The formation of crystals from nanoscale elements is a ubiquitous phenomenon in biology, geology, and materials science. But this process is not fully understood and, among other things, there are experimental difficulties inherent in its monitoring. Recently, for the first time, researchers have observed the 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 materials ranging from skeletons and shells to semiconductor materials. But many aspects of their training are shrouded in mystery. Understanding the process by which nanoparticles self-assemble into solids, for example biomineralization, can help us understand why the production of teeth, bones, etc. can go wrong and thus cause disease. On the other hand, this understanding can also be a powerful driver of innovation in technology fields.
You should know that there are three distinct stages in the growth of crystals: nucleation, post-nucleation, and growth arrest. Many studies have focused on understanding the initiation of nucleation, modeling components with different properties and varying growth stages to form high-quality crystals.
However, key determination of the kinetics, crystal morphology, and properties of post-nucleation growth processes remains limited 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 falling down the nano-stairs to form the characteristic stacked layers of a crystal, gliding before finally falling into place. This new knowledge can be used to design new materials, including thin films for electronic applications. Published in Turnaround Magazine Nature Nanotechnology.
Change the view
As mentioned earlier, although crystallization is a ubiquitous phenomenon, how crystals form remains a mystery. Common representations of crystals are in the form of salt, sugar, snowflakes and precious stones such as diamonds. The building blocks that make up these crystalline materials—atoms, molecules, or ions—are highly ordered in lattices and stacked on top of each other to form a three-dimensional solid.
Until now, researchers have studied crystallization by looking at very large particles called colloids. But observing the self-organization of colloids into crystals gives no information about the behavior of atoms. While crystals have flat, uniform surfaces, crystalline structures made of micron-sized colloids, which are 10 to 100 times larger than nanoparticles, adopt rough, uneven surfaces.
Northwestern’s Erik Luiten, who led the theoretical and computational work to explain the observations, says in a contacted : ” Colloids are much larger than atoms and it is doubtful that they follow the same steps during crystallization. The analogy of colloids with atoms is not real “.
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 could not see individual atoms falling.
A test tour to fill in the gaps
To better understand the crystallization process, Luiten and his colleagues turned to nanoparticles. Recent advances in the development of liquid-phase transmission electron microscopy (TEM) have made it possible to visualize nanoparticles in real time as they form solids. Co-author of the study Qian Chen’s team was responsible for optimizing the process to ensure that the electron beam would allow the particle to be viewed without damaging it.
The researchers first visualized crystal formation with advanced computer simulations. Next, they performed experiments with liquid-phase TEM to observe the nanoparticles self-assembling in real time. They used nanoparticles of different shapes — cubes, spheres, and interstellar cubes — to study how shape affects behavior.
Video : Animation illustrating in-plane and out-of-plane growth modes for hollow gold nanocube crystals inside a liquid-phase transmission electron microscope chamber. (© Eric Luiten and Qian Chen)
In the experiments, the researchers observed that the particles collided with each other and then stuck together to form layers. The formation of the crystal structure was done in a layer-by-layer fashion, with the particles first forming a horizontal layer and then accumulating vertically. Sometimes the particles break briefly to fill a lower layer.
Luijten explains: They run, then hesitate on the edge before falling. It’s like a diver teetering on the edge of a diving board “. She concludes:
By observing nanoparticles, 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 transfer electron microscopy visualization of layer-by-layer growth of smooth surface crystal from hollow gold nanocubes. Surface particles of the growing crystal are monitored (central positions covered by yellow dots). (© Eric Luiten and Qian Chen)
The researchers hope the information will help engineers design new materials, including thin-film materials used to make flexible electronic components, light-emitting diodes, transistors and solar cells.
Source: Nature Nanotechnology
