Daqin Chen

Enhanced luminescence of Mn4+:Y3Al5O12 red phosphor via impurity doping

Currently, white light-emitting-diodes converted using Ce3+:Y3Al5O12 phosphor suffer from the shortage of red component and the easy aging of the organic silicone binder. Herein, a novel and non-rare-earth doped Mn4+:Y3Al5O12 red phosphor was synthesized by a traditional solid-state reaction. This phosphor can emit red luminescence attributed to Mn4+:2E → 4A2 spin-forbidden transition in the 600–700 nm spectral region and can be efficiently excited by both the commercially available near-ultraviolet and blue chips. Impressively, Mg2+, Ca2+, and Ge4+ dopants were found to be beneficial for enhancing Mn4+ luminescence, and the related mechanisms were systematically discussed. Furthermore, Mn4+:Y3Al5O12 embedded inorganic glass ceramic was successfully fabricated to replace the phosphor in organic silicone as the color converter, and a stacking geometric configuration by sequentially coupling a Ce3+:Y3Al5O12 glass ceramic and a Mn4+:Y3Al5O12 glass ceramic with an InGaN blue chip was designed to explore its possible application in warm white light-emitting diodes.

A review on Mn4+ activators in solids for warm white light-emitting diodes

Currently, the major commercial white light-emitting diodes consist of a blue-emitting chip and Y3Al5O12:Ce3+ yellow phosphor. However, the shortage of a red emitting component in the constructed device makes it difficult to realize warm white light with a high color-rendering index and low correlated color temperature. In this mini-review article, we provide an overview of recent progresses in developing Mn4+ doped red phosphors for promising applications in warm white light-emitting diodes. Firstly, the spectroscopic properties of Mn4+ in solids, including electronic and vibronic energy-level structures, crystal-field parameters as well as thermal stability, were briefly discussed. And then the related physical and chemical synthesis strategies were introduced in detail. Afterwards, Mn4+ doped phosphors, such as oxides, fluorides as well as glass ceramic composites, and their impact on improving the photoelectric performance of white light-emitting diodes were summarized. Finally, several challenges and perspectives for exploring novel and high-performance Mn4+ doped red phosphors will be presented.

Garnet-based Li6CaLa2Sb2O12:Eu3+ red phosphors: a potential color-converting material for warm white light-emitting diodes

To alleviate the issues of low thermal stability and high correlated color temperature, exploring an inorganic color converter with both yellow- and red-emissions to replace the conventional resin/silicone-based phosphor converter for obtaining high-power warm white light-emitting diodes is highly desired. In this study, a series of garnet-based Li6CaLa2−2xEu2xSb2O12 (x = 0.1–1.0) red phosphors have been successfully synthesized by a conventional high-temperature solid-state method. The microstructure and luminescence properties were systematically investigated by X-ray diffraction, emission/excitation spectra, luminescence lifetimes and temperature-dependent decays. The as-synthesized phosphors exhibited a highly efficient red luminescence at 611 nm corresponding to the 5D0–7F2 electric dipole transition of Eu3+, and the luminescence monotonously enhanced as the Eu3+ content was increased to 100 mol%. The absence of concentration quenching was ascribed to the large Eu3+–Eu3+ distance (7.048–7.105 A) and subsequently the hindering of unwanted energy migration among them in the Li6CaLa2Sb2O12 crystalline lattice. Impressively, the Li6CaLaEuSb2O12 phosphor showed an excellent thermal stability with only 9.7% emission loss when the recording temperature was increased from 293 K to 553 K. To evaluate the suitability of Li6CaLa2Sb2O12:Eu3+ as a red converter, both the garnet-based Y3Al5O12:Ce3+ yellow and Li6CaLa2Sb2O12:Eu3+ red phosphor co-doped glass ceramics were successfully fabricated by a low-temperature co-sintering technique. Importantly, the adverse energy transfers between Ce3+ and Eu3+ were efficiently suppressed due to the spatial separation of Ce3+ in Y3Al5O12 and Eu3+ in Li6CaLa2Sb2O12 in the crystal lattice. As a consequence, the quantum yield of the glass ceramic reached as high as 89.3%, and the constructed white light-emitting diode exhibited an optimal luminous efficacy of 101 lm W−1, a correlated color temperature of 5449 K and a color rendering index of 73.7. It is expected that the developed Li6CaLa2−2xEu2xSb2O12 red phosphors and the related glass ceramics should have potential applications in high-power warm white light-emitting diodes.

Highly Sensitive Dual-Phase Nanoglass-Ceramics Self-Calibrated Optical Thermometer

A strategy to achieve high sensitivity of noncontact optical thermometer via the structure design of nanoglass-ceramic and the usage of Ln(3+) (Ln = Eu, Tb, Dy) luminescence as reference signal and Cr(3+) emission as temperature signal was provided. Specifically, the synthesized dual-phase glass-ceramics were evidenced to enable spatially confined doping of Ln(3+) in the hexagonal GdF3 nanocrystals and Cr(3+) in the cubic Ga2O3 nanoparticles, being beneficial to suppressing detrimental energy transfer between Ln(3+) and Cr(3+) and thus significantly enhancing their luminescence. As a consequence, completely different temperature-sensitive luminescence of Ln(3+)4f → 4f transition and Cr(3+) 3d → 3d transition in the present glass-ceramic resulted in obvious variation of Cr(3+)/Ln(3+) fluorescence intensity ratio with temperature and strikingly high detecting temperature sensitivity of 15-22% per K. We believe that this preliminary study will provide an important advance in exploring other innovative optical thermometry.

MoS2 Nanosheet-Modified CuInS2 Photocatalyst for Visible-Light-Driven Hydrogen Production from Water.

Exploiting photocatalysts respond to visible light is of huge challenge for photocatalytic H2 production. Here, we synthesize a new composite material consisting of few-layer MoS2 nanosheets grown on CuInS2 surface as an efficient photocatalyst for solar H2 generation. The photocatalytic results demonstrate that the 3 wt % MoS2 /CuInS2 photocatalyst exhibits the highest H2 generation rate of 316 μmol h(-1)  g(-1) under visible light irradiation, which is almost 28 times higher than that of CuInS2 . Importantly, the MoS2 /CuInS2 photocatalyst shows a much higher photocatalytic activity than that of Pt-loaded CuInS2 photocatalyst. The enhanced photocatalytic activities of MoS2 /CuInS2 photocatalysts can be attributed to the improved charge separation at the interface of MoS2 and CuInS2, which is demonstrated by the significant enhancement of photocurrent responses in MoS2 /CuInS2 photoelectrodes. This work presents a noble-metal-free photocatalyst that responds to visible light for solar H2 generation.

Large-scale room-temperature synthesis and optical properties of perovskite-related Cs4PbBr6 fluorophores

Currently, metal–halide perovskite semiconductors have attracted enormous attention for their excellent optical performance. However, challenging issues, such as the ability to perform large-scale synthesis as well as the thermal/moisture stability, limit their practical applications. Herein, we developed an inhomogeneous interface reaction strategy in a liquid–liquid immiscible two-phase system to realize the large-scale room temperature synthesis of novel perovskite-related Cs4PbBr6 semiconductors. Although the sizes were on the micrometer scale, the Cs4PbBr6 products exhibited bright green luminescence with a narrow line-width originating from exciton recombination confined in PbBr64− octahedra, and the photoluminescence quantum yields reached 40–45% owing to a large exciton binding energy of 222 meV. Furthermore, temperature cycling experiments demonstrated their excellent thermal stability with repeatable and reversible luminescence, and moisture-resistance experiments showed ∼65% of quantum yield loss after exposure to air for one month. Finally, a prototype white light-emitting diode device with a low correlated color temperature of 3675 K and a high color rendering index of 83 was constructed using green emissive Cs4PbBr6 and red emissive Eu2+:CaAlSiN3 phosphors, certainly indicating its promising applications in the optoelectronics field.

Interface engineering of a noble-metal-free 2D–2D MoS2/Cu-ZnIn2S4 photocatalyst for enhanced photocatalytic H2 production

Accelerating the charge separation of semiconductor photocatalysts remains a great challenge to develop highly efficient solar-to-H2 conversion systems. Here, 2D Cu2+-doped ZnIn2S4 (Cu-ZnIn2S4) nanosheets modified with 2D MoS2 are designed and prepared via solution chemical routes. Detailed characterization reveals that the specially designed unique 2D–2D structure is critical to the high photocatalytic performance for solar H2 generation. Benefiting from the presence of a large 2D nanojunction in the 2D–2D photocatalyst, the MoS2/Cu-ZnIn2S4 has an increased contact surface area for charge transfer. The improved charge separation is demonstrated by the significant enhancement of photocurrent responses. It is found that the 2D–2D MoS2/Cu-ZnIn2S4 photocatalyst at a 6 wt% MoS2 loading amount exerts a 5463 μmol h−1 g−1 H2-evolution rate under visible light irradiation (λ > 420 nm) with an apparent quantum yield of 13.6% at wavelength λ = 420 nm in 0.1 M ascorbic acid aqueous solution. This activity far exceeds those of noble metal (such as Pt, Ru, Pd or Au) loaded-Cu-ZnIn2S4 photocatalysts. The results demonstrate that the construction of a 2D nanojunction is a promising strategy to accelerate charge separation and enhance the photocatalytic performance of semiconductor photocatalysts for solar H2 generation.

Intense multi-state visible absorption and full-color luminescence of nitrogen-doped carbon quantum dots for blue-light-excitable solid-state-lighting

Currently, the excitation-wavelength-dependent photoluminescence of traditional carbon dots does not constitute true tuning and their absorptions show gradual attenuation in the visible region. Herein, we report a facile strategy to realize intense visible absorption and full-color emissions of nitrogen doped carbon dots via the control of surface nitriding. The quantum yields of the carbon dots in aqueous solution reached 49.2%, 30.6% and 30.3% for blue, yellow and red emissions, respectively. Structural characterizations and spectroscopic analyses verify that three diverse emitting states, i.e., sp2 carbon core, CO and CN related surface defects, are responsible for the multi-state absorptions and tunable emissions of the carbon dots. The ability to truly tune luminescence into the red wavelength region with strong visible absorption enables these carbon dots to improve the correlated color temperature and color rendering index of traditional phosphor-converted white light-emitting diodes.

Achieving efficient Tb3+ dual-mode luminescence via Gd-sublattice-mediated energy migration in a NaGdF4 core–shell nanoarchitecture

A strategy to realize dual-mode luminescence from identical Tb3+ is provided via Gd-sublattice-mediated energy migration and core–shell engineering techniques. By optimizing the structure of a NaGdF4:Yb/Tm@NaGdF4:Ce/Tb nanoarchitecture, both upconversion and downshifting emissions, originating from 5D4 → 7F6,5,4,3 transitions of Tb3+, are achieved through Yb3+ → Tm3+ → [Gd3+]n → Tb3+ and Ce3+ → [Gd3+]n → Tb3+ energy transfer processes, respectively.

A dual-functional upconversion core@shell nanostructure for white-light-emission and temperature sensing

A strategy to simultaneously achieve white-light-emission and temperature sensing via a 980 nm excitable upconversion core@shell nanoarchitecture design was provided. Specifically, the prepared Yb/Ho/Ce: NaGdF4@Yb/Tm: NaYF4 active-core@active-shell nanocrystals enabled the spatially confined doping of Ho3+ in the core and Tm3+ in the shell and thus greatly reduced the adverse energy transfers between them, leading to intense upconversion emissions for both Ho3+ and Tm3+ activators. Notably, introducing Ce3+ into the core resulted in the competition of radiation transitions from the Ho3+: 5S2, 5F4 green-emitting states and Ho3+: 5F5 red-emitting one, which was beneficial to tune the red to green intensity ratio and ultimately realize white-light luminescence. Temperature-dependent upconversion emission spectra of the core@shell samples evidenced the joint contribution of Ce3+ in the core and Tm3+ in the shell to improve sensitivity for temperature detection. As a consequence, the core@shell nanostructure was demonstrated to have a high temperature sensitivity (2.4% K−1) and excellent signal discriminability (3040 cm−1), being potentially applicable as an optical thermometric material.