1. Introduction

Periodically structured plasmonic systems, including arrays of metal nanoparticles, arrays of dielectric nanoparticles on a metal film, nanostructured metal or metal-dielectric films, represent a class of optical materials bearing a huge potential for both fundamental investigations and development of applications [1]. Their optical/plasmonic properties, determined by excitation of surface plasmons, offer many ways to control and manipulate light-matter interactions at the nanoscale, mainly through size, shape and arrangement of their constituents. A particular type of periodic plasmonic nanostructure is represented by metal films deposited over ordered arrays of dielectric microspheres, known in the literature as metal film over nanospheres (MFoN) [2], metal-coated microspheres [3], metal film over colloidal crystals [4], hybrid photonic-plasmonic crystals [5], or metal half-shell arrays [6]. Note that, although some papers used the term ‘nanospheres’, while other ‘microspheres’ for the dielectric colloidal spheres under the metal films, they deal with colloidal spheres having diameters of few hundred nanometers. We will stick to the MFoN term throughout this paper, for simplicity.

These MFoN came to attention mainly due to work in VanDuyne’s group, which demonstrated the reliability of MFoN as Surface-Enhanced Raman Scattering (SERS) substrates [7]. Thorough investigations of their optical properties were however not performed at that time. Later on, the optical transmission properties started to raise interest, due to the interesting behavior observed in MFoN, resembling the extraordinary optical transmission (EOT) discovered in the late 2000s [8]. It was demonstrated that MFoN exhibit an EOT-like transmission band, tunable by the dielectric sphere size, despite not having actual holes along the transmission direction [3,9,10]. Meanwhile, besides practical SERS applications [11,12], these structures gained attention as plasmonic platforms for surface plasmon resonance spectroscopy [4,13,14] or surface-enhanced/metal-enhanced fluorescence studies [15,16].

While the transmission of MFoN has been studied in the context of EOT, there has been little attention to understanding the overall optical response, i.e. by studying not only the transmittance, but also the reflectance and absorbance spectra of these structures. The dependence of both transmittance and reflectance on sphere diameter and film thickness, over wide ranges, are currently not available. The few studies that analyze reflectance spectra [10,14,17] only show a single configuration of the MFoN in terms of thickness of the deposited film and microsphere size. The absorbance spectra are particularly relevant for surface-enhanced spectroscopy (Raman - SERS and fluorescence - SEF) applications, as high absorbance can indicate that radiation is being trapped within the MFoN structure in the form of surface plasmons, either localized or propagative. This strong absorbance caused by plasmons is accompanied by large electric field enhancements near the metallic surface, facilitating absorption, emission or scattering in nearby molecules and leading to enhancements of orders of magnitude in SERS or SEF intensity. The high SERS enhancements achievable by plasmonic effects allow therefore to detect and analyze ultra-low analyte concentrations, based on their fingerprint-like vibrational spectra. Hence, SERS became a serious candidate for the development of sensing and analytical applications in fields such as medicine or environment.

Very often, microfluidic technology is employed for sample manipulation in analytical applications, due to several advantages including the small sample volumes required and high throughput achievable in an automatized and repeatable manner. Consequently, the development of application based on SERS combined with microfluidics, also known as lab-on-a-chip SERS, is currently a hot research topic. Indeed, more and more SERS-in-microfluidics applications are currently being developed, as demonstrated by many recent publications [18–21], and overviewed in some review papers [22].

At the same time, the development of solid SERS substrates, from the optical and plasmonic properties point of view, has generally been done in ambient conditions, namely in air. The same holds true for MFoN, as their optical properties in fluids (e.g. aqueous environment) have not been studied so far. It is known however, that the optical properties of metallic nanostructures strongly depend on the refractive index of the medium, and generally the surface plasmon resonances are considerably redshifted when these structures are immersed in water, as compared to air. Moreover, different effects can arise from the higher index medium, such as changes in the near-field coupling strength in plasmonic dimers, or changes in the width of the plasmon resonances due to matching of the refractive index on different sides of a nano-patterned film [23], or more efficient excitation of plasmonic surface lattice resonances [24]. Therefore, detailed studies of the optical properties of nanostructured SERS substrates immersed in aqueous environment are needed in order to explore their optical/plasmonic response, and optimize their plasmon-induced field enhancements required in SERS.

Here we study the optical response of MFoN by FDTD simulations, in both air and water, for a wide range of sphere diameters and metal film thicknesses. Our two main objectives are i) to analyze the dependence of the optical response of MFoN on structural parameters (sphere size, film thickness, film morphology), and ii) to analyze the optical response of MFoN in water, with SERS-in-microfluidics applications in mind. We begin by validating our modeling approach against experimental results for AuFoN structures. Then we determine the transmittance, reflectance and absorbance spectra, in both air and water, for arrays of microspheres having diameters in the range 200 - 1000 nm, coated by gold films having thicknesses in the range 40–200 nm. We discuss similarities and differences between both the far-field and near-field optical response of AuFoN in air and water. Interestingly, for larger spheres and film thicknesses we identify an additional absorbance band, of different nature than the better-known main absorption band. Finally, we analyze the role of the metal type (Au, Ag, Cu, Al), and the impact of film morphology on the optical spectra.

2. Methods

2.1 FDTD modeling

The interaction between electromagnetic radiation and AuFoN was simulated using the Ansys Lumerical FDTD software. We simulated a monolayer of polystyrene spheres arranged in a close-packed hexagonal structure, on a silica substrate (Figs. 1(d) and 1(e)). A finite array composed of 90 dielectric spheres was simulated. The monolayer was covered with a gold film. As expected, the gold would not only cover the spheres, but would also fall in the spaces between the spheres. Material properties were used from Palik for SiO₂ (glass) and from Johnson and Christy (1972) for Au (Gold). Refractive indexes of 1.59, 1 and 1.333 were used for polystyrene, air and, respectively, water. The Au film deposited on top of the polystyrene spheres was represented as ellipsoidal caps. The definition of the ellipsoidal caps was based on two parameters: dtF and dzF, representing, as percentages with respect to the Au film thickness, the increase in ellipsoid radius in the x- and y- directions compared to the sphere radius and, respectively, the vertical upward shift of the ellipsoid center. The Au below the plane of the spheres’ centers is then removed. In simulations presented here we use dtF=40 and dzF=50. Perfectly matched layer (PML) boundary conditions were used for all boundaries. A mesh accuracy factor of 6 was used and a conformal variant 1 mesh refinement. Through the AuFoN and the surrounding area (50 nm above and below), a finer mesh was defined, with a spatial resolution equal to D/100, where D is the diameter of the spheres. A plane wave source, linearly polarized along the x direction was used, of wavelength between 450 nm and 1000 nm. For sphere diameters above 500 nm, additional simulations for the 1000 nm - 2000 nm spectral range were performed. Simulations were performed for both air and water media, spanning sphere diameters between 200 nm and 1000 nm, every 100 nm, and Au film thickness between 40 nm and 200 nm, every 20 nm.

 

Fig. 1. (a) SEM and (b) AFM images of AuFoN; (c) measured and modeled transmittance spectra of AuFoN made of 50 nm Au on 500 nm spheres, bare colloidal crystals, and a flat Au film; (d) snapshot of the simulation setup in Lumerical; (e) schematic representation of the simulation setup in x-y, x-z and y-z transects; (f) measured and modeled reflectance spectra for 50 nm Au on 500 nm spheres.

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2.2 Sample fabrication and characterization

Arrays of polystyrene microspheres were prepared by convective self-assembly from aqueous colloidal suspensions, as previously described [14]. These were then coated by gold films, through thermal evaporation or sputtering under vacuum. Transmittance (T) measurements were performed at normal incidence in unpolarized light, with a compact spectrometer (Ocean Optics) coupled by optical fiber to an optical microscope. Reflectance (R) was measured at quasi-normal incidence on a Jasco V-530 spectrophotometer equipped with a SLM-468S reflectivity extension. Scanning electron microscopy images were obtained by a JEOL JSM 6700F. AFM measurements were performed on a WITec alpha system.

3. Results and discussion

3.1 Optical properties of AuFoN

Figure 1(a) and 1(b) presents the typical morphology of AuFoN fabricated by convective self-assembly, followed by metal coating through thermal evaporation. The close-packed hexagonal arrangement of the spheres is clearly visible, as well as some deviations and imperfections. The structure used for the modeling is presented in Figs. 1(d) and 1(e). Our choice of a finite array of about 90 spheres is not very common, and until few years was probably difficult to handle by usual desktop computers. We propose that it is a better suited option than the periodic boundary conditions typically used, because the periphery of this finite structure takes the role of imperfections that exist in the real AuFoN (Fig. 1(a)). Consequently we found this model to better capture the experimentally observed features. In Supplement 1, Fig. S1 presents a comparison between a transmittance spectrum simulated with a unit cell and periodic boundary conditions and the spectrum simulated with the finite array. In Supplement 1, Fig. S2, the electric field distributions in the two cases are also presented: in the center of the finite array the fields are identical to the periodic case, while they are strongly distorted on the edge of our finite array.

Figure 1(c) shows a comparison between measured and simulated transmittance spectra for AuFoN made of 50 nm Au on 500 nm polystyrene spheres, together with the bare colloidal crystal (CC) made of 500 nm spheres and a flat Au film 50 nm thick. There is a good agreement between the two datasets, the main features of the measured data being well captured by the model. A vertical shift exists between modeled and measured data, which may be at least partly due to measurement calibration. The main feature in the transmission of bare sphere arrays is the transmission stop-band, observed around 610 nm, which appears due to coupling of incident light into eigenmodes of the array, i.e. light travelling along the chains of spheres. For the AuFoN, the main feature of the transmission spectrum is the transmission band, observed in the range 625–725 nm, associated to the EOT phenomenon, as discussed previously [3,10]. A comparison for typical reflectance spectra is shown in Fig. 1(f) for AuFoN with 50 nm Au on 500 nm spheres. The spectra are again very similar, with the model being able to reproduce all the features (local minima and maxima) observed experimentally. The main feature of the reflectance spectrum is a broad reflectance dip, with the minimum in the 700–800 nm region. The position of this minimum in the modeling result is strongly influenced by the choice of some fine structural parameters involved in the model definition of the Au film on top of the spheres, and will be discussed later. The very good overall matching between simulation and experiment supports the validity of the model, making it thus appropriate for further studies and analyses aiming at understanding the role of different structural parameters and for optimization of the plasmon-induced enhancement of optical processes (e.g. Raman scattering in SERS of emission in SEF) by AuFoN. The AuFoN structure is a rather complex one, the system being composed of three parts: the dielectric sphere array, the metal film on top, and an array of triangular metal particles formed on the substrate. Therefore, some questions can arise concerning the individual role played by each of these three arrays. Additional simulations with either triangular metal particles removed (Supplement 1, Fig. S3) or spheres removed (Supplement 1, Fig. S4), evidenced the following main conclusions: the role played by the triangular particles on the overall optical response is negligible, while the sphere arrays plays a major role in the transmission but a minor one on the reflectance spectra. For thin films (e.g. 50 nm) the effect on reflectance is stronger than for thick films (e.g. 200 nm).

We further analyze into more detail the modeling results and discuss the behavior exhibited by AuFoN in water. Figure 2 shows the far-field and near-field optical response of AuFoN with 60 nm Au on 500 nm spheres, in both air and water. As it can be observed the broad reflectance dip is shifted with respect to the transmission band, therefore, a main absorbance band is usually observed in the same spectral region as the reflectance dip (indicated by red arrows in Figs. 2(a) and 2(d)). The reflectance and absorbance spectra are red-shifted, about 210 nm, when moving from air to water. The transmission spectrum on the other hand, is only slightly affected, the EOT transmission band peak shifts by 30 nm, but is much less pronounced in water compared to air. The maximum absorbance is attributed to excitation of surface plasmons. This can be seen by looking at the simulated electric field distributions near the AuFoN surface. Strong electric fields are found in the junctions between two adjacent Au caps. These fields have exceptionally high values at the maximum absorbance wavelength compared to an off-maximum wavelength, located 200 nm to the red of the maximum, for both air and water (Figs. 2(b), 2(c), 2(e) and 2(f)). Based on these results, and taking into account knowledge from previous published experimental studies on SERS or SEF optimization of AgFoN [15,25], we can conclude that the absorbance band is a good indicator of the wavelengths at which these AuFoN will be efficient as substrates for plasmon-enhanced spectroscopy, such as SERS or SEF. Besides the redshift, when placing the AuFoN in water, the absorbance band is also broadened, which apparently expands the spectral range of field enhancement, as suggested by comparing pairs of Figs. 2(b)-(c) and 2(e)-(f). The experimental behavior of AuFoN in water, and the good agreement between simulation and experiment in water are presented in Supplement 1, Fig. S5.

 

Fig. 2. (a) Transmittance, reflectance and absorbance of 60 nm Au on 500 nm spheres in air; (b) electric field magnitude at the maximum absorbance wavelength (indicated by the red arrow in a), and (c) electric field magnitude at a wavelength 200 nm away from the maximum absorbance (indicated by the black arrow in a); (d), (e), (f) similarly, in water.

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3.2 Dependence of the optical response of AuFoN on structural parameters

Figure 3 illustrates the dependence of the optical properties of AuFoN on Au film thickness, showing the transmittance, reflectance and absorbance of AuFoN of 500 nm spheres and Au film thickness between 40 nm and 200 nm, in both air and water. The spectra for a bare CC, and for a flat film of 40 nm, 80 nm and 120 nm are shown as references. AuFoN exhibit a transmission dip at the same wavelengths as the bare CC, followed by a region of higher transmittance that has been associated to EOT. This transmittance band can be observed for the 40 nm and 80 nm Au film thickness, while for larger film thickness the transmittance becomes very low. Compared to the reflectance of a flat gold film, abroad reflectance dip is observed for the entire thickness range. The wavelength of its minimum, and that of the absorbance maximum decrease as the Au thickness increases, this decrease being faster for thinner films and much slower for the thicker Au films. Interesting, the absorbance value at the maximum remains rather constant, with only small fluctuations near 0.9. Same observations are valid for both air and water. Concerning the differences between the air and water cases, one can observe that for the air case the absorbance maximum position changes by 230 nm form the 40 nm to the 200 nm Au film, while in water this change is of 280 nm.

 

Fig. 3. Transmittance, reflectance, and absorbance spectra of AuFoN made of 500 nm spheres and Au films with thickness between 40 and 200 nm (solid lines), bare CC (gray dashed line) and flat Au film with thicknesses between 40 and 120 nm (dotted lines): (a), (c), and (e) in air; (b), (d), and (f) in water.

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Figure 4 shows the dependence of transmittance, reflectance and absorbance on sphere size, for a fixed Au film thickness of 60 nm, in both air and water. As the sphere size increases, most optical features become more red-shifted, moving into the IR region. A broadening of the absorbance band is observed as bands move towards the IR, accompanied by a decrease of the absorbance value at the maximum, from 0.92 at 645 nm to 0.78 at 1340 nm, going from 400 nm to 1000 nm spheres. The behavior is similar for AuFoN in water environment.

 

Fig. 4. Transmittance, reflectance, and absorbance spectra of AuFoN made of spheres with diameters between 200 and 1000 nm coated by60 nm thick Au film: (a), (c), and (e) in air; (b), (d), and (f) in water.

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To give an overview, the position of the main absorbance peak of AuFoN was mapped as a function of sphere diameter and Au film thickness. This is plotted in Fig. 5, for both air and water. The plots show that the features identified so far – blue-shift of the main absorbance maximum with increasing Au thickness, red-shift of the maximum with increasing sphere size, and red-shift when moving from air to water – are consistent across our set of simulations. This information can be used when designing SERS applications in air or in microfluidics. The large absorbance linked to the excitation of surface plasmons will bring a larger SERS enhancement factor. The laser used for excitation needs to match the plasmon band of the SERS substrate. Even more specifically, some studies have shown that SERS enhancements are maximized when the plasmon band of the substrate is positioned between the frequency of the excitation laser and that of the analyzed Raman bands, such that both the incident and Raman-scattered photons can be enhanced [26]. For a given setup available in a laboratory (e.g. laser wavelength), suitable parameters can be chosen in terms of Au thickness and sphere size in order to maximize the SERS enhancement. It can also be observed that the most widely-encountered laser wavelengths used for SERS on gold surfaces, 633 nm and 785 nm (marked by dash lines in Fig. 5) can be effectively targeted: spheres in the range 300–700 nm for air applications, and spheres in the range 230–500 nm for microfluidic (water) applications, across the whole thickness range investigated.

 

Fig. 5. The wavelength of the main absorbance maximum, as a function of sphere diameter and Au film thickness in (a) air and (b) water. Structures for which there is no apparent maximum have been masked out.

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3.3 Secondary absorbance band

In addition to the main absorption peak analyzed so far, a very interesting finding of this work concerns the observation of a secondary absorption maximum, at shorter wavelengths than the main absorption maximum. This secondary absorbance band becomes visible for larger sphere sizes (above 800 nm in air and 600 nm in water) and larger Au film thicknesses (above 160 nm air and 140 nm in water). It can be seen in Fig. 6, where the absorbance in air for AuFoN made of 200 nm Au on 800–1000 nm spheres is plotted. This secondary absorbance maximum also exhibits the same dependence on sphere size, i.e. it redshifts with sphere size increase. The inset in Fig. 6 shows a clear correlation between the spectral positions of the main and secondary absorbance bands across the whole range of simulations.

 

Fig. 6. Absorbance spectra of AuFoN made of spheres with diameters between 800 and 1000 nm and 200 nm Au film in air. The arrows indicate the wavelengths of the main absorption maximum ($\mkern5.2mu\raisebox{-.3pt}{\rotatebox{20}{\hbox{$\bar{}$}}}\mkern-9.45mu\lambda_1$, black arrow) and secondary absorption maximum ($\mkern5.2mu\raisebox{-.3pt}{\rotatebox{20}{\hbox{$\bar{}$}}}\mkern-9.45mu\lambda_2$, red arrow) for the 1000 nm spheres, for which electric fields are shown in Fig. 7. Inset shows a scatterplot of $\mkern5.2mu\raisebox{-.3pt}{\rotatebox{20}{\hbox{$\bar{}$}}}\mkern-9.45mu\lambda_2$ against $\mkern5.2mu\raisebox{-.3pt}{\rotatebox{20}{\hbox{$\bar{}$}}}\mkern-10.55mu\lambda_1$, for all simulations in which a secondary absorption peak was identified; black line shows a linear fit through this data (slope=0.8, intercept=−88 nm).

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To better understand the nature of this secondary absorbance maximum, Fig. 7 shows the electric field distributions at the two absorbance maxima for 200 nm Au on 1000 nm spheres. By analyzing the electric field magnitude E, one can notice that, while the electric field at the main absorbance band exhibits two lobes, it exhibits four lobes at the secondary absorbance band. This distribution indicates the presence of multipolar surface plasmon modes at the secondary absorbance. By further looking at the E_X and E_Z components, we find that the high values of the electric field at the junctions between two adjacent Au caps are mainly x-polarized, while the high fields positioned higher on top of the spherical caps are due to fields along the z axis. For the secondary absorbance maximum, the components along the x and z directions are better separated, leading to distinct field distributions.

 

Fig. 7. (a) E; (b) E_X; (c) E_Z at the main absorbance maximum (black arrow in Fig. 6), for 1000 nm spheres coated by 200 nm Au, in air; (d) E; (e) E_X; (f) E_Z at the secondary absorbance maximum (red arrow in Fig. 6) for 200 nm Au on 1000 nm spheres, in air.

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A further question concerning the secondary absorbance peak is whether the dielectric sphere arrays underneath the metal film plays a role. To answer this we performed an additional simulation on the AuFoN made of 200 nm Au on 1000 nm spheres, in which the spheres were removed. Results, displayed on Fig. 8, indicate that the sphere arrays doesn't play a role, thus both absorbance maxima are determined by the interconnected semi-shell morphology of the Au film.

 

Fig. 8. Absorbance spectra of AuFoN made of 1000 nm spheres coated by 200 nm Au film, and Au structure only, with polystyrene spheres removed.

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The existence of several absorbance peaks has not been shown in the literature before. However we suspect that in some studies authors may have analyzed it or exploited it in SERS, but without having an overview of the optical response over a broad spectral range. Our findings, discussed above, on the secondary absorbance band can explain some apparently contradicting experimental results in the literature. As example, Styles et al. [27] found that the absorbance band of AgFoN made of 540 nm spheres coated by 220 nm Ag peaks at about 575 nm, which is consistent with the behavior we observed for the main absorbance band. On the other hand, Lin et al. [28] found a minimum reflectance (equivalent to maximum absorbance) at about 550 nm for 1000 nm spheres coated by 120 nm Ag film. In light of our results discussed above, we believe that in this case the authors were analyzing a secondary absorbance band, or even a higher-order multipolar mode. The electric field distribution found in their analyses, having a structured nature that better resembles Fig. 7(d) than Fig. 7(a), also supports this claim.

3.4 Tuning the optical response of MFoN by film morphology and metal type

Next we analyze the effect of changing some parameters defined in the construction of the Au semi-shell caps, dtF and dzF, described in Section 2.1. By increasing dtF or dzF, the angle formed between two Au caps at their junction increases, as well as the amount of Au deposited in that region between two caps. This has the potential to impact the transmittance, reflectance and absorbance spectra. Results are presented in Fig. 9 for a few selected representative cases, for an AuFoN made of 50 nm Au on 500 nm spheres. The transmittance is only slightly affected, showing some minor intensity and spectral shape modifications. Interestingly, the effect on the reflectance, and consequently on the absorbance spectra is quite significant. While the evolution of the reflectance below 650 nm remains largely similar, the minimum in reflectance, corresponding to maximum absorbance, strongly shifts with changing parameters, ranging between 730 nm for dtF=40, dzF=100 to about 950 nm for dtF=20, dzF=50. Our interpretation of these observations is given in the following. The sphere array has a main role in the enhanced transmission peak, where the fields are distributed inside the spheres, below the metal caps (Supplement 1, Fig. S2), thus are not sensitive to the fine morphology details on the top of the metal film. Therefore, the transmission is not strongly sensitive to those morphology changes. The fields at the maximum absorbance on the other hand are located exactly where those morphology changes occur, at the regions where neighbor metal caps meet (Supplement 1, Fig. S6). The field distributions show that, for the different cases analyzed in Fig. 9, the fields are located mostly above the metal film or can penetrate more or less below the film. The spatial extension of the enhanced fields is also different, with more localized fields for the sharper metal caps intersection. It is then no surprise that these effects are accompanied by a wavelength change, as observed.

 

Fig. 9. (a) Schematic representation of two adjacent spheres covered with Au film, for different pairs of parameters dtF and dzF; (b) transmittance, (c) reflectance and (b) absorbance for 500 nm spheres coated by 50 nm Au, for different pairs of parameters dtF and dzF.

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This result has a certain practical importance, as it points to an interesting alternative for controlling the optical response of the MFoN, by adjusting the fine morphology of the metal film. As example, thermal evaporation and magnetron sputtering offer different degree of directionality of the deposition, so the proposed approach is practically feasible. The suitable set of modeling parameters to best describe the MFoN structure may therefore be dependent on the deposition technique. These observations also explain why simulated transmittance matches much better the experimental data than the reflectance does. Further investigation would be needed in this respect to compare deposition techniques in terms of the fine morphology of the Au caps and its impact on optical response.

Another parameter that might be used to tune the optical response of MFoN is the choice of metal of the deposited film. The response of MFoN using several frequently used metals were simulated. In Fig. 10, the absorbance of AuFoN is compared to that of AgFoN, AlFoN and CuFoN for a 200 nm film deposited on 1000 nm spheres. The existence of two maxima identified so far for AuFoN is visible irrespective of the metal chosen. Moreover, the position of the two maxima remain in the same range. The choice of metal offers some degree of tunability mainly for the main absorbance band, which can be shifted by tens of nm, with negligible shifts for the secondary maximum.

 

Fig. 10. Absorbance of MFoN made of 1000 nm spheres coated by 200 nm thick film, by using different metals for the film composition: gold (Au), silver (Ag), copper (Cu) and aluminum (Al).

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4. Conclusions

The dependence of the optical properties of AuFoN on sphere diameter and Au film thickness were analyzed in both air and water environments. We found that the main absorbance peak shifts by varying these parameters across the visible and the near-IR spectral range. A blue shift of the main absorbance band is found when increasing the Au film thickness, and a red-shift when increasing sphere diameter. A red-shift and slight broadening of the absorbance band is also found when changing the medium from air to water. These results can be used to tune the plasmon-induced absorbance of AuFoN in order to maximize the performance of the desired spectroscopy-based sensing application, SERS or SEF, exploiting common lasers in the visible range. We further showed that the triangular particles on the substrate have a negligible influence on the overall response, while the role of the photonic colloidal crystal is crucial for transmittance and only minor for reflectance. For larger spheres and thicker films, which have the main absorbance band towards the IR, a secondary absorbance band, emerging at shorter wavelengths than the main absorbance band, was identified. This absorbance band, of multipolar nature, can be tuned across the visible range, and has a distinct electric field distribution compared to the main absorbance band, which might be exploited in both fundamental studies and sensing applications.

We have also determined that fine details of the film morphology, namely the metal deposited at the junction between two neighbor spheres, has a strong impact on the main absorbance band spectral position: the sharper the angle formed by the two touching metal caps (implying also less metal deposited in this area) the more redshifted the plasmon band is. The metal type can also modify the plasmon resonance, mostly for the main absorbance band, but not for the secondary one. These results contribute to a better overall understanding of the optical / plasmonic properties of MFoN, their dependence on sphere size (200 - 1000 nm) and film thickness (40–200 nm) across a wide spectral range (450–2000 nm), and their behavior in aqueous environment, useful for developing microfluidic SERS applications.