Sol–gel-derived titania-hydroxypropylcellulose hybrid thin films of high refractive indices: solution components affecting the refractive index and uncracking critical thickness

Optically transparent, ca. 200–800 nm thick TiO2-hydroxypropylcellulose (HPC) hybrid thin films were prepared from Ti(OC3H 7 i )4–HPC–HCl–H2O–C3H 7 i OH solutions by the sol–gel method, where the as-deposited films were dried at 120 °C. The effects of the amount of HPC, H2O and HCl in the starting solutions on the refractive index and uncracking critical thickness of the films were studied, where the effects on the critical thickness was discussed on the basis of in situ stress measurements during heating. The increase in HPC content increased the critical thickness and lowered the refractive index. The increase in HCl content resulted in a decrease in critical thickness and an increase in refractive index. Larger H2O contents gave rise to a maximum in critical thickness while the refractive index was unaffected. Such variation in critical thickness with varying solution compositions was demonstrated to result from the differences in in-plane stress generated during heating. By optimizing the processing parameters an 810 nm thick TiO2–HPC hybrid film of a refractive index of 1.84 was obtained.