Ultrathin clean overlayers are often under mechanical stress because the atomic structures of the substrate and the adsorbate cannot be matched without strain. If the stress energies are comparable to the interface and surface energies, the system is heteroepitaxial. The self-assembly phenomena caused by stress release mechanisms of heteroepitaxial systems have application potential in nanotechnology. In this thesis we study two phenomena where the overlayer adopts a certain structure to release stress. In the first phenomenon the material forms islands on the supporting surface while in the second case the overlayer stays flat but nucleates a set of defects. The computational modeling used in the studies is based on coarse-grained, atomistic methods. We apply a simplified, two-dimensional model on the islands. It turns out that the model can describe all the common growth modes and the most frequent island shape agrees with the literature. According to the model, the dependence of the most likely island shape on the coverage is determined by the balance between the surface and the stress energies and it does not depend significantly on the wetting films beneath the island. One can fit the model to a simple analytic formula which is capable of reproducing the numerical calculations. It can be said that the model used in this thesis is now understood fairly well. Our preliminary studies indicate that the generalization of the model for three-dimensional nanowires are in good agreement with the two-dimensional results. When we investigate defect generation in flat thin films, we choose to model a specific system in detail. To study copper on the close-packed palladium surface, we pick another physical model than what was used with the islands because a model which has been specifically taylored for these two materials can be compared to the experiments directly. It turned out that the overlayer forms stacking faults already in the submonolayer coverage regime. The finding is consistent with experimental data but calls for a new interpretation for the overlayer structure.
|Translated title of the contribution||Stress relaxation in supported metallic nanostructures|
|Publication status||Published - 2010|
|MoE publication type||G5 Doctoral dissertation (article)|
- thin films
- computational physics