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Complex 3D metallic structures can now be manufactured at nanoscale

19 October 2012

Material fabrication relies on the controlled deformation of metals by industrial processes such as bending, shearing, and stamping. But is this technology transferable to the nanoscale?

Scientists from Aalto University in Finland and the University of Washington in the US have now demonstrated this to be possible. By combining ion processing and nanolithography they have managed to create complex three-dimensional structures at nanoscale.

The discovery follows from a quest for understanding the irregular folding of metallic thin films after being processed by reactive ion etching.

"We were puzzled by the strong-width-dependent curvatures in the metallic strips. Usually initially-strained bilayer metals do not curl up this way", explains Aalto University's Khattiya Chalapat.

The puzzle began to unravel when Chalapat noticed, together with Dr. Hua Jiang, that the Ti peak was absent from the EDX spectra of folded Ti/Al bilayers.

Further experiments at OV Lounasmaa Laboratory confirmed that the strips bend upward with strong width-dependent curvatures if their bottom layer is made more reactive to ions than the top surface.

In nature, similar geometrical effects take place in self-organisation directly observable to the human eye. When the flower stem of a blooming dandelion is cutinto small strips then put them in water, and the strips will fold with observable width-dependent curvatures due to differences in the water absorption between the inside and outside parts of the stem.

The team attempted to find a way to adapt these natural processes to nanofabrication. They found that a focused ion beam can locally induce bending with nanoscale resolution.

The technology has various applications in the fabrication of nanoscale devices. The structures are surprisingly resilient; the team found them to be quite sturdy and robust under a variety of adverse conditions, such as electrostatic discharge and heating.

"Because the structures are so small, the coupling and the magnitude of typical nanoscale forces acting on them would be commensurately small," says Docent Sorin Paraoanu, the leader of the Kvantti research group at Aalto University.

"As for applications, we have demonstrated so far that these structures can capture and retain particles with dimensions of the order of a micrometer. However, we believe that we are just scratching the tip of the iceberg; a comprehensive theory of ion-assisted self-assembly processes is yet to be reached," notes Paraoanu.

Microfabrication was undertaken at the Aalto University research facilities for microfabrication and the imaging was undertaken at Micronova Centre for Micro and Nanotechnology and the Nanomicroscopy Centre in Finland. 

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