A stability study of transparent conducting WO3/Cu/WO3 coatings with antimicrobial properties

Abstract

WO3/Cu/WO3 coatings are transparent electrodes, but conductivity and transmittance have been observed to decrease with time. This paper reports the improved stability of WO3/Cu/WO3 coatings deposited by magnetron sputtering on glass and polyethylene terephthalate substrates. The stability issues due to Cu oxidation and migration can be addressed by adjusting the deposition parameters. Lowering the sputtering pressure results in denser WO3 films, confirmed by spectroscopic ellipsometry, and thus more stable coatings. The coatings retain their properties in an inert atmosphere, indicating that Cu oxidation is the main reason for the decrease in conductivity, rather than its migration observed by X-ray photoelectron spectroscopy. Optical property modeling is used to optimize the thickness of the three-layer coatings to obtain the highest figure-of-merit for a transparent electrode. A structure of glass/WO3 (70 nm)/Cu (10 nm)/WO3 (45 nm) gives a sheet resistance of 14 Ω/sq. and a light transmittance of 65% at 600 nm. In addition, the antimicrobial properties of these coatings are revealed. A decrease up to 105 of the gram-negative Escherichia coli and gram-positive Staphylococcus aureus bacterial colony formation units is found for several WO3/Cu/WO3-based coatings. In the case of the MS2 (Emesvirus zinderi) bacteriophage, a decrease in infectious particles for up to 104 plaque-forming units is obtained. The results indicate that more stable samples also had higher antimicrobial activity.