Heterogeneous Nanoplasmonic Amplifiers for Photocatalysis’s Application: A Theoretical Study

Abstract

The higher cost of Ag and Au and their resonance frequency shift limitation opened the way to find an alternative solution by developing new nanohybrid antenna based on silicon and silicon dioxide coated with metallic nanoparticles. The latter has been recently solicited as a promising configuration for more large-scale plasmonic utilisation. This work reports a multitude of fascinating new phenomenon on LSPR on silicon antenna wires coated with core-shell nanospheres and the studying of the nanoplasmonics amplifiers to control optical and electromagnetic properties of materials. The LSPR modes and their interaction with the silicon nanowires are studied using numerical methods. The suggested configuration offers resonance covering the UV-visible and NIR regions, making them an adaptable addition to the nanoplasmonics toolbox.

1. Introduction

Metal-based nanomaterials are a key element in the development of advanced nanotechnology. These nanomaterials are promising candidates for a variety of applications including optical sensors and biosensors technology [1,2,3,4], photochemistry and photocatalysis [5,6,7,8]. Particularly, metallic nanoparticles NPs attracted substantial interest due to their Localized Surface Plasmon Resonance (LSPR). LSPR properties are very promising to enhance the photocatalytic activities in semiconductors, metal–organic framework (MOF), 2D materials, metals, and perovskites [9,10,11,12,13,14,15,16,17]. Recently, different studies have been reported on the importance of plasmonic metallic nanostructures on the engineering of their properties, e.g., the charge transfer, solar absorption, surface chemistry, stability, selectivity, etc. [9,18,19,20].

LSPR are the oscillations of electron clouds around NPs. Due to light excitation of the NPs, electronic oscillation is induced, and the NP behaves as an electromagnetic dipole re-emitting light coherently at the same frequency. While a part of this emitted light is scattered to the far field, the other part is concentrated in the nanoscale vicinity of the metal surface of NPs leading to nanoplasmonic NPs [21,22,23,24,25]. As a result, the concentrated light generates a strong intensity of the electromagnetic field (enhanced field (EF)) of several orders of magnitude achieved either on the apex of NPs or by approaching two (dimer), three (trimer), four (quadrimer) or multiple NPs. Another feature of nanoplasmonic NPs is the exited wide range of wavelengths, extending from the ultraviolet (UV)-visible (V) to near infrared (NIR) [26,27]. Therefore, they are known as excellent UV-Vis-NIR emitters. The enhancement intensity around NPs is strongly dependent on the nanoparticles chemical composition [8,28,29] as well as their geometric features (radius and orientation) [30,31,32,33], the latter stay the most interesting factors to be considered firstly in any nanoplasmonic amplifiers.

Pure metallic NPs are the most used nanomaterials as plasmonic amplifiers due to the high rate of reproducibility and durability of, i.e., Au. Unfortunately, some have limited surface chemistry such as Ag that degrades due to unstable chemical composition, high cost and are deficient optical absorbers as catalysts [13,34]. By coupling a catalytic material as a protective self-assembled monolayer to the nanoplasmonic NPs, the efficiency of plasmonic NPs is improved by tuning the plasmon resonance higher/lower wavelength therefore expanding the absorption response frequency of NPs [13,31,34]. Besides, the amplification on the adjacent surface of NPs and catalytic material may be increased, leading to a hot-carrier generation [35,36,37]. Consequently, it is paramount to study more configurations of plasmonic amplifiers that are characterized by lower cost as well as chemically environmentally friendly. In addition, it is necessary to optimize the geometric configuration of the nanoparticles in order to enhance their electric amplification factor (EF), thus making the materials useful in several applications such as surface enhanced Raman spectroscopy (SERS) and photocatalysis [5,35,37,38,39].

We demonstrate LSPR behaviour in the interparticle junction of plasmonic antennas of Si and SiO₂ decorated with metallic Ag/Au core-shell NPs through a finite element simulation. This method not only explains the plasmon modes and tunability, but also provides a quantitative description of the electric field distribution surrounding the Ag cores/Au shells at the nanoscale. Therefore, we suggest a model of variable distance between Ag cores/Au shells, variable material antenna to investigate the enhancement factor, absorption cross-section, LSPR sensitivity and tunability. We demonstrate that the antenna’s configuration exhibit dramatically increased photocatalytic activity over their individual components. The developed approach provides for independent control of plasmon tunability and intensity and paves the way for an efficient heterogenous plasmonic photocatalysts.

2. Results and Discussion

We start our numerical simulation by optimization of the configuration of silicon nanoantenna decorated by Ag/Au core-shell nanospheres. We studied the quantitative enhancement factor and plasmon modes when varying the interparticle distance between the surfaces of the gold shell in the interparticle junction in the z axis by keeping the distance between the surfaces of the gold shell in xy axis constant to 1 nm. This was followed by a quantitative description of the significance of using shell nanolayer on pure silver. We compared pure gold and silver and heterogeneous nanostructure of core-shell nanospheres that decorate the Si wire nanoantenna. We then compared the effect of the surrounding medium covered by different chemical nature of solvents by studying the tuning sensitivity ($\Delta \lambda$_max/$\Delta$n) that is defined as the change of the plasmon resonance peak position ($\Delta \lambda$_max), expressed in nm, as a function of refractive index change ($\Delta$n), expressed in Refractive Index Unit (RIU).

2.1. Study of Geometrical Parameters

This section summarizes the geometrical parameter effects on the electromagnetic field enhancement and absorption cross section of the 3D model of silicon nanoantenna surrounded by nanospherical silver/gold core-shell. Figure 6e,f presents the distribution of electromagnetic field enhancement (E_loc/E_inc) in the silicon nanoantenna surrounded by nanospherical silver/gold core-shell. The configuration in the xz plane and in the xy plane is presented to better show the improvement of the localized field enhancement. The maximum field enhancement is ~400 for interparticle distance d_int in the xy plane =15 nm and it is located both between the spherical silver nanoparticles and between the silicon nanoantenna and nanospherical silver/gold core-shell. The origin and parameters that influence the electromagnetic field enhancement are described below.

2.1.1. Effect of Interparticle Distance d_int

We fixed the interparticle distance between Ag/Au core-shell in the xy plane to 1 nm and studied the effect of variable interparticle distance (d_int) in the zy plane between nanospherical Ag/Au core-shell with a constant radius equal to 3 nm Ag core/2 nm Au shell and for 70 nm length, 10 nm radius of silicon nanoantenna are depicted in Figure 1a–e. Five resonant plasmon bands are observed, two with high intensity and three with average intensity. The three plasmon band denoted in the figure by (plasmon mode 1 (P.M.1), plasmon mode 2 (P.M.2) and plasmon mode 3 (P.M.3) are initially located (d_int= 15 nm) in the visible regions at 515.0 nm, 624.5 nm and 681.0 nm, respectively.

The two other plasmon bands denoted by plasmon mode 4 (P.M.4) and plasmon mode 5 (P.M.5) are located in the near-infrared regions at 822.1 nm and 927.0 nm, respectively, for d_int = 15 nm. By varying the interparticle distance in the zy plane from 15 nm to 25 nm, we noticed different variations and shifts of the LSPR plasmon modes. Figure 1f summarizes the plasmon resonance band shift as we modify the interparticle distance. As the interparticle distance increases from 15 nm to 25 nm, the resonant plasmon modes of LSPR in the visible regions P.M.1, P.M.2 and P.M.3 redshifts of 50.0 nm, 4.0 nm and 18.3 nm, respectively. However, the resonant plasmon modes of LSPR in the NIR regions P.M.4 and P.M.5 blue shifts of 29.8 nm and 24.4 nm, respectively, as the interparticle distance increases. The plasmon mode shift can be explained by the strong interaction of the lower energy level with the highest ones in the energy level diagram presented in Figure 2a with red dashed lines. For small distances, the symmetrically modes (bonding) and asymmetrically modes (anti-bonding) are governing the interaction modes [40,41,42]. The figure is a presentation of a part of the heterogeneous antenna in the yz plane. The latter explains the energy modes coupling of the plasmon and it strongly depends on the interparticle distance between the surfaces of the nanoshell and the contact regions with the antenna nanowire. This is the localization of hot electrons presented in red in the 3D simulated model. To describe the possible coupling between the plasmon modes also defined as the hybridized modes for $l>0$, Equation (1), summarize all the symmetric (${\omega }_{+}$) and antisymmetric (${\omega }_{-}$) coupling modes in the case of metallic core-shell nanostructures [40,41]:

$${\omega }_{l\mp }=\frac{{\omega }_s^2}{2}\left[1\mp \frac{1}{2l+1}\sqrt{1+4l\left(l+1\right){\left(\frac{r_1}{r_2}\right)}^{2l+1}}\right]$$

(1)

For a given azimuthal number m, ${\omega }_s$ is the surface plasmon frequency ${\omega }_s=\sqrt{\frac{2\pi e^2{n}_0}{m}}$ with $n_0$and $e$ being the charge density and the electron charge, respectively.

Figure 2. (a) Energy level diagram of the coupling of plasmon modes of the heterogeneous antenna. (b) 3D simulation of the electromagnetic field enhancement (near-field enhancement) in the silicon nanoantenna surrounded by a Ag/Au core-shell nanospheres. The model is the air domain for different interparticle distances (d_int = 15 nm, 18 nm, 20 nm, 23 nm, 25 nm), silicon nanoantenna radius r_Si = 10 nm and length L_Si = 70 nm, Ag/Au core-shell nanospheres radius r_Ag@Au= 5 nm. The inset shows the red line where the electromagnetic field was collected on the surface of the Si nanoantenna.

Figure 2. (a) Energy level diagram of the coupling of plasmon modes of the heterogeneous antenna. (b) 3D simulation of the electromagnetic field enhancement (near-field enhancement) in the silicon nanoantenna surrounded by a Ag/Au core-shell nanospheres. The model is the air domain for different interparticle distances (d_int = 15 nm, 18 nm, 20 nm, 23 nm, 25 nm), silicon nanoantenna radius r_Si = 10 nm and length L_Si = 70 nm, Ag/Au core-shell nanospheres radius r_Ag@Au= 5 nm. The inset shows the red line where the electromagnetic field was collected on the surface of the Si nanoantenna.

In terms of enhancement factor, Figure 2b presents the distribution of the enhancement factor E_loc/E₀ on the cross section of the silicon nanoantenna. The distance in the xzy plane is fixed to 1 nm. However, the interparticle distance in the zy plane was varied from 15 nm to 25 nm. Small interparticle distances of 15 nm, 18 nm and 20 nm give the maximum field enhancement distribution of 420. However, beyond 20 nm interparticle distance, the enhancement factor drops to 63, thus 6 times lower. These results are obvious since as the interparticle gap decreases, dimer effect influences the field and intensity decreases and plasmon mode coupling improves the electromagnetic field interactions.

2.1.2. Effect of the Ag/Au Core-Shell Radius (Only Ag, Only Au, Au @1 nm, Au @2 nm and Au@3 nm)

The quantitative understanding of the plasmonic heterogeneous nanostructures has led to the design of highly sensitive nano-antennas by the proper selection of materials along with the optimization of size, shape anisotropy and surface chemistry of the structures. However, their performance depends on the excitation of the localized surface plasmon resonance (LSPR), which is able to modify and enhance the light–matter interaction. In particular, the choice of core shell nanospheres as a decoration of the silicon nanoantenna will be explained in the following. Figure 3 depicts the enhancement factor at variable shell thickness 1 nm, 2 nm and 3 nm when we use pure silver and pure gold. Gold core gives the minimum (g = 114) enhancement factor with respect to silver core (g = 138). This is obvious since silver is known to be the best amplifier for SERS application. However, the decoration of silver with 1 nm, 2 nm and 3 nm of a gold reinforce the enhancement factor. Specifically, 1 nm Au shell gives the most dramatic improvement more than 3 times greater than pure silver. Overall, 2 nm Au shell is 2 times better with respect pure silver and Au@3 nm is 1.5 times greater than pure silver. Hence, the best optimized configuration is the Si antenna decorated with Ag/Au core-shell with the dimensions of (4 nm–1 nm).

In terms of LSPR plasmon resonance, the core shell configuration exhibits two plasmon modes in the UV and visible range. The maximum plasmon enhancement for pure gold is 692 nm, the pure silver is at 589 nm, the Ag/Au@1 nm is at 689 nm. The Ag/Au@2 nm is equal to 731 nm and the Ag/Au@3 nm is equal to 651 nm. Thus, the plasmon resonance can be better tuned in the near-infrared regions with the Ag/Au@2 nm configuration.

2.2. Best Material Antenna for Photo Catalysis and SERS Enhancement

Silicon Si or Silicon Dioxyde SiO₂ Antenna?

Other important parameters which should be taken into account in any nanoplasmonic antenna is the chemical nature of the structure. In the previous section, we only studied silicon as a material of the antenna. Herein, we varied the nanoantenna material to silicon dioxide to investigate the LSPR variation. For this study, we selected the optimized geometry from the previous section, the Si radius is 7 nm and 70 nm long. The Ag/Au core-shell was fixed to 4 nm and the interparticle distance d_int = 15 nm in the zy plane and to 1 nm in the xy plane. Figure 4a depicts the electromagnetic field enhancement E_loc\E₀ on the surface of the 3D simulated model of the up SiO₂ nanoantenna with Ag/Au core-shell, bottom Si nanoantenna with Ag/Au core-shell for 330 nm and 670 nm laser excitation. The maximum enhancement factor presented with red colour is equal to 400 and the minimum enhancement factor in blue is 0 as depicted by the toolbar on the right. We present the distribution of the electromagnetic field enhancement for only two laser excitations, 330 nm and 670 were the plasmon resonance occur. According to the figure, a direct comparison between Si and SiO₂ for 670 nm excitation laser confirms that the silicon antenna presents a larger enhancement in red of the electric field on the vicinity of Ag/Au core-shell nanospheres. To better understand the electromagnetic field distribution, we present in Figure 5b the variation of enhancement factor on the surface of the Si and SiO₂ nanoantenna at variable wavelength. The maximum plasmon band for the SiO₂ nanoantenna is within the UV range, particularly at 371 nm with g = 335. However, using Si antenna, we can tune the plasmon resonance to the visible range with a maximum plasmon band at 648.7 nm and g$\approx$400. This study confirms that in order to tune the plasmon resonance in the UV, visible or near-infrared range, the material must be chosen appropriately and properly.

Table 1. Comparison between the enhancement factor of this work and other reported work.

Reported Studies	Configuration	Excitation Wavelength (nm)	EF
This work	Ag/Au core-shell on Si	~650	~105
C. Zhang et al. [43]	Au@Ag/Si	~532	~107
D. Wu et al. [44]	Ag/Au core shell	~507	~105
F. Zhang et al. [45]	Ag–SiO2 nanomace arrays	~514.5	~105

shows the comparison between the enhancement factor of this work and those reported elsewhere.

2.3. LSPR Wavelength Tunability

The localized surface plasmon resonance (LSPR) is sensitive to a number of factors, including the refractive index of the surrounding environment. In order to understand its sensitivity with the refractive index, we immersed the heterogenous antenna of Si decorated with Ag/Au core shell in different solvents. We kept the same shape of the structure: Si antenna radius is r_Si= 7 nm and the Ag/Au core shell is r_Au@Ag = 5 nm radius. The separation distance d_int between the surfaces of the shells is 1 nm in the xy plane and d_int = 15 nm in the zy plane. Figure 5a shows the calculated absorption cross section of the silver cores/gold shells immersed in different solvents (n₁ = 1.33, n₂ = 1.36, n₃ = 1.39, n₄ = 1.43, n₅ = 1.47, n₆ = 1.51, n₇ = 1.62) as a function of the wavelength variation from 200 nm to 1100 nm. The complex structure displayed a significant redshift of the all LSPR resonant plasmon modes when it is immersed in a solvent with higher refractive index as can be seen in Figure 5a. In this wavelength range, the position of the peak shift depends strongly on the real part of the permittivity and the enhancement is maximized therefore as presented in Equation (2): [46].

$${\epsilon }^{,}=-2{\left(n_{sol}\right)}^2$$

(2)

where n_sol is the refractive index of the solvent. We define therefore the sensitivity of plasmon modes as the variation of plasmon mode wavelength with the respect to the solvent with refractive index n = 1.33 (water) at variable refractive index. Figure 5b presents the sensitivity of heterogeneous nanoantenna as the refractive index changes. The relative shift in peak position,$\Delta {\lambda }_{max}$ (With reference to the water solvent) is linearly dependent on the refractive index change of the solvents. We can then obtain the sensitivity factor from the linear plot between $\Delta {\lambda }_{max}$ an $n_{sol}$ that is the slope of the resonant plasmon mode of LSPR. The calculated slope is presented in the inset of Figure 5b. Sensitivity of the heterogeneous nanoantenna in the visible range is 69.5 nm per RIU, 250.1 nm per RIU and 324.2 nm per RIU for P.M.1, P.M.2 and P.M.3, respectively. In the NIR range, sensitivity of nanoantenna is 402.2 nm per RIU for P.M.4 and 230.1 nm per RIU for P.M.5. The high sensitivity of the heterogenous nanoantenna confirms that we can tune its resonant plasmon modes in the visible to near infrared to cover a large band of spectral regions.

3. Methods

We used Finite Element Method (FEM) to solve the heterogeneous geometry of antennas proposed in this study as depicted in Figure 6a,b. FEM was successfully used to understand the optical, chemical and electromagnetic properties of NPs at the nanoscale [47]. Based on the analysis of mathematical models, this method simulates the behaviour of complicated geometries. FEM method is based on Maxwell’s equations that are integrated in the COMSOL Multiphysics software to perform our extended simulations, exploiting, in particular, the software wave optic interface covering the modelling of electromagnetic fields and waves in the frequency domain. The interface proceeds, first, by formulating the Maxwell’s Equation (3):

$${\mu }_r{}^{-1}\left[\nabla \left(\nabla \stackrel{\rightharpoonup }{E}\right)-{\nabla }^{2 }\stackrel{\rightharpoonup }{E}\right]-k_0{}^2\left({\epsilon }_r-\frac{j\sigma }{\omega {\epsilon }_0}\right)\stackrel{\rightharpoonup }{E}=0$$

(3)

where $\omega$, ${\mu }_r$ and $\sigma$ are the excitation frequency, the relative permeability (fixed to 1) and the electrical conductivity, respectively. In these calculations, the relative dielectric permittivities corresponding to the optical frequencies ${\epsilon }_r\left(\omega \right)={\left(n\left(\omega \right)+ik\left(\omega \right)\right)}^2={\epsilon }^{\prime }\left(\omega \right)-i{\epsilon }^{″}\left(\omega \right)$, n and k are real numbers defining the refractive index and the extinction coefficient, respectively, for the electric displacement. They are defined from references in Table 2. $\omega$ is the excitation frequency of the incident light. ${\epsilon }^{\prime }$ and ${\epsilon }^{″}$ are the real and the imaginary parts of the relative permittivity, respectively. In addition, the permittivity of free space is indicated by${\epsilon }_0$and the wave number in free space is indicated by $k_0$, being $k_0=\raisebox{1ex}{$\omega $}\!\left/ \!\raisebox{-1ex}{$c_0$}\right.$ (with $c_0$ the speed of light in vacuum). Finally, in these calculations, we assume an ideal matched layer absorbing the propagating wave in the interior of the computational region and taking reflections in the interior interface. Then, the finite element method is used by the software to solve the equation and, finally, it discretizes the equation in numerically stable edge elements. The meshing is defined by minimum element size = 1.5 × 10⁻¹¹ and maximum size =1.5 × 10⁻⁹ in order to have a better resolution of the thin regions and to better capture the field enhancement in the nanogap. A presentation on the fine mesh is presented in Figure 6c,d. The simulated model is a three-dimensional (3D) configuration. The nanoantenna is defined as a nanowire with 10 nm radius r_Si and 70 nm length L. The nanowire is decorated with silver/gold core-shell nanospheres with 5 nm radius denoted as r_Ag@Au. In spite of the long-time compiling, 3D is an accurate way to have real behaviour of nanoantennas. The electromagnetic effect occurs first around the Ag/Au nanospheres. The local field is further enhanced, and a dipole is induced leading to the enhancement of the Raman scattering in the nanogap. Therefore, the $G_1=g^2$ is defined as $\frac{{\left|E_{loc}({\omega }_0)\right|}^2}{{\left|E_0({\omega }_0)\right|}^2}$ were $E_{loc}$and $E_{0 }$are the local electric fields in the presence and absence of nanospheres, respectively. Then, a mutual excitation from the system of the nanospheres at a resonant frequency induces an enhanced apparent Raman polarizability defined as $G_2=\frac{{\left|E_{loc}({\omega }_r)\right|}^2}{{\left|E_0({\omega }_r)\right|}^2}$. As a result, the simulated enhanced Raman scattered light G also defined as the enhancement factor from the structure of silicon nanoantenna surrounded by Ag/Au core-shell nanospheres Equation (4) is presented as follows:

$$G=G_1({\omega }_0)G_2({\omega }_r)=\frac{{\left|E_{loc}({\omega }_0)\right|}^2{\left|E_{loc}({\omega }_r)\right|}^2}{{\left|E_0({\omega }_0)\right|}^2{\left|E_0({\omega }_r)\right|}^2} \approx \frac{{\left|E_{loc}({\omega }_r)\right|}^4}{{\left|E_0({\omega }_r)\right|}^4}$$

(4)

The approximation only holds true for small Raman shifts for which the enhancement of the excitation wavelength and the signal wavelength can be expected to have the same enhancement factor. Figure 6e,f depicts the resulting enhancement factor g = E_loc\E₀ for the corresponding parameters: Si nanoantenna radius r_Si = 10 nm, length L = 70 nm, Ag/Au core-shell radius r_Ag@Au = 5 nm with r_Au = 1 nm and r_Ag = 4 nm, interparticle distance d_int = 1 nm in xy plane and interparticle distance dint in the zy plane is equal to 15 nm and the wavelength excitation ${\omega }_r=670 \mathrm{nm}.$ The color bar on the right shows the minimum enhancement factor in blue = 0 and the maximum enhancement factor presented in red = 400. In the contact regions between the silicon nanowire antenna and the core-shell Ag-Au nanospheres, the electromagnetic field enhancement is maximized due to the dimer effect that leads to hot electrons generation [5,6,35,36,37]. In the xy plane, the gap between the core/shell Ag-Au is equal to 1 nm, and this also leads to a maximum enhancement factor in the interparticle vicinity. It should be noted that the smallest physically meaningful gap that corresponds to the length between two gold atoms Au–Au, which is approximately 288.9 pm~0.3 nm distance was respected in this study [48]. This 3D model with the new geometrical configuration will be great of benefit to fabricate the 3D hybrids nanoantenna with specific enhancement properties.

Table 2. Values of the refractive index and extinction coefficient used in the simulation.

Materials	Ag	Au	Si	SiO2
n, k coefficients	Johnson and Christy 1972 [49]	Johnson and Christy 1972 [49]	H. H. Li [50]	Ghosh 1999 [51]

Table 2. Values of the refractive index and extinction coefficient used in the simulation.

Materials	Ag	Au	Si	SiO2
n, k coefficients	Johnson and Christy 1972 [49]	Johnson and Christy 1972 [49]	H. H. Li [50]	Ghosh 1999 [51]

Figure 6. (a) 3D configuration model of Si nanonatenna surrounded with Ag/Au cores-shell nanosphere. (b) Dimension of the cross section in the xz plane: Si nanoantenna radius r_Si = 10 nm, length L = 70 nm, Ag/Au core-shell radius r_Ag@Au = 5 nm, interparticle distance d_int = 1 nm in xy plane and interparticle distance d_int in the zy plane varies between 15 nm and 25 nm. In this figure d_int = 15 nm. (c,d) mesh of the 3D configuration in the xy plane (up) and xz plane (down) (e,f) Distribution of the electric field enhancement Eloc/E₀ on the surface of heteregeounous nanoantenna.

Figure 6. (a) 3D configuration model of Si nanonatenna surrounded with Ag/Au cores-shell nanosphere. (b) Dimension of the cross section in the xz plane: Si nanoantenna radius r_Si = 10 nm, length L = 70 nm, Ag/Au core-shell radius r_Ag@Au = 5 nm, interparticle distance d_int = 1 nm in xy plane and interparticle distance d_int in the zy plane varies between 15 nm and 25 nm. In this figure d_int = 15 nm. (c,d) mesh of the 3D configuration in the xy plane (up) and xz plane (down) (e,f) Distribution of the electric field enhancement Eloc/E₀ on the surface of heteregeounous nanoantenna.

4. Conclusions

Heterogeneous nanoplasmonics structures, i.e., core-shell Ag@Au metallic decorating silicon nanowire was introduced in this study as a new model for photo catalysis and SERS application. In particular, the reduction of size of metal nanostructures to confine the electrons of the excited conduction band lead to a great improvement of the electric field enhancement and tunability near the nanostructures. Therefore, controlling the size of the nanostructures provides greater control over these collective excitations of LSPR. Finally, we confirmed that by coupling a catalytic material such as silicon a protective self-assembled monolayer to the nanoplasmonic NPs, the efficiency and sensitivity of the heterogeneous antenna by tuning the plasmon resonance and enhancement factor are improved.