Preparation of Electrochemical and Surface Plasmon Resonance

J. Phys. Chem. C 2008, 112, 10883–10888

10883

Preparation of Electrochemical and Surface Plasmon Resonance Active Interfaces: Deposition of Indium Tin Oxide on Silver Thin Films Sabine Szunerits,*,†,‡ Xavier Castel,§ and Rabah Boukherroub‡ Laboratoire d’Electrochimie et de Physicochimie des Mate´riaux et des Interfaces, CNRS-INPG-UJF, 1130 rue de la piscine, B.P. 75, 38402 St. Martin d’He`res Cedex, France, Institut de Recherche Interdisciplinaire (IRI, USR CNRS-3078) and Institut d’Electronique, de Microe´lectronique et de Nanotechnologie (IEMN, CNRS-8520), Cite´ Scientifique, AVenue Poincare´ - B.P. 60069, 59652 VilleneuVe d’Ascq, France, and Institut d’Electronique et de Te´le´communications de Rennes (IETR)-UMR CNRS 6164, 18 rue H. Wallon, B.P. 406, 22004 Saint-Brieuc Cedex 1, France ReceiVed: March 25, 2008

The paper reports on the fabrication and characterization of chemically and optically stable silver-based surface plasmon resonance (SPR) interfaces. The SPR interfaces are obtained by a radio frequency (rf) sputtering technique of titanium (Ti), silver (Ag), and indium tin oxide (ITO) targets in one run at ambient temperature. The best SPR signal in terms of shape and minimum intensity was observed for Ti (5 nm)/Ag (38 nm)/ITO (4 nm) interface. The role of ITO is primarily to protect the silver interface from chemical degradation. In addition, it provides an electrochemical addressable oxide-based interface. 1. Introduction The potential of surface plasmon resonance (SPR) sensors to characterize thin films and to monitor interfacial processes was recognized in the late 1970s.1–3 Some years later, Liedberg et al.4 were able to sense immunoglobulin antibodies by observing the changes in the critical angle when the antibodies were selectively bound to a gold film. Since then, SPR sensors have been used to study many kinds of ligand-receptor interactions, including protein-ligand, antibody-antigen, protein-carbohydrate, protein-DNA, DNA-DNA, and cell adhesion interactions.5–9 The choice of the metal layer is critical for SPR sensing, as the refractive index n of the metal film influences considerably the shape of the plasmon curve.10–13 Suitable metals include silver, gold, copper, and aluminum.10,14,15 Gold is most commonly used as it possesses highly stable optical and chemical properties. Silver provides the sharpest SPR signal (small Full Width at Half-Maximum, FWHM) with an increased penetration length.13,15–17 However, the chemical instability of silver in air and particularly in aqueous solutions makes it difficult to record reliable optical signals.18 Still, silver can be used for SPR sensing if it is protected by a thin, dense, and chemically stable overlayer. For example, functional alkanethiols have been used to form self-assembled monolayers on a silver surface, protecting the interface and allowing further surface modifications.11,12,19–22 On the other hand, azo-dye-incorporated polyionene films (20 nm in thickness)23 and aluminum tris(8-hydroxyquinoline) dyes (30 nm in thickness)24 were spin-coated on silver SPR chips to protect the silver interface. An interesting alternative using bimetallic silver/gold layers has also been investigated to prepare * To whom correspondence should be addressed: tel +33 4 76 82 65 52; e-mail [email protected]. † Laboratoire d’Electrochimie et de Physicochimie des Mate ´ riaux et des Interfaces, CNRS-INPG-UJF. ‡ Institut de Recherche Interdisciplinaire (IRI, USR CNRS-3078) and Institut d’Electronique, de Microe´lectronique et de Nanotechnologie (IEMN, CNRS-8520). § Institut d’Electronique et de Te ´ le´communications de Rennes (IETR)UMR CNRS 6164.

stable SPR chips. The strategy takes advantage of both metals: the Ag underlayer sharpens the SPR signal, while the gold overlayer being in contact with the solution protects the silver from oxidation, due to its high chemical stability.25–27 In many biosensors, silane coupling chemistry on oxide substrates (glass, silicon, indium tin oxide) is used for immobilization of biomolecules. A way to take advantage of SPR spectroscopy for monitoring the course of surface reactions and the coupling chemistry developed for glass is to coat the noble metal with a thin oxide layer. Some attempts for the fabrication of such SPR chips, consisting of metal films coated with thin silicon dioxide layers, are reported in the literature.28–33 We have recently shown that stable thin films of SiOx can be deposited on thin gold films by plasma-enhanced chemical vapor deposition (PECVD).34–36 The resulting Au/SiOx chips exhibited good SPR signals and showed electrochemical activity.29 Furthermore, the Au/SiOx interfaces were successfully applied for polarization modulation infrared reflection-absorption spectroscopy (PM IRRAS) structural analysis of dimyristoylphosphatidylcholine (DMPC) bilayers deposited on the silica films.37,38 Transparent conducting oxide films such as indium tin oxide (ITO) display several interesting features such as optical transparency, electrical conductivity, and excellent adhesion properties to metals.39–44 Different techniques have been used to synthesize ITO films: sol-gel-based processes,45,46 spray hydrolysis,47 pulsed laser deposition,48–50 direct current (dc) magnetron sputtering,51,52 and radio frequency (rf) sputtering.53,54 The deposition of ITO on silver is highly suitable, as the conducting oxide film will be beneficial in several ways: (i) it will protect the silver film and hinder its chemical degradation over time; (ii) the increased penetration length of the silver plasmon waves together with a sharp SPR signal will be preserved; (iii) it will allow the combination of silane coupling chemistry with SPR spectroscopy, and (iv) the surface can be used for E-SPR based applications. The main motivation of this work is to develop a simple and reproducible method to grow ITO-conducting films with controlled thicknesses on silver-based SPR chips to protect the silver

10.1021/jp8025682 CCC: $40.75  2008 American Chemical Society Published on Web 07/01/2008

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Figure 1. Schematic diagram of multilayer interface: Silver is sandwiched between Ti and ITO thin films.

from oxidation. Titanium, Ag, and ITO films were deposited by rf sputtering at ambient temperature.55 This approach offers a possibility for multilayer deposition (several targets are incorporated in the deposition chamber) with the advantage of avoiding any diffusional and/or cracking problems between the layers due to the lack of sample heating.56 ITO thin films of 4-10 nm were irreversibly deposited on silver-based SPR interfaces and investigated for their potential applications for SPR and electrochemical-SPR sensing. The optimum silver and ITO thicknesses were evaluated in terms of chemical stability, electrical resistivity, electrochemical charge transfer, and the shape and position of the SPR signal. 2. Experimental Section 2.1. Materials. Potassium chloride (KCl) and potassium hexacyanoferrocyanide [Fe(CN)64-] were obtained from Aldrich and used without further purification. 2.2. Preparation of Silver/ITO Composite Slides. Ti/Ag/ ITO composite interfaces were formed on freshly cleaned glass slides (76 × 26 × 1 mm3; n ) 1.58 at λ ) 633 nm) by rf sputtering machine (Plassys MP 450S) at 8 × 10-8 mbar base pressure (turbomolecular rotary pump system). The deposition chamber contains three sputtering targets (75 mm in diameter), enabling the deposition of multilayers in a complete run without breaking the vacuum: (1) titanium disk (99.995% purity), (2) silver disk (99.99% purity), (3) In2O3-SnO2 ceramic disk (In2O3 90% w/w; SnO2 10% w/w, 99.999% purity). The deposition temperature is measured with a thermocouple set behind the sample holder. Metal deposition is carried out at rf power of 13.56 MHz under argon atmosphere with the following parameters: (1) Ti (rf power ) 150 W, total pressure ) 0.011 mbar, O2/Ar ratio ) 0%, deposition speed ) 20 nm min-1; (2) Ag (rf power ) 150 W, total pressure ) 0.011 mbar, O2/Ar ratio ) 0%, deposition speed ) 121 nm min-1). During ITO deposition, an Ar/O2 gas mixture is introduced into the chamber and the following sputtering parameters were used: rf power ) 38 W, total pressure ) 0.012 mbar, O2/Ar ratio ) 0.051%, deposition speed ) 0.6 nm min-1. The temperature does not exceed 25 °C in all cases. In this work, the optimized parameters in terms of shape and minimum intensity of the SPR chip were obtained for a multilayer with a Ag thin film of 38 nm sandwiched between 5 nm Ti and ITO films with a thickness ranging from 4 to 10 nm (Figure 1). 2.3. Instrumentation. 2.3.1. Electrochemical Surface Plasmon Resonance. Electrochemical experiments were performed with an Autolab potentiostat 30 (Eco Chemie, Utrecht, The Netherlands). The electrode cell is not a conventional three-electrode well, but the double-channel cell of the Autolab SPRINGLE Instrument (Eco Chemie, Utrecht, The Netherlands), allowing simultaneous SPR and electrochemical measurements to be performed. The configuration of this equipment is described elsewhere.57 In short,

Szunerits et al. polarized laser light (λ ) 670 nm) is directed to the bottom side of the sensor disk via a hemispheric lens placed on a prism (BK7 having a refractive index of n ) 1.52) and the reflected light is detected by a photodiode. The angle of incidence is varied by use of a vibrating mirror with a frequency of 44 Hz. SPR curves were scanned on the forward and backward movement of the mirror, and the minima in reflectance were determined and averaged. The instrument is equipped with an electrochemical open cuvette system of 20-150 µL sample volume where an Ag/AgCl reference electrode, a platinum counterelectrode, and a fixed contact point to the Ag or ITO layer of the sensor chip is incorporated. The active electrode surface is 0.07 cm2. Digi Sim 3 was used to simulate the cyclic voltammograms by use of the following parameters: D ) 7.6 × 10-6 cm2 s-1, R ) 0.5, and E° ) 0.155 V. 2.3.2. Sheet Resistance Measurements. The sheet resistance R of the samples is measured by use of a standard four-point probe configuration with a 225 Keithley current source and a 7050 Schlumberger microvoltmeter. 2.3.3. Atomic Force Microscopic Measurements. The samples were imaged with a Dimension 3100 model atomic force microscope (AFM; Veeco, Santa Barbara, CA) equipped with a Nanoscope IV controller (Digital Instruments) under ambient conditions. Single-beam silicon cantilevers (AFM-TM Arrow, Nanoworld) with spring constants of ∼42 N m-1 and resonant frequencies of ∼250 kHz were used. All AFM images were acquired in tapping mode at a constant force of 5-50 pN. 2.3.4. X-Ray Diffraction Measurements. An X-Ray diffractometer (Seifert 3003 PTS) in θ-2θ mode was used to determine the structure and orientation of the multilayers. A parallel beam configuration has been used with a Ge(220) monochromator mounted on the primary beam (λKR1 ) 1.54056 Å). Data were recorded between 10° and 90° with a step angle of 0.01° and a scan rate of 0.15° min-1. This slow recording had improved the signal/noise ratio. ITO indexation was obtained in reference to JCPDS file 06-0416 (In2O3 exhibits a cubic bixbyite structure with a lattice parameter a ) 10.118 Å) and Ag indexation with JCPDS file 04-0783 (Ag crystallizes in face-centered cubic structure with a lattice parameter a ) 4.0862 Å). 3. Results and Discussion 3.1. Surface Plasmon Resonance Characteristics of Ti/ Ag and Ti/Ag/ITO Interfaces. Thin films of silver (30-50 nm) were deposited on clean glass slides coated with 5 nm Ti adhesion layer by rf sputtering at ambient temperature. The shape and minimum of the SPR signal depend largely on the thickness and overall roughness of the deposited silver layer. Figure 2A shows the SPR signals of silver layers of different thicknesses recorded in water, immediately after the silvercoated glass slides were in contact with ambient temperature. An optimal SPR signal in terms of sharpness and minimum intensity was obtained for Ag films with a thickness of ∼40 nm. The SPR signal could be slightly further improved when silver was deposited directly on the glass interface without an additional Ti adhesion layer. However, these interfaces were found to be mechanically unstable and not suitable for the preparation of reliable SPR chips. The best SPR signal was observed for a silver thickness of 38 nm. The optical signal is changing significantly over time due to the chemical oxidation of the interface, visible to the naked eyes as white spots on the surface. This instability is demonstrated by following the change of the SPR signal over 2 h when the interface was immersed in water (Figure 2B). An abrupt change of the SPR signal occurs

E-SPR Active Interfaces: Deposition of ITO on Ag

J. Phys. Chem. C, Vol. 112, No. 29, 2008 10885

Figure 3. Change of SPR signal with increasing ITO film thickness on Ti (5 nm)/Ag (38 nm) SPR interfaces: (dotted line) experimental data, (solid line) fit; n(prism) ) 1.52, n(Ti) ) 2.4 + i3.313 (d ) 5 nm), n(Ag) ) 0.14 + i4.75 (d ) 38 nm), n(ITO) ) 2 + i0.001 (d ) 0, 4-10 nm).

TABLE 1: SPR Features of Ti (5 nm)/Ag (38 nm) Interfaces Coated with ITO Thin Films of Different Thicknessesa

Figure 2. (A) Influence of silver thickness on SPR signal: (gray squares) 50 nm, (gray circles) 45 nm, (black circles) 40 nm, (black squares) 35 nm, (open circles) 30 nm. (Dotted line) experimental data, (solid line) fit; n(prism) ) 1.52, n(Ti) ) 2.4 + i3.313 (d ) 5 nm), n(Ag) ) 0.14 + i4.75 (d ) 30-50 nm). (B) Stability of the silver film with time, immersion in water; Ti (5 nm)/Au (38 nm) (black) and Ti (5 nm)/Au (38 nm)/ ITO (10 nm) (gray).

in the first minutes and stabilizes after 40 min, due most likely to surface passivation by formation of a thin oxide layer. The chemical oxidation of the silver thin film in ambient conditions can be prevented by coating the silver interface with a thin protecting layer. For this purpose, thin films of ITO were deposited on the silver-based SPR chip by rf sputtering at ambient temperature. One of the advantages of the rf sputtering technique is the possibility of multilayer deposition, as several targets can be incorporated in the deposition chamber. ITO was thus deposited immediately after thin silver films have been formed, reducing the risk of silver oxidation. As the deposition process was performed at room temperature, any diffusional and/or crack problems between the layers were avoided.56 Figure 3A displays the change of the SPR signal with increasing ITO film thickness. The resonance angle Θ is shifted to higher angles and the curves are broadened, seen in an enlarged full width at half-maximum (FWHM) (Table 1). While Ti (5 nm)/Ag (38 nm) shows a FWHM ) 2.25, addition of 10 nm ITO increases the FWHM to a value not higher than 3.53. For comparison, the FWHM for an optimized Ti (5 nm)/Au (50 nm) SPR chip is much higher (5.54). Thin ITO layers with thicknesses up to 10 nm can be consequently deposited on silver-based SPR interfaces without a considerable negative effect on the SPR signal. This is due to the higher imaginary part of silver (n′′ ) i4.75) compared to gold (n′′ ) i3.313). Silver SPR interfaces coated with 10 nm ITO exhibit optical signals, which are stable over time (Figure 2B). The SPR signal does not change significantly in contact with water, in contrast to naked silver interfaces (without ITO coating). This points toward almost

sample

ITO, nm

Θ, deg

intensity

FWHM

1 2 3 4 5 6 7 8

0 4 5 6 7 8 9 10

66.38 67.83 68.23 68.66 69.13 69.64 70.14 70.64

0.019 0.023 0.023 0.0198 0.0165 0.023 0.0198 0.0198

2.25 2.79 2.84 2.93 3.12 3.25 3.39 3.53

a

Extracted from Figure 3.

pinhole-free ITO films. Indeed, silver interfaces coated with 10 nm ITO did not show significant oxidation even after 1 month in contact with water. 3.2. Topographic Characteristics of Ti/Ag and Ti/Ag/ITO Interfaces. The topography of the silver SPR chips before and after ITO coating was investigated by AFM. Figure 4A displays the tapping-mode AFM image of a silver surface immediately after exposure to ambient conditions. The surface is composed of grains with an average size of 80 nm and a surface roughness rms of ∼2.04 nm. Deposition of 4 and 10 nm ITO films on the surface induces changes in the surface roughness. The AFM image of the surface covered with 4 nm thick ITO film exhibits increased roughness (rms ∼ 5.87 nm) compared to the native surface (Figure 4B), while 10 nm thick films show rms ∼ 7.30 nm (Figure 4C). 3.3. X-ray Diffraction Measurements of Ti/Ag and Ti/Ag/ ITO Interfaces. The crystalline orientation of the heterostructures was investigated by XRD. The diffraction peaks of the Ti(5 nm)/Ag(38 nm) correspond to the (111) and (222) diffraction planes of Ag, indicating a perfect {111} texture of the Ag thin film (Figure 5A). The 2θ experimental peak positions fit perfectly with theoretical ones, indicating that the Ag layer grows without strain. The ratio of the intensity of the (222)/(111) diffraction peaks is 0.05, much lower than the value reported in the standard JCPDS file of 0.12. The deposited Ag layer exhibits thus the thermodynamically most stable (111) surface facets of the fcc crystals. Reflection from the underlying Ti layer has not been observed. Additional ITO growth is induced by the {111} facets of the Ag underlayer. The deposition of ITO results in an additional low-intensity (222) diffraction peak (Figure 5B). The small diffraction single peak does not allow us to draw any conclusion regarding the orientation of ITO crystallites.

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Szunerits et al.

Figure 4. AFM images of (A) Ti (5 nm)/Ag (38 nm), (B) Ti (5 nm)/Ag (38 nm)/ITO (4 nm), and (C) Ti (5 nm)/Ag (38 nm)/ITO (10 nm) interfaces.

3.4. Electrical and Electrochemical Characterization. Figure 6 shows the evolution of the sheet resistance for Ti (5 nm)/ Ag (30-53 nm) and Ti(5 nm)/Ag (30-53 nm)/ITO (4 nm) (Figure 6A) and Ti (5 nm)/Ag (38 nm)/ITO (0-10 nm) (Figure 6B). A multilayer structure such as Ti/Ag/ITO can be schematically described by three resistances in parallel,58 and the sheet resistance R of the interface can be described by

1 1 1 1 + + ) R RTi RAg RITO with RTi, RAg, and RITO being the resistance of the Ti, Ag, and ITO films, respectively. An increase of the silver film thickness leads to a decay61 of the total sheet resistance for the Ti (5 nm)/ Ag interfaces (Figure 6A). The sheet resistance of the bilayer is governed by the silver resistivity with FAg ) 1.587 µΩ cm (at 20 °C) rather than the thin Ti film (FTi )39 µΩ cm at 20 °C).59,60 The deposition of 4 nm of ITO on the Ti (5 nm)/Ag (30-54 nm) interfaces (Figure 6A) increases the sheet resistance to some extent; however, R decreases in the same manner as in the Ti/Ag case. The increase in the sheet resistance of the Ti (5 nm)/Ag (30-53 nm)/ITO (4 nm) interface could be due to the formation of a thin silver oxide layer at the Ag/ITO interface

or through a damaged Ag/ITO interface. Silver is a soft metal and the deposition of ITO via pulverization onto the Ag interface is realized with a relatively high energy. To distinguish between the two possibilities, we replaced the silver interlayer by gold, as this interface is less prone to surface oxidation. For Ti (5 nm)/Au (38 nm) a sheet resistance of R ) 0.88 ( 0.01 Ω sq-1 was obtained, while the presence of 4 nm ITO increased the sheet resistance to R ) 0.96 ( 0.01 Ω sq-1, indicating therefore a damage of the interface during ITO deposition rather than oxide formation. As seen in Figure 2A, an optimal SPR signal in terms of sharpness and minimum intensity is observed for a silver thickness of ∼40 nm. The development of the sheet resistance of the Ti (5 nm)/Ag (38 nm)/ITO (0-10 nm) seen in Figure 6B indicates that the thin ITO film with a resistivity of FITO ) 0.49 mΩ cm (at 20 °C) is not altering the overall resistance of the interface and that the resistivity is governed by the underlying silver film.59,60 Cyclic voltammetry (CV) in the presence of an electroactive species such as Fe(CN)64- is an additional valuable tool for testing conducting oxide electrodes. Figure 7 compares the first i/E responses of a thin silver film electrode without and with 4

E-SPR Active Interfaces: Deposition of ITO on Ag

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Figure 7. Cyclic voltammetry curves in Fe(CN)64- (10 mM) in KCl (0.1 M/water) of a 38 nm thick textured Ag film deposited on glass with 5 nm Ti adhesion layer: (black line) Ti/Ag, (black dotted line) Ti/Ag/4 nm ITO, (gray line) Ti/Ag/10 nm ITO; scan rate 0.05 V s-1.

Figure 5. X-ray diffraction patterns of (A) Ti (5 nm)/Ag (38 nm) bilayer and (B) Ti (5 nm)/Ag (38 nm)/ITO (10 nm) interface.

signal in contact with water as seen in Figure 2B. As expected, Fe(CN)64- is oxidized on the bare silver electrode in a oneelectron process with an estimated apparent rate of k0app ) 0.005 cm s-1 and peak separation, ∆Ep, of 90 mV. This higher ∆Ep is due most likely to the high film resistivity of the thin silver film as observed on gold.30 The presence of 10 nm of ITO on silver decreases slightly the electron transfer rate to k0app ) 0.0025 cm s-1. However, in contrast to gold/SiOx interfaces where the presence of SiOx limited the diffusion of Fe(CN)64to the gold electrode interface and served as a barrier for electron transfer, ITO shows a highly conductive behavior in accordance with the electrical measurements. On the other hand, while a constant sheet resistance to R ) 0.58 ( 0.01 Ω sq-1 (Figure 6B) was determined independent of the ITO thickness, the presence of the thinnest ITO films of 4 nm results in CVs with decreased current and enlarged peak separation. The cyclic voltammetry results are not in contradiction with the resistivity measurements. The four-point probe used for the resistivity measurements penetrates all layers (ITO, Ag, and Ti) and measures the global sheet resistance; CV, on the other hand, is more sensitive to the conductivity of the electrical interface in contact with the electrolyte (ITO in our case) and thus to the homogeneity of the formed ITO layer. Conclusion

Figure 6. Sheet resistance measurements: (A) Ti (5 nm)/Ag as a function of Ag thickness (b) and with additional 4 nm ITO (9); (B) Ti (5 nm)/Ag (38 nm)/ ITO as a function of the ITO thickness.

The deposition of thin, optically transparent, and electrochemical-conducting ITO films on silver-based SPR interfaces has been demonstrated. It is based on room-temperature rf sputtering with multiple targets in the deposition chamber. The resulting multilayers are chemically stable over time, an important property for use of the interface as a sensing platform. In contrast to previously reported SPR interfaces based on silicon oxide, the electrochemical behavior of the ITO thin films is highly improved and is comparable to that of metals. This allows such an interface to be used in electrochemical-based SPR experiments. The presence of surface hydroxyl groups on ITO will also allow the use of the well-known glass chemistry to be adapted in an easy way to such interfaces, opening the way of novel SPR interfaces. The sensitivity of the silver-based SPR interfaces is currently under investigation in our laboratory.

and 10 nm ITO overlayers. The i/E response is unchanged after 10 CV cycles for the Ti (5 nm)/Ag (38 nm)/ITO interfaces, while the electrochemical response on the Ti (5 nm)/Ag (38 nm) deteriorates over time due to the change of the metal interface. This is in accordance with the change of the SPR

Acknowledgment. The Agence Nationale de la Recherche (ANR, Project “LSPR”), the Centre National de la Recherche Scientifique (CNRS), the Insitut National Polytechnique de Grenoble, and the Nord-Pas-de Calais region are gratefully acknowledged for financial support.

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