Silver nanoparticles are generated in glass by a dry process. First silver ions are driven into the glass by electric field-assisted ion exchange. Subsequent annealing in air led to the formation of silver nanoparticles beneath the surface of the glass. A thin slice of the cross section of the sample was prepared. This visualization of the depth profile facilitated optical analysis of the embedded layer containing silver nanoparticles to be preformed. We observed that there were narrower plasmon bands close to the sample surface and wider plasmon bands in lower layers. It is attributed to the formation of larger nanoparticles with lower number density close to the surface and slightly smaller nanoparticles with higher number density in the depth of the sample.
© 2011 OSA
Glasses and other dielectrics containing metal nanoparticles are of interest due to their unique linear and nonlinear optical properties. These properties are dominated by the strong surface plasmon resonances (SPRs) of the metal nanoparticles. The spectral position and shape of these SPRs can be designed within a wide spectral range throughout the visible and near-infrared spectra by choice of the metal and the dielectric matrix [1,2] or by manipulation of the size  shape  and spatial distribution  of the metal clusters. Therefore, these compound materials are promising candidates for many applications in the field of photonics [6–12].
Glass with embedded nanoparticles has traditional been fabricated using ion-exchange technique. Ion-exchange process has been the subject of basic and applied research for the potential exploitation of new dopants in the preparation of passive and active glasses . In the ion-exchange technique, metal dopants are usually introduced into the glass by immersing the matrix in a molten salt bath containing the dopant ions [14–16]. For instance, soda-lime float glass (72.2 SiO2, 14.2 Na2O, 0.71 K2O, 6.5 CaO, 4.42 MgO, 1.49 Al2O3, 0.13 Fe2O3, 0.4 SO3, in wt.-%) is the favorite substrate for the Ag+–Na+ ion-exchange process [15,16]. For the ion- exchange process, a glass substrate is placed in a mixed melt of AgNO3 and KNO3 (or AgNO3 and NaNO3) at 400 °C. The metallic ions are driven into the glass due to the chemical potential gradient, and replace alkali ions of the matrix that are released into the melt. In this way, metal concentration values well beyond the solubility limits may be achieved without clustering. The thickness of the glass substrate, time of the ion-exchange process, and weight concentration of AgNO3 in the melt determined the concentration and distribution of Ag+ ions in the glass. Thermal annealing of the ion-exchanged glass in a H2 reduction atmosphere (or air), typically at 400–650 °C (depending on the process), then results in the reduction of silver ions and formation of spherical silver nanoparticles. For glass containing iron ions (Fe2+ and Fe3+), and for annealing in a non-reducing atmosphere (air) the following two thermoreducing reactions have been considered : a) Fe2+ → Fe3+ + e-, and b) Ag+ + e- → Ag0.
Thermal-assisted ion exchange however proved to be an ineffective technique when dealing with multivalent ion species, as is the case of cobalt and gold . Multivalent ions come to substitute monovalent alkali ions (mostly Na) of the matrix, requiring structural modifications that come to depend crucially on the local composition of the glass.
An alternative approach and somehow less explored technique is to use an external electric field to assist the migration, making it possible to perform ion-exchange with multivalent ions, that are driven by the gradient in the electrochemical potential. In this configuration, the metal dopant supplier is a metallic film directly deposited on the glass matrix. This technique prevents inter-diffusion between the ionic species as in this case the dopant ions coming from the film replace the alkali ions of the glass matrix. Currently, Cu–Na and Ag–Na are the most studied field-assisted processes [17–21], with application in the fields of luminescent glasses, light waveguide technology and SERS.
Recently, dry fabrication of silver nanocomposites glasses using solid-state field-assisted diffusion process and post-annealing were reported [22,23]. In both cases, the authors successfully produced silver nanocrystals embedded glass. An important issue in this context is the fabrication of large area homogenous samples. Knowledge of the shape and thickness of the nanoparticles containing layer is also of paramount importance for some applications [e.g., 1,2,7–12]. In this paper, we demonstrate a three-step dry technique for the fabrication of scalable, homogenous (island-free) metal-glass nanocomposites; glass with embedded silver nanoparticles. We present a cross-section image of one such sample with an unprecedented clarity and optically analyze the depth profile of the uniformly distributed embedded layer containing silver nanoparticles.
2. Experimental methods
As a substrate Schott B270 super-white soda-lime glass with a thickness of 1 mm was used. The composition of the sample (in wt-%) was (69.2) SiO2, (9.8) Na2O, (9.5) CaO, (7.6) K2O, (2.8) BaO, (1.1) Al2O3 . The iron content in this glass is considered to be very low. The weight of the employed glass sample was 2.6026 g. One side of the glass (a circular area with a diameter of 18 mm) was coated with a fast drying silver suspension (Agar 301: very fine silver flakes dispersed in isopropanol).
Step I of fabrication: The sample was then annealed at 300 °C for ~30 minutes. The coated silver layer formed a homogeneous stable solid film of beige color and with a thickness of ~15 μm. After coating and annealing, the weight of the sample was increased by 29.1 mg. The thickness and weight increase after annealing indicated the formation of silver oxide (Ag2O with density of 7.14 g/cm3).
Step II of fabrication: The sample was then pressed between two metal electrodes (circular shape and with a diameter of 18 mm). In order to improve the contact a piece of graphite foil was inserted between the glass and the negative electrode. This has also the advantage that the substances coming out of the glass do not pollute the electrodes. Also graphite forms a non-blocking cathode since it accepts alkali ions. The electrodes with the samples were placed inside an oven and connected to a high-voltage power supply, with the positive voltage connected to the Ag site. The experimental setup for field-assisted diffusion apparatus is shown in Fig. 1(a) . The sample was placed inside the oven at 300 °C. A voltage of 1 kV was then applied across the sample for an hour. The current-time dynamics of the process was monitored throughout the experiment.
Step III of fabrication: After the diffusion process, the residual silver film was mechanically removed from the surface. This as-diffused sample was then annealed in air at 550 °C for 48 hours. This resulted in the formation of embedded spherical silver nanoparticles as will be discussed later.
Visualization of the depth profile: In order to visualize the depth profile of silver particles the sample was cut and a thin slice was prepared. For this, the sample was embedded in an epoxy resin (Specifix-20, Struers Limited) to prevent chipping of the glass and to make it physically manageable for grinding, polishing etc. The resin cures at room temperature. The section has been polished on both sides and was ~35 μm thick.
Optical characterization: The optical characterizations of the sample were performed using a JASCO V-670 UV/VIS/NIR Spectrophotometer, a microscope spectrophotometer [MPM 800 D/UV, Zeiss] equipped with a rectangular diaphragm of 10μm × 100μm, and KEYENCE Digital Microscope VHX-1000.
3. Results and discussion
Current as a function of time during the field-assisted diffusion is shown in Fig. 1(b). As it can be seen the current starts rising rapidly to ~420 μA/cm2 and then reaches a maximum value of ~600 μA/cm2 during the course of the process. Integrating the current over time gives the total charge transfer of ~2.01 A⋅s/cm2.
The field-assisted diffusion can be understood as an electro-chemical process in a solid-state cell [25–27]. At the applied temperature (300 °C) the cations in the glass, mainly Na+, K+, Ca2+, become mobile (sodium is particularly known to be mobile at elevated temperatures). The applied direct current (dc) electric field leads to an ionic current flow and depletion of alkali and alkaline ions under the anode. This results in a space-charge region with a strong electric field, which drives silver into the glass. Here, the oxidized silver film under anode acts as a source for silver ions. Cations are moving towards the cathode where they are neutralized, leaving negative voids near the anode. This pave the way for silver ions (Ag+) to start migrating into the glass matrix and fill the voids left by the alkali ions. For instance and in the case of sodium, it is known that the covalent character of Ag-O bond is higher than that of Na-O bond [28,29]. Therefore, the force constant for Ag-O is higher. This causes the force constant for Si-O to be lower for Ag-Si-O NBO (nonbridging oxygen) than for Na-Si-O NBO [28,29].
The amount of cations in the glass matrix is limited and they will deplete after a period. Close to the anode a large part of the current is carried by silver anions. The total charge transfer of 2.01 A⋅s/cm2 is equivalent to the charge of 2.24 mg/cm2 of Ag+ ions (This is a rough estimate for the amount of silver brought into the glass, neglecting thermal diffusion, other components of the current and silver ions of higher valence).
In Step I of the fabrication the silver film was annealed and oxidized in air. Therefore, and as it can be seen from Fig. 1(b), in Step II of the fabrication the current is ionic and basically caused by the silver ions moving into the glass from the anode and alkali and alkaline ions moving out of the glass at the cathode. Our observed current-time dynamics is in contrast to the previous works [22,26] where Step I, namely annealing of the silver film in air before field-assisted diffusion, has not been performed. This led to the observation of a sharp rise in current in a few minutes followed by slow decrease . The authors attributed their observations to the higher rate of the oxidizing reaction of silver on the anode at the beginning of the process to that of the migration rate of silver ions into the substrate.
It has been suggested earlier that the amounts of metallic ions penetrating into the glass matrix depend on the applied voltage and temperature [30,31]. This allows controlling the doping process. Here, the current after an hour became nearly constant. For the purpose of this experiment, after an hour the voltage was disconnected and the residual silver film was removed from the anode surface. The sample was transparent. Figure 2(a) shows the transmission spectra of the original glass sample (blue dashed line), and of the as-diffused sample (black dotted line). This indicates that after Step II the silver is predominantly in ionic state.
Post annealing of the sample in air for 48 hours and at 550 °C resulted in the change of color of the treated area as shown in Fig. 2(b). The extinction spectrum of the sample is shown in Fig. 2(a)-red line. This is the familiar surface plasmon resonance band, here peaking at ~410 nm and indicating the formation of spherical silver nanoparticles in the treated area.
Therefore, as a result of the post-annealing process silver ions (Ag+) are further reduced to silver atoms (Ag0), which then in turn formed silver nanoparticles with larger sizes. For annealing in air and given the considerable coloration observed and that our glass substrate is considered to be iron-free, it would be unrealistic to believe that small level of any other impurity can be responsible for the reduction mechanism. Therefore, we consider that the electrons required for silver reduction are extracted from atoms that are intrinsic to the glass, namely nonbridging oxygen (NBO) atoms  via the following reactions [29,32,33]:
Given the large amount of silver in our sample (intense coloration and strong SPR band), and in addition to the above reactions the following reduction reaction may also take place :
Making a thin slice of the sample and examining the profile of the nanoparticles-containing layer provides the means for further optical analysis. Figure 3 shows the cross section of this layer, distributed over four lines. The border region between the treated and untreated area (edge of the electrode) can easily be identified. The thickness of the nanoparticle-containing layer is ~230 μm, and as it can be seen the nanoparticles-containing layer is homogenously distributed throughout the sample. Only at the border (located in the top left corner of the slice) the profile is slightly disturbed due to the edge effects. This is where one of the edges of the electrode was placed. The lower layer is darker (dark yellow) in comparison to the area close to the surface (light yellow).
Figure 4 shows the absorbance spectra taken from different depths of the thin slice using the microscope spectrophotometer with a rectangular diaphragm of 10μm × 100μm. The spectra were taken every 10 μm across the cross section (20 spectra in total). The depth where each spectrum was measured is assigned to the horizontal-axis, while the wavelength is assigned to the vertical-axis. The darker contours in Fig. 4 indicate higher absorbance. Each contour line is labeled with an individual value. In all depths shown in this figure there is a plasmon band centered around 410 nm, corresponding to the formation of nanoparticles with diameters ranging from ~6 to 12 nm [1,23,34]. Fitting the sum of the spectra shown in Fig. 4 to the absorption spectrum measured through the whole sample (Fig. 2(a)-red line) gave a thickness of ~33.1 μm. This value is in good agreement with the thickness of the thin slice of ~35 μm.
The plasmon band is narrower for the area closer to the surface and becomes wider for lower layers. This is attributed to the formation of larger particles with lower number density closer to the surface (due to the higher probability of the reactions 1 and 2 in the near surface layer [29,33]) and smaller nanoparticles with higher number density in the deeper layers (reduction reaction 3).
Here, due to the high absorption, the SPR band was mostly cut off making it very difficult to register the exact peak position and its small red shift caused by the spill-out effect of the nanoparticles . The spill-out effect of the electrons will lead to the volume-average mean electron density decreasing and consequently peak position red shifting. It is more prominent with decreasing the particle size . Although we were not able to register the red shift caused by the spill-out effect, nevertheless broadening of the plasmon band in the depth of the sample can clearly be seen from the spectra presented in Fig. 4. It is known that the width of the plasmon band shows the decay time of the coherent motion of the electrons constituting the plasmon upon external excitation [1,36]. Previously, it has been shown that in the vicinity of the SPR band of silver nanoparticles (due to the small imaginary part of their interband susceptibility), the plasmon band at its FWHM has the same value as the frequency of electron collisions . Therefore, decrease in particle size can result in the increase of the frequency of electron collisions and hence broadening of the plasmon band .
We hence believe that our study uniquely shows the formation of larger particles with lower number density closer to the surface and smaller nanoparticles with higher number density in the deeper layers. We are currently working towards fabrication of samples with different content of inclusions for in-depth experimental and theoretical studies of the diffusion profile.
In summary, dc electric field-assisted fabrication of homogenous (island-free) silver-doped nanocomposites glass in air and via a three-step technique was demonstrated. We presented a cross-section image of the fabricated nanocomposite with an unprecedented clarity showing the depth profile of the metallic nanoparticles. This allowed optical analysis of the embedded layers. Our study uniquely shows the formation of larger particles with lower number density closer to the surface and smaller nanoparticles with higher number density in the deeper layers.
It is known that the amount of ions penetrating into the glass matrix and shape of the diffusion profile should depend on the process parameters, namely applied voltage and temperature . In addition of controlling the doping process, annealing parameters (such as temperature and duration) create the condition for metal-based nanocluster formation. It is believed that the size of the nanoparticles and the depth profile of the nanoparticles-containing layer strongly depend on the parameters of the annealing process and composition of the glass substrate (e.g., concentration of the reducing agent in the depth ).
Work is in progress to fabricate nanocomposite samples using our three-step solid-state field-assisted diffusion technique with different diffusion and annealing parameters and study their effects on the depth profile, number density and size of the inclusion. It is important since size and spatial distribution (volume fill factor and number density) of inclusions directly influences their optical properties that in turn are of paramount importance for some applications in optoelectronics [e.g., 1,2,7–14].
This work was conducted under the aegis of the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom (EP/I004173/1). Amin Abdolvand is an EPSRC Career Acceleration Fellow at the University of Dundee.
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