UC Berkeley

This dissertation contains two parts. The goal of the first part is to understand the experimental observation that the friction forces between a 2D material and an amorphous substrate are sufficient to stabilize tensile and compressive strains of the order of 1\% within the 2D material. An idealized atomic scale model featuring stick-slip friction is proposed to explore the interaction between a 2D material and a substrate. The atomic scale model shows that the observed strains can be sustained by bonds with strengths of the order of van der Waals (vdW) bonds. The observed distribution of strain and force in the strain-stabilized structure suggests that the maintenance of strain can be attributed to atoms located along the boundary, within a specified width range, which corresponds well to the prediction made by a continuum model.

In the second part, the observations of interesting atomic scale structures that include vortices, anti-vortices, and other more complicated structures in the force vectors revealed by the model in the first part are discussed. The centers of the vortices and anti-vortices are identified by calculating the winding number at each atomic position. Vortices and anti-vortices are found to annihilate with each other during structural relaxation, but a significant number of vortices and anti-vortices are preserved with the friction on the order of vdW interaction. Similar simulations are repeated for a system without the friction, and the interesting structures of vortices and anti-vortices cannot be maintained. Furthermore, the size of the vortices is quantified by defining a directional correlation function to investigate the evolution of those intriguing patterns, which concludes that the average size of the vortices will increase as the system stabilizes. It is also found that vortices and anti-vortices exist in the atomic displacements, which suggests the possibility to observe those structures experimentally. Finally the effect of those structures on friction is investigated by applying constant force and constant velocity in this stick-slip friction model.