Nanoscale ‘Pins’ Enhance SERS

Lynn M. Savage

Surface-enhanced Raman spectroscopy (SERS) is still a young technique, but it shows promise for elemental and chemical analysis in the semiconductor and biology fields, among others. It has been well established that using silver or gold nanoscale particles increases the sensitivity of the technique, but doing so requires mixing them with the substance to be analyzed — not always easy or convenient.

A scanning electron microscope image shows the basic structure of several “nanopins” set among a larger array. The pins could replace silver and gold nanoparticles in surface-enhanced Raman spectroscopy experiments. Reprinted with permission of the American Chemical Society.

In addition, the random distribution of the nanoparticles and of the analyte results in low reproducibility and low efficiency.

Now Xiang Zhang and his colleagues at University of California, Berkeley, have reported a reproducible way to construct nanoscale objects, dubbed “nanopins,” that may replace random scatterings of gold and silver particles in SERS setups.

The investigators spin-coated a 150-nm-thick layer of poly(methylmethacrylate) (PMMA) onto a substrate composed of a quartz wafer coated with indium tin oxide. They etched the PMMA using an electron beam system from FEI Co. of Hillsboro, Ore., set to emit a high spot dose of 40,000 fC. Because the energy of the beam is highest at its center and weaker around its perimeter, the polymer cross-links more extensively near the centerline and etches away the surrounding material, leaving a cylindrical structure.

They made an array of the cylinders, then evaporated 20 nm of gold onto it and lifted the excess gold and PMMA away with acetone. The result was an array of dielectric nanopins that appear like two-toned thumbtacks: a gold base with a gold-capped pin composed of the cross-linked PMMA. Scanning electron microscopy revealed that the central cylinder is ~300 nm tall and has an elliptical cross section with an ~100-nm short axis and an ~150-nm long axis; the base is ~500 nm in diameter. Numerical simulation showed a large volume of a relatively large field enhancement, with the maximum peak of 18.2 times the amplitude of incident light and 331.2 times its intensity.

Importantly, the optical response of one nanopin (the hybridization of the individual resonances of the cap and the base) can be tuned by designing the dimensions of the cap and base and the distance between them. Along with the easy adjustability of the distances between the nanopins within an array, these properties of the constructs likely mean that the resonance frequency of the system can be tuned to optimize the sensitivity of surface-enhanced Raman spectroscopy experiments.

Nano Letters, ASAP edition, March 8, 2007.