Responsive nanomaterials composed of gold nanoparticles (AuNPs) and temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels offer the promise of designing smart materials that can change color in response to varying thermal or photothermal stimuli. Typical PNIPAM/AuNP hybrids are heavily loaded with AuNPs. Here, we demonstrate that hybrids with an average loading of three to five AuNPs per PNIPAM sphere exhibit peak extinction shifts of over 150 nm and color change from red to purple to gray as the temperature increases from 25 to 50 °C. We observe that the time scale for spectral shifts is offset from that for hydrophobic collapse of the PNIPAM spheres. Facilitated by the low loading density, we combine kinetic studies of the changes in the extinction spectra with finite-difference time-domain simulations to show that the location of AuNPs relative to the PNIPAM sphere at different stages of collapse is a key variable accounting for the time and temperature dependence of the experimental data.

Driven by the desire to understand energy transfer between plasmonic and catalytic metals for applications such as plasmon-mediated catalysis, we examine the spatially resolved electron energy-loss spectra (EELS) of both pure Au nanoprisms and Pt-decorated Au nanoprisms. The EEL spectra and the resulting surface-plasmon mode maps reveal detailed near-field information on the coupling and energy transfer in these systems, thereby elucidating the underlying mechanism of plasmon-driven chemical catalysis in mixed-metal nanostructures. Through a combination of experiment and theory we demonstrate that although the location of the Pt decoration greatly influences the plasmons of the nanoprism, simple spatial proximity is not enough to induce significant energy transfer from the Au to the Pt. What matters more is the spectral overlap between the intrinsic plasmon resonances of the Au nanoprism and Pt decoration, which can be tuned by changing the composition or morphology of either component.

We report the self-assembly of ultrasmooth AuxAg1–x nanoparticles with homogeneous composition via pulsed laser-induced dewetting (PLiD). The nanoparticles are truncated nanospheres that sustain unique plasmonic features. For the first time an electron energy loss spectroscopy (EELS) study elucidating the size and composition effects on the plasmonic modes of truncated AuxAg1–x nanospheres is carried out. EELS characterization captures a linear red-shift in both bright and dark modes as a function of the atomic fraction of Au and a progressive red-shift of all modes as the size increases. The results are interpreted in the context of Mie theory and electron beam simulations. Armed with the full plasmonic spectrum of the AuxAg1–x system, the truncated spheres and their ordered arrays synthesized via PLiD have promise as elements in advanced photonic devices.

Electron energy-loss spectroscopy (EELS) provides detailed nanoscopic spatial and spectral information on plasmonic nanoparticles that cannot be discerned with far-field optical techniques. Here we demonstrate that EELS is capable of mapping the relative phases of individual localized surface plasmons that are hybridized within nanoparticle assemblies. Within the context of an effective plasmon oscillator model, we demonstrate the relationship between the self-induced back-force on the electron due to the plasmon and the EEL probability and use this to present a rubric for determining the relative phases of hybridized localized surface plasmons in EELS. Comparison between the analytical oscillator model, experiment, and numerical electrodynamics simulation is made across a variety of nanoparticle monomer, dimer, and trimer systems.

Energy transfer from plasmonic nanoparticles to semiconductors can expand the available spectrum of solar energy-harvesting devices. Here, we spatially and spectrally resolve the interaction between single Ag nanocubes with insulating and semiconducting substrates using electron energy-loss spectroscopy, electrodynamics simulations, and extended plasmon hybridization theory. Our results illustrate a new way to characterize plasmon–semiconductor energy transfer at the nanoscale and bear impact upon the design of next-generation solar energy-harvesting devices.

Recent experiments report observations of quantum interference between plasmon resonances, inviting descriptions of plasmon–photon interaction using methods from quantum optics. Here we demonstrate, using a Heisenberg–Langevin approach, that the radiation emitted from the localized surface plasmon resonances of a mixed-metal heterodimer may exhibit observable, beat frequency interferences at a far-field detector, known as quantum beats. This prediction represents a correspondence between V-type atoms of quantum optics and the familiar heterodimer system of plasmonics. We explore this analogy in depth and find that although both systems support quantum beats, the heterodimer emits photons in bunches due to the bosonic nature of the plasmon. This highlights a significant difference between the properties of atomic and plasmonic systems.

Motivated by the need to study the size dependence of nanoparticle–substrate systems, we present a combined experimental and theoretical electron energy loss spectroscopy (EELS) study of the plasmonic spectrum of substrate-supported truncated silver nanospheres. This work spans the entire classical range of plasmonic behavior probing particles of 20–1000 nm in diameter, allowing us to map the evolution of localized surface plasmons into surface plasmon polaritons and study the size dependence of substrate-induced mode splitting. This work constitutes the first nanoscopic characterization and imaging of these effects in truncated nanospheres, setting the stage for the systematic study of plasmon-mediated energy transfer in nanoparticle–substrate systems.

The Fano interference phenomenon between localized surface plasmon resonances (LSPRs) of individual silver nanocubes is investigated using dark-field optical microscopy and electron-energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). By computing the polarization induced by the electron beam, we show that the hybridized modes responsible for this Fano interference are the same as those present in the resonance-Rayleigh scattering spectrum of an individual nanocube on a substrate.

Nanomaterials exhibiting plasmonic optical responses are impacting sensing, information processing, catalysis, solar, and photonics technologies. Recent advances have expanded the portfolio of plasmonic nanostructures into doped semiconductor nanocrystals, which allow dynamic manipulation of carrier densities. Once interpreted as intraband single-electron transitions, the infrared absorption of doped semiconductor nanocrystals is now commonly attributed to localized surface plasmon resonances and analyzed using the classical Drude model to determine carrier densities. Here, we show that the experimental plasmon resonance energies of photodoped ZnO nanocrystals with controlled sizes and carrier densities diverge from classical Drude model predictions at small sizes, revealing quantum plasmons in these nanocrystals. A Lorentz oscillator model more adequately describes the data and illustrates a closer link between plasmon resonances and single-electron transitions in semiconductors than in metals, highlighting a fundamental contrast between these two classes of plasmonic materials.Keywords: doped nanocrystals; photodoping; plasmons; quantum dots; zinc oxide

A consequence of thermal diffusion is that heat, even when applied to a localized region of space, has the tendency to produce a temperature change that is spatially uniform throughout a material with a thermal conductivity that is much larger than that of its environment. This implies that the degree of spatial correlation between the heat power supplied and the temperature change that it induces is likely to be small. Here, we show, via theory and simulation, that through a Fano interference, temperature changes can be both localized and controllably directed within certain plasmon-supporting metal nanoparticle assemblies. This occurs even when all particles are composed of the same material and contained within the same diffraction-limited spot. These anomalous thermal properties are compared and contrasted across three different nanosystems, the coupled nanorod–antenna, the heterorod dimer, and the nanocube on a substrate, known to support both spatial and spectral Fano interferences. We conclude that the presence of a Fano resonance is not sufficient by itself to induce a controllably nanolocalized temperature change. However, when present in a nanosystem of the right composition and morphology, temperature changes can be manipulated with nanoscale precision, despite thermal diffusion.Keywords: Fano resonances; nanoscale heat diffusion; thermal hot spots; thermoplasmonics;

The optical-frequency magnetic and electric properties of cyclic aromatic plasmon-supporting metal nanoparticle oligomers are explored through a combination of scanning transmission electron microscopy (STEM)/electron energy-loss spectroscopy (EELS) simulation and first-principles theory. A tight-binding-type model is introduced to explore the rich hybridization physics in these plasmonic systems and tested with full-wave numerical electrodynamics simulations of the STEM electron probe. Building from a microscopic electric model, connection is made at the macroscopic level between the hybridization of localized magnetic moments into delocalized magnetic plasmons of controllable magnetic order and the mixing of atomic pz orbitals into delocalized π molecular orbitals of varying nodal structure spanning the molecule. It is found that the STEM electrons are uniquely capable of exciting all of the different hybridized eigenmodes of the nanoparticle assembly—including multipolar closed-loop ferromagnetic and antiferromagnetic plasmons, giant electric dipole resonances, and radial breathing modes—by raster scanning the beam to the appropriate position. Comparison to plane-wave light scattering and cathodoluminescence spectroscopy is made. The presented work provides a unified understanding of the complete plasmon eigenstructure of such oligomer systems as well as of the excitation conditions necessary to probe each mode.Keywords: cathodoluminescence (CL) spectroscopy; electron energy-loss spectroscopy (EELS); magnetic plasmon resonances; scanning transmission electron microscopy (STEM)

Through numerical simulation, we predict the existence of the Fano interference effect in the electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) of symmetry-broken nanorod dimers that are heterogeneous in material composition and asymmetric in length. The differing selection rules of the electron probe in comparison to the photon of a plane wave allow for the simultaneous excitation of both optically bright and dark plasmons of each monomer unit, suggesting that Fano resonances will not arise in EELS and CL. Yet, interferences are manifested in the dimer’s scattered near- and far-fields and are evident in EELS and CL due to the rapid π-phase offset in the polarizations between super-radiant and subradiant hybridized plasmon modes of the dimer as a function of the energy loss suffered by the impinging electron. Depending upon the location of the electron beam, we demonstrate the conditions under which Fano interferences will be present in both optical and electron spectroscopies (EELS and CL) as well as a new class of Fano interferences that are uniquely electron-driven and are absent in the optical response. Among other things, the knowledge gained from this work bears impact upon the design of some of the world’s most sensitive sensors, which are currently based upon Fano resonances.Keywords: cathodoluminescence; electron energy loss spectroscopy; Fano resonances; plasmonics

We extend our previous quantum many-body Green’s function formalism to characterize the deformations induced in the electronic structure of a quantum emitter when it strongly couples with a plasmon-supporting environment at finite frequency. Through infinite-order perturbation theory, we predict the emergence of subtle yet observable changes in the plasmon-dressed molecule’s frontier orbitals, orbital energies, and low-lying electronic excitations when the molecular and plasmonic systems are resonantly coexcited. These distortions, which predominately arise from the finite-frequency image interaction, point to new chemical and optical properties beyond those of the vacuum molecule and bear impact upon resonant plasmon-enhanced molecular spectroscopies and hot-electron-driven chemical catalysis. We propose an experiment capable of testing our predictions.

Super-resolution optical imaging of Rhodamine 6G surface-enhanced Raman scattering (SERS) and silver luminescence from colloidal silver aggregates are measured with sub-5 nm resolution and found to originate from distinct spatial locations on the nanoparticle surface. Using correlated scanning electron microscopy, the spatial origins of the two signals are mapped onto the nanoparticle structure, revealing that, while both types of emission are plasmon-mediated, SERS is a highly local effect, probing only a single junction in a nanoparticle aggregate, whereas luminescence probes all collective plasmon modes within the nanostructure. Calculations using the discrete-dipole approximation to calculate the weighted centroid position of both the |E|2/|Einc|2 and |E|4/|Einc|4 electromagnetic fields were compared to the super-resolution centroid positions of the SERS and luminescence data and found to agree with the proposed plasmon dependence of the two emission signals. These results are significant to the field of SERS because they allow us to assign the exact nanoparticle junction responsible for single-molecule SERS emission in higher order aggregates and also provide insight into how SERS is coupled into the plasmon modes of the underlying nanostructure, which is important for developing new theoretical models to describe SERS emission.Keywords: discrete-dipole approximation; hot spots; plasmon; silver luminescence; surface-enhanced Raman scattering

A computational analysis of the electron- and photon-driven surface-plasmon resonances of monomer and dimer metal nanorods is presented to elucidate the differences and similarities between the two excitation mechanisms in a system with well-understood optical properties. By correlating the nanostructure’s simulated electron energy-loss spectrum and loss-probability maps with its induced polarization and scattered electric field we discern how certain plasmon modes are selectively excited and how they funnel energy from the excitation source into the near- and far-field. Using a fully retarded electron-scattering theory capable of describing arbitrary three-dimensional nanoparticle geometries, aggregation schemes, and material compositions, we find that electron energy-loss spectroscopy (EELS) is able to indirectly probe the same electromagnetic hot spots that are generated by an optical excitation source. Comparison with recent experiment is made to verify our findings.Keywords: electron energy-loss spectroscopy; hot spots; localized surface plasmon; scanning transmission electron microscopy; silver nanoparticles

The electromagnetic scattering properties of Ag nanoparticle aggregates known to be antennas for single-molecule surface-enhanced Raman scattering are investigated from a continuum electrodynamics perspective. High-resolution mappings of the spatial, spectral, and polarization dependence of the volumes of the aggregate’s electromagnetic hot spots reveal multiple active regions for enhanced Raman scattering activity by molecular chromophores. Further analysis of these regions using maps of polarization surface-charge density shows that some hot spots are due to the collective and phase-coherent excitation of localized surface-plasmon resonances, whereas others derive from interfering plasmonic excitations resulting from scattering from gaps and surfaces. The latter are still capable of generating intense local fields at certain excitation energies, whereas the former tend to provide the most spatially delocalized regions of high electromagnetic-field strength.Keywords: hot-spot volume; localized surface plasmons; surface-enhanced Raman scattering;