Article pubs.acs.org/JPCC
Plasmon Hybridization and Field Confinement in Multilayer Metal− Dielectric Nanocups Maj Frederiksen,† Vladimir E. Bochenkov,†,‡ Michael B. Cortie,§ and Duncan S. Sutherland*,† †
Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Denmark Chemistry Department, Lomonosov Moscow State University, Moscow, Russia § Institute for Nanoscale Technology, University of Technology Sydney, Sydney, Australia ‡
S Supporting Information *
ABSTRACT: Large-area arrays of dispersed multilayer gold−dielectric nanocups were fabricated by colloidal lithography and studied by extinction spectroscopy. Hybridization of the elemental plasmons of the individual nanocups gave rise to new resonance peaks in the visible and near-infrared regions of the extinction spectrum. Transmission electron microscopy was used to confirm the fabricated structure geometry, and the optical properties of the arrays were studied by UV−vis−NIR spectroscopy and finite-difference time-domain (FDTD) simulations. The nature of the resonances was elucidated from Efield plots and charge plots showing clear hybridized modes. We observe a dominant hybridized dipolar mode combining a bonding and antibonding mode at the two caps. A high-energy antibonding (antisymmetric) quadrupolar mode of an individual nanocup is revealed through hybridization with an elemental mode on the second nanocup. A lowenergy tunable cavity mode with a very small mode volume is observed in the near-IR range.
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INTRODUCTION Noble-metal nanoparticles are known for their unique optical properties in the visible and near-infrared regions where their optical response is strongly dominated by the collective oscillations of the conduction electrons known as localized surface plasmon resonances (LSPRs). Functional properties including spectral tunability, high scattering cross sections, and local field enhancements1 have made plasmonic structures of interest in applications and research fields as diverse as photovoltaics,2 chemical and biological sensors,3,4 metamaterials,5 and medical treatment.6 The spectral position and line shape of an LSPR depend on the size, shape, and dielectric environment of the supporting nanoparticle. There exists a broad array of fabrication tools which has enabled the study of the properties of differently shaped nanoparticles such as disks, shells, semishells, and star shapes.7−10 The optical extinction of many structures can be described in terms of dipolar and narrower quadrupolar modes.11−18 Recently, the plasmonic response of complex metallic structures consisting of several individual metallic elements has become a topic of intense research interest.19−25 Tunable optical responses and strong electric field enhancement arise from the coupling of the elemental plasmon resonances. The phenomenon can be described via a model termed plasmon hybridization,26 an electromagnetic analogue of molecular orbital theory. Hybridization results in splitting of the plasmon resonances into bonding (lower energy) and antibonding (higher energy) modes, where the coupling strength and energy of the resulting hybridized modes critically © 2013 American Chemical Society
depend on the energy matching and spatial configuration of the elemental resonances. In a layered metal−insulator−metal nanostructure geometry the insulator thickness can easily be controlled to give very small and well-controlled spatial separations. This has been utilized in the fabrication of multilayer nanoshells,22 stacked disks,23,27 stacked rings,28 and stacked double crescents.24 In all of the systems new resonances are observed that can be attributed to the bonding and antibonding combinations of elemental plasmon modes of the individual structures, in agreement with the plasmon hybridization model. In addition to the strong hybridization of elemental modes, the thin insulator layer in the metal−insulator−metal geometry can result in highly effective subwavelength confinement of light, for example, through cavity plasmon modes of extremely small mode volumes and high local field enhancements.29 Plasmonic semishells consisting of a dielectric spherical core asymmetrically coated with a thin metallic layer have attracted significant recent attention due to their tunable optical properties and ease of fabrication. The resonances of semishells can be tuned through varying the core size and metal layer thickness analogous to that in core−shell structures.22,30 Furthermore, the broken-symmetry geometry of a semishell offers additional approaches to tune the optical properties as the fractional height (solid angle) of the semishell has a significant impact on the resonances.9,31 The semishells have Received: March 15, 2013 Revised: July 1, 2013 Published: July 3, 2013 15782
dx.doi.org/10.1021/jp402613u | J. Phys. Chem. C 2013, 117, 15782−15789
The Journal of Physical Chemistry C
Article
the same electrolyte concentration were used on identically treated substrates to produce samples with comparable particle distributions. Very few dimers, trimers, or larger aggregates (
