Crystallization of Binary Mixtures in Photothermal Self-Assembly

We experimentally investigate crystal formation induced by light-driven thermal convection. In our experiment, gold nanoparticles (15 nm) absorb light and generate a temperature gradient, which induces a convection flow. The smaller gold nanoparticles travel alongside the flow, while the larger particles (0.8 μm) settle near the bottom of the sample chamber, forming a self-assembled crystalline monolayer. We found that the number of particles (0.8 μm) in the monolayer rises with increasing light intensity; however, crystallization only occurs when a critical number of particles have gathered. This behavior resembles the two-step nucleation event observed in colloidal self-assembly. Additionally, we are investigating how a binary mixture of particles with different diameters, such as (1 μm & 10 μm) and (2 μm & 10 μm), assemble under photothermal convection. We observed that the larger particles (10 μm) form crystals at lower light intensities, while the smaller particles (1 μm & 2 μm) crystallize rapidly at higher light intensities. Stronger hydrodynamic interactions and the influence of drag in larger particles, combined with the impact of two-step nucleation in smaller particles, likely cause the intensity-dependent crystallization dynamics.