ICYS Annual Report 2023

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Jian XU1. Outline of ResearchPersistent phosphors, a kind of specific luminescent materials that can exhibit “self-sustained” persistent luminescence (PersL) in the dark for minutes, hours or even days after ceasing external excitation, have achieved a big commercial success for civil applications, such as emergency signage, safety indication, luminous paints, watch dials, etc. [1]. It features energy storage capabilities, for which not all the energy absorbed upon external excitation is immediately emitted as light. A fraction of absorbed energy is used to transfer a charge carrier from the luminescent center to a so-called trap center, generating a metastable state. The trapping-detrapping process of charge carriers is regarded as the key mechanism dominating the performance (luminescent duration and intensity) of persistent phosphors [2]. Based on this mechanism, once energy level locations of lanthanide ions in a specific host are understood, design novel persistent phosphors based on this mechanism becomes feasible. Here, vacuum referred binding energy (VRBE) diagram [3] of 15 lanthanides provides a strong predicting power since the characteristic variation in electron and hole trapping depths of lanthanide ions are given by the shape of the two zigzag curves representing the ground states of Ln2+/3+. Because the zigzag shape of two curves remains almost unchanged in different hosts due to the shielding effect of 5s2 and 5p6 orbitals on 4f14 orbitals, once the binding energy of the ground state for one lanthanide ion relative to the conduction band (CB) or valence band (VB) is determined, those of 4f levels of all other lanthanides can be estimated well by constructing this diagram.2. Research ActivitiesAccording to the VRBE diagram of the most famous garnet host, YAG (see Fig. 1), Eu3+, whose relevant 2+ ground state locates at the lowest energy among all Ln2+ ions. So Eu3+ is barely selected as a candidate of trap center due to its far deep trap depth (ε), which represent the energy gap between the ground state of Eu2+ and the bottom of CB. However, once lowering the energy level of CB edge to decrease the trap depth from the ground state of Eu2+, Eu3+ can also act as an efficient electron trap to greatly enhance the Cr3+ PersL. Inspired by this diagram, if the Al3+ sites in YAG are fully substituted by Ga3+ with larger ionic radius, Eu3+ owns the high possibility as an electron trap since the CB edge is decreased from -1.53 eV (YAG) to -2.36 eV (YGG) in the VRBE diagram, and the “ε” value of 1.58 eV in YGG is considered as the optimal one for PersL working at room temperature (RT). Therefore, in this work, two Cr3+-Eu3+ co-doped YGG transparent ceramics are prepared with different Eu3+ doping concentrations (0.05/0.5 mol%), in order to reveal the feasibility of using Eu3+ as electron trapping centers for enhancing Cr3+ PersL. Persistent luminescent decay curves of Cr3+ singly-doped and Cr3+-Eu3+ co-doped YGG transparent ceramics after ceasing UV illumination for 5 min at RT are shown in Fig. 2a, in which the decay curve of the widely used ZGO:Cr deep-red persistent phosphor under the same experimental condition is also plotted as a reference. Judging from the different decay behaviors of singly/co-doped samples, Cr3+ singly-doped sample shows higher initial intensity with shorter duration, and both of the Cr3+-Eu3+ co-doped samples show lower initial intensity with longer duration. Especially for the co-doped samples, after ceasing the UV light for over 2 hours, the radiance of PersL still keep higher value than ZGO:Cr, proving the powerful PersL performance the main thermoluminescence (TL) glow peaks in YGG:Cr are mainly located under RT, the different decay behavior after Eu3+ co-doping suggests totally different trap distributions, and such distribution is also concentration dependent as proved from the sample imaging after stopping UV illumination (see Fig. 2b) ICYS Annual Report 2023 region. Since the deep-red in Fig. 1. Vacuum referred binding energy (VRBE) diagram of Y3Al5O12 (YAG)-Y3Ga5O12 (YGG) hosts demonstrating the energy level locations between 4f states of Ln2+/3+ ions and conduction/valance bands (“ε” represents the energy gap between the conduction band bottom and ground state of Eu2+)Fig. 2. (a) Persistent luminescence decay curves (in unit of radiance) monitoring Cr3+ PersL of the YGG:Cr, YGG:Cr-Eu0.05, YGG:Cr-Eu0.5 ceramic samples (ZGO:Cr ceramic sample as a reference); (b1-b6) images of the YGG:Cr-Eu0.05 and YGG:Cr-Eu0.5 ceramic samples under and after mercury lamp (254 nm, 6 W output) illumination for 5 min, (photographs were taken by a digital camera (EOS kiss X5, Canon), and the settings remained constant: exposure time: 1/20 s for PL and 15 s for PersL, ISO value: 1600, and aperture value (F-value): 5.0).References[1] J. Xu and S. Tanabe, J. Lumin., 205 (2019) 581.[2] T. Matsuzawa, et al., J. Electrochem. Soc., 143 (1996) 2670.[3] P. Dorenbos, Phys. Rev. B. 37 (2013) 035118.Research Digest 29Enhancement of Cr3+ Persistent Luminescence by Eu3+ Sensitization in Garnet Transparent Ceramics