Nonclassical Crystallization of an Al₂O₃Film by Positively Charged Secondary Nanoparticles during Aerosol Deposition

Thin film growth through nonclassical crystallization with charged nanoparticles as building blocks has been studied in chemical vapor deposition and physical vapor deposition. A similar mechanism might be applied to aerosol deposition (AD), with which the mechanism for the evolution of dense ceramic films at room temperature has not been clearly understood. Aerosol particles fracture into fragments of secondary nanoparticles during AD. In this work, secondary particles passing between the two parallel biased and grounded electrodes were captured by the transmission electron microscopy (TEM) grid membrane on each electrode, and it was revealed that they were mainly positively charged. Neutral secondary nanoparticles, not deflected by the electric field, produced a porous film on the silicon substrate, whereas charged secondary nanoparticles produced a dense film. The positive charge of the secondary nanoparticles appeared to enhance the plastic deformation required for the evolution of dense films during AD, which was supported by ab initio calculations.