Hot‐Electron Effects in Plasmonics and Plasmonic Materials

The coherent motion of free charge carriers, called surface plasmons, near the surface of good conductors is extensively used for efficient trapping, confinement, and exploitation of light on the nanoscale, which is impossible to achieve by other means.1 While until recently, the main focus of plasmonics was on the manipulation of electromagnetic fields, the free-carriers forming surface plasmons have amazing properties themselves. When a surface plasmon decays, its energy gets transferred to an electron–hole pair, and for a short period, these carriers stay “hot” — they are in a non-equilibrium energy distribution for lifetimes well below a picosecond. This can be exploited if these carriers are extracted from the plasmonic nanostructures before thermalization to the lattice occurs. Proof-of-concept demonstrations have been developed over the last three years to reveal fascinating applications in areas such as surface-enhanced catalysis (e.g., water splitting), photodetectors without bandgaps, and nanoscale control over chemical reactions. Additionally, hot electrons can generate heat locally, which can also be harvested to induce useful chemical, biological, and physical processes, including hyperthermia for cancer treatment and modification of magnetic properties for data storage applications. Understanding and achieving control over these hot-carrier processes via geometric design of plasmonic nanostructures, the choice of the plasmonic material and the design of the interfaces between plasmonic and semiconductor and molecular materials, will open new avenues for the development of broadband photocatalysis, enhanced nanoscale optoelectronic devices, nanoscale sources of electric and magnetic fields for data storage applications, nanoscale and ultrafast temperature control, and many other technologies of our century. This special issue covers some of the most pertinent and timely topics of plasmonic hot carrier effects and applications. Concepts, approaches, challenges and perspectives to plasmonic hot-carrier chemistry are discussed in detail, exemplifying the role of surface states and charge transfer at the nanoscale, and heterogeneous catalysis based on hot-electron effects (article numbers 1700004 and 1700191). The role and advantages of new plasmonic materials, such as transition-metal nitrides, are discussed, which enable plasmonic devices with enhanced performance for solar energy conversion (article number 1601031). Control over hot-carrier generation and their use as probes for the local environment are discussed in the context of nanoparticle shapes (article number 1600907) and enabled by new experimental techniques capable of studying hot-electron chemistry of few molecules and interfaces (article numbers 1700195 and 1600594). The generation of hot electrons is a quantum effect and strongly size and material dependent, opening opportunities for optimization of its sensitivity. The technique of hot-electron microscopy is promising for studies of nanoscale junctions and charge transfer in multilayered materials. Understanding of hot-carrier dynamics in various nano­structures is important for both optimization of related properties, such as optical nonlinearities directly related to the carrier relaxation, as well as hot electrons kinetic properties influencing their transfer to neighboring molecules and materials (article numbers 1600914, 1700299, and 1700141). For example, the lifetime and dynamics of hot carriers in layered two-dimensional materials and heterostructures can be controlled by influencing different scattering mechanisms and interlayer interactions. This can pave the way for plasmonic heterostructures with improved hot-carrier transport, which have now been identified (article number 1600914). The distinct transient behaviors of the chiral response of the molecules resulting from energy relaxation processes of hot electrons can be engineered and controlled on ultrafast (picosecond) scales via the excitation of hot electrons (article number 1700299). Geometry dependent hot-carrier relaxation in plasmonic metamaterials allows influencing both the magnitude of the third-order nonlinear response as well as its dynamics (article number 1700141). In turn, the observation of the Kerr-type nonlinearity may serve as a probe for hot-carrier behavior. The articles in this special issue by no means cover all fields of plasmonic hot carriers and their applications, but serve as a cross-section of different activities at the beginning of the road to their bright future. Much more is to come, as hot-carrier plasmonics is well on the way to develop into one of the most exciting crossroads between physics, chemistry, and materials science. London, May 2017 Anatoly V. Zayats is the head of the Experimental Biophysics and Nanotechnology Group at the Department of Physics, King's College London. His current research interests are in the areas of nanophotonics, plasmonics and metamaterials, nonlinear and ultrafast optics and spectroscopy, and optical properties of surfaces, thin films, semiconductors and low-dimensional structures. Stefan Maier is the Lee-Lucas Chair of Experimental Physics and the Head of the Experimental Solid State Group at Imperial College London. His current research focuses on plasmonics, hot-electron chemistry, and dielectric nanophotonics.