Fluctuation spectroscopy of step edges on Pt(111); Crossover from bulk to surface diffusion

By step fluctuation spectroscopy, using low-energy electron microscopy LEEM, we investigate step energies and relaxation on clean Pt(111) surface at temperatures above half the melting temperature Tm (range 1190K<T<1520K). The average step stiffness is about 210 meV/nm, weakly temperature dependent, and fairly isotropic with mainly a sixfold angular variation. Consequently, the step free energies are highly isotropic.
Below 1400K the relaxation times vary with wave vector q as q3, and above 1400K as q2, while the observed activation energy changes from ~1.25 to ~2.75eV. A discussion shows that surface diffusion is responsible for the rates at low temperatures, and that bulk vacancy diffusion becomes dominant above 1400K; the surface process is being short-circuited by a faster flow of bulk vacancies. Known bulk diffusion coefficient for Pt is consistent with this interpretation. The analysis find that the surface mass diffusion coefficient is Ds=2-5x10−4 exp(−1.2±0.1 eV/kBT) cm2/s.
It is also observed for Pt(111) at 1400 K and above that neighboring steps react to form multisteps that retain capillary characteristics. We determine the multistep stiffnesses, b~n, and so estimate their line free energies, assumed isotropic.