"Tailor-Made"
inhibitors in crystal growth: a Monte Carlo simulation study
Author(s): van Enckevort WJP (van
Enckevort, Willem J. P.), Los JH
(Los, Jan H.)
JOURNAL OF PHYSICAL CHEMISTRY
C Volume:
112 Issue: 16 Pages:
6380-6389
Abstract:
The impact of tailor-made inhibitors,
that is, additives with
horizontal and downward bonds similar to the growth units but with
weaker or repelling bonds upward, on the growth and etching of crystals
is studied. Kinetic Monte Carlo simulations applied to the {001} face
of the solid-on-solid Kossel crystal are used. For thermally roughened
faces, the measured reduction of growth rate as a function of inhibitor
concentration and bond strength is described by a simple analytical
model, considering an averaged blocking efficiency for each inhibitor
molecule adsorbed on the crystal surface. In contrast to the rough
faces, for flat faces, growth kinetics is no longer linear, and a dead
supersaturation zone develops, where growth is almost blocked. The
width of the dead zone as a function of step free energy and inhibitor
concentration in the mother phase largely follows Cabrera and
Vermilyea's theory of step pinning by adsorbed impurities.
Concentration measurements show that the fraction of inhibitor
molecules incorporated into the crystal lattice increases for
increasing supersaturation up to a maximum value at the end of the dead
zone, after which it diminishes again. Etching of flat faces again
reveals nonlinear kinetics and the presence of a dead undersaturation
zone, where etching is prohibited. This observation is explained using
a one-dimensional model, in which the process of etching is considered
as a stripping off of surface steps by kink propagation in the presence
of inhibitor molecules.