Plasmonic Self-Assembled Colloidal Magnetic Resonators

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[Under construction - Nick Schade (fall 2009)]

Plasmonic self-assembled colloidal magnetic resonators have been demonstrated in this study. These are clusters of metallic colloidal nanoparticles that support magnetic dipole resonances at near-infrared frequencies. Each cluster, or trimer, consists of three gold nanospheres coated with polymer that have undergone self-assembly from solution. The resulting structures permit separation gaps between the particles of about two nanometers, which surpasses the resolution obtainable through traditional approaches such as lithography. This result demonstrates that self-assembly offers a promising route to low cost and complex three dimensional metamaterials.

General Information

Keywords: colloid, self-assembly, plasmon

Authors: Jonathan A. Fan, Chih Hui Wu, Jiming Bao, Rizia Bardhan, Naomi J. Halas, Vinny Manoharan, Gennady Shvets, and Federico Capasso.

Date: 2009 (in review).

School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA.

Department of Physics, University of Texas at Austin, 1 University Station C1600, Austin, TX 78712, USA.

Department of Electrical and Computer Engineering, University of Houston, N308 Engineering Building 1, Houston, TX 77204, USA.

Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, MS-366, Houston, TX 77005, USA.


Diagram from Fan paper.
Figure 1. Diagram of the self-assembly of metallic colloidal particles into a trimer, with the LC circuit analogy.

Researchers are interested in colloidal metallic nanostructures because they can support surface plasmons, or oscillations of free electrons in the metal that couple with the electromagnetic field. The plasmonic properties of nanostructures depend strongly on their geometric and material properties. Precision at the nanometer scale is necessary to tune the resonant frequencies supported by these structures and self-assembly of the colloidal particles is one way that this could be achieved. Here the authors have assembled trimers of three dielectric-coated gold nanoparticles with controllable gap separations and measured their electromagnetic properties with correlated scattering spectroscopy.

The magnetic resonant behavior of these structures makes sense when one considers the notion that the structure is analogous to an LC circuit (see Figure 1) consisting of nanocapacitors (the separation gaps) and nanoinductors (the metal nanoparticles themselves, which individually support a plasmon resonance). To build these structures, gold nanoshells are first synthesized around silica cores. The plasmon resonance can be tuned by varying the ratio of the core and shell radii. A dielectric coating defines the gap separation in the clusters.

These polymer-coated nanoshells are then assembled by placing a droplet of solution onto a hydrophobic substrate and allowing it to dry at room temperature. The droplet separates into smaller droplets as it dries, some of which contain three nanoshells, which in the end will assemble into a trimer because surface tension causes them to remain enclosed inside the droplet. Thus, the trimer is held together by Van der Waals forces once it forms and is the entropically favored arrangement for the three particles. The optical properties of the structures may then be probed using dark field spectroscopy.

Diagram from Fan paper.
Figure 2. Electrostatic simulation of a trimer. (A) Resonant wavelengths for different modes as a function of gap separation. (B) Electric potential (coloring) and displacement currents (vector field) of the first order electrostatic modes. The magnetic dipole mode supports a circulating displacement current.

It is possible to solve for the electrostatic modes of the trimer structure using a numerical finite element method, and these can be used for comparison with and interpretation of the spectral data. The resonant mode energies can be predicted as a function of gap separation with this technique, and one can model the structures to understand the magnetic and electric dipole resonance modes (see Figure 2). With decreasing gap separation, the capacitive coupling increases, causing all the resonant energies to decrease.

Scattering spectra can be compared with predictions based on these simulations. There is strong agreement between the data and simulations except for some effects in the data that can be attributed to geometric perturbations due to asymmetries at the trimer gaps. In order to mitigate these effects, higher quality spherical particles or larger gap sizes will be necessary.

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