Direct structural and chemical characterizations of metal oxide nanoparticles using high-voltage electron microscopy

Abstract

Nanoscale materials continue to be extensively studied as they exhibit unique behaviour compared with their bulk counterparts. These emerging functional nanomaterials often have atomic order limited to the nanoscale. Unfortunately, although great strides toward making complex functional nanomaterials have been accomplished, the powerful methods that we have for solving the atomic and chemical structure of bulk crystals fail for such materials. No broadly applicable, quantitative, robust, and direct analytical methods exist to solve the structures and chemistries of the nanomaterials simultaneously. Hence, to develop analytical methods to make direct interpretation for the nanomaterials allowed with high precision has been eagerly demanded, which is called as “nanostructure problem”. In the world of nanostructures, transmission electron microscopy (TEM)-based methods are positioned to play a crucial role by providing direct information on some nanomaterials. Of the microscopic techniques, the contrast based on the high-angle scattering of electrons (Z-contrast) in scanning transmission electron microscopy (STEM) allows the elements with a high value of the nuclear charge Z to be imaged, but this technique does not provide an elemental information about light elements such as oxygen. In addition, it is not yet feasible to image the light atoms in the nanoparticles even by aberration $(C_s)$ -corrected high-resolution transmission electron microscopy (HRTEM) because sizes of most nanoparticles stand off the thickness range possible for directly interpretable atomic column imaging condition. The native resolution of a HRTEM can be improved by increasing its electron beam energy as well as reducing the spherical aberration of its objective lens. These methods have enabled the resolution of oxygen atoms in many cases of bulk metal oxides. The bulk metal oxides in the TEM usually have a continuous thickness variation from edge to interior of the specimen due to its ...