures Cl/Br and Br/I, 0 ≤ m ≤ 1) hybrid perovskite quantum dots (QDs). Impressively, cation substitution of Cs+ by FA+ was beneficial in finely tuning the band gap and in exciton recombination kinetics, improving the structural stability, and raising the absolute quantum yields up to 85%. With further assistance of anion replacement, full-spectral visible emissions in the wavelength range of 450-750 nm; narrow full width at half-maxima, and a wide color gamut, encompassing 130% of National Television System Committee television color standard, were achieved. Finally, Cs1-mFAmPbX3-polymer films retaining multicolor luminescence are prepared and a prototype white light-emitting diode device was constructed using green Cs0.1FA0.9PbBr3 and red Cs0.1FA0.9Br1.5I1.5 QDs as color converters, certainly suggesting their potential applications in the optoelectronics field.

Phase-transition-induced giant enhancement of red emission in Mn4+-doped fluoride elpasolite phosphors

Mn4+-Doped red fluoride phosphors have attracted significant interest of researchers because of their excellent luminescence properties that can address the issue of the lack of red light components in commercial white light-emitting-diodes (WLEDs). Herein, we synthesized two novel Mn4+-based phosphors based on two different K2LiAlF6 phases (cubic and trigonal). The quantum efficiency of cubic K2LiAlF6:Mn4+ was only 3.2%, but it underwent a dramatic increase to 87.5% when the trigonal phase changed to cubic K2LiAlF6. The trigonal phase was gradually transformed to the cubic phase by controlling the HF volume concentration; this led to an increase in the Mn content and enhancement of the emission intensity. The analysis of the experimental spectra of the synthesized phosphors was facilitated by the crystal field calculations of the K2LiAlF6:Mn4+ energy levels. A good agreement between the theoretical and experimental data was achieved. Special attention was paid to the structure of these two phases and the phase transition mechanism. The morphologies, compositions, and temperature-dependent photoluminescence properties were investigated in detail. The as-prepared phosphors showed excellent thermal and chromatic stabilities. Finally, the optimized K2LiAlF6:Mn4+ phosphor was blended with the YAG:Ce3+ yellow phosphor and a blue LED to fabricate WLEDs. The color rendering index (Ra) of this new WLED is 86, and the correlated color temperature (CCT) is 3498 K. These findings indicate that the K2LiAlF6:Mn4+ phosphor is a promising red phosphor for applications in WLEDs.

Inverse thermal quenching effect in lanthanide-doped upconversion nanocrystals for anti-counterfeiting

It is common that thermal quenching in luminescent materials inevitably occurs with an elevation in temperature. Herein, an inverse thermal quenching effect in lanthanide-doped upconversion nanocrystals was reported. Specifically, defects with an appropriate energy state were designed in Yb/Er (Ho, Tm) doped Na3ZrF7 nanocrystals to actualize a thermal-induced enhancement in luminescent intensity. Owing to this special temperature dependent optical property, our results reveal that these kinds of materials are very suitable for high security anti-counterfeiting applications.

Tunable Optical Properties and Enhanced Thermal Quenching of Non-Rare-Earth Double-Perovskite (Ba1–xSrx)2YSbO6:Mn4+ Red Phosphors Based on Composition Modulation

Non-rare-earth Mn4+-doped double-perovskite (Ba1- xSr x)2YSbO6:Mn4+ red-emitting phosphors with adjustable photoluminescence are fabricated via traditional high-temperature sintering reaction. The structural evolution, variation of Mn4+ local environment, luminescent properties, and thermal quenching are studied systematically. With elevation of Sr2+ substituting content, the major diffraction peak moves up to a higher angle gradually. Impressively, with increasing the substitution of Ba2+ with Sr2+ cation from 0 to 100%, the emission band shifts to short-wavelength in a systematic way resulting from the higher transition energy from excited states to ground states. Besides, this blue-shift appearance can be illuminated adequately using the crystal field strength. The thermal quenching of the obtained solid solution is dramatically affected by the composition, with the PL intensity increasing 16% at 423 K going from x = 0 to 1.0. The w-LEDs component constructed by coupling the UV-LED chip with red/green/blue phosphors demonstrate an excellent correlated color temperature (CCT) of 3404 K, as well as color rendering index (CRI) of 86.8.

Reverse synthesis of CsPbxMn1−x(Cl/Br)3 perovskite quantum dots from CsMnCl3 precursors through cation exchange

Successful development of all-inorganic CsPbX3 (X = Cl, Br, or I) perovskite quantum dots (PQDs) has been witnessed due to their unique optical and electrical properties. However, the heavy usage of Pb element in PQDs has limited the commercial application of these PQDs, and the preparation of low-Pb or Pb-free PQDs is imperative. In this study, we report room-temperature reverse synthesis of CsPbxMn1−xCl3 PDQs from CsMnCl3 precursor nanocrystals through cation exchange. Interestingly, two-step phase transformation processes from hexagonal CsMnCl3 to rhombohedral Cs4PbxMn1−xCl6 and finally to cubic CsPbxMn1−xCl3 are evidenced. With an increase in the reaction time, Mn2+ ions in the PQD lattice are gradually replaced by Pb2+ ions. Notably, Mn2+ d → d broadband luminescence is not detected for the Cs4PbxMn1−xCl6 product, but it is intense for the CsPbxMn1−xCl3 sample owing to efficient energy transfer from CsPbCl3 to Mn2+. Moreover, it is found that the introduction of Br− into the reaction system will prohibit the complete phase transformation from rhombohedral Cs4PbxMn1−x(Cl/Br)6 to cubic CsPbxMn1−x(Cl/Br)3. By controlling the ratio between the CsMnCl3 precursor and PbBr2, tunable luminescence is easily achieved due to the combined emissions from both CsPb(Cl/Br)3 PQDs and Mn2+-emitting centers.

CsPbX3 (X = Br, I) perovskite quantum dot embedded low-melting phosphosilicate glasses: controllable crystallization, thermal stability and tunable emissions

In this work, low-melting (700 °C) phosphosilicate glasses embedded with CsPbX3 (X = Br, I) perovskite quantum dots (QDs) were fabricated via a glass crystallization strategy. The as-prepared CsPbBr3 QDs@glass nanocomposites exhibited typical green luminescence assigned to exciton recombination radiation and outstanding thermal stability due to the protection of the robust inorganic glass host. Multi-color tunable emissions in the spectral range of 500–750 nm were achieved by modifying the molar ratio of halide sources in the glass. As a proof-of-concept experiment, a light-emitting diode device was constructed by coupling the green-emitting CsPbBr3 QDs@glass and red-emitting CsPbBr2I QDs@glass with a commercial InGaN blue chip, yielding a bright white light emission with excellent optoelectronic performance. It is expected that the investigated CsPbX3 QDs@glass nanocomposites may find promising application as color converters in solid-state-lighting.

In Situ Crystallization Synthesis of CsPbBr3 Perovskite Quantum Dot-Embedded Glasses with Improved Stability for Solid-State Lighting and Random Upconverted Lasing

All-inorganic cesium lead bromide CsPbBr3 perovskite quantum dots (QDs) are emerging as potential candidates for their applications in optoelectronic devices but they suffer from poor long-term stability due to their high sensitivity to UV irradiation, heat, and especially to moisture. Although great advances in improving stability of perovskite QDs have been achieved by surface modification or encapsulation in polymer and silica, they are not sufficiently refrained from external environment due to nondense structures of these protective layers. In this work, in situ nanocrystallization strategy is developed to directly grow CsPbBr3 QDs among a specially designed TeO2-based glass matrix. As a result, QD-embedded glass shows typical bright green emission assigned to exciton recombination radiation and significant improvement of photon/thermal stability and water resistance due to the effective protecting role of dense structural glass. Particularly, ∼90% of emission intensity is even remained after immersing QD-embedded glass in water up to 120 h, enabling them to find promising applications in white-light-emitting device with superior color stability and low-threshold random upconverted laser under ambient air condition.

Near-infrared to short-wavelength upconversion temperature sensing in transparent bulk glass ceramics containing hexagonal NaGdF_4: Yb^3+/Ho^3+ nanocrystals

Transparent glass ceramics containing hexagonal NaGdF4: Yb3+, Ho3+ nanocrystals were successfully fabricated via self-crystallization, which was further confirmed by XRD, TEM, HRTEM, and STEM-HADDF, as well as upconversion (UC) emission spectra. Impressively, GC750 exhibited fascinating upconversion luminescence, and the corresponding 5F1/5G6 and 5F2,3/3K8 states of Ho3+ were proven to be thermally coupled energy levels (TCELs), resulting in high temperature-sensitive behaviors based on fluorescence intensity ratio (FIR) for optical thermometry. As a consequence, a high relative sensitivity of 1.43%·K−1 at 390 K was achieved, offering a great potentially application in optical thermometry.