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Molybdenum oxide films offer a rich variety of properties for diverse applications, but exclusive synthesis of desired phases is a major challenge. Here, we demonstrate that oxygen flow ratio fO2 = [O2]/[Ar + O2] is crucial not only for phase selection of non-layered monoclinic MoO2 and layered orthorhombic α-MoO3 but also for controlling grain size and preferred orientation. Both mica and sapphire support exclusive MoO2 formation for 0.15 ≤ fO2 ≤ 0.25 at deposition temperatures Tdep = 400 and 500 °C, while α-MoO3 forms only at Tdep = 400 °C for 0.35 ≤ fO2 ≤ 0.5. Within the fO2 windows favoring each phase, high fO2 fosters large grains with out-of-plane 0k0 texture, except for MoO2 on c-sapphire at Tdep = 500 °C, where no fO2-texture correlation is discernible. These findings provide a framework for rational synthesis of single-phase monoclinic MoO2 and orthorhombic MoO3 with control over texture and microstructure to access desired properties. 

Controlling nanoporosity to favorably alter multiple properties in layered crystalline inorganic thin films is a challenge. Here, we demonstrate that the thermoelectric and mechanical properties of Ca 3 Co 4 O 9 films can be engineered through nanoporosity control by annealing multiple Ca(OH) 2 /Co 3 O 4 reactant bilayers with characteristic bilayer thicknesses (b t ). Our results show that doubling b t , e.g. , from 12 to 26 nm, more than triples the average pore size from ∼120 nm to ∼400 nm and increases the pore fraction from 3% to 17.1%. The higher porosity film exhibits not only a 50% higher electrical conductivity of σ ∼ 90 S cm −1 and a high Seebeck coefficient of α ∼ 135 μV K −1 , but also a thermal conductivity as low as κ ∼ 0.87 W m −1 K −1 . The nanoporous Ca 3 Co 4 O 9 films exhibit greater mechanical compliance and resilience to bending than the bulk. These results indicate that annealing reactant multilayers with controlled thicknesses is an attractive way to engineer nanoporosity and realize mechanically flexible oxide-based thermoelectric materials.