Mechanisms for Sliding Friction of Single Molecules Using Scanning Tunneling Microscopy at Room Temperature

We discuss the controlled translation of individual molecules across a surface in two dimensions under ultrahigh vacuum conditions using scanning tunneling microscopy (STM). Based on STM observations and STM elastic scattering quantum chemical (ESQC) plus molecular mechanic calculations, a number of key features for translation are elucidated including the role of molecular flexure. In particular we suggest that single molecule motion via slip-stick, rolling, sliding, and jumping actions can be produced and visualized at the single molecule limit. Additionally we observe ``wear'' at the single molecule level where a ligand is removed through the action of pushing the molecule over a step. These unique insights provide the framework for a bottom-up approach to friction accessible to theoretical modeling.