Surface scattering contribution to the plasmon width in embedded Ag nanospheres

Sensitive, Stable, and Recyclable ZnO/Ag Nanohybrid Substrates for Surface-Enhanced Raman Scattering Metrology

Surface-enhanced Raman scattering is a practical, noninvasive spectroscopic technique that measures chemical fingerprints for varieties of molecules in multiple applications. However, synthesizing appropriate substrates for practical, long-term applications of this method has always been a challenging task. In the present study, we show that ZnO/Ag nanohybrid substrates may act as highly stable, sensitive, and recyclable substrates for surface-enhanced Raman scattering, as illustrated by the detection of methylene blue, selected as a test dye molecule with self-cleaning functionalities. Specifically, we demonstrate the detection enhancement factor of 3.7 × 107 along with exceptional long-term stability explained in terms of the localized surface plasmon resonance from the Ag nanocrystals embedded into the chemically inert ZnO nanoparticles, constituting the nanohybrid. Significantly, these substrates can be efficiently cleaned and regenerated while maintaining their high performance upon recycling. As a result, using these substrates, up to 10-12 M detection sensitivity has been demonstrated, enabling the accuracy required in modern environmental monitoring, bioassays, and analytical chemistry. Thus, ZnO nanoparticles with embedded Ag nanocrystals constitute a novel class of advanced nanohybrid substrates for use in multiple applications of surface-enhanced Raman scattering metrology.

Coaction effect of radiative and non-radiative damping on the lifetime of localized surface plasmon modes in individual gold nanorods

Revealing the coaction effect of radiative and non-radiative damping on the lifetime of the localized surface plasmon resonance (LSPR) mode is a prerequisite for the applications of LSPR. Here, we systematically investigated the coaction effect of radiative and non-radiative damping on the lifetime of the super-radiant and sub-radiant LSPR modes of gold nanorods using time-resolved photoemission electron microscopy (TR-PEEM). The results show that the lifetime of the LSPR mode depends on the length of the gold nanorod, and the different variation behavior of an LSPR mode lifetime exists between the super-radiative mode and the sub-radiative one with the increase of nanorod length (volume). Surprisingly, it is found that the lifetime of the super-radiant LSPR mode can be comparable to or even longer than that of the sub-radiant LSPR mode, instead of the usual claim that a sub-radiant LSPR mode has a longer life than the super-radiant mode. Those TR-PEEM experimental results are supported by finite-difference time-domain simulations and are well explained by the coaction effect with the calculation of the radiative and non-radiative damping rate with the increase of the nanorod volume. We believe that this study is beneficial to build a low-threshold nano-laser and ultrasensitive molecular spectroscopy system.

Tunable Multipolar Surface Plasmons in 2D Ti3C2 T x MXene Flakes

2D Ti₃C₂ T _x MXenes were recently shown to exhibit intense surface plasmon (SP) excitations; however, their spatial variation over individual Ti₃C₂ T _x flakes remains undiscovered. Here, we use scanning transmission electron microscopy (STEM) combined with ultra-high-resolution electron energy loss spectroscopy (EELS) to investigate the spatial and energy distribution of SPs (both optically active and forbidden modes) in mono- and multilayered Ti₃C₂ T _x flakes. With STEM-EELS mapping, the inherent interband transition in addition to a variety of transversal and longitudinal SP modes (ranging from visible down to 0.1 eV in MIR) are directly visualized and correlated with the shape, size, and thickness of Ti₃C₂ T _x flakes. The independent polarizability of Ti₃C₂ T _x monolayers is unambiguously demonstrated and attributed to their unusual weak interlayer coupling. This characteristic allows for engineering a class of nanoscale systems, where each monolayer in the multilayered structure of Ti₃C₂ T _x has its own set of SPs with distinctive multipolar characters. Moreover, the tunability of the SP energies is highlighted by conducting in situ heating STEM to monitor the change of the surface functionalization of Ti₃C₂ T _x through annealing at temperatures up to 900 °C. At temperatures above 500 °C, the observed fluorine (F) desorption multiplies the metal-like free electron density of Ti₃C₂ T _x flakes, resulting in a monotonic blue-shift in the SP energy of all modes. These results underline the great potential for the development of Ti₃C₂ T _x-based applications, spanning the visible-MIR spectrum, relying on the excitation and detection of single SPs.

The nonmonotonous shift of quantum plasmon resonance and plasmon-enhanced photocatalytic activity of gold nanoparticles

The surface plasmon resonance (SPR) of metal nanoparticles exhibits quantum behaviors as the size decreases owing to the transitions of quantized conduction electrons, but most studies are limited to the monotonous SPR blue-shift caused by off-resonant transitions. Here, we demonstrate the nonmonotonous SPR red-shift caused by resonant electron transitions and photocatalytic activity enhanced by the quantum plasmon resonance of colloidal gold nanoparticles. A maximal SPR wavelength and the largest photocatalytic activity are observed in the quantum regime for the first time for the gold nanoparticles with a diameter of 3.6 nm. Theoretical analysis based on a quantum-corrected model reveals the evolution of SPR with quantized electron transitions and well explains the nonmonotonous size-dependencies of the SPR wavelength and absorption efficiency. These findings have profound implications for the understanding of the quantum nature of the SPR of metal nanoparticles and their applications in areas ranging from photophysics to photochemistry.

Diffuse Surface Scattering in the Plasmonic Resonances of Ultralow Electron Density Nanospheres

Localized surface plasmon resonances (LSPRs) have recently been identified in extremely diluted electron systems obtained by doping semiconductor quantum dots. Here, we investigate the role that different surface effects, namely, electronic spill-out and diffuse surface scattering, play in the optical properties of these ultralow electron density nanosystems. Diffuse scattering originates from imperfections or roughness at a microscopic scale on the surface. Using an electromagnetic theory that describes this mechanism in conjunction with a dielectric function including the quantum size effect, we find that the LSPRs show an oscillatory behavior in both position and width for large particles and a strong blue shift in energy and an increased width for smaller radii, consistent with recent experimental results for photodoped ZnO nanocrystals. We thus show that the commonly ignored process of diffuse surface scattering is a more important mechanism affecting the plasmonic properties of ultralow electron density nanoparticles than the spill-out effect.

Optical enhancement of plasmonic activity of catalytic metal nanoparticles

Plasmon-assisted direct photocatalysis through enhanced light absorption in catalytic metal nanoparticles. Enhancement is achieved by coupling the plasmon resonance of a silver nanoantenna to that of a catalytic metal nanoparticle.