Sep 2, 2011
Nanoscale copy, cut and paste
Focused ion beams (FIBs) are an indispensable tool in nanotechnology and especially in the up and coming fields of nano-optics and plasmonics. Researchers at the University of Minnesota-Twin Cities in Minneapolis and ETH Zurich have now used the technique in conjunction with a method called template stripping to engineer complex 3D metallic nanostructures whose features can be controlled to within less than 2 nm. Normally, such a feat is extremely difficult because direct FIB milling in metallic films exposes their rough grains, something that ultimately degrades device performance.
Plasmonics is a relatively new branch of photonics that exploits surface plasmons – collective oscillations of electrons that propagate on the surface of a metal. Plasmons can interact strongly with light scattered off patterns etched into the metallic surface, as long as the pattern dimensions are smaller than the wavelength of the light.
However, one problem is that plasmons are also scattered and absorbed by irregular bumps on a surface, which inevitably reduces the efficiency of plasmonic devices. And techniques traditionally used to fabricate nanostructures, like FIBs – which are used to carve out patterns on the metal surface – typically produce such uneven surfaces. What's more, implanted ions (another unwanted side effect of this technique) also lead to plasmons being absorbed.
Sang-Hyun Oh of Minnesota and colleagues say that they have now found a way around these hurdles. The researchers have employed FIB to fabricate sets of plasmonics devices with arbitrary arrangements of nanoscale bumps, grooves and apertures whose heights and depths can be controlled to within just 1–2 nm. Their method involves depositing metals (such as copper, gold or silver) onto a patterned silicon template wafer and then applying an adhesive to the imprinted metal. The metal and adhesive are pulled away together from the silicon to create an extremely smooth yet patterned surface that shows high-quality plasmonic resonances – that is, with very sharp and intense resonance peaks.
Using its technique, the team was able to make smooth, double-sided "bull's eye" devices – well known plasmonics structures that consist of a nano-aperture and concentric gratings that are perfectly aligned on both sides of a silver film. "Surprisingly, such a complex structure can be made with our one-step, peel-off processing technique," Oh told nanotechweb.org. "What's more, combining template stripping with an FIB instrument allows us to 'copy' metallic nanostructures from the patterned silicon mould with high fidelity by 'cutting', 'moving' and even 'pasting' these metallic films in situ."
Nathan Lindquist, lead author of the paper published in Nano Letters, says that the template-stripping method avoids roughness and contamination issues because the FIB patterns the template rather than the metallic film itself. "The end result is a pristine, ultra-smooth metallic surface that has superior optical properties in which the plasmon waves can propagate with low loss, at nearly their theoretical limits," he explained. FIB is very easy to perform in the silicon template – almost like "cutting into butter", according to the researchers – since it has a pure crystal structure. Metallic films, on the other hand, have many randomly distributed polycrystalline grains and each grain orientation can have a different FIB milling rate. This is what leads to the rough surfaces invariably observed in FIB treated metallic films.
And that was not all; because the plasmonic properties of the structures made closely match those produced in computer models, they can be used to study what happens to these nanostructures when they are heated, stretched, twisted or overcoated with other materials (such as biomolecules or quantum dots, for instance), claim the researchers. "Being able to make many identical copies of these structures with finely tuned resonances will be essential for such studies," said co-author David Norris of ETH Zurich.
About the author
Belle Dumé is contributing editor at nanotechweb.org