Controlling the crystallinity and nonlinear optical properties of transparent TiO2–PMMA nanohybrids

Abstract

Titania–polymer nanohybrid thin films represent a new class of potential materials for optoelectronic applications. While most such nanohybrid thin films lack control in crystallinity, we report in this paper transparent nanohybrids of titania-polymethyl methacrylate (TiO₂–PMMA) thin films having a remarkably enhanced nanocrystallinity. Post-treatments with water vapor at relatively low temperatures were applied on these thin films, following in situ sol–gel polymerization. They promoted rearrangement of flexible Ti–O–Ti bonds leading to enhanced crystallization of the TiO₂ phase. The degree of TiO₂ crystallinity in the resulting nanohybrid films was studied by using XRD, FTIR, UV–Vis spectroscopies and HRTEM. Both linear and nonlinear optical responses increase with the enhancement of TiO₂ crystallinity in the nanohybrids. The highest two-photon absorption coefficient (β) and nonlinear refractive index (n₂) were observed for the nanohybrid thin films with highest TiO₂ crystallinity, as confirmed by open and closed aperture Z-scan techniques using 250 fs laser pulses at 800 nm, having a value of 2260 cm GW⁻¹ and 6.2 × 10⁻² cm² GW⁻¹, respectively.