“Zincone” Zinc Oxide−Organic Hybrid Polymer Thin Films Formed by Molecular Layer Deposition

Hybrid organic−inorganic polymer thin films of the form (−O−Zn−O−C2H4−)n have been deposited from diethyl zinc and ethylene glycol using molecular layer deposition (MLD) over a range of substrate temperatures between 100 and 170 °C. Infrared transmission confirms that the films consist of Zn−O and ethylene-oxide units. In analogy with known alucone polymers of the form (−O−Al−O−R−)n, the zinc-based hybrid material is an example of a “zincone” polymer. In situ quartz crystal microbalance analysis indicated that the sequential surface reactions of diethyl zinc and ethylene glycol are sufficiently self-limiting and saturating to enable well-controlled MLD growth. Quantitative analysis of in situ quartz crystal microbalance and film thickness results indicate that ethylene glycol molecules can undergo a “double reaction” where the OH groups on both ends of the diol react with available Zn−C2H5 surface sites to produce a relatively inert bridging alkane. The mass uptake per MLD cycle during Zn−hybrid film deposition decreases with increasing reaction temperature. Infrared transmission spectroscopy also shows that Zn−organic hybrid films are stable in dry air. However, the as-deposited ZnO−hybrid material could be hydrolyzed by H2O (for example, in ambient) resulting in films consisting of zinc oxide and zinc hydroxide with some carbon remnants. Spectroscopic ellipsometry indicates the thickness of hydrolyzed films increases linearly with reaction cycles, and scanning probe and transmission electron microscopy images show the hydrolyzed ZnO−hybrid film coating is uniform and conformal. The transmission electron micrographs also show the hydrolyzed Zn−hybrid films contain nanoscale porosity. These results suggest new pathways to fabricate organic−inorganic hybrid materials, including metal-organic framework structures.