Developing electron ptychography by using 2D materials

Published : 26 March 2019

The aim of this thesis is to perform numerical and experimental developments on a promising technique of Electron Microscopy : Electron Pytchography.

Ptychography consists in (a) acquiring a series of coherent diffraction patterns obtained with a small probe that is scanned with overlaps over the region of interest and (b) analysing numerical and iteratively all these diffraction patterns. The coherence of the beam and the numerical reconstruction allow to retrieve structural information much smaller than the beam size. For instance in 2011, by using an electron beam of about 15nm, gold atomic columns distant of about 0.23 nm where imaged. Theoretically, the limit of resolution is the wavelength of the incident beam , which is about 2.5 pm for an electron beam accelarated with an electric potential of 200kV. In the fields of X-rays and light optics, important results and developments have been recently performed, mainly because even with no lenses it is possible to reconstruct numerically not only the image of the object but also to compute the complex wave functions of the incendent and exit beams. However, most the actual ptychography software assume a weak interaction between the incident beam and the object, the so called phase object approximation which neglects multiple interactions. This approximation is good for X-rays and visible light, but it works rarely for electron beam, unless the object is very thin as in a monolayer 2D-material.

It is why in this thesis we propose to (1) develop a new software that would take into account multiple interactions (2) use more and more complex 2D materials. 2D monolayers, will allow to optimise the experimental set-up on the electron microscopes and to test the existing ptychography software. More complex 2D materials involving several layers and functionnalized layers will allow to test the new software. We hope that this thesis will allow to reconstruct the 3D atomic structures of multilayer 2D-materials and give information on the charges and chemical bonds. These would be great breakthroughs in material science characterisation.

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