PhD - Leuven | More than two weeks ago
Advance the 3D atomic-scale analysis of oxide materials by laser-assisted atom probe tomography
The ultimate dream of a scientist is to determine the species and position of every single atom in any given material. This greatly helps in understanding the materials (functional or even more exotic) properties, such as electronic conductivity, magnetism or superconductivity. In this respect, Atom Probe Tomography (APT) emerged as a very promising characterization technique due to its high 3D spatial resolution (down to a few ångström) and extremely high elemental sensitivity.
In a nutshell, APT is based on the concept of controlled ionization and removal of atoms from the surface of a very sharp needle (apex diameter < 150 nm) by the combined effect of an ultra-short voltage or laser pulse (ps) and a high electric field. Through the ions’ flight time towards the detection system, we can identify the elements present in the needle. By collecting the ions on a 2D detector, and by tracking their arrival sequence, we can reconstruct the 3D volume of the analyzed sample with sub-nm resolution. The advent of laser-assisted Atom Probe Tomography (LAPT) enabled to perform atomic scale characterization of even poorly or non-conductive materials (such as semiconductors and oxides) for which the voltage-pulsed approach is impossible. This opened the field of APT for e.g. semiconductor and energy storage (battery) technologies.
Notwithstanding the experimental successes, leading to its widespread implementation, one needs to improve on the experimental success rates, reliability, spatial and quantification accuracy. It is our vision to tackle these challenges by gaining a better understanding of the fundamental processes underlying LAPT analysis of non-conductive materials. Several intriguing questions need to be answered, for instance, laser irradiation facilitates the field evaporation of semiconductors and oxides even if the laser photon energy is (significantly) smaller than the materials’ band gap, whereby light adsorption should normally not occur. Moreover, because of strong (quantum) localization effects of the absorbed light in the nanoscale tips, one needs to also consider the processes of heat transfer (on the ps to ns time scale) to explain the spatio-temporal–dependent evaporation processes. It remains cumbersome (or impossible) to determine the effective temperature at the nanoscale tip during evaporation. The relevance of this becomes obvious when considering temperature-assisted atomic diffusion processes, which would alter the atoms’ position during analysis, and ultimately deteriorate spatial accuracy.
In this research project, we aim to investigate the short-pulsed (fs – ps) laser-nanotip interaction of wide band-gap materials by i) (experimentally) evaluating the tip temperature (and its evolution) during the laser pulse, ii) investigating laser power effects on the field evaporation and iii) correlating the field-evaporation behavior with the electronic, optical and thermal properties of the material under analysis. These properties are possibly affected by the applied voltage (field) or laser energy (temperature) in LAPT, which can be manifested through for instance band bending or a metal-to-insulator transition. We focus on various (doped) oxide compounds which function e.g. as dielectrics in semiconductor technology or present a significant ionic/electronic conductivity for energy storage and photovoltaic applications.
There is an extensive playground to be explored in search of a deeper understanding of the relevant processes in LAPT. As such, the candidate can partake in experimental work (tip fabrication, APT analysis...), data-analysis (3D reconstruction and analysis...) and possibly physical modelling (theoretical, simulation...), which can be further specified at the start of the research project with input from the candidate.
Required background: Physics, Nanoscience and Nanotechnoloy, Engineering Science or equivalent
Type of work: 70% Experimental, 30% Theoretical
Supervisor: Claudia Fleischmann
Co-supervisor: Andre Vantomme
Daily advisor: Jeroen Scheerder
The reference code for this position is 2021-008. Mention this reference code on your application form.