Plasmonic Control and Stabilization of Asymmetric Light Scattering from Ag Nanocubes on TiO₂

When plasmonic nanoparticles are placed on a highly refractive semiconductor substrate, we can expect three different effects: (i) resonance mode splitting, (ii) asymmetric light scattering based on the split modes, and (iii) site-selective nanoetching due to plasmon-induced charge separation (PICS) at the nanoparticle-semiconductor interface. Here, we develop novel photofunctional materials by taking advantage of those three effects. More specifically, we control the asymmetric scattering of Ag nanocubes on TiO₂ by PICS, so as to develop materials for photodrawing of one-way visible translucent images and multicolor scattering images. The one-way visible translucent images, which are translucent scattering images visible only from the back side, are drawn by anaerobic bottom-selective etching of the Ag nanocubes. For drawing the multicolor scattering images, a scattering color of Ag nanocubes is changed from yellow to green by the anaerobic bottom-selective etching and from yellow to red by aerobic nonselective etching. We also theoretically and experimentally examined the contribution of a possible thermal effect to the nanoetching, and revealed that the contribution is negligible; Ag nanocubes on TiO₂ are stable even at 473 K for 2 h in the dark, whereas the theoretically expected temperature increase under light is less than 1 K. In addition, we developed methods to stabilize the Ag nanocubes by inactivating PICS. When Ag nanocubes on TiO₂ are coated with a thin polymer layer, PICS is decelerated and the stability is improved. Replacing TiO₂ with diamond, which does not accept electrons from plasmonic nanoparticles, is also an effective method to stabilize the nanocubes.