Atoms wandering on surfaceslead complex lives. For example, they face many restrictions on their freedom to move. High walls sometimes leave only one road open, and that road may have checkpoints. Such walls and checkpoints are a consequence of the crystal structure of the surface and the interaction between the surface atoms and the wandering atom. The crystal structure in turn is controlled by the nature of the bonding—metallic, covalent, ionic—between atoms in the crystal.
When atoms deposited on a surface move easily, they find the optimum crystal sites and form high‐quality films, a process now visible in the scanning tunneling microscope.