Colloidal particles exhibit intriguing collective behavior beyond their individual properties when they are organized into colloidal matter. Treating colloidal particles as individual atoms, researchers are exploring new strategies to build complex colloidal structures for new functional devices. In particular, the organization of colloidal particles with sizes comparable to the optical wavelength (also known as optical matter) will enable diverse novel optical performances, including structured color imaging, Fano resonator, and chiral metamaterials. However, the optical properties of colloidal matter are highly sensitive to the geometric configurations, which requires rigorous and precise structural control and gives rise to the challenge in the development of assembly approach.
Herein, we propose a light-directed particle-to-particle assembly technique - opto-thermophoretic assembly (OTA) - to build colloidal matter at a low optical power. To enable the optical manipulation and assembly, we add an ionic surfactant (i.e., cetyltrimethylammonium chloride) into the particle suspension, which serves as a surface charge source, a macro ion, and a micellar depletant above its critical micelle concentration. Taking advantage of the thermophoretic migration of different ionic species in the fluidics and the plasmon-enhanced photon-phonon conversion, we create a thermoelectric field to manipulate colloidal atoms at single-particle resolution. Specifically, the depletion of the ionic micelles provides a strong attractive force to achieve tunable inter-particle bonding. The OTA strategy is applicable to colloidal particles in a wide range of sizes, shapes, and materials.
The OTA strategy can release the rigorous design rules required in the existing assembly techniques and enriches the structural complexity in colloidal matter, which will open a new window for rational design of functional colloidal devices. As an example, we demonstrate the application of the OTA approach in the reconfigurable assembly of colloidal chiral metamolecules. Selecting plasmonic metallic nanoparticle and dielectric silicon nanoparticles as the building block, we succeeded in the assembly of colloidal metamolecules with optical chirality. Beyond the geometry chirality well known in traditional chiral materials, we demonstrate the composition chirality using colloidal particles with different materials, but with the same size. Specifically, the colloidal metamolecules can be switched between their left-handed and right-handed configurations through controlling the light field. In addition, we study the origin of chirality in the colloidal metamolecules by rationally changing their configurations, and thus controlling the inter-particle interaction.