PhD project: Surface dynamics of nanostructures under a thermal gradient: from basic mechanisms to thermal tweezers
Laboratory: Centre Interdisciplinaire de Nanoscience de Marseille, France
Supervisor: Stefano Curiotto, 06 62 92 28 86, stefano.curiotto@cnrs.fr
This PhD project deals with the study of thermomigration of surface nanostructures.
Atomic diffusion, that is a random motion of atoms in the absence of any force field, is modified under a thermal gradient in bulk materials. This phenomenon is known as thermomigration (TM). The first experimental observations showed that homogeneous mixtures of atoms, molecules or particles of different kind become inhomogeneous in a thermal gradient, because the different species respond differently to the thermal gradient.
TM is often considered as an undesirable phenomenon, because it affects the stability of materials by changing the atomic concentrations. For instance, TM changes the mechanical stability of alloys used for soldering, and is very detrimental in electronics, where electric currents lead to thermal gradients due to local heating by the Joule effect. TM is also studied in the context of pore removal in ceramics during the sintering fabrication. The effect of TM has been studied in bulk solid materials but on surfaces it is almost unknown. In nanomaterials, weak thermal gradients can lead to strong effects because of the small amount of matter, and of the importance of surface effects. For instance, Xie et al. have proposed and demonstrated a method based on TM to grow in a controlled way metallic nanowires directly from the surface of a hot solid by simply touching the solid with a cold tip and drawing it back [Nature Comm. 10 (2019) 4478]. Within this project the PhD student will study how nano-objects displace and organize on surfaces under the effect of a thermal gradient. We have already obtained some promising results on the motion of 2D islands on Si under thermomigration, these results are a solid base for the development of the PhD project.
Activity: The PhD student will study the dynamics of nanostructures at surfaces such as 2D islands and 3D nanodroplets on different substrates, thanks to the low energy electron microscope (LEEM) available at CINaM. This powerful instrument allows taking images with a typical frequency of 1 Hz, a lateral resolution of some nanometers and a contrast in height that makes surface steps visible. It is thus perfectly suited to investigate the kinetics of surface phenomena. The thermal gradient will be applied with a specific sample holder and/or using a laser beam. Kinetic Monte Carlo simulations will complement the experiments. The PhD grant is based on a project financed by the ANR (Thermotweez).
Requested background: good knowledge in condensed matter physics, motivation for experimental work. The candidate should be graduated with high grades in physics, materials science or equivalent degree.