Shyam Katnagallu

High Fidelity Reconstruction of Experimental Field Ion Microscopy Data by Atomic Relaxation Simulations

A key question in materials science is to identify how atomic scale structural and chemical motifs determine the properties of materials. To address this question, atomic-scale investigations of materials are instrumental. The huge advances made in the field of ion emission microscopy such as atom probe tomography (APT), allow today routine studies at the atomic scale. The predecessor to APT, field ion microscopy (FIM) [1] invented by E.W. Müller in 1951, was the first technique to image individual atoms on the surface of a sharp metallic needle. A tomographic FIM or 3D FIM employs a controlled field evaporation of atoms from the surface of a cryogenically cooled sharp needle-shaped specimen held at high enough electric field, while constantly imaging the field evaporating surface by an ionizing imaging gas. From a modelling perspective atomistic simulations such as density functional theory (DFT), molecular dynamics (MD) and Monte Carlo methods, nowadays also provide detailed insight into the atomic structure of materials. The current study aims to combine such atomic scale experimental investigations with corresponding atomic scale simulations with the aim to obtain specific structural atomic motif features for which the experimental reconstruction of the atomic positions alone is not precise enough. These experiment informed simulations, or simulation enhanced experiments, cannot only save computation time as the simulations are conducted directly on experimentally obtained atomic configurations but it can also qualify the FIM technique as the most precise available tool for the full tomographic and crystallographic real space imaging of atoms. The suitability of the extracted experimental atomic scale data for such informed atomistic simulations is also explored here.

Atomistic Simulations of Surface Effects Under High Electric Fields

The dynamics of a surface changes drastically when it is exposed to an intense electric field. In the case of a metal, an external electric field will induce a charge on the surface, leading to interaction between the field and the surface. This can lead to the deformation of the surface due to tensile stress from the interaction, but also more subtle effects, such as lowered barriers for diffusion of surface ad-atoms, field assisted evaporation of atoms from the surface and significant changes in the local electric field.