Ab Initio simulations of alloy surfaces under electric field

Project Description

Electric fields effects when applied to metal and alloy surfaces are crucial in understanding the behaviour of the material in corrosion and also characterization routines such as atom probe and field ion microscope. The application of electric field often causes charge redistribution on materials surfaces. Such effects are only sufficiently described when simulated through an ab initio theory such as Density Functional Theory (DFT). Field evaporation behviour of pure metals and alloys was evaluated. The high electric fields are applied by a Generalized dipole correction method implemented in S/PHI/nX, a DFT framework. In this project we developed routines to simulate field evaporation and field ionization behaviour on non trivial high Miller index surfaces, as seen in reality, from DFT. During this project, I developed a good understanding of the working of DFT and the underlying physics. My participation in this project imporved my programming skills and also gave me mentorship oppurtunity.

Related Publications

• "Understanding atom probe's analytical performance for iron oxides using correlation histograms and ab initio calculations" - New Journal of Physics Mar 2024. View Publication
  Field evaporation from ionic or covalently bonded materials often leads to the emission of molecular ions. The metastability of these molecular ions, particularly under the influence of the intense electrostatic field (10^10 Vm-1), makes them prone to dissociation with or without an exchange of energy amongst them. These processes can affect the analytical performance of atom probe tomography (APT). For instance, neutral species formed through dissociation may not be detected at all or with a time of flight no longer related to their mass, causing their loss from the analysis. Here, we evaluated the changes in the measured composition of FeO, Fe2O3 and Fe3O4 across a wide range of analysis conditions. Possible dissociation reactions are predicted by density-functional theory (DFT) calculations considering the spin states of the molecules. The energetically favoured reactions are traced on to the multi-hit ion correlation histograms, to confirm their existence within experiments, using an automated Python-based routine. The detected reactions are carefully analysed to reflect upon the influence of these neutrals from dissociation reactions on the performance of APT for analysing iron oxides.
• "Accurate computation of chemical contrast in field ion microscopy" - Physical Review B April. 2023. View Publication
  We present a computational approach to simulate local contrast observed in field ion microscopy (FIM). It is based on density-functional theory utilizing the Tersoff-Hamann approach as done in scanning tunneling microscopy (STM). A key requirement is the highly accurate computation of the surface states' wave-function tails. To refine the Kohn-Sham states from standard iterative global solvers we introduce and discuss the EXtrapolation of Tails via Reverse integration Algorithm (EXTRA). The decaying tails are obtained by reverse integration (from outside in) using a Numerov-like algorithm. The starting conditions are then iteratively adapted to match the values of plane-wave Kohn-Sham wave functions close to the surface. We demonstrate the performance of the proposed algorithm by analyzing and showing the chemical contrast for Ta, W, and Re at Ni surface.
• "Ab initio vacancy formation energies and kinetics at metal surfaces under high electric field" - Physical Review B Jan. 2023. View Publication
  Three-dimensional field ion microscopy (3D-FIM) is a powerful technique for visualizing crystalline defects at an atomic scale by recording field ion microscope images under field-evaporating conditions and reconstructing the underlying atomic configuration. However, the quantification of observed vacancies and their origins remains contentious, with debate over whether high electrostatic fields (1–5 V/Å) used in 3D-FIM might introduce artifact vacancies. Density functional theory simulations on stepped nickel and platinum surfaces with kinks suggest that contrary to previous proposals, the formation of vacancies on electrified metal surfaces under such conditions is more challenging, and the electrostatic field could impede a potential "vacancy annihilation" mechanism by introducing kinetic barriers.
• "Role of Simulations and Experiments in Analytical Field Ion Microscopy" - Microscopy and Microanlysis Feb. 2018. View Publication
  This study introduces the Analytical Field Ion Microscope (aFIM), a hybrid experimental technique merging the high spatial resolution of the Field Ion Microscope (FIM) with the chemical discrimination capabilities of Atom Probe Tomography (APT), enabling real-space, atomic-scale imaging of materials and their defects. Experimental and theoretical advancements are discussed, including the integration of time-of-flight mass spectroscopy with FIM imaging and the development of data-mining protocols correlating field ionization and field evaporation events. Additionally, the application of aFIM to model alloys reveals insights into creep deformation and the effects of alloying elements, while Density Functional Theory (DFT) calculations shed light on the impact of high electric fields on vacancies, aiming to enhance the technique's robustness for defect analysis.