Optimization of gapped nanoantennas

PEDANO, MARÍA LAURA; LI, SHUZHOU; SCHATZ, GEORGE C.; MIRKIN, CHAD A.

Buenos Aires

Workshop; XIII J. J. Giambiagi 2011 Winter School on Nanophotonics. Workshop on "Recent Advances in Nanophotonics"; 2011

U.B.A. Fac. Cs. Ex.y Nat., Depto. de Física.

Over the past six years, Professor?s Mirkin Group have been developing a novel Electrochemistry-enabled nanofabrication technique known as on-wire lithography(OWL).[1] This technique allows control of the chemical composition and architecture ofa one-dimensional wire from the nanometer to micrometer length scale.[1, 2] Periodicstructures that consist of gaps as small as 2 nm and segment lengths that span the 20nm to many micrometer length scale have been synthesized by OWL.[3] OWL isparticularly useful for creating plasmonically active structures from noble metals suchas Au and Ag.[2] In collaboration with Professor Schatzs, it has been demonstratedthat by tailoring disk (120 nm thick, 360 nm diameter) and gap sizes (30 nm) in onedimensionalAu nanostructures, one could create Raman hotspots, which have beenoptimized for spectroscopic identification purposes.[3]Gapped rods provide a unique platform for elucidating structure/functionrelationships, both for single-molecule electrochemical techniques and for surfaceenhancedRaman scattering (SERS).In this presentation we show the dependence of SERS intensities on the rodsegment length, gap topography and gap distance, for gold gapped rods with segmentlengths varying over a wide range (40-2000 nm). Significantly, we have determined thatthe SERS enhancement and localized fields in the gap simply do not monotonicallydecrease with segment length but rather are a periodic function of the segmentlength.[4] The periodic dependence is determined by the SPP wavelength. Both theoryand experiment show periodic variation of SERS intensity with segment length asdetermined by odd-symmetry plasmon multipoles. Additionally, we have determinedthat rough gaps lead to a smaller SERS enhancement than smooth gaps for thesestructures, even though the rough gaps have a larger total surface area. Excitation ofeven-symmetry modes is dipole forbidden (for polarization along the rod axis), but thisselection rule can be relaxed by roughness or, for smooth gaps, by near-field couplingbetween the rod segments.[5] Therefore, there is a strong dependence on the gap sizeif the gap is smooth, but roughness significantly reduces this sensitivity and it alsoquenches any effect of the polarization direction.From these results we can conclude that one can indeed create idealizedgeometries that have both micrometer length segments and nanogaps that lead to ahigh signal enhancement to be able to spectroscopically characterize the gapcomposition by surface-enhanced Raman scattering (SERS).References:[1] L. D. Qin, S. Park, L. Huang, C. A. Mirkin, Science 2005, 309,113.[2] X. Chen, Y. M. Jeon, J. W. Jang, L. Qin, F. Huo, W. Wei, C. A. Mirkin, J. Am. Chem. Soc.2008, 130, 8166.[3] L. D. Qin, S. L. Zou, C. Xue, A. Atkinson, G. C. Schatz, C. A. Mirkin, Proc. Natl. Acad. Sci.USA 2006, 103, 13300.[4] M. L. Pedano, S. Li, G. C. Schatz, and C. A. Mirkin. Angew. Chem. Int. Ed. 2010, 49, 78.[5] S. Li, M. L. Pedano, G. Chang, C. A. Mirkin, and G. C. Schatz. Nano Letters 2010, 10, 1722.