Rapid Assembly of Colloidal Crystals under Laser Illumination on a GeSbTe Substrate.

Optical techniques have been actively studied for manipulating nano- to microsized objects. However, long-range attraction and rapid transport of particles within thin quasi-two-dimensional systems are difficult because of the weak thermophoretic forces. Here, we introduce an experimental system that can rapidly generate quasi-two-dimensional colloidal crystals in deionized water, sandwiched between two hard plates. When a pulsed laser is irradiated on a chalcogenide phase-change material spattered on one side of the plates, the induced Marangoni-like flow causes a colloidal self-assembly in the order of tens of micrometers within the laser spot, with a transport velocity of a few tens of micrometers per second. This is due to the large thermal gradient induced by chalcogenide characteristics of high laser absorption and low thermal conductivity, and a strong hydrodynamic slip flow at the hydrophobic chalcogenide interface. Moreover, the colloidal crystals exhibit various lattice structures, depending on the laser intensity and chamber distance. For a certain range of the chamber distance, the colloidal crystal phases can be alternated by tuning the laser intensity in real time. Our system forms and deforms quasi-two-dimensional colloidal crystals at an on-demand location on a GeSbTe substrate.