Crystals make up a number of substances. For example, they are found in salt, sugar, snowflakes and gemstones. In each case, the crystals are highly ordered, layered structures. Although crystals are ubiquitous in nature, much of the information relating to how they form has remained a mystery.
This mystery has been delayered through a new breakthrough. By using optimized microscopy, researchers at Northwestern University have watched nanoparticles form crystals in real time. Described as ‘mesmerizing’, researchers have captured the process of nanoparticles self-assembling into solid materials.
The researchers spent time 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 behaviour.
It was discovered that these nanoparticles wiggle in solution and grow into crystals of various morphologies like polyhedral. 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.
This is shown through particles raining downwards, tumbling along stairsteps and sliding around before snapping into place to form a crystal’s signature stacked layers. 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.
By viewing nanoparticles, the scientists viewed particles that are larger than atoms, but smaller than colloids. So, we have completed the whole spectrum of length scales. We are filling in the missing length.
The research also has a practical application and should assist with the design new materials, including thin films for electronic applications, such as flexible electronics, light-emitting diodes, transistors and solar cells. This was made possible through the use of liquid-phase transmission electron microscopy (TEM).
The research has been published in the journal Nature Nanotechnology. The report is titled “Unravelling crystal growth of nanoparticles.”
