A new technique that produces 3D models of individual crystals has opened a window for scientists to see the subtle deviations that emerge from their otherwise perfect patterns.
Researchers at New York University (NYU) went back to the drawing board to look deep into solids made up of repeating units and determine how they grow.
With a short wavelength that is about the same size as many of the repeating units that make up crystals, scientists have long been able to use X-rays to deduce how a crystal’s components fit together by measuring the angle at which the rays are bent.
For all its ingenuity, however, X-ray crystallography has its limits, which are summarized rather neatly in the opening sentence of a new paper published in Natural materials this month: “Structures of molecular crystals are identified using scattering techniques, because we cannot look inside them.”
The paper describes a new technique that promises to finally change that – although not for crystals composed of repeating units of individual atoms.
Instead, they are crystals composed of patterns based on colloidal particles, which are large enough to see under a conventional microscope and manipulate in ways that would be impossible for atoms.
Studying such crystals has enabled 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 limitations. Difficulties in finding reliable ways to image relatively complex colloidal crystals have meant that their research to date has been largely limited to thin, simple structures formed from a single constituent particle.
In contrast, many atomic-scale crystals are composed of two or more elements and form complex, three-dimensional structures.
The new technique developed 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 charge of individual colloidal particles to pull them into crystal lattices, allowing the reliable construction of binary colloidal crystals – shaped crystals by molecules composed of two different types of particles in the same way that, for example, crystals of table salt are formed from sodium and chlorine.
The new study shows the effectiveness of seeding these individual colloidal particles with a fluorescent dye to distinguish one species from another – and, crucially, continuing to do so once they have formed crystals. This means that scientists can finally ‘look’ into a fully formed crystal and make direct observations of its innards.
As the researchers report: “We are able to distinguish all particles within a binary ionic crystal and reconstruct the entire internal 3D structure to a depth of ~200 layers.”
The NYU team reports several new findings that they have already gleaned from observations.
The process known as ‘twinning’, in which the lattices of two crystals are aligned so 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 minerals: the aforementioned alternating lattice of sodium and chlorine that forms table salt; cesium chloride, in which eight chlorine atoms form a “cage” around a single cesium atom; and the more exotic example of auricupride, a compound of copper and gold, in which each face of a cubic lattice of gold atoms is interrupted by a single copper atom, like a die with each face being one.
In each case, the team was able to make direct observations of the evolution of linked crystals, providing a direct experimental observation of how such structures form.
“This direct observation unambiguously unravels the internal complexity of the crystal structure, and elucidates the relationship between the particle interactions and the macroscopic crystal shape, including the generation and impact of defects and twinning,” the researchers report.
The group looks forward to unraveling the mysteries of crystals, more than 100 years after the discovery of X-rays gave humanity the first insight into the complexity of crystalline structure.
The research was published in Natural materials.