Is there no way to make the electron microscope more powerful?
Most images are two dimensional projections of three dimensional structures, which can miss fine details. With the explosion of new
imaging modalities, several methods can be used for
high-resolution characterization of materials in three dimensions, including atom probe tomography
(APT), discrete tomography, and electron tomography. APT offers extensive capabilities for both 3D imaging and chemical composition
measurements at the atomic scale and for instance has recently been applied to element-resolved corrosion analysis of a stainless-type glass-forming
Fe50Cr15Mo14C15B6 alloy. We have ready access to one of the first of these instruments at the NUCAPT facility.
However, APT requires needle-shaped specimens and can detect only about 60 per cent of the atoms in a sample. Furthermore, the
APT reconstruction commonly assumes that the shape of the specimen is spherical, which can result in inaccurate structure and
composition analysis for complex materials. Discrete tomography has been used to produce the 3D atomic reconstruction of
small crystalline samples, but this statistical method requires prior knowledge of the lattice structure and requires that the atoms
fit rigidly on that lattice.
To accurately determine the high-resolution 3D structure of materials without assumptions, electron tomography is the method of choice.
Since its introduction in 1968, electron tomography has been primarily used to determine the 3D structure of biological samples.
In the past decade, it has been increasingly applied in materials science and nanoscience. Based on conventional data acquisition and
tomographic reconstruction methods, the highest resolution achieved by electron tomography was around 1nm in three dimensions.
In 2012, Miao applied a novel data acquisition and image reconstruction method to electron tomography and achieved
a 3D resolution of 0.24 nm, the highest resolution ever obtained in any 3D imaging method.
Subsequently, Miao together with Marks applied this general electron tomography method to observe nearly all the atom in a
multiply-twinned Pt particle (right), and image the 3D dislocations and defects in the particle at atomic resolution.