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 homogenous 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 pre-condensed oligomeric Al(OH)x(NO3)3-x cluster species; both of which can be processed from aq. solution. The films are dense, ultra-smooth (Rrms < 1 nm, near 0.1 nm in many cases), and are atomically mixed amorphous metal-oxide alloys over a large composition range. We assess the chemical principles that favor the formation of amorphous homogenous 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.

Kast, M. G.; Cochran, E. A.; Enman, L. J.; Mitchson, G.; Ditto, J.; Siefe, C.; Plassmeyer, P. N.; Greenaway, A. L.; Johnson, D. C.; Page, C. J.; et al. Amorphous mixed-metal oxide thin films from aqueous solution precursors with near atomic smoothness. J. Am. Chem. Soc. 2016, jacs.6b11084