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Scientists create multi-layered, self-assembling nanoscale grids

23 June 2015

Scientists develop a technique that creates nano-structured grids from a variety of materials for applications ranging from anti-reflective coatings to touchscreens.

The sweeping laser induces intense heat that both accelerates polymer self-assembly and precisely aligns the nano-cylinders that form the foundation of the final grid (illustration: Brookhaven National Laboratory)

“We can fabricate multi-layer grids composed of different materials in virtually any geometric configuration,” says scientist Kevin Yager of the US Department of Energy's Brookhaven Laboratory, where the work was carried out. “By quickly and independently controlling the nanoscale structure and the composition, we can tailor the performance of these materials. Crucially, the process can be easily adapted for large-scale applications.”

The results, published online June 23 in the journal Nature Communications, could transform the manufacture of high-tech coatings for anti-reflective surfaces, improved solar cells, and touchscreen electronics.

The scientists synthesised the materials and characterised the nanoscale architectures using electron microscopy. Their technique relies on polymer self-assembly, where molecules are designed to assemble spontaneously into desired structures.

Self-assembly requires a burst of heat to make the molecules snap into the proper configurations. Here, an intensely hot laser is swept across the sample to transform disordered polymer blocks into precise arrangements in just seconds.

“Self-assembled structures tend to automatically follow molecular preferences, making custom architectures challenging,” says lead author and Brookhave researcher, Pawel Majewski. “Our laser technique forces the materials to assemble in a particular way. We can then build structures layer-by-layer, constructing lattices composed of squares, rhombuses, triangles, and other shapes.”

For the first step in grid construction, the team took advantage of their recent invention of laser zone annealing (LZA) to produce the extremely localised thermal spikes needed to drive ultra-fast self-assembly.

To further exploit the power and precision of LZA, the researchers applied a heat-sensitive elastic coating on top of the unassembled polymer film. The sweeping laser’s heat causes the elastic layer to expand (a bit like shrink-wrap in reverse) which pulls and aligns the rapidly forming nanoscale cylinders.

“The end result is that in less than one second we can create highly aligned batches of nano-cylinders,” says study co-author Charles Black. “This order persists over macroscopic areas and would be difficult to achieve with any other method.”

To make these two-dimensional grids functional, the scientists converted the polymer base into other materials. One method involved taking the nano-cylinder layer and dipping it into a solution containing metal salts. These molecules attach themselves to the self-assembled polymer, converting it into a metallic mesh. A wide range of reactive or conductive metals can be used, including platinum, gold, and palladium. Using vapour deposition, they were als able to infiltrate the polymer nano-cylinders with vaporised material, transforming them into functional nano-wires.

The first completed nano-wire array acts as the foundation of the full lattice. Additional layers, each one following variations on that same process, are then stacked to produce customised, criss-crossing configurations 10,000 times thinner than a human hair.

“The direction of the laser sweeping across each unassembled layer determines the orientation of the nano-wire rows,” says Yager. “We shift that laser direction on each layer, and the way the rows intersect and overlap shapes the grid. We then apply the functional materials after each layer forms. It’s an exceptionally fast and simple way to produce such precise configurations.”

“We can stack metals on insulators, too, embedding different functional properties and interactions within one lattice structure," adds co-author, Atikur Rahman. “The size and the composition of the mesh make a huge difference; for example, a single layer of platinum nano-wires conducts electricity in only one direction, but a two-layer mesh conducts uniformly in all directions.”

LZA is precise and powerful enough to overcome interface interactions, allowing it to drive polymer self-assembly even on top of complex underlying layers. This versatility enables the use of a wide variety of materials in different nanoscale configurations.

“We can generate nearly any two-dimensional lattice shape, and thus have a lot of freedom in fabricating multi-component nanostructures,” says Yager. “It’s hard to anticipate all the technologies this rapid and versatile technique will allow.”

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