The process of crystallization, in which atoms or molecules line up in orderly arrays like soldiers in formation, is the basis for many of the materials that define modern life, including the silicon in microchips and solar cells. But while many useful applications for crystals involve their growth on solid surfaces (rather than in solution), there has been a dearth of good tools for studying this type of growth.
Now, a team of researchers at MIT and Draper has found a way to reproduce the growth of crystals on surfaces, but at a larger scale that makes the process much easier to study and analyze. The new approach is described in a paper in the journal Nature Materials, by Robert Macfarlane and Leonardo Zomberg at MIT, and Diana Lewis PhD ’19 and David Carter at Draper.
Rather than assembling these crystals from actual atoms, the key to making the process easy to observe and quantify was the use of “programmable atom equivalents,” or PAEs, Macfarlane explains. This works because the ways atoms line up into crystal lattices is entirely a matter of geometry and doesn’t rely on the specific chemical or electronic properties of its constituents.
The team used spherical nanoparticles of gold, coated with specially selected single strands of genetically engineered DNA, giving the particles roughly the appearance of Koosh balls. Single DNA strands have the inherent property of attaching themselves tightly to the corresponding reciprocal strands, to form the classic double helix, so this configuration provides a surefire way of getting the particles to align themselves in precisely the desired way.
“If I put a very dense brush of DNA on the particle, it’s going to make as many bonds with as many nearest neighbors as it can,” Macfarlane says. “And if you design everything appropriately and process it correctly, they will form ordered crystal structures.” While that process has been known for some years, this work is the first to apply that principle to study the growth of crystals on surfaces.
Source: “Technique reveals how crystals form on surfaces”, David L. Chandler, MIT News Office