(439g) Kinetics of Nanoring Formation from Quantum Dots in Epitaxial Thin Films

Solid material nanostructures in the form of nanorings exhibit many interesting properties, including tunable plasmon resonance and spin-polarized current switching, due to the unique nature of their electronic confinement. Such nanostructures have potential for numerous technological applications in optoelectronic and magnetic data storage devices. Recent experimental studies have reported the formation of gold and silver nanorings from the corresponding epitaxial nanoclusters or quantum dots upon thermal annealing. This presentation aims at a fundamental understanding of the kinetics of nanoring formation from quantum dots in coherently strained epitaxial films and a systematic exploration of the resulting nanoring structures upon variation of the processing conditions.

We have developed a three-dimensional kinetic model for the surface morphological evolution of coherently strained heteroepitaxial thin films, accounting for curvature-driven diffusion on the film surface, biaxial lattice misfit strain in the film, and the wetting potential between the film and the substrate. Self-consistent dynamical simulations show that the competition of the surface energy with the elastic strain energy lead through a Stranski-Krastanow instability to the formation of nanoclusters on the film surface, with the surface morphology gradually reaching a steady state under the action of the wetting potential. However, due to the difference in the thermal expansion coefficients between the film and substrate materials, the temperature increase during the heating of the film induces additional biaxial strain in the film due to thermal mismatch, triggering further morphological evolution of the nanoclusters upon thermal annealing and their transformation into nanorings. Our simulation results provide a fundamental comprehensive interpretation of the experimental reports in the literature.

Having validated our model, we explored further the morphological dynamics of nanoclusters in epitaxial layers under thermal annealing. We have found that the annealing temperature, the size of the nanocluster prior to thermal annealing, and the thickness of the wetting layer of the heteroepitaxial system have a strong effect on the morphological evolution of the nanoclusters during thermal annealing. Our dynamical simulations demonstrate that multiple concentric nanorings, structurally similar with the reported concentric nanorings fabricated by liquid droplet epitaxy, can form from sufficiently large nanoclusters at sufficiently high annealing temperatures. Finally, we provide a fundamental explanation of multiple concentric nanoring formation from quantum dots on epitaxial film surfaces based on weakly nonlinear stability analysis. Our modeling and simulation study sets the stage for designing experimental protocols toward precise control of complex nanoring structures on epitaxial film surfaces.