Since the realization that the tips of scanning probe microscopes can inter
act with atoms at surfaces, there has been much interest in the possibility
of building or modifying nanostructures or molecules directly from single
atoms(1). Individual large molecules can be positioned on surfaces(2-4), an
d atoms can be transferred controllably between the sample and probe tip(5,
6). The most complex structures(7-11) are produced at cryogenic temperature
s by sliding atoms across a surface to chosen sites. But there are problems
in manipulating atoms laterally at higher temperatures-atoms that are suff
iciently well bound to a surface to be stable at higher temperatures requir
e a stronger tip interaction to be moved. This situation differs significan
tly from the idealized weakly interacting tips(12,13) of scanning tunnellin
g or atomic force microscopes. Here we demonstrate that precise positioning
of atoms on a copper surface is possible at room temperature. The triggeri
ng mechanism for the atomic motion unexpectedly depends on the tunnelling c
urrent density, rather than the electric field or proximity of tip and surf
ace.