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prepared anatase and rutile nanoparticulate samples via TiCl
4
hydrolysis using
either ordinary or
17
O-enriched water with subsequent heat treatment under vac-
uum. These samples were investigated in the temperature range 100-160 K. The
presence of
17
O atoms in the anatase or rutile crystal lattice caused significant line
broadening of the EMR signals monitored at 100 K. This was attributed to the
electron and hole sites, due to the interaction of the unpaired electron with a
17
O
nucleus. They have further reported that an increase in the temperature leads to a
significant decline of the EMR signals and hence they could conclude that the
stability of the photo-generated charge carriers is limited.
12.5.2 Fluorite Nanoparticles Activated by Er
3+
Ions
Irisova et al. [
38
] have reported EMR of CaF
2
nanoparticles activated with Er
3+
ions
and attributed two intense lines to be originated from surface defects. The EMR line
due to intrinsic CaF
2
surface defects can be used to estimate the grain size of the
sample. Authors have reported most of the Er
3+
centers to be nominally high
symmetry cubic centers perturbed by the disorder near the surface.
Conclusions
This chapter is an attempt to review the applications of EMR spectroscopy in
nanoparticle characterization. Mainly, EMR of metallic nanoparticles and
oxide nanoparticles has been the subject matter of this chapter. As for as
metallic nanoparticles are concerned, the EMR characterization is capable of
providing very useful information on magnetic properties but the spin-lattice
relaxation is not completely understood. In particular, the effect of particle
diameters and the matrix on the spin-lattice relaxation in metallic
nanoparticles requires series of investigations.
Further, many studies on magnetic nanoparticles have used classical
ferromagnetic approach (FMR) to describe the dynamics of magnetization.
However, this is still challenging to explain line shape and their temperature
dependence. Noginova et al. [
39
] have proposed an alternative model where
magnetic nanoparticles are considered as a giant spin and the EMR signal was
assumed to be the sum of contributions of quantum transitions between
energy levels associated with the projections of the giant spin onto the
direction of the magnetic field. This model could explain some of the general
tendencies observed in the intermediate size spin systems. Their work is
concentrated about Fe
2
O
3
system. More experimental and theoretical work
is needed on different systems to understand the transition from quantum to
classical case. Many of the metal oxides, in particular nano-TiO
2
and ZnO,
have been studied by EMR spectroscopy due to their bactericidal and
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