Spectroscopy and Imaging of Plasmonic Modes Over a Single Decahedron Gold Nanoparticle: A Combined Experimental and Numerical Study Pabitra Das and Tapas Kumar Chini* Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India * S Supporting Information ABSTRACT: Employing cathodoluminescence (CL) spec- troscopy and imaging in a field emission gun (FEG) scanning electron microscope (SEM), we study localized surface plasmon (LSP) modes on individual tilted gold nano- decahedron sitting on a silicon substrate. We experimentally resolve three distinct LSP modes in the far-field radiation acquired via CL. The experimental spectra and intensity maps of plasmon modes are in excellent agreement with the spectra and 2D-CL image obtained from finite difference time domain (FDTD) simulations. Detail analysis reveals the signature of a quadrupolar surface plasmon mode in addition to the two dipolar modes along azimuthal and polar direction of the decahedron. The experimental method and the theoretical formalism presented here provide useful insight into the plasmonic behavior of complex shaped metal nanoparticle supported by a high index substrate. ■ INTRODUCTION The optical response of noble metal nanoparticles (MNPs) is governed mainly by the concept of the collective oscillation of conduction electrons in MNP, known as localized surface plasmon (LSP) or particle plasmon. LSP when excited 1-3 resonantly with a particular wavelength of the exciting light or evanescent wave associated with fast-moving electrons, can lead to strong light absorption and scattering. Another striking feature of the LSP resonance is the considerable localized enhancement of the near-field amplitude at the nanostructured metal surface allowing electromagnetic (EM) energy to be confined at the subwavelength length scale and similar enhancement of the far-field radiation intensity. These enhanced EM fields have several interesting effects, such as enhanced fluorescence, 4 enhanced photocarrier generation, 5 and surface enhanced Raman scattering (SERS), 6 that can have potential applications in biosensing, photovoltaics, and single molecule detection. Often the local EM field enhancement in the plasmonic structures is confined spatially on length scales of ∼10-50 nm and varies strongly with the morphology and composition of nanoparticles. Consequently, investigation of the electromagnetic field distribution associated with nano- particle SPs (more specifically, LSPs) requires an experimental probe not only of sufficient spectral resolution but also of sufficient degree of spatial resolution for deeper understanding of light-matter interaction at the nanometer length scale. 7-10 Mapping the spatial variation of the photon emission 11-16 is a direct probe of resonant modes of plasmonic nanostructures and, consequently, provides a direct way to map the local electric fields. Due to the simplicity of the instrumental setup, a large body of the MNP optical studies have been performed using optical dark-field microscopy (DFM), 17-20 which provides excellent spectral resolution. However, DFM is constrained by the diffraction limit to a spatial resolution of about half a wavelength and, consequently, it cannot be used to image the spatial profile of the plasmon resonances on length scales below ∼200 nm. Near-field scanning optical microscopy (NSOM), 14 on the other hand, can achieve a resolution of ∼20 nm or slightly better but (in the case of NSOM) is constrained by the requirement of fabricating very sharp tips, and furthermore, interaction of the tip with light in the structure often perturbs the LSP mode, 22 making it challenging to probe the detailed spatial profile of the resonance. So, the search for a nonperturbing probe to directly image the plasmonic local field with high spatial resolution remains a challenging task. Alternatively, electron beam based tools, 3 such as cathodolu- minescence (CL) spectroscopy and imaging 11-13,15,16 through the detection of emitted photons in a scanning/transmission electron microscope (SEM/TEM) or electron energy loss spectroscopy (EELS) 23-25 through the detection of energy loss suffered by the inelastically scattered transmitted electrons in a TEM, are shown to constitute an excellent probe of plasmons that allows capturing even subnanometer resolution informa- tion in the spatial domain. Although EELS has been shown as the best single particle spectroscopy technique to probe plasmons on a MNP with unmatched spatial and spectral resolution, 9,25 it suffers from the drawback that samples must be electron transparent (typically below 100 nm), which Received: October 19, 2012 Revised: November 12, 2012 Published: November 20, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 25969 dx.doi.org/10.1021/jp3103782 | J. Phys. Chem. C 2012, 116, 25969-25976