Diffusion of Pd adatoms on W surfaces and their interaction with steps

The diffusion of individual Pd adatoms on the (1 1 0), (2 1 1), and (1 1 1) surfaces of W has been investigated by field ion microscopy. Besides the most fundamental process, i.e. diffusion of single atoms across perfectly defined single-crystal terraces, we have also studied an equally important surface atomic process, i.e. diffusion of atoms in the vicinity of surface steps. The behavior of an adatom approaching the step from the upper terrace has also been studied and discussed. The extra reflective barriers for the step-down motion of Pd atoms on W(1 1 0), (2 1 1), and (1 1 1) surfaces are determined to be 0.27±0.02, 0.29±0.03, 0.61±0.01 eV, respectively. The behavior of adatoms approaching the step from the lower terraces has also been observed. The activation energy for the step-up motion of Pd atom at W(1 1 1) steps is derived to be 1.84±0.07 eV. The potential energy of moving toward the W(1 1 1) pole of the tip exhibits a free energy anisotropy of 0.013 eV/4.47 Å. Such adatom behavior in relation to pyramidal facet growth is also discussed.