Thermionic converters generate electricity from thermal energy in a power cycle based on the vacuum emission of electrons. While thermodynamically efficient, practical implementations are limited by the extreme temperatures required for electron emission (>1500 K).Here we show how metal nanostructures that support resonant plasmonic absorption enable an alternative strategy. The high temperatures required for efficient vacuum emission can be maintained in a subpopulation of photoexcited "hot" electrons during steady-state optical illumination, whereas the lattice temperature of the metal remains within the range of thermal stability, below 600 K. We have also developed an optical thermometry technique based on anti-Stokes Raman spectroscopy that confirms these unique electron dynamics. Thermionic devices constructed from optimized plasmonic absorbers show performance that can outcompete other strategies of concentrated solar power conversion in terms of efficiency and thermal stability.

Nanoscale metamaterials exhibit extraordinary optical properties and are proposed for various technological applications. Here, a new class of novel nanoscale two‐phase hybrid metamaterials is achieved by combining two major classes of traditional plasmonic materials, metals (e.g., Au) and transition metal nitrides (e.g., TaN, TiN, and ZrN) in an epitaxial thin film form via the vertically aligned nanocomposite platform. By properly controlling the nucleation of the two phases, the nanoscale artificial plasmonic lattices (APLs) consisting of highly ordered hexagonal close packed Au nanopillars in a TaN matrix are demonstrated. More specifically, uniform Au nanopillars with an average diameter of 3 nm are embedded in epitaxial TaN platform and thus form highly 3D ordered APL nanoscale metamaterials. Novel optical properties include highly anisotropic reflectance, obvious nonlinear optical properties indicating inversion symmetry breaking of the hybrid material, large permittivity tuning and negative permittivity response over a broad wavelength regime, and superior mechanical strength and ductility. The study demonstrates the novelty of the new hybrid plasmonic scheme with great potentials in versatile material selection, and, tunable APL spacing and pillar dimension, all important steps toward future designable hybrid plasmonic materials.

Hierarchically porous metal–organic frameworks (HP‐MOFs) facilitate mass transfer due to mesoporosity while preserving the advantage of microporosity. This unique feature endows HP‐MOFs with remarkable application potential in multiple fields. Recently, new methods such as linker labilization for the construction of HP‐MOFs have emerged. To further enrich the synthetic toolkit of MOFs, we report a controlled photolytic removal of linkers to create mesopores within microporous MOFs at tens of milliseconds. Ultraviolet (UV) laser has been applied to eliminate “photolabile” linkers without affecting the overall crystallinity and integrity of the original framework. Presumably, the creation of mesopores can be attributed to the missing‐cluster defects, which can be tuned through varying the time of laser exposure and ratio of photolabile/robust linkers. Upon laser exposure, MOF crystals shrank while metal oxide nanoparticles formed giving rise to the HP‐MOFs. In addition, photolysis can also be utilized for the fabrication of complicated patterns with high precision, paving the way towards MOF lithography, which has enormous potential in sensing and catalysis.

Significant recent interest in plasmonic nanomaterials is based on the ability to use the strong resonant absorption to produce large transient populations of photoexcited non-equilibrium “hot” carriers that can then be employed in novel classes of photochemical reactions and more general optoelectronic detection schemes and power cycles. In this Feature Article, we outline nanoscale design features that allow for systematic control over photothermalization in plasmonic materials, connecting the microscopic mechanism of absorption, photoexcitation, relaxation, and thermal emission with the electronic temperature and lattice temperature of a metal during steady state illumination. Further, we show how anti-Stokes Raman spectroscopy can provide a quantitative measure of the energy distribution of the hot electrons and the surrounding lattice temperature, as well as indicate the electron–phonon coupling constant of hot electrons, all under optical conditions relevant to emerging hot electron devices, i.e., relatively low fluence, continuous wave (CW) excitation. A major insight from our experiments is the presence of a sustained subpopulation of hot electrons at an elevated temperature in comparison with the majority of the conduction electrons in the metal. In conjunction, we show what features of nanoscopic geometries give rise to the largest population and longest-lived hot electrons, as required for the goals of optimizing electron dynamics in developing applications of plasmonic hot electrons.

Optical and photochemical power converters based on resonant absorption in metal nanostructures generally employ a mechanism whereby optically excited "hot" carriers are injected over a Schottky barrier at a semiconductor or molecular interface. This process is inefficient because most of the excited carriers relax and thermalize with the lattice before they can be collected. Here we outline an alternative strategy that can take better advantage of both optically excited and thermalized electrical carriers by leveraging the tunneling transport phenomenon across metal junctions that concentrate and absorb light preferentially on one side of a nanoscale gap. We have developed a general description for electron transport within a parabolic conduction band approximation accounting for both thermal (Fermi−Dirac) and nonthermal contributions to the steady-state electronic energy distribution that results from optical excitation. A nonzero current density is predicted when the excited-state distribution of carriers is dissimilar on opposite sides of a tunnel junction, with electrons emitted from the electrode that absorbs more light. An increase of the short-circuit photocurrent and the associated open-circuit voltage at elevated temperatures indicates a cooperative interaction between thermal and nonthermal excitation mechanisms. We also use full wave optical simulations (FDTD method) to demonstrate a simple device design for obtaining optical power conversion efficiency that is competitive with conventional photovoltaic devices.