|Ph.D Student||Amram Dor|
|Subject||Morphology and Microstructure Evolution in Au-Fe Bilayers|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Eugen Rabkin|
This research thesis revolves around a chronological journey through the “life” of a thin bilayer on a substrate (gold/iron/sapphire). Three main “stops” in this journey, each relies on the conclusions of the previous, lead to an applicative research goal - obtaining core(iron)-shell(gold) nanoparticles by annealing the films at elevated temperatures (600-1100°C).
1. As-deposited microstructure of thin films: we obtained single-crystalline gold thin films on sapphire, having exceptional crystal quality and thermal stability, by using the iron layer as a “seed”. The seed layer accommodates the large difference between the lattice parameters of gold and sapphire, which would otherwise result in a typical polycrystalline film containing many structural defects.
2. Anisotropic dewetting: upon annealing, the films agglomerate into particles due to poor adhesion between metals (gold, iron) and the ceramic substrate (sapphire - aluminum oxide). At temperatures lower than the films’ melting temperature, this is termed ‘solid-state dewetting’. The unique microstructure of the films leads to a unique dewetting behavior, which had not been observed before. We developed a quantitative model which accounts for surface-energy and diffusion anisotropies (dependence of those properties on crystallographic direction), and described the dewetting kinetics in the films well.
3. Phase transformations in micro- and nanoparticles: when dewetting of a single-crystalline film is complete, structurally-perfect particles are obtained. Phase transformations in alloy particles (gold/iron) are expected to proceed differently from bulk systems due to the “size effect”. We explored two transformations: (1) precipitation of iron from a gold solid-solution; (2) the a?g transformation of iron and iron-gold alloys. Our main conclusion was that, contrary to the current paradigm, phase transformation proceed differently even in sub-micrometer-sized particles, and not only in nanometer-sized particles, where capillary forces dominate. Particularly, segregation (migration of a solute atom to a surface/interface to reduce its energy) of gold to all surfaces and interfaces of iron nanoparticles greatly affected the kinetics and morphology of the phase transformations.
We capitalized on the latter main result to fulfil the applicative research goal by employing a segregated gold layer as the shell, demonstrated the ability to bind organic molecules to their surfaces, and explored their magnetic properties. Such nanoparticles could find promising uses in bio-medical, data storage and catalytic applications, due to the unique combination of a magnetic iron core and an inert gold shell. Compared to other fabrication methods, we suggested a simple process which leads to nanoparticles with a high degree of purity and structural perfection.