Silica Overcoating of CdSe/CdS Core/Shell Quantum Dot Nanorods with Controlled Morphologies

CdSe/CdS core/shell quantum dot nanorods (QDNRs) exhibit anisotropic optical properties. Overcoating QDNRs with silica (SiO2) shells is desirable for protecting the surface of the inorganic core, imparting dispersibility in water, and allowing functionalization with silanes. While several methods have been developed for encapsulating spherical CdSe-based quantum dots in SiO2, extension of these approaches to QDNRs has been limited. Here, we report a reverse microemulsion approach for controlled deposition of SiO2 overcoatings with thicknesses of 2-12 nm onto CdSe/CdS QDNRs with aspect ratios of up to 19. Addition of poly(ethylene glycol) silane during SiO2 deposition terminates the reaction and allows facile control over the shell thickness, especially for thinner shells. By independently controlling the amounts of tetraethyl orthosilicate, ammonium hydroxide (NH4OH), and water, morphological control is achieved, giving uniform SiO2 shells or heterogenenous deposition onto the ends of QDNRs as lobed structures, nanolollipops (single lobe) or nanodumbbells (double lobes). Especially for high-aspect ratio QDNRs, we report breakage of the double-lobed structures. The SiO2 overcoating process causes a reduction in the fluorescence quantum yield of approximately 20%, which we attribute to use of NH4OH as a catalyst in this reaction.