The Effect of Surface Roughness on the Extinction Spectra and Electromagnetic Fields around Gold Nanoparticles
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1087-V01-08
The Effect of Surface Roughness on the Extinction Spectra and Electromagnetic Fields around Gold Nanoparticles Shuzhou Li, and George C. Schatz Department of Chemistry, Northwestern University, 2145 Sheridan Rd, Evanston, IL, 60208
ABSTRACT Electromagnetic enhancement arising from plasmon resonance excitation plays a major role in surface-enhanced Raman spectroscopy (SERS), and as a result nanoparticle morphology can significantly affect SERS intensities. In this paper we have calculated these enhancements as well as extinction spectra using the discrete dipole approximation for a system consisting of a dimer of gold disks that is made using on-wire lithography. Including surface roughness in the calculations leads to SERS enhancements for the disks whose dependence on disk spacing and thickness is in agreement with experimental measurements, with a maximum enhancement when the thickness of the disk and the disk-disk gap are 100 nm and 32 nm, respectively. These results are in better agreement with experiments than earlier estimates based on flat surfaces.
INTRODUCTION When molecules are absorbed on metallic nanoparticles, the Raman signal can be enhanced by several orders of magnitude, a process known as surface-enhanced Raman spectroscopy (SERS). SERS has been applied broadly, including biosensing and solar cell applications,1 and the enhancement mechanisms have been extensively studied by experiments and theories for the past 30 years.2 Among the different mechanisms, enhancement in the electromagnetic field due to plasmon excitation in the metal nanoparticle is generally considered to play the dominant role.3 When the metal particles are significantly smaller than the incident wavelength, the SERS enhancement factor from the electromagnetic mechanism is proportional to g4 (g=|E|/|E0|) where E and E0 are the enhanced and incident fields around the particle surfaces, respectively.4 These enhanced fields can be obtained analytically or numerically.5 Although such calculations have been done numerous times, there are relatively few cases where quantitative comparisons with measured SERS intensities have been possible, in part because the surface morphology has a significant effect on the SERS intensity, which is hard to control and characterize for length scales (few nm) that are important in SERS.6 When particles aggregate, the electric field can be enhanced at their junctions such that even single molecules can be detected.7 However, it is extremely hard to precisely control the aggregation of nanoparticles in condensed phases. On-wire lithography (OWL) is a recently developed method that allows precise control of the composition and geometry of nanodisks.8 Qin et. al. have systematically studied the optical properties of gold dimers fabricated by OWL.6 These properties can also be modelled by numerical methods, such as finite difference methods and the discrete dipole approximation (DDA).9 Theoretical calculations and experimental measurements can therefore be compared directly for isolated nanodis
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