Watch nanoparticles grow into crystals

Most people are familiar with crystals in the forms of salt, sugar, snowflakes and sparkling gems, such as diamonds. Although crystallization is a ubiquitous phenomenon, exactly how crystals form has remained a mystery. The building blocks — atoms, molecules or ions — that compose crystalline materials are highly ordered, forming lattices of equally spaced building blocks. These lattices then stack on top of each other to form a three-dimensional solid material.

“The stacking of atoms into regular arrangements is the reason that crystals have smooth, flat faces,” Luijten said. “That’s why they break along straight edges.” 

Up until now, researchers have studied crystallization by examining much larger particles called colloids. But watching colloids self-arrange into crystals did not give insights into how atoms behave. Whereas crystals have flat, uniform surfaces, crystalline structures made from micron-sized colloids tend to adopt non-uniform, rough surfaces.

“Colloids are so much larger than atoms that it is doubtful they follow the same steps when crystallizing,” Luijten said. “So, they do not teach us what atoms do. The analogy of colloids to atoms doesn’t really hold.” 

To glean deeper insights into the crystallization process, Luijten, Chen and their teams turned to nanoparticles. Recent advances to improve liquid-phase TEM have made it possible to view nanoparticles in real time as they form solid materials. Chen’s team spent years optimizing the process to ensure the electron beam could view the particles without damaging them. In the new study, the researchers used differently shaped nanoparticles — cubes, spheres and indented cubes — to explore how shape affects behavior.

The researchers first visualized crystal formation with advanced computer simulations, which were performed by Northwestern graduate students Ziwei Wang and Garrett Watson as well as postdoctoral fellow Tine Curk. Then they performed experiments with liquid-phase TEM to watch the nanoparticles self-assemble in real time. In the experiments, the researchers noticed the particles collided into each other, sticking together to form layers. Then, to form the layer-by-layer crystalline structure, the particles first formed a horizontal layer and then stacked vertically. Sometimes, after sticking to each other, the particles briefly detached to fall onto a layer below.

“They run along and then hesitate at the edge before falling,” Luijten said. “It’s like a diver hesitating at the edge of a diving board. I can’t believe we can actually see this. We have never seen the actual growth process before — only the result.” 

Luijten said this information will help engineers design new materials. The insight specifically could help with the design of thin-film materials, which are often used to build flexible electronics, light-emitting diodes, transistors and solar cells.

“Knowing how particles come together will enable us to control the shape of a surface,” Luijten said. “Do you want a flat or rough surface? Changing particle shape or how fast the particles fall can affect the surface.”