Entropy-driven self-assembly of tethered Janus nanoparticles on a sphere

Understanding the effect of curvature and topological frustration on self-assembly yields insight into the mechanistic details of the ordering of identical subunits in curved spaces, such as the assembly of viral capsids, growth of solid domains on vesicles, and the self-assembly of molecular monolayers. However, the self-assembly of nanoparticles with anisotropic surface topology and compartmentalization on curved surfaces remains elusive. By combining large-scale molecular simulations as well as theoretical analysis, we demonstrate here that the interplay among anisotropy, curvature, and chain conformation induces tethered Janus nanoparticles to self-assemble into diverse novel structures on a sphere, including binary nanocluster (CB), trinary nanocluster (CT), nanoribbon (RN) and hexagon with centered reverse (HR), which are mapped on a phase diagram related to the length asymmetry of tethered chains and Janus balance of the nanoparticles. The dynamical mechanism for the formation of these structure states is analyzed by examining the detailed kinetic pathways as well as free energy. We also show that the centered-reverse state is more prone to emerging around the topological defects, indicating the defect-enhanced entropy effect on a curved surface. Finally, the analytical model that rationalizes the regimes of these structure states is developed and fits simulations reasonably well, resulting in a mechanistic interpretation based on the order through entropy. Our findings shed light on curvature engineering as a versatile strategy to tailor the superstructures formed by anisotropic building blocks toward unique properties.