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Deposits of exposed and embedded silver nanoparticles were grown on Si(100) and silica substrates by laser ablating high-purity silver and SiO2 targets in vacuum using a femtosecond Ti:sapphire laser delivering 45 fs pulses at 804 nm and 1 kHz repetition rate. The effect of the laser fluence and irradiation time on the obtained nanostructures was investigated using several fluences between 650 mJ/cm2 and 3.2 J/cm2 and deposition times in the range of 1-20 minutes. Optical response of the deposits was characterized using optical absorption spectroscopy and the surface morphology was studied by scanning electron microscopy (SEM). Samples with the optimal optical response were obtained by depositing three successive Ag/SiO2 bilayers at the main laser wavelength (804 nm) under vacuum at substrate room temperature. They were composed of silver nanoparticles with an average diameter of 6 nm and a narrow size distribution; each layer of these nanoparticles was then separated by silica layers of approximately 100 nm. The laser fluence and deposition time for Ag (SiO2) were 650 mJ/cm2 (3.2 J/cm2) and 1 min (10 min), respectively.
© 2014 Optical Society of America
1. Introduction
Composite materials containing metal nanoparticles have attracted much attention during past decades [1–3]. Due to their unique linear and nonlinear optical properties, they have found various applications in different fields of science and technology [4–9]. For instance, they can be used for cancer treatments [10,11], catalysis [12,13], photo-heat conversion [14], second harmonic generation [15] and biological or chemical sensing [16]. Moreover, the significant increase in the local electric field produced in the surroundings of the particle [17–19] is useful for surface-enhanced Raman scattering (SERS) [20] and several linear [21] and nonlinear [22–24] optical processes. Optical properties of metal nanoparticles are governed by the localized surface plasmon resonance (LSPR); i.e., collective oscillations of the free electrons. Hence, they are strongly dependent on the size, shape, concentration, and distribution of the nanoparticles as well as on the properties of the surrounding matrix [25]. Colloid chemists have gained excellent control over particle size for several spherical metal compositions [26] but the surface of these nanoparticles is likely to be contaminated with reaction by-products such as anions and reducing agents, which can interfere with subsequent stabilization and functionalization steps. On the other hand, such a fine control of the size distribution remains elusive for most physical deposition techniques.
Femtosecond pulsed laser deposition (fs PLD) has proved to be the most notable exception to this rule. This technique has been successfully used for the deposition of nanoparticles composed of a wide variety of materials [27], both for fundamental research and for technological applications [28]. Ultrashort laser pulses offer high material removal efficiency and high deposition rates of nanometer scale particles free of microscopic particulates and therefore fs PLD constitutes an attractive procedure for the fabrication of nanostructured deposits. However, it appears that the nature of nanoparticles (NPs) grown by fs PLD strongly depends on the material and deposition conditions and therefore various studies [29–36] have tried to elucidate the different processes involved, showing the possibility of using fs PLD as a general route to NP formation. Both, metal and semiconductor NPs have been synthesized by this technique [37–50]. However, somewhat surprisingly, reports of fabrication of plasmonic nanocomposites by fs PLD are rather scarce [51–55].
In this paper we report on the fabrication of silver nanoparticles on Si(100) and SiO2 substrates by fs PLD under various conditions. Firstly, NPs were deposited either as a single layer directly on the surface or as part of an Ag/SiO2/…/Ag/SiO2 multilayer structure. It has been shown that considerable gains of the LSPR intensity can be obtained for the latter case; subsequent annealing further enhances the optical response. Surface structures of prepared deposits were studied by means of scanning electron microscopy (SEM) and the optical response was analyzed using optical absorption spectroscopy. Moreover, we have studied the deposition of Ag particles and SiO2 layers as a function of laser fluence, finding that for the former material there is not any apparent dependence on this parameter, whereas there is a nearly-linear dependence of the deposition rate on laser fluence for the latter case. Extrapolation of the experimental values of deposition rate reveals a threshold for the silica deposition around 650 mJ/cm2. To the best of our knowledge, this is the first report on deposition of embedded plasmonic nanoparticles by fs PLD. This kind of deposits has the advantage of producing NPs that are protected from the environment by the silica matrix.
2. Experimental
The experimental setup consisted of a PLD stainless steel chamber evacuated by a turbomolecular pump down to 2 × 10−4 Pa. Silver (99.999% of purity) and silica (OH content below 10 ppm and total impurity content below 30 ppm) targets were provided by Heraeus, S.A. and EMS, respectively. Targets were placed on a rotating sample holder to avoid crater formation by repetitive laser irradiation. Pulses of 45 fs were produced by a Ti:Sapphire amplified laser system (Spectra Physics) centered at 804 nm operating at a repetition rate of 1 kHz. The temporal profile of the 804 nm pulses was diagnosed by second harmonic autocorrelation whereas energy control was performed by a variable attenuator wheel. The laser beam was focused onto the target using a 25 cm focal length lens at 35° with different fluences. Substrates were ultrasonically degreased in acetone and methanol for 5 minutes. Typical deposit times were 1 and 10 minutes for Ag nanoparticles and silica layers, respectively.
Surface morphology of the deposits was examined by SEM in a Carl Zeiss Auriga 60 FIB/SEM XBeam system that is equipped with a Cobra-focused gallium ion beam column, a Schottky field emission gun and a Gemini electron column. Samples for the study of the transversal sections were prepared in the same instrument following a conventional preparation method. Size distribution and average diameters of Ag NPs were determined by analysis of the SEM micrographs.
Optical absorption spectra were measured at room temperature using a compact spectrometer, QE6500 (Ocean Optics Inc.), configured with a multichannel array detector for measuring simultaneously the whole spectrum in the range 200-850 nm with a spectral resolution better than 1 nm.
2.1. In situ reflectance measurements
Several techniques are possible in order to obtain the reflection and transmission coefficients of the electromagnetic field in multilayered structures, but one of the most elegant approaches is to employ matrix methods. Multilayer structures with isotropic and homogeneous media and parallel-plane interfaces can be described by 2x2 matrices due to the fact that the equations governing the propagation of the electric field are linear and that the tangential component of the electric field is continuous [56,57]. Consider a plane wave incident from left at a general multilayer structure having m layers between a semi-infinite transparent ambient and a semi-infinite substrate, as schematically described in Fig. 1.Each layer j (j = 1, 2, …, m) has a thickness dj and its optical properties are described by its complex index of refraction ñj = nj + i kj (or, alternatively, by the complex dielectric function ), which is a function of the wavelength (energy) of the incident light.
Fig. 1 Schematic representation of the multilayer model used to analyze the reflectance data measured in situ. In this model we considered m layers between a semi-infinite transparent ambient and a semi-infinite substrate.
The optical electric field at any point in the system can be resolved into two components corresponding to the resultant total electric field; one component propagating in the positive x direction and one in the negative x direction, which at a position x in layer j are denoted and , respectively. An interface matrix (matrix of refraction) then describes each interface in the structure:
where tjk and rjk are the Fresnel complex transmission and reflection coefficients at interface jk. For light with the electric field perpendicular to the plane of incidence (s-polarized or TE waves) the Fresnel complex reflection and transmission coefficients are defined by:
and for light with the electric field parallel to the plane of incidence (p-polarized or TM waves) as:
being , n0 the refractive index of the transparent ambient, ϕ0 the angle of incidence, and ϕj the angle of refraction in the layer j. The layer matrix (phase matrix) describing the propagation through layer j can be written as:
where , λ is the wavelength of light in vacuum and is the phase factor of layer j, which represents the phase change experienced by the wave as it traverses the layer. Now, by using the interface matrix and the layer matrix described in Eqs. (1) and (4), we can obtain the total system transfer matrix (scattering matrix) S, which relates the electric field at ambient side and substrate side by:
and S can be written as:
When light is incident from the ambient side in the positive x direction there is no wave propagating in the negative x direction inside the substrate, which means that . For the total layered structure the resulting complex reflection and transmission coefficients can be expressed by using the matrix elements of the total system transfer matrix of Eq. (6) as:
Equation (7) is valid for both, TE and TM, polarizations and the total transmittance and reflectance can be calculated as and , respectively. For the case of our in situ measurements, Eqs. (1)-(7) greatly simplify and the total reflectance of the system Ambient/Silica/Silicon can be calculated as:
and the phase factor for the silica layer is:
3. Results and discussion
3.1. Silver
The effect of the laser fluence and the deposition time on the size and shape of the deposited nanoparticles has been studied. As for the first parameter, we found no direct relationship between the laser fluence and the size of the nanoparticles in the studied interval (650 mJ/cm2 – 3.2 J/cm2). On the other hand, the influence of deposition time is shown in Fig. 2. For shorter times [1-2 minutes, Fig. 2(a)] one can clearly appreciate the growth of isolated nanoscale particles with good separation among them (so that there is no coupling between the surface plasmons of neighboring particles). However, for longer times [5 minutes or more, Fig. 2(b)] it is evident the appearance of larger precipitates that seem to be formed by the coalescence of several nanoparticles.
Fig. 2 SEM micrographs, illustrating typical sizes and morphologies of silver nanoparticles, obtained from samples with deposition times of (a) one and (b) five minutes with 45 fs laser pulses centered at 804 nm and 1 kHz repetition rate.
The effect of deposition time is better exemplified in Fig. 3, where the probability histograms are depicted for short [1 minute, Fig. 3(a)] and long [5 minutes, Fig. 3(b)] deposition times. Identical laser fluences of 650 mJ/cm2 were used in both cases. As can be seen, nanoparticles are rather small in the first case (mean diameter around 6 nm), whereas in the second case the increase in size is evident (more frequent diameter circa 35 nm). Moreover, the Gaussian-like distribution observed for the shorter deposition times changes its aspect to a more lognormal-like distribution as the deposition time increases.
Fig. 3 Size histogram of the nanoparticles produced by fs laser ablation, obtained from samples with deposition times of (a) one and (b) five minutes. Insets represent the typical optical spectra obtained from both types of samples.
More important for plasmonic applications is the evolution of the LSPR of the deposited particles (inset in both histograms of Fig. 3). When the particles are well separated (i.e., for the shorter deposition times), a typical silver LSPR peak is observed: narrow and located in the neighborhood of 400 nm. Unfortunately, it is very weak due to the low concentration of nanoparticles in this case. However, for the closer and bigger particles (i.e., longer deposition times), the LSPR peak is more intense, but, at the same time, it appears heavily modified (widened and shifted to longer wavelengths) due to the coupling of the surface plasmons of the neighboring particles.
Unfortunately, none of these two cases are satisfactory for most applications because the presence of strong and unperturbed LSPR signals are required. Hence, once we obtained satisfactory size distributions, we tried to improve the deposition method to fabricate multilayers of embedded nanoparticles.
3.2. Silica
The material chosen to cover the silver nanoparticles was silica; we selected this material mainly because of its high transparency. An added advantage of silica is that it is the same material of the substrate (even silicon substrates are covered by a thin silica layer), which improves the adherence of the grown layer. However, this choice has some problems, which partly explains why there are just a few works in the literature reporting deposition of silica by means of PLD. Namely, deposition of silica is much more complicated than that of silver because (i) this material is transparent to the laser wavelength, considerably reducing the ablation rate, and (ii) we want to deposit a continuous (and thick) silica layer, whereas for silver we just want isolated particles.
The first option that we tried was working at the third harmonic of the laser (268 nm) to increase absorption, but this strategy did not work because the fluence loss (even at the highest fluence obtained at 268 nm of 250 mJ/cm2) nullified any gains that could be obtained from the increase in absorption. So, we studied the deposition rate as a function of laser fluence for the original wavelength (804 nm). To achieve a good characterization, we measured in situ the reflectance of the silica layer, which allowed us to follow all the growth process over time. The evolution of the thickness of the silica layer for the different laser fluences is shown in Fig. 4.Note that, although the target is rotating continuously, deposition rate drops considerably after a few minutes and it is necessary to reposition the laser spot. This is observed in the graphs as small “jumps” of the deposition rate.
Fig. 4 (a) Evolution of the thickness of the silica layer, ΔdSiO2, for the different laser fluences used, and (b) deposition rate as a function of laser fluence. An ablation threshold of around 200 mW is deduced from the linear extrapolation of the data in (b). Thickness of the silica layer was obtained from in situ reflectance measurements.
The dependence of the ablation process on the laser fluence can be determined more easily if we plot the deposition rate [slope of the thickness variation in Fig. 4(a)] as a function of laser fluence [Fig. 4(b)]. As can be clearly seen, the dependence of the deposition rate on laser fluence is approximately linear. A linear fit of the experimental data allowed us to determine the threshold below which no ablation is expected. In our case, a value of about 650 mJ/cm2 was obtained.
3.3. Multilayers
To improve the optical response we prepared multilayer deposits of silver and silica. After some tests, we found that the optimal configuration was obtained for three successive Ag/SiO2 bilayers. Silver deposits were made at a laser fluence of 650 mJ/cm2 for one minute, whereas silica layers were grown using a fluence of 3.2 J/cm2 for 10 minutes. Under these deposition conditions we expected silver nanoparticles with a size distribution as shown in Fig. 3(a) and intermediate silica layers of approximately 100 nm. A typical transversal cross section of these multilayer structures is depicted in Fig. 5.
Fig. 5 SEM micrograph, showing the transversal section of a multilayer structure (Ag/SiO2/Ag/SiO2/Ag/SiO2) deposited on a silica substrate. The upper denser layer is an aluminum layer deposited to prevent charge accumulation in the SEM.
An example of the optical response of these multilayer deposits is shown in Fig. 6. Spectra obtained from silver monolayers for deposition times of 1 and 5 minutes are also depicted, for comparison purposes. As can be seen, the improvement in the plasmonic response for the multilayer deposits is dramatic: the intensity of the LSPR peak increases over four times with respect to the equivalent monolayer and it appears centered around 410 nm, closer to the value expected for spherical silver nanoparticles embedded in silica (400 nm). Moreover, there is not any apparent coupling between the surface plasmons of neighboring particles, as occurs in the monolayer deposited during 5 minutes.
Fig. 6 Typical optical responses obtained from a silver monolayer grown during one (black line) and five (red line) minutes and an Ag/SiO2/Ag/SiO2/Ag/SiO2 multilayer, as-deposited (green line) and after one hour of annealing at 600 °C (blue line).
These embedded nanoparticles have the added advantage that they are protected by the silica layer, preventing oxidation or that they are otherwise affected by the environment. Taking advantage of this extra protection, we applied a thermal annealing in air (at 600°C for an hour) to the multilayer sample (blue line in Fig. 6) and the optical response improved almost 50% with respect to the as-deposited multilayer sample (green line in Fig. 6). Moreover, the LSPR peak slightly blue-shifted to 400 nm, which indicates that now the shape of the Ag nanoparticles is closer to a sphere. It should be noted that the same annealing process applied to the unprotected particles would dampen the LSPR signal (not shown) because they would be completely oxidized under these conditions. Grown silica layers exhibited a good stoichiometry as deposited, but the annealing in air is also expected to improve the silica structure and composition.
4. Conclusions
In this work we have fabricated silver nanoparticles embedded in silica by femtosecond PLD. The size distribution of the obtained particles is very good, as it is evident from SEM micrographs and the optical response. In addition, we have determined the dependence of the deposition rate of silica as a function of laser fluence, finding that there is a threshold around 650 mJ/cm2 below which no ablation occurs. Finally, we have demonstrated that by depositing alternating layers of Ag and SiO2 one can obtain samples with excellent optical response and essentially no agglomeration of the metal nanoparticles. This is important because such accumulation would produce interactions between the LSPR of neighboring particles, dampening the optical response. Finally, it is also found that a thermal annealing in air further enhances the plasmonic response of the nanoparticles.
Acknowledgments
This work was partially funded by the Spanish ministry MINECO, projects AIC-A-2011-0718, MAT-2012-38541, CTQ2008-02578 and CTQ2012-37404-C02-01. O.P.R. is grateful with Moncloa Campus of International Excellence (UCM-UPM) for the PICATA postdoctoral fellowship. Facilities provided by the Centro de Láseres Ultrarrápidos at Universidad Complutense de Madrid are gratefully acknowledged.
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