'Incomprehensible' birth of supercrystal explained

The 'supercrystal' develops when tiny nanocrystals form a perfectly ordered surface one layer thick. In this super-matrix, the structure of the atoms -- A, B, A, B -- precisely follows that of the nanocrystals itself. "But how such a neatly ordered super-matrix could be born from all of those nanocrystals was incomprehensible to us," says Prof. Daniël Vanmaekelbergh from Utrecht University. "Now that we have insight into how the matrix is formed, we can conduct much more focused research into how we can make the structures that we would like to have."

Mysterious

To make the superstructure, the nanocrystals are dissolved in an oleaginous fluid that floats on a layer of coolant. As the oil evaporates, the nanocrystals appear to form a neat hexagonal structure on the surface of the water. But according to Vanmaekelbergh, something mysterious occurs: the nanocrystals rotate simultaneously and systematically into a pseudo-hexagonal structure. "It's as if they're synchronised swimmers," he explains.

Lego blocks

Only then do they make contact, and the nanocrystals 'click' together like Lego blocks to form a surface of a single, perfect layer. Until now, this mechanism has only been observed in metals, which are a completely different material.

X-ray radiation

It was not easy for the researchers to determine this surprising mechanism, as nanocrystals are too small to observe with an optical microscope. So the PhD candidates Jaco Geuchies and Carlo van Overbeek developed an experiment that followed the formation of the superstructure using X-ray radiation. With each change in the structure, the X-ray radiation was refracted in a different way. The researchers could then derive the movement of the nanocrystals from the changes in refraction.

Semiconductors

The nanocrystals are semiconductors that are ideally suited for switching electric currents on and off. Forming specific perfect superstructures from these kinds of nanocrystals can dramatically increase the speed of the electronic current through the material.

Important step

Graphene offers perhaps the most spectacular current speed of any material, but graphene is not suitable for use in electronic switches. So the researchers went looking for a material with a structure similar to that of graphene, but with atoms or nanocrystals that have better characteristics for electronic switches. "That is why it is such an important step that we now understand how these interesting structures are formed," according to Vanmaekelbergh.

Important step

Graphene offers perhaps the most spectacular current speed of any material, but graphene is not suitable for use in electric switches. So the researchers went looking for a material with a structure similar to that of graphene, but with atoms or nanocrystals that have better characteristics for electronic switches. "That is why it is such an important step that we now understand how these interesting structures are formed," according to Vanmaekelbergh.