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44958-AC5
Step-Edge Barriers on Channeled Surfaces

Gert Ehrlich, University of Illinois (Urbana-Champaign)

         As part of our effort to examine bounded surfaces, detailed analyses have been carried out of adatom diffusion we have observed with the field ion microscope on a dislocated W(110) surface. These analyses have revealed unusual and quite unexpected behavior. On a defect-free surface, the preferred path for diffusion is along the <111> directions, with both equally probable. It turned out that this is not true on the dislocated surface. The mean-square displacement along [ ] amounted to 4.00±0.49 [a /2]², where a is the lattice constant, but less than one fourth of that, 0.93±0.12 [a /2]², in the [ ] direction. Also surprising is the fact that on the dislocated surface, but not on a flat one, diffusivities differ remarkably with location. On the lower ramp of the dislocated surface, on the right, movement is more than three times faster than on the upper ramp, on the left. These effects, as well as the fact apparent in Fig. 1 that  [ ] lines along occupied surface sites are tilted with respect to the lines on the left, are very likely the result of strains around the dislocation core which have not yet been worked out.

We have also analyzed the behavior of asymmetric random walks on a surface and derived the moments of the displacement. In a process illustrated in Fig. 2, jumps to nearest-neighbor positions at the right occur at the rate and to the left at the rate , together with second-neighbor jumps to the right and  to the left. The second moment, for example, in such a walk turns out to be

                           ,

but higher moments have been obtained as well. Also derived has been the distribution of displacements,

                  ;

a thorough analysis of diffusion processes is now possible.

G. Antczak and P. Joźwik, Langmuir 24, 9970 (2008)

G. Antczak and G. Ehrlich, J. Chem. Phys. In Press (2008)

Fig. 1. Map of surface sites occupied by adatom in diffusion at 340 K. a) Grids associated with the  [ ] direction at the right are tilted with respect to those on the left. b) Grid lines in the [ ] direction are parallel, with the right matching the left.

Fig. 2. Schematic of a one-dimensional random walk, showing jump rates to nearest as well as next-nearest neighbors.

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