DIRECT OBSERVATION OF PLATINUM ATOMS ON PT(111) CLUSTERS

Using a helium cooled field ion microscope (FIM), it has been possible to observe individual platinum atoms on Pt(111) surfaces and clusters without affecting atomic behavior. Pt atoms diffuse at temperatures T greater than or equal to 80 K, and occupy fee sites exclusively; hcp sites are filled only by depositing atoms on a cold Pt(111) surface (T similar to 20 K). The two different binding sites are readily identif iable by the characteristic orientation of the triangular image spot p roduced by the adatom, even though the distribution of intensity over the image spot is sensitively dependent upon the voltage. Observations of the motion of Pt atoms on top of clusters reveal behavior much mor e interesting than assumed in theories of crystal growth. At temperatu res T less than or equal to 95 K, Pt atoms diffusing over the central region of platinum clusters are prevented from reaching the cluster ed ges. The zone close to the cluster edges similar to 3 nearest-neighbor spacings wide can only be penetrated at higher temperatures. At these temperatures, Pt adatoms rapidly diffuse to the edge, where they are trapped, leaving this outer zone empty of atoms. Along the edges, plat inum atoms diffuse quite rapidly at temperatures much below those requ ired for incorporation into the crystal.