Barrier(less) islands

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Science  07 Dec 2018:
Vol. 362, Issue 6419, pp. 1111
DOI: 10.1126/science.aav7009

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Crystal nuclei—the minute collections of molecules needed to spark crystal growth—are small, short-lived, and generally unobserved. Classical nucleation theory, a model for the earliest stages of crystallization, is an accounting of the free-energy debits for creating an interface between crystals and the medium from which they grow, and the free energy credits for enlarging the interior of the crystal (1, 2). On page 1135 of this issue, Chen et al. (3) report the growth of one-dimensional (1D) peptide crystals in rows on crystalline substrates, which subsequently assemble laterally row by row into films (and ultimately 2D arrays) (see the figure). Growth rates along and perpendicular to the rows, measured by scanning probe microscopy, support the absence of an activation barrier that typically must be surmounted to overcome the surface energy. This unexpected challenge to the classical nucleation theory arises from high-resolution imaging that captures kinetic measurements for very small molecular aggregates (4). No matter the sophistication of our models, looking ever more closely at growing crystals often reveals unanticipated mechanisms (5).