Plasmonics enables a wealth of applications, including photocatalysis, photoelectrochemistry, photothermal heating, optoelectronic devices, and biological and chemical sensing, while encompassing a broad range of materials, including coinage metals, doped semiconductors, metamaterials, 2D materials, bioconjugates, and chiral assemblies. Applications in plasmonics benefit from the large local electromagnetic field enhancements generated by plasmon excitation, as well as the products of plasmon decay, including photons, hot charge carriers, and heat. This special topic highlights recent work in both theory and experiment that advance our fundamental understanding of plasmon excitation and decay mechanisms, showcase new applications enabled by plasmon excitation, and highlight emerging classes of materials that support plasmon excitation.
The light-driven collective motion of electrons in materials with a small positive imaginary and negative real dielectric function is known as a plasmon, and the exploration of phenomena associated with plasmon excitation has yielded a vast field of research known as plasmonics. Plasmons are characterized as either propagating plasmons (or plasmon polaritons) or localized surface plasmons, dictated by both the dimensions of the plasmonic material relative to the wavelength of the exciting radiation as well as the nature of the excited electron density wave. In the former case, materials with both sub-wavelength and extended dimensions (such as thin films or nanowires) support electron oscillations that propagate along the extended dimension of the nanostructured material and persist over length scales much longer than the wavelength of the exciting light. The latter, localized surface plasmons, are excited in nanoparticles with sub-wavelength dimensions, and result in electron density waves that are confined at the nanoparticle surface. Within the broader field of plasmonics, there is a diversity of sub-fields due to the various processes associated with plasmon excitation and decay, including large electromagnetic field enhancements localized at the nanostructure surface (>10–104 more intense than the exciting light), elevated thermal profiles, the production of energetically excited (or “hot”) charge carriers, and photon scattering at or near the plasmon resonance. These processes enable a large range of applications, including surface-enhanced spectroscopies, photocatalysis, photothermal imaging/therapies, and optical devices, to name a few. While most early experiments in the field of plasmonics focused on coinage noble metals, such as gold and silver, more recent work has expanded the materials toolbox to include aluminum, copper, doped semiconductors, and even alkali and alkaline earth metals.
This special issue of the Journal of Chemical Physics highlights the diversity of plasmon-associated phenomena, materials, and applications by bringing together a broad cross section of work at the forefront of the field. In total, the contributions provide the reader with an appreciation of the breadth of this exciting field, and (we hope) will stimulate new directions of research. To help the reader navigate this collection, we summarize each of the 32 contributions below, focusing first on theoretical approaches to understanding plasmonic phenomena, then introducing more fundamental experimental work, and finishing with studies that point toward plasmon-enabled applications.
The first set of theoretical works in this issue focuses on the relationship between signal enhancement and plasmonic materials. Cristiano and co-workers address the possibility of using plasmon-enhanced electromagnetic fields to increase photocurrents in hybrid structures composed of Mo dichalcogenide layers and Au nanocrystals.1 The set of Au nanocrystals used in this study includes spherical dimers, nanorods, and nanostars. The authors demonstrate that these nanocrystals can amplify the photocurrents in the Mo dichalcogenide substrates. This study provides us with useful rules for designing nanomaterials with plasmonically enhanced photocurrent responses. The article by Chen and Jensen focuses on the chemical mechanism in surface-enhanced Raman scattering (SERS), which provides additional signal enhancement beyond the well-established electromagnetic mechanism for plasmon-enabled sensing.2 A new theoretical framework developed in this article allows us to interpret the chemical mechanism in terms of so-called Raman bonds. The density functional theory (DFT) based model demonstrates that the Raman bonds and the plasmonic mode spectrum directly influence the chemical enhancement effect in SERS. Alternatively, Dey et al. focus on another class of plasmonic sensors based on the refractive index sensitivity of the plasmon resonance.3 Their work uses finite difference time domain (FDTD) numerical simulations to study the effect of symmetry-breaking in hollow Au nanoprisms, showing plasmon hybridization conditions and potential for improving the refractive index sensitivity in these materials.
The next set of contributions concerns developments in the theory of light–matter interactions, which remains an active field of research during the last decade. The study by Sánchez-Barquilla et al. focuses on the challenging theoretical problem of strong coupling between quantum emitters and confined electromagnetic modes.4 The authors develop and apply a cumulant expansion method to treat the Heisenberg equations of motion. In this way, they can adequately describe the interaction of a quantum emitter and complex photonic modes at modest computational cost. The contribution by Itoh and co-workers describes the anti-crossing effect in the coupled exciton–plasmon resonance in a hybrid system composed of a plasmonic dimer and dye molecules.5 For these studies, they combined classical electrodynamics with a coupled oscillator model. The paper by Prajapati et al. presents an experiment and related modeling for interesting hybrid nanostructures based on ZnO nanorods decorated with plasmonic nanoparticles and semiconductor quantum dots.6 The focus of this contribution is on the inter-particle interactions, which lead to the enhancement of emission from the ZnO component. In their contribution, Cortes et al. present an elegant theoretical formalism: the non-Hermitian approach for the exciton–plasmon systems in the presence of dissipation and dephasing.7 The exciton–plasmon model represents one of the central problems in quantum plasmonics, and this paper should be useful for many theoretical and experimental groups in this field. Finally, Hata et al. provide a theoretical framework for a new strategy to generate up-converted light from a two-level system coupled with a plasmonic nanocavity by rapidly truncating an applied laser.8
The excitation of energetic (hot) electrons in plasmonic nanostructures, which is an active ongoing direction of research, is the focus of the next set of articles. The paper by Román Castellanos et al. concerns the well-known challenge of calculating the rates of generation of interband carriers in noble nanocrystals.9 The authors found that in small Ag nanocrystals, both d-to-sp and sp-to-sp transitions contribute to hot-carrier production. Fedorov et al. develop a theory of plasmonic resonances in metal nanoparticles linked by conductive molecular bridges.10 They demonstrate that the frequency of the charge-transfer plasmons in such systems strongly depends on the molecular bridge parameters. Finally, the study by Lio et al. models heat generation in plasmonic nanoheaters supported by flexible substrates.11 This interesting system offers the possibility of active control of plasmonic and photothermal responses using mechanical stress. In sum, the theoretical articles within this special issue showcase the diversity of phenomena enabled by plasmon excitation, a theme that continues in the experimental work described next.
The visualization of the near-field of metallic nanostructures is essential to further understand the underlying physics behind electromagnetic enhancement and to optimize the design of the plasmonic nanostructures for practical applications. Suzuki et al. visualize the plasmon mode patterns in a single gold nanorod using three-dimensional scanning near-field optical microscopy, showing that high order plasmon modes are resonantly excited in the nanorod, a result that is further supported by discrete dipole approximation (DDA) simulations.12 Vu Thi et al. directly investigate the local electric-field distribution of short range ordered nanoholes on gold membrane using a cathodoluminescence technique and finite element method (FEM) simulations.13 Dai et al. explore the spatial near-field distribution of plasmonic modes in silver micro-pyramids by multiphoton photoemission electron microscopy (mP-PEEM), illustrating that the plasmonic near-field distributions vary with the photon energy, light polarization, and phase in coherent two pulse excitation.14
The local field enhancement of plasmonic nanoparticles is particularly useful in enhancing light generation from species near plasmonic nanostructures. Gürdal and co-workers study the second harmonic generation in a hybrid system consisting of a nonlinear crystal covered with dense plasmonic nanostructures.15 The near-field generated by the plasmonic nanostructures was utilized to enhance the near-surface SHG signal from the nonlinear crystal. Tawa’s et al. track neuronal activity using plasmon-enhanced voltage-sensitive dye (VSD) imaging.16 The authors show the benefits of employing a plasmonic gold dish to enhance the dye’s emission, detecting the spontaneous network activity at the single cell level. Heiderscheit et al. use single nanoparticle techniques to study the role of overlapping the plasmon resonance and a dye’s emission wavelength in the enhancement of electrogenerated chemiluminescence (ECL).17 The authors reveal conditions to optimize such coupling, reaching an average of 10-fold enhancement in the ECL signal. Beyond enhancing photon generation from nearby molecules, plasmonic nanostructures also scatter light at their plasmon resonance. Ohnishi et al. investigate the spectral sensitivity of short range ordered gold nanohole arrays to pressure and temperature.18 They demonstrate a remarkable sensitive spectral response of gold nanohole arrays toward its surrounding pressure and temperature.
The initial stages of plasmon decay have received increased attention over the past few years. Brown and Hartland examine the chemical interface damping (CID) phenomena for propagating surface plasmons polaritons of thiol-coated Au nanostripes.19 By combining microscopy and imaging techniques, the authors show the influence of the stripe geometry and molecular structures in the CID and they compared this phenomenon between localized and propagating plasmon modes. On a similar note, enhanced Landau damping for the generation of hot-carriers from plasmon decay is investigated in this issue on two very different systems: graphene on a gold film20 and multilayers of gold nanoparticles.21 In the first case, Prakash et al. use ultrafast spectroscopy to monitor the efficient hot-carrier generation in graphene when coupling with the plasmon modes of an underlying nanohole array patterned in a gold thin film.20 In the second case, Hoeing et al. exploit the benefit of reduced radiative damping of dark-plasmon modes for the efficient generation of hot-carriers.21 Finally, Hogan and Sheldon compare the photothermalization dynamics in copper and gold nanostructures, finding that more abundant and energetic carriers are generated in copper structures.22 This could benefit future plasmonic catalysis research and applications.
Plasmon-induced charge transfer processes at the interface between metallic nanoparticles and semiconductors have been widely applied in photochemistry, photodetection, and photovoltaic applications. Sandeep et al. probe the role of protecting ligands and their hole-accepting abilities in the Fermi level equilibration and overall reactivity of plasmonic-semiconductor systems.23 Oshikiri and co-workers study the diffusion potential at the interface of plasmonic nanoparticles and a semiconductor by manipulating the interface dipole using photoelectric measurements.24 The diffusion potential formed between the plasmonic nanoparticles and surrounding media can be controlled by only one-unit cell interface dipole layer. Deshpande et al. investigate photoresponsivity of plasmonic graphene hybrid nanomaterials as photodetectors in the ultraviolet region.25 Effective charge carrier separation was observed in the Ag–graphene hybrid photodevice based on local doping. Moving to dynamic studies, ultrafast transient spectroscopy offers a way to study the fast timescale dynamics of the plasmon-induced charge carriers, as shown in the work of Okazaki et al. on metal/semiconductor heterojunctions.26 In this work, the carrier dynamics of plasmonic hematite photoanodes are studied by femtosecond transient absorption spectroscopy under near-infrared excitation, demonstrating efficient charge carrier generation in hematite decorated with gold nanostructures. In addition to charge transfer between metals and semiconductors, plasmon excitation can also generate charge transfer to nearby molecules, promoting photolytic reactions, as shown by Lin et al.27 In their work, they used low temperature scanning tunneling microscopy (STM) to study O2 dissociation on irradiated silver and found signatures of direct charge transfer that promoted photolysis.
In addition to plasmon-generated hot carriers, which are exploited in the experiments described above, plasmon excitation and decay also leads to heat generation. Measuring heat at plasmonic interfaces has recently re-emerged as a hot topic (pun intended) due to its potential for novel applications in photocatalysis. Shrestha et al. present a thermal imaging technique based on temperature-dependent Er3+ emission. By using steady state and time resolved measurements, the authors are able to map the heat generation and dissipation of Au nanostructures in both water and air.28 Marques et al. investigate the role that temperature plays in the decomposition kinetics of 5-Bromouracil and show a trade-off between laser fluence and nanoparticle stability that limits the ability to control the decomposition kinetics.29 These studies highlight both the promise and limitations of using plasmonic materials for controlling reactions.
Finally, new synthetic methods expand the range of materials available for plasmonic applications. Phan-Quang et al. develop a direct overgrowth of MoS on Au nanoporous colloids under ambient conditions, a cost-efficient synthesis pathway that leads to materials with enhanced electrocatalytic performance for hydrogen evolution.30 Asselin et al. show the possibility to decorate Mg nanoplates and nanorods with other plasmonic and catalytic materials such as Au, Ag, Pd, and Fe by using galvanic replacement.31 Cho et al. describe the colloidal synthesis of fluorine and tin co-doped indium oxide (F,Sn:In2O3) highly monodisperse 10 nm nanocubes (NCs) with tunable IR range plasmon resonances.32 Moreover, the authors performed a detailed and elegant study of the NCs near field coupling when moving to the monolayer regime. By expanding our materials toolbox, we expect new fundamental insights and new applications to emerge as we continue to push the frontier of plasmonics forward.