A multi-timescale model predicts the spherical-to-cubic morphology crossover of magnetron-sputtered niobium nanoparticles

Sputter-based cluster beam deposition offers a solvent-and effluent-free physical method to synthesize nano-particles (NPs) with tailored characteristics. Despite the broad capabilities to control the NP architecture via multiple parameters such as magnetron power, gas pressure and flow, type of gas, etc., the technique is still far from being at hand because of insufficient understanding of the processes of NP nucleation, growth, and transport. Here, we synthesize Nb NPs of spherical (diameter 32 nm) and cubic (side length 19 nm) shape, depending on pressure, and perform a detailed structural characterization. A known analytical model of the NP growth and thermalization was modified to account for ion bombardment and non-ideal atomic sticking in the plasma. The model was enforced with ad hoc multi-timescale molecular dynamic simulations to unveil the peculiarities of the growth of Nb NPs and explain the difference in their morphology.