Sibu C. Padmanabhan

We report here (i) a method for the synthesis of highly monodisperse silica@gold@silica core–shell–shell (CSS) particles (N,N-dimethyldodecylamine (DMDDA), as a catalyst for the outer silica shell formation, for the reproducible synthesis of monodisperse CSS particles avoiding the formation of the unwanted side-products such as core-free silica particles and aggregated particles with multiple cores. A rational chemical reaction mechanism describing the pathways involved in the formation of coherent silica shells, demonstrating the roles of both the amines, is proposed. The creation of partial wetting/de-wetting sites on gold nanoparticle surfaces by the adsorbed DAP molecules, the surface silanol modification by DAP and the slow hydrolysis–condensation reactions catalyzed by DMDDA are found to be responsible for the selective deposition of silica (heterogeneous nucleation) only onto the existing silica@gold core–shell particles leading to the formation of monodisperse CSS particles. The optical reflectance of the MDPC formed from these CSS particles shows an angle-dependent blue-shift that perfectly obeys the Bragg–Snell law predictions for photonic crystals. The dielectric contrast modification and the possible metal plasmonic mode-coupling in the MDPC also result in a high gap-to-midgap ratio (23%) as compared to the PC made of neat silica particles of comparable size (9%).

The microwave (MW)-assisted formation of ZnO micro-javelins from zinc nitrate and urea in aqueous solution is described. The particles (named as ‘micro-javelins’ because of their high aspect ratio and needle-like tips) grow hexagonally with well-defined facets in the 〈010〉 direction and pointed tips in (0001) direction. Powder X-ray diffraction patterns show the appearance of a strikingly dominant (1000) orientation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations reveal the morphological evolution of these hexagonal ZnO particles with time. The effect of precursor concentrations, counterion type and MW irradiation power and their consequent influence on pH and Zn²⁺ ion concentration are investigated. A mechanism for the formation of the micro-javelins is postulated. The microwave induced supersaturation of Zn(OH)⁺ species under the weakly basic pH condition and the initial growth through the (000) direction (oxygen-rich face) are proposed to be the key factors that dictate the formation of these ZnO micro-javelins. The present one-step microwave process is a straightforward and a reproducible method for the bulk synthesis of defect-free ZnO micro-javelins, which would find potential applications in microelectronic devices (e.g. lasers, cantilevers in surface probing equipment, etc.).