A new technique that produces 3D models of individual crystals has opened a window for scientists to see the subtle deviations that appear in their perfect designs.
Researchers from New York University (NYU) went back to the drawing board on how to look deep inside solids made of repeating units and determine how they grow.
With a short wavelength roughly the same size as many of the repeating units that make up crystals, X-rays have long allowed scientists to infer how a crystal’s components fit together by measuring the angle at which the rays are scattered.
However, for all its ingenuity, X-ray crystallography has its limits, which are well summed up by the introduction to a new paper published in Materials of Nature this month: “Molecular crystal structures are identified using scattering techniques because we cannot see inside them.”
The paper describes a new technique that promises to eventually change that fact—though not for crystals made up of repeating units of individual atoms.
Instead, it deals with crystals composed of patterns based on colloidal particles, which are large enough to be seen under a conventional microscope and manipulated in a way that would be impossible for atoms.
The study of such crystals has allowed advances in the understanding of crystal dynamics. The researchers cite experiments with colloidal structures that shed light on the formation and evolution of dislocations within crystal structures.
Like X-ray crystallography, this technique has limits. Difficulties in finding reliable ways to image relatively complex colloidal crystals have meant that their study has so far been largely limited to thin and simple structures formed from a single constituent particle.
Many atomic-scale crystals, in contrast, are composed of two or more elements and form complex, three-dimensional structures.
The new technique developed by the NYU team promises to allow the study of colloidal analogues of these relatively complex lattices. The technique builds on some of the team’s previous work, in which they developed a process called “polymer attenuated coulombic self-assembly,” or PACS.
PACS uses the individual electrical charges of colloidal particles to attract them into crystal lattices, allowing the reliable construction of binary colloidal crystals—the crystals formed from molecules composed of two different types of particles in the same way that, say, table salt crystals form from sodium and chlorine.
The new study demonstrates the effectiveness of seeding these individual colloidal particles with a fluorescent dye to distinguish one species from another—and, crucially, continues to do so after they have formed crystals. This means that scientists can finally “look inside” a fully formed crystal and make direct observations of its interior.
As the researchers report, “We are able to distinguish all particles within a binary ionic crystal and reconstruct the complete internal 3D structure down to 200 layers deep.”
The NYU team reports some new findings they’ve already gleaned from the observations.
The process known as “twinning,” where two crystal lattices align in such a way that they share constituent parts along a common plane, has long been of interest to scientists.
The researchers describe the creation of colloidal crystals that reproduce the atomic-scale cubic structures of several different minerals: the aforementioned alternating lattice of sodium and chlorine that forms table salt; cesium chloride, where eight chlorine atoms form a “cage” around a single cesium atom; and the somewhat more exotic example of auricupride, a compound of copper and gold, where each face of a cubic lattice of gold atoms is dotted with a single copper atom, like a chair where each face is one.
In each case, the team was able to make direct observations of the evolution of twinned crystals, thus providing a direct experimental observation of how such structures arise.
“This direct observation unambiguously reveals the intrinsic intricacies of the crystal structure, elucidating the relationship between particle interactions and the macroscopic crystal shape, including the occurrence and influence of defects and twinning,” the researchers report.
The group is looking forward to unlocking the mysteries of crystals, more than 100 years after the discovery of X-rays gave mankind the first hint of the intricacies of crystal structure.
The research was published in Materials of Nature.
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