Ledge site

Surface diffusion can be studied with a wide variety of methods using both macroscopic and microscopic techniques of great diversity.98 Basically three methods can be used. One measures the time dependence of the concentration profile of diffusing atoms, one the time correlation of the concentration fluctuations, or the fluctuations of the number of diffusion atoms within a specified area, and one the mean square displacement, or the second moment, of a diffusing atom. When macroscopic techniques are used to study surface diffusion, diffusion parameters are usually derived from the rate of change of the shape of a sharply structured microscopic object, or from the rate of advancement of a sharply defined boundary of an adsorption layer, produced either by using a shadowed deposition method or by fast pulsed-laser thermal desorption of an area covered with an adsorbed species. The derived diffusion parameters really describe the overall effect of many different atomic steps, such as the formation of adatoms from kink sites, ledge sites... 

Carter and Ward have identified a surface-mediated nucleation mechanism that involves a geometric shape match between planes of a ledge site on the substrate and planes of prenucleation aggregates. They have applied these concepts to the directed nucleation of polymorphs. ° This work provides us with the attractive possibility that a library of organic seeds can be used to control polymorphism, or to search for unknown polymorphs. Interpretations of molecular assemblies between solute and additive (seeds, substrate surfaces, excipients, and solvents) molecules may prove beneficial in selectively crystallizing or screening polymorphic systems. 

Model of a metal surface showing atoms at (a) surface sites (b) kink sites (c) ledge sites, and (d) adsorption sites [40]. 

Listed in Table 3.1 are values of the adsorption energies AG determined for several different crystalline systems. In general, adsorption energies of impurities at ledge sites are found to be on... 

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