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Near-infrared surface plasmon resonance (SPR) spectra were collected of thin multilayer films of aluminum-doped zinc oxide (AZO) / silver (Ag) / AZO on BK-7 glass in the Kretschmann configuration in air, with the silver layer thickness varying from 5 nm to 50 nm. The SPR results were interpreted by modeling the reflectance with a five-layer transfer-matrix method, with the aid of a simplex algorithm. The model indicated that the Ag plasma frequency was significantly higher than the bulk value, possibly due to Schottky effect charge transfer from the AZO to the Ag layer. Continuous silver films were made as thin as 10 nm, indicating an inhibition of metal island formation for Ag deposited on AZO.
©2012 Optical Society of America
1. Introduction
Surface plasmon resonance (SPR) for biosensing has traditionally been performed on noble metal surfaces such as gold and silver. These have a few advantages over other kinds of metals: they display SPR bands in both the visible and near-infrared frequencies, their surface chemistry is well-characterized and they are easily functionalized, and films are readily prepared by a variety of deposition methods. However, noble metal films suffer from a number of disadvantages. First, they are mechanically fragile and are easily degraded by handling. Second, they tend to form discrete islands, rather than a continuous film, when the film thickness is less than a critical value. Third, silver films tarnish rapidly in ambient conditions. Because of these problems, there has been a drive to produce SPR-compatible films that are chemically and mechanically robust. One alternative is to use new materials; SPR has been demonstrated on highly conducting indium tin oxide (ITO) [1–6], titanium nitride [7], various metals [8, 9], and transition metal silicides [10]. Several other materials such as graphene are under consideration [9]. Another alternative is to use layers of other materials to mediate the undesired behavior. Layers underneath the SPR-active film can reduce the onset of island formation [11, 12], and layers above can improve the chemical and mechanical stability [13–22]. The latter approach will be discussed herein.
1.1 Sandwich thin films
The thin films used in this study can be described as “sandwich” films, with an ABA layer structure. For typical sandwich films described in the literature, A is an insulating or weakly conducting transparent metal oxide or similar compound, such as ZnO, In2O3, ZnS, SnO2, or TiO2. Doped variants, such as ITO, are also used. The B layer is metal, typically Ag, Al, Au, or Cu. Films like these were originally developed for heat-reflecting transparent window coatings [23–30], where they provide greater heat reflection than single films of conducting metal oxides and greater transparency than single films of metals [31]. Sandwich films have also seen widespread use as transparent electrodes for electronic devices [32–41], where they provide a much lower resistivity than can be achieved with conducting metal oxides alone, while maintaining comparable transparency. Similar structures are also used for plasmonic waveguides [42, 43].
Another advantage of using the sandwich layer structure is in the prevention of island formation. Most transition metals, when deposited on insulating substrates, form a discontinuous film with an island-like structure when the amount of deposited material is less than a critical value. On bare glass substrates, silver does not form a continuous film less than approximately 25 nm thick, but if an AZO base layer is included, silver can form continuous films as thin as 10 nm [11]. This behavior can be interpreted as the wetting or non-wetting of the substrate surface by the metal. Greater wetting (a lower contact angle) means the metal film will transition from island-like to continuous at lower coverage, so that continuous films can be made thinner. As a result, there has been a considerable amount of research concerning the conditions that favor wetting. For a given metal, wetting is improved on less insulating substrates [44]. More specifically, as the substrate's band gap (Eg) decreases, plasma frequency increases, and high frequency dielectric constant (ε∞) increases, the work of adhesion increases and wetting improves, most likely due to increased charge transfer between the metal and the substrate [45, 46]. Thus, metal island formation is inhibited on the low band gap, moderately high dielectric constant ZnO (Eg = 3.3 eV and ε∞ = 4.1) compared to, for example, SiO2 (Eg = 8.9 eV and ε∞ = 2.4), α-Al2O3 (Eg = 8.9 eV and ε∞ = 2.9), and MgO (Eg = 7.8 eV and ε∞ = 2.95) [44, 45]. Island formation is expected to also be inhibited on doped ZnO (e.g. AZO, GZO) compared to pure ZnO, as the dopants increase the plasma frequency [47]. Note that transition metal seeding can also improve wetting [11, 44, 48, 49], without increasing the film transmissivity.
To date we have not found any reports of an SPR study on sandwich films. However, there are multiple reports of SPR with a single overlayer. Bao et al. [13] reported SPR sensing on 55 nm Ag films with variable thickness ZnO overlayers. Other kinds of overlayers have also been used for SPR sensors, including a silicon-carbon alloy [14], ITO [15], amorphous carbon [16, 17], antimony-doped SnO2 [18, 19], and silicates [20–22]. In addition, the approach we have taken to extracting information from the SPR spectra by modeling appears to be unique.
1.2 Surface plasmon resonance
Surface plasmon resonance (SPR) is a phenomenon in which electrons oscillate at the interface between a conducting and a dielectric medium. When this oscillation is driven by electromagnetic radiation, an electromagnetic wave called the Surface Plasmon Polariton (SPP) is generated, which can be understood as an electromagnetic wave that has been confined to the conducting surface, decaying exponentially into both of the surrounding media. The wavevector of the SPP is highly dependent on the electronic properties of the conductor used, and on the refractive index of the material close to the interface. Because of this, SPR has been used in numerous applications that detect a change in the index of refraction of one of the two materials. The most prominent example is the use of SPR for biosensing, in which binding of biomolecules to the surface causes a small change in the refractive index of the dielectric medium near the interface, which can be detected by a change in the reflectivity from the metal surface. Several review articles and book chapters have been published on this topic [50–53].
In this paper, we have used Kretschmann's geometry [54, 55], in which a thin conducting film is attached to a glass prism, and the light strikes the film through the prism. This configuration is well-suited for biosensing work, as it permits a relatively high-volume flow cell to be attached on top of the SPR-active conducting material. However, the goal of the current study was not to demonstrate biosensing capability, but to assess the optical and material effects of the sandwich thin film configuration.
2. Materials and methods
2.1 Film deposition
Three-layer films of AZO, Ag, and AZO were deposited on glass microscope slides ultrasonically cleaned in acetone for 5 min. prior to use. To achieve high quality thin Ag films it is beneficial to employ a seed layer. ZnO is well known as an excellent seed layer to promote the growth of adherent, crystalline films of Ag [12]. AZO was chosen instead of undoped ZnO for this work to facilitate sputtering. The top layer of AZO was included as a preventative measure against agglomeration of the Ag films into islands when exposed to atmosphere.
The AZO films were deposited from a 99.99% purity metallic Al:Zn target (4% atomic Al) and the Ag films from a 99.99% purity Ag targets. Prior to all depositions the chamber base pressure was below 1 × 10−6 mTorr using a cryogenic pump. All layers were deposited without any heating of the substrate, other than what is provided by the sputtering process. The microscope slides were rotated to achieve improved uniformity with the magnetrons positioned 5” from the substrate platen. The AZO layers were deposited using a 2” magnetron by pulsed DC magnetron sputtering at 50 W at a frequency of 200 kHz and positive pulse time of 1.6 µs with 5 mTorr Ar and 0.40 mTorr O2. Four point probe and Hall Effect testing found the AZO films to be highly resistive for the thicknesses employed in this testing. The Ag films were deposited from a 3” magnetron with 150 W applied DC power at 5 mTorr Ar. A shutter in front of the substrate allowed both targets to be presputtered for >2 minutes prior to film deposition. The layer thicknesses were estimated based on a combination of deposition rate measured by QCM, total thickness measured by profilometry, and optical modeling of individual films in TFCalc.
2.2 SPR analysis
The films were analyzed via a Kretschmann geometry SPR setup in the near infrared. The film substrate was attached to a 60° triangular BK-7 glass prism (nD = 1.517; n = 1.506 at λ = 1 μm) using index matching fluid (nD = 1.520, Cargille Laboratories Inc.). The prism/film assembly was mounted in a custom-built SPR unit (GWC Technologies, Inc.) attached to an FTIR spectrometer (Thermo Scientific). The light from the instrument was routed through a CaF2 beamsplitter, through a linear polarizer, reflected by the film in the Kretschmann configuration on a θ-2θ stage, and detected by an extended-range InGaAs detector. Angle of incidence and polarization were controlled manually. The instrument software collected 32 scans at 32 cm−1 resolution within the spectral range 4200 cm−1 to 11000 cm−1 wavenumber (2.38 μm to 0.909 μm wavelength) at each angle of incidence. The spectra are presented on the left side of Figs. 1 and 2 as p-polarized reflectance divided by s-polarized reflectance (Rp/Rs). No other spectral processing or manipulation has been performed on the experimental spectra.
Fig. 1 Experimental (left) and theoretical (right) FTIR-SPR spectra from 4200 cm−1 to 11000 cm−1 (2.38 μm to 0.909 μm) for (A) the 23 nm AZO / 5 nm Ag / 23 nm AZO film, (B) the 23 nm AZO / 10 nm Ag / 23 nm AZO film, (C) the 23 nm AZO / 11.2 nm Ag / 23nm AZO film, and (D) the 42.0 nm AZO / 13.9 nm Ag / 22.4 nm AZO film. The summary for all films is shown in Table 1.
Fig. 2 Experimental (left) and theoretical (right) FTIR-SPR spectra from 4200 cm−1 to 11000 cm−1 (2.38 μm to 0.909 μm) for (A) the 21.5 nm AZO / 15.8 nm Ag / 48.5 nm AZO film, (B) the 27.5 nm AZO / 20.0 nm Ag / 27.1 nm AZO film, (C) the 23 nm AZO / 30 nm Ag / 23nm AZO film, and (D) the 23 nm AZO / 50 nm Ag / 23 nm AZO film. The summary for all films is shown in Table 1.
3. Modeling
3.1 SPR spectra
Computer simulations of the spectra were performed in Matlab, using a multilayer transfer-matrix formalism [56, 57] for reflectance from a 5-layer model (Glass / AZO / Ag / AZO / Air). The model assumes the layers are homogeneous parallel planes, and that there are no magnetic effects. The glass refractive index, as a function of wavenumber, was taken from the Schott, Inc. website, and interpolated with a cubic spline. The air refractive index was simulated as constant 1.000. The silver dielectric function was initially taken to be that of the bulk material, but the modeling results were poor, so the spectra were re-modeled using a Drude dielectric function:
This dielectric function (Eq. (1) has three adjustable constants: ε∞, ωp, and Γ, which were treated as unknowns in the modeling program. Finally, the AZO dielectric function was treated as an unknown adjustable constant (εAZO), without an imaginary component. Using the Drude model for AZO did not improve the modeling results. The relevant constants (ε∞, ωp, Γ, and εAZO) were adjusted according to a simplex algorithm that sought to minimize the variance between the experimental and theoretical spectra (Figs. 1 and 2). The experimental spectra were baseline corrected before this process took place.
3.2 Bulk silver
For comparison, the values of ε∞, ωp, and Γ were obtained from a Drude model fit to the bulk silver dielectric function (Fig. 3 ). Several data sources were used: ATI: Acree Tech Inc. spectroscopic ellipsometry data; Sopra: Sopra S. A. database; Ordal: Ordal, et al. [58]; J&C: Johnson and Christy [59]. These dielectric functions and their fit are displayed in Fig. 3, and the results are also listed in Table 1 . For the fitting procedure, the Drude dielectric function (Eq. (1) was split into its real and imaginary components, such that ε = ε1 + iε2:
The fitting procedure for the bulk silver dielectric function used the same simplex algorithm as for the SPR spectra, and sought to fit both the real (Eq. (2) and imaginary (Eq. (3) components simultaneously. To achieve this, the following value (Q, Eq. (4) was minimized, based on the coefficient of determination (R2, Eq. (5) of the two fits: In Eq. (5), are the experimental data points, are the modeled data points, and is the average of all. Note that this definition of R2 is technically only valid for linear regression, and we have used it here only to produce an acceptable fit to the data. To avoid bias toward the data sets with higher populations, the fitting was done independently for each data source, and the results were averaged.Fig. 3 Complex dielectric function (ε = ε1 + iε2) for bulk silver from 4000 to 11290 cm−1 wavenumber (2.5 μm to 0.886 μm wavelength), from the following data sources: ATI: Acree Tech Inc.; Sopra: Sopra S. A. Database; Ordal: Ordal, et al. [58]; J&C: Johnson and Christy [59]. Model: Drude model fit, using the means across data sources of the parameters ε∞, ωp, and Γ given by the method in the text. The reported R2 values are the averages. SPR: Average result from SPR modeling (cf. Table 1).
Table 1. Summary of fitted parameters for the AZO/Ag/AZO films
3.3 Sensitivities
The sensitivities of these films to refractive index changes were estimated by theoretical modeling, in a similar fashion as Homola et al. [50]. Due to the low refractive index of the BK7 glass used (nD = 1.518), and the limited angle range accessible by our instrument, it is not possible to study the sensitivity directly on these films. Using the material constants obtained by fitting (Table 1), three of the sandwich films, corresponding to 10nm, 20nm, and 50nm Ag, were modeled on Schott SF10 glass (nD = 1.728) in water. A 20nm silver film was also modeled without AZO layers. The dispersion of the water was taken into account by interpolating the complex dielectric function [60]. Modes analyzed included both angle interrogation (with wavenumber fixed at 7600 cm−1), and wavenumber and wavelength interrogation (with angle fixed at 52.4°). The results are shown in Table 2 .
Table 2. Estimated sensitivities of the films to refractive index changes. Sensitivities expressed in units per refractive index unit (RIU): Sθ: angle interrogation, Sν: wavenumber interrogation, Sλ: wavelength interrogation.
4. Transmission
Optical transmission spectra of the films were collected between wavelengths of 190 nm and 1100 nm, by placing the films in a homemade sample holder placed in the beam of the spectrometer. The glass was not transparent below approximately 260 nm. The results are shown in Fig. 4 .
5. Results
The results from the FTIR-SPR measurements are shown alongside the results from the modeling in Figs. 1 and 2. In all but the 5 nm film, the spectral minima have strong angle dependence, which is characteristic of a typical SPR response in the film system. It is easy to show that this SPR response is due to the silver layer by modeling using a three-layer system omitting the AZO layers (Glass / Ag / Air), and using the bulk dielectric function for Ag. Such models do show some spectral features, and appear somewhat similar to the theoretical results for the five-layer model shown in Figs. 1 and 2, although the precise locations and intensities of the resonances do not match. In contrast, similar features are not seen for AZO alone, by any reasonable estimation of its properties. The five-layer theoretical model appears to accurately predict most of the spectral features for the films with Ag thickness greater than 5 nm. However, the experimental spectra have a curved background, with higher Rp/Rs at larger wavenumbers, that is not predicted by the model. This effect is possibly due to scattering losses in Rs from film roughness. The model assumes the films are perfectly smooth, and does not estimate scattering. In addition, plain BK7 glass in the absence of any film does not show this effect, indicating that the effect is not an instrumental artifact. To correct for this, the curved background was subtracted from the experimental results before calculating the goodness of fit.
The effects of changing the thickness of the Ag layer are readily apparent. For the 5 nm Ag film (Fig. 1(A)), no SPR features are present at any angle accessible by the instrument, and the spectra display a larger intensity variation than is observed for the other films. These effects are similar to what is observed in the SPR spectra of layer-by-layer deposited films of silver and gold nanoparticles on glass, when the density of the nanoparticles is not high, and is possibly indicative that the film is not electrically contiguous. As the Ag films are made thicker (Figs. 1 and 2), the SPR responses gain intensity, the peak widths narrow, and the sensitivity to angle of incidence increases. These effects are consistent with theory. Finally, when the Ag film is thicker than 30 nm (Fig. 2(C) and 2(D)), the SPR responses lose intensity due to damping. By 70 nm Ag, no spectral features are evident (not shown).
The modeling results indicate that the values of ε∞, ωp, and Γ are significantly higher than for bulk silver for all films (Table 1 and Fig. 3). The experimental-theoretical match is good. In contrast, modeling using the bulk dielectric function of Ag produces poor results. In addition, the dispersion of the glass has also been accounted for, and the dielectric function of the AZO has been allowed to vary from film to film. Therefore, we believe that these values are the result of a real effect in the film, and are not simply due to a neglected aspect of the modeling.
The estimations of sensitivity to changes in refractive index are shown in Table 2. Although the silver layer thickness plays a role in the sensitivity by wavenumber and wavelength interrogation, it is clear from the results that the plain silver film has the highest sensitivity for all interrogation modes. Although only three sandwich films were modeled, the sensitivities of the other sandwich films are expected to be comparable. Note that due to the water dispersion, the wavenumber and wavelength interrogation mode sensitivities are nonlinear with refractive index.
The transmission spectra are presented in Fig. 4. As the silver layer thickness increases, the films become less transparent. For all but the 5 nm Ag film, the films show a maximum transmittance intensity between 360 and 500 nm, which blue-shifts as the thickness of the film increases. Also in these films, the transmittance decreases uniformly at wavelengths higher than 500 nm, due to increased reflection. In contrast, the 5 nm film transmittance remains between 60 and 80% for all wavelengths higher than 500 nm, which is most likely an indication that the film is discontinuous and does not support planar SPR. For all films, the minima seen at approximately 265 nm (270 nm for 5 nm Ag) are most likely attributable to silver inter-band transitions.
6. Discussion
It is clear from the results that the optical properties of the silver are substantially different than bulk silver when in the sandwich configuration. While the higher damping constant is expected, the higher plasma frequency and high-frequency dielectric constant seem to contradict initial expectations; we would expect these values to be slightly lower, rather than higher, due to the inevitable imperfections of the deposition process. As explained below, one possible cause of the higher plasma frequency is the Schottky effect. If this is true, it could be a facile way to systematically engineer the plasma frequency in other SPR devices.
First, the plasma frequencies of the films were observed to be higher than that of bulk silver. One possible explanation is that there is charge transfer from the surrounding AZO to the Ag layer (a band-bending or Schottky effect). This effect is related to the difference in work functions of the two materials, and would likely be small because the work functions of Ag and AZO are similar (4.14-4.46 eV for Ag, depending on crystal orientation, and 3.7-4.62 eV for AZO, depending on doping) [61–63]. Unfortunately, there is not enough information to discern the magnitude of the charge transfer; even if work functions are chosen arbitrarily, the width of the transition region and the charge carrier density in the AZO must be known to discern the amount of additional electron density in the silver layer [64]. Additional experiments will need to be conducted to determine these parameters.
Second, the damping constant (Γ) was also observed to be higher than that of bulk silver. This can be explained in terms of lower electron mobility (μ), to which Γ shares an inverse relationship:
Here, m is the charge carrier effective mass, e is the elementary charge constant, and μ is the mobility. Note that the mobility is frequency dependent in visible frequencies. In turn, the low mobility can be explained by the silver having a finite grain size, which introduces grain boundaries that reduce the electron mean free path. This explanation is in itself not particularly remarkable, as grain boundaries are expected in a sputtered polycrystalline film. However, note that Γ increases dramatically for the 5 nm silver film, which could be indicative of islanding or a similar kind of discontinuity. In addition, Γ should theoretically decrease with the thickness of the Ag layer [65], which is not seen in our results. This inconsistency is yet to be explained. The damping constant may also be artificially higher due to beam divergence in the instrument, causing a slight spread in the incident angles. This effect has not been accounted for in the modeling. The fact that Γ remains relatively constant for films as thin as 10 nm supports that the films are continuous down to 10 nm, which is a direct consequence of the sandwich stack.Third, the high-frequency dielectric constant (ε∞) was also observed to be higher than that of bulk silver. The cause of this is currently unknown. Its effect on the Drude dielectric function is simple: it is an additive scaling term for the real part (ε1). Essentially, the higher plasma frequency causes ε1 to diverge significantly from the bulk value at lower frequencies, while the higher ε∞ brings ε1 closer to the bulk value at higher frequencies (Fig. 3). This could potentially indicate that the effect causing the higher plasma frequency is frequency-dependent.
In the modeling, the AZO dielectric function was modeled as constant, with no dispersion. Substituting a dispersive dielectric function (the Drude model, Eq. (1) did not improve the modeling results. However, if the AZO is made more conductive in the future, it may be necessary to use a conductive dielectric function model.
The lower sensitivity of the sandwich films compared to plain silver (Table 2) can be understood as a consequence of the top AZO layer, which acts as a barrier, preventing the analyte from reaching the most sensitive region closest to the metal. This same barrier is anticipated to provide better chemical and mechanical stability to the films, though proving this will require additional experiments.
The observations of onset of a SPR response as a function of film thickness is predicted theoretically, but there have been limited means to study this effect experimentally. We recently demonstrated a similar effect for ITO thin films [3]. In the case of ITO the skin depth, which lies between 100 and 150 nm at wavenumbers less than 7,000 cm−1, is much greater than Ag, which has a skin depth of 23 nm between 6,000 and 12,000 cm−1. The skin depth correlates roughly to the thickness, which gives the onset of the SPR. The SPR as a function of thickness is observed at ca. 120 nm for ITO [3]. Here, we can predict that the onset of SPR will be less than 10 nm for Ag. In both conducting materials a similar decrease in the SPR effect is also observed as the film thickness increased above twice the skin depth of the film. The maximum effect is observed when the film thickness is approximately 25% greater than the skin depth. One of the significant differences between Ag and ITO is observed in the very thin films. In ITO we have observed the equivalent of the localized surface Plasmon resonance (LSPR), which we have called the capacitive Plasmon resonance to indicate that charge separation is normal to the film. This is observed at ca. 8900 cm−1 in highly conductive ITO. Since the SPP of Ag is much higher and is outside of the spectral window of observation, no such effect is observed in ultrathin Ag films, in which the film thickness is significantly less than the skin depth.
In conclusion, using non-conductive AZO layers to prevent islanding of an SPR-active silver layer in a sandwich configuration seems to be a viable method of fabricating a robust and transparent SPR device, but there are still experiments that need to be performed. First, the sandwich configuration could potentially offer a previously unrealized method of controlling the observed plasma frequency and high-frequency dielectric constant, and thus the SPR responses, of the silver layers. Characterizing the dependence of these properties as a function of AZO thicknesses, doping levels, and deposition parameters is a high priority. Second, we are interested in functionalization of the AZO in order to construct a device. In principle, functionalization could be easily performed with thiols, which are known to form high-quality self-assembled monolayers on AZO [66]. Third, in order to use a device in an aqueous environment with the current angle range of our instrument, it is necessary to deposit the film system on a material with a higher index of refraction. Lastly, it remains to be determined whether the films are resistant to tarnishing, oxidation, corrosion, and scratching. Initial results are promising, but will require a long-term study compared to a control.
7. Conclusions
The films produced here are continuous down to 10 nm thickness of Ag. In contrast, films of silver on plain glass show islanding up to approximately 20-25 nm thickness. The surrounding AZO layers are able to inhibit islanding.
The modeling process, using a five-layer transfer-matrix method in conjunction with a simplex algorithm, was able to extract the dielectric function of the silver layers. This process could be applied to other SPR-active materials, which would allow us to more accurately estimate the effects of thin-film deposition.
In the ultra-thin Ag films studied using FT-SPR, the plasma frequency of the silver layer was demonstrably higher than its bulk value. If this can be shown to be due to the Schottky effect, it opens a technological door: film systems could be deliberately engineered to utilize this effect for SPR work.
Acknowledgments
This work was supported in part by the National Science Foundation grant number 554076. The authors would like to acknowledge Stephen Weibel, Marta Cerruti, and Misun Kang for their helpful contributions.
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