Abstract:
The morphology and the growth mechanism of the {100} surfaces of 2Zn-insulin
crystals were investigated by in situ and ex situ atomic force microscopy
(AFM). The dominant growth mechanism of the insulin crystals is the formation
and lateral expansion of two-dimensional (2D) nuclei on the growing surfaces.
Several types of dislocation spirals, such as conventional single, double
and triple spirals as well as spirals generated by a planar defect, were
also observed on the surfaces. Besides those, growth by three-dimensional
(3D) insulin islands with mutilayer stacks, which suddenly appeared on
the surface, occasionally occurred. Every step was separated from each
other with a height of about 3.5 nm, which corresponds to that of a single
layer composed of insulin hexamers. An anisotropy of step velocity was
revealed from a series of sequential images of growing crystal surfaces.
The step velocity of the multilayer stacks was about 50% of that of a single
step, which implies that only little of the diffusion fields exist. On
the other hand, the degeneration behavior of the multilayer steps provided
evidence for the Schwoebel effect, i.e. surface diffusion slightly operates
as the mechanism of solute transport towards the steps. Some topics, such
as the growth rate controlling factor, aggregation phenomena and the occurrence
of point and planar defects, are also discussed on the basis of the observations.