Illusory spirals and loops in crystal growth.

Proceedings of the National Academy of Sciences of the United States of America (2013-10-09)
Alexander G Shtukenberg, Zina Zhu, Zhihua An, Misha Bhandari, Pengcheng Song, Bart Kahr, Michael D Ward
RESUMEN

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 pathological L-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.

MATERIALES
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Sigma-Aldrich
L-Cystine, ≥98% (TLC), crystalline
Sigma-Aldrich
L-Cystine, from non-animal source, meets EP testing specifications, suitable for cell culture, 98.5-101.0%
Sigma-Aldrich
L-Cystine, ≥99.7% (TLC)
SAFC
L-Cystine
SAFC
L-Cystine
Supelco
L-Cystine, certified reference material, TraceCERT®
Cystine, European Pharmacopoeia (EP) Reference Standard
Sigma-Aldrich
L-Cystine, produced by Wacker Chemie AG, Burghausen, Germany, ≥98.5%