Inorg. Chem. 2023, 06, 13, 10021–10028, https://doi.org/10.1021/acs.inorgchem.3c01405

The discovery of violet-excitable blue-emitting phosphor is a significant breakthrough for the development of phosphor-converted full-spectrum white light-emitting diodes (WLEDs). However, the application of most known violet-excitable blue-emitting phosphors is limited by their low external quantum efficiency (EQE). In this work, we reported on how the EQE values of Eu²⁺-doped Ba(K)-β-Al₂O₃ blue-emitting phosphor can be significantly improved through lattice site engineering. By partially substituting K⁺ for Ba²⁺, the Eu²⁺-occupied crystallographic site changes and the coordination polyhedron of Eu²⁺ shrinks, leading to the increase of crystal field splitting. Consequently, the excitation spectrum exhibits a continuous red shift to match the violet excitation, which enhances the PL intensity of solid solution phosphor (Ba_0.4K_1.6)_0.84Al₂₂O_35−α:0.32Eu²⁺ ((B_0.4K_1.6)_0.84AO:Eu) by 1.42 times compared to that of the end-member Ba_1.68Al₂₂O_35−α:0.32Eu²⁺ (B_1.68AO:Eu) phosphor. Correspondingly, under the 400 nm violet light excitation, the EQE of optimal blue-emitting (B_0.4K_1.6)_0.84AO:Eu phosphor is up to 53%. Additionally, the phosphor also shows excellent resistance to luminescence thermal quenching (95% at 150 °C). Finally, the WLED fabricated based on (B_0.4K_1.6)_0.84AO:Eu and commercial green and red phosphors exhibited an ultra-high color rending index with Ra = 95.5 and R1–R15 >90. This work offers guidance for tuning the spectral properties of phosphors through lattice site engineering.

Light:Science & Applications, 2023, 07, 25, https://www.nature.com/articles/s41377-023-01219-x

The increasing demand for high-contrast biological imaging, non-destructive testing, and infrared night vision can be addressed by the development of high-performance NIR light-emitting materials. Unlike lanthanide (Ln³⁺) with sharp-line multiplets and isolated Cr³⁺ with NIR-I emission, this study reports the first-ever NIR-II broadband luminescence based on the intervalence charge transfer (IVCT) of Cr³⁺-Cr³⁺ aggregation in gallate magentoplumbite. In particular, LaMgGa₁₁O₁₉:0.7Cr³⁺ exhibits dual-emission (NIR-I, 890 nm and NIR-II, 1200 nm) with a full width at half maximum (FWHM) of 626 nm under 450 nm blue LED excitation. Moreover, this dual-emission exhibits anti-thermal quenching behavior (432% @ 290 K), attributed to the energy transfer among multiple Cr³⁺ centers. Cryogen absorption spectra, lifetimes decay (2.3 ms), and electron paramagnetic experiments reveal the NIR-II luminescence of the Cr³⁺-Cr³⁺ → Cr²⁺-Cr⁴⁺ IVCT transition. The application of LaMgGa₁₁O₁₉:0.7Cr³⁺ in NIR-II biological imaging as an optical contrast agent, non-destructive testing, and night vision is demonstrated. This work provides new insights into broadband NIR-II luminescence under UV-NIR excitation based on the IVCT of Cr³⁺-Cr³⁺ aggregation.

Advanced Optical Materials 2023, 08, 31， https://doi.org/10.1002/adom.202301579

Near-infrared (NIR) phosphors are enablers for NIR phosphor-converted light-emitting diodes (pc-LEDs). However, fewer NIR-emitting phosphors with both high internal/external quantum efficiency (IQE/EQE) and thermal stability are discovered, which obstructs the promotion of NIR pc-LEDs. Herein, by partially replacing Al³⁺ in K-β-Al₂O₃:2Cr³⁺ with Ga³⁺, the photoluminescence (PL) intensity of the solid solution K_1+δ(Al_0.4Ga_0.6)₁₁O₁₇:2Cr³⁺, (KA_0.4G_0.6O:Cr) phosphor is increased 2.75 and 1.25 times that of end-members K_1+δAl₁₁O₁₇:2Cr³⁺ (KAO:Cr) and K_1+δGa₁₁O₁₇:2Cr³⁺ (KGO:Cr). The IQE/EQE of optimal KA_0.4G_0.6O:Cr reaches 88.9%/50.8% with high thermal stability (77.4%@150 °C). The PL intensity enhancement is due to the Al/Ga-6O octahedral volume and distortion variation caused by the substitution of Ga³⁺ for Al³⁺ in K_1+δ(Al_1-y,Ga_y)₁₁O₁₇:2Cr³⁺ (KA_1-yG_yO:Cr), which leads to the forbidden d–d transition being broken and crystal field strength varied. Finally, a NIR pc-LED device fabricated based on KA_0.4G_0.6O:Cr NIR-emitting phosphor and blue chip reaches an electro-optical efficiency of 16.3% under a drive current of 100 mA. Meanwhile, non-destructive detection and plant germination applications of the NIR pc-LED are demonstrated. These results prove that KA_0.4G_0.6O:Cr is a promising NIR phosphor for diverse applications.

Chem. 2023, 08,04, 14402–14410, https://doi.org/10.1021/acs.inorgchem.3c02134

The growing demand for spectroscopy applications in the areas of bioimaging, food quality analysis, and temperature sensing has led to extensive research on infrared light sources. It is crucial for the design of cost-effective and high-performance systems that phosphors possess the ability to absorb blue light from commercial LEDs and convert the excitation energy to long-wavelength infrared luminescence. In this work, we obtained Yb³⁺ luminescence with visible light response by utilizing the energy transfer from Cr³⁺ to Yb³⁺ in Mg₂GeO₄. After the introduction of Yb³⁺, intense NIR luminescence peaking at 974 nm can be achieved with an increasing intensity. The local structure analysis was performed to investigate the preferential occupation of Yb³⁺ ions and the energy transfer process in Mg₂GeO₄. Considering the properties of thermally coupled anti-Stokes and Stokes emissions of Yb³⁺ and the sensitive variation of the emission intensity, the potential application of Mg₂GeO₄:Cr³⁺, Yb³⁺ as thermometers was demonstrated.

Dalton Trans., 2023,52, 9368-9376, https://doi.org/10.1039/D3DT01209C

Mixing cations with different chemical properties to induce the generation of asymmetric structures is a new approach for tuning the optical properties of hybrid organic–inorganic metal halides (HOIMHs). In this study, zero-dimensional (C₉N₃H₁₅)(C₉H₁₃SO)MBr₆ (M = Bi/Sb, [C₉N₃H₁₅]²⁺ = [(C₄N₂H₁₀)(C₅NH₅)]²⁺ and [C₉H₁₄SO]⁺ = [CH₃(C₆H₄)OS(CH₃)₂]⁺) are synthesized. Two different cations cause both compounds to crystallize in the polar space group P2₁2₁2₁, thus resulting in significant phase matchable second harmonic generation under a 1064 nm laser excitation. Thus, (C₉N₃H₁₅)(C₉H₁₃SO)BiBr₆ and (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibit intensities that are approximately 1.8 and 1.7 times that of KH₂PO₄, respectively. The results of density functional theory calculations show that both (C₉N₃H₁₅)(C₉H₁₃SO)BiBr₆ and (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibit direct bandgaps of 2.95 and 2.81 eV, respectively. Additionally, because of the distortion of the inorganic octahedra, (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibited bright yellow emission at room temperature, which is attributed to ns² fluorescence emission. We believe that the symmetry of the HOIMH crystal structure can be broken by introducing spatially differentiated bifunctional organic cations, which consequently enables even-order nonlinear activities.

J. Mater. Chem. C, 2023,11, 48-96, https://doi.org/10.1039/D2TC02639B

Ce³⁺/Eu²⁺ activated luminescent materials offer a versatile platform for precise emission light manipulation through structure control on the basis of the composition–structure–property correlations. To date, Ce³⁺/Eu²⁺ activated phosphors have been well developed as an indispensable component in the lighting industry and display systems due to their superior performance. Meanwhile, many persistent phosphors contain Ce³⁺/Eu²⁺ together with other lanthanide or transition-metal co-dopants. It is therefore of great importance to focus on their similarities and gain insight into the interplay effect of the local structure and electronic structure on emission peak modulation and persistent duration elongation. Here, we review the theoretical and experimental progress in the discovery and optimization of Ce³⁺/Eu²⁺ activated LEDs and persistent phosphors. The Dorenbos model on f–d transitions and the latest developments in the correlation of the local structure and luminescence characteristics are elaborated to give an overall vision on the composition–structure–property correlations in Ce³⁺/Eu²⁺-activated phosphors. Particular attention is devoted to highlighting the critical role of the electronic structure in tuning the properties of phosphors. The development and optimization routines of some typical phosphors are expounded, with an emphasis on phosphor design principles, aiming at providing inspirations for tailoring and optimizing the properties of Ce³⁺/Eu²⁺-activated phosphors toward specific applications. Finally, we propose an outlook toward potential theory developments and future material discovery.

J. Mater. Chem. C, 2023,11, 8196-8203, https://doi.org/10.1039/D2TC05554F

Hybridized local and charge-transfer (HLCT) emitters are promising for the realization of high-performance blue organic light-emitting diodes (OLEDs). However, the rational design of efficient HLCT emitters remains challenging. Here, we present two blue emitters (TAP1 and TAP2) with the HLCT state through the construction of D–π–A molecules. Theoretical calculations reveal the large overlap and partial separation of “hole” and “particle” orbitals for S₁ excited states, demonstrating the HLCT nature of the emitters. Together with the clear solvatochromic phenomenon, we propose that introducing the π-conjugated anthracene unit into an appropriate donor–acceptor segment is an effective strategy to finely regulate the locally excited and charge transfer components and construct versatile HLCT emitters. Employing TAP2 and TAP1 as emitters, the blue OLEDs exhibit good color purity, high efficiency, and extremely low efficiency roll-offs with Commission internationale de l’éclairage coordinates of, respectively, (0.14, 0.10) and (0.15, 0.11), narrow full width at half maximum values of 45 nm and 52 nm, maximum external quantum efficiency values of 5.53% and 5.16%, and efficiency roll-off values of 0.5% and 1% at the practical brightness of 1000 cd m⁻². By virtue of the transient photoluminescence decay curves, excited state energy levels and natural transition orbital analysis, we classify that the reverse intersystem crossing at high-lying excited states from T₂ to S₁ serves as an efficient approach to harvest triplet excitons and thus boost the exciton utilization efficiency. The superior properties of HLCT emitters in this work provide inspiration for the rational design of promising materials for high-performance blue OLEDs.

Dalton Trans., 2023,52, 5119-5126, https://doi.org/10.1039/D2DT04067K

Organic–inorganic hybrid metal halides have been extensively studied because of their great potential in optoelectronics. Herein, we report three hybrid metal halides (Bmpip)₂ZnBr₄, (Bmpip)₂CdBr₄, and (Bmpip)₈Pb₁₁Br₃₀ (where Bmpip⁺ is 1-butyl-1-methyl-piperidinium, C₁₀H₂₂N⁺). (Bmpip)₂ZnBr₄ and (Bmpip)₂CdBr₄ crystallize in the P2₁/c space group with zero-dimensional crystal structures with [MBr₄]²⁻ (M = Zn, Cd) tetrahedra isolated by Bmpip⁺. (Bmpip)₈Pb₁₁Br₃₀ crystallizes in the triclinic space group P

 with one-dimensional corrugated chains constructed from face-sharing [PbBr₆]⁴⁻ octahedra. All of the compounds exhibit excellent ambient and thermal stability. Under UV excitation, all three compounds exhibit very broad emissions. Temperature-dependent photoluminescence measurements indicate that the broad emissions of (Bmpip)₂ZnBr₄ and (Bmpip)₂CdBr₄ can be attributed to both the organic cations and self-trapped excitons (STEs) and that the emission of (Bmpip)₈Pb₁₁Br₃₀ is assigned to STEs. Density functional theory calculations reveal that the three compounds adopt a direct band gap. This work enriches our understanding of the structure types of hybrid metal halides while revealing their diverse emission mechanisms.

J. Mater. Chem. C, 2023,11, 8462-8469, https://doi.org/10.1039/D3TC00528C

An efficient Cr³⁺-activated broadband near-infrared (NIR) phosphor is the key enabler to integrate compact NIR light-emitting diodes (pc-LEDs) for food testing and medical detection applications. High luminescence efficiency is key for the practical applications of phosphors. Herein, we design a series of efficient Cr³⁺-doped Ca₃Al_2−ySc_yGe₃O₁₂ garnet phosphors to induce octahedral distortion for enhancing NIR luminescence efficiency. We adopt the best fitted ideal polyhedron to characterize the octahedral distortion and found that the degree of distortion reaches its maximum at y = 0.2 and decreases with y deviating from 0.2. As a result, the composition with y = 0.2 shows the highest luminescence intensity. The composition dependence of the octahedral distortion coincides with that of the luminescence intensity, indicating that the luminescence properties of Ca₃Al_2−ySc_yGe₃O₁₂:Cr³⁺ are closely related to the octahedral distortion. The luminescence internal/external quantum efficiency (IQE/EQE) is significantly enhanced from 65.4%/25.7% in Ca₃Al₂Ge₃O₁₂:0.04Cr³⁺ to 85.4%/33.5% in Ca₃Al_1.8Sc_0.2Ge₃O₁₂:0.04Cr³⁺. The optimal Ca₃Al_1.8Sc_0.2Ge₃O₁₂:0.04Cr³⁺ phosphor exhibits excellent luminescence thermal stability (∼91% at 423 K) and high NIR output power (38.2 mW at 100 mA) with an electro-optical conversion efficiency of 13.7%. This work provides a strategy for enhancing the NIR luminescence of Cr³⁺.

Inorg. Chem. Front., 2023,10, 2430-2437, https://doi.org/10.1039/D2QI02489F

With the pursuit of healthy lighting, full-spectrum white light-emitting diodes (WLEDs) fabricated with violet chips and tri-color phosphors have been put forward. However, the excitation bands of most reported blue phosphors are located in the ultraviolet (UV) region, which hinders the development of full-spectrum lighting. In this work, by partially introducing N³⁻ into a matrix, a series of Ba_0.697Al_10.914O_17.232-3y/2N_y:0.16Eu²⁺ (BAON_y:Eu) blue phosphors with red-shifted photoluminescence excitation (PLE) spectra were synthesized. Under the excitation of 400 nm violet light, the internal/external quantum efficiency (IQE/EQE) values of the optimal sample BAON_1.0:Eu were calculated to be 80%/52%, while the retained integrated emission intensity at 150 °C can be 95% of that at room temperature. The WLED device fabricated by coating BAON_1.0:Eu and other commercial phosphors on a violet chip achieved an ultra-high color rendering index (R_a = 95.4). These results indicate that our synthesized BAON_1.0:Eu can be an excellent candidate blue phosphor for full-spectrum WLED lighting.