Terminated cluster growth is a physical vapor deposition technique that allows fabrication of metallic and conducting oxide nanoparticles with a very narrow size distribution. This type of source operates on the principle of quenching a hot metal vapor in a flowing stream of cool inert gas. The supersaturated vapor cools down due to frequent collisions with inert gas atoms, which leads to condensation and formation of clusters and nanoparticles. The classical nucleation theory can be used to describe the homogeneous nucleation and growth of nanoparticles in the aggregation zone. According to this formalism, the change in free energy ΔG associated with the aggregation of n atoms from the metal vapor into a nanoparticle is given by the energy spent forming the nanoparticle surface and the chemical potential variation Δµ during vapor condensation. A maximum in ΔG and thus optimum condensation conditions are achieved in a narrow area of the aggregation zone where the temperature of the gas phase, i.e. metal vapor plus inert gas mixture, favors nucleation. Under these circumstances, the formation of embryos takes place following a three body collision process between two sputtered atoms and one Ar atom. After nucleation, nanoparticle growth takes place by successive two body collisions between the nanoparticles and the individual atoms in the metal vapor, or by coagulation, i.e. inelastic particle-particle collisions.
However, heterogeneous nucleation occurs much more often in nature, as it requires a lower variation in free energy. It takes place in preferential sites such as phase boundaries, contaminants, ions, surfaces, or it is triggered by chemical reactions. Creation of oxide nanoparticles from gas condensation of a metal vapor in the presence of oxygen is a prominent example of heterogeneous nucleation. In this case, oxidation of the metal atoms is the first step in the nanoparticle formation process. This allows for a high throughput of metal oxide nanoparticles.
A quadrupole mass filter in line with the nanoparticle source allows us analyzing and further filtering the nanoparticle beam with an ultimate size resolution of 2%, allowing precise particle size definition to be achieved. The produced nanoparticles tend to possess one additional electronic charge. This allows them to be electrostatically manipulated either through deflection, focusing, or acceleration.
For more details see "Size and composition-controlled fabrication of thermochromic metal oxide nanocrystals" César Clavero, Jonathan L Slack and André Anders Journal of Physics D: Applied Physics 46, 362001 (2013).
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