NUE 2003-2004

Initial Growth Spiral

This image shows a growth spiral on the surface of a natural graphite crystal from Namibia. There is also a self-assembled array of micro-scale growth hillocks (low hills) at the corners of the spiral's steps. A possible (and as yet unverified) mechanism for the formation of these growth hillocks is based on impurities. As illustrated in the animation, imagine that there are impurities on the surface of the graphite that are mobile (free to diffuse around the surface).

Middle Stage Growth Spiral

Growth of the graphite takes place primarily at the steps of the spiral. As carbon atoms attach at the steps, the steps develop kinks which appear to move along the step as carbon atoms attach at the kinks. The advancing kinks and steps move the impurities out of the way (crystallization purification) and tend to concentrate the impurities at the spiral corners. As the concentration of the impurities increases at the corners, they interfere with the spiral growth.

Outer Growth Spiral

Eventually they can "pin" the step at the corner. As the pinned impurities eventually get covered in carbon layers they could nucleate screw dislocations (atomic-scale defects), which could lead to hillock formation at the step corners.