Summary

Up to now transmission electron microscopy (TEM) was more focused on producing images or diffraction patterns of objects. Of course, some pioneering techniques like holography or focal series or ptychography have tried to reconstruct the electron waves functions at the exit surface of the observed objects, but their results and applications were quite limited or, as far as ptychography is concerned, quite rare. The availabilities of new pixelated 2D-detectors ^[1] and new algorithms to analyse series of image/diffraction data are now opening a new area of research in TEM: determining the electron waves and the object potentials at different planes of the observed object should become the main objective of quantitative TEM. Applied at an atomic level, this method, which is presently named ptychography, but I would like to generalize as neuscopies should be able to resolve the 3D atomic structure of any piece of matter that is thin and resistant enough to be traversed by an electron beam. The student will analyse experimental and simulated data with different ‘ptychography’ software and eventually participate in the acquisition of some experimental data.

Full description of the subject

As this research field is new for the laboratory, there should be room for 2 internships that will work in collaborations  (internship 1: Application to stacks of 2D material, internship 2: improvement of algorithms). However, depending of the personalities and the number of the candidates, the probability of have only one internship is high. Indeed, a motivated and talented candidate could perform what is described below by spreading his realizations both in the present internship and in a following already funded PhD. Firstly, the student(s) will learn the basic ideas behind ‘ptychography’ and understand the 3 existing software that will be used. They should become familiar with real and reciprocal spaces, Fourier transformation and diffraction, the multislice approach used to solve the Schrödinger equation describing the propagation of the electron in matter and much more. They should determine the strengths and limits of the 3 available software by analyzing different experimental and simulated data. Most the data will come from 2D-materials, like single or multi doped MoS2 layers. Indeed, two of the software are assuming a weak interaction between the electron beam and the sample, which is only verified when the sample is thin enough. One of this software has been developed by Dr Favre-Nicollin at ESRF, the European Synchrotron Research Facilities in Grenoble as ptychography has become a powerful technique in X-ray diffraction (making lens for X-ray beams is difficult and ptychography is a way to create images without the use of lenses). Discussions and collaborations with Dr Favre-Nicollin should take place. The first aim of these numerical studies will be to determine how robust are these software in the field of electron microscopy. As published in [1], the numerical reconstruction should compute the potential describing the interaction between the electron beam and the sample with a resolution of about 20 pm, that is to say we should determine not only the atomic structure of the 2D-layers with their eventual defects (such as vacancies, dopants, stacking fault…) but also the potential and electric fields in between the atoms, i.e. get information on the atomic bonds. The second aim would be to evaluate with the third software what kind of 3D information can be obtained from a single direction of observation; or do we really need a second direction of observation to get reliable 3D information (?).

Requested skills
Internship 1 (Application to stacks of 2D material): A physicist having a good theoretical background and enjoying playing with computer should be well adapted for this internship.
Internship 2 (Improvement of algorithms): A mathematician or computer science student.

Keywords

Images, wave functions, Schrödinger Equation, Transmission Electron microscopy, ptychography, 2D materials

Contact
Jean-Luc Rouvière
Phone: 07 82 50 07 49
Team: LEMMA