Correlation of 2D-layer Superstructures with Nanofriction -- A Scanning Probe Microscopy Study

Friction is everywhere in our daily life. Excellent skiers are in some cases friction experts, due to their skilled performance, to control the friction through the contact area between snow and ski. However, at the nanoscale, the control of friction becomes more complicated, with the situations of strong adhesive interaction, atomic contact and stick-slip motion. Especially for superstructures which are recently focused like magic-angle graphene, the study on friction for two-dimensional superstructures is an attractive topic and will help to understand the essences and behaviors of nanoscale friction practically and theoretically.
Thanks to the development of atomic force microscopy based on the principle of interaction force detection, the friction force at the nanoscale is able to be measured with a sharp tip (atomically well defined) on clean surfaces. In this thesis, friction force microscopy in ultrahigh vacuum at room temperature is applied to investigate the friction performance of two superstructures, as well as non-contact atomic force microscopy for topography evaluation. The first part is based on reconstructed KBr on Ir(111), indicating a novel periodicity different from cubic configurations as we know. It is assumed to be formed by the reason of both a strong interaction from the iridium substrate and a proper lattice match between KBr and Ir(111) at certain rotation angles. After the tuning with the double-layer stack of KBr plus graphene on Ir(111), the reconstruction of KBr disappears with the recovery of the cubic configuration. It causes the friction drastically reduced in comparison to the previous reconstructed state on pure Ir(111). The second part investigates the frictional properties of different graphene moiré patterns on Pt(111). When applying a lower normal load, all the graphene patterns present ultralow friction with a stable sliding. However, if increasing the normal load continuously, this stable state will be replaced by a dissipated regime with the surging friction force and huge plowing steps. This sudden transition is much earlier occurring for the larger patterns than the smaller ones, where a new parameter of the critical normal load is proposed to describe this distinction. It suggests that the smaller moiré patterns have higher resistance against the normal load, which may be attributed to a strong in-plane stiffness and therefore a weak puckering effect.