Micelle-Assisted Formation of Nanoparticle Superlattices and Thermally Reversible Symmetry Transitions

Nanoparticle superlattices (NPSLs) are of great interest as materials with designed emerging properties depending on the lattice symmetry as well as composition. The symmetry transition of NPSLs depending on environmental conditions can be an excellent ground for making new stimuli-responsive functional materials. Here, we report a spherical micelle-assisted method to form exceptionally ordered NPSLs which are inherently sensitive to environmental conditions. Upon mixing functionalized gold nano particles (AuNPs) with a nonionic surfactant spherical micellar solution, NPSLs of different symmetries such as NaZn13, MgZn2, and AlB2-type are formed depending on the size ratio between micelles and functionalized AuNPs and composition. The NPSLs 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. This approach may open up new possibilities for NPSLs as stimuli-responsive functional materials.