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 (TMO_x) including VO_x, CrO_x, MnO_x, Fe₂O₃, CoO_x, NiO, CuO_x, 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 (R_rms < 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 V_0.1Cr_0.1Mn_0.1Fe_0.1Zn_0.1Al_0.5O_x and V_0.2Cr_0.2Fe_0.2Al_0.4O_x with R_rms ≈ 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.