The researchers, led by Yuebing Zheng of the University of Texas at Austin, began by dispersing colloidal atoms of different materials and sizes in a solvent (water). They then added the ionic surfactant cetyltrimethylammonium chloride into the suspension. This process yields positively charged particles, positive micelles and negative ions.

Next, they shine a laser on a gold film (immersed in the suspension) to create a thermal hot spot. The temperature is highest at the hot spot and gradually decreases away from it in the surrounding medium, thus creating a temperature gradient. Micelles, ions, and particles migrate along this temperature gradient in a process called thermophoresis and this migration confines the colloidal particles at the hot laser spot. What is more, as micelles move towards the colder region, this also generates an osmotic pressure that helps bond the particles together.

“By exploiting these physics processes, we can then selectively capture colloidal particles in the suspension and organise them into diverse colloidal superstructures at will,” explains team member and lead author of the study Linhan Lin.

General technique

The technique is a general one and can be used to build an assortment of colloidal superstructures with complex configurations using colloidal atoms of a wide range of materials and sizes, he adds. “For example, we succeeded in building a 1D hybrid chain, a 2D hybrid lattice, and a 2D double-layer ‘Saturn ring’ built from polystyrene beads that are 2 and 0.96 mm in diameter. As well as assembling spherical particles, we also assembled non-spherical particles into 2D colloidal matter.

“We assembled a gold nanoparticle and a polysytrene bead into a heterogeneous dimer too. This dimer is also known as an optoplasmonic molecule, where photonic-plasmonic coupling can lead to interesting optical phenomena.”

Being able to tune interparticle bonding and precisely control colloidal configurations in this way, we are now able to assemble colloids with sizes comparable to or smaller than the wavelength of light, says Lin. “These particles strongly respond to light, and could be useful for making photonic colloidal devices, such as colloidal lasers or devices with optical chirality.”

Better than previous colloidal assembly techniques

The new technique is better than previous colloidal assembly techniques in many ways, Zheng tells “Techniques such as self-assembly and field-directed assembly do feature high throughput but they can only produce limited structural configurations. Although field-directed assembly does produce structures that can be tuned to some extent, only colloidal particles with specific physical (electric or magnetic) properties respond to external fields.”

The team, which includes researchers from Johns Hopkins University and Pennsylvania State University, is now busy applying its opto-thermophoretic assembly approach to functional colloids such as plasmonic and semiconducting nanoparticles with different configurations in an effort to better the understand how particles in a suspension couple. “We are also building different functional optical devices with designer optical properties using our technique,” reveals Zheng.

The technique is detailed in Science Advances DOI: 10.1126/sciadv.1700458.