New Technique to Visualize 3D Structure of Crystals

An international group of researchers from New York University has developed a novel method of visualizing them that is similar to having X-Ray vision. With this new method, which they have fittingly termed “Crystal Clear,” scientists can view individual crystal units and generate dynamic three-dimensional models by combining the use of transparent particles, microscopes, and lasers. The study was published in the journal Nature Materials.

This is a powerful platform for studying crystals, previously, if you looked at a colloidal crystal through a microscope, you could only get a sense of its shape and structure of the surface. But we can now see inside and know the position of every unit in the structure.

Stefano Sacanna, Study Principal Investigator and Professor, Department of Chemistry, New York University

Atomic crystals are solid materials whose constituents are arranged in a repetitive, ordered pattern. Sometimes, an atom goes missing or out of position, causing a flaw. The arrangement of atoms and imperfections determines the characteristics of different crystalline materials, from table salt to diamonds.

Many scientists, including Sacanna, examine crystals made up of tiny spheres known as colloidal particles rather than atoms. Colloidal particles are tiny—often around a micrometer in diameter or dozens of times smaller than a human hair—but considerably bigger than atoms, making them easier to observe under a microscope.

A See-Through Structure

The researchers realized the importance of seeing colloidal crystals as part of their continuing investigation into how they originate. The team, led by Shihao Zang, a PhD student in Sacanna’s lab and the study’s first author, set out to develop a method for seeing the building blocks inside crystals.

They initially created transparent colloidal particles and labeled them with dye molecules, allowing each particle to be distinguished under a microscope based on their fluorescence.

A microscope alone would not allow the researchers to look within a crystal, so they used confocal microscopy. This imaging method employs a laser beam to scan through the material, producing targeted fluorescence from dye molecules.

This displays each two-dimensional plane of a crystal, which can be placed on top of one another to create a three-dimensional digital model and determine the position of each particle. The models can be rotated, cut, and disassembled to examine the crystals and identify flaws.

In one series of examinations, the researchers applied their imaging technology to crystals formed when two of the same type of crystal grow together—a process known as “twinning.” When scientists investigated inside models of crystals with structures similar to table salt or a copper-gold alloy, they discovered the common plane of the adjoined crystals. This flaw causes these specific shapes. This shared plane revealed the molecular origin of twinning.

Crystals in Motion

In addition to studying static crystals, this new technology enables scientists to see crystals as they evolve. For example, what occurs when crystals melt? Do particles reorganize and defects move? In an experiment in which the researchers melted a crystal with the structure of the mineral salt cesium chloride, they were shocked to see that the defects remained stable and did not migrate as predicted.

The researchers also employed computer simulations to construct crystals with the same properties to evaluate their experiments on static and dynamic crystals. This confirmed that their “Crystal Clear” approach accurately captured what is within crystals.

In a sense, we are trying to put our own simulations out of business with this experiment if you can see inside the crystal, you may not need simulations anymore.

Glen Hocky, Study Co-Corresponding Author and Study Co-Corresponding Author, Department of Chemistry, New York University

Hocky is also a faculty member in the Simons Center for Computational Physical Chemistry at NYU.

Now that scientists can visualize the interior of crystals, they can better investigate their chemical history and formation, perhaps paving the way for better crystals and photonic materials that interact with light.

Sacanna added, “Being able to see inside crystals gives us greater insight into how the crystallization process works and can perhaps help us to optimize the process of growing crystals by design.”

Study co-authors include Adam Hauser and Sanjib Paul of NYU. The study was funded by the US Army Research Office, with additional support from the National Institute of Health, and used NYU IT High Performance Computing resources, including those supported by the Simons Center for Computational Physical Chemistry at NYU.

Journal Reference:

‌Zang, S., et al. (2024) Enabling three-dimensional real-space analysis of ionic colloidal crystallization. Nature Materials. doi:10.1038/s41563-024-01917-w