A new technique that creates 3D models of individual crystals has opened a window for scientists to see the subtle variations that appear in their otherwise perfect patterns.
New York University (NYU) researchers have gone back to the drawing board to peer deep into 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 bent.
However, for all its ingenuity, X-ray crystallography has its limits, which are rather neatly summarized by the opening sentence of a new paper published in Natural materials this month: “Molecular crystal structures are identified using scattering techniques because we cannot see into them.”
The paper describes a new technique that promises to finally change that—though not for crystals composed of repeating units of single atoms.
Instead, it refers to crystals composed of patterns based on colloidal particles that are large enough to be seen under a conventional microscope and manipulated in ways that would not be possible for atoms.
The study of such crystals has enabled advances in the understanding of crystal dynamics. Scientists cite experiments with colloidal structures that shed light on the origin and evolution of dislocations in crystal structures.
Like X-ray crystallography, this technique has its 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, simple structures formed from a single fundamental particle.
Many atomic-scale crystals, on the other hand, are made up of two or more elements and form complex three-dimensional structures.
A new technique pioneered by the NYU team promises to enable 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 electrical charges of individual colloidal particles to draw them into crystal lattices, which enables the reliable construction of binary colloidal crystals – formed crystals 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, continue to do so once they form crystals. This means that scientists can finally “look inside” a fully formed crystal and directly observe its innards.
As the researchers report, “We are able to resolve all particles in a binary ionic crystal and reconstruct the full internal 3D structure down to a depth of ~200 layers.”
The NYU team reports several new findings they’ve already gleaned from the observations.
A process known as “twinning,” in which the lattices of two crystals align in such a way that they share components 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 makes up table salt; cesium chloride, where eight chlorine atoms form “cages” around a single cesium atom; and the somewhat more exotic example of auricupride, a copper-gold compound where each face of a cubic lattice of gold atoms is punctuated by a single copper atom, like a cube where each side is one.
In each case, the team was able to directly observe the evolution of twinned crystals, thus providing a direct experimental observation of how such structures form.
“This direct observation unambiguously reveals the intrinsic complexities of the crystal structure and elucidates the relationship between particle interactions and the macroscopic crystal form, including the formation and impact of defects and twins,” the researchers report.
The group is looking forward to unlocking the secrets of crystals, more than 100 years after the discovery of X-rays gave mankind the first hint of the complexity of crystalline structure.
The research was published in Natural materials.