Article pubs.acs.org/JPCC
Preventing the Degradation of Ag Nanoparticles Using an Ultrathin a‑Al2O3 Layer as Protective Barrier G. Baraldi,† M. Carrada,‡ J. Toudert,† F. J. Ferrer,§ A. Arbouet,‡,∥ V. Paillard,‡,∥ and J. Gonzalo*,† Instituto de Ó ptica, CSIC, Serrano 121, 28006 Madrid, Spain CEMES-CNRS, 29 rue Jeanne Marvig, BP94347, 31055 Toulouse, France § Centro Nacional de Aceleradores (University of SevillaCSIC), Av. Thomas A. Edison 7, 41092 Sevilla, Spain ∥ Université Paul Sabatier, Université de Toulouse, 29 rue Jeanne Marvig, 31055 Toulouse, France † ‡
S Supporting Information *
ABSTRACT: We compare the morphology and optical response of plasmonic nanostructures produced by pulsed laser deposition, consisting of a 2D distribution of Ag nanoparticles exposed to air or buried under an amorphous Al2O3 layer whose thickness is tuned in the 0.5 to 14 nm range. We observe that the covering process leads to drastic changes in Ag content, which are interpreted in terms of sputtering of Ag atoms promoted by the incoming Al ions. This Ag sputtering process is avoided as soon as the nanoparticles are embedded under a subnanometer-thick layer of amorphous Al2O3. Meanwhile, the spectral position of the nanoparticles’ characteristic surface plasmon resonance, measured immediately after the film growth, is not significantly affected by the deposition of the covering layer. Nevertheless, the resonance band associated with uncovered Ag nanoparticles has vanished after 12 months, as a result of their oxidation. Embedding the nanoparticles under a subnanometer-thick layer of amorphous Al2O3 is enough to avoid the observed atmospheric aging processes as well as to preserve the features of their surface plasmon resonance. The results presented here are therefore promising in view of the pulsed laser deposition-based elaboration, at the wafer scale, of robust and stable tailor-made plasmonic substrates that may potentially present high electromagnetic coupling with their environment due to the very small distance to the nanostructure surface.
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INTRODUCTION Noble metal (Ag or Au) nanostructures show unique optical properties such as resonant optical absorption, scattering, and near-field enhancement that are related to the excitation of surface plasmon resonances (SPRs).1 Strong SPR-induced nearfields have been shown to play a key role in the enhancement of Raman (SERS effect) or fluorescence signals of active molecules or nanostructures.2−5 However, due to the shortrange of SPR-enhanced near-fields,6−9 it is usually required to locate the active nanostructures at a distance of a few nanometers of the metal nanoparticles (NPs). In this sense Whitney,8 Kiel,9 and Standridge10 aimed at quantifying the range of plasmonic interaction through dielectric protective layers deposited on the NPs and found out that it is of the order of NP diameter or even shorter. In this context, simple substrates for SERS or metal-enhanced fluorescence, as well as plasmonic nanostructures suitable for implementation in photovoltaic devices, can be obtained by burying a single layer of noble metal NPs with tuned size, shape, and © 2013 American Chemical Society
organization, at a few-nanometer depth in a transparent dielectric matrix.10 Moreover, in the case of Ag NPs, which have been shown to suffer from aging processes due to oxidation that may damp11 or shift12 the SPRs and related nearfield enhancement, the dielectric matrix must also act as an efficient protective barrier against chemical degradation.10,13−15 Among the different chemical or physical methods suitable for the fabrication of single layers of metal NPs encapsulated between thin dielectric layers,9,13,16,17 automatized physical deposition techniques have shown a considerable potential for controlling the nature, size, shape, and organization of the NPs and, thus, the features of the SPR and its related near-field.18−20 These techniques rely on the alternate deposition (usually in vacuum) at wafer scale of metal and dielectric species, the latter being deposited on the just-grown metal NPs. Although much Received: February 8, 2013 Revised: April 15, 2013 Published: April 30, 2013 9431
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Figure 1. Upper row: Plan-view TEM images of (a) UN, (c) CN-1270, and (e) CN-5060 films. Lower row: Distribution of the in-plane long axis length (L) of the NPs for (b) UN, (d) CN-1270, and (f) CN-5060 films. The inset in panel a is a high resolution TEM image, showing that Ag NPs are polycrystalline.
atmospheric agents and sputtering is achieved for NPs buried under an a-Al2O3 layer thinner than 1 nm.
effort has been made to understand and control the growth of metal NPs using these techniques, little has been reported to demonstrate their supposed potential for covering metal NPs with ultrathin and efficient protective dielectric layers. Dielectric cover deposition has recently been shown to affect Ag NPs’ shape and size, due to shape relaxation,21 sputtering effects,22 and matrix−metal NP intermixing effects.23 Correlation between the surface roughness of a dielectric covering layer sputtered onto Ag NPs and the morphology of the granular metal layer has also been demonstrated.24 Nevertheless, these studies involved relatively thick covering dielectric layers (around 10 nm thick) which are hardly suitable for SERS or metal-enhanced fluorescence applications, and no information about the midterm stability of the NPs against atmospheric agents was reported. Pulsed laser deposition (PLD) is a powerful technique for growing metal NPs with a controlled plasmonic response embedded under nanometer-thick amorphous aluminum oxide (a-Al2O3) layers with low porosity and high transparency.19,22,23 In this work, we thus investigate the potential of these a-Al2O3 layers as protective barriers against atmospheric aging processes, such as chemical degradation, in the case of Ag NPs smaller than 10 nm in size that usually present a strong chemical sensitivity. The effect of the covering layer on the midterm chemical stability of the NPs is studied and compared to that of uncovered NPs by analyzing the evolution of the optical response of the nanostructures during 12 months. In addition, it is evidenced that covering the NPs with a-Al2O3 affects the Ag content. In light of sputtering yield modeling, we explain this effect in terms of sputtering of Ag by incoming Al+ ions during the deposition of the covering layer.22 We show that efficient protection against chemical degradation due to
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EXPERIMENTAL SECTION Alternate pulsed laser deposition (PLD) was performed in vacuum (10−4 Pa) using an ArF excimer laser (λ = 193 nm, τ = 25 ns full-width half-maximum) which was focused to ablate alternatively polycrystalline Al2O3 and Ag rotating targets at room temperature. A ∼5 nm thick a-Al2O3 buffer layer was first deposited on the rotating substrate, placed 38 mm away from the targets. A single layer of Ag NPs was then grown on the aAl2O3 surface by ablating the Ag target using 300 laser pulses. The Ag NPs were covered in situ, immediately after their growth, with an a-Al2O3 layer having a variable thickness adjusted by varying the number of pulses on the Al2O3 target in the range from 160 to 5060. For comparison, an uncovered film with the same buffer thickness and the same number of laser pulses on the Ag target was fabricated. More details about the deposition procedure can be found elsewhere.19,23 From now on we will identify the films involving uncovered and covered Ag nanostructures as UN and CN-xxx, respectively, where xxx stands for the number of pulses on the Al2O3 target used to deposit the covering layer. Different substrates (Si, carbon coated mica, fused silica or glass) were used to allow a complete morphological, structural, and optical characterization of the films. Ag NPs’ nucleation and growth was not affected by the use of different substrates as they were always deposited on the a-Al2O3 buffer layer. A Cary 5000 spectrophotometer was used to measure the absorbance (Abs = log10(1/T) where T is the transmittance) spectra of the films in the 300 to 800 nm range where the resonant behavior of Ag NPs is expected to occur. Measurements were performed at normal incidence, and the spectra measured for fused silica substrates were considered as 9432
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Figure 2. Cross section TEM images of (a) CN-1270 and (b) CN-5060 films deposited on Si substrates. The sketches at the sides of the images show the thicknesses of the buffer and covering a-Al2O3 layers.
1c and 1e) are very different from that of the UN film. After deposition of the capping layer, only isolated NPs are observed while no chainlike structures are seen. In this case the NPs’ inplane projected shape is circular or slightly elliptical with the inplane long axis randomly oriented. The distribution of L is very similar for CN-1270 and CN-5060 and more homogeneous than in the UN film, as it is deduced by the narrower histogram presented in Figures 1d and 1f. L remains in the 3−7 nm range, leading to an average value ⟨L⟩ ∼ 5 nm. With respect to the UN film (Figure 1a), the NP density has increased up to 2.2 × 104 NPs μm−2 whereas the surface coverage has decreased to 25%. The structure of the other films, such as CN-510 (Figure S1, Supporting Information), is similar to that observed for the CN-1270 and CN-5060 films. Figure 2 shows cross section TEM images corresponding to (a) CN-1270 and (b) CN-5060 films. We observe that the Ag NPs (dark contrast regions) are embedded between two regions with lighter contrast that corresponds to the buffer and capping a-Al2O3 layers. Beneath the buffer layer a very thin brighter layer of native SiO2 is also observed in both cases. The NPs lie on a plane placed at 5.1 nm from the silicon substrate, and neither subimplanted nor dispersed Ag in the matrix could be observed. NP height (H) is 3.6 ± 0.2 nm in both cases. Thus, if we take into account the value of L (Figures 1d and 1f), we can conclude that covered NPs are slightly flattened ellipsoids, the average L/H ratio being ∼1.4. Finally, Ag NPs are buried under covering layers of different thicknesses in the CN-1270 and CN-5060 films: in the first case, the surface to NP distance is 0.5 nm, while in the second case it is 14.3 nm. It is worth noting that the covering layer shows a flat surface in both cases, and that no corrugation replicating the buried NPs is observed. The results of the TEM characterization presented in Figure 1 provide evidence that nanostructures covered with a-Al2O3 show different NP morphologies, surface density, and surface coverage compared to the uncovered ones. These differences, particularly in the case of surface coverage, suggest that the deposition of the covering layer may affect the Ag content, [Ag], in the nanostructures. We have analyzed this possibility by RBS, and the results obtained are presented in Figure 3, where [Ag] is shown as a function of the number of pulses used to deposit the covering a-Al2O3 layer. [Ag] is close to 7 × 1015 atoms cm−2 for the UN film (0 pulses on Al2O3) and decreases almost exponentially as the number of pulses increases until it reaches a value of ∼3.5 × 1015 atoms cm−2 for ≈1270 pulses on Al2O3 (i.e., CN-1270) to remain approximately constant for a larger number of pulses. Figure 4a shows the experimental absorbance spectra of the UN and CN-1270 films, measured immediately after their deposition (denoted as “as-grown” conditions). An absorption
baselines. In order to study the temporal stability of the optical response against atmospheric agents, absorbance was measured just after deposition (as-grown sample) and two, five, and twelve months later. During this period of time the samples were kept in laboratory boxes in air. X-ray photoelectron spectra were recorded in an ESCALAB 210 spectrometer working in the pass energy constant mode at a value of 50 eV and using a Mg Kα (hν = 1253.6 eV) X-ray source. Binding energies (BE) were referenced to the Al 2p line at a BE of 73.8 eV. The base pressure of the analysis chamber was maintained below 3 × 10−7 Pa during data acquisition. The morphology and thickness of the capping layer and the size, morphology, and organization of the covered and uncovered Ag NPs have been investigated by transmission electron microscopy (TEM) in both plan-view and crosssection configuration using a field emission FEI Tecnai F20 equipped with a spherical aberration corrector and a TEM-FEG CM20 both operating at 200 kV. Plan-view samples were prepared by floating off films from carbon-coated mica substrates in deionized water and picking up on Cu grids. Cross-sectional specimens were prepared by mechanical polishing and ion milling using the standard procedure. Finally, the silver atomic content, [Ag], was measured in the films grown on Si substrates by Rutherford backscattering spectrometry (RBS) using a 2.0 4He2+ ion beam and a surface barrier Si detector, placed at 165°. The spectra obtained have been analyzed with the SIMRNA 6.0 code,25 the error in the determination of [Ag] being 2%.
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RESULTS Figure 1 shows plan-view bright field TEM images of (a) UN, (b) CN-1270, and (c) CN-5060 films. In these images the dark contrast corresponds to Ag NPs whereas lighter regions correspond to a-Al2O3. In the case of the UN film, Ag forms NPs with almost circular projected shape, that can be either isolated or form apparently elongated chainlike structures that are composed of adjacent and/or overlapped particles. Particle overlapping is visible in most of the chainlike structures, appearing as a darker contrast which corresponds to a higher amplitude contrast. The darker zones in the isolated Ag NPs are due to diffraction contrast. The length distribution of the in-plane long axis (L) for the UN film is presented in Figure 1b. Most NPsthe isolated onespresent L values ranging from 2 to 8 nm, while a small fractionthe NP aggregatesshow an in-plane long axis that can be as long as 22 nm. The NP surface density is ∼1.3 × 104 NPs μm−2, and their surface coverage is close to 50%. The inset in Figure 1a shows a high resolution TEM image, showing that Ag NPs are polycrystalline. The morphologies of the CN-1270 and CN-5060 films (Figures 9433
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incidence, using a multilayer model. The absorbance was then derived from T as explained in the Experimental Section. The NPs were considered as monodisperse ellipsoids in weak point dipole−dipole interaction,26 with two of their principal axes being oriented in the single layer’s plane. They were assumed to present a random in-plane orientation and an isotropic in-plane organization, with a mean (center-to-center) interparticle distance Λ. The NPs were assumed to be embedded in a homogeneous medium with a dielectric function εm = εa‑Al2O3 = 2.72.27 The dielectric function of the NPs was derived from that of bulk Ag28 and corrected for finite size effects.29 Figure 4b shows the calculated absorbance at normal incidence for slightly flattened and in-plane elongated Ag NPs (L = 5 nm, inplane short axis length B = 4.5 nm, H = 3.6 nm) with Λ = 8 nm. Using this set of parameters that corresponds well to the nanostructure of the CN films as seen by TEM (Figures 1 c−f), one observes a single in-plane SPR band peaking at around 480 nm, in good agreement with the SPR peak position reported in Figure 4a for the CN-1270 film. In view of the more complex nanostructure of the UN film (Figures 1a,b) due to the presence of elongated chainlike structures, it could be surprising to observe a SPR band peaking at nearly the same wavelength (λ ≈ 465 nm) as for the CN1270 film. In order to investigate this peculiar result, we have performed additional effective medium calculations. First, we have used the previously described model and approximated the chainlike structures (as those seen in Figure 1a) as ellipsoidal Ag NPs with a strong in-plane elongation. The NPs were assumed to be embedded in an average medium mixing the response of vacuum and a-Al2O3 (εm = 1.82), standing, as a
Figure 3. [Ag] content, as determined by RBS, in the films as a function of the number of pulses used to deposit the covering a-Al2O3 layer.
band peaking at λ ≈ 465 nm is seen for the CN-1270 film. This position is typical of the SPR of well separated, slightly flattened or in-plane elongated Ag NPs, as effectively observed by TEM in the case of the CN films (Figures 1c−f), and surrounded by a medium with a moderate refractive index. In order to emphasize this fact, we have performed simulations of the optical response of a single layer of Ag NPs embedded in a homogeneous a-Al2O3 medium under the form of a thin film (i.e., covered NPs). The calculations were performed in the quasi-static limit, which is a reasonable assumption regarding the small size of the NPs involved in this work compared to the wavelength of the visible light. As a first approximation, the single layer of Ag NPs was modeled as an effective medium, whose effective dielectric function εeff in the layer’s plane was used to calculate the transmittance T of the film at normal
Figure 4. (a) Absorbance spectra measured at normal incidence for the as-grown UN film (black dashed curve) and the CN-1270 film (blue solid curve). (b) Absorbance spectrum at normal incidence of a single layer of slightly flattened and in-plane elongated ellipsoidal Ag NPs (L = 5 nm, inplane short axis length B = 4.5 nm, H = 3.6 nm) embedded in a-Al2O3, simulated using an effective medium model. The NPs were assumed to present a random in-plane orientation with two of their principal axes in the layer’s plane and an isotropic in-plane organization with a mean centerto-center interparticle distance Λ = 8 nm. (c) Calculated in-plane longitudinal SPR-peak wavelength of the effective extinction coefficient as a function of L (with B = 4.5 nm, H = 3.6 nm, and Λ = 25 nm) for both the uncovered (open squares) and covered (solid squares) cases. Simulations were performed using the same effective medium model as in panel b, the uncovered NPs being assumed to be embedded in an average homogeneous medium mixing the dielectric functions of vacuum and a-Al2O3. (d) Granfilm simulation of the absorbance at normal incidence of 6 nm spherical Ag NPs deposited on a-Al2O3 and organized on a square array as a function of Λ. 9434
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first approximation, for the case of uncovered NPs. The evolution of the (in-plane longitudinal) SPR position was studied as a function of L (with B = 4.5 nm, H = 3.6 nm, and Λ = 25 nm), as plotted in Figure 4c. It is seen that an increase in the NPs’ in-plane elongation (i.e., an increase in L at fixed B) yields a fast shift of the SPR toward the near-infrared. Chainlike nanostructures behaving optically as strongly elongated NPs would therefore yield in-plane longitudinal SPR contributions peaking between 500 nm and the near-infrared (as shown in Figure 4c, open squares). As a consequence, a polydisperse assembly of NPs with a broad shape distributionincluding NPs with a strong in-plane elongation (as seen in Figures 1a and 1b)would present a broad SPR peaking at λ ≥ 500 nm. However, the measured SPR band of the UN film (Figure 4a) is quite narrow and is centered at ∼465 nm. Therefore, it seems unlikely that optical measurements on the UN film probe elongated coalesced structures. Instead, Figure 4c suggests that the SPR spectral position reported for the UN film could be more likely attributed to NPs with weakly anisotropic shapes. In view of the dense NP packing revealed in Figure 1a, one also has to consider the effect of near-contact coupling between weakly anisotropic NPs on their optical response. It is wellknown that the SPR modes of densely packed NPs can peak at wavelengths very different from those of the same NPs taken separately.2 For a given NP size, shape, and embedding medium, the SPR wavelengths vary as a function of the separation distance between NPs. It has been shown30 that the spectral position of the longitudinal SPR absorption band in a dimer of spherical Ag NPs (15 nm in diameter) shifts slowly when the (gap between NPs)/(NPs diameter) ratio decreases to values as small as 0.1. In the following, we evaluate the consequences of interparticle coupling on the SPRs’ spectral position in the case of a single layer of smaller Ag NPs (as those concerned in this paper). At such aim we have calculated the absorbance spectra of densely packed assemblies of Ag NPs using the Granfilm code.31,32 This code permits determination of the effective optical response of a single layer of monodisperse NPs in the quasi-static size regime, supported on a substrate and displaying a rotational shape with a vertical revolution axis. It relies on the accurate calculation of the electric potential inside and outside of the NPs using a multipole expansion method, the influence of the substrate being taken into account using the method of image charges. Therefore, it is a useful tool in cases where the point dipole approximation cannot be made, for instance for supported assembly NPs with truncated spherical or spheroidal shapes. Although it seems that it has been mainly used up to now for diluted systems of NPs, it allows taking into account coupling between NPs through the dipole and quadrupole contribution of their near-field, thus suggesting that it can be used in the case of densely packed NPs. The absorbance was calculated at normal incidence for a simplified model system consisting of 6 nm spherical Ag NPs deposited on a-Al2O3 and organized on a square array. Dipole and quadrupole contributions were taken into account for interactions, while the multipole order for calculation of the quasi-static electric potential of the NPs was increased until convergence was reached. Calculations were done as a function of the interparticle distance (array pitch Λ). As seen in Figure 4d, decreasing Λ until the NPs are almost in contact (0.5 nm gap) only induces a slight red-shift (from ∼350 nm to ∼390 nm) of the dipolar in-plane SPR band. Therefore, for the NP sizes involved in this work, near-contact coupling between NPs might induce only a moderate in-plane SPR red-
shift when compared to the same NPs taken separately. The measured spectral position for the SPR (465 nm) is thus consistent with the excitation of NPs separated by very small gaps and presenting a slightly nonspherical shape (weak inplane elongation and/or flattened shape), which might be the building blocks of the chainlike structures. Indeed, the shape and interaction-induced red-shift with respect to the case of separated spherical Ag NPs could sum and drive the in-plane SPR toward 465 nm. One has nevertheless to bear in mind that the previous simulations, which do not take into account exactly the complex nanostructure of the UN film (Figure 1a), only allows a qualitative interpretation of the SPR position. Finally, taking the previous results into account, one would expect a slight red-shift of the SPR band when covering the NPs if their shape and size were unchanged upon covering as it is shown in Figure 4c, in which effectively the SPR’s spectral position has also been plotted in the case of covered NPs. The fact that this red-shift is not observed could be explained by the increase in the interparticle separation that tends to blue-shift the SPR (Figure 4d) and could compensate the red-shift provoked by the increase in εm. Figure 5 shows the experimental evolution of the absorbance of the UN and CN-1270 films as a function of the exposure
Figure 5. Aging effect on the absorbance spectra measured at normal incidence for the UN film (black and open symbol curves) and the CN-1270 film (blue and solid symbol curves) as a function of the exposure time to air.
period to air. In the case of the UN film, the amplitude of the SPR band intensity decreases and slowly shifts toward shorter wavelengths during the months following deposition, until it almost disappears. The amplitude decreases by a factor of 14 and the resonance shifts from 465 to 435 nm after 12 months. On the contrary, in the case of covered NPs (CN-1270), no change either in amplitude or in position is detected after 12 months. CN-1270 is the film with the thinnest capping layer for which no changes in optical response are observed. For a lower number of pulses on the Al2O3 target, the amplitude of the SPR decreases as in the case of the UN film. Figure 6 shows the XPS spectra measured for the UN and CN-1270 films 12 months after deposition, at the (a) Al 2p, (b) O 1s, (c) Ag 3d, and (d) S 2p levels. The corresponding peak energies are listed in Table 1. The relative intensities of the Al, O, and Ag signals are different for the UN and CN-1270 films, due to their different in-depth distribution of species in the near-surface thin region from which photoelectrons escape. The modified Auger parameter (α*) has been calculated from the M4VV and M5VV Ag signals in order to get absolute information about the oxidation state of Ag.33 The obtained values are 726.03 and 722.54 for the CN-1270 and UN films respectively. 9435
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Figure 6. XPS spectra of UN (dashed black curves) and CN-1270 (solid blue curves) films corresponding to (a) Al 2p, (b) O 1s, (c) Ag 3d, and (d) S 2p levels.
Table 1. Element Binding Energiesa
mainly ions, having kinetic energies large enough to promote sputtering of material deposited in previous pulses.22 The evolution of [Ag], with the number of laser pulses on the Al2O3 target (Figure 3), suggests that sputtering takes place during the covering of Ag NPs and, moreover, that the amount of Ag removed per pulse decreases as the thickness of the covering layer increases. Since analytical models of sputtering such as Zalm’s or Yamamura and Tawara’s 34,35 do not allow considering the “shielding” effect associated with the growth of an a-Al2O3 layer that slowly deposits over and between the Ag NPs, we have considered the model developed in a previous work22 that is based on the SRIM 2008 software,36 which allows taking into account this effect to evaluate the extent of sputtering while covering Ag NPs. However, SRIM 2008 does not allow considering discontinuous layers like NPs. Thus, the single layer of NPs is modeled by a silver continuous thin film, the results being weighted by the Ag surface coverage determined from TEM images (Figure 1).22 The thickness of the Ag layer is 3.6 ± 0.2 nm, which corresponds to the height of Ag NPs as deduced from Figure 2. In addition, we have considered a cohesive energy value corrected for the finite size effects that reduce the stability of the Ag NPs with respect to bulk Ag (EcAgNPs ≈ 2.96 eV for the Ag NPs’ morphology shown in Figure 1a).22 Finally, we have considered the bulk material values for Ag density (ρ = 10.47 g cm−3), lattice binding energy (3 eV), and displacement energy (25 eV). The modeled structure is completed by adding under the Ag NPs an amorphous alumina buffer layer, an amorphous silica layer, and an infinite silicon substrate (Figure 2). The density of the SiO2 layer is ρ = 2.95 g cm−3 and that of a-Al2O3 the value previously reported for films produced by PLD under similar experimental conditions (ρ = 2.95 g cm−3).37 With regard to the species responsible for the sputtering of Ag, we consider that this is induced by Al+ ions, since they are the most relevant species present in the Al2O3 laser generated plasma in terms of concentration and kinetic energy.38 Thus, we consider in our calculations the Al+ ion density per pulse (QAl+ = 6.3 × 1013
binding energy (eV) element
UN
CN-1270
Al (2p) O (1s) Ag (3d3/2) Ag (3d5/2) S
73.9 530.8 374.1 368.1 160.9
73.8 530.5 373.3 367.3
a
Binding energies (in eV) of the relevant elements found in the UN and CN-1270 films.
Al and O peak positions are similar for both UN and CN1270 films. In contrast, this is not the case for Ag, the Ag peak of the CN-1270 film being shifted to lower binding energies compared to that of the UN film, while the gap between the split levels (3d3/2 and 3d5/2) is 6 eV in both cases. Finally, we observe that S is only detected in the case of UN films (Figure 6d).
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DISCUSSION The deposition of the covering layer has an important impact on the morphology of Ag NPs as it is shown in Figure 1. Chainlike structures cannot be seen in the CN films, and only single NPs with an elliptic or circular in-plane projected shape (Figures 1c and 1e) and narrow size distributions are observed (Figures 1d and 1f). TEM characterization shows that the CN films present a surface density of NPs twice higher than the UN film, and a significantly lower NP surface coverage. From the data obtained by RBS and presented in Figure 3, we propose that these morphological changes are related not to a redistribution of the deposited Ag but to a loss of Ag from the nanostructures induced by the deposition of the covering layer. After ∼1000 laser pulses on the Al2O3 target, the total loss of Ag is close to 50% of the initial deposited amount. The plasma produced during nanosecond laser ablation is characterized by the presence of a significant fraction of species, 9436
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ions cm−2) and the Al+ kinetic energy function distribution obtained, under similar ablation conditions, in an earlier work.38 The inset in Figure 7 shows the calculated Ag sputtering yield (Ag SY) as a function of the kinetic energy of incident Al+
silver sulfide has been reported for NPs grown by lithography or NPs deposited through solution.12,42,43 In this context, XPS data obtained from UN and CN-1270 films 12 months after deposition, presented in Figure 6, allow us to point out three main facts. First, Figures 6a and 6b confirm that the XPS signal coming from O is mainly related to the matrix, as otherwise expected. Indeed, in both cases the measured binding energy for Al and O corresponds to the values measured for Al2O3.44 Second, Figure 6c gives qualitative information about the oxidation state of the Ag NPs. In the case of covered NPs (CN1270 film) the binding energy of the Ag 3d5/2 level is 367.3 eV and the corresponding modified Auger parameter α* is 726.03, thus confirming that Ag NPs are made of metallic Ag.44 However, in the case of uncovered NPs (UN film) the Ag 3d5/2 peak is located at 368.1 eV and the corresponding modified Auger parameter α* is 722.54. These results, far from the ones typical of metallic Ag, confirm the oxidation of the Ag NPs.44 The determination of the compound that is formed is beyond the scope of this work, and it would require a more detailed chemical analysis. Nevertheless, the presence of sulfur that has been evidenced in the UN film, as shown in Figure 6d, could involve the presence of silver sulfide or sulfate.44 Finally, Ag NPs covered with a 0.5 nm thick a-Al2O3 layer are safe from any type of oxidation, and no sulfur is detected. This layer thus avoids the deterioration of Ag NPs that takes place when they are exposed to air. This protection effect might result from the low porosity of the a-Al2O3 layers, achieved through pulsed laser deposition, which avoids any contaminant diffusion. Finally from Figure 5 we observed that the degradation of the Ag NPs can be monitored through the evolution of their optical response, which consists of a decrease in their SPR amplitude and a slight blue-shift of their SPR peak wavelength. As it has been proposed in the literature, this evolution of the SPR can be understood in terms of the formation of a shell around unprotected metal NPs due to the reaction of the contaminants with Ag whose thickness increases with time.12 As the thickness of the shell increases, the NP metal core shrinks. Thus, a smaller Ag volume contributes to absorption, which decreases the SPR peak intensity. In addition, the reduced NP size (
