Nanofabricated periodic arrays of silver elliptical discs as SERS substrates

Signal enhancement observed in surface-enhanced Raman spectroscopy (SERS) is attributable to the presence of noble-metal nanostructures on substrate surfaces. The rational development of SERS-active substrates depends critically on the homogeneity and intensity of surface plasmon resonances, properties that are strongly dependent on both the morphology and dielectric properties of the metals and composite materials making up the SERS substrates. Enhancement can be controlled by the shape, size, and spacing of metallic nanoparticles. Previous studies in our group have shown that arrays of elliptical nanodiscs have promising geometries for this purpose. Using electron beam lithography (EBL), we fabricate close-packed arrays of these discs with lateral dimensions ranging from 300:50 to 300:300 nm (long axis : short axis). The arrays are composed of a negative photoresist that, once the lithography process is complete, are coated with a noble metal through physical vapor deposition (PVD). In this work, optimum thickness and deposition rate of noble metal are determined for these substrates. The lithographically produced nanopatterns are studied by Raman spectroscopy to examine the effect of altering the elliptical aspect ratio on SERS activity, while scanning electron microscopy (SEM) is used to examine pattern surfaces post lithographic development and post noble-metal deposition. Atomic force microscopy (AFM) is used to inspect the roughness of substrate surfaces. Reproducibility between different arrays of the same pattern ranges from 12 to 28%. Homogeneity of our uniform-morphology EBL/PVD-fabricated substrates is examined and compared to our random-morphology polymer nanocomposite substrates. Using rhodamine 6G as an analyte, an increase in SERS signal is noted as the aspect ratio of ellipses goes from 6:1 to 6:6. Our experimental data, in terms of trends in SERS activity, correlate with trends in field enhancements calculated using a simple electrostatic model and with the magnitude of the broad red-shifted spectral continuum observed for the substrates. Copyright © 2008 John Wiley & Sons, Ltd.