Advanced Functional Materials, 2023, 01, https://doi.org/10.1002/adfm.202209275

Mechanoluminescence (ML), as an optical response to deformation stimuli, shows great potential in high-end stress sensing, ultrasonic field visualization, and multidimensional anti-counterfeiting. However, processive practical applications in bio-medicine are constrained by the discovery of near-infrared (NIR) ML materials. Unlike lanthanides (Ln³⁺) with sharp multiplets, two kinds of Cr³⁺-doped NIR ML materials, gallate spinel (ZnGa₂O₄:Cr³⁺, Zn₃Ga₂GeO₈:Cr³⁺) and gallate magnetoplumbite (SrGa₁₂O₁₉:Cr³⁺) are here reported. Owing to the intrinsic cation antisite defects and cation vacancies in the matrix, these materials exhibit bright NIR ML under a relatively low load (20 N). In particular for SrGa₁₂O₁₉:Cr³⁺ (750 nm, peak; 100 nm, FWHM) with low persistent luminescence (PersL) interference, the ML behavior can be further rejuvenated under UV and sunlight irradiation. SrGa₁₂O₁₉:Cr³⁺ also shows bright NIR emission under photo- and thermo-stimulation. Owing to their excellent tissue penetration and concealment capability, NIR ML materials show great potential in the fields of bio-medicine and anti-counterfeiting.

Inorg. Chem. 2023, 02, 07, 3018–3025, https://doi.org/10.1021/acs.inorgchem.2c03653

Zero-dimensional (0D) In-based organic–inorganic metal halides (OIMHs) have received growing interest in recent years as promising luminescent materials. However, the high efficiencies of 0D In-based OIMHs are all dependent on Sb doping in the existing literature. Here, we report a novel 0D In-based OIMH (C₁₀H₂₂N₂)₂In₂Br₁₀, which exhibits intrinsic broadband emission (610 nm), and the photoluminescence quantum yield (PLQY) can reach 70% without Sb doping. (C₁₀H₂₂N₂)₂In₂Br₁₀ shows a typical 0D structure with three different In–Br polyhedra (two octahedra and one tetrahedron) separated by large organic cations. Based on the optical property measurements and theoretical calculations, we demonstrate that (C₁₀H₂₂N₂)₂In₂Br₁₀ is an indirect semiconductor with a band gap of 3.74 eV, and the In–Br inorganic moiety is primarily responsible for the intense emission of (C₁₀H₂₂N₂)₂In₂Br₁₀. Interestingly, the unique double octahedral configuration in (C₁₀H₂₂N₂)₂In₂Br₁₀ may enhance the structural distortion and stimulate the self-trapped excitons (STEs), leading to the related high PLQY. Our work provides a novel 0D In-based OIMH with high-efficiency intrinsic emission, which is helpful for understanding the structure–PL relationships of hybrid halides.

Laser & Photonics Reviews 2023, 02, 08, https://doi.org/10.1002/lpor.202200999

For the conventional persistent luminescence (PersL) charging processes, carriers are photo-pumped to the conduction band (CB) or high-energy excited states (HES) under short-wavelength UV or coherent near-infrared (NIR) laser excitation. Herein, electron tunneling charging behavior is reported in Cr³⁺, Sm³⁺ co-doped NIR PersL magnetoplumbite SrGa₁₂O₁₉, which allows for efficient charging by incoherent visible light. First, the electrons are efficiently captured by the neighboring Ga^II-O²⁻ electron–hole trap centers via a tunneling process, and then these excited electrons are transferred to shallow traps via a persistently energetic optical pump. This work further optimizes the PersL performance via engineering the energy band through partial substitution of In³⁺ for Ga³⁺. Consequently, tunneling charging occurring near the neighboring Cr³⁺-traps dimers enables Sr(Ga,In)₁₂O₁₉:Cr³⁺,Sm³⁺ to display brighter NIR PersL (≈760 nm, peak; ≈100 nm, FWHM) than gallate spinel under sunlight irradiation. This work provides insights into electron tunneling charging under low-energy excitation for NIR PersL, which may inspire more PersL explorations for practical applications.

J. Appl. Cryst. (2023). 56, 884-888, https://doi.org/10.1107/S160057672300328X

Polyhedron ligands have been frequently used to characterize the ligand environment in inorganic materials. With a cation or anion as the central atom, its bonded counterparts are set as the vertices of the polyhedron. Generally, a series of geometrical parameters of the polyhedron, such as the average bond length and volumetric size, are extracted to illustrate the subtle change of local structure on compositional substitution. Polyhedral distortion analysis is widely adopted in research fields relying on a strong structure–property relationship. For example, in rare-earth or transition-metal element doped luminescent materials, the energy levels of the excited d electrons are highly dependent on the polyhedral distortion of the ligand. As a result, the spectral excitation/emission peaks of the luminescence center can be modulated by composition substitution, which usually induces polyhedral distortion due to the mismatch between ionic radii (Wang et al., 2016; Chen et al., 2017; Zhao et al., 2022). Therefore, characterization of polyhedral distortion constitutes an important part in the crystal structure analysis of inorganic materials.

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.