Modena - 29.06.2018 - S3 Seminar Enzo Rotunno
Date and Time: Friday June 29, 2018 - 11.00
Venue: S3 Seminar Room, Third Floor, Physics Building, FIM Department

Speaker: Enzo Rotunno

Title: Controlling the propagation along atomic columns by intelligent electron beam shaping

Abstract:Most of the beams used so far in electron microscopy are normally shaped by a hard aperture whose radius is selected in order to limit the aberrations effect leading to a beam shape close to an Airy disc. The introduction of holographic electron beam shaping has completely changed this paradigm allowing for the engineering of probes with different kinds of complex wave fronts.
In this work we report on a detailed analysis of the propagation of electron beams having different shapes in a model system, namely a [100] oriented cubic GaN crystal.
The analyses are based on the comparison between multislice simulations and a reformulated Bloch wave based on the concept of Transverse Energy as the only quantum number. This model allows for a quantitative description of the propagation on the bases of the free space properties of each beam.
We first consider the special case of the 0-th order Bessel beam compared to the ordinary STEM probe in order to understand in details the “pendellösung” oscillation: the discrete momentum spectrum of the Bessel beams produces, as a result, a very strong selection on the excited Bloch states that can be exploited to characterize the beam propagation. Finally, we will introduce the actual diffraction-free solution of the propagation of beams in a crystal to highlight the difference with the Bessel beam. In fact, shaping the beam as an approximate 1s state, which can be considered as a Gaussian beam, allows for the minimization of the diffraction/pendellösung effects observed in both aperture limited and Bessel probes.
We then foresee the possibility to engineer the pendellösung oscillation of the probe in STEM experiments aiming at a 3D reconstruction of the sample. In a recent article we demonstrated that we were able to define the channeling condition by tuning the beam convergence. This can be use to explore different thickness ranges inside the specimen. We also proposed a mathematical framework to reconstruct the atomic distribution of guest atoms along the imaged columns analyzing the information from a set of experiments performed using different probe settings. As a case study, we used the InxGa1-xN (x<10%) alloy.

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