Investigation of the physical, optical, and chemical properties of phase segregated AlCoOx thin films from a novel hexol-type cluster.

The use of a novel inorganic nanoscale cluster (Al[(µ-OH)2Co(NH3)4]3(NO3)6) was investigated for its utility as a precursor for AlCoOx films. Mixed-metal aluminum and cobalt oxide thin films were solution deposited from the novel cluster solution via the spin-coating method on Si (100) and quartz substrates. The films were annealed at increasing temperatures up to 800 °C, and characterization of these films via TEM and XRD confirms binary Co3O4 crystalline phase present in an amorphous Al2O3 network. Films are relatively smooth (Rrms < 4 nm), polycrystalline, and demonstrate a tunable optical response dominated by Co3O4 with two electronic transitions.

Amorphous Mixed-Metal Oxide Thin Films from Aqueous Solution Precursors with Near-Atomic Smoothness.

Thin films with tunable and homogeneous composition are required for many applications. We report the synthesis and characterization of a new class of compositionally homogeneous thin films that are amorphous solid solutions of Al2O3 and transition metal oxides (TMOx) including VOx, CrOx, MnOx, Fe2O3, CoOx, NiO, CuOx, and ZnO. The synthesis is enabled by the rapid decomposition of molecular transition-metal nitrates TM(NO3)x at low temperature along with precondensed oligomeric Al(OH)x(NO3)3-x cluster species, both of which can be processed from aq solution. The films are dense, ultrasmooth (Rrms < 1 nm, near 0.1 nm in many cases), and atomically mixed amorphous metal-oxide alloys over a large composition range. We assess the chemical principles that favor the formation of amorphous homogeneous films over rougher phase-segregated nanocrystalline films. The synthesis is easily extended to other compositions of transition and main-group metal oxides. To demonstrate versatility, we synthesized amorphous V0.1Cr0.1Mn0.1Fe0.1Zn0.1Al0.5Ox and V0.2Cr0.2Fe0.2Al0.4Ox with Rrms ≈ 0.1 nm and uniform composition. The combination of ideal physical properties (dense, smooth, uniform) and broad composition tunability provides a platform for film synthesis that can be used to study fundamental phenomena when the effects of transition metal cation identity, solid-state concentration of d-electrons or d-states, and/or crystallinity need to be controlled. The new platform has broad potential use in controlling interfacial phenomena such as electron transfer in solar-cell contacts or surface reactivity in heterogeneous catalysis.