Silver Nanocubes Coated in Ceria:Core/Shell Size Effects on Light-Induced Charge Transfer

Plasmonic sensitization of semiconductors is an attractive approach to increase light-induced photocatalytic performance; one method is to use plasmonic nanostructures in core@shell geometry. The occurrence and mechanism of synergetic effects in photocatalysis of such geometries are under intense debate and proposed to occur either through light-induced charge transfer (CT) or through thermal effects. This study focuses on the relation between the dimensions of Ag@CeO2 nanocubes, the wavelength-dependent efficiency, and the mechanism of light-induced direct CT. A 4-mercaptobenzoic acid (4-MBA) linker between core and shell acts as a Raman probe for CT. For all Ag@CeO2 nanocubes, CT increases with decreasing excitation wavelength, with notable increase at and below 514 nm. This is fully explainable by CT from silver to the 4-MBA LUMO, with the increase for excitation wavelengths that exceed the Ag/4-MBA LUMO gap of 2.28 eV (543 nm). A second general trend observed is an increase in CT yield with ceria shell thickness, which is assigned to relaxation of the excited electron further into the ceria conduction band, potentially producing defects

Optical properties of strongly interacting supported silver nanocube monolayers

Plasmonic properties of monolayers of strongly interacting silver nanocubes (AgNC) with controlled interparticle spacing are investigated. Uniform monolayers with controlled particle densities are made using the Langmuir-Blodgett technique with passive phospholipid spacers, such as dioleoyl phosphatidylcholine (DOPC). Both extinction intensity and wavelength of dipole-dipole coupling modes are tuned via particle spacing. The refractive indices of the substrates are used to tune dipolar and interparticle coupling modes via deposition onto thin films of silicon (0 - 25nm). By varying silicon film thickness it is possible to shift and control peak widths and position for both the dipole and interparticle dipole-dipole coupling modes. Control of plasmon shifts and interparticle spacing is applied towards the optimization of SERS substrates. SERS substrates using a Rhodamine B label are tuned at different excitation wavelengths which are in resonance with either the plasmon dipole, fluorescent dye, or interparticle coupling mode. Substrates display reproducible enhancement across multiple sites. This work presents methodology to design and optimize uniform silver nanocube SERS substrates through tuning of plasmon shifts and particle spacing

Probing the anisotropy of SERS enhancement with spatially separated plasmonic modes in strongly coupled silver nanocubes on a dielectric substrate

The utilization of substrate-particle interactions provides a route to tuning the optical properties of strongly coupled supported plasmonic nanoparticles. In this work fine

Substrate-induced effects on the plasmonic properties of strongly coupled silver nanocubes

The behaviour of the plasmonic modes of supported strongly coupled silver nanocubes is studied. Silver nanocube monolayers with controlled particle density were fabricated via the Langmuir-Blodgett technique and deposited on substrates with varying refractive indices. Substrates include glass, thin films of silicon, and titanium oxide on glass. The dipolar and bonded dipolar modes are red shifted with increasing refractive index of the substrate. Surface-enhanced Raman spectroscopy (SERS) is used as a tool to probe the electric field enhancements of the silver nanocube monolayers. SERS enhancement of silver nanocube monolayers is found to be highly substrate dependant, typically decreasing with increasing refractive index of the underlying substrate. This work aims to find the source of this enhancement decrease, and distinguishes between effects related electromagnetic enhancement and effects caused by the optics of the Raman spectroscopy system itself

Probing the Anisotropy of SERS Enhancement with Spatially Separated Plasmonic Modes in Strongly Coupled Silver Nanocubes on a Dielectric Substrate

The utilization of substrate–particle interactions provides a route to tuning the optical properties of strongly coupled supported plasmonic nanoparticles. In this work fine control over interparticle and particle–substrate interactions is demonstrated using Langmuir–Blodgett monolayers of silver nanocubes deposited onto titanium oxide (TiO_<i>x</i>) thin films of varying thickness. By using two Raman reporters, a Rhodamine-B (RhB) and benzenethiol (BT), surface-enhanced Raman spectroscopy (SERS) independently examines electromagnetic (EM) enhancement at the substrate/nanocube interface (RhB) and at the surface of the cubes, where the label is predominately 20–40 nm away from the dielectric substrate (BT). For RhB the SERS enhancement factor (EF) drops as much as an order of magnitude on 20 nm TiO_<i>x</i> with respect to glass. However, for BT, a maximum SERS EF of (2.5 ± 0.4) × 10⁵ was observed on TiO_<i>x</i> compared to (1.5 ± 0.4) × 10⁵ on glass, an increase of 60%. Control over the organization of the nanocube monolayer reveals that maximum enhancement occurs in small, discrete clusters of nanocubes as opposed to large aggregates. Fine control over the optical properties and near-field EM distribution of coupled nanostructures can be accomplished through tuning of the dielectric properties of the substrate yielding a route to optimizing properties for field-enhanced plasmonic applications

Plasmonic properties of weakly interacting silver nanocubes on high refractive index substrates

In the present work we investigated the properties and behavior of plasmonic modes of silver nanocube monolayers with respect to reflection and transmission of visible radiation. Uniform monolayers of low particle densities were created using the Langmuir-Blodgett technique using the phospholipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) as a passive spacer. Dipole-dipole coupling modes were avoided by depositing at low pressures to ensure sufficient spacing between the nanocubes. The refractive index sensitivities of plasmonic modes for monolayers on glass, silicon thin films, and bulk silicon wafers were measured using varying solutions of water and ethylene glycol. By varying the refractive index of the substrates it is possible to investigate the relative contribution of plasmonic modes with respect to absorption of the incident signal

Fine tuning of plasmonic properties of monolayers of weakly interacting silver nanocubes on thin silicon films

Plasmonic properties, such as refractive index sensitivity (RIS), surface enhancement of the Raman signal (SERS), fluorescence quenching, and photocatalytic activity, of monolayers of weakly interacting monodisperse silver nanocubes were qualitatively modified in a very well controlled manner by supporting them on thin silicon films with varying thickness. Such fine tunability is made possible by the strong dependence of the nanocube dipolar (D) and quadrupolar (Q) plasmon mode hybridization on the refractive index of the supporting substrate. By increasing the Si film thickness from zero to ∼25 nm we were able to "shift" the D resonance mode by up to 200 nm for ∼80 nm cubes without significantly affecting the Q mode. The silicon supported nanocubes showed a significant improvement in RIS via the Q mode with a figure of merit greater than 6.5 and about an order of magnitude enhancement of the SERS signal due to the stronger electric field created by the D mode. Such substrates also showed a ∼10 times decrease in rhodamine 6G fluorescence as well as the rates of amorphous carbon formation. The study proposes a new way to design and engineer plasmonic nanostructures

Dynamics of nanocubes embedding into polymer films investigated: Via spatially resolved plasmon modes

Integration of nanoparticles into thin films is essential for the development of functional materials, studies of fundamental interfacial processes, and exploitation of inherent properties from the particles themselves. In this work, we systematically investigate the process of incorporation of silver nanocubes into thin polystyrene films at temperatures just above the polymer glass transition. The process of nanocrystal incorporation can be precisely monitored via far-field spectroscopy to observe the response of spatially resolved hybrid plasmon modes. Each plasmon resonance has a distinct dynamic range and maximum sensitivity forming a complementary set of nanorulers that operates over a distance comparable to the edge length of the cubes. The approach explored in this work is a general robust method for the development of long-range polychromatic nanorulers

Unusually Sharp Localized Surface Plasmon Resonance in Supported Silver Nanocrystals with a Thin Dielectric Coating

An unusually sharp localized surface plasmon resonance (sLSPR) is observed for a monolayer of glass-supported silver nanocubes coated with a thin, 5–20 nm, Al₂O₃ film. The resonance becomes significantly narrower and stronger while losing optical anisotropy and sensitivity to the surroundings with increasing overlayer thickness. Surface-enhanced Raman scattering excitation profiles indicate an additional enhancement to the electric field brought in by the sLSPR. The resonance is thought to originate from a Fano-like constructive interference between the quadrupolar and dipolar LSPR modes in supported silver nanocubes leading to enhanced light extinction. This phenomenon is of significance for plasmon-induced charge-transfer processes in photovoltaics and catalysis

Hybridized plasmon resonances in core/half-shell silver/cuprous oxide nanoparticles

Core/shell nanoparticles are of interest due to their attractive optical, electronic, and catalytic properties. By combining these core/shell features with physical anisotropy of the shell-by selectively capping only a portion of the core-nanostructures with unique properties can be formed. This work presents the synthesis and investigates the plasmonic properties of silver nanocube (AgNC)/cuprous oxide core/half-shell nanoparticles. The developed partial shell growth technique is suitable for any room temperature two-step core/shell synthesis. Using this technique AgNC/Cu2O core/shell nanoparticles were formed with a distinct half-shell morphology, either pyramidal or cubic, where the geometry of the half-shell can be precisely controlled by selecting specific synthesis conditions. Furthermore, the cuprous oxide half-shells induced hybridization of the plasmon modes in the silver core and thus enabled spatial and spectral manipulation of plasmon resonances for nanoparticles in suspension. The proposed core/half-shell morphology will be particularly advantageous for directed assembly, formation of dimers for SERS sensing, or as individual particles for catalysis