This is all a bit of arcana that
continues to inform our work with materials and the theory holds up
surprisingly well.
It is always good when a perfectly
good theoretical framework holds up in the face of a rigorous empirical test. Imagine generating theory if you were to start
with this piece of empirical support.
It is good work.
Crystal mysteries spiral
deeper
by Staff Writers
New York NY (SPX)
Oct 25, 2013
New York University chemists have discovered crystal growth complexities, which at first glance appeared to confound 50 years of theory and deepened the mystery of how organic crystals form. But, appearances can be deceiving.
Their findings, which appear in the latest
edition of Proceedings of the National Academy of Sciences, have a range of implications
-- from the production of pharmaceuticals and new electronic materials to
unraveling the pathways for kidney stone formation.
The researchers focused on L-cystine
crystals, the chief component of a particularly nefarious kind of kidney stone.
The authors hoped to improve their understanding of how these crystals form and
grow in order to design therapeutic agents that inhibit stone formation.
While the interest in L-cystine crystals is
limited to the biomedical arena, understanding the details of crystal growth,
especially the role of defects -- or imperfections in crystals -- is critical
to the advancement of emerging technologies that aim to use organic crystalline
materials.
Scientists in the Molecular Design Institute
in the NYU Department of Chemistry have been examining defects in crystals
called screw dislocations -- features on the surface of a crystal that resemble
a spiraled ham.
Dislocations were first posed by William
Keith Burton, Nicolas Cabrera, and Sir Frederick Charles Frank in the late
1940s as essential for crystal growth. The so-called BCF theory posited that
crystals with one screw dislocation would form hillocks that resembled a spiral
staircase while those with two screw dislocations would merge and form a
structure similar to a Mayan pyramid -- a series of stacked "island"
surfaces that are closed off from each other.
Using atomic force microscopy, the Molecular
Design Institute team examined both kinds of screw dislocations in L-cystine
crystals at nanoscale resolution. Their results showed exactly the opposite of
what BCF theory predicted -- crystals with one screw dislocation seemed to form
stacked hexagonal "islands" while those with two proximal screw
dislocations produced a six-sided spiral staircase.
A re-examination of these micrographs by
Molecular Design Institute scientist Alexander Shtukenberg, in combination with
computer simulations, served to refine the actual crystal growth sequence and
found that, in fact, BCF theory still held. In other words, while the
crystals' physical appearance seemed at odds with the long-standing theory,
they actually did grow in a manner predicted decades ago.
"These findings are remarkable in that
they didn't, at first glance, make any sense," said NYU Chemistry
Professor Michael Ward, one of the authors of the publication.
"They appeared to contradict 60 years of
thinking about crystal growth, but in fact revealed that crystal growth is at
once elegant and complex, with hidden features that must be extracted if it is
to be understood. More importantly, this example serves as a warning that first
impressions are not always correct."
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