Whereas on the clean Pt(111) surface Pt grows in a multilayer (3D) mode at 300 and 400 K, preadsorption of an ordered p(2x 2) oxygen overlayer leads to a high quality layer-by-layer (2D) growth with the oxygen Aoating 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 height for the motion of Pt adatoms across step edges. This facilitates the interlayer mass transport and thus the layer-by-layer growth.PACS numbers: 68.55.Jk, 61.16.Ch, 68.55.Ce, 68.55.Ln Recently surfactants have attracted considerable interest in both heteroand homoepitaxial growth by increasing the smoothness of the deposited films [1][2][3][4].

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The nucleation of Pt films grown by ion beam assisted deposition on Pt(111) has been studied by scanning tunneling microscopy. At temperatures T≥200 K, the simultaneous ion bombardment during vapor phase deposition leads to a substantial increase in the island number density caused by nucleation at ion impact induced adatom clusters. This increase is observed even in the higher temperature range where after ion bombardment alone the ion impact induced adatom clusters are no longer present.