Kahr and Ward Groups Unravel Mysteries of Crystal SpiralsNYU Chemistry Professors Bart Kahr and Michael D. Ward report new findings in the Proceedings of the National Academy of Sciences (PNAS, October 7, 2013 doi:10.1073) in a study called, "Illusory Spirals and Loops in Crystal Growth." Coming out of the Molecular Design Institute in the Department of Chemistry, the research was spearheaded by Research Scientist Alexander Shtukenberg, and co-authored by Zina Zhu, Zhihua An, Misha Bhandari and Pengcheng Song. The work provides an example of crystal growth at the nanoscale, wherein features that appear to contradict 50 years of crystal growth theory actually turn out to support that theory, if features are unmasked at the molecular scale. The research is highlighted on the NYU Research site, as "Crystal Mysteries Spiral Deeper."
The theory of dislocation-controlled crystal growth identifies a continuous spiral step with an emergent lattice displacement on a crystal surface; a mechanistic corollary is that closely spaced, oppositely winding spirals merge to form concentric loops. In situ atomic force microscopy of step propagation on pathologicalL-cystine crystals did indeed show spirals and islands with step heights of one lattice displacement. We show by analysis of the rates of growth of smaller steps only one molecule high that the major morphological spirals and loops are actually consequences of the bunching of the smaller steps. The morphology of the bunched steps actually inverts the predictions of the theory: Spirals arise from pairs of dislocations, loops from single dislocations. Only through numerical simulation of the growth is it revealed how normal growth of anisotropic layers of molecules within the highly symmetrical crystals can conspire to create features in apparent violation of the classic theory.
This research was supported by the National Science Foundation, the NYU Materials Research for Science & Engineering Center (also NSF), and an NYU Graduate School of Arts and Science Horizon Fellowship, awarded to author Zina Zhu.
|Bart Kahr||Mike Ward||Alexander Shtukenberg||Zina Zhu||Misha Bhandari ||Pengcheng Song||Zhihua An|
NYU Research Site
Updated on 11/18/2013