Discover this applications:
What is Ptychography?
Ptychography is a computational method of microscopic imaging which has been shown to be the most robust of the CDI variants. The coherent source of illumination interacts with the sample and generates interference patterns in the far-field where different diffracted beams overlap. The data is collected either by shifting the sample (in the case of X-rays) or shifting the illuminating beam (in the case of electrons). The key to the method is to oversample the information – lateral shifts must be small enough to allow partial overlapping of the probe positions. Computation methods are then used to reconstruct the phase from the wealth of information available, hence solving the phase-detection problem.
In electron microscopy, ptychography is performed as a scanning transmission experiment. The sample is illuminated by a cone of focused or controllably defocused electrons which scans the sample. A 2D diffraction pattern is collected for each point of a scan (the final dataset is 4 dimensional). Direct or iterative reconstruction methods are then used to reconstruct the phase of the sample. In our example, a scan of a 2D material, MoS2, was performed with an 80kV electron probe and the reconstruction clearly shows a presence of dopant Rhenium atoms. Additionally, ptychography allows post-acquisition correction of beam aberrations which has a great potential for microscope affordability in the future.
[MoS2 2D material with Re dopants imaged by electron ptychography with MerlinEM detector at 80kV illumination. Image courtesy Dr Shoucong Ning, NUS, Singapore]
Ptychography benefits from the speed of direct radiation detectors
Modern synchrotron sources provide a massive increase in coherent flux. Coherent diffraction imaging (CDI) overcomes the resolution limit of X-ray optics to determine structures to better than 100 nm resolution.
The advantage of ptychography is that the spatial resolution achieved can be higher than that achieved with conventional optics.
Ptychography benefits from the high frame rate capabilities of hybrid pixel detectors for on-the-fly scans.