  We report numerical investigations of a three-dimensional model of diffusive
growth of fine particles, the internal structure of which corresponds to
different crystal lattices. A growing cluster (particle) is immersed in, and
exchanges monomer building blocks with a surrounding medium of diffusing
(off-lattice) monomers. On-surface dynamics of the latter is accounted for by
allowing, in addition to detachment, monomer motion to the neighboring vacant
crystal sites, according to probabilistic rules mimicking local thermalization.
The key new feature of our model is the focus on the growth of a single
cluster, emerging as a crystalline core, without development of defects that
can control large-scale growth modes. This single, defect-free core growth is
imposed by the specific dynamical rules assumed. Our results offer a possible
explanation of the experimentally observed shape uniformity, i.e., fixed,
approximately even-sized proportions, in synthesis of uniform colloids and
nanoparticles. We demonstrate the basic principles of well-defined particle
shape emergence in such growth. Specifically, several shapes are possible for a
given crystal structure. Formation of shapes that follow the crystal symmetry
and are uniform, can be a result of the nonequilibrium nature of the growth
process. The shape of a growing particle can be controlled by varying the
relative rates of kinetic processes, as well as by adjusting the concentration
of monomers in the surrounding medium.
