Whereas on the clean Pt(111) surface Pt grows in a multilayer (3D) mod
e at 300 and 400 K, preadsorption of an ordered p(2 x 2) oxygen overla
yer leads to a high quality layer-by-layer (2D) growth with the oxygen
floating on the film. In contrast to other surfactants oxygen can be
completely removed from the film surface at the growth temperature. It
is demonstrated that the presence of oxygen reduces the barrier heigh
t for the motion of Pt adatoms across step edges. This facilitates the
interlayer mass transport and thus the layer-by-layer growth.