Au@Ag Core–Shell Nanorods Support Plasmonic Fano Resonances

Abstract

In this work, we investigated experimentally and theoretically the plasmonic Fano resonances (FRs) exhibited by core-shell nanorods composed of a gold core and a silver shell (Au@Ag NRs). The colloidal synthesis of these Au@Ag NRs produces nanostructures with rich plasmonic features, of which two different FRs are particularly interesting. The FR with spectral location at higher energies (3.7 eV) originates from the interaction between a plasmonic mode of the nanoparticle and the interband transitions of Au. In contrast, the tunable FR at lower energies (2.92-2.75 eV) is ascribed to the interaction between the dominant transversal LSPR mode of the Ag shell and the transversal plasmon mode of the Au@Ag nanostructure. The unique symmetrical morphology and FRs of these Au@Ag NRs make them promising candidates for plasmonic sensors and metamaterials components. Localized surface plasmon resonances (LSPRs) of metal nanoparticles, defined as collective oscillations of free electrons, have been extensively studied in recent years due to their versatility for a variety of applications, such as sensing, energy harvesting, and catalysis 1-5. In particular, all-plasmonic Fano resonances (FRs) have attracted great interest during the past decade 6-8. FRs are a type of resonant scattering phenomenon that gives rise to an asymmetric line-shape 9,10. They have considerable potential for applications like waveguiding 11,12 , subwavelength optical imaging 13,14 , low-loss metamaterials preparation 12,15 , chemical and biological sensing 12,16,17 , and energy harvesting 18,19 , to name a few. FRs are produced by the coupling of a discrete state with a continuum-e.g., between a narrow and a wide plasmon mode-and several plasmonic nanostructures have been proposed to display them 20. Structural symmetry-breaking is the most common approach because it induces a non-uniform electromagnetic environment around the nanostructure, leading to the effective coupling between broad and narrow multipolar plasmon resonances. Examples of this approach are non-concentric multilayered nanoshells 21-23 , heterodimer nanostructures 6,24,25 , ring-disk nanocavities 26-28 , full nanocavities 29 , nanoparticle clusters 7,30-32 , and nanocrystals supported on substrates 16,33. The main disadvantage of this strategy is that the complex and/or asymmetric nanostructures are fabricated using intricate and expensive techniques 27 , and/or only work under specific conditions 34 , which largely reduce their applicability. In contrast, the generation of plasmonic FRs in highly symmetric metal nanoparticles is much more challenging; indeed, only a few examples of them exist, such as bimetallic nanoparticles 25,35,36 , metallic nanoshells 8,21 , and in some metal@dielectric 37-39 an all-dielectric 40 core-shell nanostructures. However, these systems are easier to fabricate and are thus more attractive from the application point of view. In this work, we report the observation of two different FRs on core-shell nanorods (NRs) composed of a Au core and a Ag shell (...

Controlled Alloying of Au@Ag Core–Shell Nanorods Induced by Femtosecond Laser Irradiation

“…Likewise, the transversal mode variation is also reflected in the simulated plasmon bands, in particular the loss of the Fano-resonance modes. [41] Based on the simulated optical properties, the blueshift of the longitudinal band observed for the hot-dog-like NRs compared to as-synthesized ones (from 800 to 760 nm) can be explained as a result of the preferential loss of Ag from the tips and modification of the cuboidal shell morphology. Indeed, these combined effects might compensate for the increase in aspect ratio, from 3.5 to 3.7.…”

“…The synthesis of Au@Ag NRs with an LSPR band at 800 nm started from the growth of single-crystal gold nanorods (Au NRs) with a longitudinal LSPR band at 1000 nm (see the Experimental Section and Figure S1, Supporting Information), followed by overgrowth of an Ag shell, which resulted in a blueshift of the longitudinal LSPR to 800 nm. [41][42][43] Indeed, by tuning the dimensions of the Ag shell, we can control the position of the longitudinal plasmon band to match the fs-laser wavelength (Figure 1A,B). For Au NRs of 102 ± 8 nm in length and 18 ± 2 nm in width, the growth of an Ag shell of 6 ± 2 nm in thickness at the sides was appropriate to shift the LSPR band from 1000 down to 800 nm (Figure 1A,B and Figure S1, Supporting Information).…”

“…Synthesis of Au@Ag NRs-Au@Ag NRs at 800 nm: The nanoparticles were prepared following a previously reported seeded growth method. [41] Briefly, a given volume of the Au NR solution (≈60 µL) was added to 10 mL of a 25 × 10 −3 m CTAC solution (final absorption of the Au NRs in the growth mixture at 400 nm of 0.4 optical path: 1 cm). To precisely adjust the LSPR band at 800 nm, a calibration curve was previously obtained by varying the concentration of Au NRs in solution (absorption at 400 nm between 0.2 and 0.6).…”

Sharp Spectral Variations of the Ultrafast Transient Light Extinction by Bimetallic Nanoparticles in the Near‐UV

“…The deposition of Ag leads to the appearance of additional peaks in the low‐wavelength part of the spectrum. In the spectral range under investigation, results reported in the literature for similar bimetallic NP samples [ 2,4,12,34 ] exhibit four dominant resonance peaks. This is also the case here for the samples with the thinnest (0.8eq) and thickest (8eq) Ag shell.…”

“…Fortunately, the outstanding optical properties of silver NPs can be combined with the convenience of gold NP synthesis resulting in AuNRcore and Ag-shell hybrid bimetallic nanostructures (AuNR@ Ag). [3,4,[12][13][14][15][16] The Ag deposits preferentially along the transverse dimension of the AuNR, leading to cuboid-shaped AuNR@Ag NPs. Their optical properties can be tuned from pure AuNRs' [6] to pure Ag cuboids' [17] by tailoring their equivalent Ag:Au molar ratio.…”