Nonlinear Optical Plasmonic Core-Shell Nanocavities – Supporting Information

Synthesis of BaTiO 3 /Au core-shell nanocavities. The principle of the synthesis of BaTiO3/Au core-shell nanocavities is shown in Fig. S1. All chemicals were of reagent-grade purity and were purchased from Sigma-Aldrich unless specified. 100 nm tetragonal BaTiO 3 nanoparticles were provided by Paul Bowen of the Powder Technology Laboratory at EPFL. Ultraviolet spectroscopy indicated a band gap at 3.2 eV (data not shown) for the BaTiO 3 nanoparticles. Milli-Q grade deionized water was used throughout the procedure. The synthesis of BaTiO 3 /Au core-shell nanostructures was based on seeded Au growth, which consists of the following three steps. 1. Introduction of primary amine on the surface of BaTiO 3 nanoparticles. In order to remove the surface Ba 2+ ions that prevents silane binding, 30 mg 100 nm BaTiO 3 nanoparticles (10 12 particles/cm 3) were added to 9 mL 1 M nitric acid solution. The solution was ultrasonicated for 10 minutes for sufficient surface ion removal and dispersion of the nanoparticles. The particles were then washed twice with deionized water using centrifuge at 8000× g for 10 minutes each, after which the particles were transferred into a 9 mL solution that consists of 7.65 mL ethanol, 1.25 mL deionized water, and 0.1 mL ammonia (30%). The mixture was placed on a hotplate and heated to 60°C under vortex stirring. After the solution temperature was stabilized, 8 µL of Aminopropyltriethoxysilane (APTES) was injected under vortex stirring. The reaction was allowed to proceed by stirring the solution at 60°C for at least four hours. After the introduction of primary amine, the solution was washed four times with ethanol and the