3 research outputs found
Radioluminescence Sensitization in Scintillators and Phosphors: Trap Engineering and Modeling
The role of charge carrier trapping
in determining radioluminescence
(RL) efficiency increase during prolonged irradiation of scintillators
has been studied by using YPO<sub>4</sub>:Ce,Nd as a model material.
The Nd<sup>3+</sup> ions act as efficient electron traps minimizing
the role of intrinsic defects. Different Nd contents were considered
in order to point out the correlation between the trap concentration
and the detected RL efficiency dose dependence. RL measurements as
a function of temperature clarified the role of the trap thermal stability
in determining the shape and the magnitude of such effect. We propose
also a model based on trap filling which is able to describe accurately
the complex processes which are involved
Eu Incorporation into Sol–Gel Silica for Photonic Applications: Spectroscopic and TEM Evidences of α‑Quartz and Eu Pyrosilicate Nanocrystal Growth
The problem of Eu
incorporation into silica as dispersed dopants,
clusters, separate-phase nanoparticles, or nanocrystals, which is
of key importance for applications in the fields of lasers and scintillators,
is faced by applying to sol–gel silica doped with nine different
Eu<sup>3+</sup> concentrations (0.001–10 mol % range) various
spectroscopic techniques, including crystal field and vibrational
mode analysis by means of Fourier transform absorption and microreflectivity
(in the 200–6000 cm<sup>–1</sup> and 9–300 K
ranges), radioluminescence, and Raman scattering studies at 300 K.
The variety of methods revealed the following concordant results:
(1) amorphous Eu clusters grow when the Eu concentration is increased
up to 3 mol % and (2) Si–OH groups are completely removed and
ordered phase separation occurs at 10 mol % doping, as suggested by
the remarkable narrowing of the spectral lines. Comparison with polycrystalline
Eu oxide, Eu silicates, and α-quartz spectra allowed the unequivocal
identification of Eu<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> pyrosilicate
and α-quartz as the main components of nanocrystals in 10 mol
% Eu-doped silica. Such conclusions were brilliantly confirmed by
transmission electron microscopy and electron diffraction analysis.
Phonon coupling and anharmonicity were analyzed and are discussed
for a few vibrational modes of nanocrystals
Size-Dependent Luminescence in HfO<sub>2</sub> Nanocrystals: Toward White Emission from Intrinsic Surface Defects
Defect engineering operated on metal
oxides by chemical and structural
modifications may strongly affect properties suitable for various
applications such as photoelectrochemical behavior, charge transport,
and luminescence. In this work, we report the tunable optical features
observed in undoped monoclinic HfO<sub>2</sub> nanocrystals and their
dependence on the structural properties of the material at the nanoscale.
Transmission electron microscopy together with X-ray diffraction and
surface area measurements were used to determine the fine structural
modifications, in terms of crystal growth and coalescence of crystalline
domains, occurring during a calcination process in the temperature
range from 400 to 1000 °C. The fit of the broad optical emission
into spectral components, together with time-resolved photoluminescence,
allowed us to identify the dual nature of the emission at 2.5 eV,
where an ultrafast defect-related intrinsic luminescence (with a decay
time of a few nanoseconds) overlaps with a slower emission (decay
of several microseconds) due to extrinsic Ti-impurity centers. Moreover,
the evolution of intrinsic visible bands during the material transformation
was monitored. The relationship between structural parameters uniquely
occurring in nanosized materials and the optical properties was investigated
and tentatively modeled. The blue emissions at 2.5 and 2.9 eV are
clearly related to defects lying at crystal boundaries, while an unprecedented
emission at 2.1 eV enables, at relatively low calcination temperatures,
the white luminescence of HfO<sub>2</sub> under near-UV excitation