= 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.<Positioning single atoms with a scanning tunnelling microscope by Don Eigler and Erhard Schweizer>[1990 abstract: low-temperature positioning of individual xenon atoms on single-crystal nickel]{ref}
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