NANO LETTERS
Real-Space Mapping of the Strongly Coupled Plasmons of Nanoparticle Dimers
2009 Vol. 9, No. 10 3619-3625
Deok-Soo Kim,† Jinhwa Heo,‡,§ Sung-Hyun Ahn,† Sang Woo Han,| Wan Soo Yun,‡ and Zee Hwan Kim*,† Department of Chemistry and BK21 DiVision of Chemistry, Korea UniVersity, Seoul 136-701, Korea, DiVision of AdVanced Technology, Korea Research Institute of Standards and Science, Daejeon 305-600, Korea, Department of Chemistry, Gyeongsang National UniVersity, Jinju 660-701, Korea, and Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 305-701, Korea Received June 9, 2009; Revised Manuscript Received July 13, 2009
ABSTRACT We carried out the near-field optical imaging of isolated and dimerized gold nanocubes to directly investigate the strong coupling between two adjacent nanoparticles. The high-resolution (∼10 nm) local field maps (intensities and phases) of self-assembled nanocube dimers reveal antisymmetric plasmon modes that are starkly different from a simple superposition of two monomeric dipole plasmons, which is fully reproduced by the electrodynamics simulations. The result decisively proves that, for the closely spaced pair of nanoparticles (interparticle distance/ particle size ∼0.04), the strong Coulombic attraction between the charges at the interparticle gap dominates over the intraparticle charge oscillations, resulting in a hybridized dimer plasmon mode that is qualitatively different from those expected from a simple dipole-dipole coupling model.
Because of its potential for chemical sensing applications as well as its fundamental physical interest, the near-field coupling between two neighboring nanoparticles has been one of the most extensively studied topics in nanoplasmonics.1-5 The presence of coupling in dimeric nanoparticle is experimentally manifested by the red-shift of the scattering resonances and by the strongly enhanced spectroscopic signals (such as surface-enhanced Raman scattering,6-10 SERS) from molecules placed between the two neighboring nanoparticles. Currently, these spectral signatures of the coupling form the physical basis of many biochemical and chemical sensors,2,8 yet detailed mechanisms behind the interparticle coupling are yet to be established. When the interparticle distance is smaller than the excitation wavelength yet significantly larger than the sizes of nanoparticles, a simple dipole-dipole interaction satisfactorily explains the observed spectra of the coupled plasmons. This regime is relevant in most of the “plasmon ruler” applications in which the red shift of the scattering spectra * To whom correspondence should be addressed. E-mail: zhkim@ korea.ac.kr. Tel.: +82-2-3290-3142. Fax +82-2-3290-3121. † Korea University. ‡ Korea Research Institute of Standards and Science. § Gyeongsang National University. | KAIST. 10.1021/nl901839f CCC: $40.75 Published on Web 07/22/2009
2009 American Chemical Society
provides the long-range (a few tens or hundreds of nanometers) distance between the two nanoparticles linked by large polymeric molecules such as DNAs.2 On the other hand, when the interparticle distance is much smaller than the sizes of the nanoparticles, more complicated behaviors are to be expected. This regime is most relevant to SERS measurements6–10 in which two or more self-assembled nanoparticles (with sizes of 20-100 nm) are usually separated from each other by a monolayer of molecules with a thickness of less than a few nanometers. Unlike in the case of the weak dipole-dipole coupling regime in which the symmetries of the monomer plasmons are mostly retained, the resulting coupling may induce significant asymmetry in the plasmon modes of nanoparticles to give new “hybridized” plasmon modes.11 It is only recently that researchers started to uncover the intricate details of the coupling at such small distances. For example, Atay et al.,12 Danckwert et al.,13 and Lassiter et al.3 observed abrupt changes in the resonancefrequencies and nonlinear responses when two nanoparticles are brought very close together, which suggest the importance of higher multipolar interactions between the nanoparticles. Thus far, a majority of investigations on interparticle coupling have primarily focused on the spectroscopy of coupled plasmons. However, many details of strongly coupled plasmons still remain largely unverified thus far. High-
resolution near-field microscopic studies on the coupled plasmons may provide direct information on how the plasmons of the individual nanoparticles transform into the coupled plasmons. Herein, we employ phase- and polarization-sensitive scattering apertureless near-field scanning optical microscopy (ANSOM)14-21 to directly investigate the plasmon modes of closely spaced dimeric nanoparticles. The high-resolution (∼10 nm) near-field intensity and phase maps of dimeric nanoparticles reveal unique antisymmetric plasmon modes that are drastically different from those of the monomeric nanoparticles, which directly proves that the strong capacitive coupling1,3 dominates over the intraparticle dipolar charge oscillations. We employ single crystalline gold nanocubes (with an edge length of ∼100 nm) as our primary unit nanoparticles for three practical reasons. First, owing to their structural symmetry, the nanocubes support degenerate plasmon modes that may be easily recognized from the nearfield images, and any external perturbation (including the interparticle coupling) of the degeneracy can be identified by the spatial changes of the plasmons. Second, nearmonodisperse nanocubes have far more reproducible optical as well as morphological properties than generic nanospheres, which allows us to reliably compare the plasmon modes of monomeric and dimeric nanoparticles. Third, atomically flat surfaces of nanocubes allow the near-field probes to fully access the exposed surfaces of the dimers without significant artifacts. To show that our observation is general, and not special to the dimeric cubes, we also carried out the similar measurements on dimeric nanospheres as well. Threedimensional finite-difference time-domain (FDTD) electrodynamics simulations of plasmon modes are also carried out in order to validate our interpretation. Figure 1a presents the scanning electron-microscopy (SEM) images of chemically synthesized (Supporting Information-A) single-crystal gold nanocubes studied herein, which show their near-monodispersive sizes (average size ∼100 nm, rms size distribution
