AuNRs@Pd were synthesized via bottom-up epitaxial Pd growth, incorporating Pd precursor (H2PdCl4), reducing agent (ascorbic acid, AA), surfactant (cetyltrimethylammonium bromide, CTAB), and hydrochloric acid (HCl) into the AuNR solution18,35. In this study, two different Pd concentrations of 1 and 2 mM were used to synthesize AuNRs@Pd with different shell thicknesses and morphologies. Structural characterizations of the nanoparticles were conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figures 1A and S1 depict SEM images of CTAB-capped AuNRs with an average size of 72.31 (± 7.254) nm × 25.90 (± 2.276) nm (Fig.  S2). The thick AuNRs@Pd2mM, synthesized using 2 mM of Pd precursor, showed coarse and bumpy shell structures, as shown in the high-resolution TEM (HR-TEM) results (Figs. 1B and S3). The formation of the thick Pd shell on AuNRs@Pd2mM formed through layer-by-layer epitaxial growth before encountering tensile strain owing to lattice mismatch between Pd and Au lattices18. Consequently, Pd islands emerged to alleviate the stress caused by tensile strain on the Pd lattice. The thickness of the Pd islands extending from the core to the tip of thick AuNRs@Pd2mM varied between 3.164 and 5.763 nm. Decreasing the Pd precursor concentration to 1 mM endorsed the formation of thin AuNRs@Pd1mM with a smoother Pd shell (Figs. 1C and S4). SEM results revealed that the mean aspect ratio (AR) of thick AuNRs@Pd2mM was determined at 2.47 (± 0.341) (Fig. 1D). However, the AR of thin AuNRs@Pd1mM was 2.67 (± 0.354) exhibited greater value than that of thick AuNRs@Pd2mM (Fig. 1D). This difference in AR was attributed to the thickness of the Pd shell, which influenced the dimensions of the bimetallic nanoparticles in terms of length and width.Fig. 1(A) SEM image of CTAB-capped AuNRs utilized for the synthesis of AuNRs@Pd. (B) SEM image of thick AuNRs@Pd2mM exhibits bimetallic nanoparticle structure with a bumpy surface. (C) SEM image of thin AuNRs@Pd1mM depicts a notably smooth formation of the Pd shell on the surface. (D) The normal distributions of AuNR (purple), thin AuNRs@Pd1mM (green), and thick AuNRs@Pd2mM (orange) aspect ratios (AR) reveal the decrement in AR as the Pd concentration used increases.We examined the distribution of Au and Pd on AuNRs@Pd2mM using HR-TEM coupled with energy-dispersive X-ray spectroscopy (EDX). The elemental mapping images obtained from EDX revealed the presence of Pd islands on the AuNR surface, with Au and Pd distributed across the bimetallic nanoparticle (Fig. 2). Furthermore, Pd exhibited a preference for growth on Au (001) and Au (111) facets before reaching another lattice and undergoing reduction36. This observation aligns with the elemental mapping results of thick AuNR@Pd2mM, indicating a higher concentration of Pd at the tip, as depicted in Fig. 2.Fig. 2(A–C) TEM–EDX results of thick AuNRs@Pd2mM, revealing Pd deposition along the AuNR surface, with a preference for Pd deposition at the tip region, as evidenced through the elemental mapping of Pd (highlighted in red in C).Subsequently, we investigated the scattering properties of the synthesized AuNRs@Pd using single-particle DF microscopy and spectroscopy to eliminate the ensemble averaging of nanoparticles. Figure 3A displays the DF scattering image of single thin AuNRs@Pd1mM, with the corresponding scattering spectra, denoted by circles in Fig. 3A, presented in Fig. 3B. Meanwhile, Fig. 3C depicts the DF scattering image of a single thick AuNR@Pd2mM, while the DF scattering spectra of representative single particles are shown in Fig. 3D. In complex plasmonic bimetallic nanostructures, the plasmonic resonance could be classified as the superradiant mode due to its strong coherence with incident light. Conversely, the subradiant mode in the spectra of this bimetallic nanostructure corresponded to the peak in the higher energy spectral range37. The superradiant peak was visible at low energy area. Additionally, a Fano dip was observed between the two peaks of the AuNRs@Pd scattering spectra, which arose from the overlapping peak between the narrow LSPR peak of Au and the wide scattering peak of Pd18,38. In the scattering spectra of thin AuNRs@Pd1mM, we observed a tendency for the superradiant peak intensity to be higher compared to the subradiant peak intensity (Fig. 3B). Conversely, thick AuNR@Pd2mM exhibited a higher intensity at its subradiant mode (Fig. 3D). The thicker Pd shell attenuated the characteristic of Au LSPR spectra, leading to a lower intensity of the superradiant scattering peak. Therefore, the thickness of the Pd shell is proven to influence the shape of scattering spectra of the bimetallic AuNRs@Pd. Besides the observed intensity reduction in the Au spectra, the introduction of the Pd shell appeared to have a noticeable effect on the scattering intensity of the nanoparticle observed through DF microscopy. As illustrated in Fig. 3A and C, the DF image of AuNRs@Pd1mM with high Pd contents on the shell, proven by a pronounced subradiant peak in its DF scattering spectra, exhibits a significant decrease in scattering intensity observed through DF microscopy (Fig. 3B and D).Fig. 3(A) DF scattering image and (B) LSPR scattering spectra of thin Pd shell AuNRs@Pd1mM exhibit the presence of two distinct peaks, namely superradiant (low energy) and subradiant (high energy), separated by the Fano dip. (C) DF scattering image and (D) LSPR scattering spectra of thick Pd shell AuNRs@Pd2mM demonstrate higher intensity of the superradiant peak compared to its subradiant peak intensity.The appearance of the subradiant and the superradiant peaks in the scattering spectra of AuNRs@Pd induced the existence of the Fano resonance, depicted as a dip. While multiple factors might contribute to the occurrence of the Fano resonance, the overlap between two peaks, the broad Pd peak and the narrow Au LSPR peak, was the cause of the Fano resonance in the single-particle scattering spectra of AuNRs@Pd. Based on the results, the Fano energy of thick AuNR@Pd2mM in a water medium was smaller than that of thin AuNRs@Pd1mM, which were 2.2969 (± 0.0277) and 2.3142(± 0.0316) eV, respectively. Consistent with our previous findings, an increase in the concentration of Pd on the surface induced a blueshift of the superradiant peak18. The coupling between the Au and Pd peaks induced a phenomenon in which the superradiant peak, primarily associated with the Au resonance, underwent a shift towards higher energy level. Numerous prior investigations have confirmed the presence of the broad Pd peak within the wavelength range of 345–441 nm (equivalent to 2.81–3.59 eV), while the energy values of the Pd peak are typically higher than those of the Au LSPR peak (approximately 1–2 eV)39,40,41,42,43. Consequently, the reduced energy difference between the two peaks \({(E}_{Au}-{E}_{Pd})\) led to the appearance of a Fano dip \({(E}_{Fano})\) at lower energy levels when additional Pd was introduced to the surface of AuNR, as indicated using Eq. (1)18:$${E}_{Fano}\propto \frac{{\left(q{\Gamma }_{Pd}+ {E}_{Au}- {E}_{Pd}\right)}^{2}}{{\left({E}_{Au}-{E}_{Pd}\right)}^{2}+ {\Gamma }_{Pd}^{2}}$$
(1)
In Eq. (1), q denotes the Fano parameter, and \({\Gamma }_{Pd}\) describes the linewidth of the continuous mode, which in this case the Pd scattering peak. The optimal value of q was determined by assessing the normalized Fano cross-section, calculated using Eq. (2). In this expression, \(\upvarepsilon \) represents the reduced energy, which is derived by dividing the difference between the Pd peak and the Fano resonance energy by the linewidth of Pd peak18,44.$${\sigma }_{Fano}= \frac{{\left(q+\upvarepsilon \right)}^{2}}{\left({q}^{2}+1\right)\left({\varepsilon }^{2}+1\right)}$$
(2)
The results revealed a distinctive dip at lower energy compared to the peak energy of Pd, as reported in numerous prior studies (\(E-{E}_{Pd}<0\))39,40,41,42,43. Consequently, the \(\varepsilon \) value aligns with the observed trend of AuNRs@Pd scattering spectra when the q value is within the range of 0 < q < 118.We then investigated the effect of RI of the surrounding medium on subradiant, superradiant, and Fano resonance modes in single AuNRs@Pd. This study on the RI effect was conducted using air (RI = 1.000), water (RI = 1.333), and oil (RI = 1.518) as the surrounding medium (Figs. S5–S10). Scattering peak analysis was conducted using the IF method to determine the peak scattering energy of both peaks and the curvatures of the constructing peaks (subradiant and superradiant peaks). This mathematical approach was conducted by deriving the spectra curve of the bimetallic nanoparticle scattering spectra to obtain their first and second-order derivation curves. The first-order derivative IF points are located where the curvatures change sign, indicating the subradiant and superradiant peaks, as well as the Fano dip. Additional IF points are located where the concavity of the spectra changes sign, indicating changes in the slope of the constructing peaks, occurring when the second derivative of intensity with respect to wavelength or photon energy equals zero. Prior to calculation, the scattering peaks were smoothed using Savitzky–Golay (SG) smoothing to improve the signal-to-noise ratio18,45. This method was chosen for its underlying principle, which utilizes a locally-fitting data approach using polynomials through the method of least squares. This smoothing approach facilitates the smoothing of the scattering peak while preserving the shape of the single-particle scattering spectra45. The first, second, and third rows in Fig. 4A–C depict the scattering spectra of single AuNRs@Pd1mM, along with their respective first and second-order derivatives in different media. The maxima of the superradiant plasmon scattering peak of AuNRs@Pd1mM, denoted as IF B, were located at 1.982 (± 0.065), 1.967 (± 0.052), and 1.930 (± 0.032) eV in air, water, and oil, respectively. Meanwhile, the maxima of the subradiant plasmon scattering peak of AuNRs@Pd1mM, indicated as IF F, were located at 2.461 (± 0.038), 2.444 (± 0.0415), and 2.425 (± 0.0783) eV for air, water, and oil, respectively. These results demonstrate the redshift of the peaks alongside the increment of the local dielectric constant, aligning with previous reports18,46,47. The Fano dip, represented by IF D, was located at 2.291 (± 0.021), 2.292 (± 0.025), and 2.326 (± 0.099) eV for air, water, and oil, respectively. These results confirm that the blueshift of the Fano dip on AuNRs@Pd is associated with the increase in RI. IF A and IF C represent the x-intercepts of the second derivatives of AuNR@Pd1mM scattering spectra within the superradiant plasmon peak regime. The LSPR IF values of A and C were 1.797 (± 0.044) and 2.123 (± 0.064); 1.784 (± 0.041) and 2.101 (± 0.049); and 1.744 (± 0.034) and 2.078 (± 0.031) eV in air, water, and oil, respectively, ordered from lowest to highest RI. These findings illustrate the redshift of the inflection peaks value along with the increment of the local dielectric constant, similar with numerous previous reports33,46,48. Meanwhile, IF E and IF G describe the points where the second derivative of the scattering peak crosses the x-axis on the subradiant scattering peak regime of AuNR@Pd1mM. The observation on IF E indicated a blueshift coinciding with an increase in the dielectric constant, contrary to the tendencies observed at other second-order derivation IFs, which were 2.379 (± 0.028); 2.395 (± 0.061); and 2.416 (± 0.069) eV in air, water, and oil, respectively, ordered from the lowest to the highest RI. In thin AuNRs@Pd1mM, IF E describes the change in concavity of the subradiant peak close to the dip region, which has relatively low intensity compared to its superradiant peak. Therefore, the observed blueshift in IF E was predominantly caused by the coupling between the superradiant and subradiant peaks. Concurrently, the IF G demonstrated less noticeable shifts compared to any other IFs across different RI environments (Fig. 4D).Fig. 4(A–C) Inflection point method applied to the single-particle scattering spectra of thin AuNR@Pd1mM across three different local RIs (air, water, and oil) (first row), along with its first (second row) and second (third row)-order derivatives. (D) Resulting inflection points plotted against the three local RIs for points A–G.To further investigate the influence of the Pd shell thickness on single AuNRs@Pd scattering peak across different dielectric constant conditions, DF microscopy was performed on single thick AuNRs@Pd2mM using similar medium variants. In this context, a high proportion of Pd will significantly dampen the superradiant plasmon peak scattering intensity of the bimetallic nanoparticles, identified as the plasmonic peak of Au, causing a shift to a lower wavelength (blueshift)49. Additionally, the epitaxial growth of the Pd shell beyond its critical thickness leads to the formation of islands, resulting in a bumpy Pd shell on the Au core surface50. The surface evolution of the bimetallic nanoparticles results in the weakening and blueshift of the plasmon resonance peaks18,50. Consequently, the scattering peaks of AuNRs@Pd2mM exhibit a more pronounced blueshift compared to AuNR@Pd1mM, as shown in IF B, which corresponds to the superradiant peak energy, and IF F, which corresponds to the subradiant peak energy of the bimetallic nanoparticle scattering spectra (Figs. 4D and 5D).Fig. 5(A–C) Inflection point method applied to the single-particle scattering spectra of thick AuNR@Pd2mM across three different local RIs (air, water, and oil) (first row), along with its first (second row) and second (third row)-order derivatives. (D) Resulting inflection points plotted against the three local RIs for points A–G.The first, second, and third rows in Fig. 5A–C depict the scattering spectra of single AuNRs@Pd2mM, along with their respective first and second-order derivatives in different media. The maxima of the superradiant plasmon scattering peak of AuNRs@Pd2mM indicated as IF B were located at 2.056 (± 0.053), 2.046 (± 0.053), and 2.035 (± 0.067) eV for air, water, and oil, respectively. Compared to AuNRs@Pd1mM, the sensitivity of IF B of AuNRs@Pd2mM toward RI changes was less pronounced owing to the attenuation of Au plasmon scattering peak (Fig. 5D). The damping of the Au core plasmon and the increase in scattering peak of the AuNRs@Pd subradiant peak upon light irradiation were attributed to the thicker Pd shell coating on the surface of Au core27. However, changes in the concavity of the thick AuNRs@Pd2mM in the superradiant regime with respect to RI changes were more visible compared to its peak shift, as evidenced by the IF A results with values of 1.842 (± 0.039), 1.826 (± 0.044), and 1.815(± 0.053) eV in air, water, and oil, respectively, ordered from smallest to highest. The sensitivity of IF A was 1.27 times higher compared to IF B, with values of 51.345 (± 2.768) meV.RIU−1 (R2 = 0.9971) and 40.41 (± 9.130) meV.RIU−1 (R2 = 0.9514), respectively (Fig. 6). The coupling effect between superradiant and subradiant peaks in AuNRs@Pd2mM scattering spectra led to a shift of the IF C towards higher energy levels along with the increment of RI. The peaks for subradiant plasmon peak of thick AuNRs@Pd2mM, denoted as IF F, showed less sensitivity compared to thin AuNRs@Pd1mM, with values located at 2.619 (± 0.013), 2.615 (± 0.019), and 2.587 (± 0.024) eV for air, water, and oil, respectively (Fig. 6). Aligned with the superradiant peak results, its IFs from the second-order derivative on subradiant peak regime showed redshifts for IF G and IF E along with an increase in the local dielectric constant, with values of 2.809 (± 0.027) and 2.476 (± 0.026); 2.807 (± 0.019) and 2.466 (± 0.021); and 2.759 (± 0.017) and 2.431 (± 0.016) eV for air, water, and oil, respectively. The sensitivity of the subradiant peak concavity in thick AuNRs@Pd2mM also demonstrated a higher degree of responsiveness to changes in RI compared to its peak. The surface roughness of plasmonic nanoparticles has been shown to induce broadening of the LSPR peak, thereby reducing its sensitivity to changes in the surrounding dielectric constant of bimetallic nanoparticles49. Our previous work has demonstrated that Pd deposition on the Au surface reduces the scattering intensity of bimetallic nanoparticles, as evidenced by the observed decrease in energy in real-time DF microscopy imaging results18. Consequently, AuNRs@Pd2mM with thicker shells exhibit lower sensitivity in IFs A-C and IFs E–G compared to AuNRs@Pd1mM with thinner Pd shells.Fig. 6RI sensitivity of inflection points toward varying local RI media in (A) thin AuNRs@Pd1mM and (B) thick AuNRs@Pd2mM spectra.Similar to the results for thin AuNR@Pd1mM, IF D, which described the Fano dip, showed blueshifts with values of 2.256 (± 0.034); 2.292 (± 0.041); and 2.302 (± 0.033) eV for air, water, and oil, respectively (Fig. 5D). However, the sensitivity of IF D to RI changes in AuNRs@Pd2mM was 1.52 times higher compared to AuNRs@Pd1mM, with the repeatability of IF D in each medium slightly higher than that of the thin AuNRs@Pd1mM (Figs. 6 and S11). A comprehensive comparison of sensitivity between thin AuNRs@Pd1mM and thick AuNRs@Pd2mM across different local RI is presented in Fig. 6. The data reproducibility of each IF point across various refractive indices exhibited relatively good results, proving the reliability of the IF method in determining spectral changes in the multimodal scattering peaks of single AuNRs@Pd (Fig. S11).
Elucidating refractive index sensitivity on subradiant, superradiant, and fano resonance modes in single palladium-coated gold nanorods
