Giant O₂‑Induced Photoluminescence Modulation in Hierarchical Titanium Dioxide Nanostructures

We demonstrate exceptionally large modulation of PL intensity in hierarchical titanium dioxide (TiO₂) nanostructures exposed to molecular oxygen (O₂). Optical responsivities up to about 1100% at 20% O₂ concentrations are observed in hyperbranched anatase-phase hierarchical structures, outperforming those obtainable by commercial TiO₂ nanopowders (up to a factor of ∼7 for response to synthetic air) and significantly improving the ones typically reported in PL-based opto-chemical gas sensing using MOXs. The improved PL response is discussed in terms of the specific morphology of hierarchical structures, characterized by simultaneous presence of small nanoparticles, large surface areas, and large voids. These characteristics guarantee an optimal interplay between photogenerated charges, PL-active centers, and adsorbed gas molecules. The results highlight the potentialities offered by hierarchical structures based on TiO₂ or other MOXs and open interesting scenarios toward the development of all-optical and/or hybrid (opto/electrical) chemical sensors with improved sensitivity

Direct Evidence of Polar Ordering and Investigation on Cytophilic Properties of Pyroelectrified Polymer Films by Optical Second Harmonic Generation Analysis

Pyroelectrification (PE) has been proposed as a low-cost electrode-free tool for orienting and aligning dipole molecules in polymer layers, by means of the electric field produced by a pyroelectric crystal under appropriate thermal stimulations. Probing and assessing the coherent polarization arrangement is of fundamental importance for the process control and optimization but is also a challenging task. In fact, the probing operation must be noninvasive and avoid any disarrangement the dipoles and thus without interfering with the resulting surface charges. Here we show that the PE-induced polar ordering can be probed <i>in situ</i> by an electrode-free analysis based on the measurement of the intensity of the second-harmonic optical wave generated by the polymer film. In fact, the results show a substantial enhancement of the second-order susceptibility of the polymer layer, caused by the PE-induced dipoles alignment. Moreover, thanks to this approach, it is demonstrated the ability of PE process to polarize polymer layers even in the “noncontact configuration”, i.e., when a dielectric spacer (glass substrate) is placed between the pyroelectric crystal and the polymer film