3 research outputs found
Controlled Electron–Hole Trapping and Detrapping Process in GdAlO<sub>3</sub> by Valence Band Engineering
Two
different trapping and detrapping processes of charge carriers
have been investigated in GdAlO<sub>3</sub>:Ce<sup>3+</sup>,Ln<sup>3+</sup> (Ln = Pr, Er, Nd, Ho, Dy, Tm, Eu, and Yb) and GdAlO<sub>3</sub>:Ln<sup>3+</sup>,RE<sup>3+</sup> (Ln = Sm, Eu, and Yb; RE
= Ce, Pr, and Tb). Cerium is the recombination center and lanthanide
codopants act as electron-trapping centers in GdAlO<sub>3</sub>:Ce<sup>3+</sup>,Ln<sup>3+</sup>. 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 Ce<sup>3+</sup>. On the other
hand, Sm<sup>3+</sup>, Eu<sup>3+</sup>, and Yb<sup>3+</sup> act as
recombination centers, while Ce<sup>3+</sup>, Pr<sup>3+</sup>, and
Tb<sup>3+</sup> act as hole-trapping centers in GdAlO<sub>3</sub>:
Ln<sup>3+</sup>,RE<sup>3+</sup>. 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 Pr<sup>4+</sup> or Tb<sup>4+</sup> and recombine with the trapped
electrons on Sm<sup>2+</sup>, Eu<sup>2+</sup>, or Yb<sup>2+</sup> and
yield characteristic trivalent emission from Sm<sup>3+</sup>, Eu<sup>3+</sup>, or Yb<sup>3+</sup> at ∼600, ∼617, or ∼980
nm, respectively. Lanthanum was introduced to engineer the valence
band energy and change the trap depth in Gd<sub>1–<i>x</i></sub>La<sub><i>x</i></sub>AlO<sub>3</sub>:Eu<sup>3+</sup>,Pr<sup>3+</sup> and Gd<sub>1–<i>x</i></sub>La<sub><i>x</i></sub>AlO<sub>3</sub>:Eu<sup>3+</sup>,Tb<sup>3+</sup>. The results show that the valence band moves upward and the trap
depth related to Pr<sup>3+</sup> or Tb<sup>3+</sup> decreases
Electronic Structure and Site Occupancy of Lanthanide-Doped (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> Garnets: A Spectroscopic and First-Principles Study
Photoluminescence
excitation (PLE) and emission spectra (PL) of
undoped (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> as well as Eu<sup>3+</sup>- and Ce<sup>3+</sup>-doped samples
have been investigated. The PL spectra show that Eu<sup>3+</sup> enters
into both dodecahedral (Ca, Sr) and octahedral (Y, Lu) sites. Ce<sup>3+</sup> gives two broad excitation bands in the range of 200–450
nm. First-principle calculations for Ce<sup>3+</sup> 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)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> have been constructed with the lanthanide
dopant energy levels by utilizing spectroscopic data. The Ce<sup>3+</sup> 5d excited states are calculated by first-principles calculations.
Thermoluminescence (TL) glow curves of (Ce<sup>3+</sup>, Sm<sup>3+</sup>)-codoped (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> 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 Ce<sup>3+</sup> and the absence of afterglow in (Sr,
Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub>