![]() ![]() As they were observing the first stage of growth of nanowires, they could see that the metal-semiconductor alloy they used remained liquid at a much lower temperature than its crystallization point and so they decided to investigate this phenomenon. It was during their studies, originally focusing on the growth of semiconducting nanowires, that the scientists discovered the unusual properties of these liquids. ![]() Pentagonal clusters formed at the interface exhibit a denser structure compared to solid gold and prevent the liquid from crystallization at temperatures as low as 300 Kelvin below the solidification temperature. ![]() “This constitutes the first experimental proof that pentagonal order is at the origin of supercooling”, explains Tobias Schülli.ĭroplet of a gold-silicon liquid alloy on a silicon (111) surface. The team performed the control experiment with the same liquid exposed to three-fold and four-fold coordinated surfaces, which reduced the supercooling effect dramatically. “We studied what happened to the liquid in contact with a five-fold coordinated surface”, explains Tobias Schülli, first author of the paper. Their findings confirmed that a strong supercooling effect took place. The researchers from the CEA, CNRS and ESRF studied the structure of a particular liquid, a gold-silicon alloy, in contact with a specially decorated silicon (111) surface, where the outermost layer of the solid featured pentagonal atomic arrangements. Until today there was no experimental proof that this five-fold coordinated structures are at the origin of supercooling. In this example, pentagons are an obstacle to crystallization. In the two-dimensional analogue, a plane cannot be filled by pentagons only, whereas triangles, rectangles or hexagons can fill a plane perfectly. This is not possible with five-fold coordinated clusters. To form a crystal however, one needs a structure that can be repeated periodically, filling the entire space. Models propose that a significant fraction of the atoms in liquids arrange in five-fold coordinated clusters. Over the last 60 years the very existence of deep supercooling has led to speculations that the internal structure of liquids could be incompatible with crystallization. Supercooling was discovered already in 1724 by Fahrenheit, but even today the phenomenon remains a subject for intense discussions. Controlling solidification behaviour is important for applications ranging from hail prevention up to technological processes such as welding and casting or even the growth of semiconductor nanostructures. In everyday life, though, there is usually some crystalline impurity in contact with the liquid that will trigger the crystallization process, and therefore the freezing. Clouds at high altitude are a good example for this: they contain tiny droplets of water that, in the absence of seed crystals do not form ice despite the low temperatures. ![]() Supercooled liquids are trapped in a metastable state even well below their freezing point, which can only be achieved in liquids that do not contain seeds that may trigger crystallization. ![]()
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