Wet chemical processes offer high throughput, but the nanoparticle component has to be biocompatible or water soluble. On the other hand, lithography tools such as electron beam lithography and focused ion beams allow well defined and well positioned nanodots, but are limited by low throughput and small patterning regions.

Melting induced fragmentation

To address these issues, researchers at Princeton University, US, have developed a novel and simple way of making periodic metal nanodot arrays over large areas. The group has dubbed its low-cost, high-throughput technique "melting induced fragmentation" (MIF).

First, the scientists use nanoimprint lithography (NIL) to pattern metal (gold) nanogratings on a substrate. Next, the team melts the linear structures with a single laser pulse, which fragments the gratings into arrays of round and periodic metal nanodots. The formation of the metal dot array is attributed to the Rayleigh instability in a narrow line of liquid.

System upgrade

To further improve the periodicity of the nanodots, the researchers use pre-patterned substrates under the grating to regulate the fragmentation process. Pre-patterned shallow trenches perpendicular to the metal grating encourage the molten metal to flow into the trench/grating cross-points to minimize the system energy. As a result, the period of the nanodots in the direction of the original nanograting is determined by the period of the shallow trenches rather than the natural MIF, which leads to nanodot arrays with regular periodicity.

The method demonstrated by the Princeton group inherits the low-cost, high-throughput advantages of NIL. In addition, the thermal effect of a single 20 ns laser pulse on the substrate is negligible, which makes the MIF technique suitable for different support materials including plastics. The simple fabrication method could be extended to other metals and has a wide range of applications in areas such as magnetics, plasmonics, surface enhanced Raman scattering and other photonic devices.

The researchers presented their work in Nanotechnology.