Directional Scattering and Sensing with Bimetallic Fanocubes: A Complex Fano-Resonant Plasmonic Nanostructure

Concentric nanostructures provide a unique architecture to manipulate light by modification of their internal geometry with minimal changes to their overall size. In this work, we have theoretically examined, using finite difference time domain simulations, the plasmonic properties of a concentric cubic nanostructure consisting of a silver (Ag) core, silica (SiO₂) interlayer, and gold (Au) shell. These “bimetallic fanocubes” display two separate geometry dependent Fano resonances in the visible and in the near-infrared. We employed a plasmon hybridization model to understand the origin of the spectral features and observe distinct hybridized modes contributed by the edges and corners, which is unique to the cubic geometry. Specifically, we note that the “nonbonding” mode that is essentially dark and not observable in spherical concentric nanostructures is enhanced in the bimetallic fanocubes. We show the far-field properties, and Fano resonances of the fanocubes can be tuned by altering the thickness of the silica layer, the thickness of the Au shell, and by breaking symmetry. Further, we have examined the refractive index sensing and directional scattering abilities of the fanocubes to ultimately enable their use in a range of applications, harnessing their absorption and scattering properties

Facile Chemical Approach to ZnO Submicrometer Particles with Controllable Morphologies

We have developed a simple wet-chemistry approach to fabricating ZnO submicrometer particles with unique morphologies including rings, bowls, hemispheres, and disks. The size and morphology of the particles can be conveniently tailored by varying the concentrations of the zinc precursor. The reaction temperature, pH, and concentration of ammonia are also found to play critical roles in directing the formation of these particle morphologies. These submicrometer particles exhibit strong white-light emission upon UV excitation as a result of the presence of surface defect states resulting from the fabrication method and synthesis conditions

Metallic Nanoshells with Semiconductor Cores: Optical Characteristics Modified by Core Medium Properties

It is well-known that the geometry of a nanoshell controls the resonance frequencies of its plasmon modes; however, the properties of the core material also strongly influence its optical properties. Here we report the synthesis of Au nanoshells with semiconductor cores of cuprous oxide and examine their optical characteristics. This material system allows us to systematically examine the role of core material on nanoshell optical properties, comparing Cu2O core nanoshells (εc ∼ 7) to lower core dielectric constant SiO2 core nanoshells (εc = 2) and higher dielectric constant mixed valency iron oxide nanoshells (εc = 12). Increasing the core dielectric constant increases nanoparticle absorption efficiency, reduces plasmon line width, and modifies plasmon energies. Modifying the core medium provides an additional means of tailoring both the near- and far-field optical properties in this unique nanoparticle system

Optically-Driven Collapse of a Plasmonic Nanogap Self-Monitored by Optical Frequency Mixing

A nanoparticle separated from a metallic surface by a few-nanometer thick polymer layer forms a nanoscale junction, or nanogap. Illuminating this structure with ultrashort optical pulses, exciting the plasmon resonance, results in a continuous, monitorable collapse of the nanogap. The four-wave mixing signal generated by this illumination of the nanogap provides a simultaneous monitoring of the collapse, increasing dramatically upon gap closure. Collapse is irreversible, occurring with simultaneous ablation of the dielectric from the junction

Au Nanorice Assemble Electrolytically into Mesostars

Star-shaped mesotructures are formed when an aqueous suspension of Au nanorice particles, which consist of prolate hematite cores and a thin Au shell, is subjected to an electric current. The nanorice particles assemble to form hyperbranched micrometer-scale mesostars. To our knowledge, this is the first reported observation of nanoparticle assembly into larger ordered structures under the influence of an electrochemical process (H2O electrolysis). The assembly is accompanied by significant modifications in the morphology, dimensions, chemical composition, crystallographic structure, and optical properties of the constituent nanoparticles

Au Nanorice Assemble Electrolytically into Mesostars

Star-shaped mesotructures are formed when an aqueous suspension of Au nanorice particles, which consist of prolate hematite cores and a thin Au shell, is subjected to an electric current. The nanorice particles assemble to form hyperbranched micrometer-scale mesostars. To our knowledge, this is the first reported observation of nanoparticle assembly into larger ordered structures under the influence of an electrochemical process (H2O electrolysis). The assembly is accompanied by significant modifications in the morphology, dimensions, chemical composition, crystallographic structure, and optical properties of the constituent nanoparticles

Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry

With the advent of novel nanoscale devices, fast and reliable thermal mapping with high spatiotemporal resolution is imperative for probing the characteristics of phonons and evaluating the local temperature at the nanoscale. In this work, Raman spectroscopy is employed as a rapid and noncontact optical thermometry technique to investigate phononic properties of macroscopic assemblies of monodisperse palladium oxide (PdO) nanocrystals. PdO has been extensively employed in high temperature catalytic devices; however, the phonon behavior which determines the thermal stability of PdO remains unexplored thus far. Our study focuses on homogeneous, large-scale assemblies of monodisperse 4 and 10 nm nanocrystals synthesized using colloidal chemistry to understand size-dependent effects on the measured thermal properties. By monitoring the Raman peak shifts, peak broadening, and alterations in peak intensities as a function of laser power and particle concentration, a size-dependent trend is observed attributable to confinement of optical phonons within nanocrystal grain boundaries and laser-induced heating, both influenced by nanocrystal size. This study correlates size-dependent single-particle heating effects with size-dependent interparticle heat transfer under laser irradiation and is enabled by controlled nanocrystal synthesis

Fluorescence Enhancement by Au Nanostructures: Nanoshells and Nanorods

Metallic nanoparticles influence the quantum yield and lifetime of adjacent fluorophores in a manner dependent on the properties of the nanostructure. Here we directly compare the fluorescence enhancement of the near-infrared fluorophore IR800 by Au nanoshells (NSs) and Au nanorods (NRs), where human serum albumin (HSA) serves as a spacer layer between the nanoparticle and the fluorophore. Our measurements reveal that the quantum yield of IR800 is enhanced from ∼7% as an isolated fluorophore to 86% in a NSs−HSA−IR800 complex and 74% in a NRs−HSA−IR800 complex. This dramatic increase in fluorescence shows tremendous potential for contrast enhancement in fluorescence-based bioimaging

Enhancement in Organic Photovoltaics Controlled by the Interplay between Charge-Transfer Excitons and Surface Plasmons

In this work, we investigate plasmonic enhancement in poly­(3-hexylthiophene):phenyl-C61-butyric acid methyl ester organic photovoltaics (OPVs) by integrating shape- and size-controlled bimetallic gold core–silver shell nanocrystals (Au–Ag NCs) into the poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate hole-transport layer. We observed that the best-performing Au–Ag NC-incorporated OPVs improved the power conversion efficiency by 9% via a broadband increase in photocurrent throughout the visible spectrum. Our experimental and computational results suggest that the observed photocurrent enhancement in plasmonic OPVs originates from both enhanced absorption and improved exciton dissociation and charge collection. This is particularly achieved by placing metal NCs near the interface of the active layer and hole-transport layer. The impedance spectroscopy results suggest that Au–Ag NCs reduce recombination and also increase the internal exciton to carrier efficiency by driving the dissociation of bound charge-transfer states to free carriers

Ultrafast Excited-State Dynamics in Shape- and Composition-Controlled Gold–Silver Bimetallic Nanostructures

In this work, we have examined the ultrafast dynamics of shape- and composition-controlled bimetallic Au/Ag core/shell nanostructures with transient absorption spectroscopy (TAS) as a function of Ag layer thickness (0–15 nm) and pump excitation fluence (50–500 nJ/pulse). Our synthesis approach generated both bimetallic nanocubes and nanopyramids with distinct dipolar plasmon resonances and plasmon dephasing behavior at the resonance. Lifetimes obtained from TAS at low powers (50 nJ/pulse) demonstrated minimal dependence on the Ag layer thickness, whereas at high power (500 nJ/pulse) a rise in electron–phonon coupling lifetime (τ₁) was observed with increasing Ag shell thickness for both nanocubes and nanopyramids. This is attributable to the stronger absorption of the 400 nm pump pulse with higher Ag content, which induced higher electron temperatures. The phonon–phonon scattering lifetime (τ₂) also rises with increasing Ag layer, contributed both by the increasing size of the Au/Ag nanostructures as well as by surface chemistry effects. Further, we observed that even the thinnest, 2 nm, Ag shell strongly impacts both τ₁ and τ₂ at high power despite minimal change in overall size, indicating that the nanostructure composition also strongly impacts the thermalization temperature following absorption of 400 nm light. We also observed a shape-dependent trend at high power, where τ₂ increased for the nanopyramids with increasing Ag shell thickness and nanostructure size, but bimetallic nanocubes demonstrated an unexpected decrease in τ₂ for the thickest, 15 nm, Ag shell. This was attributed to the larger number of corners and edges in the nanocubes relative to the nanopyramids