Phase reconstruction using fast binary 4D STEM data

Example modulus and phase CBED patterns

Researchers from the department's David Cockayne Centre for Electron Microscopy, The Rosalind Franklin Institute and Diamond Light Source have published in vol 116 of Applied Physics Letters their investigation into applying focused probe ptychography using binary 4D datasets obtained using scanning transmission electron microscopy (STEM).    

The bit depth of the counting detector was reduced to permit a decrease in the dwell time and to increase the frame rate, which reduced the electron exposure of the sample for a given beam current.  Atomically resolved phase contrast of an aluminosilicate zeolite was observed from sparse diffraction patterns with isolated individual electrons, which demonstrates the potential of binary ptychography as a low-dose 4D STEM technique.

The intensity in the STEM detector plane was recorded on a pixelated detector, resulting in a 4D dataset.  Two of these dimensions corresponded to the real-space probe position and two to the reciprocal space position in the convergent  beam electron diffraction pattern recorded at the detector plane, with the transmission function of the aluminosilicate zeolite sample retrieved without the need for real or virtual phase plates, with reduced noise from unwanted higher spatial frequencies.  

The 1-bit counting mode on the MerlinEM system recorded 10 times faster than a 12-bit counting mode, which is within one order of typical conventional annular dark-field imaging speeds.  Importantly, the results demonstrate that fast pixelated detectors do not need a high dynamic range to provide high-quality phase reconstructions, and that faster detectors should enable high throughput, low dose 4D STEM phase imaging in the near future.  Furthermore, multi-frame acquisition combined with image requisition techniques is being used more often to increase the signal-to-noise ratio of STEM images, and faster scanning will allow this approach to be used with pixelated detectors.