Microwave-Driven Crystallographic Alignment for Producing Metal (M) Thin Films with Highly-Oriented M(111) Surface.

Production of metal thin films exhibiting controlled crystallographic alignment is essential for catalysts, sensors, and substrates for the epitaxial growth of 2D materials (graphene, h-BN, metal chalcogenides, etc.). Conventional methods to produce aligned metal films necessitate specific substrates and high-temperature processing, which limits their applicability to thin films and accessible metal species. This study introduces microwave (MW) annealing as a novel technique to induce crystallographic alignment in Au thin films. MW-annealing enables effective out-of-plane alignment of the Au(111) facet within a few minutes at lower temperatures than thermal annealing, irrespective of the type of substrate and without concerns related to the thin film dewetting. The underlying mechanism of this alignment is investigated through experimental and numerical studies, revealing the role of MW-induced residual stress and strain fields, which additionally promote exposure of the Au(111) planes to the surface in comparison to thermal annealing. The highly-oriented Au(111) film is used as a strain-relieved substrate for heteroepitaxial growth of other (111) aligned metal (Ag, Al, and Cu) thin films. This work offers a scalable strategy for achieving crystallographically aligned metal thin films that can be used for sensors and a strain-relieved substrate used in heteroepitaxy.