The physics of the light-tip interaction in laser-assisted Atom Probe Tomography

The ultimate dream of a materials analysist 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, ferro-electricity, 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 and a high electric field. Through the ions’ flight time towards the detection system, we can identify the elements present in the needle (Fig. a). 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 (L-APT) 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 L-APT analysis of semiconducting and other 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 (Fig. b). 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.

Several challenges remain to be surmounted to unlock L-APT’s full potential, in which the exact nature of the tip-light interaction plays a key role. To tackle this topic, you have the possibility to explore an extensive playground, including experimental work (tip fabrication, APT analysis...), data-analysis (3D reconstruction and analysis...) and possibly physical modelling (theoretical, simulation...).