Seeing Double: Coupling between SubstrateImageChargesandCollective Plasmon Modes in Self-Assembled Nanoparticle Superstructures (original) (raw)
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One-Dimensional Coupling of Gold Nanoparticle Plasmons in Self-Assembled Ring Superstructures
Nano Letters, 2009
Plasmon coupling in ordered metal nanoparticle assemblies leads to tunable collective surface plasmon resonances that strongly depend on the interparticle distance. Here we report on the surface plasmon scattering of polystyrene-functionalized 40 nm gold nanoparticles selfassembled into close-packed rings. Using single particle dark-field scattering spectroscopy, we observed strong near-field coupling between neighboring nanoparticles, which results in red-shifted multipolar plasmon modes highly polarized along the ring circumference. Correlated optical spectroscopy and scanning electron microscopy of individual rings with different diameters revealed that the plasmon coupling is independent of ring curvature and mostly insensitive to the local nanoparticle arrangement. Our results further suggest that a one-dimensional gold nanoparticle assembly yields long-range collective plasmonic properties similar to those of metallic nanowires.
Journal of Materials Chemistry, 2008
This article provides a review of our recent Rayleigh scattering measurements on single metal nanoparticles. Two different systems will be discussed in detail: gold nanorods with lengths between 30 and 80 nm, and widths between 8 and 30 nm; and hollow gold-silver nanocubes (termed nanoboxes or nanocages depending on their exact morphology) with edge lengths between 100 and 160 nm, and wall thicknesses of the order of 10 nm. The goal of this work is to understand how the linewidth of the localized surface plasmon resonance depends on the size, shape, and environment of the nanoparticles. Specifically, the relative contributions from bulk dephasing, electron-surface scattering, and radiation damping (energy loss via coupling to the radiation field) have been determined by examining particles with different dimensions. This separation is possible because the magnitude of the radiation damping effect is proportional to the particle volume, whereas, the electron-surface scattering contribution is inversely proportional to the dimensions. For the nanorods, radiation damping is the dominant effect for thick rods (widths greater than 20 nm), while electron-surface scattering is dominant for thin rods (widths less than 10 nm). Rods with widths in between these limits have narrow resonances-approaching the value determined by the bulk contribution. For nanoboxes and nanocages, both radiation damping and electron-surface scattering are significant at all sizes. This is because these materials have thin walls, but large edge lengths and, therefore, relatively large volumes. The effect of the environment on the localized surface plasmon resonance has also been studied for nanoboxes. Increasing the dielectric constant of the surroundings causes a red-shift and an increase in the linewidth of the plasmon band. The increase in linewidth is attributed to enhanced radiation damping.
Light scattering of interacting gold nanorods
physica status solidi (b), 2009
The optical field intensity of light scattering from nanorods of gold has been imaged at distances that are intermediate between the near-field and far-field regimes using a near-field scanning optical microscope (NSOM). For scattering from isolated nanorods the Fraunhofer diffractive behaviour is modified slightly by the dipolar nature of metal nanoantannae as would be expected at these imaging distances. However, when the nanorods are brought into close proximity, interactions between the nanorods alter the scattering behaviour substantially creating large field intensities between the structures. By sampling the field with the near-field microscope tip scanned at different heights, detailed maps of the scattering profile can be generated.
Plasmonic Shaping in Gold Nanoparticle Three-Dimensional Assemblies
The Journal of Physical Chemistry C, 2013
When a large number of similar gold particles are organized into complex architectures, the dipolar plasmon spectrum of the individual plasmonic entities gives rise to a broader, red-shifted feature centered around 750 nm. In this work, we show that superstructures fabricated using the convective assisted capillary force assembly method (CA-CFA) and excited at that wavelength display a subwavelength patterning of their optical field intensity that results from the self-consistent coupling between the colloidal nanoparticles. First, we demonstrate the fabrication of shape-controlled threedimensional assemblies of metallic nanocrystals using the CA-CFA method. In a second step, the absorption band resulting from the mutual coupling between the metallic building blocks is exploited to excite a coupled plasmon mode and map the twophoton luminescence (TPL) by scanning a tightly focused light beam. Highly resolved TPL images show that the morphology of the plasmonic particle assemblies has a strong impact on their optical response. A model based on a rigorous optical Gaussian beam implementation inside a generalized propagator derived from a three-dimensional Green dyadic function accurately reproduces the TPL maps revealing the influence of interparticle separation and thus coupling between the individual particles. Finally, we show that the spatial distribution of the electric field intensity can be controlled by tuning the linear polarization of the optical excitation.
Hyperspectral dark-field microscopy of gold nanodisks
Micron, 2015
The light scattering properties of hexagonal and triangular gold nanodisks were investigated by means of Cytoviva hyperspectral dark-field microscopy, exploring the huge enhancement of the scattered waves associated with the surface plasmon resonance (SPR) effect. Thanks to the high resolution capability of the dark-field microscope, the SPR effect turned it possible to probe the individual nanoparticles directly from their hyperspectral images, extrapolating the classical optical resolution limit, and providing their corresponding extinction spectra. Blue spectral shifts involving the in-plane dipolar modes were observed for the hexagonal gold nanodisks in relation to the triangular ones, allowing their spectroscopic differentiation in the dark-field images.
Stacked Gold Nanorectangles with Higher Order Plasmonic Modes and Top-Down Plasmonic Coupling
The Journal of Physical Chemistry C, 2014
We present stacked hollow nanostructures created using electron beam lithography (EBL) that act as optical scattering sites with a complex combination of local surface plasmon resonances and top-down electromagnetic hotspots due to the incorporation of the third dimension into their construction. These hollow rectangular gold nanotructures with gold caps show a significant red-shift in their main scattering peak as compared to the solid structures. Finite-difference time-domain modeling shows that the plasmonic response of these structures is dominated by higher order plasmonic modes and that the strength of these modes is shown to vary according to whether a cap is present. The higher order dipolar mode caused by the capped nanostructure results in manifold increase in the intensity of the electric field compared to the quadrupolar mode from a solid rectangle. This analysis provides important information on how complex plasmonic resonances respond to structural changes which will be useful in future studies that utilize these coupled resonances for detection or light manipulation. In addition, the stacking scheme presents a new route for modifying the optical response of plasmonic nanostructures through top-down plasmonic coupling which may yield plasmon resonance modes not observed in common 2D nanostructures along with significant increases in the local electric fields of these open "hotspots".
Collective plasmon modes in a compositionally asymmetric nanoparticle dimer
2011
The plasmon coupling phenomenon of heterodimers composed of silver, gold and copper nanoparticles of 60 nm in size and spherical in shape were studied theoretically within the scattered field formulation framework. In-phase dipole coupled σ -modes were observed for the Ag-Au and Ag-Cu heterodimers, and an antiphase dipole coupled π -mode was observed for the Ag-Au heterodimer. These observations agree well with the plasmon hybridization theory. However, quadrupole coupled modes dominate the high energy wavelength range from 357-443 nm in the scattering cross section of the D=60 nm Ag-Au and Ag-Cu heterodimer. We demonstrate for the first time that collective plasmon modes in a compositionally asymmetric nanoparticle dimer have to be predicted from the dipole-dipole approximation of plasmon hybridization theory together with the interband transition effect of the constitutive metals and the retardation effect of the nanoparticle size. Copyright 2011 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License.
Correlated Structure and Optical Property Studies of Plasmonic Nanoparticles
The Journal of Physical Chemistry C, 2011
This article provides a review of our recent studies of single metal nanoparticles and single nanoparticle clusters aimed at correlating the structural and plasmonic properties of the same entity. The correlation between the structure and the optical properties arising from the localized surface plasmon resonance (LSPR) on single nanoparticles from various samples is described. Nanoparticles of different materials (Ag and Au) and shapes (spheres, cubes, triangles) are considered. Experiments were carried out using transmission electron microscopy (TEM), dark-field spectroscopy, and surface-enhanced Raman spectroscopy (SERS). Results of those measurements were compared with electrodynamics calculations to provide insight into the interpretation and physical meaning of the experimental results. We examine correlated studies of triangular nanoparticle arrays to highlight the significance of single entity measurements over ensemble-averaged measurements. Furthermore, we show how an examination of statistics on large data sets helps draw quantitative structureÀLSPR relationships. We also show that implementing SERS in correlated measurements improves the understanding of factors important in determining SERS enhancements. Finally, we extend the scope of correlated measurements to the tracking and controlled manipulation of single nanoparticles, thus paving the way for in vivo diagnostics using nanomaterials.
Optics Express, 2010
Dark field microspectroscopy is the primary method for the study of plasmon modes of individual metallic nanostructures. Light from a plasmonic nanostructure typically scatters with a strong angular and modal dependence, resulting in significant variations in the observed spectral response depending on excitation and collection angle and polarization of incident light. Here we examine how spectrally dependent radiation patterns arising from an individual plasmonic nanoparticle, positioned on a dielectric substrate, affect the detection of its plasmon modes. Careful consideration of excitation and collection geometry is of critical concern in quantitative studies of the optical response of these nanoparticle systems.