Controlled Electron–Hole Trapping and Detrapping Process in GdAlO₃ by Valence Band Engineering

Two different trapping and detrapping processes of charge carriers have been investigated in GdAlO₃:Ce³⁺,Ln³⁺ (Ln = Pr, Er, Nd, Ho, Dy, Tm, Eu, and Yb) and GdAlO₃:Ln³⁺,RE³⁺ (Ln = Sm, Eu, and Yb; RE = Ce, Pr, and Tb). Cerium is the recombination center and lanthanide codopants act as electron-trapping centers in GdAlO₃:Ce³⁺,Ln³⁺. Different lanthanide codopants generate different trap depths. The captured electrons released from the lanthanide recombine at cerium via the conduction band, eventually producing the broad 5d–4f emission centered at ∼360 nm from Ce³⁺. On the other hand, Sm³⁺, Eu³⁺, and Yb³⁺ act as recombination centers, while Ce³⁺, Pr³⁺, and Tb³⁺ act as hole-trapping centers in GdAlO₃: Ln³⁺,RE³⁺. In this situation, we find evidence that recombination is by means of hole release instead of the more commonly reported electron release. The trapped holes are released from Pr⁴⁺ or Tb⁴⁺ and recombine with the trapped electrons on Sm²⁺, Eu²⁺, or Yb²⁺ and yield characteristic trivalent emission from Sm³⁺, Eu³⁺, or Yb³⁺ at ∼600, ∼617, or ∼980 nm, respectively. Lanthanum was introduced to engineer the valence band energy and change the trap depth in Gd_1–<i>x</i>La_<i>x</i>AlO₃:Eu³⁺,Pr³⁺ and Gd_1–<i>x</i>La_<i>x</i>AlO₃:Eu³⁺,Tb³⁺. The results show that the valence band moves upward and the trap depth related to Pr³⁺ or Tb³⁺ decreases

Electronic Structure and Site Occupancy of Lanthanide-Doped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ Garnets: A Spectroscopic and First-Principles Study

Photoluminescence excitation (PLE) and emission spectra (PL) of undoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ as well as Eu³⁺- and Ce³⁺-doped samples have been investigated. The PL spectra show that Eu³⁺ enters into both dodecahedral (Ca, Sr) and octahedral (Y, Lu) sites. Ce³⁺ gives two broad excitation bands in the range of 200–450 nm. First-principle calculations for Ce³⁺ on both dodecahedral and octahedral sites provide sets of 5d excited level energies that are consistent with the experimental results. Then the vacuum referred binding energy diagrams for (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ have been constructed with the lanthanide dopant energy levels by utilizing spectroscopic data. The Ce³⁺ 5d excited states are calculated by first-principles calculations. Thermoluminescence (TL) glow curves of (Ce³⁺, Sm³⁺)-codoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ samples show a good agreement with the prediction of lanthanide trapping depths derived from the energy level diagram. Finally, the energy level diagram is used to explain the low thermal quenching temperature of Ce³⁺ and the absence of afterglow in (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂

Storage of Visible Light for Long-Lasting Phosphorescence in Chromium-Doped Zinc Gallate

ZnGa₂O₄:Cr³⁺ presents near-infrared long-lasting phosphorescence (LLP) suitable for in vivo bioimaging. It is a bright LLP material showing a main thermally stimulated luminescence (TSL) peak around 318 K. The TSL peak can be excited virtually by all visible wavelengths from 1.8 eV (680 nm) via d–d excitation of Cr³⁺ to above ZnGa₂O₄ band gap (4.5 eV–275 nm). The mechanism of LLP induced by visible light excitation is entirely localized around Cr_N2 ion that is a Cr³⁺ ion with an antisite defect as first cationic neighbor. The charging process involves trapping of an electron–hole pair at antisite defects of opposite charges, one of them being first cationic neighbor to Cr_N2. We propose that the driving force for charge separation in the excited states of chromium is the local electric field created by the neighboring pair of antisite defects. The cluster of defects formed by Cr_N2 ion and the complementary antisite defects is therefore able to store visible light. This unique property enables repeated excitation of LLP through living tissues in ZnGa₂O₄:Cr³⁺ biomarkers used for in vivo imaging. Upon excitation of ZnGa₂O₄:Cr³⁺ above 3.1 eV, LLP efficiency is amplified by band-assistance because of the position of Cr³⁺⁴T₁ (⁴F) state inside ZnGa₂O₄ conduction band. Additional TSL peaks emitted by all types of Cr³⁺ including defect-free Cr_R then appear at low temperature, showing that shallower trapping at defects located far away from Cr³⁺ occurs through band excitation