Nanocrystals are nanometre-sized spheres consisting of regularly arranged atoms. Owing to their advantageous properties, they are on the rise in several technologies. Semiconductor nanocrystals, for instance, are used in new generation television screens. More recently, so-called intermetallic nanocrystals, in which two different metals combine to form a crystal lattice, have made a name for themselves as they promise improved and unique applications. Those applications range from catalysis to data storage and medicine.
In theory, there are tens of thousands of possible combinations of metals that could make up such nanocrystals, with a correspondingly large number of different material properties. So far, however, it has only been possible to make nanocrystals out of a few such pairings. A team of researchers at ETH Zurich led by Maksym Yarema and Vanessa Wood at the Institute for Electronics have now developed a new technique that, in principle, allows one to realize nearly all possible combinations of intermetallic nanocrystals. Their results were recently published in the scientific journal Science Advances.
Surprisingly intuitive method
“Our method is simple and intuitive – so intuitive, in fact, that we were surprised that no one had had this idea before us”, says Yarema. In conventional procedures for producing nanocrystals made of a single metal, the metal atoms are introduced in molecular form, for instance as salts, into a solution in which the nanocrystals then form. “Theoretically that can also be done with two different metals, but in practice it is difficult, or even impossible, to combine distinctly dissimilar metals in the reactor”, Yarema explains. Therefore, the ETH scientists resorted to a procedure that has been used for centuries: amalgamation, a particular kind of fusing or blending metals.
Liquid metals
Amalgams are particularly well-known from dentistry, where they are used as a filling material, and also from gold mining. In both cases, liquid mercury is added to dissolve other metals (for tooth fillings, a mixture of copper, zinc and silver). However, amalgamation also works with any other liquid metal. Besides mercury, which is liquid even at room temperature, there are a number of metals with relatively low melting points, such as gallium (30 degrees centigrade), indium (157 degrees) or tin (232 degrees).
Amalgamation approach for nanocrystals
Yarema and his colleagues use the amalgamation approach at the nanoscale. The reaction starts with the dispersion of nanocrystals containing a single metal, for instance silver. Then, the atoms of the second metal – say, gallium – are added in molecular form (in this case as amides, a compound of carbon, hydrogen, and nitrogen), while the mixture is heated to around 300 degrees.