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2-D Materials

2-D crystals grow with no energy barrier

Counterintuitive finding in row-by-row growth mechanism suggests ways of coaxing hard-to-crystallize molecules to form ordered films

by Mitch Jacoby
December 16, 2018 | A version of this story appeared in Volume 96, Issue 49

Molecular model of peptides forming an ordered structure.
Credit: PNNL
By assembling a crystal (yellow) one row at a time, peptides bypass the energy barrier usually associated with crystal nucleation.
Video of peptides forming an ordered structure.
Credit: Science
An atomic force microscopy study finds that when 2-D crystals grow by adding one row at a time, as seen here with peptides assembling on a surface, the usual energy barrier disappears.

Scientists have been growing and studying crystals for so long, you would think the ordered structures had given up all their secrets long ago. Not so. A team of researchers has just discovered that the energy barrier that usually needs to be surmounted by a tiny crystal as it nucleates and grows can be bypassed if the structure forms as a 2-D crystal that grows by adding one row at a time (Science 2018, DOI: 10.1126/science.aau4146). The finding may lead to new strategies for engineering crystalline films used in microelectronic devices. Led by Yu Huang of the University of California, Los Angeles, and James De Yoreo of Pacific Northwest National Laboratory, the team used atomic force microscopy and computations to study the mechanism by which peptides in solution form chains and add, row by row, to a growing peptide crystal on a molybdenum disulfide surface. Conventional wisdom indicates that the energy cost of forming an interface between a crystal nucleus and the surrounding liquid outweighs the energy savings earned by enlarging the crystal’s interior. The zero-barrier finding may seem counterintuitive to crystal growers, the team notes, but it was predicted more than 100 years ago by thermodynamics pioneer J. Willard Gibbs.


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