Aug 12, 2011
Template stripping produces nanohole arrays
Following Thomas Ebbesen's discovery of extraordinary light transmission over 10 years ago, scientists have been busy investigating plasmonic nanohole arrays - in which the phenomenon occurs - for applications in biochemical sensing, Raman spectroscopy and photovoltaics. One major problem, however, is that it is still relatively difficult to produce metallic nanoholes, which, at present, can only be patterned using expensive lithography and etching processes. Now, a team of researchers at the University of Minnesota-Twin Cities may have overcome this problem because they have succeeded in producing centimetre-sized ultra-smooth nanoholes arrays by depositing metal on a reusable silicon mould. The technique could help speed up the development of nanohole arrays devices, they say.
Norwegian physicist Thomas Ebbesen discovered extraordinary light transmission (EOT) in 1998. He found that, contrary to what was then believed, it was possible to transmit light through nanosized apertures in metal films under certain conditions. Scientists now know that this phenomenon involves surface plasmon polaritons – surface excitations on a metal that involve billions of electrons.
Compared with crystalline silicon wafers, however, metal films are difficult to pattern. They cannot be plasma etched and as-deposited metal films have rough surfaces. Silicon wafers, on the other hand, can be engineered to nearly atomic-scale perfection. Ideally, researchers would like to reproduce the nanoscale patterns found on silicon wafers in metallic films. This would allow them to take advantage of well known and widely used silicon processing technologies and avoid patterning metal films directly.
Luckily, such a technique exists. It is called template stripping and is already beloved of scanning probe microscopists who use it to make unpatterned ultra-smooth metal films. It can also be employed to make patterned metals, as David Norris' and Sang-Hyun Oh's teams already demonstrated in 2009.
Now, the same group has gone a step further and shown that template stripping can make centimetre-sized plasmonic nanohole arrays in optically thick (>100 nm) metal films, and that these structures can then be used for plasmonic biosensing.
The researchers began by making a silicon mould that contains deep nanoscale pits produced using nano-imprint lithography. "During top-down metal evaporation, these pits force the silver atoms to form the nanohole patterns," Oh told nanotechweb.org. "We can then easily peel off the patterned metal film using a sticky material such as epoxy, or even Scotch tape," he added. "In this way, only a single lithography and etching step is needed in the entire process of producing hundreds of identical arrays." More importantly, such template-stripped films are as smooth as silicon wafers – typically with 1–2 nm high roughness – something that reduces unwanted scattering of light in the arrays and subsequent damping of surface plasmon waves.
Normally, silver is not biocompatible – gold is better in this respect – but the researchers overcame this problem by coating the freshly stripped silver nanoholes with thin silica shells. An advantage of doing this is that proteins, antibodies and other biomolecules can then easily be attached to the silica for subsequent biosensing applications. "For biosensing, the quality of these molecular recognition elements is as important as the raw performance of the underlying solid-state sensor," explained Oh. "With ultra-smooth silver films and silica shells, we now have a nice platform to interface high-performance plasmonic nanohole sensors with a variety of biomolecules and lipid membranes."
Hatice Altug of the University of Boston, who was not involved in the work says: ""Plasmonic nanohole arrays have recently attracted much attention as they are promising for wide range of applications - for example, these subwavelength-sized holes could be ideal for biosensing applications. However, their widespread use also depends on their being built using high-throughput, low-cost and large-area nanofabrication techniques that produce plasmonics structures of a high optical quality. In this regard, the template-stripping method described in this new work is very practical and should be widely adapted in the plasmonics field."
Teri Odom of Northwestern University (again not involved in this research) adds: "This is clever way to achieve large area, ultra-flat, silver nanohole arrays that can easily be integrated into real-time sensing platforms. Their innovation lies in the deposition of metal on single crystalline silicon templates followed by template stripping. Another really nice aspect of the work is that the silicon template can be reused to make many copies of metal nanohole arrays."
Oh's team, which includes researchers from the Los Alamos National Laboratory, agrees and says that the template-stripping technique could benefit biologists and chemists who want to use plasmonic nanostructures without having to rely on expensive lithographic processing. "Once they possess a master mould, all they need is access to a metal evaporator and glue," said Oh. "Beyond nanohole arrays, we have previously shown that template stripping can produce a wide range of metallic nanostructures, including ultra-sharp tips, gratings, and multi-layer metamaterials. We hope that more researchers will now practice this simple yet powerful technique to produce ultra-smooth metals with high throughput."
The work was published in ACS Nano.
About the author
Belle Dumé is contributing editor at nanotechweb.org