Ptychographic Algorithms for Phase Recovery in 4D Scanning Transmission Electron Microscopy

Abstract: In Momentum-resolved Scanning Transmission Electron Microscopy (4D STEM), a convergent electron beam is raster-scanned across a think specimen in 2D in real space. The corresponding 2D diffraction pattern, in momentum space, to each point is recorded, forming a 4D data set. Information decoding process can follow thereafter to produce an image of the specimen in real space. Ptychography is reconstruction algorithm that allow the extraction of the probe wavefunction and the multiplicative object transmission function of the specimen. Ptychography is implemented through direct and iterative schemes. Some of which are the extended Ptychographic Iterative Engine (ePIE), the Wigner Distribution Deconvolution (WDD) and the simpler version of WDD, the Single Side-Band (SSB). This thesis gives an overview of STEM ptychography giving examples of its experimental and simulated implementations. The different ptychographic reconstruction methods are explored in a mathematical framework when applicable. Finally, an SSB reconstruction was made using an original script for simulated data of MoS2 monolayer. Moreover, four-dimensional data was recorded using a STEM instrument. A natural step following this research would be the implementation of the WDD algorithm.