The formation of crystals from nanoscale constituents is a ubiquitous phenomenon in biology, geology, and materials science. But this process is not fully understood, among other things because of the experimental difficulties inherent in its observation. Recently, for the very first time, researchers have observed this process of self-assembly of nanoparticles into solid materials. The fascinating videos obtained show the particles falling into place to form the characteristic stacked layers of a crystal.
Crystals play an important role in the formation of substances ranging from skeletons and shells to semiconductor materials. But many aspects of their training are shrouded in mystery. Understanding the process of self-assembly of nanoparticles into solid materials can, for example, help understand why biomineralization, producing teeth, bones, etc., can go wrong and thus cause disease. On the other hand, this understanding can also be a powerful driver of innovations in technological fields.
You should know that there are three distinct phases in the growth of crystals: nucleation, post-nucleational growth and growth arrest. Many studies have focused on understanding the onset of nucleation and producing high quality crystals by sampling constituents with different attributes and varying growth conditions.
However, the kinetics of post-nucleation growth processes, an important determinant of crystal morphology and properties, has remained underexplored due to the experimental challenges associated with real-time nanoscale imaging.
Recently, a research team from Northwestern University and the University of Illinois imaged the crystal growth of nanoparticles of different shapes. The videos show them tumbling down nano-staircases and sliding before finally falling into place to form the characteristic stacked layers of a crystal. This new knowledge might be used to design new materials, including thin films for electronic applications. The breakthrough was published in the journal Nature
Nanotechnology.
Change point of view
As mentioned earlier, although crystallization is a ubiquitous phenomenon, exactly how crystals form has remained an enigma. The common representation of crystals is in the form of salt, sugar, snowflakes, and precious stones, such as diamonds. The building blocks—atoms, molecules, or ions—that make up these crystalline materials are highly ordered in lattices, and stack on top of each other to form a three-dimensional solid material.
So far, researchers have studied crystallization by looking at much larger particles called colloids. But observing colloids self-arrange into crystals gives no information regarding the behavior of atoms. While crystals have flat, uniform surfaces, crystal structures made from micron-sized colloids, which are 10 to 100 times larger than nanoparticles, tend to adopt rough, non-uniform surfaces.
Northwestern’s Erik Luijten, who led the theoretical and computational work to explain the sightings, says in a communiqué : « Colloids are much larger than atoms, it is doubtful that they follow the same steps during crystallization. The analogy of colloids with atoms does not really hold ».
In the past, they have also used X-ray crystallography or electron microscopy to visualize single layers of atoms in a crystal lattice. But they were unable to watch the atoms fall into place individually.
An experimental tour de force to fill in the gaps
To better understand the crystallization process, Luijten and his colleagues turned to nanoparticles. Recent advances to improve liquid-phase transmission electron microscopy (TEM) have made it possible to visualize nanoparticles in real time as they form solid materials. The team of Qian Chen, co-author of the study, was responsible for optimizing the process to ensure that the electron beam might allow the particles to be seen without damaging them.
Researchers first visualized crystal formation with advanced computer simulations. Next, they performed experiments with liquid-phase TEM to observe the nanoparticles self-assemble in real time. They used nanoparticles of different shapes — cubes, spheres, and recessed cubes — to explore how shape affects behavior.
VIDEO : Animation illustrating in-plane and out-of-plane growth modes for concave gold nanocube crystals inside a liquid-phase transmission electron microscope chamber. (© Erik Luijten and Qian Chen)
In the experiments, the researchers noticed that the particles collided with each other and then stuck together to form layers. The formation of the crystalline structure was done layer by layer, the particles first formed a horizontal layer and then piled up vertically. Sometimes the particles broke off briefly to fill in a lower layer.
Luijten explique : « They run along then hesitate at the edge before falling. It’s like a diver who hesitates at the edge of a diving board “. She concludes:
By observing nanoparticles, we observe particles larger than atoms, but smaller than colloids. Thus, we have completed the entire spectrum of length scales ».
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VIDEO : Liquid-phase transmission electron microscope view of the layer-by-layer growth of a smooth-surfaced crystal from concave gold nanocubes. Surface particles on the growing crystal are tracked (center positions covered with yellow dots). (© Erik Luijten and Qian Chen)
The researchers believe this information will help engineers design new materials, including thin-film materials, which are often used to produce flexible electronic components, light-emitting diodes, transistors and solar cells.