Flexible SiO2 nanofilms assembled on poly(ethylene terephthalate) substrates through a room temperature fabrication process for nanoscale integration

Abstract

We propose a room-temperature and ambient-pressure fabrication process of SiO₂ nanofilms on flexible poly(ethylene terephthalate) (PET) substrates. Polymer Langmuir–Blodgett (LB) films containing silsesquioxane units (p(DDA/SQ22)) are successfully photo-oxidized into SiO₂ nanofilms without PET degradation by UV-ozone treatment. Thicknesses of the obtained SiO₂ nanofilms are tuned by 0.4 nm per monolayer, indicating that this method affords SiO₂ nanofilms with a precisely controlled thickness. Photo-oxidized SiO₂ nanofilms have good transparency and high electric resistance (ca. 10⁻⁹ S cm⁻¹) even on flexible PET substrates. The resulting SiO₂ nanofilms also serve as a protecting layer for PET substrates against photo-oxidizing degradation. Thus, PET films retain optical transparency even after UV-ozone treatment. Results of an investigation of the photo-oxidization mechanism reveal that UV-ozone induced conversion from silsesquioxane to SiO₂ comprises two photochemical reaction processes: (1) decomposition of organic moieties in p(DDA/SQ22) LB films and (2) subsequent photo-oxidation of residual silsesquioxane units. The time constant of the second reaction strongly depends on the initial film thickness. This sequential and thickness-dependent conversion indicates that the conversion is controlled by the diffusion of extrinsic oxygen sources into the films. The results show that this method is promising for the coating of transparent insulating SiO₂ layers for flexible optoelectronic device applications.