(INVITED) Review of luminescent properties of Ce³⁺-doped garnet phosphors: New insight into the effect of crystal and electronic structure

The absorption and luminescence properties (centroid shift and crystal field splitting of 5d orbitals) of Ce³⁺-doped garnets are summarized from the viewpoints of chemical composition (electron negativity and optical basicity) and the local crystal structure of the Ce³⁺ ion (bond length and distortion). Clear trends exist between (1) the centroid shift of 5d energy (εc) and the optical basicity of the host garnets and between (2) the crystal field splitting of the lowest 5d1-5d2 levels (Δ₁₂) and a new distortion parameter obtained from the crystal structure data. The data of quantum yield and quenching temperatures of Ce³⁺-doped garnets are also considered, indicating the thermal ionization process as the main mechanisms for the quenching process. Based on our recent experimental results, to prove the thermal ionization process, the principle and the most important features of the photocurrent excitation (PCE) and thermoluminescence excitation (TLE) spectra measurements are summarized. Finally, a general trend for the quenching temperature of Ce³⁺ luminescence in the garnets is discussed in terms of the energy gap between the lowest 5d₁ level and the conduction band bottom obtained from the vacuum referred binding energy (VRBE) diagram

Development of Ce³⁺ and Li⁺ co-doped magnesium borate glass ceramics for optically stimulated luminescence dosimetry

Magnesium tetraborate, MgB₄O₇, is an attractive host material for dosimetry due to the two characteristics: its low effective atomic number (Zeff = 8.4) and high neutron capture cross-section of the ¹⁰B isotope. Particularly, Ce³⁺ and Li⁺ ions co-doped MgB₄O₇ has shown optically stimulated luminescence (OSL) signal comparable to that of Al₂O₃:C, which is a well-known OSL dosimetry material. In this work, for further improvement of the dosimetric properties, a new synthesis route for MgB₄O₇:Ce³⁺-Li⁺ is described: glass-ceramic (GC) MgB₄O₇: Ce³⁺- Li⁺ samples were prepared by heat treatment of the magnesium borate glass with the composition, 25MgO–72B₂O₃–3Li₂O-0.3Ce³⁺. The prepared GC samples show UV-blue radioluminescence assigned to the Ce³⁺ 5d → 4f transition under X-ray irradiation and two thermoluminescence (TL) glow peaks related to the shallow and deep electron traps. Although fading of the TL and OSL signal was observed due to electron release from the shallow traps, the electrons captured by deep traps were stable at room temperature. Particularly, GC samples annealed at 750 °C and 800 °C, named GC750 and GC800, showed stable OSL up to 10 h following β-ray irradiation, after an initial fading mainly due to the presence of shallow traps in the material. From the viewpoint of this fading ratio, GC750 and GC800 showed potential as a practical OSL dosimeter

Thermochromic Narrow Band Gap Phosphors for Multimodal Optical Thermometry: The Case of Y³⁺-Stabilized beta-Bi₂O₃:Nd³⁺

The design and development of effective luminescent thermal sensors have been driving technological progress in many different fields ranging from catalysis to biology and microelectronics, to name a few. The ratiometric concept of using the ratio between two luminescent emissions of lanthanide-doped phosphors allows overcoming some limitations resulting from the single emission-based thermometers. A fundamental requirement for the development of effective luminescent thermometers relies on efficient luminescence output, which is not always accessible. Therefore, alternative methods to probe the temperature in a reliable and simple way are still a challenge. Despite the conventional limits of using narrow band gap materials as hosts for lanthanoid ions, a smart design allows for the development of unusual phosphors with appealing properties. By taking advantage of the narrow band gap of Bi₂O₃ polymorphs, here we demonstrate the potential of the tetragonal Y-stabilized β-Bi₂O₃:Y³⁺, Nd³⁺ system as a multimodal thermometer combining the conventional Boltzmann thermometry based on Nd³⁺ together with the thermochromism of the host. With the aim of testing this new concept, the temperature dependence of the reflectance spectra was investigated. Moreover, from the application point of view, the chromaticity variations of the material described by means of simple thermometric parameters such as the ratio a*/b* and the hue angle hab are demonstrated to be particularly promising and already implemented in software commonly used worldwide. The results suggest the potential of the strategy of combining narrow band gap semiconductors with lanthanoid ions to design reliable and multimodal thermal sensors, paving the way to a new family of thermochromic and luminescent thermal sensors

Near-infrared long persistent luminescence of Er3+ in garnet for the third bio-imaging window

生体透過率の高い波長で長時間光り続ける蛍光体の開発に成功 --光源を必要としない近赤外域での生体分子可視化に--. 京都大学プレスリリース. 2016-11-29

Feature issue introduction: Persistent and photostimulable phosphors - an established research field with clear challenges ahead

Persistent phosphors have the ability to emit light long after the excitation has ended, typically by using thermal energy to liberate previously trapped charges. Alternatively, also photons can be used for the detrapping, leading to optically stimulated luminescence (OSL). This particular field of phosphor research has seen a strong expansion over the past two decades, with a steady growth of the materials library, an improved structural and luminescence characterization and the development of novel applications. Despite this success, clear challenges lie ahead in terms of a deeper understanding of the trapping mechanism and an associated optimization of the energy storage capacity being crucial for many applications. This focus issue “Persistent and Photostimulable Phosphors” within Optical Materials Express features papers presented at the third International Workshop on Persistent and Photostimulable Phosphors (IWPPP 2015) held at the University of Texas at Arlington

Photostimulation induced persistent luminescence in Y₃Al₂Ga₃O₁₂:Cr³⁺

Cr³⁺-activated Y₃Al₂Ga₃O₁₂ garnet (YAGG:Cr³⁺) persistent phosphor has been recently reported as a potential candidate material for in vivo imaging application. Temperature dependence of photoluminescence (PL) spectra and thermostimulated luminescence (TSL) glow curves with several conditions, especially photostimulation wavelength dependence, were carefully investigated with the perspective of deep trap utilization for long-term in vivo imaging. The PL spectrum showed typical Cr³⁺ emission due to ²E→⁴A₂ and transitions) does not suffer from temperature quenching up to 600 K. From the TSL glow curve measurements, it was found that the persistent luminescence cannot be activated by visible light excitation. However, photostimulation induced persistent luminescence by red to near-infrared light can be possible in this material

Advance in Red-emitting Mn⁴⁺-activated Oxyfluoride Phosphors

稳定可靠的高光子能量发光(620~650 nm)红光荧光粉, 对于构建低色温、高显指荧光粉转换型白光发光二极管(WLED)至关重要。Mn⁴⁺激活红光荧光粉是当前 WLED 用荧光粉研究热点之一。本文介绍了 Mn⁴⁺离子的能级跃迁与光致发光特性, 详细叙述了目前所报道的七种 Mn⁴⁺激活含 d⁰/d¹⁰/s⁰离子氧氟化物系列红色荧光粉(如Na₂WO₂F₄:Mn⁴⁺等)的制备方法、晶体结构及其发光特性。目前⁴⁺在氧氟化物结构中得到强 R 线(零声子线)发光的情况少, 微观配位体仍是[MnF₆]或[MnO₆], 其化学稳定性和量子效率研究也很缺乏。最后对 Mn⁴⁺激活氧氟化物红光荧光粉的研究进行了展望。The stable and reliable red phosphor with high-photon energy emission (620–650 nm) is critical for the fabrication of the phosphor-converted white light-emitting diode (WLED) with low correlated color temperature and high color rendering index. Mn⁴⁺-activated phosphor is an emerging kind of red-emitting phosphor for WLED. Herein, the energy levels transition and photoluminescence characteristics of the Mn⁴⁺ ion were introduced; then, the preparation, crystal structure and luminescent properties of as-far reported seven kinds of Mn⁴⁺-doped oxyfluoride red phosphors (such as Na₂WO₂F₄:Mn⁴⁺) containing d⁰/d¹⁰ or s⁰ cations were reviewed. Currently, only in quite rare case of oxyfluoride, Mn⁴⁺ was found to exhibit strong R-line emission, with local coordination remaining as either [MnF₆] or [MnO₆]. The studies on the chemical stability and quantum efficiency of Mn⁴⁺ -doped oxyfluoride phosphors are still insufficient. Finally, we prospected the future development of Mn⁴⁺-doped oxyfluoride phosphor