Preparation and Characterization of Silver Substrates Coated with

May 12, 2009 - †Laboratoire d'Electrochimie et de Physicochimie des Mat´eriaux et des ... BP 75, 38402 St. Martin d'H`eres Cedex, France, ‡Instit...
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Preparation and Characterization of Silver Substrates Coated with Antimony-Doped SnO2 Thin Films for Surface Plasmon Resonance Studies )

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:: Mael Manesse,†,§ Rosendo Sanjines,‡,* Valerie Stambouli,§ Corentin Jorel, Bernard Pelissier, Marcin Pisarek,^ Rabah Boukherroub,# and Sabine Szunerits†,#,*

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† Laboratoire d’Electrochimie et de Physicochimie des Mat eriaux et des Interfaces (LEPMI), CNRS-INPGUJF, 1130 rue de la piscine, BP 75, 38402 St. Martin d’H eres Cedex, France, ‡Institut de Physique de la Mati ere Condens ee (IPMC), SB/EPFL, Station 3, CH-1015 Lausanne, Switzerland, §LMGP, Laboratoire des Mat eriaux et du G enie Physique, INP Grenoble, Minatec, 3 parvis Louis N eel, BP 257, 38016 Grenoble Cedex 1, France, Laboratoire des Technologies de la Micro electronique (LTM), CNRS, 17 avenue des Martyrs, 38054 Grenoble, France, ^Institute of Physical Chemistry, Polish Academy of Sciences (Physical Chemistry of Materials Center), Kasprzaka 44/52, 01-224 Warsaw, Poland, and #Institut de Recherche Interdisciplinaire (IRI, USR-3078) and Institut d’Electronique, de Micro electronique et de Nanotechnologie (IEMN, CNRS-8520), Cit e Scientifique, Avenue Poincar e - B.P. 60069, 59652 Villeneuve d’Ascq, France

Received February 10, 2009. Revised Manuscript Received April 15, 2009 This paper reports on the preparation of silver/antimony-doped tin oxide (Ag/SnO2:Sb) hybrid interfaces using magnetron sputtering and their characterization. The influence of the Sn target composition (doping with 2 or 5% Sb) on the electrochemical and electrical characteristics of the hybrid interface was investigated using X-ray photoelectron spectroscopy (XPS), sheet resistance measurements, cyclic voltammetry, scanning tunneling microscopy (STM) and surface plasmon resonance (SPR). The best interface in terms of electrical conductivity and SPR signal is a hybrid interface with a 8.5 ( 0.3 nm thick SnO2:Sb layer obtained from a Sn target with 2% Sb deposited on 38 nm thick silver film. Different strategies to link functional groups onto the Ag/SnO2:Sb interface are also presented.

1. Introduction The label-free detection scheme based on the principle of surface plasmon resonance (SPR), has become exceedingly popular in analytical science.1,2 The choice of the metal layer where plasmon waves are generated is critical for sensitive SPR sensing. The refractive index n and particularly the imaginary part n00 of the thin metal film considerably influences the shape of the final plasmon curve.3,4 Gold is most commonly used, as it possesses stable optical and chemical properties. In the case of silver, the SPR signal shows a small full width at half-maximum (fwhm) value and thus sharp signal with an increased penetration length.5 However, the chemical instability of silver in air and particularly in aqueous solutions makes it difficult to use the interface for sensing.6 Protecting the silver interface with thin, dense, and chemically stable overlayers is an interesting alternative to benefit from the sensitive SPR response of silver as well from an optical stabilized SPR signal.7-9 Our group and others have successfully prepared oxide-based (SiOx, ITO, SnO2) *To whom correspondence should be addressed. E-mail: sabine.szunerits@ enseeg.inpg.fr. Tel:(+33) 04 76 82 65 52. Fax: (+33) 04 76 82 66 30. (1) Homola, J.; Yee, S. S.; Gauglitz, G. Sens. Actuators, B 1999, 54, 3–15. (2) Wolfbeis, O. S.; Homola, J. Surface Plasmon Resonance Based Sensors; Springer: Berlin/Heidelberg, 2006. (3) Hutter, E.; Cha, S.; Liu, J.-F.; Park, J.; Yi, J.; Fendler, J. H.; Roy, D. J. Phys. Chem. B 2001, 105, 8–12. (4) Lecaruyer, P.; Canva, M.; Rolland, J. Appl. Opt. 2007, 46, 2361. (5) Hutter, E.; Fendler, J. H.; Roy, D. J. Phys. Chem. B 2001, 105, 11159–11168. (6) Kooyman, R. P. H.; Kolkman, H.; Van Gent, J.; Greve, J. Anal. Chim. Acta 1998, 212, 35. (7) Uznanski, P.; Pecherz, J. J. Appl. Polym. Sci. 2002, 86, 1459. (8) Winter, G.; Barnes, W. L. App. Phys. Lett. 2006, 88, 051109. (9) Zynio, S. A.; Samoylov, A. V.; Surovtseva, E. R.; Mirsky, V. M.; Shirshov, Y. M. Sensors 2002, 2, 62. (10) Szunerits, S.; Castel, X.; Boukherreoub, R. J. Phys. Chem. C. 2008, 112, 10883. (11) Szunerits, S.; Boukherroub, R. Langmuir 2006, 22, 1660.

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SPR chips on gold10-14 and silver interfaces.10,15,16 The use of a transparent wide band semiconductor such as tin oxide (SnO2; Eg = 3.6 eV) is particularly appealing, as the doping level can be changed rather easily by the amount of incorporated pentavalent cations such as Sb5+ (substitution of Sn4+) inside the film.15 However, in the case of the Ag/SnO2:Sb interfaces obtained from a Sn target containing 5% Sb, the SPR signals were already strongly degraded, even at a film thickness of ∼8 nm. This is due to the high imaginary part n00 of the final SnO2:Sb interface.15 In this paper, we show that the SPR signal of Ag/SnO2:Sb interfaces can be improved by using Sn targets containing a decreased Sb percentage without sacrificing their good electrical and electrochemical properties. Different functionalization schemes will be discussed to make the Ag/SnO2:Sb interfaces ready for further sensing applications.

2. Experimental Section 2.1. Materials. Potassium permanganate (KMnO4), sodium periodate (NaIO4), hydrochloric acid (HCl), potassium carbonate (K2CO3), sodium hydrogensulfite (NaHSO3), phosphate buffer (PBS, 0.1 M), ruthenium(III) hexamine trichloride ([Ru(NH3)6] Cl3), N-hydroxysuccinimide (NHS), 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC), sodium dodecyl sulfate (SDS), sodium hydroxide (NaOH), sulfuric acid (12) Szunerits, S.; Boukherroub, R. Electrochem. Commun. 2006, 8, 439–444. (13) Szunerits, S.; Coffinier, Y.; Janel, S.; Boukherroub, R. Langmuir 2006, 22, 10716–10722. (14) Szunerits, S.; Nunes-Kirchner, C.; Wittstock, G.; Boukherroub, R.; Chantal, G. Electrochim. Acta 2008, 53, 7805–7914. (15) Manesse, M.; Sanjines, R.; Stambouli, V.; Boukherroub, R.; Szunerits, S. Electrochem. Commun. 2008, 10, 1041. (16) Szunerits, S.; Castel, X.; Boukherroub, R. J. Phys. Chem. C. 2008, 112, 15813.

Published on Web 05/12/2009

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Manesse et al. (H2SO4), and hexane of analytical grade were purchased from Sigma-Aldrich. 3-Aminopropyltriethoxysilane (APTES), octadecyltrichlorosilane (OTS) and undecenyltrichlorosilane (UETS) were purchased from Gelest Inc. (France). 5-(Aminoacetamido) fluorescein was obtained from Invitrogen, Inc. (France). 2.2. Preparation of Ag/SnO2:Sb Composite Slides. The formation of Ag/SnO2:Sb hybrid interfaces was carried out using magnetron sputtering in one deposition chamber containing three sputtering targets: Ti (99.995% purity), Ag (99.995% purity), and Sn (99.995%). The Ag interfaces were obtained by first sputtering 5 nm of Ti [V = 291 V; P = 23 W; I = 100 mA; p = ∼4.5  10-3 mbar; Ar (20 sccm); deposition speed = ∼24 A˚ min-1] followed by 38 nm of Ag [370 V; 17 W, 60 mA; ∼4.5  10-3mbar; Ar (20 sccm), deposition speed = ∼140 A˚ min-1]. SnO2:Sb films between 5 and 10 nm were deposited on the Ti/Ag interfaces using Sn (99.995% purity) targets doped with (i) 5% or (ii) 2% Sb (99.995% purity) (Kurt J . Lesker, Clairton, PA) using the following parameters: V = 290 V; I = 30 mA; p = ∼ 6  10-3 mbar; O2/Ar ratio = 10/20 sccm; deposition speed = ∼ 20 A˚ min-1. 2.3. Silanization of the Ag/SnO2:Sb Interface. All the surface modification steps were performed on Ag interfaces coated with a 8.5 ( 0.3 nm thick SnO2:Sb layer (formed with a Sn target doped with 2% Sb). The interface was first cleaned by UV/ozone to remove any organic contaminants on the surface and then reacted with a 10-2 - 10-3 M solution of OTS in hexane/ CCl4 (v/v: 70/30) for 2 h at room temperature. The resulting surfaces were rinsed with CHCl3 and dried under a stream of nitrogen. Amine-terminated Ag/SnO2:Sb surfaces were prepared by chemical treatment of the clean surface with 3% APTES in ethanol/water (v/v: 95/5) for 30 min under sonication. The interfaces were then washed with methanol, water (two times), and methanol and annealed for 20 min at 110 C. 17 Carboxylic-terminated Ag/SnO2:Sb surfaces were prepared in a two-step process: formation of vinyl termination followed by chemical oxidation of the terminal CdC double bonds to COOH groups. The vinyl termination was obtained by the reaction of the clean hydroxyl-terminated surface with UETS (0.2%) in hexane for 2 h at -10 C. The resulting interfaces were then rinsed with chloroform, ethanol ,and water, and dried under a stream of nitrogen. The terminal vinyl groups were oxidized using an aqueous solution of NaIO4 (195 mM), KMnO4 (5 mM), and K2CO3 (18 mM) for 24 h at room temperature. The interfaces were then immersed in a NaHSO3 (0.3 M) aqueous solution for 5 min, in a HCl (0.1M) solution for 5 min, and finally rinsed with milli-Q water. The terminal COOH groups were linked to 5-(aminoacetamido)fluorescein by first immersing the interfaces in a NHS (50 mM)/EDC (20 mM) aqueous solution for 30 min at 15 C and then reacting with a 10 μM aqueous solution of the amineterminated dye. The reaction was conducted at room temperature for 2 h. The resulting interfaces were cleaned by SDS (2%) aqueous solution for 15 min, rinsed three times with milli-Q water, and dried under a nitrogen stream. 2.5. Instrumentation. Electrochemical SPR. Cyclic voltammetry (CV) experiments were performed using an Autolab potentiostat 100 (Eco Chemie, Utrecht, The Netherlands). The electrolyte was phosphate-buffered saline (PBS, 0.1 M)/water. The electrochemical cell is the cell of the Autolab ESPRIT Instrument (Eco Chemie) allowing simultaneous SPR and electrochemical measurements to be performed. For a detailed description of the instrument, see ref 22. The instrument is equipped with an electrochemical open cuvette system of 20-150 μL sample volume, where a Ag/AgCl reference electrode, a platinum counter electrode, and a fixed contact point to (17) Coffinier, Y.; Szunerits, S.; Marcus, B.; Desmet, R.; Melnyk, O.; Gengembre, L.; Payen, E.; Delabouglise, D.; Boukherroub, R. Diamond Relat. Mater. 2007, 16, 892.

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Article the conducting layer of the sensor chip are incorporated. The active electrode surface is 0.07 cm2. The refractive indexes used were n(Ti) = 2.4 + i3.313 with d = 5 nm; n(Ag) = 0.140 + i4.581 with d = 38 nm; n(SnO2:Sb) = 1.91 + i0.643 (5%); and n(SnO2:Sb) = 1.91 + i0.249 (2%).10,15 Ellipsometry. Spectroscopic ellipsometry data in the visible range was obtained using a UVISEL Jobin Yvon Horiba Spectroscopic Ellipsometer equipped with a DeltaPsi 2 data analysis software. The system acquired a spectrum ranging from 2 to 4.5 eV (corresponding to 300 to 750 nm) with 0.05 eV (or 7.5 nm) intervals. Data were taken using an angle of incidence of 70, and the compensator was set at 45.0. Data were fitted by regression analysis to a film-on-substrate model as described by their thickness and their complex refractive indices. The values given are averaged over five measurements taken on different spots on the surface. Contact Angle Measurements. Water contact angles were measured using deionized water. We used a remote-computer controlled goniometer system (DIGIDROP by GBX, France) for measuring the contact angles. The accuracy is (2. All measurements were made in ambient atmosphere at room temperature. Sheet Resistance Measurements. The sheet resistance R of the samples is measured using a homemade four-point probe configuration at room temperature. 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 A˚). Data were recorded between 10 and 90 with a step angle of 0.01 and a scan rate of 0.3 min-1. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed using a customized Thermo Fisher Scientific Theta 300 spectrometer or a Microlab 350 instrument (nonmonochromatic) using the AlKR wavelength (1486.6 eV). Sb3d and O1s spectra were recorded with 100 eV pass energy, while survey spectra and high resolution spectra of the modified SnO2 interface were recorded using 150 and 50 eV pass energy. The pressure during analysis was 1.0  10-9 mbar. A linear or Shirley background subtraction was made to obtain XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/ Lorentzian mixed function. The measured binding energies were corrected referring to energy of C1s at 285 eV. Scanning Tunneling Microscopy (STM). STM was performed on a commercial ultrahigh vacuum STM-Omicrometer system. Fluorescence Measurements. Fluorescence (λex= 492 nm; λem = 516 nm) was recorded with an epifluorescence microscope (BX 60, Olympus) equipped with a Peltier cooled charge-coupled device (CCD) camera (Hamamatsu) and imaging software (Cell, Olympus).

3. Results and Discussion The physical properties of the transparent conducting antimony-doped tin oxide SnO2:Sb films have been investigated by optical (reflexivity and transmittance) and electrical measurements. For this purpose, 650 nm thick films have been deposited on glass at a substrate temperature of 300 C. The deposited SnO2:Sb films from two Sn:Sb targets with nominal Sb composition of 2 at % and 5 at % exhibit low resistivity values of 6.1 x10-3 Ωcm and 2.7 x10-3 Ωcm, respectively. The reflection spectra of the deposited films indicate that these films exhibit high infrared reflection with plasma resonance wavelength λp of about 1750 and 1250 nm as shown in the Figure 1. From these results and using a classical Drude theory, the optical carrier concentration is estimated to be 3.7  1020 cm-3 for the films deposited from the 2% Sb-doped Sn target and 7.2  1020 cm-3 for the films DOI: 10.1021/la900502y

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Figure 1. Reflection spectra of Sb-doped tin oxide films deposited from 2 atom % (red line) and 5 atom % (blue line) Sb-doped Sn targets.

deposited from the 5% Sb-doped target. In addition, using resistivity and carrier concentration values, the electron mobility is found to be independent of the doping level (2.8-3.0 cm2/(V s)), indicating that the electrical properties of these films are mainly controlled by the doping concentration. These results are in good agreement with those reported in the literature for Sb-doped SnO2 films.23,24 Tin (Sn) targets with 5 and 2% antimony (Sb) content were used to form thin SnO2:Sb films on clean glass slides coated with 38 nm silver (Ag) and 5 nm titanium (Ti) adhesion layer using magnetron sputtering. The film thickness was controlled by the deposition time and estimated using ellipsometry and SPR techniques. The film thickness of the SnO2:Sb layer for both Sn targets (with 2 or 5% Sb) is comparable for a similar deposition time (Table 1). Both ellipsometry and SPR techniques gave similar values of the deposited films and corroborated the deposited film thicknesses. The crystalline orientations of the different heterostructures were investigated by XRD. No diffraction patterns were observed, consistent with the presence of an amorphous coating. XPS analysis was conducted on SnO2:Sb (8.5 nm thick) layers deposited on silicon substrate to estimate the doping of the SnO2: Sb layers. Silicon was used rather than glass to avoid charging effects and to obtain stable signals over time. Figure 2 shows the XPS spectra recorded between 525 and 545 eV for Si/SnO2:Sb electrodes obtained from a 5% antimony-doped tin target (Figure 2A) and a 2% antimony-doped tin target (Figure 2B). The peaks correspond to transitions of Sb and O core levels. The large peak observed at 531 eV is asymmetric. As reported in the literature, the O1s core level peak, associated with SnO2, is situated at 530.0 eV, and that of hydroxyl groups is at 531.5532.0 eV while the Sb3d5/2 and Sb3d3/2 core levels of Sb2O3 or Sb2O5 oxides are located at about 530.0 and 539.5 eV. The Sb3d5/2 core level overlaps with that of the O1s. In order to estimate the chemical composition of the films, the 531.0 eV peak was analyzed in terms of three components attributed to oxides, hydroxyl groups, and the Sb3d5/2. The obtained atomic concentrations for Sn, Sb, and O are listed in Table 2. The ratio of oxygen atoms to the sum of all metallic atoms (Sn+Sb) is around 1.95 for the Sn target with 2% Sb and 1.79 for the 5% Sb, respectively. The low values of 0.3 and 2 Sb atom % found are rather in contradiction compared with the nominal target composition of 2 and 5 atom % and also with the optical and electrical results as discussed above. Accurate chemical composition calculations from the XPS spectra is difficult in the present case as a result of overlapping of the core level peaks, the relatively low content of the Sb in the films, and the presence of surface contaminants such as C and OH, which can substantially lowered the XPS signal emission of minor components. Additional investigations 8038 DOI: 10.1021/la900502y

sample

deposition time/min

dSPR/nm

dellispometry/nm

1a (Ag/ SnO2:Sb5%) 2a (Ag/ SnO2:Sb5%) 3a (Ag/ SnO2:Sb5%) 1b (Ag/ SnO2:Sb2%) 2b (Ag/ SnO2:Sb2%) 3b (Ag/ SnO2:Sb2%)

2.5 5.0 7.5 2.5 5.1 7.3

4.5 ( 0.3 8.3 ( 0.1 15.4 ( 0.3 5.5 ( 0.2 8.5 ( 0.2 15.0 ( 0.3

5.1 ( 0.1 8.5 ( 0.1 15.1 ( 0.1 5.4 ( 0.1 8.3 ( 0.1 15.2 ( 0.1

are need to elucidate the chemical composition of thinner (5-10 nm thick) Sb-doped SnO2 films. Electrical and Electrochemical Characterizations. A multilayer structure such as Ti/Ag/SnO2:Sb can be schematically described by three resistances in parallel, and the sheet resistance R of the interface can be described by 1 1 1 1 ¼ þ þ R RTi RAg RSnO2:Sb with RTi, RAg, and RSnO2:Sb being the resistance of the Ti, Ag, and SnO2:Sb films, respectively. The sheet resistance of the Ti/Ag bilayer is governed by the silver resistivity (FAg = 1.587 μΩ 3 cm at 20 C) rather than that of the thin Ti film (FTi = 39 μΩ 3 cm at 20 C).18,19 Figure 3 displays the evolution of the resistivity of Ag/SnO2:Sb composite interfaces with increasing SnO2:Sb film thickness for Sn targets with 5 or 2% Sb content. The resistivity increases in both cases with increasing SnO2:Sb thickness and reaches a limit when the layer is ∼10 nm thick. Figure 3 shows clearly that the resistivity is independent of the doping level of the layer. This suggests that the apparent resistivity of the hybrid Ti/ Ag/SnO2:Sb interface is determined by the silver layer resistivity rather than the coating oxide. This was confirmed by CV. CVs recorded on interfaces 2a and 2b (Table 1) using Ru(NH3)63-/4as the redox mediator are in accordance with electrical measurements in the sense that no difference in the electron transfer rate was observed. Compared to uncoated silver, both interfaces show slightly retarded electron transfer kinetics due to the increased resistivity of the hybrid interface (Figure 4). STM Studies. STM investigations were performed on freshly sputtered films. Figure 5 shows the surface morphology of Ti/Ag and Ti/Ag/SnO2:Sb (8.3 nm, using 2 at% target) films. The surface of the Ti(5 nm)/Ag(38 nm) layered film exhibits silver grains of average size of 50 nm, the surface roughness, defined as the root-mean-square (rms), is in the range of 2-3 nm, and the average high is 2.2 nm. As this layer is covered with 10 nm thick SnO2 film, the surface morphology shows elongated grains with poor lateral resolution, and the roughness is slightly higher (rms = 4-5 nm). The poor lateral resolution is characteristic of low electrical conducting surfaces. These results show that the Ag under layer was full covered by the deposition of 8.3 nm thick SnO2 film. Surface Plasmon Resonance. The new interfaces were investigated using SPR in the scanning mode. Figure 6A shows the SPR signals of the Ag/SnO2:Sb composite interfaces using a Sn target doped with 2% Sb. As expected, with increasing SnO2:Sb film thickness, the resonance angle ΘSPR shifted to higher angles and an increase of the absorbed light intensity at ΘSPR together with an increase in the fwhm of the SPR signal were observed (Figure 6B). As seen from the change of surface plasmon angle in (18) Handbook of Chemistry and Physics, 82nd ed.; CRC Press: New York, 2001. (19) Minami, T. MRS Bull. 2000, 25, 38–44.

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Figure 2. High-resolution XPS spectra (in the 525 to 545 eV energy range) of SnO2:Sb (8.5 nm) as a function of the Sb doping level: (A) 5% Sb-doped Sn target; (B) 2% Sb-doped Sn target. Table 2. Atomic Composition Determined by XPS Analysis of the SnO2:Sb (8.5 nm thick) Layers Deposited on Silicon Substrate As a Function of the Doping Level of the Sputtering Target atomic concentration/% layer

Sn

Sb

O

2% Sb in Sn target 5% Sb in Sn target

33.5 33.8

0.3 2.0

66.2 64.2

Figure 4. CV curves in Ru(NH3)63+ (10 mM)/PBS (0.1 M) aqueous solution for a 50 nm Ag film deposited on glass with 5 nm Ti adhesion layer without coating (black line), (A) coated with ∼8.5 nm SnO2:Sb using a Sn target containing 2% Sb (gray dotted line) or 5% (black dotted line), and (B) coated with 5.5 nm (gray), 8.5 nm (black dotted), and 15 nm SnO2:Sb (gray dotted) using a Sn target containing 2% Sb; scan rate: 0.05 V s-1; A = 0.07 cm2.

Figure 3. Sheet resistance measurements of Ti (5 nm)/Ag (38 nm)/ SnO2:Sb as a function of SnO2:Sb thickness and doping level. The gray line corresponds to layers obtained with a 2% Sb-doped Sn target, the black line corresponds to the layers deposited with a 5% Sb-doped Sn target.

Figure 6B, the final SPR signal of a 4.5 nm thick Ag/SnO2:Sb (5%) corresponds roughly to that of a two times thicker SnO2:Sb (2%) film and shows the improvement which has been obtained using the 2% Sb doped Sn target. This difference results from the higher imaginary refractive index of SnO2:Sb (5%) (n00 = i0.643),15 in contrast to n00 = i0.249 for SnO2:Sb (2%). We showed recently that a 4.5 nm thick SnO2:Sb (5%) film on Ag is not homogeneous and does not protect the underlying Ag film completely.15 The SPR signal changes over time as a result of the surface passivation by local silver oxide formation. The presence of a 8.3 nm thick SnO2:Sb film did, however, protect the Ag substrate from oxidation. Such an interface is chemically and optically stable and shows well-defined electrochemical characteristics. It can be concluded that using a Sn target with reduced Sb content allows protection of thin silver films, providing at the same time well-defined SPR characteristics without sacrificing the electrical and electrochemical characteristics. The quality of the interface was further checked by immersion in different solutions over time. The immersion of the interface for 6 h into ethanol, acetonitrile, hexane, and into aqueous solution with Langmuir 2009, 25(14), 8036–8041

Figure 5. STM images of freshly sputtered Ti/Ag and Ti/Ag/ SnO2:Sb (8.3 nm from 2 atom % target) films. The scanned area is 400  400 nm2. (A) Ti/Ag film, tunneling conditions I = 0.5 nA, U = 0.5 V. (B) Ti/Ag/SnO2:Sb film, tunneling conditions I = 0.2 nA, U = +1.5 V.

a pH < 11 at room temperature did not result in any degradation of the multilayer structure. At higher pH, the SnO2:Sb film dissolved, however, over time. This interface, Ag/SnO2:Sb (8.3 nm) obtained with the Sn target containing 2% Sb was thus used for further surface modifications. Surface Functionalization. In addition to presenting a platform compatible for electrochemical SPR, the 8.3 nm thick SnO2:Sb (2%) film was subjected to different surface treatments. The surface was functionalized in the first instance with three different silanes: APTES, OTS, and UETS. Prior to chemical silanization, the oxide surface was treated with UV-ozone to DOI: 10.1021/la900502y

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Figure 6. (A) Reflected light intensity versus incident angle curves for 38 nm thick silver layers on glass with a 5 nm titanium adhering layer before and after deposition of SnO2/Sb of different thickness with 2% Sb target: dashed lines are experimental results; full lines are fitted curves. Fitting parameters: n(prism) = 1.52, n(titanium) = 2.4 + i3.313 with d = 5 nm, n(silver) = 0.140 + i4.581 with d = 38 nm, n(SnO2/Sb) = 1.91 + i0.643 (for 5% Sb), and n(SnO2/Sb) = 1.91 + i0.249 (for 2% Sb). Fitting program: Winspall 2.01. (B) change of resonance angle (circles) and fwhm (squares) of a film formed from a 2% (blue) or 5% Sb target (black). Scheme 1. Schematic Illustration of the Functionalization Strategy of SnO2:Sb Interfaces

generate surface hydroxyl groups required for covalent coupling of the silanes. The chemically modified surfaces were characterized using contact angle measurements. The initial silver substrate exhibits a high contact angle (Θ = 78). After deposition of a thin (8.3 nm thick) SnO2/Sb layer on the silver film, the contact angle decreased to a value below 30, as expected for a hydrophilic oxide surface. After UV-ozone treatment, the water droplet completely wetted the surface. Chemical treatment of the oxide surface with APTES generated a surface terminated with primary amine groups, as shown recently by one of us, and led to an increase of the contact angle to 48. Coupling of OTS to Ag-SnO2/Sb interface led, on the other hand, to a hydrophobic surface with a contact angle of 104, in agreement with the nature and the chemical composition of the coupled organic molecule. The value is slightly lower than expected for a dense and ordered monolayer (∼110).20 There are several advantages linked to the presence of carboxylic groups on the interface being that amine-terminated targets can be directly linked to the interface using well-known chemistry. Stambouli et al. have recently reported on the linking of amine-terminated DNA onto aerosol pyrolysis formed SnO2 films.21 The approach was based on the use of glutaraldehdye as the linker between APTES-modified SnO2 and amine-terminated DNA. Even though this approach works satisfactorily, there are mainly two drawbacks. One is linked to the formation of (20) Wassermann, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (21) Stambouli, V.; Labeau, M.; Matko, I.; Chenevier, B.; Renault, O.; :: Guiducci, C.; Chaudouet, P.; Roussel, H.; Nibkin, D.; Dupuis, E. Sens. Actuators, B 2006, 113, 1025.

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polymers using APTES. Using longer functional alkyl chain silanes, such as UETS, limits this side reaction. Second, glutaraldehdye, besides being toxic, is prone to form polymers. The formed imine bond is also chemically not stable and has to be reduced in an additional step using NaBH4 solutions. This is not the case using EDC/NHS chemistry. Scheme 1 shows the different chemical steps to form surface carboxyl groups on Ag/SnO2:Sb hybrid interfaces. After a UV-ozone treatment, the interface was reacted with UETS. This resulted in an important increase of the contact angle to 101, in agreement to the value reported by Wasserman et al.20 The terminal vinyl groups were further oxidized to carboxylic groups by an aqueous solution of KMnO4/NaIO4. As expected, the resulting surface displays a more hydrophilic character due to the COOH groups, with a contact angle value below 40. XPS analysis was conducted to confirm the success of the reaction. Figure 7A displays the XPS survey of an acid-terminated interface showing peaks at 285, 532, 102, 155, 25, 489, and 715 eV due to C 1s, O 1s, Si 2p, Si 2s, Sn 4d, Sn 3d, and Sn 3p, in agreement with the chemical composition of the grafted molecule. The presence of Si indicates the success of the silanization reaction. To confirm the presence of Si-C and O-Si-O bonds, the high-resolution XPS spectrum of the Si2p band was deconvoluted (Figure 7B). The fitted results show the presence of three components: 99.8 eV (Si-Si), 102.4 (Si-C), and (22) Wink, T.; Van Zuilen, S. J.; Bult, A.; van Bennekom, W. P. Anal. Chem. 1998, 70, 827–832. (23) J. Ma, J.; Hao, X.; Ma, H.; Xu, X.; Yang, Y.; Huang, Sh.; Zhang, D.; Cheng, Ch. Solid State Commun. 2002, 121, 345. (24) Shanthi, E.; Banerjee, A.; Dutta, V; Chopra, K. L. J. Appl. Phys. 1982, 53, 1615.

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demonstrated. After the linking of 5-(aminoacetamido)fluorescein to the terminal COOH groups using EDC/NHS, an additional XPS band due to N 1s is observed. The success of the surface functionalization with the fluorescence molecule was furthermore corroborated with the fluorescence image seen in Figure 7C.

4. Conclusion

Figure 7. (A) XPS survey spectra of Ag/SnO2:Sb (8 nm) modified (a) with COOH groups and (b) after linking 5-(aminoacetamido) fluorescein probe. (B) High-resolution Si2p XPS spectrum of a COOH-modified interface. (C) Fluorescence image after linking of 5-(aminoacetamido)fluorescein to COOH groups.

104.6 (Si-O) in accordance with the incorporation of a functional silane on the SnO2 interface. The availability of COOH groups to react in an aminolysis with amine-reactive molecules was finally

Langmuir 2009, 25(14), 8036–8041

The deposition of thin films of SnO2:Sb onto silver substrates by magnetron sputtering using different Sb-doped Sn targets (2 or 5%) is described. The influence of the doping level on the electrical, electrochemical, and SPR properties of the resulting interfaces was investigated. The best results in terms of electrochemical and SPR characteristics, in addition to chemical stability, were observed for a 8.5 ( 0.3 nm thick SnO2:Sb (2%) film deposited on Ag. Surface functionalization using organosilicon coupling chemistry was demonstrated on such hybrid interfaces for linking functional molecules to the surface. The Ag-SnO2:Sb interface will open new opportunities in the field of biosensors with the possibility of combining electrochemical and SPR measurements on the same chip. Acknowledgment. The Agence Nationale de la Recherche (ANR), the Institut National Polytechnique de Grenoble (INPG, BQR 2006), the Centre National de la Recherche Scientifique (CNRS), and the Nord-Pas-de Calais region are gratefully acknowledged for financial support.

DOI: 10.1021/la900502y

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