8 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>
Luminescence and Energy Transfer between Ce<sup>3+</sup> and Pr<sup>3+</sup> in BaY<sub>2</sub>Si<sub>3</sub>O<sub>10</sub> under VUV–vis and X‑ray Excitation
A detailed investigation on photoluminescence
properties and energy transfer (ET) dynamics of Ce<sup>3+</sup>, Pr<sup>3+</sup>-doped BaY<sub>2</sub>Si<sub>3</sub>O<sub>10</sub> is provided
along with the potential X-ray excited luminescence application. The
luminescence properties of Pr<sup>3+</sup> are studied in VUV–UV–vis
spectral range at low temperature, and the spectral profiles of Pr<sup>3+</sup> <sup>3</sup>P<sub>0</sub> and <sup>1</sup>D<sub>2</sub> emission
lines are determined using time-resolved emission spectra. Upon 230
nm excitation, the electron population from Pr<sup>3+</sup> 4f5d state
to its 4f<sup>2</sup> excited state is discussed in detail. As Pr<sup>3+</sup> concentration rises, Pr<sup>3+</sup> <sup>3</sup>P<sub>0</sub> and <sup>1</sup>D<sub>2</sub> luminescence possess different concentration-related
properties. The incorporation of Ce<sup>3+</sup> in the codoped sample
produces the strong Ce<sup>3+</sup> luminescence under 230 nm excitation,
which is the combined result of Pr<sup>3+</sup> 4f5d → Ce<sup>3+</sup> 5d ET and Ce<sup>3+</sup> intrinsic excitation. On the other
hand, the increasingly strong ET of Ce<sup>3+</sup> 5d → Pr<sup>3+</sup> 4f<sup>2</sup> results in the decrease of Ce<sup>3+</sup> emission intensity and the gradual deviation of Ce<sup>3+</sup> luminescence
decay from the single exponential in the system. By employing the
Inokuti–Hirayama model, the dipole–dipole interaction
is confirmed as the predominant multipolar effect in controlling this
ET process, and the value of <i>C</i><sub><i>DA</i></sub> is determined to be 9.97 × 10<sup>–47</sup> m<sup>6</sup>·s<sup>–1</sup>. Finally, the relatively low scintillation
light yield of Ce<sup>3+</sup>-doped BaY<sub>2</sub>Si<sub>3</sub>O<sub>10</sub> material impedes its application potential in the
scintillator field, and the cosubstitution of Pr<sup>3+</sup> results
in the observable decline of scintillation performance
Electronic Properties of Ce<sup>3+</sup>-Doped Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub>: A Combined Spectroscopic and Theoretical Study
Photoluminescence properties of Ce-doped
Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub> crystals prepared
by a solid-state reaction
method are first investigated with excitation energies in the vacuum-ultraviolet
(VUV) to ultraviolet (UV) range. Six bands are observed in the excitation
spectrum of the Ce<sup>3+</sup> 5d → 4f emission at 15 K. The
highest energy band is attributed to the host excitonic absorption,
from which the band gap energy of the host is estimated to be around
7.2 eV. The four lowest energy bands are assigned to the 4f<sub>1</sub> → 5d<sub>1–4</sub> transitions of Ce<sup>3+</sup> located
on the three distinct Sr<sup>2+</sup> sites in Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub> with almost equal probability,
based on a comparison between excitation band maxima energies and
4f → 5d transition energies obtained from wave-function-based
CASSCF/CASPT2 calculations with spin–orbit coupling on Ce-centered
embedded clusters. The 4f<sub>1</sub> → 5d<sub>5</sub> transition,
not observed in the low-temperature excitation spectrum, is found
to be overshadowed by a nearby defect-related excitonic absorption.
On the basis of present experimental and calculated results for Ce-doped
Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub>, the energy-level
diagram for the 4f ground states and the lowest 5d states of all trivalent
and divalent lanthanide ions on the Sr<sup>2+</sup> sites of Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub> is constructed and
discussed in association with experimental findings
Vacuum Referred Binding Energy Scheme, Electron–Vibrational Interaction, and Energy Transfer Dynamics in BaMg<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:Ln (Ce<sup>3+</sup>, Eu<sup>2+</sup>) Phosphors
The
host structure and the synchrotron radiation VUV–UV
luminescence properties of samples BaMg<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> (BMSO):Ln (Ce<sup>3+</sup>, Eu<sup>2+</sup>) at different
doping levels and different temperatures were investigated in detail.
Three important aspects are studied to elucidate the luminescence
properties of samples: (1) the vacuum referred binding energy (VRBE)
scheme is constructed with the electron binding in the BMSO host bands
and in the Ce<sup>3+</sup> and Eu<sup>2+</sup> impurity levels with
the aim to explain the different thermal stabilities of Ce<sup>3+</sup> and Eu<sup>2+</sup> emissions; (2) the electron–vibrational
interaction analysis on the narrow Eu<sup>2+</sup> emission indicates
a weak electron–phonon interaction in the current case; (3)
by using three models (Inokuti–Hirayama, Yokota–Tanimoto,
and Burshteĭn models) at different conditions, the energy transfer
dynamics between Ce<sup>3+</sup> and Eu<sup>2+</sup> was analyzed.
It reveals that the energy transfer from Ce<sup>3+</sup> to Eu<sup>2+</sup> via electric dipole–dipole (EDD) interaction is dominant
while energy migration between Ce<sup>3+</sup> is negligible. Finally,
the X-ray excited luminescence spectra of samples BMSO:Ce<sup>3+</sup>/Eu<sup>2+</sup> are collected to evaluate their possible scintillator
applications
The Effect of Sr<sup>2+</sup> on Luminescence of Ce<sup>3+</sup>-Doped (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>
A series of Ce<sup>3+</sup>-doped (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub> phosphors
with different Ce<sup>3+</sup> and Ca<sup>2+</sup>/Sr<sup>2+</sup> concentrations were prepared by a high temperature solid-state reaction
technique. To get insight into the structure–luminescence relationship,
the impact of incorporation of Sr<sup>2+</sup> on structure of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub> was first investigated via Rietveld
refinement of high quality X-ray diffraction (XRD) data, and then
the VUV–UV excitation and UV–vis emission spectra of
(Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup> were
collected at low temperature. The results reveal that the crystal
structure evolution of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup> has influences on band gaps and Ce<sup>3+</sup> luminescence
properties including 4f–5d<sub><i>i</i></sub> (<i>i</i> = 1–5) transition energies, radiative lifetime,
emission intensity, quantum efficiency, and thermal stability. Moreover,
the influence of Sr<sup>2+</sup> content on the energy of Eu<sup>3+</sup>–O<sup>2–</sup> charge-transfer states (CTS) in (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Eu<sup>3+</sup> was studied
in order to construct vacuum referred binding energy (VRBE) schemes
with the aim to further understand the luminescence properties of
(Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup>.
Finally, X-ray excited luminescence (XEL) spectra were measured to
evaluate the possibility of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup> as a scintillation material
High Light Yield of Sr<sub>8</sub>(Si<sub>4</sub>O<sub>12</sub>)Cl<sub>8</sub>:Eu<sup>2+</sup> under X‑ray Excitation and Its Temperature-Dependent Luminescence Characteristics
In this work, we first investigate
the relationship between temperature
and lattice parameters by means of Rietveld refinement and then demonstrate
its impact on the luminescence peak position of Eu<sup>2+</sup> in
Sr<sub>8</sub>(Si<sub>4</sub>O<sub>12</sub>)ÂCl<sub>8</sub>. It is
found that with increases in temperature, lattice expansion takes
place without significant distortion of the coordination around Eu<sup>2+</sup>. As a result, the crystal field splitting of the Eu<sup>2+</sup> 5d state decreases. At the same time, with the experimental
data of the full width at half-maximum of Eu<sup>2+</sup> emission
at different temperatures and the infrared spectrum, the effective
phonon frequency is evaluated and the main vibration motions are determined
using first-principles calculation. Due to the high light yield under
X-ray excitation and the excellent thermal stability of luminescence
intensity and decay, a further optimized sample Sr<sub>7.7</sub>Eu<sub>0.3</sub>(Si<sub>4</sub>O<sub>12</sub>)ÂCl<sub>8</sub> could be a
potential scintillation material