K. Vojisavljević

Defect induced variation in vibrational and optoelectronic properties of nanocrystalline ZnO powders

Structural disorder of ZnO nanopowders with mean crystallite size down to 15 nm, produced by mechanical activation in high energy mills, has been analyzed by x-ray diffraction and Raman spectroscopy. The influence of such disorder on optical and electronic properties of activated ZnO nanopowders has been investigated using photoluminescence spectroscopy and spectroscopic ellipsometry. A revised interpretation of the resonant enhancement of the first and second order Raman scattering by the E1(LO) phonons in highly disorder ZnO nanopowders has been proposed. Detailed analysis of resonant Raman effects in ZnO powders under sub band gap excitation has given valuable information about defect induced electronic states in the band gap of ZnO. Systematic trend in the electron–phonon coupling strength, with the correlation length which depends on lattice disorder in ZnO, has been also demonstrated.

Structural characterization of mechanically milled ZnO: influence of zirconia milling media

Zinc oxide nanoparticles were obtained by milling in a planetary ball mill with a zirconia milling assembly for up to 5 h in air. The samples were characterized by scanning electron microscopy, x-ray diffraction (XRD) and Raman spectroscopy methods. The deviation of the lattice parameters from single crystal values was related to defect creation and increase of strain inside the hexagonal lattice of milled ZnO nanoparticles. The observed redshift and peak broadening of the major first-order Raman modes were ascribed to the formation of intrinsic defects by mechanical milling combined with the effects of phonon confinement in nanosized powders. To investigate the type of intrinsic defects and impurities introduced during milling, it was necessary to analyze both milled and thermally treated ZnO. After thermal treatment, the intensity of the Raman spectra increased and the peak positions reverted to values similar to those in unmilled ZnO powder, pointing to defect annihilation. XRD patterns of sintered samples confirmed the existence of zirconia impurities and the Rietveld analysis revealed a small amount of zirconium introduced in the ZnO crystal lattice on the Zn sites or interstitial sites. The large influence of those impurities on the micro-Raman spectra of thermally treated samples was observed in this study.

Microstructural characterization of mechanically activated ZnO powders

In this paper, changes of microstructural characteristics of disperse systems during mechanical activation of zinc oxide (ZnO) have been investigated. ZnO powder was activated by grinding in a planetary ball mill in a continuous regime in air during 300 min at the basic disc rotation speed of 320 rpm and rotation speed of bowls of 400 rpm but with various balls‐to‐powder mass ratios. During ball milling in a planetary ball mill, initial ZnO powder suffered high‐energy impacts. These impacts are very strong, and large amounts of microstructural and structural defects were introduced in the milled powders. The morphology and dispersivity of particles and agglomerates of all powders were investigated by scanning electron microscopy and scanning transmission electron microscopy. The specific surface area of initial ZnO powder was determined as 3.60 m2 g−1 and it increased to 4.42 m2 g−1 in mechanically activated powders. An increase of the ball‐to‐powder mass ratio led to a decrease of particle dimensions as well as increased the tendency for joining into quite compact agglomerates, that is aggregates, compared with the very loose, soft initial agglomerates. The obtained results pointed out that activation of ZnO powders produces a highly disperse, nano‐scaled mixture of small particles, that is crystallites with sizes in the range of 10–40 nm. Most of these particles are in the form of aggregates with dimensions of 0.3–0.1 μm. The crystallite and aggregate size strongly depend on milling conditions, that is ball‐to‐powder mass ratio, as shown in this investigation.

Microstructural Properties of PZT Thin Films Deposited on LaNiO3-Coated Substrates

The modified polymeric precursor method (Pechini method) was successfully used for the preparation of epitaxial and polycrystalline ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films. Films were deposited on LaNiO3 (LNO) – coated silicium (1 0 0) and platinum substrates (Pt (1 1 1)/Ti/SiO2/Si) by spin coating technique. LNO electrodes were also prepared by the Pechini method and treated under different thermal treatment conditions to obtain films with different structural and microstructural properties. Investigation of PZT microstructure was performed as a function of orientation and morphology of the bottom electrode, as well as of thermal treatment conditions. Grain size and morphology were analyzed by AFM, while the quality and orientation of PZT films were determined by GIXRD analysis. It has been found that the proposed thermal treatment on a hot plate, with slow heating rate and long annealing time, can result in the formation of epitaxial PZT films on Si and LNO-coated Si substrates.