Control of morphology in inert-gas condensation of metal oxide nanoparticles

We report preparation conditions to obtain different morphologies of as-deposited refractory metal-oxide nanoparticles using inert-gas condensation with CO₂ laser heating. The micrometer-scale morphology of the nanoparticles depends on the specific metal oxide, the buffer gas composition and pressure, and the target-to-substrate distance. These parameters control the extent to which a plume of nonagglomerated nanoparticles can reach a deposition substrate. Buffer gas pressure has the largest influence for a given material, with lower pressures producing a dense columnar morphology and higher pressures resulting in an open networked morphology. An estimate based on the geometry of the gas-phase plume and experimental results for Y₂O₃ nanoparticles produced in 4 Torr N₂ gives a critical concentration of tens of nanoparticles per μm³ for the transition of agglomerates versus isolated nanoparticles reaching a deposition substrate.