Nanoplasmonic Spectroscopy of Single Nanoparticles Tracking Size and Shape Effects in Pd Hydride Formation

Sammanfattning: Localized surface plasmon resonance (LSPR) is a phenomenon of collective oscillation of conduction electrons in metal nanoparticles smaller than the wavelength of light that is used for its excitation. Plasmonic metal nanoparticles are able to confine light to extremely small volumes around them, i.e. below the diffraction limit. This gives rise to strongly localized and enhanced electromagnetic fields in so-called “hot spots” of the plasmonic nanoparticle. These hot spots usually correspond to the edges, sharp corners or tips of monomer structures, and, in case of coupled multimer arrangements, to the antenna junctions. Plasmonic hot spots are highly advantageous for sensing, since any object that is inserted there will influence the optical resonance of the system via coupling to the local field. Placing a well-defined catalytic nanoobject in the hot spot of a plasmonic nanoantenna offers thus unique possibilities to obtain detailed information about the role of specific features (e.g. facets, size, shape or relative abundance of low-coordinated sites, etc.) of that particle for its functionality/activity at the single particle level. Consequently, there is an increasing interest to use plasmonic antennas as a tool to investigate catalytic processes in/on single functional nanomaterials in situ. Single particle measurements are possible with the use of dark-field scattering spectroscopy, since plasmonic nanoparticles efficiently scatter light and are easily observable in the dark-field microscope. In this context, this work was dedicated to: 1) Development of a fabrication method for making plasmonic nanoantenna structures with the possibility to place a nanoparticle of interest (catalyst) in the hot spot of the antenna. 2) Investigation of the role of size and shape in hydride formation thermodynamics of wet-chemically synthesized single palladium (Pd) nanocrystals. The latter was possible by attaching the Pd nanocrystal to a plasmonic nanoantenna (gold sphere) by means of electrostatic self-assembly. The role of size was investigated for Pd nanocubes ranging from 20 to 50 nm. The role of shape was considered by modulating the Pd nanocrystal shape from cube to rod to octahedron.