Atomic-scale characterization of embedded and supported nanostructures by scanning transmission electron microscopy

Dispersed metal/oxide particles in or on a support matrix are the key structures determining the properties of many scientifically and technologically important materials. Two widely used examples of them which were investigated in this thesis are embedded oxide precipitates in high temperature superconductors (HTS) and supported-metal clusters in heterogeneous catalysis. The superconductive properties of HTS depend on the flux pinning properties of the nanostructures embedded in the host matrix. With the same analogy, the catalytic properties of heterogeneous catalysts, such as the activity and selectivity, strongly depend on the structure parameters. The focus of this thesis is on developing STEM techniques for the characterization of embedded and supported nanostructures.;To obtain three-dimensional information for the spatial and size distribution of the nanostructures embedded in the superconductor matrix material, STEM tomography was employed. The effect of various image-processing techniques on the visibility of tomographic reconstructions was investigated. The distribution uniformity, position and size of the particles were observed to be dependent on the interaction of the particles with the twin boundaries. It was observed that the larger particles are generally located on more than one twin boundary, moreover, the particle size is smaller on the twin boundaries shared by several particles. This suggests that the growth of the particles is determined by fast twin boundary diffusion and the formation of the large particles might be prevented by altering the temperature-time parameters of the production processing.;Zeolites are prototypical crystalline nanoporous materials that provide supports for transition-metal cations for catalytic applications. As a part of this thesis, aberration-corrected STEM under low-dose imaging conditions was used to image and determine the locations of metal nanoclusters and individual metal atoms within the intra-crystalline spaces of zeolites, with atomic resolution. By using STEM in combination with multislice image simulations, the active sites of metal catalysts anchored in a zeolite crystal were imaged. Furthermore, a possible cluster formation mechanism was deduced by comparing the locations of mononuclear iridium species and iridium clusters anchored within zeolites.;To fully understand the functionality of nanostructures, we need to be able to identify the location and species of each individual atom within a single nanostructure. In this thesis, it was also demonstrated that aberration-corrected STEM can be used to identify the individual atoms, map the full nanocluster structure in three dimensions with atomic resolution, and determine the changes in the positions of individual metal atoms in images that were recorded sequentially by examining supported bimetallic clusters that are only on the order of 1 nm in size.;This thesis highlights that STEM is an excellent method for determining embedded/supported nanostructures in three-dimension with single atomic sensitivity and unprecedented detail. Therefore, STEM combined with theoretical dynamical image simulations and image processing (reconstruction, image enhancement) techniques can provide answers to some of the challenging questions posed in the field of the coated superconductors and supported metal catalysts.