Jul 14, 2009
Laser pulse transforms metal grating into nanodot array
Metal nanodots are useful in many fields such as catalysis, environmental remediation, DNA detection and high-density data storage, to give just a few examples. Periodic arrays are required for certain applications and major fabrication methods include self-assembly-based wet chemical processes and lithography-based nanopatterning.
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.
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.
About the author
Dr Qiangfei Xia received his PhD in electrical engineering from Princeton University and is currently a research associate at Hewlett-Packard Labs in Palo Alto, CA. His research interests include nanoimprint lithography, nanofabrication and their applications in nanoscale devices and system integration. Dr Stephen Y Chou is the Joseph C Elgin Professor of engineering and head of the NanoStructure Laboratory at Princeton University. Dr Chou is a world leader, pioneer and inventor in a broad range of nanotechnologies, among which are nanoimprint lithography, quantized magnetic disks (bit-patterned media), ultra-small transistors and sub-wavelength optical elements.