Mechanical, electrical and antibacterial properties of polycrystalline ZnO films passivated with ZnS

Zinc oxide (ZnO) nanostructures, due to their optical and electrical properties, are ideal systems for design of various devices such as: light emitting diodes, transistors or sensors of UV light, selected gases and... [ view full abstract ]

Zinc oxide (ZnO) nanostructures, due to their optical and electrical properties, are ideal systems for design of various devices such as: light emitting diodes, transistors or sensors of UV light, selected gases and biosensors. Among the various ZnO nanomaterials, the nanolayers obtained by the sol-gel process and subsequent calcination have attracted considerable interest in different applications as a result of their facile synthesis, polycrystalline nature and a high surface area to volume ratio.
This study presents properties of polycrystalline ZnO nanofilms obtained by a spin-coating process followed by calcination in air and further passivation by a zinc sulfide (ZnS) layer in a gas-phase of H2S [1]. The ZnO/ZnS nanostructures were characterized by Fourier Transform Infrared and Raman spectroscopies, Scanning Electron Microscopy and Atomic Force Microscopy. The thin ZnS coating mimicked the shape of the ZnO crystals and did not significantly change the morphology of the ZnO film.
Moreover, ZnS coating of ZnO films improved their stability in liquids, electrical conductivity and mechanical properties. ZnS as water-insoluble semiconductor enhanced the stability of ZnO layer in biological liquids. After sulfidation process ZnO/ZnS films were stable in water for over 7 days, while ZnO nanolayers were water soluble. Nanohardness and elastic modulus of the samples and nanowear tests were performed using nanoindentation methods and revealed improvement of mechanical properties after sulfidation process. The electrical conductivity of the ZnO nanolayers, investigated by Conductive-Atomic Force Microscopy and van der Pauw method, increased after ZnS coating formation as a result of surface defect passivation and removing oxygen molecules, which can trap free carriers. Additionally, ZnO and ZnO/ZnS films showed antimicrobial properties against Escherichia coli.
Our findings indicate that ZnO surface passivation is required for designing future devices to improve its stability at different environments and enhance its mechanical, electrical and antibacterial properties.

Acknowledgements
Financial support from Polish Ministry of Science and Higher Education (Grant “Iuventus Plus” 0018/IP2/2015/73, 2015-2017) and Nation Centre for Research and Development (PBS1/A9/13/2012) is gratefully acknowledged.

[1] The synthesis, characterization and ZnS surface passivation of polycrystalline ZnO films obtained by the spin-coating method, A. Baranowska-Korczyc et al., J. Alloy and Comp. (in press).