Micelle-assisted formation of nanoparticle superlattices and thermally reversible symmetry transitions

Abstract

Nanoparticles are of great interest because of their surface-to-volume ratio as well as unique/outstanding properties, which can be applied to a broad range of applications such as nanoparticle-based catalysts, energy storage materials, optical device, and bio-sensors. The studies on various methodologies have been extensively explored for the fabrications of nanoparticle superlattices (the highly regular arrangement of nanoparticles). Recently, their new unique and emergent properties have been reported when the nanoparticles assembled into superlattices. Their properties are also dependent on the symmetry of nanoparticle superlattices. The symmetry transition of nanoparticles depending on environmental conditions can be an excellent ground for making new stimuli-responsive functional materials. In other words, the fabrication and control for the nanoparticle superlattices are essentially required to open up new possibilities for next-generation nanodevices. This dissertation reports a spherical micelle-assisted method to form exceptionally ordered nanoparticle superlattices, which are inherently sensitive to environmental conditions. Upon mixing functionalized gold nanoparticles (AuNPs) with a nonionic surfactant spherical micellar solution, nanoparticle superlattices of different symmetries such as NaZn13, MgZn2, and AlB2-type are formed depending on the size ratio and composition ratio between micelles and functionalized AuNPs. The nanoparticle superlattices formed by the spherical micelle-assisted method show thermally reversible order-order (NaZn13–AlB2) and order-disorder (MgZn2–isotropic) symmetry transitions, which are consistent with the Gibbs free energy calculations for binary hard-sphere model. The approaches may open up new possibilities for other metallic, semiconducting and magnetic nanoparticle systems, which can be used in a variety of applications.