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

Abstract

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 Gd1–x La x AlO3:Eu3+,Pr3+ and Gd1–x La x AlO3:Eu3+,Tb3+. The results show that the valence band moves upward and the trap depth related to Pr3+ or Tb3+ decreases.

Charge carrier trapping processes in lanthanide doped LaPO4, GdPO4, YPO4, and LuPO4

“…Since all Ln 3+ codopants locate at the La 3+ site, we will assume that the frequency factor s remains a constant. 7,32 The trap depths for the other codopants then Er 3+ were determined by using T m from column 2 of Table 1 and solving Eq. (1) with β=5 K/s.…”

“…For storage phosphors applied in X-ray imaging, deep traps (~2 eV) are needed to avoid thermal fading at room temperature (RT) 5 . Relatively shallow traps (<~0.7 eV) are required to generate RT afterglow 6,7 . So, if we can control the trap depth of holes or electrons, then in principle one may engineer or deliberate design storage and afterglow properties.…”

Toward tunable and bright deep‐red persistent luminescence of Cr3+ in garnets

“…As mentioned in the Introduction, codoping strategy using lanthanide ions as additional electron traps has been widely used to further enhance the PersL intensity of persistent phosphors. 3,25,27,28 However, considering fabricating at least 15 different codoping samples with different lanthanide codopants as a trial and error method for one target host, which is an inefficient approach relying on serendipity, employ theoretical predictions, especially energy-level locations of lanthanide ions in the target host is a much more efficient way to realize this purpose. Therefore, we again take the GSGG:Cr sample as a typical example to demonstrate how to select suitable lanthanide ions as sensitizers by utilizing the HRBE diagram as a highly efficient prediction tool.…”

“…Therefore, once the binding energy of the 4 f ground state (GS) for one lanthanide ion relative to the conduction band (CB) or the valence band (VB) is determined, those of 4 f levels of all other lanthanides in a certain material host can be estimated fairly well by constructing either host‐referred binding energy (HRBE) or vacuum‐referred binding energy (VRBE) diagrams . Since lanthanide ions are widely adopted as electron trap centers (hole trap centers in rare cases) for developing novel persistent phosphors or enhancing PersL intensities in present/reported persistent phosphors, these theoretical prediction diagrams can be a very useful guide to choose proper lanthanide ions as efficient foreign traps inducing desirably long PersL …”

“…Therefore, once the binding energy of the 4f ground state (GS) for one lanthanide ion relative to the conduction band (CB) or the valence band (VB) is determined, those of 4f levels of all other lanthanides in a certain material host can be estimated fairly well by constructing either host-referred binding energy (HRBE) or vacuum-referred binding energy (VRBE) diagrams. [22][23][24] Since lanthanide ions are widely adopted as electron trap centers (hole trap centers in rare cases 25,26 ) for developing novel persistent phosphors or enhancing PersL intensities in present/reported persistent phosphors, these theoretical prediction diagrams can be a very useful guide to choose proper lanthanide ions as efficient foreign traps inducing desirably long PersL. 3,22,25,27,28 In this work, photoluminescence (PL), PersL, and thermoluminescence (TL) properties of six different garnets are investigated in detail, and we introduce a facile way to develop Cr 3+ -doped garnet persistent phosphors using both the crystal field engineering to tune the emission band of Cr 3+ The starting powder was mixed by a ball milling method with anhydrous alcohol for 1 hour.…”