Enhancement of Light Transmission through Silver Nanoparticles

Article pubs.acs.org/JPCC

Enhancement of Light Transmission through Silver Nanoparticles R. Brenier* Université de Lyon, F-69000; Univ. Lyon1, Laboratoire PMCN; CNRS, UMR 5586; F69622 Villeurbanne Cedex, France ABSTRACT: This article reports experiments showing that the transmittance of silver nanoparticles on ZrO2-coated silica substrates is enhanced by functionalization of the silver surface with a self-assembled monolayer of perfluorodecanethiol. The transmittance can become surprisingly higher than that of the oxide substrate over a wide wavelength range (300−680 nm). The maximum of transmittance can even reach that of the naked silica. This enhancement is against what is expected from the usual modification of the extinction cross-section of noble metal nanoparticles upon coating with a dielectric shell. Large decrease of both reflectance and absorptance are related to the increase of transmittance. Conversely, the transmittance of silver nanoparticles on silica substrates, without ZrO2, decreases upon silver functionalization, in a classical way. An effective medium description of the functionalized silver nanoparticles is presented. This approach, still to be improved, reveals the major role of the interferences of light in the nanoparticle films, in the phenomenon of transmittance enhancement. The respective effects of light scattering by the nanoparticles, of the silver functionalization and of ZrO2 are discussed.

1. INTRODUCTION The phenomenon of extraordinary optical transmission was first reported by Ebbesen et al.1 for a flat silver film containing a periodic array (square lattice) of subwavelength holes, deposited on a quartz substrate. For such an optically thick film, the transmission spectrum, at normal incidence, is composed of a series of maxima separated by minima of transmittance, in the visible and near-infrared ranges. The transmittance of some maxima is greater than expected from the area of the holes struck by light, and this comparison defines the enhancement of transmittance. The basic mechanism proposed by Ebbesen et al.2,3 involves diffraction of light by the array of holes and coupling to surface plasmon polariton modes (SPP). Further studies have shown that enhanced optical transmission could be obtained through periodic4 or aperiodic5,6 arrays of holes with rotational symmetries, as well. Another crucial series of research has focused on the role of the hole shape7 suggesting the occurrence of resonances linked to the holes themselves, different from SPP modes. So, situations where no SPP could be involved were studied: randomly distributed holes6,8,9 or single hole in a metal.10 Silver layers without any hole, but optically thin and bearing a grating11 or randomly distributed nanoparticles,12 also are examples of systems exhibiting enhanced light transmission. In the fields of plasmonics and optoelectronics,13 silver nanoparticles have aroused much interest since several decades. The reasons lie in their high efficiency to interact with light and in their tunable optical properties. The large electromagnetic fields occurring near their surfaces have strong implications in nano-optics.14 These features are due to the localized surface plasmons (LSP) of the individual nanoparticles. When the © 2012 American Chemical Society

nanoparticles are regularly spaced in a monolayer, far-field dipolar interactions can lead to new collective optical effects and narrow plasmon resonances have been observed.15 Nanoparticles or nanoshells arranged in dense periodic arrays16 are submitted to near-field interactions, and high order modes control the optical properties. In this field, studies are scarce, but recent calculations17 have shown that such monolayers can support various types of LSP, void-like and sphere-like with poor quality factors, and delocalized Bragg-like plasmons with higher quality factors. Owing to the presence of pores in the lattice allowing for light tunnelling, these modes are responsible for multiple transmission resonances. Experimentally, some studies have been devoted to the identification of plasmon modes by reflectivity measurements in metallic nanovoid18,19 or hollow sphere20 arrays. Owing to the versatility of the collective properties of the noble metal nanoparticles, a wide range of promising optical applications is possible. In this way, arrays of gold nanospheres can be used in metamaterials exhibiting electromagnetically induced transparency.21 Recently, Kravets et al.22 have elaborated and studied a metamaterial made of randomly distributed silver nanoparticles in a dielectric matrix with strong absorption of light, useful for energy conversion. Very recently, we have studied the possibility of making bifunctional surfaces bearing both superhydrophobic and plasmonic properties.23 First, a film of disordered silver nanoparticles was elaborated from the method of oxideinitiated silver nanoparticle generation.24 In a series of Received: October 28, 2011 Revised: December 20, 2011 Published: January 30, 2012 5358

dx.doi.org/10.1021/jp210374j | J. Phys. Chem. C 2012, 116, 5358−5366

The Journal of Physical Chemistry C

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ethanol−heptane (2/3:1/3 v/v) for the second one, mixing ethanol−heptane (1/3:2/3 v/v) for the third one, and pure heptane for the fourth one. Finally, the samples were dried under the laboratory hood and immediately introduced in a Lambda900 spectrophotometer from Perkin-Elmer for measuring its transmittance and reflectance with an incident angle of 4°. After optical analysis, the effect of pure heptane was tested on each sample. The samples were immersed in a bottle containing heptane for one hour, dried, and analyzed by spectrophotometry. These operations were repeated until obtaining stable optical properties. Afterward, the silver nanoparticles were functionalized by grafting a self-assembled-monolayer of 1H,1H,2H,2H-perfluorodecanethiol (PFDT). For this operation, the samples were immersed in a bottle containing PFDT in heptane (2 mM) for the desired time, then rinsed in pure heptane, and finally dried. Silver films were also simultaneously deposited on one face of a series of naked and ZrO2-coated silica substrates by electron-gun evaporation, under a base vacuum of 10−4 Pa. The film thickness was 25 nm, determined by profilometry. The transmittance and reflectance spectra of these samples were measured before and after silver functionalization with PFDT SAM.

experiments, the substrate was ZrO2-coated Pyrex. Second, a self-assembled monolayer of perfluorodecanethiol (PFDT SAM) was grafted on the surface of the nanoparticles as a low surface energy material. These surfaces were characterized by atomic force microscopy (AFM), which revealed that most of the nanoparticle sizes were between 40 and 80 nm. The optical properties were measured by spectrophotometry. The reflectance was found slightly lower than that of naked Pyrex around the wavelength λ = 400 nm. This intriguing optical effect suggested promising enhancement of light transmission associated with antireflectivity. The aim of the present article is, then, to report a more comprehensive study of these optical properties. In the present work, the same conditions as in our previous article23 were used for generating the nanoparticles, but Pyrex was replaced by silica substrates in order to extend the optical measurements in the UV range. First, as PFDT SAM grafting takes place in heptane, the stability of the nanoparticle films in this solvent has been carefully studied as a function of the immersion time. Second, the optical properties of the samples have been measured for different generation times of the nanoparticles, between 8 and 24 h, as a function of the duration of SAM grafting. Then, the effect of the ZrO2 layers was tested by comparing the optical spectra evolutions upon SAM grafting of samples with and without ZrO2. In a third part, the silver nanoparticle films were replaced by a continuous and optically thin (25 nm) silver film obtained by electron-gun evaporation, in order to clarify the role of the nanoparticles. Finally, it will be shown that precious clues on the physical mechanisms at work in our experiments can be obtained from simulations of light interferences in the multilayers, considering the silver nanoparticle films as effective media.

3. RESULTS AND DISCUSSION 3.1. Stabilization of Silver Films in Pure Heptane. The transmittance spectrum of the sample obtained after generation of silver nanoparticles on silica substrate for 17 days is depicted in Figure 1 (spectrum 1). The dip of transmittance at λ = 243

2. EXPERIMENTAL SECTION Silica substrates of dimensions 7.5 × 2.5 × 0.2 cm3 were used. Some were coated with ZrO2 layers by sol−gel. According to Urlacher et al.,25 the sol was elaborated from the mixing of Zr n-propoxide 70% solution in propanol, isopropanol, and acetylacetone. The substrates were dipped in the sol and withdrawn at the speed of 8 cm/minute. After drying at 100 °C in air for 10 min, the coated substrates were annealed in a furnace under oxygen atmosphere. They were directly introduced for 30 min into the furnace preheated at 400 °C, then cooled down. The thicknesses of the ZrO2 films determined by profilometry were of 60 nm. Then, the operations of nanoparticle generation, spectrophotometry measurements, silver stabilization in pure heptane, and functionalization with PFDT SAM were performed under minimum loss of time in order to minimize possible silver oxidation. The formation of the silver nanoparticles on the two opposite faces of the substrates was achieved exactly as in our previous publication.23 Briefly, ethanolic solutions with 5 mM AgNO3 were prepared. The substrates were soaked in a bottle of solution, which was placed in an electric oven preheated at the temperature of 32 °C for the chosen generation time. This time was 8, 13, or 24 h for the ZrO2-coated silica substrates. The naked silica substrates had to be treated for the much longer time of 17 days because silica is an oxide much less efficient than ZrO2 for promoting Ag+ reduction.24 For rinsing, the substrates covered with silver nanoparticles were transferred (without drying) from the elaboration solution successively into four bottles containing pure ethanol for the first one, mixing

Figure 1. Transmittance spectra of (s) naked silica substrate, (1) silver nanoparticles generated for 17 days on silica substrate, and then, (2) stabilized by immersion for 2 h in pure heptane.

nm is due to the naked substrate (spectrum s). The maximum of transmittance at λ ≈ 325 nm corresponds to the nearly vanishing of the dielectric function of bulk silver. The dip at λ = 407 nm is to be attributed to the dipolar plasmon resonance of the nanoparticles in mutual interactions and interacting with the substrate. After immersion of the sample in pure heptane for successive durations of one hour, the plasmon resonance is blue-shifted by Δλ, and the transmittance at the resonance is increased. These two effects rapidly saturate with the immersion time, and after 2 h, the stable spectrum 2 is obtained (Δλ = −12 nm). The interpretation of the transmittance evolution is straightforward: immersion in heptane removes a fraction of the silver nanoparticles from the sample. To fix ideas, within the approximated framework of 5359



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Figure 2. (A) Transmittance and (B) reflectance spectra of (0) 56.6 nm thick ZrO2 films on both sides of the silica substrate, (1) silver nanoparticles generated for 24 h on the oxide, and then, (2) stabilized by immersion for 2 h in pure heptane.

a dilute colloidal solution,26 the nanoparticle loss can be estimated to about 30%. The reflectance of these samples (not shown) is closed (hardly lower) than that of the naked silica substrate (spectrum s in Figure 4B) all over the wavelength range. A nanoparticle loss during immersion in heptane is also evident from the transmittance spectra of all the samples with silver nanoparticles generated on ZrO2-coated silica substrates. In these cases, the phenomenon saturates for an immersion time of two hours, as well. For example, in Figure 2A,B, respectively, the transmittance and the reflectance of a sample containing silver nanoparticles generated for 24 h on the ZrO2 layers are presented, before (spectra 1) and after immersion for two hours in heptane (spectra 2). The transmittance and reflectance spectra of the ZrO2-coated silica substrate (spectra 0) exhibit, respectively, a peak and a dip at λ = 260 nm coming from the Fabry−Perot (FP) resonance the ZrO2 layers. In Figure 2A, both the transmittance spectra 1 and 2 contain an unresolved double peak around λ = 300 nm. The low wavelength component of this peak is the FP resonance of the ZrO2 layers, and the high wavelength component corresponds to the vanishing of the dielectric function of bulk silver. In the wavelength range 330−650 nm, around the plasmon resonance, both the transmittance and the reflectance increase after the sample immersion in heptane, and the nanoparticle loss can roughly be estimated to 14%. So, the nanoparticles are more strongly linked on ZrO2 than on silica. As for the samples made of a silver film on silica or on ZrO2coated silica substrates from electron-gun evaporation, the effect of immersion in heptane for successive durations of one hour is negligible on both transmittance and reflectance. 3.2. Optical Effects of Silver Functionalization by PFDT SAM. In this section, all the samples have been immersed in pure heptane for two hours in order to stabilize the silver nanoparticles, before spectrophotometry analysis and before silver functionalization by PFDT SAM. 3.2.1. Silver Nanoparticles on Silica. In Figure 3, the transmittance spectra of the silver nanoparticles generated for 17 days on the silica substrate are presented, before (spectrum 1) and after silver functionalization for 3 days (spectrum 2). Indeed, the optical evolution is so rapid over the first dozens of minutes that the transmittance spectrum measured after one hour is very close to the one measured after three days and was removed from Figure 3 for a better clarity. During SAM grafting, the reflectance of the sample (not shown) remains unchanged.

Figure 3. Transmittance spectra of (s) naked silica substrate, (1) silver nanoparticles generated for 17 days on silica substrate, after stabilization in heptane, and then, (2) after functionalization by PFDT SAM for 3 days.

In the UV range, a hollow appears in the transmittance spectrum at λ = 282 nm during SAM grafting. This feature, also present as a shoulder in the absorptance spectrum (not shown), corresponds to a new mode of light absorption in the films. We have verified that PFDT molecules in heptane exhibit an absorption band around λ = 230 nm (not shown) in agreement with other sulfur-containing organic compounds.27 Then, the red-shift of this band up to 282 nm is likely related to the chemical binding of PFDT molecules to silver. The peak of transmittance attributed to the vanishing of the dielectric function of bulk silver is slightly red-shifted from 325 to 331 nm by the nanoparticle functionalization. The truncation of the low wavelength side of the peak by the absorption band at λ = 282 nm likely is the reason of this effect. The modifications of the dipolar plasmon band of the silver nanoparticles upon functionalization consist in a red-shift by 11 nm of the resonance and a decrease of the transmittance at the resonance from T1 to T2 by a factor ξ = T2/T1 = 0.93. Qualitatively, these two modifications are in perfect agreement with the often reported modifications of the extinction crosssection of noble metal nanoparticles induced by a solvent, a dielectric host medium, or shell.28,29 The special case of silver nanoparticles obtained by nanosphere lithography and modified with alkanethiol SAM was carefully studied by Van Duyne et al.30 These authors have demonstrated that the induced redshift of the plasmon resonance varies linearly with the chain length of the SAM. For decanethiol, which bears a chain of the 5360



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Figure 4. (A) Transmittance, (B) reflectance, and (C) absorptance spectra of (s) naked silica substrate, (0) 56.6 nm thick ZrO2 films on both sides of the silica substrate, (1) silver nanoparticles generated for 8 h on the oxide, after stabilization in heptane, and then, (2) after functionalization by PFDT SAM for 1 h and (3) after functionalization by PFDT SAM for 3 days. (D) Transmittance, (E) reflectance, and (F) absorptance spectra of (s) naked silica substrate, (0) 56.6 nm thick ZrO2 films on both sides of the silica substrate, (1) silver nanoparticles generated for 24 h on the oxide, after stabilization in heptane, and then, (2) after functionalization by PFDT SAM for 1 h and (3) after functionalization by PFDT SAM for 3 days.

functionalization. Transmittance, reflectance, and absorptance spectra are also shown, respectively, in Figure 4D−F, for the sample with silver nanoparticles generated for 24 h. For the sample with the nanoparticle generation time of 13 h, the spectra are not shown because they are intermediate. The bandgap of ZrO2 is responsible for the large drop of transmittance and increase of absorptance for wavelengths lower than 230 nm. A slight hollow in transmittance and a shoulder in absorptance spectra at λ = 243 nm (spectra 1) are due to silica (spectra s).

same length as PFDT, a red-shift by 19 nm was measured. The refractive index of PFDT (1.33),31 lower than that of decanethiol (1.42), could explain that only 11 nm was found in our case. Nevertheless, no variation of extinction crosssection (and so, of transmittance) was reported by Van Duyne et al.,30 unlike us. 3.2.2. Silver Nanoparticles Generated on ZrO2-Coated Silica. Transmittance T, reflectance R, and absorptance A = 1 − T − R are depicted as a function of the wavelength λ, respectively, in Figure 4A−C, for the sample with silver nanoparticles generated for 8 h, after different durations of 5361



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In the UV range, the absorption band at λ = 282 nm, due to the chemical binding between PFDT and silver as for the nanoparticles on silica, progressively grows in the absorptance spectra during SAM grafting, going with a profound dip in the transmittance spectra, whatever the generation time of the nanoparticles. However, no related feature can be detected in the reflectance spectra. After nanoparticle functionalization, the FP resonance of the ZrO2 layers is still present as a dip in the reflectance spectra 2 and 3 around λ = 265 nm. Unfortunately, the corresponding peak of transmittance is hidden by the strong absorption band at λ = 282 nm. The peak of transmittance in spectra 3 of Figure 4A (λ = 330 nm) and D (λ = 340 nm) comes from the vanishing of the dielectric function of bulk silver. As for the nanoparticles on silica (see Figure 3), the low wavelength side of the peak is likely truncated by the absorption band at λ = 282 nm. This peak identification will be confirmed by the optical simulation in section 3.3. A noticeable point is that the transmittance in the presence of the grafted nanoparticles can be higher than that of the naked ZrO2-coated silica substrate. For the silver generation time of 8 h, that occurs over the wide wavelength range 300− 680 nm. The maximum value of the transmittance close to λ = 330 nm even quasi-reaches the transmittance of naked silica (spectrum s). Upon PFDT SAM grafting, the reflectance (spectra 2 and 3 in Figure 4B,E) decreases all over the wavelength range. Around λ = 300 nm, the reflectance becomes rather low, lower than that of naked silica. The increase of transmittance upon silver nanoparticle coating by a dielectric, over the near UV and visible ranges including the plasmon band, have never been reported to the best of our knowledge and constitutes the main result of this article. The role of the ZrO2 intermediate films has been proved crucial in the related behaviors of the transmittance, reflectance, and absorptance of our multilayers upon the silver functionalization by PFDT SAM. Let us now test the role of the silver film structure itself just by replacing the nanoparticles by a continuous and optically thin film. 3.2.3. Continuous Silver Films Deposited on Silica and on ZrO2-Coated Silica. The reflectance spectra of a 25 nm thick silver film deposited by electron-gun evaporation on a ZrO2coated silica substrate are given in Figure 5, before (spectrum 1) and after (spectrum 2) functionalization by PFDT SAM for 3 days. Spectra 3 and 4 are the respective transmittance spectra. No evidence of any absorption band at λ = 282 nm due to the chemical binding between PFDT and silver can be detected. The reason certainly lies in the surface density of PFDT molecules much weaker on this flat silver surface than on the 3dimensional surface of the silver nanoparticle films studied in the previous sections. Indeed, the transmittance is quasi-unchanged and the reflectance very slightly decreased by the PFDT functionalization. We have found a similar behavior (not shown) for the same silver film on silica without ZrO2. These behaviors are in good agreement with optical simulations of multiple interferences of light in the multilayers, considering that silver is coated with a dielectric layer of thickness 1.7 nm and refractive index 1.33.31 Therefore, even with the presence of ZrO2, the transmittance cannot be increased by the functionalization if the silver film is not nanostructured. This conclusion agrees with the results reported by Wang et al.12 These authors have shown that the transmittance of ZnO/Ag/ZnO multilayers can be enhanced

Let us first consider the effect of the nanoparticle generation time on the different spectra prior to silver functionalization (spectra 1 in all the windows of Figure 4). The generation of the silver nanoparticles on the oxide induces a decrease of the sample transmittance and an increase of its absorptance all over the wavelength range, whatever its duration. The reflectance is mainly decreased by nanoparticle generation between 250 nm and a limit around 500 nm depending on the generation time, while it is increased for wavelengths greater than this limit. The amplitude of all these spectra variations increases with the generation time of the silver nanoparticles, as expected from the increase of the surface density of these latter ones. After the silver generation time of 8 h, the peak of transmittance at λ = 265 nm (and the corresponding dip of reflectance) is the FP resonance of the ZrO2 layers. After the silver generation time of 24 h, this latter peak is still present but shouldered on its high wavelength side by the peak due to the vanishing of the dielectric function of bulk silver. The local minimum of transmittance and maximum of absorptance between 400 and 500 nm roughly locate the plasmon resonance of the silver nanoparticles in the situation of multiple interferences of light. These two extrema do not coincide since the slope of the reflectance spectrum is different from zero within this wavelength range. Nevertheless, with respect to the nanoparticles on silica, the minimum of transmittance in the visible range is red-shifted by 40 and 56 nm for the nanoparticle generated on ZrO2 for, respectively, 8 and 24 h. These large shifts are mainly to be attributed to the so-called substrate effect due to an induced charge distribution in the substrate and was widely studied for isolated nanoparticles.32,30,28 In this latter case, the red-shift of the plasmon resonance increases with the refractive index of the substrate (about 1.47 and 1.9 for silica and ZrO2,33 respectively), as observed in our case. Consider now the effects of PFDT SAM grafting of the silver surface. The optical properties of the samples first strongly change after silver functionalization by PFDT SAM for 1 h, whatever the generation time of the nanoparticles (spectra 2 in Figure 4). Then, the evolution goes on slower and can be considered completed after 3 days (spectra 3 in Figure 4). The slow progress of the optical properties is linked to the diffusion of the PFDT molecules inside the porous structure of the silver nanoparticle films. The substantial evolution between 1 h and 3 days, even for the lowest generation time of 8 h, conversely to the previous case of the nanoparticles on silica, is the evidence that the silver porous films are much thicker on ZrO2 than on silica. This feature is another confirmation that ZrO2 is much more efficient for promoting silver generation than silica. For wavelengths higher than 350 nm, around the plasmon resonance, the transmittance progressively increases during SAM grafting (Figure 4A,D). Its local minimum is red-shifted by Δλ and increased by the factor ξ = T2/T1 previously defined. For the respective generation times of 8 and 24 h, Δλ = 118 nm, ξ = 1.24 and Δλ = 28 nm, ξ = 1.52, once the grafting has been completed after 3 days. Then, the factor ξ is significantly higher than 1, increasing with the generation time. In the vicinity of the plasmon resonance, the absorptance is substantially decreased between 1 h and 3 days of functionalization (Figure 4C,F). It becomes very weak in the case of the silver nanoparticles generated for 8 h, in relation with the rather high transmittance of this sample. As for the local maxima of absorptance, they are blue-shifted upon functionalization. 5362



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nanoparticles are accounted for. Nevertheless, when the nanoparticles are supported, the substrate effect is ignored. The absorption band due to Ag−PFDT binding around λ = 282 nm is not considered either. We have used the values from Johnson and Christy38 for the optical constants of bulk silver, corrected for surface dispersion effects in small nanoparticles.32 According to Doyle,35 when the nanoparticles are assembled, in the vicinity of the dipole plasmon resonance, their mutual interactions occur via dipole−dipole coupling. Then, the effective dielectric function εef of randomly distributed nanoparticles with dipolar electric polarizability α can be approximated owing to Maxwell−Garnett approach by εef = εm(α3 + 2f α)/(α3 − f α)

where εm is the dielectric constant of the host medium, and f is the volume fraction of silver, related to the density of nanoparticles N by

Figure 5. Optical spectra of a 25 nm thick silver film deposited by electron-gun evaporation on a 56.6 nm thick ZrO2 film on silica; reflectance before (1) and after (2) functionalization by PFDT SAM for 3 days; transmittance before (1) and after (2) functionalization by PFDT SAM for 3 days.

f = 4/3πa3N This model lead to successful interpretations in the literature,22,35,39 but in our case, it did not allow for satisfactory fitting of both transmittance and reflectance experimental spectra, despite the introduction of a size distribution of the nanoparticles. The main reason of this failure owes much to the fact that the substrate effect on the plasmon resonance is ignored and that the refractive index of ZrO2 is rather high. Nevertheless, the model gives useful trends presented below, allowing for a better understanding of the optical role of the ZrO2 layers and the coating of the nanoparticles with PFDT SAM. We especially focus on the unusual variations of transmittance due to SAM grafting. The calculated transmittance of a sample made of silver nanoparticles on both sides of a silica substrate is depicted in Figure 6A (spectrum 1). To fix ideas, the film thickness has been reasonably chosen to 40 nm, the nanoparticle radius to 20 nm, and the density of nanoparticles N to 2.10−7 nm−3. The filling factor is then f = 0.67%. The grafting of PFDT SAM on the silver nanoparticles is simulated by a shell of thickness 1.7 nm and refractive index 1.33 (spectrum 2) or by embedding the nanoparticles in a host medium with refractive index 1.33 (spectrum 3). In Figure 6C, the imaginary part of the refractive index Nim of the nanoparticle film (spectrum 1) shows off the evolution of the plasmon resonance due to shell coating (spectrum 2) or to embedding in the host medium (spectrum 3). In every case, the plasmon resonance coincides with a sharp dip in transmittance clearly related to light absorption. Coating the silver nanoparticles with the shell produces a red-shift of Δλ = 5 nm (experimentally, Δλ = 11 nm) and a slight decrease of transmittance at the resonance by a factor of ξ = 0.974 (experimentally, ξ = 0.930). Although a nonperfect agreement, this hypothesis of nanoparticle modification is closer to the experimental plasmon resonance evolution than the second hypothesis consisting of embedding the nanoparticles in a host. Indeed, in this latter case, a much too important red-shift by 39 nm and a significant decrease of transmittance at the resonance by a factor of ξ = 0.868 can be calculated. A crucial point is that the factor ξ is always lower than 1 as expected from the usual effect of dielectric media on the extinction cross-section of nanoparticles.29 Is it then possible to find conditions leading the factor ξ to values higher than 1 as we have experimentally observed for samples containing 56.6 nm thick ZrO2 layers (see Figure 4A

with respect to ZnO/Ag because the Ag layer was not smooth but contained a large number of silver islands. According to the authors, the silver islands work as a grating allowing for the coupling of light to the SPP modes of the silver film at the entrance and light re-emission at the exit. In our case, such a mechanism cannot be invoked for explaining the enhancement of transmittance induced by the functionalization of the silver nanoparticles by PFDT SAM. Moreover, no Bragg-like plasmon can be involved because our assemblies of nanoparticles are disordered. Indeed, this latter feature is well suited for considering the silver nanoparticle film as an effective medium in the framework of the Maxwell−Garnett approach. So, in the next section we adopt the macroscopic point of view of multiple interferences of light transmitted and reflected at each interface of the multilayered samples.22 3.3. Optical Simulations. Simulations of transmittance, reflectance, and absorptance of multilayers were performed owing to Abeles method,34 in order to better understand the above-reported experimental results. The refractive index of ZrO2 films and silica substrate could be well fitted using Cauchy’s law in good agreement with previous measurements33 (compare spectra 0 in Figures 4A and 6B, for example) on the wavelength range 300 to 800 nm. For wavelengths lower than 300 nm, no attempt was made for taking into account for the band gap of the oxides. A thickness of 56.6 nm was found for the ZrO2 films, in good agreement with the value obtained from profilometry (60 nm). All these fitted parameters were then used in the simulations of the optical properties of the oxides (spectra 0 in Figure 6) of samples containing silver nanoparticles (spectra 1 to 3 in Figure 6). The dielectric function of the silver nanoparticle films was calculated in the framework of extended Maxwell−Garnett theory.35 The dipolar electric polarizability α of an isolated nanoparticle is related to the coefficient a1 of the Mie theory36 by α = i3a1/(2k3)

where k is the wavenumber, k = 2πnma/λ, nm being the refractive index of the host medium and a the radius of the nanoparticle. For a nanoparticle coated with a shell, a1 is given by Bohren.37 Then, in this model, the effects of the electric charges induced in the host medium or the shell on the 5363



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Figure 6. Simulated optical properties. Spectra (s) refer to silica substrates, (0) to ZrO2-coated silica substrates, (1) to naked silver nanoparticles, (2) to silver nanoparticles coated with a shell of thickness 1.7 nm and refractive index 1.33, (3) to silver nanoparticles embedded in a host medium with refractive index 1.33. (A) Transmittance spectra of a sample made of 40 nm thick silver nanoparticle films on both sides of a silica substrate. The density of the nanoparticles is N = 2 × 10−7 nm−3, and their radius is a = 20 nm. (B) Transmittance spectra of a sample made of 40 nm thick silver nanoparticle films on both sides of a 56.6 nm thick ZrO2-coated silica substrate. The density of the nanoparticles is N = 2 × 10−7 nm−3, and their radius is a = 20 nm. (C) Imaginary part of the refractive index Nim of the nanoparticle film. The density of the nanoparticles is N = 2 × 10−7 nm−3, and their radius is a = 20 nm. (D) Transmittance spectra of a sample made of 160 nm thick silver nanoparticle films on both sides of a silica substrate. The density of the nanoparticles is N = 2.5 × 10−8 nm−3, and their radius is a = 40 nm. (E) Transmittance spectra of a sample made of 160 nm thick silver nanoparticle films on both sides of a 56.6 nm thick ZrO2-coated silica substrate. The density of the nanoparticles is N = 2.5 × 10−8 nm−3, and their radius is a = 40 nm. (F) Imaginary part of the refractive index Nim of the nanoparticle film. The density of the nanoparticles is N = 2.5 × 10−8 nm−3, and their radius is a = 40 nm.

and 4D)? The effect on transmittance produced by the introduction of such layers on both sides of the silica substrate, without modifying the features of the silver nanoparticle top films used in the above calculations, is shown in Figure 6B to be compared to Figure 6A. Clearly, this effect consists of an overall

decrease of the oxide transmittance (spectrum 0) and of the sample transmittances for the naked (spectrum 1), shell-coated (spectrum 2), or (to a less extent) embedded (spectrum 3) nanoparticles. The reason lies in the refractive index of ZrO2 (∼1.9) being much greater than that of silica (∼1.47), which 5364



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silica (spectrum s), as in the experiments. Therefore, the highest peaks of transmittance for wavelengths between 200 and 350 nm in the experimental spectra 2 and 3 of the functionalized Ag nanoparticles, in Figure 4A,D, do not come from the red-shift of the Fabry−Perot resonance due to the ZrO2 layers but come from the vanishing of the dielectric function of bulk silver.

enhances the reflectance. Now, let us compare the different spectra inside Figure 6B. It appears that embedding the nanoparticles in the host increases the transmittance outside the sharp plasmon resonance, but the transmittance is not changed at the resonance itself (ξ = 1). Therefore, a first conclusion emerges: the introduction of the ZrO2 layers is not a sufficient condition for increasing the factor ξ above 1. Indeed, changing the features of the nanoparticle films themselves seems the only possible way for increasing the factor ξ above 1. Nevertheless, coating the silver nanoparticles with the shell is a model always leading to a factor ξ lower than 1. In the model of embedded nanoparticles, the factor ξ cannot be increased above 1 just by varying the density N and (or) the thickness of the silver nanoparticle films. The key parameter of the problem is the radius of the nanoparticles. For example, increasing this radius up to 40 nm (close to the observed high limit value23) for a nanoparticle film of thickness 160 nm, keeping the filling factor f at the value of 0.67% as was previously, leads to the calculated transmittance spectra depicted in Figure 6D,E, respectively, without and with the ZrO2 films. In both cases, comparing spectra 3 and 1 indicates that, at the plasmon resonance, the transmittance of the multilayer containing the embedded silver nanoparticles is greater than that of the multilayer containing the naked silver nanoparticles (ξ = 1.08). Then, a second conclusion can be drawn: the introduction of the ZrO2 layers is not a necessary condition for increasing the factor ξ above 1. This latter property does not come from the FP resonance of the ZrO2 films. In the simulations depicted in Figure 6D,E, the reason for having the factor ξ greater than 1 cannot be the decrease of light absorption in silver because the imaginary part of the refractive index Nim of the nanoparticle films does not decrease (compare spectra 1 and 3 in Figure 6F). Therefore, the enhancement of transmittance upon silver functionalization is a property of the interference phenomena within the silver films themselves. Indeed, the scattering cross-section of light by the silver nanoparticles is roughly multiplied by 3 as the nanoparticle radius is increased from 20 to 40 nm. As ξ < 1 with the former size and ξ > 1 for the latter one, the simulations show that the scattering of light by the silver nanoparticles basically governs the interference phenomena. Here is the true role of ZrO2: it has a high efficiency for silver generation with respect to silica. The optical role of the ZrO2 layers is, indeed, minor. Nevertheless, it is worth noticing that our simulations significantly underestimate the factor ξ since its experimental value lies between 1.24 and 1.52. The simulation of the transmittance confirms the peak identification given in section 3.2 (Figure 4A,D) for wavelengths between 200 and 350 nm. Consider the transmittance simulation of Figure 6E, for example. These calculations do not take into account for the absorption band due to Ag−PFDT binding, so all the spectra 0, 1, 2, and 3 contain the FP resonance of the ZrO2 layers at the same location around λ = 250 nm. In addition, spectra 1, 2, and 3 contain the peak of transmittance around λ = 320 nm corresponding to the vanishing of the dielectric function of bulk silver. Moreover, without the presence of the ZrO2 layers (Figure 6D), only this latter peak is present, as expected. In both cases with and without ZrO2 films, embedding the silver nanoparticles in the PFDT host significantly enhances the peak of transmittance around λ = 320 nm up to the transmittance level of the naked

4. CONCLUSIONS In this article, a new way for enhancing the transmission of light through a disordered assembly of silver nanoparticles is reported. The strategy consists in grafting with PFDT SAM the nanoparticles obtained on ZrO2-coated silica substrates via the chemical method of oxide-initiated silver nanoparticle generation. The grafting takes place progressively; so, its effects can be controlled. The chemical binding of PFDT molecules to silver upon grafting of PFDT SAM is proved by absorption of light. In order to better understand the mechanisms involved in the modifications of the optical properties by this silver functionalization, a comparison is made, on the one hand, with silver nanoparticles generated on silica substrates without the presence of ZrO2, and, on the other hand, with continuous thin silver films obtained by electron-gun evaporation. Without ZrO2, the plasmon resonance of the nanoparticles around λ = 400 nm is red-shifted, and the transmittance at the resonance is decreased during silver functionalization, in qualitative agreement with the usual modification of the extinction cross-section of noble metal nanoparticles by a dielectric environment. With the ZrO2 layers, the transmittance is increased, while the absorptance is decreased upon SAM grafting at saturation for 3 days. The transmittance can become higher than that of the oxide substrate for wavelengths between 300 and 680 nm, and even reach that of the naked silica for λ around 330 nm. The local minimum of transmittance can be enhanced by a factor as high as 1.52. These transmittance enhancements are quite unexpected from what is known about the modification of the extinction cross-section of noble metal nanoparticles due to their dielectric environment. In the same way, important related variations of the reflectance upon SAM grafting is observed. The reflectance is much decreased in the vicinity of the local transmittance minima and can be lower than that of the naked silica (antireflectivity). Our experimental study also demonstrates that the presence of the ZrO2 layers on silica is not a sufficient condition for the enhancement of transmittance upon silver functionalization. Moreover, a silver nanostructuration is necessary. Owing to the disorder of the assembly of nanoparticles, considering each nanoparticle film as an effective medium in the extended Maxwell−Garnett approximation, is a useful macroscopic point of view. Although this model does not allow for a detailed fit of our experimental results, mainly because the substrate effect on the plasmon resonance is ignored, it reveals the main elements underlying the enhancement of the transmittance upon SAM grafting. This intriguing phenomenon is due to the interferences of light within the silver nanoparticle films themselves: the nanoparticles behave as embedded and not just coated by the PFDT dielectric, on the one hand; their size has to be large enough for presenting a significant scattering cross-section, on the other hand. Indeed, these features are due to the chemical role of ZrO2 being more efficient than silica for the generation of silver, but the optical role of ZrO2 is minor. 5365



Article

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Taking into account that our elaboration method of silver nanoparticles is simple, at low cost, and that the chemistry of thiol SAM is versatile, the work reported here offers new opportunities in the fields of nano-optics and plasmonics.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 04 72 43 29 87. Fax: 04 72 43 26 48. E-mail: roger. [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

I am grateful to N. Terrier (Plateforme Nano-Lyon, INL) for silver thin film deposition.

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Enhancement of Light Transmission through Silver Nanoparticles

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