Versatile template-directed synthesis of gold nanocages with a predefined number of windows.

Highly symmetrical gold nanocages can be produced with a controllable number of circular windows of either 2, 3, 4, 6 or 12 via an original fabrication route. The synthetic pathway includes three main stages: the synthesis of silica/polystyrene multipod templates, the regioselective seeded growth of a gold shell on the unmasked part of the silica surface and the development of gold nanocages by dissolving/etching the templates. Electron microscopy and tomography provide evidence of the symmetrical features of the as-obtained nanostructures. The optical properties of nanocages with 4 and 12 windows were measured at the single particle level by spatial modulation spectroscopy and correlated with numerical simulations based on finite-element modeling. The new multi-step synthesis approach reported here also allows the synthesis of rattle-like nanostructures through filling of the nanocages with a guest nano-object. With the potential to adjust the chemical composition, size and geometry of both the guest particle and the host cage, it opens new routes towards the fabrication of hollow nanostructures of high interest for a variety of applications including sensing devices, catalytic reactors and biomedicine.

High-Pressure Effect on the Optical Extinction of a Single Gold Nanoparticle.

When reducing the size of a material from bulk down to nanoscale, the enhanced surface-to-volume ratio and the presence of interfaces make the properties of nano-objects very sensitive not only to confinement effects but also to their local environment. In the optical domain, the latter dependence can be exploited to tune the plasmonic response of metal nanoparticles by controlling their surroundings, notably applying high pressures. To date, only a few optical absorption experiments have demonstrated this feasibility, on ensembles of metal nanoparticles in a diamond anvil cell. Here, we report a nontrivial combination between a spatial modulation spectroscopy microscope and an ultraflat diamond anvil cell, allowing us to quantitatively investigate the high-pressure optical extinction spectrum of an individual nano-object. A large tuning of the surface plasmon resonance of a gold nanobipyramid is experimentally demonstrated up to 10 GPa, in quantitative agreement with finite-element simulations and an analytical model disentangling the impact of metal and local environment dielectric modifications. High-pressure optical characterizations of single nanoparticles allow for the accurate investigation and modeling of size, strain, and environment effects on physical properties of nano-objects and also enable fine-tuned applications in nanocomposites, nanoelectromechanical systems, or nanosensing devices.

Fano Interference in the Optical Absorption of an Individual Gold-Silver Nanodimer.

Fano resonances are central features in the responses of many systems including atoms, molecules, and nanomaterials. They arise as a consequence of interferences between two channels, most frequently associated with two system modes. In plasmonic materials, Fano interferences between optical modes have been shown, experimentally and theoretically, to induce narrow features in their scattering spectra. By investigating individual silver-gold heterodimers, we first experimentally demonstrate that Fano interference is also a key effect in the optical absorption of plasmonic nano-objects, in agreement with theoretical predictions. Conversely to previously investigated systems, the two interacting modes at the origin of absorptive Fano effect are mostly localized on either one or the other dimer component. Experimental results were obtained by selectively monitoring the optical absorption of one dimer component using a two-color nonlinear time-resolved technique. This also opens the way to full optical far-field noncontact investigations of charge or energy exchanges between nano-objects with a spatial resolution much smaller than the optical wavelength.