Scientists at INM – Leibniz Institute for New Materials found out that certain nanoparticles assemble into groups as if they were atoms. Like the atoms of metals or noble gases, they form specific structures depending on their number. Through their findings, the researchers are now able to make precisely defined structures from nanoparticles. Normally, nanoparticles form rather disordered, often loose and fuzzy clusters. The results were recently published in the scientific magazine "Nano Letters".
The researchers assume that this unexpected behavior derives from the smallness of the nanoparticles. "We assume that the nanoparticles with a core diameter of only six nanometers show a behavior similar to atoms: They move very fast, collide with each other and attract each other", explains Tobias Kraus, head of the Structure Formation Group. Therefore, they can assemble almost as orderly as atoms.
Depending on the number of nanoparticles, the scientists can now predict which three-dimensional lattice are formed by the particles. "Imagine that clusters with 20 particles look like a sphere, whereas 40 particles arrange rather like a cube and 60 particles form a pyramid", explains Kraus, who holds degrees in materials science and chemical engineering. It is possible to produce specific shapes by defining the quantity of the nanoparticles in the production process. "Since nanoparticles arranged as a sphere have different properties than nanoparticles arranged as a cube, we can influence properties by the number of the particles", says Kraus. "A rather elongated cluster may not fit through the pores of a filter, for example, although it contains more particles than a spherical cluster."
The scientists use a well-established principle to force the nanoparticles into this highly ordered structure. To begin with, all gold nanoparticles must be of the same size, which is achieved in a classic preparation procedure: The researchers dissolve little bars of gold in a concentrated acid, combine the dissolved gold with organic molecules and add surface-active substances. When heating this mixture, the scientists obtain nanoparticles with a diameter of six millionths of a millimeter. The nanoparticles swim in oil, which is then dispersed into droplets. Each droplet contains several nanoparticles. "As these droplets evaporate, the space for the nanoparticles is increasingly reduced so that they assemble in an orderly manner and form the ordered clusters", says Kraus.
In the future, the group will integrate various particles into the clusters, each of them having a different task. This may be a first step to building microscopic machinery from particles.