Au@Ag Core–Shell Nanorods Support Plasmonic Fano Resonances

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.

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][22][23] , heterodimer nanostructures 6,24,25 , ring-disk nanocavities [26][27][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][38][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 (Au@Ag NRs). In this system, the spectrally localized LSPR modes of the Ag shell (the discrete levels) couple to the Au interband transitions (the continuum), showing remarkable tunability of the FRs. Although these Au@Ag NRs had been previously synthesized by a colloidal seed-mediated method 41,42 , their plasmonic modes had not been identified so far as FRs.

Results and Discussion
In order to investigate the role of Ag overgrowth over Au NRs on the emergence of FRs, Au@Ag NRs with different amounts of Ag were synthesized (see Methods section), as shown in Fig. 1. The cuboid morphology displayed by the Au@Ag NRs had already been previously observed due to stabilization of the {100} facets of Ag ffc nanocrystals 41 . The dimensions and composition of the different nanostructures are collected in Table 1.
The optical response of these nanostructures is depicted in Fig. 2. Au NRs exhibit the typical LSPRs, with one peak located at 1000 nm and the other one at 510 nm, corresponding to the longitudinal and transversal plasmon modes, respectively. On the other hand, Au@Ag NRs present richer plasmonic features. For instance, the presence of the longitudinal plasmon mode can still be observed, but it blue-shifts with the increasing Ag content (i.e., with the decreasing aspect ratio, see Table 1). More interestingly, two clear FRs arise, with approximate spectral locations at 3.7 eV (335 nm) and 2.92 (425 nm)-2.75 eV (450 nm); hereafter, we will refer to them as FR1 and FR2, respectively. The typical charge distribution is very different for both FRs, as can be seen in Fig. 2b,c.
To better understand the origin of the FRs, we have to discuss first the LSPR modes of the cubic morphology. A cube sustains an infinite number of plasmon modes 16 , but six of them account for 96% of the total oscillator strength 43 . Indeed, the FRs observed in the Au@Ag NRs can be ascribed to the two LSPR modes with the lowest energy, which represent almost 70% of the total oscillator strength 43 , as the other modes fall in the region of interband transitions of Ag. The charge distribution for these two main modes is represented schematically in Fig. 2d,e. In both modes, charges are concentrated on the corners of the cube 43,44 , with charges of the same sign located on opposite sides. However, mode II presents also charges of opposite sign located on the edges of the cube 16 . Both modes have a large electric dipole moment and couple strongly to light, with oscillator strengths of 0.44 and 0.24 for plasmon modes I and II, respectively 43 . However, the dominant mode (I) exhibits remarkable tunability compared to the other mode 16 . This is particularly true when the cube is elongated in one direction 45 ;   Table 1. Longitudinal LSPRs and dimensions of the Au@Ag NRs and the Au NRs used to seed their growth. The volume percentages given in the last two columns represent, respectively, the Au and Ag content in the different Au@Ag NRs.
the main plasmon splits into a longitudinal and a transversal mode (Fig. 2f,g), with the former red-shifting as a function of the aspect ratio while the displacement for the latter is practically null. Figure 3 shows the FDTD simulations performed to help us understand the origin of the FRs. No FR is observed when the light is polarized along the longitudinal axis of the nanoparticles (Fig. 3a,b) whereas both FRs appear with transversal polarization (Fig. 3b). FR1 only appears in the Au@Ag NRs (Fig. 3c,d), which suggests that it is caused by the interaction between a plasmonic mode of the nanoparticle and the interband transitions of Au. In fact, this FR is very similar to that observed previously for a spherical core-shell plasmonic nanostructure,  www.nature.com/scientificreports www.nature.com/scientificreports/ ascribed to coupling of the Ag shell anti-bonding mode, which corresponds to the negative parity of the dipoles; i.e., the antisymmetric field (see Fig. 2b) 25 .
On the other hand, FR2 is present for both the Au@Ag NRs and the Ag shell. Hence, it can be ascribed to the interaction between the dominant transversal LSPR mode of the Ag shell and the transversal plasmon mode of the whole nanostructure. This latter transversal mode can be clearly seen in Fig. 3c,d for a nanostructure with identical aspect ratio but with the dimensions reduced to 1/3 those of the original particle. The scattering is negligible for this small particle and, hence, the transversal plasmon mode is observed without the FR.
In order to obtain additional insights into the characteristics of FR2, we fitted it with the expressions developed by Gallinet and Martin for FRs on a continuum-like wide LSPR mode having a Lorenzian shape 20,46 : The two terms of Eq. (1) represent, respectively, the symmetric pseudo-Lorentzian line shape (subscript 's') and the Fano-like asymmetric line shape (subscript 'a'). In this equation, a is the maximum amplitude of the Lorentzian resonance, E s is the resonance energy position, and Γ s is its approximate spectral width. Likewise, for the asymmetric FR, E a is the position of the resonance center, Γ a gives an approximation of its spectral width, q is the asymmetry parameter, and b is the modulation damping parameter originating from intrinsic losses. To better fit our spectra, we added a sigmoidal term [B 1 + A 1 /(1 + exp(−A 2 *(E − E 0 )))] to account for the interband absorption of Au 8 . Here B 1 , A 1 , A 2 , and E 0 are the offset, amplitude, slope, and position of the sigmoid, respectively. As can be seen in Fig. 3d, the fit of the Fano profile is excellent. The isolated profile of the FR (i.e., the asymmetric term) is shown in the inset of Fig. 3d. This clearly confirms that the features of the spectrum of Au@ Ag NRs are dominated by two FRs, one of them produced by the interaction of a plasmonic mode of the Ag shell with the interband transitions of Au and the other by the interaction between two plasmonic modes of the whole Au@Ag NRs.
Finally, we have studied the behavior of the FRs as a function of the refractive index of the medium, to evaluate their potential use as sensors (Fig. 4). First, as depicted in Fig. 4a, we dispersed Au 8 @Ag 92 NRs in three different media (see Methods section): water (n = 1.33), isopropyl alcohol (n = 1.377) and chloroform (n = 1.466). As can be seen in the inset of this figure, FR2 and the longitudinal plasmon mode exhibit a very similar dependence on n, but is slightly more sensitive for the former. FR1, on the other hand, shows no appreciable variations on position or shape for the different media. Moreover, we performed FDTD simulations of the optical response of the same Au@Ag NR for light polarized transversal to the nanostructure and different refractive indices (between 1 and 2, including the experimental values). From a fit to this data with the model described by Eq. (1), we extracted the position and shape of FR2 (Fig. 4b) and the transversal LSPR mode (not shown). As can be seen in the inset of Fig. 4b, their shift is very similar but, again, it is slightly larger for FR2. Hence, we can conclude that for these nanostructures FR2 is slightly better than the LSPR for sensing applications whereas FR1 is almost independent of the environment.

conclusions
In summary, we have shown that a nanostructure composed of a Au NR covered with a silver shell is a very simple symmetrical nanoparticle that exhibits two different FRs. In addition to geometric simplicity, these nanoparticles are easy to synthesize using a standard colloidal method. Therefore, the configuration we are proposing is a very advantageous alternative to previous systems exhibiting FRs, surpassing them in simplicity, tunability, and intensity of the resonances. We have also shown that the tunable FR located at lower energies is strongly dependent on the refractive index of the surrounding medium. This may open the door to a number of applications, such as in sensing, where FRs have often been proposed as a good alternative to LSPRs but which have been seldom realized experimentally due to the complexity of the systems able to generate intense FRs.
Synthesis of Au NRs. The Au NRs were prepared using a seeded growth method with some modifications 47  Synthesis of Au@Ag NRs. The nanoparticles were prepared following a seeded growth method with some modifications 41 . Briefly, 500 µL of 0.01 M AgNO 3 and 50 µL of a 0.01 M ascorbic acid solution were added to 10 mL of a 25 mM CTAC solution. The concentration of Au NRs in the growth mixture was varied to obtain different Ag shell thicknesses (Table 2). Then, the mixture was heated up to 65 °C and left undisturbed for 12 h. Finally, the nanoparticles were centrifuged for 30 min at 5000 rpm and redispersed in 10 mL of a 25 mM CTAC solution.
Polyethylene glycol functionalization of Au@Ag NRs. An aqueous solution of poly(ethylene glycol) methyl ether thiol (6 kDa) previously sonicated for 5 min was added to Au@Ag NRs (1 nM, 5 mL) stabilized with CTAC (1 mM) under stirring for 3 h. Then, the excess of free ligand was removed from the solution by four centrifugation cycles (6500 rpm, 90 min). In each cycle, the supernatant was removed and the precipitate was redispersed in the same volume of solvent. The order of solvent transfer was water, isopropyl alcohol and chloroform.
Transmission electron microscopy (TEM). Low magnification TEM images were obtained on a JEOL JEM-1400PLUS transmission electron microscope operating at an acceleration voltage of 120 kV. Carbon-coated 400 square mesh copper grids were used. For TEM grid preparation, 1.5 mL of the mixture was centrifuged (in 1.5 mL Eppendorf tubes) and redispersed in 1.5 mL of a 1 mM CTAC solution. Then, the nanoparticles were centrifuged again (same parameters) and redispersed in 20 μL of a 1 mM CTAC solution. Finally, 3 μL of water and 1 μL of the nanoparticle suspension were deposited on a carbon-coated 400 square mesh copper grid (placed on Parafilm) and allowed to dry slowly. www.nature.com/scientificreports www.nature.com/scientificreports/ UV-vis-NIR Spectroscopy. Extinction spectra were recorded on a UVICONXL spectrophotometer (Bio-Tex Instruments). All experiments were carried out at 298 K using quartz cuvettes with an optical path length of either 2 mm or 1 cm.

Optical Simulations.
Optical response and near-field enhancements were calculated using the method of finite differences in the time domain (FDTD), as implemented in the free software package MEEP 48 . In this method, Maxwell equations are solved by a second-order approximation. Space is divided into a discrete grid, the Yee grid 49 , and the fields are evolved in time using discrete time steps. A schematic representation of the geometry used for the calculation is shown in Fig. 5. Simulations were performed for the nanostructures oriented along the three Cartesian axes. In all calculations, we employed a spatial resolution of 0.5 nm. For the refractive index, we applied the bulk values of Ag and Au fixed by Johnson and Christy 50 , using a Drude term and five Lorentzians 51 . For the calculations in Fig. 3, the refractive index of the surrounding medium was fixed at 1.33 (water) whereas different values between 1 and 2 where used to study the sensing capabilities of the FRs. In Fig. 5, d 1 and d 2 are, respectively, the rod diameter and the box side. Likewise, L 1 and L 2 represent the length of the rod and the box, respectively. The edges of the box were rounded with a radius of 2 nm, to make it more similar to the experimental structures (see Fig. 1).

Data availability
The data generated and analysed during the current study will be made available from the corresponding author on reasonable request.   Table 2. Volume of Au NR stock solution added to seed the growth of Au@Ag NRs and the resulting Au concentration in the growth mixture. Figure 5. 2D schematic representation of the 3D model used to simulate the optical response of the core-shell nanoparticles. An x-polarized plane wave impinges over the nanostructure and the flux is measured on all the faces of a parallelepiped containing the whole nanostructure (the blue rectangle) to determine the scattering, absorption, and extinction efficiencies (Q sca , Q abs , and Q ext , respectively). The simulation is repeated three times, with the plasmonic nanoparticle aligned along the three Cartesian axes. The simulation area is surrounded by a perfectly matched layer (PML) to mimic an infinite space.