Atom-by-atom imaging and spectroscopy with the
scanning transmission electron microscope
Ondrej L. Krivanek
Nion, Kirkland, WA, USA (www.nion.com) and
Dept. of Physics, Arizona State University, Tempe, AZ, USA

Kaffe kl. 14.45.

Probing matter atom-by-atom with an ultra-small yet very intense beam of
electrons in a scanning transmission electron microscope (STEM) has recently
become possible, principally because of 5 developments, several of which we have
pioneered at Nion:

1) Correction of electron-optical aberrations has resulted in electron beams that are
smaller than the typical atom, which means that the beam can be focused on one
atom at a time.

2) Aberration correction has allowed this performance to be maintained at low beam
energies (30-80 keV), which do not cause knock-on radiation damage in materials
such as graphene. As a result, very large electron doses can be used, leading to
relatively noise-free images and spectra from individual atoms.

3) Improved cold field emission electron guns (CFEGs) are giving coherent probe
currents as high as 0.5 nA, and sub-atomic electron probes of 0.2 - 1 nA.

4) Ultra-high vacuum (UHV, pressure < 1x10-9 torr) has become available in the
sample chamber, allowing samples to be examined without contamination and
without contamination-caused beam-assisted chemical etching.

5) Samples such as graphene and monolayer BN have become readily available,
allowing atoms to be imaged in non-overlapping configurations, rather than
imaging whole columns of atoms, as was the case with thicker samples.

These advances will be reviewed and illustrated by practical examples, such as
annular dark field (ADF) imaging, electron energy loss spectroscopy (EELS) and
energy-dispersive X-ray spectroscopy (EDXS) of individual substitutional atoms in
graphene. The EEL spectra will be shown to contain fine structures that provide
information about the local environment of individual atoms.
Our progress on a new instrumentation project – reaching 30 meV and smaller
energy EELS resolution with a nm-sized electron probe – will also be described.