The history of plasmonics is inextricably linked with our understanding of the unique properties of metals arising from their abundance of free, conduction-band electrons. These free electrons, whose wavefunctions are delocalized throughout the material, are responsible for the most characteristic features of metals, such as their high reflectivity and excellent thermal and electrical conductivities. A surprisingly large number of observations about metals can be explained using the Drude model, where the free electrons are treated as a gas-phase plasma. The Drude model yields a complex-valued permittivity ε(ω), whose real component is negative for frequencies below the plasma frequency. It is the negative values of Re[ε(ω)] that give rise to surface-bound electromagnetic waves known as surface plasmon–polaritons (SPPs) and localized surface plasmons (LSPs), which underlie the myriad applications of plasmonic materials.
Surface-bound electromagnetic waves were first observed in the late 1950s by bombarding metal films with fast electrons and for many years remained mostly in the domain of solid state physics. However, the nanorevolution that began in the 1980s and 1990s provided powerful new tools for the synthesis, characterization, and theoretical modeling of nanostructured materials. It was quickly realized that the most exciting optical properties of nanostructured plasmonic materials were attributed to the excitation of the SPPs. The surface-enhanced Raman scattering (SERS) effect, in particular, introduced SPPs and later LSPs to the chemical physics community, requiring careful consideration of the interaction between surface plasmons and molecular systems.
The articles highlighted in this special topic illustrate that the fascination with plasmonic materials and their chemical applications remains as strong as ever, despite decades of sustained research.
Focused electron beams as a probe of matter with nanometer-scale spatial resolution have continued to find increasing importance in the investigation of plasmonic nanostructures. Herein, Borodinov et al. presented a pan-sharpening approach to enhance hyperspectral electron energy loss spectroscopy (EELS) analysis of complex plasmonic nanostructures.1 They were able to acquire two EELS datasets, one of high spatial resolution and the other of high spectral resolution, and combine them to create a new dataset with the benefits of both. By using stimulated electron energy loss and gain spectroscopy, Collette et al. imaged the optically stimulated near-field of the bonding and antibonding LSP modes of gold nanorod dimers with varying aspect ratios to tune the hybrid LSP resonances to the applied laser energy.2 Mousavi M. et al. investigated the impact of hierarchical geometry on the surface plasmon response of a cross section of a wrinkled gold structure using EELS, with emphasis placed on how structural disorder and specific features in the film may affect the system’s eigenmodes.3 They found that micrometer-sized structural features have only a small effect on the surface plasmon resonance response, whereas nanofeatures strongly affect the resonant modes. By using two-color photoemission electron microscopy (PEEM) and counterpropagating SPP generation on a 2D nanohole array, Crampton et al. were able to visualize SPPs in real space and explored the efficacy of single and dual component SPP steering elements on controlling their directionality, which is important to the development of miniaturized optical circuitry.4 By controlling the linear laser polarization and using angle-resolved photoelectron velocity map imaging, Pettine et al. achieved the continuous control of the distribution of the excitation and emission of hot electrons on the azimuthal plane of a gold nanoshell supported on a substrate.5 These results demonstrate a highly predictive level of understanding of the underlying physics and possibilities for ultrafast spatiotemporal control over hot carrier dynamics.
Understanding the optical response of plasmonic materials is a prerequisite to understanding the interactions between a metal and molecule. For instance, Bitton et al. studied the effect of chromium adhesion layer on the spectral linewidths of bowtie-structured plasmonic cavities.6 They found that a reduction in the thickness of the adhesion layer could decrease the linewidths of both bright and dark plasmonic modes and they were able to achieve the smallest linewidth without any adhesion layer. Mizobata et al. developed near-field transmission and reflection spectromicroscopy to reveal the absorption and scattering characteristics of a single silver nanoplate.7 They found that the optical characteristics and the wavelength dependency of the optical contrast originate from the scattering and absorption properties of the plasmons. By using time-dependent density functional tight binding, Liu et al. investigated the optical properties of monomers and dimers of face centered cubic Ag nanoparticles and determined trends for absorption peak shifts and how these depend on the interparticle distance.8 Payne et al. investigated the role of material permittivity and surface damping effects on the quantitative morphometric analysis of hundreds of defect-free ultra-uniform individual Au nanospheres by using optical extinction microscopy.9 They found that the single crystal dataset is the best to describe the material permittivity and the surface damping parameter of the nanoparticles. Zhao et al. described two mechanisms, intraband plasmon-mediated radiative decay and interband electron–hole recombination, accounting for gold nanorod photoluminescence (PL) resulting from nonlinear laser excitation of single-nanoparticle PL.10 They found that nonlinear excitation can lead to energy and polarization modulation of nanoparticle optical signals that are not observed using linear excitation. Wang et al. observed avalanche multiphoton photoluminescence (AMPL) from coupled Au–Al nanoantennas under intense laser pumping.11 Compared with ordinary metallic photoluminescence, AMPL shows more than one order of magnitude emission intensity enhancement and distinct spectral features. Oliveira et al. presented in detail the template stripped-based method for fabrication of plasmon-tunable tip pyramid (PTTP) probes in a highly reproducible way.12 Compared with typical gold micropyramids, PTTP can achieve an enhancement of at least one order of magnitude higher and an imaging resolution of about 20 nm. Fiederling et al. used a plasmonic tip modeled by a single positively vs negatively charged silver atom to map an immobilized molecule, i.e., tin(II)phthalocyanine (SnPc).13 Thereby, they evaluated the impact of static charges localized on the tip’s frontmost atom on the tip-enhanced Raman spectroscopy (TERS) signal and the lateral resolution. Kumar et al. characterized plasma functionalized graphitic flakes using TERS. By exploiting the nanoscale spatial resolving power of the measurement, they were able to characterize disorder in graphitic nanomaterials on the nanoscale with three different levels of nitrogen functionalization.14 A perspective on TERS, highlighting recent progress in understanding reacting and dynamic systems ranging from simple model reactions to complex processes with practical applications, is contributed by Sartin et al.15
Enhancement of Raman scattering and PL from colloidal CdSe/CdS nanoplatelets deposited on arrays of Au nanodisks was investigated by Milekhin et al.16 In particular, they characterize the nanoplatelet’s SERS and surface-enhanced PL intensity profiles and interpret their shape in terms of the Au nanodisk LSP spectrum vs different excitation and emission enhancement mechanisms. By combining fluorescence lifetime imaging microscopy, scanning confocal fluorescence imaging, Rayleigh scattering imaging, optical microscopy, and finite difference time domain simulations, Farcau et al. identified the spatial locations where surface-enhanced fluorescence effects occur in linear arrays of silver half-shells.17 This work represents the first such study of polarization-sensitive hybrid colloidal photonic–plasmonic crystals. Using a combination of zeta potential and SERS measurements, Xi and Haes studied the impacts of functional group protonation on monosubstituted benzene derivatives with amine, carboxylic acid, or hydroxide.18 They find that pH variations induce carboxylate protonation and electron redistribution that weaken molecular affinity, leading to geometric changes in the way the molecule is oriented with respect to the surface of its supporting plasmonic gold nanostar.
The theoretical understanding of molecules in the proximity of a strong local field is important to understand the novel spectral features observed in such a confined region. For example, Gong et al. presented a computational method based on quantum-mechanical theory to fully describe the interaction between confined SPPs and adsorbed molecules at the interface of a metal and a dielectric.19 The breakdown of the conventional dipole approximation emerges at a high confinement factor in metal/dielectric interfaces, allowing efficient harvesting of SPPs with low excitation energies and increasing the efficiency of solar energy conversion by dye molecules. Using the excitation wavelength-dependent SERS and density functional theory (DFT) calculations, Prakash explored Herzberg–Teller selection rules on the charge-transfer effect in SERS of perylene tetracarboxylic diimide adsorbed on an Ag nano-island film.20 They observed three resonances, including molecular, localized surface plasmon, and charge transfer, with 514.5 nm excitation. Chen and Jensen presented a frequency dependent Raman bond model based on damped response theory to partition the Raman intensity to interatomic charge flow modulations or Raman bonds.21 The model quantifies the interference between the molecular resonance mechanism, the charge transfer mechanism, and the electromagnetic mechanism and interprets the electromagnetic mechanism as charge flow modulations in the metal. In an effort to better model nanoplasmonic responses in the time domain, Dall’Osto et al. developed a theoretical approach to explicitly include the time-dependent dielectric functions of noble metal nanoparticles into their time domain-boundary element method simulations. Their approach captures the presence of many transition frequencies, such as in gold, and helps to provide a clearer understanding of dynamical processes that are more naturally viewed in the time domain.22
SERS and TERS have found important applications in fields related to biological, gas–solid, and UHV conditions. For example, Wallace et al. demonstrated two mechanical routes for controlled positioning and inserting nanosensors into the tissue and discussed two means of focusing on the nanosensors both before and after they are inserted into the tissue.23 This method can serve as a crucial starting point for future measurements involving SERS nanosensors inserted in complex tissues. By electrochemical deposition of Pt overlayers onto SERS-active uniform Au nanoparticle films, Ge et al. investigated the adsorption kinetics of CO at the gas–solid interface by considering the complex dynamic coupling and shielding effects.24 They found that the adsorption of linear bonded CO follows Langmuir adsorption behavior, whereas multiply bonded CO did not. Utilizing the subnanometer resolved TERS technique, Li et al. were able to explicitly resolve the subtle different adsorption configurations and structural deformations of CPP molecules on metal substrates with different crystallographic orientations.25 Single nanohood structures were observed on isotropic surfaces, and “Möbius-like” features and symmetric bending structures were observed on anisotropic surfaces.
Plasmon resonances have been increasingly used to enhance the detection sensitivity of nonlinear spectroscopy methods. For instance, Zong et al. presented a theoretical model to describe the nondispersive line shapes observed in plasmon-enhanced coherent anti-Stokes Raman scattering (PECARS) using 4-mercaptopyridine adsorbed on a self-assembled Au nanoparticle substrate and aggregated Au nanoparticle colloids as model systems.26 Decoupling of the photoluminescence background of the substrate and high local electric field enhancements lead to a better signal intensity of PECARS compared with that in plasmon-enhanced stimulated Raman scattering (PESRS). Olson et al. reported a comprehensive experimental and theoretical study of the lower-wavenumber vibrational modes in the surface-enhanced hyper Raman scattering (SEHRS) of Rhodamine 6G (R6G) and its isotopologue R6G-d4.27 They were able to perform a detailed analysis of the complex vibronic coupling effects in R6G and found the importance of surface orientation for characterizing the system. Shen et al. experimentally designed a multiband plasmon-enhanced second-harmonic generation (SHG) platform of three-dimensional MDM (3D-MDM) nanocavities that consist of thin ZnO films integrated with silver mushroom arrays.28 ZnO materials in intracavity regions can not only modify mode couplings near fundamental wavelengths but also serve as an efficient nonlinear dielectric allowing amplification of SHG signals at several wavelengths simultaneously.
Taken together, these 28 articles highlight a diversity of exciting new advances in the spectroscopy and microscopy of plasmonic systems using a variety of optical, scanning probe, and electron beam techniques. From basic scientific investigation to applications in biology, catalysis, solar energy harvesting, and sensing, it is clear that plasmonic systems offer tremendous potential, and we expect the future to continue to find new opportunities for plasmonic phenomena impacting an even greater range of fields.