Solid-State Dewetting of Gold Aggregates/Islands on TiO₂ Nanorod Structures Grown by Oblique Angle Deposition

A composite film made of a stable gold nanoparticle (NP) array with well-controlled separation and size atop a TiO₂ nanorod film was fabricated via the oblique angle deposition (OAD) technique. The fabrication of the NP array is based on controlled, Rayleigh-instability-induced, solid-state dewetting of as-deposited gold aggregates on the TiO₂ nanorods. It was found that the initial spacing between as-deposited gold aggregates along the vapor flux direction should be greater than the TiO₂ interrod spacing created by 80° OAD to control dewetting and produce NP arrays. A numerical investigation of the process was conducted using a phase-field modeling approach. Simulation results showed that coalescence between neighboring gold aggregates is likely to have caused the uncontrolled dewetting in the 80° deposition, and this could be circumvented if the initial spacing between gold aggregates is larger than a critical value s_min. We also found that TiO₂ nanorod tips affect dewetting dynamics differently than planar TiO₂. The topology of the tips can induce contact line pinning and an increase in the contact angle along the vapor flux direction to the supported gold aggregates. These two effects are beneficial for the fabrication of monodisperse NPs based on Rayleigh-instability-governed self-assembly of materials, as they help to circumvent the undesired coalescence and facilitate the instability growth on the supported material. The findings uncover the application potential of OAD as a new method to fabricate structured films as template substrates to mediate dewetting. The reported composite films would have uses in optical coatings and photocatalytic systems, taking advantage of their ability to combine plasmonic nanostructures within a nanostructured dielectric film.