Partial Phase-Separation in the Initial Growth Stage of Exsolution-Active SrTi_0.95-xNi_0.05Nb_xO_3-δ Thin Films

Heavily doped perovskite oxide thin films have emerged as versatile catalysts based on exsolution processes, which yield functional nanoparticles, but also introduce dopant inhomogeneities and internal phase separation. Here, we resolve the initial-stage of thin film growth in heavily Ni-doped and (Nb,Ni)-co-doped SrTiO_3-δ utilizing scanning tunneling microscopy and spectroscopy (STM/S). While the growth of co-doped SrTiO_3-δ follows statistically distributed island nucleation, Ni-doped SrTiO_3-δ (STNi) exhibits a distinct pentagonal or hexagonal pattern of monolayer islands. A similar pattern is observed in thicker films by transmission electron microscopy (TEM). STS reveals Ni-dopant clusters forming from the onset of the thin film growth and serving as nucleation points for embedded nanostructures at increasing film thickness. Quantitative analysis reveals that in the as-grown monolayers, approximately 20% of the Ni ions segregate into nanoclusters, while the remaining ∼80% are dissolved in the perovskite lattice. Upon reduction (600°C, UHV), the entire Ni content of the sub-monolayer STNi is found to nucleate as metallic nanoparticles at the surface, revealing two exsolution pathways from the perovskite lattice as well as from pre-defined clusters. These findings provide deeper insights into the nucleation processes in heavily doped oxide and further reveal the role of dopant inhomogeneities for metal exsolution reactions.