Titania-PMMA nanohybrids of enhanced nanocrystallinity

Nanohybrid thin films consisting of titania nanoparticles embedded in a suitable polymer matrix represent a new class of functional materials for optoelectronic applications. For instance, our previous studies show that titania-poly(methylmethacrylate) or TiO₂-PMMA nanohybrids derived from in situ sol-gel technique are promising as nonlinear optical materials since they can demonstrate a very fast recovery time of ∼1.5 picosecond and a large third-order nonlinear susceptibility, χ⁽³⁾ up to 1.93 × 10^-9 esu, as observed by pump-probe and Z-scan techniques. However, it was realized that the conventional in situ sol-gel technique employed led to a largely amorphous TiO₂ phase in the PMMA matrix. In order to overcome this limitation, we have performed two modification approaches making use of high pressure water vapor, i.e.: pre-and post-hydrothermal treatments applied on the sol-gel precursors. The degree of TiO₂ crystallinity in the resulting nanohybrids was studied by XRD, FTIR, and TEM. Both approaches can enhance the nanocrystallinity TiO₂ phase considerably and at the same time preserve the integrity of polymer matrix. It was found, however, that while the nanohybrid thin films derived from the pre-hydrothermal treatment show a high absorption in the visible length, those derived from the post-hydrothermal treatment are highly transparent. Nonlinear optical measurement using the Z-scan technique on the latter sample shows a significant enhancement in χ⁽³⁾ as high as 5.27 × 10^-9 esu.