Amplified Generation of Hot Electrons and Quantum Surface Effects in Nanoparticle Dimers with Plasmonic Hot Spots

Plasmonic excitations in optically driven nanocrystals are composed of excited single-particle electron–hole pairs in the Fermi sea. In large nanostructures, most of the excited plasmonic electrons have relatively small excitation energies due to the conservation of linear momentum. However, small optically driven nanocrystals may have large numbers of hot electrons with large energies. In this study, we develop the concept of hot electron generation further by considering the effect of a plasmonic hot spot. Plasmonic hot spots are areas in a nanostructure with highly inhomogeneous and enhanced electric fields. In our model of a nanoparticle dimer, the hot spot region appears near the gap between the nanoparticles. We then apply the quantum formalism based on the density matrix to describe this system. We show that the electromagnetic enhancement and the nonconservation of linear momentum in the hot spot of the nanoparticle dimer lead to strongly increased rates of generation of energetic (hot) electrons. The rates of hot electron generation grow faster than the absorption cross section and the electromagnetic enhancement factor with the decrease of the gap between the nanoparticles. This happens due to the breaking of the linear momentum conservation of electrons in the hot spot regions. We also show that hot electron generation effect leads to the quantum mechanism of surface-induced absorption in nanocrystals that is an intrinsic property of any confined plasmonic system. The results obtained in this study can be useful for understanding and designing plasmonic photodetectors and hybrid materials for efficient photocatalysis

Mie Sensing with Neural Networks: Recognition of Nano-Object Parameters, the Invisibility Point, and Restricted Models

In this work, we use artificial neural networks (ANNs) to recognize the material composition, sizes of nanoparticles and their concentrations in different media with high accuracy, solely from the absorbance spectrum of a macroscopic sample. We construct ANNs operating in the following two schemes. The first scheme is designed to recognize the dimensions and refractive indices of dielectric scatterers in mixed ensembles. The second ANN model simultaneously recognizes the dimensions of gold nanospheres in a mixture and the refractive index of a matrix. A challenge in the first scheme arises at and near the invisibility point, i.e., when the refractive index of nanoparticles is close to that of the medium. Of course, particle recognition in this regime faces fundamental physical limitations. However, such recognition near the invisibility point is possible, and our study reveals its unique properties. Interestingly, the recognition process for the refractive index in the vicinity of the invisibility point shows very small errors. In contrast, the errors for the recognition of the radius grow strongly near this point. Another regime with limited recognition occurs when the extinction spectra are not unique and can correspond to different realizations of nanoparticle mixtures. Regarding multi-particle or polydisperse solutions, the ML-based models should in such cases be rationally restricted to maintain the feasibility of the recognition process. Overall, the recognition schemes proposed and investigated by us can find their applications in the field of sensing

Localization of Excess Temperature Using Plasmonic Hot Spots in Metal Nanostructures: Combining Nano-Optical Antennas with the Fano Effect

It is challenging to strongly localize temperature in small volumes because heat transfer is a diffusive process. Here we show how to overcome this limitation using electrodynamic hot spots and interference effects in the regime of continuous-wave (CW) excitation. We introduce a set of figures of merit for the localization of excess temperature and for the efficiency of the plasmonic photothermal effect. Our calculations show that the local temperature distribution in a trimer nanoparticle assembly is a complex function of the geometry and sizes. Large nanoparticles in the trimer play the role of the nano-optical antenna, whereas the small nanoparticle in the plasmonic hot spot acts as a nanoheater. Under the specific conditions, the temperature increase inside a nanoparticle trimer can be localized in a hot spot region at the small heater nanoparticle and, in this way, a thermal hot spot can be realized. However, the overall power efficiency of local heating in this trimer is much smaller than that of a single nanoparticle. We can overcome the latter disadvantage by using a trimer with a nanorod. In the trimer assembly composed of a nanorod and two spherical nanoparticles, we observe a strong plasmonic Fano effect that leads to the concentration of optical energy in the small heater nanorod. Therefore, the power efficiency of generation of local excess temperature in the nanorod-based assembly greatly increases due to the strong plasmonic Fano effect. The Fano heater incorporating a small nanorod in the hot spot has obviously the best performance compared to both single nanocrystals and a nanoparticle trimer. The principles of heat localization described here can be potentially used for thermal photocatalysis, energy conversion and biorelated applications

Broadband Absorbing Exciton–Plasmon Metafluids with Narrow Transparency Windows

Optical metafluids that consist of colloidal solutions of plasmonic and/or excitonic nanomaterials may play important roles as functional working fluids or as means for producing solid metamaterial coatings. The concept of a metafluid employed here is based on the picture that a single ballistic photon, propagating through the metafluid, interacts with a large collection of specifically designed optically active nanocrystals. We demonstrate water-based metafluids that act as broadband electromagnetic absorbers in a spectral range of 200–3300 nm and feature a tunable narrow (∼100 nm) transparency window in the visible-to-near-infrared region. To define this transparency window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped silicon nanocrystals as opaque optical absorbers (“optical wall”) in the UV and blue-green range of the spectrum. Water itself acts as an opaque “wall” in the near-infrared to infrared. We explore the limits of the concept of a “simple” metafluid by computationally testing and validating the effective medium approach based on the Beer–Lambert law. According to our simulations and experiments, particle aggregation and the associated decay of the window effect are one example of the failure of the simple metafluid concept due to strong interparticle interactions

Differentiating Plasmon-Enhanced Chemical Reactions on AgPd Hollow Nanoplates through Surface-Enhanced Raman Spectroscopy

Plasmonic photocatalysis demonstrates great potential for efficiently harnessing light energy. However, the underlying mechanisms remain enigmatic due to the transient nature of the reaction processes. Typically, plasmonic photocatalysis relies on the excitation of surface plasmon resonance (SPR) in plasmonic materials, such as metal nanoparticles, leading to the generation of high-energy or “hot electrons”, albeit accompanied by photothermal heating or Joule effect. The ability of hot electrons to participate in chemical reactions is one of the key mechanisms, underlying the enhanced photocatalytic activity observed in plasmonic photocatalysis. Interestingly, surface-enhanced Raman scattering (SERS) spectroscopy allows the analysis of chemical reactions driven by hot electrons, as both SERS and hot electrons stem from the decay of SPR and occur at the hot spots. Herein, we propose a highly efficient SERS substrate based on cellulose paper loaded with either Ag nanoplates (Ag NPs) or AgPd hollow nanoplates (AgPd HNPs) for the in situ monitoring of C–C homocoupling reactions. The data analysis allowed us to disentangle the impact of hot electrons and the Joule effect on plasmon-enhanced photocatalysis. Computational simulations revealed an increase in the rate of excitation of hot carriers from single/isolated AgPd HNPs to an in-plane with a vertical stacking assembly, suggesting its promise as a photocatalyst under broadband light. In addition, the results suggest that the incorporation of Pd into an alloy with plasmonic properties may enhance its catalytic performance under light irradiation due to the collection of plasmon-excitation-induced hot electrons. This work has demonstrated the performance-oriented synthesis of hybrid nanostructures, providing a unique route to uncover the mechanism of plasmon-enhanced photocatalysis