Molecular Coatings for Stabilizing Silver and Gold Nanocubes under Electron Beam Irradiation

We study the degradation process of closely spaced silver and gold nanocubes under high-energy electron beam irradiation using transmission electron microscopy (TEM). The high aspect ratio gaps between silver and gold nanocubes degraded in many cases as a result of protrusion and filament formation during electron beam irradiation. We demonstrate that the molecular coating of the nanoparticles can act as a protective barrier to minimize electron-beam-induced damage on passivated gold and silver nanoparticles

Molecular Coatings for Stabilizing Silver and Gold Nanocubes under Electron Beam Irradiation

We study the degradation process of closely spaced silver and gold nanocubes under high-energy electron beam irradiation using transmission electron microscopy (TEM). The high aspect ratio gaps between silver and gold nanocubes degraded in many cases as a result of protrusion and filament formation during electron beam irradiation. We demonstrate that the molecular coating of the nanoparticles can act as a protective barrier to minimize electron-beam-induced damage on passivated gold and silver nanoparticles

Real-Time Dynamics of Galvanic Replacement Reactions of Silver Nanocubes and Au Studied by Liquid-Cell Transmission Electron Microscopy

We study the galvanic replacement reaction of silver nanocubes in dilute, aqueous ethylenediaminetetraacetic acid disodium salt (EDTA)-capped gold aurate solutions using <i>in situ</i> liquid-cell electron microscopy. Au/Ag etched nanostructures with concave faces are formed <i>via</i> (1) etching that starts from the faces of the nanocubes, followed by (2) the deposition of an Au layer as a result of galvanic replacement, and (3) Au deposition <i>via</i> particle coalescence and monomer attachment where small nanoparticles are formed during the reaction as a result of radiolysis. Analysis of the Ag removal rate and Au deposition rate provides a quantitative picture of the growth process and shows that the morphology and composition of the final product are dependent on the stoichiometric ratio between Au and Ag

Edge-Gold-Coated Silver Nanoprisms: Enhanced Stability and Applications in Organic Photovoltaics and Chemical Sensing

We report a facile synthetic route for edge-gold-coated silver nanoprisms (GSNPs) and their comprehensive optical and structural characterization. The GSNPs exhibit remarkably high stability toward chemical etching and excellent performance as both optical antennae for light-harvesting applications and refractive index sensors. We show that when embedded into a photovoltaic bulk heterojunction film of poly­(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM), plasmonic GSNPs act as optical antennae to substantially enhance light absorption in the active organic solar cell layer. We measure a ≈7-fold enhancement in the polaron generation yield through photoinduced absorption spectroscopy. Owing to the high stability, large sensitivity factors, and strong field enhancement effect, these GSNPs exhibit great potential as optical probes for sensing and photovoltaic applications. We also show that the refractive index sensing figure of merit (FoM) of GSNPs can reach 4.05 RIU^–1 and suggest based on finite-difference time-domain (FDTD) calculations that the FoM of GSNPs could reach even higher values with better control of particle dispersity

Synthesis of Spiky Ag–Au Octahedral Nanoparticles and Their Tunable Optical Properties

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly­(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 10⁴ field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM)

Nanoplasmonics: Classical down to the Nanometer Scale

We push the fabrication limit of gold nanostructures to the exciting sub-nanometer regime, in which light–matter interactions have been anticipated to be strongly affected by the quantum nature of electrons in metals. Doing so allows us to (1) evaluate the validity of classical electrodynamics to describe plasmonic effects at this length scale and (2) witness the gradual (instead of sudden) evolution of plasmon modes when two gold nanoprisms are brought into contact. Using electron energy-loss spectroscopy and transmission electron microscope imaging, we investigated nanoprisms separated by gaps of only 0.5 nm and connected by conductive bridges as narrow as 3 nm. Good agreement of our experimental results with electromagnetic calculations and LC circuit models evidence the gradual evolution of the plasmonic resonances toward the quantum coupling regime. We demonstrate that down to the nanometer length scales investigated classical electrodynamics still holds, and a full quantum description of electrodynamics phenomena in such systems might be required only when smaller gaps of a few angstroms are considered. Our results show also the gradual onset of the charge-transfer plasmon mode and the evolution of the dipolar bright mode into a 3λ/2 mode as one literally bridges the gap between two gold nanoprisms

Synthesis of Spiky Ag–Au Octahedral Nanoparticles and Their Tunable Optical Properties

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly­(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 10⁴ field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM)

Synthesis of Spiky Ag–Au Octahedral Nanoparticles and Their Tunable Optical Properties

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly­(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 10⁴ field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM)

Real-Time Imaging of the Formation of Au–Ag Core–Shell Nanoparticles

We study the overgrowth process of silver-on-gold nanocubes in dilute, aqueous silver nitrate solution in the presence of a reducing agent, ascorbic acid, using <i>in situ</i> liquid-cell electron microscopy. Au–Ag core–shell nanostructures were formed via two mechanistic pathways: (1) nuclei coalescence, where the Ag nanoparticles absorbed onto the Au nanocubes, and (2) monomer attachment, where the Ag atoms epitaxially deposited onto the Au nanocubes. Both pathways lead to the same Au–Ag core–shell nanostructures. Analysis of the Ag deposition rate reveals the growth modes of this process and shows that this reaction is chemically mediated by the reducing agent

Real-Time Imaging of the Formation of Au–Ag Core–Shell Nanoparticles

We study the overgrowth process of silver-on-gold nanocubes in dilute, aqueous silver nitrate solution in the presence of a reducing agent, ascorbic acid, using <i>in situ</i> liquid-cell electron microscopy. Au–Ag core–shell nanostructures were formed via two mechanistic pathways: (1) nuclei coalescence, where the Ag nanoparticles absorbed onto the Au nanocubes, and (2) monomer attachment, where the Ag atoms epitaxially deposited onto the Au nanocubes. Both pathways lead to the same Au–Ag core–shell nanostructures. Analysis of the Ag deposition rate reveals the growth modes of this process and shows that this reaction is chemically mediated by the reducing agent