Enhai Song

Highly Efficient and Thermally Stable K3AlF6:Mn4+ as a Red Phosphor for Ultra-High-Performance Warm White Light-Emitting Diodes

Following pioneering work, solution-processable Mn4+-activated fluoride pigments, such as A2BF6 (A = Na, K, Rb, Cs; A2 = Ba, Zn; B = Si, Ge, Ti, Zr, Sn), have attracted considerable attention as highly promising red phosphors for warm white light-emitting diodes (W-LEDs). To date, these fluoride pigments have been synthesized via traditional chemical routes with HF solution. However, in addition to the possible dangers of hypertoxic HF, the uncontrolled precipitation of fluorides and the extensive processing steps produce large morphological variations, resulting in a wide variation in the LED performance of the resulting devices, which hampers their prospects for practical applications. Here, we demonstrate a prototype W-LED with K3AlF6:Mn4+ as the red light component via an efficient and water-processable cation-exchange green route. The prototype already shows an efficient luminous efficacy (LE) beyond 190 lm/W, along with an excellent color rendering index (Ra = 84) and a lower correlated color temperature (CCT = 3665 K). We find that the Mn4+ ions at the distorted octahedral sites in K3AlF6:Mn4+ can produce a high photoluminescence thermal and color stability, and higher quantum efficiency (QE) (internal QE (IQE) of 88% and external QE (EQE) of 50.6%.) that are in turn responsible for the realization of a high LE by the warm W-LEDs. Our findings indicate that the water-processed K3AlF6 may be a highly suitable candidate for fabricating high-performance warm W-LEDs.

Tailored Near-Infrared Photoemission in Fluoride Perovskites through Activator Aggregation and Super-Exchange between Divalent Manganese Ions

Biomedical imaging and labeling through luminescence microscopy requires materials that are active in the near‐infrared spectral range, i.e., within the transparency window of biological tissue. For this purpose, tailoring of Mn2+–Mn2+ activator aggregation is demonstrated within the ABF3 fluoride perovskites. Such tailoring promotes distinct near‐infrared photoluminescence through antiferromagnetic super‐exchange across effective dimers. The crossover dopant concentrations for the occurrence of Mn2+ interaction within the first and second coordination shells comply well with experimental observations of concentration quenching of photoluminescence from isolated Mn2+ and from Mn2+–Mn2+ effective dimers, respectively. Tailoring of this procedure is achieved via adjusting the Mn–F–Mn angle and the Mn–F distance through substitution of the A+ and/or the B2+ species in the ABF3 compound. Computational simulation and X‐ray absorption spectroscopy are employed to confirm this. The principle is applied to produce pure anti‐Stokes near‐infrared emission within the spectral range of ≈760–830 nm from codoped ABF3:Yb3+,Mn2+ upon excitation with a 976 nm laser diode, challenging the classical viewpoint where Mn2+ is used only for visible photoluminescence: in the present case, intense and tunable near‐infrared emission is generated. This approach is highly promising for future applications in biomedical imaging and labeling.

Multifunctionalities of near-infrared upconversion luminescence, optical temperature sensing and long persistent luminescence in La3Ga5GeO14:Cr3+,Yb3+,Er3+ and their potential coupling

The multifunctionalities of La3Ga5GeO14:Cr3+,Yb3+,Er3+ (LGG:Cr3+,Yb3+,Er3+) with respective functions of near-infrared (NIR) upconversion (UC) luminescence, optical temperature sensing (OTS) and long persistent luminescence (LPL) were carried out and investigated in detail, which makes the materials attractive for bioapplications. The NIR UC luminescence at ∼830 nm with deep penetration in tissues is ascribed to the 4T2 → 4A2 transition of Cr3+. The OTS function based on the intensity ratio variation of 2H11/2 → 4I15/2 to 4S3/2 → 4I15/2 transitions of Er3+ ion detected that the thermal effects of the UC material caused by the laser irradiation ranges from 307 to 332 K for pumping power of 60 to 86 mW mm−2. It also showed LPL of Cr3+ with a defect trap depth of ∼0.77 eV below the conduction band, benefiting excitation-free and noise-free imaging in tissues. Additionally, anomalous temperature dependant UC emission behaviours of Cr3+ and Er3+ in this material were also characterized and discussed, which can be understood by the configurational coordinate (CC) model and the complex forward and backward energy transfer processes among the dopants, respectively. The potential coupling of these functions was further discussed, such as UC induced NIR LPL, which would repetitively restrengthen the fading LPL signals and alert the operators to thermal damage of the biosystems by the NIR laser in the tissue imaging.

Waterproof Narrow-Band Fluoride Red Phosphor K2TiF6:Mn4+ via Facile Superhydrophobic Surface Modification

With unique and efficient narrow-band red emission and broadband blue light absorption characteristics, Mn4+-activated fluoride red phosphors have gained increasing attention in warm white LEDs (WLEDs) and liquid crystal display (LCD) backlighting applications, whereas the intrinsic hygroscopic nature of these phosphors have inevitably limited their practical applications. Herein, a waterproof narrow-band fluoride phosphor K2TiF6:Mn4+ (KTF) has been demonstrated via a facile superhydrophobic surface-modification strategy. With the use of superhydrophobic surface modification with octadecyltrimethoxysilane (ODTMS) on KTF surfaces, the moisture-resistance performance and thermal stability of the phosphor KTF can be significantly improved. Meanwhile, the absorption, and quantum efficiency did not show obvious changes. The surface-modification processes and mechanism, as well as moisture-resistance performances and luminescence properties, of the phosphors have been carefully investigated. It was found that the luminous efficiency (LE) of the modified KTF was maintained at 83.9% or 84.3% after being dispersed in water for 2 h or aged at high temperature (85 °C) and high humidity (85%) atmosphere (HTHH) for 240 h, respectively. The WLEDs fabricated with modified KTF phosphor showed excellent color rendition with lower color temperature (2736 K), higher color rendering index (CRI, Ra = 87.3, R9 = 80.6), and high luminous efficiency (LE = 100.6 lm/W) at 300 mA. These results indicate that hydrophobic silane coupling agent (SCA) surface modification was a promising strategy for enhancing moisture resistance of humidity-sensitive phosphors, exhibiting great potential for practical applications.

A yellow-emitting phosphor of Mn2+-doped Na2CaP2O7

A yellow-emitting Na2CaP2O7:Mn(2+)phosphors have been synthesized by solid state reaction. The crystal structure, photoluminescence properties as well as concentration quenching mechanism have been investigated. The (4)T1-(6)A1 emission of Mn(2+)in Na2CaP2O7 phosphor ranges from 500 to 650 nm and exhibits a red shift while increasing the Mn(2+)concentration. The crystal field strength is calculated based on the combination of excitation spectrum and Tanabe-Sugano diagram. The chromaticity coordinates of Na2CaP2O7:Eu(2+), Mn(2+)phosphors were discussed in order to develop the potential application in white light-emitting diodes (LEDs).

Transition Metal-Involved Photon Upconversion

Upconversion (UC) luminescence of lanthanide ions (Ln3+) has been extensively investigated for several decades and is a constant research hotspot owing to its fundamental significance and widespread applications. In contrast to the multiple and fixed UC emissions of Ln3+, transition metal (TM) ions, e.g., Mn2+, usually possess a single broadband emission due to its 3d 5 electronic configuration. Wavelength‐tuneable single UC emission can be achieved in some TM ion‐activated systems ascribed to the susceptibility of d electrons to the chemical environment, which is appealing in molecular sensing and lighting. Moreover, the UC emissions of Ln3+ can be modulated by TM ions (specifically d‐block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln3+ owing to the well‐shielded 4f electrons and tuneable energy levels of the TM ions. The electric versatility of d 0 ion‐containing hosts (d 0 normally viewed as charged anion groups, such as MoO6 6‐ and TiO4 4‐) may also have a strong influence on the electric dipole transition of Ln3+, resulting in multifunctional properties of modulated UC emission and electrical behaviour, such as ferroelectricity and oxide‐ion conductivity. This review focuses on recent advances in the room temperature (RT) UC of TM ions, the UC of Ln3+ tuned by TM or d 0 ions, and the UC of d 0 ion‐centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.

Tunable multiple emissions in manganese-concentrated sulfide through simultaneous tailoring of Mn-site coordination and Mn-Mn pair geometry

In contrast to generally single-band visible emission feature from Mn2+, simultaneous visible (VIS) and near-infrared (NIR) multiple emissions are demonstrated in Mn2+ concentrated sulfide (MnS) by only involving a single crystallographic site. Upon varying the Mn2+-site coordination and/or Mn-Mn pairs geometry in different structural MnS, the multiple emissions from divalent manganese can be easily tuned from 575 to 720 nm (VIS) or from 880 to 900 or 1380 nm (NIR), respectively. The excitation spectroscopy and the luminescent decay, together with crystal structural analyses, are employed to investigate the electronic transition and the excited state dynamics of these Mn2+ concentrated systems. It is found that the VIS and NIR emissions can be ascribed to the isolated Mn2+ ion and exchange coupled Mn-Mn pair center, respectively. The effect of crystal field and bridging geometry, as well as temperature on the exchange coupled Mn2+ pairs NIR emissive center, is also investigated in detail. This work not on...

Selective enhancement of red upconvesion luminescence of Er 3+ by doping with Mn 2+ ions

Well-defined KZnF₃:Yb³⁺, Er³⁺, Mn²⁺ nanocubes were successfully prepared by a facile solvothermal method. The XRD and SEM measurements results show that the cubes have good crystallinity and the average cubic size is about 40 nm The incorporation of Mn²⁺ ions into KZnF₃:Yb³⁺, Er³⁺ leads to the great increase in the red to green luminescence intensity ratios from 10:1 to 214:1 and the red emission was enhanced about 20 times. The selective enhancement of red upconversion luminescence of Er³⁺ can be ascribed to the exchange energy transfer interactions between Er³⁺ ions and Yb³⁺-Mn²⁺ dimers. The case of KZnF₃:Yb³⁺, Er³⁺, Mn²⁺ provides a new strategy to get the pure red upconversion luminescence, which show potential applications in the fields of lighting, displays and biological nano labels.