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Stamping out SERS substrates Researchers turn to electron-beam lithography and nanotransfer printing to control the shape and spacing of SERS substrates.
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ix centuries ago, monks crafted beautifully detailed books, one by one, that only royalty could afford. It took Johannes Gutenberg’s invention of the movable-type printing press to bring books within reach of the masses. Today, the same holds true for nanoscale substrates for surface-enhanced Raman spectroscopy (SERS). Techniques such as reactive ion etching (RIE) and electron-beam lithography (EBL) combined with vapor deposition create the well-defined arrays of noble-metal nanoparticles needed for SERS, but the expense of these one-ata-time methods makes such arrays too costly for most applications. Nahla Abu Hatab, Jenny Oran, and Michael Sepaniak at the University of Tennessee have taken Gutenberg’s approach by using EBL to build a nanotransfer printing (nTP) block that can stamp out welldefined arrays of nanoscale silver SERS substrates on PDMS (ACS Nano 2008, DOI 10.021/nn7003487). SERS is gaining popularity as a technique capable of single-molecule detection that works in conjunction with nanoparticulate or nanostructured noble-metal substrates. At the nanoscale level, the substrates are not uniform sheets of material but arrays of individual particles or other features. The intensity of the SERS signal depends strongly on the size, shape, and spacing of the features, and researchers have struggled to fabricate them reproducibly. “With nTP, we have a scalable method to create well-ordered arrays of substrates with controlled size, shape, and spacing,” says Sepaniak. The researchers designed 2D models of squares, triangles, or ellipses of a defined size and spacing. Feature dimensions ranged from 100 to 300 nm with variable nanoscale spacings. After the designs were transferred to an EBL system, the stamps were created by exposing a 2-in.-thick silicon wafer spin2304
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SEM image shows silver-coated stamps (left column), stamps after printing (middle column), and silver nanoparticles on a PDMS substrate (right column).
coated with negative photoresist to an electron beam to remove the exposed material. Dense arrays of 250-nm-high pillars averaged 40 × 40 µm and were spaced 500 μm apart on the wafer. Electron-beam resists are adhesive polymers, so the researchers coated the etched wafers with a metal-releasing film of heptakis(6-O-tert-butyldimethylsilyl-2,3-di-O-acetyl)-β-cyclodextrin (H-β-CD). The film stuck to the resist. After a layer of silver was deposited via physical vapor deposition to create isolated silver nanoparticles on the pillars, the stamp was pressed gently onto a PDMS film. When peeled from the stamp, the PDMS substrate retained the nanoparticle arrays. The stamps were used to create arrays in triplicate. After each use, the wafers were washed with dilute tetrachloro auric acid to remove any remnants of silver and then were recoated with Hβ-CD and silver. “For larger runs, we’d probably want to create a more robust stamp using RIE,” says Sepaniak. The researchers relied on SEM to assess the
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fidelity of the stamping process; it enabled them to determine that a 50-nmthick layer of silver on a 50-nm-thick coating of H-β-CD produced optimal results with a minimum number of defects in the printing process. Shuming Nie at Emory University thinks that this process shows great promise for creating ordered nanostructured arrays in which researchers can control characteristics such as structure size and shape. This capability is not available with other techniques. “But in terms of detection sensitivity and enhancement factors, it is not clear whether this type of printed SERS substrate is competitive with more traditional colloids or nanofabricated surfaces,” he says. One way to improve detection sensitivity is to fine-tune the distance between the array members, and Sepaniak believes that is possible by taking advantage of PDMS’s elastomeric properties. After completing the nTP process, the researchers stretched the PDMS substrate by ≥145% of the original length. Micrographs showed that the gaps between the nanoparticles increased proportionately. Stretching the substrate before stamping should produce the opposite effect, shrinking the gap between particles when the substrate is then relaxed. Katrin Kneipp of Harvard Medical School says, “We think that bringing nanostructures very close to one another should produce significant enhancements in SERS signals, and this approach may finally enable us to produce arrays with extremely close spacing between individual metal nanoparticles.” Indeed, Sepaniak plans to create bow-tie structures, which have been suggested as interesting structures for SERS. He explains, “We’ve already created triangles 300 nm on a side, separated by a 100 nm gap, and now we’re going to attempt to close that gap to just a few nanometers.” a —Joe Alper © 2008 American Chemical Societ y
