A new technique that creates 3D models of individual crystals has opened a window into which scientists can see subtle deviations in their otherwise perfect patterns. Researchers at New York University (NYU) went back to the drawing board to look deep into solids made up of repeated parts and determine how they grow.

Because X-rays have a short wavelength that is about the same size as most of the repeating units that make up crystals, they have long allowed scientists to understand how the components of a crystal fit together by measuring the angle at which the rays are refracted.

However, for all its ingenuity, X-ray crystallography has its limits, and this is summed up quite neatly in the opening sentence of a new paper published this month. Nature Supplies: “Molecular crystal structures are identified by scattering techniques because we cannot see inside them.”

The paper describes a new technique that promises to eventually change that, though not for crystals composed of repeating single units of atoms.

Instead, they are crystals composed of patterns based on colloidal particles that are large enough to be seen under an ordinary microscope and manipulated in ways impossible for atoms.

The study of such crystals has made it possible to make progress in understanding the dynamics of crystals. Researchers cite experiments with colloidal structures that shed light on the formation and evolution of dislocations in crystal structures.

Like X-ray crystallography, this method has its limitations. Difficulties in finding reliable ways to image relatively complex colloidal crystals have meant that their research has so far been largely limited to thin, simple structures composed of a single component particle. Many atomic crystals, in contrast, consist 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 counterparts of these relatively complex lattices. The technique builds on some of the team’s previous work, where they developed a process called polymer-attenuated Coulomb self-assembly, or PACS.

PACS uses the electrical charges of individual colloidal particles to attract them into crystal lattices, enabling the reliable formation of binary colloidal crystals (crystals formed by molecules composed of two different types of particles, in the same way table salt crystals are formed, for example). . from sodium and chlorine.

The new study demonstrates the effectiveness of seeding these individual colloidal particles with fluorescent dye to distinguish one species from another, and importantly, they continue to do so once they form crystals. This means scientists can finally “look inside” a fully formed crystal and directly observe its interior.

As the researchers report, “we can distinguish all particles in the binary ionic crystal and reconstruct the entire internal 3D structure down to a depth of ~200 layers.” The NYU team reports several new findings from the observations. The process known as “twinning,” in which the lattices of two crystals align so that they share constituent parts along a common plane, has long intrigued scientists.

Researchers describe the formation of colloidal crystals that replicate the atomic-scale cubic structures of several different minerals: the aforementioned alternating lattice of sodium and chlorine that forms table salt; cesium chloride, in which eight chlorine atoms form a “cage” around one cesium atom; and the slightly more exotic example of auricuprid, a copper-gold compound in which each face of a cubic lattice of gold atoms is marked with a single copper atom, like a cube in which each face is unique.

In each case, the team was able to make direct observations of the evolution of twin crystals, thus providing a direct experimental observation of how such structures arise.

“This direct observation clearly reveals the internal intricacies of the crystal structure, elucidating the relationship between particle interactions and macroscopic crystal form, including the appearance and effect of defects and twins,” the researchers wrote.

The group looks forward to unraveling the mysteries of crystals, more than 100 years after the discovery of X-rays that gave humanity the first clue to the intricacies of crystal structure.