Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new imaging technique, tested on samples of nanoscale gold and carbon, that greatly improves images of light elements using fewer electrons. The newly demonstrated technique, dubbed MIDI-STEM, for matched illumination and detector interferometry STEM, combines STEM with an optical device called a phase plate that modifies the alternating peak-to-trough, wave-like properties (called the phase) of the electron beam.

In MIDI-STEM (right), an electron beam travels through a ringed “phase plate,” producing a high-resolution image (bottom right) that provides details about a sample containing a heavy element and light element. Details about the carbon are missing in an image (bottom left) of the sample using a conventional electron imaging technique (ADF-STEM). (Colin Ophus/Berkeley Lab)

This phase plate modifies the electron beam in a way that allows subtle changes in a material to be measured, even revealing materials that would be invisible in traditional STEM imaging. The phase plate in the MIDI-STEM technique allows a direct measure of the phase of electrons that are weakly scattered as they interact with light elements in the sample. These measurements are then used to construct so-called phase-contrast images of the elements. Without this phase information, the high-resolution images of these elements would not be possible.