(Tb,Eu)-doped
ZnO-annealed films at 1100 °C showed intense
photoluminescense (PL) emission from Eu and Tb ions. The high-temperature
annealing led to a chemical segregation and a secondary Zn-free phase
formation that is suspected to be responsible for the high PL intensity.
Large faceted inclusions of rare-earth (RE) silicates of a size of
few hundred nanometers were observed. Owing to various advanced electron
microscopy techniques, a detailed microstructural study of these nanometric
inclusions combining atomic Z contrast imaging (STEM) and precession
electron diffraction tomography (PEDT) data was carried out and resulted
in the determination of a hexagonal P63/m-type (Tb,Eu)9.43(SiO4)6O2−δ structure related to an oxy-apatite
structure. Chemical analyses from spectroscopic data (energy-dispersive
X-ray mapping and electron energy loss spectroscopy) at the atomic
scale showed that both RE elements sitting on two independent (4f)
and (6h) atomic sites have three-fold oxidation states, while refinements
of their occupancy sites from PEDT data have evidenced preferential
deficiency for the first one. The deduced RE–O distances and
their corresponding bond valences are listed and discussed with the
efficient energy transfer from Tb3+ toward Eu3+.
Codoped
(Tb,Eu) ZnO films grown by magnetron sputtering on a silicon
substrate and annealed up to 1200 °C showed intense photoluminescence
(PL) emission from Eu3+ ions. The high-temperature annealing
led to diffusion and segregation of rare earth (RE) elements toward
the bottom of the film, which induced the formation of nanometric
Zn-free inclusions responsible for remarkable PL emission intensity.
Combined electron diffraction, chemical contrast imaging, and optical
studies of these nanometric phases have been carried out. Large inclusions
of zinc silicates and RE silicates a few hundreds of nanometers in
size were observed, embedded in a silica phase. The structural determination
of these RE-rich inclusions was carried out by combining atomic Z contrast imaging (high-angle annular dark-field imaging)
and precession electron diffraction data. Upon annealing at 1200 °C,
it appeared that the structure was related to an F-type disilicate
structure. Energy-dispersive X-ray spectroscopy and electron energy
loss spectroscopy experiments were carried out to determine the ratios
between elements and the oxidation states of the RE elements in the
abovementioned inclusions. A (Tb,Eu)2Si2O7 formulation is proposed from dynamical precession electron
diffraction tomography refinements, leading to a +III valence state
for the RE species in agreement with spectroscopic results. PL modeling
is also in good agreement with the experimental data. These results
complete those obtained at 1100 °C for which the inclusions were
identified as some RE10–x
(SiO4)6O2–x
oxyapatite
structures and pointed out a combined structural and PL properties’
evolution between 1100 and 1200 °C annealing temperatures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.