Various wearable electronic devices have been developed for extensive outdoor activities. The key metrics for these wearable devices are high touch sensitivity and good mechanical and thermal stability of the flexible touchscreen panels (TSPs). Their dielectric constants (k) are important for high touch sensitivities. Thus, studies on flexible and transparent cover layers that have high k with outstanding mechanical and thermal reliabilities are essential. Herein, an unconventional approach for forming flexible and transparent cellulose nanofiber (CNF) films is reported. These films are used to embed ultralong metal nanofibers that serve as nanofillers to increase k significantly (above 9.2 with high transmittance of 90%). Also, by controlling the dimensions and aspect ratios of these fillers, the effects of their nanostructures and contents on the optical and dielectric properties of the films have been studied. The length of the nanofibers can be controlled using a stretching method to break the highly aligned, ultralong nanofibers. These nanofiber-embedded, high-k films are mechanically and thermally stable, and they have better Young's modulus and tensile strength with lower thermal expansion than commercial transparent plastics. The demonstration of highly sensitive TSPs using high-k CNF film for smartphones suggests that this film has significant potential for next-generation, portable electronic devices.

The thermo-optic coefficient (dn/dT) of inorganic-organic hybrid material films prepared by the sol-gel process of organoalkylsilanes is measured using a prism coupler equipped with a hot stage. The effect of the organic modifier on the variation of dn/dT in inorganic-organic hybrid material films has been investigated. The value of dn/dT becomes more negative with increasing molecular weight of the organic modifier or with an increase in the proportion of modifier in the sample. On the other hand, dn/dT increases with an increase in the degree of organic photopolymerization. From these results, it can be seen that the value of dn/dT in these films can be varied between −0.83 × 10 −4 /°C to −2.43 × 10 −4 /°C by changing the organic modifier concentration and type.

Light illumination processing created photoinduced reduction of refractive index as well as volume contraction in an organic-inorganic hybrid material. Whereas both refractive index and film thickness are decreased significantly by the light exposure, transmittance and anti-soiling property of the hybrid film are not affected by the exposure. Direct patterning is possible upon light illumination using the photoinduced change in thickness without any developing process.