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 CrN2 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 CrN2. 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 CrN2 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
CrR then appear at low temperature, showing that shallower
trapping at defects located far away from Cr3+ occurs through
band excitation.
Cr 3+ doped spinel compounds AB 2 O 4 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 ZnGa 2 O 4 and to a lesser extent MgGa 2 O 4 , can also be induced by visible light excitation via 4 A 2 → 4 T 2 transition of Cr 3+ , 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 ZnGa 2 O 4 and MgGa 2 O 4 . 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 3+ ions, as shown by the increasing strain broadening of EPR and XAFS peaks observed upon increasing antisite disorder. It appears that ZnAl 2 O 4 , which exhibits the largest crystal field splitting of Cr 3+ and the smallest antisite disorder, does not show considerable persistent luminescence upon visible light excitation as compared to ZnGa 2 O 4 and MgGa 2 O 4 . These results highlight the importance of Cr 3+ ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr 3+ doped AB 2 O 4 spinel compounds.
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