"Our goal is to fabricate high-performance flexible electronics," Youn Sang Kim of Ewha Womans University told nanotechweb.org. "Using surface and interface engineering techniques we hope to produce various devices based on the assembly of nanomaterials, such as displays or lab-on-a-chip systems."

Stamp and submerge
The first step in the team's nanoparticle packing process is to create a master template. Made from a PDMS-coated polymer mould, the flexible structure can withstand more than 200 imprints and is used to fabricate a replica pattern in a layer of pentaerythritol propoxylate triacrylate (PPT).

Once UV-cured, the PPT replica provides a robust template for confining the nanoparticles within a well defined area. To fill the template, the researchers simply dip the patterned PPT substrate into a dispersion of nanoparticles in deionised water.

The temperature of the solution is held at 85 °C for seven hours to allow the solvent to evaporate over the PPT replica and fall below the level of the template. To ensure that the nanoparticles remain suspended in the liquid, a stirrer is included within the set-up and is set to rotate at a rate of 100 rpm.

Three samples (a bare PPT surface, a hydrophilic-treated surface and a hydrophobic-treated surface) were used to examine the influence of surface wettability on the quality of the assembled nanoarrays. Only the bare PPT surface with a contact angle of 68.1°, which lies somewhere in between the hydrophilic (7°) and hydrophobic (99.5°) treatments, produced a selectively packed template. In this case, all of the polystyrene nanospheres were located within the defined packing zone.

The results provide further evidence that the selective ordering of the nanoparticles is affected by both the capillary forces at the drying front of the meniscus and the physical confinement of the template.

The researchers presented their work in Nanotechnology.