Plasmonic nanomaterials
Surface plasmons are produced by the optical excitation of free electrons at metal-dielectric interfaces. Gold and silver nanostructures can be designed to support tunable plasmon modes, with diverse applications such as chemical sensing, biological imaging, photonics, and energy harvesting. Many of these applications rely on localized electromagnetic fields that are generated at plasmon resonance. The field factors produced by single metal nanoparticles are relatively modest, but can intensify dramatically when nanoparticles are close together, resulting in strongly coupled plasmons.
We have studied the collective plasmonic properties of gold nanoparticle 2D arrays (formed by self-assembly) and gold nanorod 2D arrays (prepared by templated synthesis), whose optical absorbance and reflectance peaks span the range of visible and near-infrared wavelengths. Periodic nanorod arrays support localized plasmon resonances as a function of height, interparticle separation, and their dielectric medium; the gaps between nanorods act as resonant cavities that can support a harmonic series of plasmon modes. These tunable features enable the engineering of metamaterials with nonclassical optical properties; for example, nanorod arrays are capable of supporting hyperbolic plasmon modes that can enhance spontaneous emission and lasing at non-resonant wavelengths (collaboration with Shalaev group).
From left to right: gold nanorod 2D array (side and top views); vis-NIR reflectance with resonant extinctions, due to harmonics generated within periodic nanorod array; nanorods as hyperbolic metamaterial supporting spontaneous emission (lasing).
For plasmon-enhanced spectroscopy and chemical sensing modalities such as surface-enhanced Raman scattering (SERS), highly ordered metal nanostructures are not required. More commonly, SERS-active substrates rely on "hot spots" that can be generated by irregular structures or aggregates. Examples from our lab include nanoporous gold films etched with sharp edges, and magnetic gold nanoclusters (MGNCs) whose aggregation can be induced by a magnetic field that produce hot spots on demand. Controlled surface functionalization can endow metal nanostructures with supramolecular function, and can be performed in ways that direct molecular analytes toward SERS-active sites. Raman-active surface ligands are also useful for reporting the adsorption of metal ions or molecules which have no SERS signatures of their own.
