A binary system to investigate photothermally driven self-assembly of colloidal particles and nanocomposites

Photothermal convection and thermophoresis are known to drive the assembly of micro- and nanoscale particles. We demonstrate a simple experimental system using gold nanoparticles and high-intensity LED light to realize rapid, large-scale self-assembly of microscale colloidal particles and surfactant-nanoparticle composites. Photothermal heating of the gold nanoparticle (diameter d = 15 - 50 nm) suspension generates a temperature gradient, causing a convective flow and self-assembly of other particles (D = 0.7 – 10 μm) present in the solution. Interestingly, we find a transition from 2D monolayers to 3D clusters as the assembled particle size gets smaller than 1 μm. We show that the crystalline order of the assembly is determined by the surface charge of the gold nanoparticles that contributes to the screening of the repulsive interactions between the microscale colloidal particles. We demonstrate a patterned light illumination system that can be used to achieve control over the assembly size. Additionally, we use this system to drive the self-assembly of surfactant (CTAB) - gold nanoparticle composite microstructures of various 3D shapes depending on particle concentration and temperature. All these demonstrations show the potential of this binary particle system for studying non-equilibrium multiscale and multi-material assemblies.