Research Article Received: 3 April 2009 Accepted: 15 May 2009 Published online in Wiley Interscience: (www.interscience.wiley.com) DOI 10.1002/jrs.2392 A high-throughput method for controlled hot-spot fabrication in SERS-active gold nanoparticle dimer arrays Kristen D. Alexander, a* Meredith J. Hampton, b Shunping Zhang, c Anuj Dhawan, d Hongxing Xu, c and Rene Lopez a We present a high-throughput method for fabricating large arrays of surface-enhanced Raman scattering (SERS) active gold dimers. Using a large-area/low-cost nanopatterning method in conjunction with a meniscus force deposition technique, we were able to create large arrays of uniformly spaced nanoclusters comprising two 60-nm gold nanospheres. Raman measurements of a thiophenol monolayer deposited on smaller scale arrays of aligned dimers yielded enhancement factors as high as 10 9 . Polarization-controlled measurements show spectral peak heights to be 10–100 times smaller when the incident beam is polarized perpendicularly to the dimer axis, confirming that the measured enhancements arise from the ‘hot spots’ between the two nanospheres. Copyright c  2009 John Wiley & Sons, Ltd. Keywords: surface-enhanced Raman scattering (SERS); gold nanoparticles; hot spot; enhancement factor; templating Introduction The ability to detect and identify substances efficiently is of great scientific interest. This is a task for which Raman spectroscopy is well suited. However, trace Raman detection has proven difficult to achieve due to the limitations imposed by its small scattering cross section. [1] In 1977, small analyte detection started to become possible thanks to enhancements originating from molecules in the proximity of roughened metallic surfaces. [2] This effect, known as surface-enhanced Raman scattering (SERS), has been suggested to be capable of single-molecule detection. [3–5] The emergence of SERS presented an avenue around intrinsic signal strength problems and has renewed interest in Raman spectroscopy for a wide range of applications. [6–8] However, while significant progress has been made toward Raman detection of very dilute analytes, the technique is still limited due to the unreliable reproducibility of large enhancement factors. [9,10] The lack of reproducibility of enhancement factors is not surpris- ing since the regions where enhancements are particularly large (i.e. ‘hot spots’) are commonly limited to the few cubic nanome- ters of volume between adjacent nanoparticles. [11] Furthermore, several studies have shown that these enhancements are extraor- dinarily sensitive to nanocluster morphology, the wavelength and polarization of the excitation source, and, perhaps above all, the interparticle spacing and the resultant plasmon coupling coupling. [11 – 20] Although many of these studies have carried out theoretical simulations to predict the electric field enhancements arising from various types of nanoclusters, there have been few suc- cessful attempts to tease out these behaviors from real nanoscopic features since a reliable method for their timely fabrication and systematic testing remained elusive. Advances in electron and ion beam lithography have made it possible to control the morphol- ogy and location of particles down to a few nanometers, but such precision is still insufficient to test these effects. On the other hand, other groups have succeeded in linking gold nanoparticles in a controlled manner to create small clusters with known interpar- ticle spacing [21,22] but the process is low throughput and, more importantly, produces random cluster orientations that obscure the characterization process. In this paper, we experimentally address these theoretical predictions by making measurements of the enhancement factors produced by clusters of a specific size and morphology. Recognizing that the key to characterization of this effect lies in the ability to produce large quantities of nearly identical clusters, we approach this problem using a technique that deftly exploits the advantages of parallel fabrication. We direct metal nanoparticles to form large, ordered arrays of only dimers with controlled size, orientation, and placement with respect to fiducial patterns. Here, we are able to demonstrate the degree of dependence of the SERS enhancement factor on different parameters. Specifically, we measured a 10 9 enhancement factor for two closely spaced 60-nm gold nanospheres with a marked sensitivity to the polarization angle with respect to the dimer axis. ∗ Correspondence to: Kristen D. Alexander, Department of Physics and Astron- omy, University of North Carolina, Chapel Hill, NC, USA. E-mail: krisalex@physics.unc.edu a Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC, USA b Department of Chemistry, University of North Carolina, Chapel Hill, NC, USA c Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P. R. China d Department of Biomedical Engineering, Duke University, Durham, NC, USA J. Raman Spectrosc. (2009) Copyright c  2009 John Wiley & Sons, Ltd.