10 research outputs found
The importance of inversion disorder in the visible light induced persistent luminescence in Cr doped ABO (A = Zn or Mg and B = Ga or Al)
Cr doped spinel compounds ABO with A=Zn, Mg and B=Ga, Al
exhibit a long near infrared persistent luminescence when excited with UV or
X-rays. In addition, persistent luminescence of ZnGaO and to a lesser
extent MgGaO, can also be induced by visible light excitation via
A T transition of Cr, which makes these
compounds suitable as biomarkers for in vivo optical imaging of small animals.
We correlate this peculiar optical property with the presence of antisite
defects, which are present in ZnGaO and MgGaO. By using X-ray
absorption fine structure (XAFS) spectroscopy, associated with electron
paramagnetic resonance (EPR) and optical emission spectroscopy, it is shown
that an increase in antisite defects concentration results in a decrease in the
Cr-O bond length and the octahedral crystal field energy. A part of the defects
are in the close environment of Cr ions, as shown by the increasing
strain broadening of EPR and XAFS peaks observed upon increasing antisite
disorder. It appears that ZnAlO, which exhibits the largest crystal
field splitting of Cr and the smallest antisite disorder, does not show
considerable persistent luminescence upon visible light excitation as compared
to ZnGaO and MgGaO. These results highlight the importance of
Cr ions with neighboring antisite defects in the mechanism of persistent
luminescence exhibited by Cr doped ABO spinel compounds.Comment: 10 pages + supplementary (available on request
Order and disorder around Cr 3+ in chromium doped persistent luminescent AB 2 O 4 spinels
International audienc
Interplay between chromium content and lattice disorder on persistent luminescence of ZnGa2O4:Cr3+ for in vivo imaging
International audienc
Origin of the visible light induced persistent luminescence of Cr3+-doped zinc gallate
International audienc
Storage of Visible Light for Long-Lasting Phosphorescence in Chromium-Doped Zinc Gallate
International audienc
Storage of Visible Light for Long-Lasting Phosphorescence in Chromium-Doped Zinc Gallate
ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> 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 Cr<sup>3+</sup> to above ZnGa<sub>2</sub>O<sub>4</sub> band gap
(4.5 eV–275 nm). The mechanism of LLP induced by visible light
excitation is entirely localized around Cr<sub>N2</sub> ion that is
a Cr<sup>3+</sup> 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<sub>N2</sub>. 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<sub>N2</sub> 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 ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> biomarkers used for
in vivo imaging. Upon excitation of ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> above 3.1 eV, LLP efficiency is amplified by band-assistance
because of the position of Cr<sup>3+4</sup>T<sub>1</sub> (<sup>4</sup>F) state inside ZnGa<sub>2</sub>O<sub>4</sub> conduction band. Additional
TSL peaks emitted by all types of Cr<sup>3+</sup> including defect-free
Cr<sub>R</sub> then appear at low temperature, showing that shallower
trapping at defects located far away from Cr<sup>3+</sup> occurs through
band excitation