Plasmons: untangling the classical, experimental, and quantum mechanical definitions

New Nanophotonics Approaches for Enhancing the Efficiency and Stability of Perovskite Solar Cells

Over the past decade, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has experienced a remarkable ascent, soaring from 3.8% in 2009 to a remarkable record of 26.1% in 2023. Many recent approaches for improving PSC performance employ nanophotonic technologies, from light harvesting and thermal management to the manipulation of charge carrier dynamics. Plasmonic nanoparticles and arrayed dielectric nanostructures have been applied to tailor the light absorption, scattering, and conversion, as well as the heat dissipation within PSCs to improve their PCE and operational stability. In this review, it is begin with a concise introduction to define the realm of nanophotonics by focusing on the nanoscale interactions between light and surface plasmons or dielectric photonic structures. Prevailing strategies that utilize resonance-enhanced light-matter interactions for boosting the PCE and stability of PSCs from light trapping, carrier transportation, and thermal management perspectives are then elaborated, and the resultant practical applications, such as semitransparent photovoltaics, colored PSCs, and smart perovskite windows are discussed. Finally, the state-of-the-art nanophotonic paradigms in PSCs are reviewed, and the benefits of these approaches in improving the aesthetic effects and energy-saving character of PSC-integrated buildings are highlighted.

Three-atom-wide gold quantum rods with periodic elongation and strongly polarized excitons

Atomically precise control over anisotropic nanoclusters constitutes a grand challenge in nanoscience. In this work, we report our success in achieving a periodic series of atomically precise gold quantum rods (abbrev. Au QRs) with unusual excitonic properties. These QRs possess hexagonal close-packed kernels with a constant three-atom diameter but increasing aspect ratios (ARs) from 6.3 to 18.7, all being protected by the same thiolate (SR) ligand. The kernels of the QRs are in a Au1-(Au3)n-Au1 configuration (where n is the number of Au3 layers) and follow a periodic elongation with a uniform Au18(SR)12 increment consisting of four Au3 layers. These Au QRs possess distinct HOMO-LUMO gaps (Eg = 0.6 to 1.3 eV) and exhibit strongly polarized excitonic transition along the longitudinal direction, resulting in very intense absorption in the near-infrared (800 to 1,700 nm). While excitons in gapped systems and plasmons in gapless systems are distinctly different types of excitations, the strongly polarized excitons in Au QRs surprisingly exhibit plasmon-like behaviors manifested in the shape-induced polarization, very intense absorption (~106 M-1 cm-1), and linear scaling relations with the AR, all of which resemble the behaviors of conventional metallic-state Au nanorods (i.e., gapless systems), but the QRs possess distinct gaps and very long excited-state lifetimes (10 to 2,122 ns), which hold promise in applications such as near-infrared solar energy utilization, hot carrier generation and transfer. The observation of plasmon-like behaviors from single-electron transitions in Au QRs elegantly bridges the distinct realms of single-electron and collective-electron excitations and may stimulate more research on excitonics and plasmonics.

Effects of Field Strength and Silver Nanowire Size on Plasmon-Enhanced N2 Dissociation

Dissociation of the nitrogen molecule via plasmon-enhanced catalysis using noble metal nanoparticles has been investigated both experimentally and computationally in recent years. However, the mechanism of plasmon-enhanced nitrogen dissociation is still not very clear. In this work, we apply theoretical approaches to examine the dissociation of a nitrogen molecule on atomically thin Ag_n nanowires (n = 6, 8, 10, 12) and a Ag₁₉⁺ nanorod. Ehrenfest dynamics provides information about the motion of nuclei during the dynamics process and real-time TDDFT calculations show the electronic transitions and population of electrons over the first 10 s of fs time scale. The activation and dissociation of nitrogen are typically enhanced when the electric field strength increases. However, the enhancement is not always monotonic with field strength. As the length of the Ag wire increases, nitrogen is typically easier to dissociate and thus requires lower field strengths, even though the plasmon frequency is lower. The Ag₁₉⁺ nanorod leads to faster dissociation of N₂ than the atomically thin nanowires. Overall, our detailed study yields insights into the mechanisms involved in plasmon-enhanced N₂ dissociation, as well as provides information about factors that can be used to improve adsorbate activation.

First-principles study of electronic and optical properties in 1-dimensional oligomeric derivatives of telomestatin

Real-space self-interaction corrected (time-dependent) density functional theory has been used to investigate the ground-state electronic structure and optical absorption profiles of a series of linear oligomers inspired by the natural product telomestatin. Length-dependent development of plasmonic excitations in the UV region is seen in the neutral species which is augmented by polaron-type absorption with tunable wavelengths in the IR when the chains are doped with an additional electron/hole. Combined with a lack of absorption in the visible region this suggests these oligomers as good candidates for applications such as transparent antennae in dye-sensitised solar energy collection materials. Due to strong longitudinal polarisation in their absorption spectra, these compounds are also indicated for use in nano-structured devices displaying orientation-sensitive optical responses.

A comprehensive investigation of the plasmonic-photocatalytic properties of gold nanoparticles for CO2 conversion to chemicals

Understanding the interactions between plasmonic gold (Au) nanoparticles and the adsorbate is essential for photocatalytic and plasmonic applications. However, it is often challenging to identify a specific reaction mechanism in the ground state and to explore the optical properties in the excited states because of the complicated pathways of carriers. In this study, photocatalytic reduction of carbon dioxide (CO₂) to C1 products (for example, CO and CH₄) on the Au(111) nanoparticle (NP) surface was studied based on reaction pathway analysis, adsorbate reactivity, and its ability to stabilize or deactivate the surface. The calculated reaction Gibbs free energies and activation barriers revealed that the first step in CO reduction via a direct hydrogen transfer mechanism on Au(111) is the formation of formyl (*CHO) instead of hydroxymethylidyne (*COH). Furthermore, the size enhanced and symmetry sensitive optical responses of cuboctahedral Au(111) NPs on localized surface plasmon resonance (LSPR) were investigated by using time-dependent DFT (TDDFT) calculations. Although near field enhancement around cuboctahedral Au(111) NPs is only weakly dependent on the morphology of NPs, it was observed that corner sites stabilize *C-species to drive the CO₂ reduction to CO. The density of active surface states interacting with the adsorbate states near the Fermi level gradually decreases from the (111) on-top site toward the corner site of the Au(111) NP-CO system, which strongly affects the molecule's binding on catalytic sites and, in particular, electronic excitation. Finally, the spatial distribution of the charge oscillations was determined as a guide for the fabrication of Au NPs with an optimal LSPR response.

Theoretical Investigations on the Plasmon-Mediated Dissociation of Small Molecules in the Presence of Silver Atomic Wires

Plasmonic nanoparticles can promote bond activation in adsorbed molecules under relatively benign conditions via excitation of the nanoparticle's plasmon resonance. As the plasmon resonance often falls within the visible light region, plasmonic nanomaterials are a promising class of catalysts. However, the exact mechanisms through which plasmonic nanoparticles activate the bonds of nearby molecules are still unclear. Herein, we evaluate Ag₈-X₂ (X = N, H) model systems via real-time time-dependent density functional theory (RT-TDDFT), linear response time-dependent density functional theory (LR-TDDFT), and Ehrenfest dynamics in order to better understand the bond activation processes of N₂ and H₂ facilitated by the presence of the atomic silver wire under excitation at the plasmon resonance energies. We find that dissociation is possible for both small molecules at high electric field strength. Activation of each adsorbate is symmetry- and electric field-dependent, and H₂ activates at lower electric field strengths than N₂. This work serves as a step toward understanding the complex time-dependent electron and electron-nuclear dynamics between plasmonic nanowires and adsorbed small molecules.