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1. Solar-blind ultraviolet-C persistent luminescence phosphors

   https://doi.org/10.1038/s41467-020-16015-z  

   Wang, Xianli ; Chen, Yafei ; Liu, Feng ; Pan, Zhengwei ( April 2020 , Nature Communications)

   Visible-light and infrared-light persistent phosphors are extensively studied and are being used as self-sustained glowing tags in darkness. In contrast, persistent phosphors for higher-energy, solar-blind ultraviolet-C wavelengths (200–280 nm) are lacking. Also, persistent tags working in bright environments are not available. Here we report five types of Pr³⁺-doped silicates (melilite, cyclosilicate, silicate garnet, oxyorthosilicate, and orthosilicate) ultraviolet-C persistent phosphors that can act as self-sustained glowing tags in bright environments. These ultraviolet-C persistent phosphors can be effectively charged by a standard 254 nm lamp and emit intense, long-lasting afterglow at 265–270 nm, which can be clearly monitored and imaged by a corona camera in daylight and room light. Besides thermal-stimulation, in bright environments, photo-stimulation also contributes to the afterglow emission and its contribution can be dominant when ambient light is strong. This study expands persistent luminescence research to the ultraviolet-C wavelengths and brings persistent luminescence applications to light.

    

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2. Exploring the optical properties of La 2 Hf 2 O 7 :Pr 3+ nanoparticles under UV and X-ray excitation for potential lighting and scintillating applications

   https://doi.org/10.1039/c8nj00895g  

   Zuniga, Jose P. ; Gupta, Santosh K. ; Pokhrel, Madhab ; Mao, Yuanbing ( January 2018 , New Journal of Chemistry)

   New optical materials with efficient luminescence and scintillation properties have drawn a great deal of attention due to the demand for optoelectronic devices and medical theranostics. Their nanomaterials are expected to reduce the cost while incrementing the efficiency for potential lighting and scintillator applications. In this study, we have developed praseodymium-doped lanthanum hafnate (La 2 Hf 2 O 7 :Pr 3+ ) pyrochlore nanoparticles (NPs) using a combined co-precipitation and relatively low-temperature molten salt synthesis procedure. XRD and Raman investigations confirmed ordered pyrochlore phase for the as-synthesized undoped and Pr 3+ -doped La 2 Hf 2 O 7 NPs. The emission profile displayed the involvement of both the 3 P 0 and 1 D 2 states in the photoluminescence process, however, the intensity of the emission from the 1 D 2 states was found to be higher than that from the 3 P 0 states. This can have a huge implication on the design of novel red phosphors for possible application in solid-state lighting. As a function of the Pr 3+ concentration, we found that the 0.1%Pr 3+ doped La 2 Hf 2 O 7 NPs possessed the strongest emission intensity with a quantum yield of 20.54 ± 0.1%. The concentration quenching, in this case, is mainly induced by the cross-relaxation process 3 P 0 + 3 H 4 → 1 D 2 + 3 H 6 . Emission kinetics studies showed that the fast decaying species arise because of the Pr 3+ ions occupying the Hf 4+ sites, whereas the slow decaying species can be attributed to the Pr 3+ ions occupying the La 3+ sites in the pyrochlore structure of La 2 Hf 2 O 7 . X-ray excited luminescence (XEL) showed a strong red-light emission, which showed that the material is a promising scintillator for radiation detection. In addition, the photon counts were found to be much higher when the NPs are exposed to X-rays when compared to ultraviolet light. Altogether, these La 2 Hf 2 O 7 :Pr 3+ NPs have great potential as a good down-conversion phosphor as well as scintillator material. 

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3. Optical properties of undoped, Eu 3+ doped and Li + co-doped Y 2 Hf 2 O 7 nanoparticles and polymer nanocomposite films

   https://doi.org/10.1039/c9qi01181a  

   Gupta, Santosh K. ; Zuniga, Jose P. ; Abdou, Maya ; Ghosh, P. S. ; Mao, Yuanbing ( January 2020 , Inorganic Chemistry Frontiers)

   Desirable phosphors for lighting, scintillation and composite films must have good light absorption properties, high concentration quenching, high quantum efficiency, a narrow color emission, and so forth. In this work, we first show that undoped yttrium hafnate Y 2 Hf 2 O 7 (YHO) nanoparticles (NPs) display dual blue and red bands after excitation using 330 nm light. Based on density functional theory (DFT) calculations, these two emission bands are correlated with the defect states arising in the band-gap region of YHO owing to the presence of neutral and charged oxygen defects. Once doped with Eu 3+ ions (YHOE), the YHO NPs show a bright red emission, a long excited state lifetime and stable color coordinates upon near-UV and X-ray excitation. Concentration quenching is active when Eu 3+ doping reaches 10 mol% with a critical distance of ∼4.43 Å. This phenomenon indicates a high Eu 3+ solubility within the YHO host and the absence of Eu 3+ clusters. More importantly, the optical performance of the YHOE NPs has been further improved by lithium co-doping. The origin of the emission, structural stability, and role of Li + -co-doping are explored both experimentally and theoretically. DFT calculation results demonstrate that Li + -co-doping increases the covalent character of the Eu 3+ –O 2− bonding in the EuO 8 polyhedra. Furthermore, the YHOE NPs have been dispersed into polyvinyl alcohol (PVA) to make transparent nanocomposite films, which show strong red emission under excitation at 270 and 393 nm. Overall, we demonstrate that the YHO NPs with Eu 3+ and (Eu 3+ /Li + ) doping have a high emission intensity and quantum efficiency under UV and X-ray excitation, which makes them suitable for use as phosphors, scintillators and transparent films for lighting, imaging and detection applications. 

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4. On comparison of luminescence properties of La 2 Zr 2 O 7 and La 2 Hf 2 O 7 nanoparticles

   https://doi.org/10.1111/jace.16693  

   Gupta, Santosh K. ; Abdou, Maya ; Ghosh, Partha S. ; Zuniga, Jose P. ; Manoharan, Ezhilarasan ; Kim, HyeongJun ; Mao, Yuanbing ( July 2019 , Journal of the American Ceramic Society)

   Unveiling the underlying mechanisms of properties of functional materials, including the luminescence differences among similar pyrochlores A₂B₂O₇, opens new gateways to select proper hosts for various optoelectronic applications by scientists and engineers. For example, although La₂Zr₂O₇(LZO) and La₂Hf₂O₇(LHO) pyrochlores have similar chemical compositional and crystallographic structural features, they demonstrate different luminescence properties both before and after doped with Eu³⁺ions. Based on our earlier work, LHO‐based nanophosphors display higher photo‐ and radioluminescence intensity, higher quantum efficiency, and longer excited state lifetime compared to LZO‐based nanophosphors. Moreover, under electronic O²⁻→Zr⁴⁺/Hf⁴⁺transition excitation at 306 nm, undoped LHO nanoparticles (NPs) have only violet blue emission, whereas LZO NPs show violet blue and red emissions. In this study, we have combined experimental and density functional theory (DFT) based theoretical calculation to explain the observed results. First, we calculated the density of state (DOS) based on DFT and studied the energetics of ionized oxygen vacancies in the band gaps of LZO and LHO theoretically, which explain their underlying luminescence difference. For Eu³⁺‐doped NPs, we performed emission intensity and lifetime calculations and found that the LHOE NPs have higher host to dopant energy transfer efficiency than the LZOE NPs (59.3% vs 24.6%), which accounts for the optical performance superiority of the former over the latter. Moreover, by corroborating our experimental data with the DFT calculations, we suggest that the Eu³⁺doping states in LHO present at exact energy position (both in majority and minority spin components) where oxygen defect states are located unlike those in LZO. Lastly, both the NPs show negligible photobleaching highlighting their potential for bioimaging applications. This current report provides a deeper understanding of the advantages of LHO over LZO as an advanced host for phosphors, scintillators, and fluoroimmunoassays.

    

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5. Gd 3+ -activated narrowband ultraviolet-B persistent luminescence through persistent energy transfer

   https://doi.org/10.1039/D1DT00120E  

   Wang, Xianli ; Chen, Yafei ; Kner, Peter A. ; Pan, Zhengwei ( March 2021 , Dalton Transactions)

   null (Ed.)

   This work reports the realization of Gd 3+ persistent luminescence in the narrowband ultraviolet-B (NB-UVB; 310–313 nm) through persistent energy transfer from a sensitizer of Pr 3+ , Pb 2+ or Bi 3+ . We propose a general design concept to develop Gd 3+ -activated NB-UVB persistent phosphors from Pr 3+ -, Pb 2+ - or Bi 3+ -activated ultraviolet-C (200–280 nm) or ultraviolet-B (280–315 nm) persistent phosphors, leading to the discovery of ten Gd 3+ NB-UVB persistent phosphors such as Sr 3 Gd 2 Si 6 O 18 :Pr 3+ , Sr 3 Gd 2 Si 6 O 18 :Pb 2+ and Y 2 GdAl 2 Ga 3 O 12 :Bi 3+ as well as five ultraviolet-B persistent phosphors such as Y 3 Al 2 Ga 3 O 12 :Pr 3+ , Sr 3 Y 2 Si 6 O 18 :Pb 2+ and Y 3 Al 2 Ga 3 O 12 :Bi 3+ . The persistent energy transfer from the sensitizers to Gd 3+ is very efficient and the Gd 3+ NB-UVB afterglow can last for more than 10 hours. This study expands the persistent luminescence research to the NB-UVB as well as the broader ultraviolet-B spectral regions. The NB-UVB persistent phosphors may act as self-sustained glowing NB-UVB radiation sources for dermatological therapy. 

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