‘Incomprehensible’ Birth of Supercrystal Explained

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. Credit: Image courtesy of Utrecht University

Two years ago, a team led by Utrecht University in Science explaining how they had created a material with unique and extremely interesting electronic characteristics. In this ‘supercrystal’, the electrons move almost with the speed of photons, and the electric current can be switched on and off. This makes it ideal for ultra-fast electronics. But at the time, the researchers were at a loss to explain how this ‘supercrystal’ obtained its unique structure. Now they have unravelled the mystery, and it appears to involve a completely different mechanism for crystal formation. This is an important insight for research into new materials with unique electronic characteristics.

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.”

Method: 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 something mysterious occurs: the nanocrystals rotate simultaneously and systematically into a pseudo-hexagonal structure. “It’s as if they’re synchronised swimmers,” he explains. 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.

Quantitative analysis of the GISAXS and GIWAXS data.

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.

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. Graphene offers perhaps the most spectacular current speed of any material, but graphene is not suitable for use in electronic switches. So 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.
http://press.uu.nl/incomprehensible-birth-of-supercrystal-explained/