The University of Birmingham team has put its centimeter-sized hybrid metallic nanostructured arrays to the test by monitoring the shift in the chip's extinction spectrum following the addition of absolute ethanol. Initially, the surface produced a spectrum with peaks at 585.3 and 493.6 nm, but when a drop of ethanol was placed on the chip the peaks shifted to 451.3 and 618.2 nm.

Pattern transfer
To make its sensing device, the group uses a combination of interference lithography, metal deposition and etching. First, the team deposits a 20 nm chromium film on a 2 × 2 cm fused silica substrate that has been cleaned and baked (170 °C for 1 hour). Next, a 120 nm layer of AZ3100MI positive resist is spun on top of the chromium film followed by a post-application bake at 100 °C for 15 mins.

Next it's time to pattern the substrate. The researcher's interference lithography set-up is based on a Lloyd's mirror configuration. A mirror is placed normal to the substrate and illuminated with an s-polarized laser beam with a wavelength of 442 nm. Part of the incident laser beam is reflected by the mirror and interferes with the unreflected part of the beam to form an optical pattern of light and dark lines.

It's possible to reuse the substrates many times by first removing the silver layer and then depositing a fresh filmKyle Jiang, University of Birmingham

The resist is exposed by the interference fringe for 8 seconds. After rotating the sample by 90 °, the exposure is repeated to form an array of nanopillars. The resist is then developed in Microposit 303A for around 30 seconds and rinsed in deionized water. Next, the resist residue is removed using reactive ion etching and a second etching process is initiated to transfer the array pattern into the fused silica substrate.

Finally, a 40 nm thick film of silver is deposited on the fused silica nanopillars to create a periodic array of metallic nanostructures that exhibits localized surface plasmon resonance.

"Our large area structures can be divided up to compare experimental results," Kyle Jiang of the University of Birmingham's micro and nanoengineering group told "It's possible to reuse the substrates many times by first removing the silver layer and then depositing a fresh film."

Fine tuning
As Jiang points out, the pillar size can be adjusted by selecting different exposure doses and the pitch can be modified by changing the incident angle of the interference lithography beam. By fine tuning the structure, the researchers hope to obtain an array with stronger field enhancement properties.

The researchers presented their work in Nanotechnology.