Important step in research into new materials
‘Incomprehensible’ birth of supercrystal explained
Two years ago, a research team led by Utrecht University published an article 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 results of their research were published online in Nature Materials on Monday, 5 September.
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.”
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.
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.
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 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.
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.
At the bottom: images of the formation of the supercrystal by the PhD candidates and authors Jaco Geuchies and Carlo van Overbeek and final author Daniël Vanmaekelbergh. The images are available for download in low and high resolution by selecting the desired image.
'In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals'
Jaco J. Geuchies*, Carlo van Overbeek*,Wiel H. Evers, Bart Goris, Annick de Backer, Anjan P. Gantapara*, Freddy T. Rabouw*, Jan Hilhorst, Joep L. Peters*, Oleg Konovalov, Andrei V. Petukhov*, Marjolein Dijkstra*, Laurens D. A. Siebbeles, Sandra van Aert, Sara Bals and Daniel Vanmaekelbergh*
Nature Materials, DOI: 10.1038/NMAT4746, 5 September 2016
* affiliated with Utrecht University
This research was funded in part by the ESRF, Stichting FOM and the Fonds Wetenschappelijk Onderzoek Vlaanderen. Most of the experiments were conducted at the European Synchrotron Radiation Facility in Grenoble. The ESRF has an extremely powerful electron synchrotron that is capable of generating especially powerful X-ray radiation.
- Press release about the 2014 publication in Science.
- This study is in advance of the research project for which Daniël Vanmaekelbergh and his fellow UU professors Willem Kegel and Andries Meijerink have received a TOP-PUNT subsidy from NWO Chemical Sciences.