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MM: Fachverband Metall- und Materialphysik

MM 23: Methods in Computational Materials Modelling (methodological aspects, numerics)

MM 23.1: Vortrag

Mittwoch, 3. April 2019, 10:15–10:30, H44

Understanding field evaporation in atom probe tomography from a first-principles perspective — •Christoph Freysoldt, Michael Ashton, and Jörg Neugebauer — Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40273 Düsseldorf

Field evaporation is the elementary process at the heart of atom probe tomography (APT): single atoms or molecules evaporate from the sample under the influence of a very high electric field. Despite 50 years of APT experiments, a comprehensive predictive theory is still lacking.

We approach the problem in the framework of density-functional theory. From the minimum-energy path for desorption as a function of applied field, we compute critical fields and transition barriers for a variety of surface sites in good agreement with available experimental data. Our results show that atoms evaporating from within the surface at steps or kinks stay in close bonding contact at the transition state. The surface bonding characteristics of the evaporating atom is thus much more important than the electron transfer process, which was assumed to be the dominant mechanism in previous models.

From our insights, we propose a simple analytic theory. The critical zero-barrier field results from the balance between the maximum intrinsic force along the reaction coordinate and the field-induced force. The latter can be obtained directly from the change of the surface dipole along the reaction coordinate, while the former is not very sensitive to the applied field. The theory also provides barrier heights for subcritical fields.