Scientists develop ‘X-ray vision’ technique to see inside crystals

A team of researchers from New York University has created a new way to view crystals by peering inside their structures, similar to X-ray vision. Their new technique, which they aptly named “Crystal Clear “, combines the use of transparent particles and microscopes with lasers that allow scientists to see each unit that makes up the crystal and create dynamic three-dimensional models.

“This is a powerful platform for studying crystals,” says Stefano Sacanna, professor of chemistry at NYU and principal investigator of the study, published in the journal Natural materials.

“Previously, if you looked at a colloidal crystal under a microscope, you could only get an idea of ​​its shape and surface structure. But now we can see inside and know the position of each unit in the structure. “

Atomic crystals are solid materials whose constituent elements are repeatedly and orderly positioned. Occasionally, an atom is missing or out of place, resulting in a defect. The arrangement of atoms and defects is what creates different crystalline materials – from table salt to diamonds – and gives them their properties.

To study crystals, many scientists, including Sacanna, look to crystals composed of tiny spheres called colloidal particles rather than atoms. Colloidal particles are tiny – often around a micrometer in diameter, tens of times smaller than a human hair – but are much larger than atoms and therefore easier to see under a microscope.

A transparent structure

In their ongoing work to understand how colloidal crystals form, researchers have recognized the need to see inside these structures. Led by Shihao Zang, a Ph.D. student in Sacanna’s lab and first author of the study, the team set out to create a method to visualize the building blocks inside a crystal.

They first developed transparent colloidal particles and added dye molecules to mark them, allowing each particle to be distinguished under a microscope thanks to their fluorescence.

A microscope alone would not allow researchers to see the inside of a crystal. So they turned to an imaging technique called confocal microscopy, which uses a laser beam that scans the material to produce targeted fluorescence from the dye molecules.

This reveals each two-dimensional plane of a crystal, which can be stacked on top of each other to build a three-dimensional digital model and identify the location of each particle. The models can be turned, cut and taken apart to examine the inside of the crystals for possible defects.

In a series of experiments, researchers used this imaging method on crystals that form when two crystals of the same type grow together, a phenomenon known as “twinning.”

When they looked inside models of crystals with structures equivalent to table salt or an alloy of copper and gold, they were able to see the common plane of adjacent crystals, a defect that gives rise to these particular forms. This shared plan revealed the molecular origin of twinning.

Moving crystals

In addition to observing static crystals, this new technique allows scientists to visualize crystals as they change. For example, what happens when crystals melt: do the particles rearrange and the defects move? In an experiment in which the researchers melted a crystal with the structure of the mineral salt cesium chloride, they were surprised to find that the defects were stable and did not move as expected.

To validate their experiments on static and dynamic crystals, the team also used computer simulations to create crystals with the same characteristics, confirming that their “Crystal Clear” method accurately captured what was there. inside the crystals.

“In a sense, we’re trying to put our own simulations out of business with this experiment: If you can see inside the crystal, you might not need simulations anymore,” jokes Glen Hocky, assistant professor of chemistry. at NYU, a faculty member of the Simons Center for Computational Physical Chemistry at NYU and co-corresponding author of the study.

Now that scientists have a method to view the inside of crystals, they can more easily study their chemical history and how they form, which could pave the way for building better crystals and developing materials photonics that interact with light.

“Being able to see the inside of crystals gives us a better idea of ​​how the crystallization process works and can perhaps help us optimize the crystal growth process by design,” Sacanna adds.