Shine a light, build a crystal

(Press-News.org) NYU scientists are using light to precisely control how tiny particles organize themselves into crystals. 

Their research, published in the Cell Press journal Chem, provides a simple and reversible method for forming crystals that can be used to develop a new generation of adaptable materials.

Crystals—from snowflakes and diamonds to the silicon used in electronics—are made up of particles arranged in repeating patterns. To study how crystals form, some scientists use colloidal particles—microscopic spheres suspended in liquid that self-assemble into colloidal crystals. Colloidal particles are also the building blocks for advanced materials, including those used in optical and photonic technologies like sensors and lasers.

Despite the ubiquity and utility of crystals, it remains difficult to manipulate them.

“The challenge in the field has been control: crystals usually form where and when they want, and once conditions are set, you have limited ability to adjust the process in real time,” said study author Stefano Sacanna, professor of chemistry at NYU.

In their new study in Chem, the researchers found a simple and powerful way to control particles and form crystals: illuminating them.

The team added light-sensitive molecules, or photoacids, to a solution of liquid and colloidal particles. When light is shined on the photoacids, they temporarily become more acidic, which influences how they interact with the surfaces of colloidal particles. As a result, this changes the electric charge on the particles, which directly controls whether the particles attract or repel each other.

“Essentially, we used light as a remote control to program how matter organizes itself at the microscale,” said Sacanna.

In a series of experiments and simulations, the researchers showed that by adjusting light intensity, timing, and spatial patterns, they can trigger crystals to form or melt on demand, decide where crystallization happens, reshape and “sculpt” crystals, and improve their order and size to fabricate larger, more complex colloidal structures. 

“Using our photoacid gave us a surprising level of control over the attraction between particles. Just turning the light up or down a little made the difference between the particle fully sticking or being fully free,” said study author Steven van Kesteren of ETH Zürich, who conducted this work at NYU as a postdoctoral researcher in Sacanna’s lab.

“Because light is so easy to control, we could make our system do quite complex things. We could shoot light at particle blobs and see them melt under the microscope, or shine a light so that random blobs of particles ordered themselves into crystals. We could also remove specific crystals quite easily by simply unsticking the particles at that spot,” added van Kesteren.

Notably, the researchers could manipulate the system’s behavior as a “one pot” experiment and did not have to perform repeated, separate experiments with redesigned particles or many different solution and particle salt concentrations. Simply adjusting the level of was enough to cause particles to assemble or disassemble. 

Technologically, it opens a path toward creating materials whose structure, and therefore properties, can be tuned with light—for instance, photonic materials whose color or optical response can be written, erased, and rewritten on demand. Such light-programmable colloidal crystals could be used to fabricate reconfigurable optical coatings, adaptive sensors, or next-generation display and information storage technologies, where patterns and functionality are defined not during manufacturing, but dynamically by illumination.

“Our approach brings us closer to dynamic, programmable colloidal materials that can be reconfigured on demand,” said study author Glen Hocky, associate professor of chemistry and a faculty member at the Simons Center for Computational Physical Chemistry at NYU. “This system also allows us to test a number of predictions on how self-assembly should behave when interactions between particles or molecules are changing across space or time.”

Additional study authors include Nicole Smina, Shihao Zang, and Cheuk Wai Leung of NYU. The research was supported by the US Army Research Office (award W911NF-21-1-0011), the Swiss National Science Foundation (grant 217966), and the NYU Simons Center for Computational Physical Chemistry (grant 839534).

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