Past exam of the frontiers of experimental condensed matter physics course of the University of Cambridge / 2013 / Paper 1 / 1 / b / Solution

A scanning tunneling microscope can modify an adsorbate's potential landscape as well as image it. First locate the atom with a weakly perturbing tunnel-current setpoint. Bringing the tip closer strengthens tip–atom coupling. In lateral atomic manipulation, moving the tip parallel to the surface can pull or push the atom between adsorption sites; reducing the coupling afterwards leaves it at the chosen location. A change in tunneling current during the motion can signal a jump, and a subsequent low-current image checks the final arrangement.

In vertical atomic manipulation, a suitable tip interaction or voltage pulse transfers an atom between substrate and tip. The tip is repositioned and the atom is deposited elsewhere by changing that interaction again. Electric fields and inelastic tunneling excitation can also change adsorption, induce hopping or modify bonds. The useful mechanism depends on the atom, substrate, tip and bias: there is no universal pulse amplitude that works for every system. Low temperature suppresses thermal diffusion and ultrahigh vacuum helps preserve a clean, stable surface.

A motivating demonstration is IBM in atoms. Don Eigler and Erhard Schweizer used a low-temperature STM to position individual xenon atoms on a nickel surface, constructing a chosen atomic pattern. The 1990 paper reported operation at about $4\mathrm{K}$ with atomic positioning precision. This demonstrates controlled rearrangement of surface adsorbates, rather than making the image alone prove that every STM feature is an isolated atom.^[ref]