The compensation of the depolarization field in ferroelectric layers requires the presence of a suitable amount of charges able to follow any variation of the ferroelectric polarization. These can be free carriers or charged defects located in the ferroelectric material or free carriers coming from the electrodes. Here we show that a self-doping phenomenon occurs in epitaxial, tetragonal ferroelectric films of Pb(Zr0.2Ti0.8)O3, consisting in generation of point defects (vacancies) acting as donors/acceptors. These are introducing free carriers that partly compensate the depolarization field occurring in the film. It is found that the concentration of the free carriers introduced by self-doping increases with decreasing the thickness of the ferroelectric layer, reaching values of the order of 1026 m−3 for 10 nm thick films. One the other hand, microscopic investigations show that, for thicknesses higher than 50 nm, the 2O/(Ti+Zr+Pb) atomic ratio increases with the thickness of the layers. These results suggest that the ratio between the oxygen and cation vacancies varies with the thickness of the layer in such a way that the net free carrier density is sufficient to efficiently compensate the depolarization field and to preserve the outward direction of the polarization.
The formation and crystallization of disordered nanosized ZnO resulting from the thermal decomposition of nanocrystalline hydrozincite [Zn5(CO3)2(OH)6] has been observed and investigated during pulse annealing experiments up to 625 °C in air or vacuum by electron paramagnetic resonance of trace amounts of substitutional Mn2+ impurity ions, in correlation with X-ray diffraction and transmission electron microscopy measurements. The mesoporous structure of the disordered ZnO, which initially forms in air and vacuum at 225 and 175 °C, respectively, further transforms into nanocrystalline ZnO of increasing particle size and improved lattice quality at higher annealing temperatures. The crystallization process, which does not affect the concentration of the substitutional impurity ions, as well as the simultaneous presence of both disordered and crystalline phases, should be considered in further applications of the resulting nanosized ZnO.
The correlation of the lattice disorder
with the nanocrystal average
size, in ZnO nanocrystals synthesized by several different methods,
has been quantitatively monitored by line shape analysis of the multifrequency
electron paramagnetic resonance (EPR) spectra of low concentrations
of substitutional Mn2+ probing ions. The observed correlation
between the line broadening parameter of the spectrum and the average
ZnO nanocrystals size, independent of the synthesis procedure of the
ZnO nanocrystals, demonstrates the dominance of the size related strain/disorder.
On the basis of this result, a new method for determining the average
ZnO nanocrystal size from the quantitative analysis of the EPR spectra
of the Mn2+ probes was derived. The nanocrystallization
of the disordered ZnO formed by the thermal decomposition of hydrozincite
was monitored using this procedure. The observed ZnO nanocrystallite
growth kinetics at lower temperatures was described by a structural
relaxation mechanism consisting of the local ordering by rearrangements
of the atoms in the interfaces/grain boundaries, with a growth activation
energy of ∼23 kJ/mol. When the nanostructured ZnO was more
than 75% crystallized, another growth mechanism of the nanocrystals
was found to occur, driven by the reduction of the total grain boundary
energy.
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