Excitation wavelength dependent surface enhanced Raman scattering of 4-aminothiophenol on gold nanorings Jian Ye, * ab James Andell Hutchison, b Hiroshi Uji-i, b Johan Hofkens, b Liesbet Lagae, a Guido Maes, b Gustaaf Borghs a and Pol Van Dorpe a Received 20th November 2011, Accepted 20th December 2011 DOI: 10.1039/c2nr11805j Detailed understanding of the underlying mechanisms of surface enhanced Raman scattering (SERS) remains challenging for different experimental conditions. We report on an excitation wavelength dependent SERS of 4-aminothiophenol molecules on gold nanorings. SERS and normal Raman spectra, combined with well-characterized surface morphology, optical spectroscopy and electromagnetic (EM) field simulations of gold nanoring substrates indicate that the EM enhancement occurs at all three excitation wavelengths (532, 633 and 785 nm) employed but at short wavelengths (532 and 633 nm) charge transfer (CT) results in additional strong enhancements of particular Raman transitions. These results pave the way to further understanding the origin of the SERS mechanism. Introduction Raman scattering is typically very weak due to the small scat- tering cross-section of molecules. However, the surface enhanced Raman scattering (SERS) effect can result in the enhancement of Raman scattering by molecules adsorbed on rough metal surfaces. The enhancement factor (EF) can reach a level that enables single molecule detection. 1 The mechanisms behind SERS still remain a matter of controversy, but two mechanisms are extensively mentioned in the literature. In the electromag- netic (EM) mechanism, the excitation of localized surface plas- mons results in strongly enhanced local electric fields around the metal nanostructure, leading to a more intense Raman scattering of molecules near the metal surface. 2–4 In the EM mechanism, the EF of each molecule is approximately given by EF ¼ |EF ex | 2 |EF scat | 2 , where EF ex is the local electric field (EF) at the excitation wavelength and EF scat is the corresponding EF at the Stokes-shifted Raman scattering wavelength. More often, this expression is simplified by assuming that EF ex and EF scat are the same, and hence EF ¼ |EF ex | 4 . The second popular mecha- nism, called charge transfer (CT), involves the excitation of CT between analyte molecules and metal structures to give rise to a resonance Raman enhancement process. 5,6 A SERS substrate can be any plasmon-resonance-supporting nanostructure, for example, nanoshells, 7 nanocubes, 8 nano- triangles, 9 nanostars, 10 nanobowls, 11 nanosphere dimers 12 and aggregates. 13 Recently, plasmonic nanoring structures have also been proposed and demonstrated for SERS application, 14–16 because of their highly tunable optical properties over a broad spectral range and their efficient concentration of EM fields. 14 A particular advantage of the nanorings is that they can be fabri- cated by the nanosphere lithography technique, 14,16 which is a simple, productive, scalable and cheap method. Because the SERS effect mainly arises from the plasmon resonances of the nanostructures, it is typically wavelength dependent. A particular SERS substrate will therefore exhibit good enhancement in a limited excitation wavelength range. For example, a detailed excitation wavelength-scanned SERS spec- troscopic study of benzenethiol adsorbed on silver (Ag) nano- particle arrays, fabricated by nanosphere lithography, has been carried out by McFarland et al. 17 The SERS spectra were correlated with the corresponding localized surface plasmon resonance (LSPR) spectra of the nanoparticle arrays. The maximum SERS enhancement factor (EF max ) was shown to occur for excitation wavelengths which are slightly blue-shifted with respect to the LSPR wavelength of nanoparticle arrays. This is in agreement with the predictions of the EM enhancement mechanism of SERS. Additional work by Etchegoin’s group 18 and Van Duyne’s group 19,20 has made it extensively accepted that the EF max occurs at excitation wavelengths quite close to the spectral locations of the LSPR extinction maximum for indi- vidual non-interacting nanostructures. However, the extinction spectrum is not a good indicator of the maximum SERS enhancement in the case of complex nanostructures, e.g. dimers, quadrimers and non-ordered aggregates. 18 If other effects, such as CT or chemical transformation, are involved, the SERS mechanism becomes even more complicated. The full under- standing of the SERS mechanism remains a challenge. 4-Aminothiophenol (4-ATP), a Raman active probe, has been widely used in SERS because of its strong chemical affinity to Au and Ag and the large SERS signal. 21–28 Another reason of 4-ATP becoming increasingly important is that its b 2 -type Raman a imec vzw, Kapeldreef 75, Leuven, Belgium. E-mail: Jian.Ye@imec.be; Fax: +32 1628 1097; Tel: +32 1628 8795 b Chemistry Department, Katholieke Universiteit Leuven, Leuven, Belgium 1606 | Nanoscale, 2012, 4, 1606–1611 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 1606 www.rsc.org/nanoscale PAPER Downloaded by Univ Lille 1 on 08 March 2012 Published on 04 January 2012 on http://pubs.rsc.org | doi:10.1039/C2NR11805J View Online / Journal Homepage / Table of Contents for this issue