Analytical Transmission SEM for Nanomaterials

Summary:

This project develops low-energy transmission electron diffraction, imaging, and spectroscopy in the scanning electron microscope, to enable determinations of microscopic structure, defect types, and interface character in ultrathin films, nanoparticles, and nano-bio material systems, to overcome imaging and analytical challenges faced by conventional SEM and TEM methods.

Description:

Established imaging and diffraction techniques for measuring structure of nanomaterials and soft matter do not show both good contrast and high resolution, and they can cause significant material damage. This is particularly the case for isolated nanostructures such as individual nanoparticles and ultrathin films. For example, identifying the crystallographic phase of an unknown nanoparticle of diameter 5 nm to 10 nm is extremely difficult, even in the most powerful high-resolution TEMs commercially available today. The problem centers on the generation of electron scattering within small volumes. For structures in the size regime of 10 nm and smaller, electrons with energies of the order of 200 keV exhibit a mean free path for scattering that can easily approach an order of magnitude larger than the particle itself. Decreasing those energies to those typical of an SEM (~ 20 keV) decreases the mean free path to values commensurate with the particle size. As a result, more electrons will scatter, to provide the information-rich content needed to measure crystal phase, crystal orientation, defects, and internal order within nanostructures.

A fully analytical transmission-SEM, alternatively considered a low-voltage STEM, would not only fuel more thorough characterization of nanoscale structures, enabling more precise process control and material reliability, but it would make available many powerful TEM-like capabilities to a large population of SEM users worldwide, in a diverse range of applications.

Major Accomplishments:

In the first year, we collected data that suggest the important Kikuchi scattering responsible for diffraction appears to occur within a few nanometers of the exit surface of the specimen.
We have obtained t-EBSD patterns from several types of nanomaterials, including transition metal films and foils ranging in thickness from ~ 5 nm to > 300 nm, HfO₂ films of thickness < 10 nm, Fe-Co particles of diameter ~ 10 nm, Pt particles of diameter ~ 2 nm, Fe₃O₄ particles of diameter ~ 8 nm, Ag particles of diameter ~ 10 nm, Al₂O₃ particles of diameter ~ 20 nm.
A form of high-angle annular dark-field imaging in transmission revealed high-contrast substructure within nanocrystalline Fe particles created by aqueous synthesis methods.