Lab researchers have invented a thin, membrane-like film that grows "totally aligned" crystals, similar to those that make up oyster pearls, seashells, bones, teeth, and other natural, mineral-rich structures. Clusters of tiny, calcium carbonate crystals, all oriented in the same direction, spontaneously form on the film when a calcium carbonate solution is dropped on the film surface.
Materials scientists Deborah Charych and Amir Berman, and colleagues in the biomolecular materials program at LBNL's Center for Advanced Materials, reported the research in the July 28 issue of the journal Science.
The real pearls of the technology may be gentler, less expensive ways to produce the aligned materials prized by the electronics industry. Current methods of making certain hi-tech, crystalline ceramics, for instance, require heating metal ingredients to several hundred degrees Celsius, followed by a complex cooling process. Thin films, Charych says, have the potential to do the same work, but at room temperature and with water-based ingredients.
The aligned crystals made with such films could also be a valuable tool for basic research on materials structure. "By hitting aligned crystals with an aligned beam of light--for instance, from a synchrotron light source--it should be possible to probe their electronic structure in new ways," Charych says.
At the molecular level, crystals are models of organization. They are made of symmetrical layers of atoms stacked atop one another, like apples at a produce market. The regular arrangement of the atoms give rise, on a large scale, to a crystal's distinctive shape and smooth, flat faces.
Nature uses collections of tiny crystal grains to build shells, skeletons, and other hard structures. The crystals are grown on beds of proteins. Mollusks, for instance, use protein beds to lay down successive layers of calcium carbonate crystals for shells; an oyster's pearl is crafted in a similar layer-by-layer fashion around a grain of sand.
The key to making the most of crystals for structure is to direct how the grains form relative to one another among the proteins. When crystal grains are aligned--with their faces oriented in the same direction like musicians in a marching band--the resulting material is much stronger. This is one reason chalk (made of unaligned calcium carbonate) is brittle, while sea shells (made of aligned calcium carbonate and protein) are hard.
Materials scientists have tried to mimic nature's crystals by substituting the protein beds with simple acidic films. The films are made of molecular chains with charged heads and long, uncharged tails, similar to the building blocks of cell membranes. On a water surface, such chains bunch together to form thin films, with their charged heads ordered regularly on the film surface. The regularity of the heads provides a foundation for mineral atoms to crystallize.
Until now, researchers have been able to match nature only to a limited extent, growing partially aligned crystals. The crystals would all grow with the same face against the film surface, but with their other faces pointing in different directions.
Charych and Berman were able to grow totally aligned crystals by making a film made of molecules that acted cooperatively as the crystals formed. They built their film with a type of molecule called a polydiacetylene (PDA), which has a reactive bond in its uncharged tail. When PDA films are exposed to ultra-violet light, the bonds connect to one another--that is, the film polymerizes.
A crystal that forms atop one area of a PDA film causes that part of the film's structure to shift slightly. But because the chains of film are connected, the surrounding chains shift in tandem. This structural cooperation means that crystals that form on the film are in total alignment.
The researchers knew the film's structure was changing shape because the crystals caused the film to turn from blue to red. PDA films are special from other thin films in that the type of light wavelength they transmit is sensitive to changes in their underlying surface.
"We've shown that the film-crystal interface is a dynamic system," Charych says. "The color change tells us the film is not passively sitting there while the crystal forms. It actually reorganizes itself to optimize the best fit for the growing crystal."
Charych has previously used similar films to create a simple, color-based test for the flu virus. In the flu test, PDA films were topped with sialic acid sugars, the cell-surface molecules to which flu viruses bind when infecting human cells. Flu attachment to the sialic acids caused the film to change color, signaling the presence of virus.