Surface Diffusion by AN Atomic Exchange Mechanism.
Abstract
The motion of adatoms on the iridium (110) plane has been investigated using the field ion microscope (FIM). For both iridium and tungsten adatoms, motion is predominantly across the {1(')10} channels of close-packed atoms constituting the {110} plane on an fcc lattice. Quantitative experiments have established that for iridium, cross-channel motion occurs with an activation energy of 17 kcal/mole and a prefactor of 10('-6) cm('2)/sec. Since cross-channel motion was not expected, the field ion microscope has been used to investigate the structure of the surface. By direct observation, the ridge spacing is shown to be approximately twice the bulk value; this and the existence of two adsorption sites indicate that Ir(110) surfaces as formed in the FIM are (2 x 1) reconstructed, a result similar to what has been found in low-energy electron diffraction (LEED) studies. However, a structure somewhat different from the standard LEED reconstruction is proposed to account for the field ion microscope measurements. By directly measuring the mass of an adatom before and after diffusion in an atom probe, it has been established that in cross-channel diffusion for tungsten deposited on iridium (110) the adatom exchanges with one of the substrate atoms. Although this effect was predicted by lattice statics and molecular dynamics calculations (which do not take reconstruction into account), other predictions are at variance with observations on iridium (110). The energetics of diffusion for all systems on which cross-channel diffusion has been observed are compared. However, right now there does not appear to be either a simple explanation of why exchange occurs on some metals but not on others, or a pattern by which exchange could be predicted.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- March 1982
- Bibcode:
- 1982PhDT........53W
- Keywords:
-
- Physics: Condensed Matter