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Simulation For TAP

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Tomographic Atomic Probe (TAP) is a powerful tool used to analyze the structure of material at the atomic scale. It exploits the huge intensity of the electric field generated at nanometric tips subject to a large electric potential. The field at the tip is of the order of 10V/nm, which is large enough to induce ion emission. This process is well controlled for metals but remains challenging for insulators. The SIMUL team is involved in the collaborative project AQURATE with the GPM in Rouen (2015-2018).
The aim of the project is twofold. First the SIMUL team will analyze the stability and lifetime of molecular ionic fragments observed experimentally, in particular SiO²⁺, ZnO²⁺, MgO²⁺, (ZnO)₂², and also the fragments emitted from GaN, CaF₂ and LiF. The analysis requires to determine the deformation and couplings of the lowest potential energy surfaces (PES) under the influence of the electric field from the tip. Theses curves can be generated with the quantum chemical code MOLCAS. They will be further diabatized to obtain a fit of the PES in the whole space. Second, the experience of the SIMUL team in the development of model potential for molecular dynamics will be used to analyze the field emission process itself. Our aim is not yet at an accurate determination of the proportion of various species emission. We shall rather build up a model based on tight binding description of the atoms experiencing at the tip. In such a simulation the charge of each atom will be prescribed according to the local electric field, until emission is induced. Then a statistical analysis will be done to record the proportion of each fragment, including neutral ones, which cannot be detected experimentally. Our aim is here to analyze the sensitivity of the distribution to the physical parameters to shed some light on the leading processes at work for insulator emission.