Plasmonic Core/Half-Shell Nanoparticles: Exploring Half-Shell Growth for Gold, Titania, Silica and Cuprous Oxide onto Silver Nanocubes

Abstract

Core/shell nanoparticles have often been studied due to their attractive optical, electronic, and catalytic properties. By coupling these core/shell features with physical anisotropy of the shell -by selectively capping only a portion of the core -complex nanostructures with unique properties can be formed. This research investigates the synthesis and plasmonic properties of silver nanocube/metal and oxide core/shell nanoparticles with full or partial shells. The shell materials tested were gold, titania, silica, and cuprous oxide. The general method involved growing the shell on a silver nanocube (AgNC) that was partially embedded in a polystyrene thin-film supported by a glass substrate. The amount of shell coverage was dictated by the depth of embedding, and thus nanocube surface exposed and available for reaction. After in-solution shell growth, the particles were removed from the substrate and suspended in solution. The characterization of these particles by UV-vis spectroscopy and transmission electron microscopy confirmed their altered plasmonic properties and their morphology. This partial shell growth technique is ideal for any low temperature, two step core/shell synthesis. The resultant shells had quite distinct morphologies dependant on the shell material chosen. While the gold and titania shell growths are incomplete at this time, silica and cuprous oxide provided positive results. The AgNC/SiO2 core/shell nanoparticles exhibited a complete and conformal coverage of SiO2 due to undergrowth between the AgNC and the polystyrene. This undergrowth arises from mild etching of the silver during the silica growth, creating space for the silica precursor to penetrate between the core and the polymer thin-film. The AgNC/Cu2O core/shell nanoparticles formed had a distinct half-shell morphology with iv either pyramidal or cubic half-shells where the geometry of the shell is dependent upon the order of reagent addition. Furthermore, the cuprous oxide half-shells caused significant modifications to the localized surface plasmon resonance of the AgNC core and resulted in in-solution hybridization of the plasmon modes. The proposed core/half-shell morphology will be particularly advantageous in the formation of dimers for SERS sensing or as individual particles for catalysis. v

Selective Thermoplasmonic Embedment Of Supported Silver Nanocrystals

“…The PS films had a roughness of ~0.5 nm and a thickness of approximately 140 nm. These values were consistent with roughness and thickness values of PS films synthesized with the spin coating method and thus deemed adequate supports for the AgNC monolayers 37. Deposition of AgNC on to PS Thin Films via Langmuir Shaefer Deposition LS Deposition was used to deposit a monolayer of AgNCs on each of the PS coated glass substrates.…”

“…After 25 minutes the reaction was quenched as the dipolar and quadrupolar peaks had sufficiently red-shifted and a multipolar peak at 350 nm emerged, indicating the presence of AgNCs with edge length approximately 30-60 nm. 33,167 The final peak positions of the dipolar, quadrupolar and multipolar modes were 460 nm, 388 nm, and 346 nm respectively. Transmission electron microscopy (TEM) was used to determine the shape and size distribution of the AgNCs.…”

“…33 Nanocubes have multiple peaks in their extinction spectra as charge accumulates at their edges and corners inducing extra multipolar charges. 37 Silver nanocubes (AgNCs), such as the ones used in this study, typically exhibit 3 distinct peaks in their extinction spectra: a dipolar peak at ~430-460 nm, a quadrupolar peak at ~390 nm and a multipolar peak at ~350 nm. The multipolar peak in the extinction spectra of silver nanocubes corresponds to a octupolar resonance which is unique to silver cubes.…”

“…140,141 The second requirement is a surface energy requirement that can be modelled using the Helmholtz free energy of the polymer and the nanoparticle. 37 According to Kovacs and Vincent, the thermodynamic driving force behind embedment is a decrease in free energy. 146 The Helmholtz surface free energy of a single rigid particle on a flexible polymer can be modelled by the equation…”

Studying the Generation of Hot Electrons in Plasmonic Nanoparticle Monolayers

“…Once the surface was fully coated, the spin coater was spun for 30 seconds at 4000 rpm. 53,54,55 The slide was removed and placed on a large glass petri dish. These steps were repeated for each slide.…”