The migration of substances in complex systems is most commonly a result of diffusion – a mechanism in which atoms or molecules migrate from the area of high concentration to the area of low concentration so that at the end the concentration of these atoms or molecules is uniform throughout the whole medium. Such is the case of e.g. juice and water – when mixed together and left for a sufficient period of time, the juice should be uniformly distributed in the whole glass of water.
In the micro- and nanoscale, however, there are also other mechanisms of migration, frequently competing with diffusion, often with a different end result. Such is the case of silver and gold nanolayers, deposited on ultra-thin germanium wetting films.
Silver and gold thin films are widely used in various kinds of optical devices. Their performance depends on the roughness of the metal layer. However, since the adhesion of both silver and gold to most of the commonly used oxide substrates is very low, growing smooth plasmonic metal layers with thickness at the order of a few dozen nanometers or less is a challenging task.
To mitigate these phenomena, it has been proposed to use an ultra-thin germanium interlayer between the substrate and the metal, which essentially acts as a glue, thus forming a sandwich-like structure. However, In our recent studies, published in the Beilstein Journal of Nanotechnology [1] and in Surface Science [2], we have shown that germanium not only decreases the roughness of the silver and gold films but also quickly migrates from the bottom of these layers towards their surfaces in a process known as segregation.
Silver and gold nanolayers exhibit granular structure – you may think of them as a layer of ultra-small sand grains, just made of metal and very tightly packed together. The use of germanium interlayer between the substrate and the metal influences that granular structure of the metal. Instead of a homogeneous distribution of metal grains throughout the whole sample, the grains in direct contact with germanium layer are larger and those further away are smaller, and thus, greater in number. This means that the further from the germanium interlayer, there are more grain boundaries (sites where two or more different grains are in contact) and more grain boundary voids, where many small grains are in contact with each other. Germanium atoms have a lower free enthalpy when sitting in a point where more grains are in contact. In human terms, they like to be in that position more than in a simple grain boundary between two grains. As most of these points are near the surface of the metal layers, that is where the germanium atoms migrate, both in the case of silver and gold.
After two months, an overwhelming majority of germanium atoms can be found near the surface and almost none of them are at the metal-substrate interface where the Ge layer was initially deposited. This has profound consequences, one of them being a noticeable modification of the metals optical parameters (refractive index and extinction coefficient), which must be taken into account when designing optical devices.
These findings are described in the article entitled Evidence of germanium segregation in gold thin films, recently published in the journal Surface Science, and the article entitled Growth model and structure evolution of Ag layers deposited on Ge films, recently published in the Beilstein Journal of Nanotechnology. These findings are result of a collaboration of researchers from the University of Warsaw (Faculty of Physics as well as Department of Chemistry) and UTP University of Science and Technology in Bydgoszcz (Institute of Mathematics and Physics).