Fabrication and Characterization of Dielectric ZnCr2O4 Nanopowders and Thin Films for Parallel-Plate Capacitor Applications.

We report here the successful shape-controlled synthesis of dielectric spinel-type ZnCr2O4 nanoparticles by using a simple sol-gel auto-combustion method followed by successive heat treatment steps of the resulting powders at temperatures from 500 to 900 °C and from 5 to 11 h, in air. A systematic study of the dependence of the morphology of the nanoparticles on the annealing time and temperature was performed by using field effect scanning electron microscopy (FE-SEM), powder X-ray diffraction (PXRD) and structure refinement by the Rietveld method, dynamic lattice analysis and broadband dielectric spectrometry, respectively. It was observed for the first time that when the aerobic post-synthesis heat treatment temperature increases progressively from 500 to 900 °C, the ZnCr2O4 nanoparticles: (i) increase in size from 10 to 350 nm and (ii) develop well-defined facets, changing their shape from shapeless to truncated octahedrons and eventually pseudo-octahedra. The samples were found to exhibit high dielectric constant values and low dielectric losses with the best dielectric performance characteristics displayed by the 350 nm pseudo-octahedral nanoparticles whose permittivity reaches a value of ε = 1500 and a dielectric loss tan δ = 5 × 10-4 at a frequency of 1 Hz. Nanoparticulate ZnCr2O4-based thin films with a thickness varying from 0.5 to 2 µm were fabricated by the drop-casting method and subsequently incorporated into planar capacitors whose dielectric performance was characterized. This study undoubtedly shows that the dielectric properties of nanostructured zinc chromite powders can be engineered by the rational control of their morphology upon the variation of the post-synthesis heat treatment process.

X-ray topography of subsurface crystal layers.

New capabilities of the Berg-Barrett topographic method are demonstrated using a skew-asymmetric X-ray diffraction scheme for investigating structural changes near the surface of semiconductor materials. Specifying the X-ray extinction depth, the details of defects and strains are revealed with high resolution. Consequently, analysis of structural distortion of layers near the surface after various types of surface processing becomes more complete.

Structure diagnostics of heterostructures and multi-layered systems by X-ray multiple diffraction.

This article presents the results of research on multi-layered heterostructures by a modified calculation technique of multiple X-ray diffraction. The Al x In1-x Sb heterostructure and a Zn(Mn)Se/GaAs(001) multi-layered system were used as models to specify conditions for cases of coincidental coplanar three-beam or coincidental noncoplanar four-beam X-ray diffraction. These conditions provide the means for a high-precision determination of lattice parameters and strain anisotropy in layers.