The formation of NH4TiOF3 mesocrystals can be largely manipulated by controlling the colloidal and chemical stabilities of NH4TiOF3 nanoparticles as the building blocks. The factors studied in the present work are temperature and the concentration of F127 (a triblock copolymer). At 23 °C and 35 °C, the presence of 10% F127 gives rise to the formation of submicron particles, whereas, a lower concentration of F127 results in submicron particles and aggregates, and a higher concentration produces a combination of NH4TiOF3 mesocrystals and submicron particles. The process is dominated by the formation of NH4TiOF3 mesocrystals at 4 °C, where the chemical conversion from NH4TiOF3 nanoparticles to TiO2 nanoparticles is significantly slowed down. A pancake-like conformation of F127 molecules on the hydrophilic particle surfaces is adopted to explain the concentration dependent steric effect and hydrophobic attraction imparted by their PPO chains. The TiO2 nanocrystals derived from calcination show superior photocatalytic performance, in comparison to TiO2 mesocrystals, due to the much higher specific surface area.
Plasmonic nanoclusters, an ordered assembly of coupled metallic nanoparticles, support unique spectral features known as Fano resonances due to the coupling between their subradiant and superradiant plasmon modes. Within the Fano resonance, absorption is significantly enhanced, giving rise to highly localized, intense near fields with the potential to enhance nonlinear optical processes. Here, we report a structure supporting the coherent oscillation of two distinct Fano resonances within an individual plasmonic nanocluster. We show how this coherence enhances the optical four-wave mixing process in comparison with other double-resonant plasmonic clusters that lack this property. A model that explains the observed four-wave mixing features is proposed, which is generally applicable to any third-order process in plasmonic nanostructures. With a larger effective susceptibility χ (3) relative to existing nonlinear optical materials, this coherent double-resonant nanocluster offers a strategy for designing high-performance third-order nonlinear optical media.
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