Electroless Metallization of Glass Surfaces Functionalized by

Kent Ridge, Singapore 119260. Wei Huang. Institute of Materials Research and Engineering, National University of Singapore,. Research Link, Kent Ridge...
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Langmuir 2001, 17, 7425-7432

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Electroless Metallization of Glass Surfaces Functionalized by Silanization and Graft Polymerization of Aniline Yongjun Chen, E. T. Kang,* and K. G. Neoh Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 119260

Wei Huang Institute of Materials Research and Engineering, National University of Singapore, Research Link, Kent Ridge, Singapore 117602 Received June 11, 2001. In Final Form: July 26, 2001 Silanization of glass surfaces by (3-glycidoxypropyl)trimethoxysilane (GPS) provided a surface-coupled layer of functional molecules for the subsequent reaction with aniline (An). The so-modified glass surface (the An-GPS-glass surface) was then subjected to oxidative graft polymerization of aniline. The composition and microstructure of the modified glass surfaces were characterized by X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectroscopy, and atomic force microscopy (AFM). The protonationdeprotonation behavior, the interconvertible intrinsic redox states, and the spontaneous metal reduction behavior of the grafted polyaniline (PANI) chains on the glass surface (the PANI-An-GPS-glass surface) were similar to those of the PANI homopolymer. The PANI-An-GPS-glass surface with the electrolessly deposited palladium could be used to catalyze the electroless deposition of copper (the Cu-Pd-PANIAn-GPS-glass surface). The spatial distribution of the grafted PANI chains into the matrix of the electrolessly deposited metals had given rise to the strong adhesion of copper to the glass surface. Adhesion failure occurred at the interface between the copper foil backing and the epoxy adhesive when both were applied onto the Cu-Pd-PANI-An-GPS-glass surface in an attempt to peel off the electrolessly deposited metal layer and the tethered polymer layer from the glass surface.

Introduction With the rapid growth of the microelectronic industry, optoelectronic and molecular electronic materials in the form of ultrathin and nanostructured films have been actively researched.1 The common methods for introducing stable organic thin films on substrates have included the electrochemical growth of polymers on modified electrode surfaces,2 Langmuir-Blodgett deposition,3 spin coating,4 coupling of highly reactive compounds (such as the silanes),5 adsorption of substances having specific polar groups (such as the proteins),6 surface graft copolymerization,7 and plasma-enhanced deposition.8 The above techniques are indispensable for the fabrication of ultrathin films, as they also allow the molecular redesign of the substrate surfaces to impart new and specific functionalities.9-11 * To whom correspondence should be addressed. Fax: (65) 7791936. E-mail: [email protected]. (1) Irie, M.; Ohara, H.; Tsujioka, M.; Nomura, T. Mater. Chem. Phys. 1998, 54, 317. (2) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Gottesfeld, A. J. Am. Chem. Soc. 1990, 112, 6135. (3) Ruil, A., Jr.; Mattoso, L. H. C.; Mello, S. V.; Telles, G. D.; Oliveira, N. O., Jr. Synth. Met. 1995, 71, 2067. (4) Kulkami, V. G. Synth. Met. 1995, 71, 2129. (5) Xu, J. Z.; Choyke, W. J.; Yates, J. T. J. Phys. Chem. B 1997, 101, 6879. (6) Liebmannvinson, A.; Lander, L. M.; Foster, M. D.; Brittain, W. J.; Vogler, E. A.; Majkrzak, C. F.; Satija, S. Langmuir 1996, 12, 2256. (7) Chen, Y.; Kang, E. T.; Neoh, K. G.; Tan, K. L. J. Phys. Chem. B 2000, 104, 9171. (8) Zhang, Y.; Tan, K. L.; Yang, G. H.; Zou, X. P.; Kang, E. T.; Neoh, K. G. J. Adhes. Sci. Technol. 2000, 14, 1723. (9) Suzuki, M.; Kishida, A.; Iwata, H.; Ikada, Y. Macromolecules 1986, 19, 1804. (10) Lazare, S.; Srinivasan, R. J. Phys. Chem. 1985, 90, 2124. (11) Kang, E. T.; Zhang, Y. Adv. Mater. 2000, 12, 1481.

Recently, the “molecular self-assembly” phenomenon has been widely used to prepare materials with novel optical and electrical properties.12 It is believed that selfassembled monolayers (SAM) approach, which allows the control of molecular ordering, is a potentially useful technique for the construction of future advanced materials and devices.13 It is well-known that the molecular assemblies exhibiting novel or enhanced physical properties arise from cooperative phenomena. Among the techniques for self-assembling, silanization has been widely used for the modification of inorganic surfaces.14 The formation of a monolayer of functional silane molecules with a high degree of orientation akin to that of the classical Langmuir-Blodgett films is known.15 Silanes have been used in precision casting,16 cements and ceramics,17 glass frosting,18 paints and coatings,19 solgel glasses and ceramics,20 and water repellents.21 For applications in microelectronics, films of silicon dioxide are deposited on silicon substrates by the application of (12) Ulman, A. An Introduction to Ultrathin Films from Langmuir Boldgett to Self-Assembly; Academic: San Diego, CA, 1991. (13) Ulman, A. J. Mater. Educ. 1989, 11, 205. (14) Barry, A. Chemtech 1977, 7, 766. (15) Netzer, L.; Iscovici, R.; Sagiv, J. Mol. Cryst. Liq. Cryst. 1983, 93, 415. (16) Jones, C. U.S. Patent 2678282, 1954 (assigned to General Electric Corp.). (17) Emblem, H. G.; Walters, I. R. J. Appl. Chem.: Biotechnol. 1977, 27, 618. (18) Pipken, M. U.S. Patent 2596896, 1951 (assigned to General Electric Corp.). (19) Anthony, B. T. U.S. Patent 4495360, 1985 (assigned to General Electric Corp.). (20) Barry, A. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John Wile & Sons: New York, 1997; Vol. 22, p 79. (21) Depasquale, R.; Wilson, M. U.S. Patent 4648904, 1987 (assigned to SCM).

10.1021/la010866y CCC: $20.00 © 2001 American Chemical Society Published on Web 10/13/2001

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a partially hydrolyzed solution of tetraethoxysilane or methyltriethoxysilane.22 Among the electroactive polymers, polyaniline (PANI) has been of particular interest because of its environmental stability, controllable electrical conductivity, and interesting redox properties associated with the chain nitrogen.23,24 PANI also exhibits crystallinity and solution- or counterion-induced processability.25,26 Furthermore, the electrical properties of PANI can be substantially improved through secondary doping.27 PANI has potential applications not only in the areas of corrosion protection of metals,28 light-emitting devices,29 biocompatible materials30,31 and materials for catalysts, electrodes, and sensors32,33 but also in the controlling of electromagnetic reaction and in the dissipation of electrostatic charges.34,35 In the present work, we report on the approach of molecular modification of glass surface by silanization with (3-glycidoxypropyl)trimethoxysilane (GPS) (the GPSglass surface), followed by the reactive coupling of the epoxide groups of GPS with aniline (the An-GPS-glass surface), and finally by the oxidative graft polymerization of aniline onto the An-GPS-glass surface to result in a covalently grafted PANI layer on glass (the PANi-AnGPS-glass surface). The PANI-modified glass surface undergoes spontaneous reduction of palladium ions in Pd2+ solution. The surface-adsorbed palladium catalyzes the subsequent electroless plating of copper on the glass surface. Experimental Section Materials. Commercial silicate glass slides for optical microscopy applications were used as the substrates. The glass slide was sliced into rectangular strips of about 0.5 cm × 1.0 cm in size. To remove the organic residues from the surface, the glass substrates were cleaned using the “piranha” solution, a mixture of 70% concentrated sulfuric acid and 30% hydrogen peroxide. The cleaned glass strips were then rinsed thoroughly with doubly distilled water. The (3-glycidoxypropyl)trimethoxysilane (GPS) and the aniline (An) monomer were obtained from the Gelest Inc. of Tullytown, PA, and the Aldrich Chemical Co. of Milwaukee, WI, respectively. They were used as received. The structure of GPS is shown as follows:

(22) Gagnon, D. U.S. Patent 4103065, 1978 (assigned to OwensIllinois). (23) Amano, K.; Ishikawa, H.; Kobayashi, A.; Satoh, M.; Hasegawa, E.; Synth. Met. 1994, 62, 229. (24) Tan, K. L.; Kang, E. T.; Neoh, K. G. Polym. Adv. Technol. 1994, 5, 171. (25) Nicolau, Y. F.; Djurado, D. Synth. Met. 1993, 55-57, 394. (26) Angelopoulous, M.; Asturius, G. E.; Ermer, S. P.; Ray, A.; Scherr, E. M.; MacDiarmid, A. G.; Akhtar, M.; Kiss, Z.; Epstein, A. J. Mol. Cryst. Liq. Cryst. 1988, 160, 151. (27) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103. (28) Ahmad, N.; MacDiarmid, A. G. Synth. Met. 1996, 78, 103. (29) Chen, S. A.; Chuang, K. R.; Chao, C. L.; Lee, H. T. Synth. Met. 1996, 82, 207. (30) Chen, Y. J.; Kang, E. T.; Neoh, K. G.; Wang, P.; Tan, K. L. Synth. Met. 2000, 110, 47. (31) Wang, C. H.; Dong, Y. Q.; Sengothi, K.; Tan, K. L.; Kang, E. T. Synth. Met. 1999, 102, 1313. (32) Heeger, A. J.; Yang, Y.; Westerweele, E.; Zhang, C.; Cao, Y.; Smith, P. In The Polymeric Materials Encyclopedia: Synthesis, Properties and Applications; Salamone, J. C., Ed.; CRC Press Inc.: Boca Raton, FL, 1996; p 5500.

Chen et al. The solvent, N-methylpyrrolidinone (NMP), and other chemicals were of reagent grade and were obtained from the Aldrich Chemical Co. Silanization of Glass Surface with (3-Glycidoxypropyl)trimethoxysilane (GPS): GPS-Glass Surface. The GPS solution was prepared by adding 1 mL of GPS to 99 mL of acetic acid/sodium acetate buffer (pH 5). The 1 vol % GPS solution was stirred continuously for 30 min to allow for the hydrolysis reaction to proceed prior to being applied onto the glass slides. The glass slides were stored in the doubly distilled water prior to being transferred into the silane solution. This procedure kept the hydrocarbon contamination to a minimum and ensured that the slides remained completely wettable when immersed in the aqueous silane solution. The glass slides were removed from the silane solution after 10 min. They were then placed in a 60 °C constant temperature oven for 10 min. The physically adhered silane molecules were removed by rinsing with the doubly distilled water. The surface-modified glass slides were dried by pumping under reduced pressure. Coupling Reaction between Aniline and the Epoxide Group of GPS: An-GPS-Glass Surface. The coupling reaction between the aniline (An) monomer and the epoxide group of GPS was carried out at 60 °C for 6 h in ethanol solution containing the GPS-glass substrate and 60 vol % An. The experimental conditions were found to be effective in promoting the coupling of aniline to the epoxide groups of the glycidyl monomers.7 After the covalent coupling of An with the epoxide group, the glass substrate was washed thoroughly with copious amounts of NMP to remove the unreacted An monomer. The residual NMP on the An-coupled GPS-glass (An-GPS-glass) substrate was, in turn, removed by washing in ethanol. Oxidative Graft Polymerization with Aniline: PANIAn-GPS-Glass Surface. For the oxidative graft polymerization with aniline, the An-GPS-glass substrates were immersed in 0.5 M H2SO4 solution containing 0.01-0.2 M of aniline and the corresponding amount of (NH4)2S2O8 oxidant to achieve an aniline monomer to oxidant molar ratio of 1:1. The reaction was allowed to proceed at about 0 °C for 5 h. The method was thus similar to that reported in the literature for the oxidative homopolymerization of aniline to produce the conductive emeraldine (EM) salt.36,37 The grafted EM salt on the glass surface was converted (deprotonated) to the neutral EM base form by immersing and equilibrating the substrate in copious amounts of doubly distilled water. The surface-modified glass was subsequently immersed in a large volume of NMP (a good solvent for EM base) for at least 24 h with continuous stirring to remove the adhered and physically adsorbed EM base polymer. During the washing process, the NMP solvent was changed every 8 h. The polyaniline (PANI)-grafted An-GPS-glass surface (the PANI-An-GPS-glass surface) was further washed with doubly distilled water to remove the residual NMP before being dried by pumping under reduced pressure. PANI Reduction, Reduction of Palladium, and Electroless Plating of Copper. The EM state of the grafted PANI on the glass surface was reduced to the leucoemeraldine (LM) state by exposure to hydrazine for 1 h, followed by thoroughly rinsing with doubly distilled water and drying under reduced pressure.36,37 For the electroless deposition of Pd metal, the PANI-An-GPS-glass surface, with the PANI in its reduced LM state, was immersed in the palladium nitrate solution (100 mg dm-3 Pd(II) ions in 0.05 M HNO3) for 10 min. After removal from the Pd nitrate solution, the Pd laden glass substrate was rinsed thoroughly with the doubly distilled water before being dried under reduced pressure. For the electroless plating of (33) MacDiarmid, A. G.; Yang, L. S.; Huang, W. S.; Humphrey, B. D. Synth. Met. 1987, 18, 393. (34) Trivedi, D. C.; Dhawan, S. K. In Polymer Science Contemporary Themes; Sivaram, S., Ed.; Tata McGraw-Hill: New Delhi, 1991; Vol. II, p 746. (35) Joo, J.; Wu, C. Y.; Benatar, A., Jr.; Faisst, C. F.; Zegarski, J.; MacDiarmid, A. G. In Intrinsically Conducting Polymers: An Emerging Technology; Aldissi, M., Ed.; Kluwer Press: Dordrecht, The Netherlands, 1993; p 165. (36) Ray, A.; Asturias, G. E.; Kershner, D. L.; Ritchter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1989, 29, E145. (37) Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135.

Electroless Metallization of Glass Surfaces copper, the Pd-laden glass slide was immersed in the electroless copper plating bath for about 30 min at the temperature of 80 °C. The composition of the plating solution was as follows: 0.7 wt % CuSO4‚5H2O; 2.5 wt % potassium sodium tartrate; 0.4 wt % sodium hydroxide; 0.4 wt % formaldehyde.38,39 After the electroless plating of copper, the Cu-laden glass was washed thoroughly with doubly distilled water before being dried under reduced pressure. The thickness of the electroless deposited copper layer was determined gravimetrically. Adhesion Measurements. For the adhesion strength measurements, a copper sheet backing of about 0.05 mm in thickness was adhered to the Cu-laden glass surface with a commercial epoxy adhesive (Aldralite Stand, from Ciba-Geigy Chem. Co. of Cambridge, UK). The epoxy adhesive was cured at 120 °C for 5 h before the assembly was subjected to the 180°-peel adhesion strength measurement. The measurement of the 180°-peel adhesion strength was carried out on an Instron 5544 Tensile Tester from the Instron Corp. of Canton, MA. All measurements were performed at a crosshead speed of 5 mm/min. Each adhesion strength reported was the average of at least three sample measurements, which did not vary by more than ( 0.5 N/cm. Characterization of the Surface-Modified Glass. The surface-modified glass slides were characterized by X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectrum, atomic force microscopy (AFM), and conductivity measurements. XPS measurements were carried out on a Kratos Analytical AXIS HSi spectrometer with a monochromatized Al KR X-ray source (1486.6 eV photons). The X-ray source was run at a reduced power of 150 W (15 kV and 10 mA). The surface-modified glass samples were mounted on the standard sample studs by means of double-sided adhesive tapes. The core-level spectra were obtained at the photoelectron takeoff angle (R, with respect to the sample surface) of 90°. The pressure in the analysis chamber was maintained at 10-8 Torr or lower during each measurement. To compensate for surface charging effects, all binding energies were referenced to the C 1s hydrocarbon peak at 284.6 eV. In peak synthesis, the line width (full width at half-maximum or fwhm) of Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peak area ratios. The UV-visible absorption spectra were measured on the Shimadzu UV-3101PC UV-vis-NIR scanning spectrophotometer. The surface morphologies of the pristine and surfacemodified glass slides were examined on a Nanoscope IIIa atomic force microscope (AFM), using the tapping mode at a scan size of 10 µm and a scan rate of 1.0 Hz. The root-mean-square surface roughness (Ra) of the film was evaluated directly from the AFM image. The conductivity of the modified glass surface was measured by the two-probe method, using a Hioki model 3265 digital electrometer. For each conductivity value reported, at least three sample measurements were averaged.

Results and Discussion The processes of surface modification of the glass substrate by silanization with GPS, reactive coupling of the epoxide functional group of GPS with aniline, oxidative graft polymerization of aniline, and finally electroless plating of palladium and copper are shown schematically in Figure 1. Each process is described in detail below. Silanization of the Glass Surfaces with GPS: GPS-Glass Surface. Surface monolayers can be formed on glass, silicon, and certain metal substrates via chemical coupling of the silanes.40 In the present work, an epoxide group-containing silane, GPS, is coupled on the glass surface. Figure 2a,b shows respectively the wide scan and C 1s core-level spectra of the pristine glass surface. The corresponding spectra of the glass surface after GPS (38) Mance, A. M.; Waldo, R. A.; Dow, A. A. J. Electrochem. Soc. 1989, 136, 1667. (39) Ebneth, H. In Metallizing of Plastics: A Handbook of Theory and Practice; Suchentrunk, R., Ed.; ASM Int.: Materials Park, OH, 1993; Vol. 30, Chapter 3, p 35. (40) Reena, B.; Jack, Y.; Anthony, M. M.; Brian, E.; Jaqueline, K. Langmuir 1995, 11, 4393.

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silanization are shown in Figure 2c,d, respectively. The persistence of a residual C 1s core-level signal on the pristine glass surface is probably associated with the presence of adventitious carbon and hydrocarbons. The C 1s peak intensity in the wide scan spectrum of the glass surface increases substantially after GPS silanization (compare part c to part a of Figure 2). The CH:CO peak area ratio of the silanized glass surface in Figure 2d is about 1:1.3, which is somewhat lower than the theoretical CH:CO peak area ratio of 1:2 for the hydrolyzed and coupled GPS. The deviation probably has resulted, at least in part, from the presence of adventitious carbon and hydrocarbons on the pristine glass surface. The surface morphologies of the pristine glass and the GPS-silanized glass (GPS-glass) are revealed by the respective AFM images of Figure 3a,b. The root-meansquare surface roughness (Ra) of the GPS-glass surface is about 2.8 nm, which is higher than that of about 1.5 nm for the pristine glass surface. The increase in Ra value is consistent with the presence of coupled GPS on the glass surface. Generally, the thickness of the coupled polysiloxane layer is determined by the concentration of the silane solution. It has been suggested that deposition of silane onto glass from a 0.25 vol % solution could result in about eight molecular layers.14 The multilayers could be either interconnected through a loose network structure or intermixed or both. The orientation of the functional groups is generally horizontal, not necessarily planar, on the surface of the substrate.14 In this work, the silane concentration used (1 vol %) is higher than 0.25 vol %. The high silane concentration ensures the formation of GPS multilayers (eight layers and above) on the glass substrate. The thickness of the multilayers is probably less than the probing depth of the XPS technique (∼7.5 nm for an organic matrix41) since the CH:CO peak area ratio of the silanized glass surface is lower than the theoretical CH:CO peak area ratio of the hydrolyzed and coupled GPS, as stated earlier. Some of the adsorbed layers could have been removed in the subsequent washing process. Increasing the GPS solution concentrations to above 1 vol % and up to about 4 vol % does not appear to cause a further increase in the multilayer thickness, as suggested by the lack of any significant changes in the CH:CO ratio. Thus, the GPS-glass surface prepared with 1 vol % GPS was used for the subsequent coupling reaction. Reactive Coupling of Aniline (An) with the Epoxide Moieties of GPS: An-GPS-Glass Surface. The GPS-silanized glass surface can be further functionalized through reactions with amines, including the less reactive aromatic amine, such as aniline (An). The reaction is allowed to proceed in ethanol since alcohols can act as catalysts in the ring opening reaction of the epoxides.42 Figure 4a,b shows the respective C 1s and N 1s core-level spectra of the GPS-glass surface after reactive coupling with An in a 60 vol % ethanol solution of An. The coupling reaction was allowed to proceed at 60 °C for 6 h (see Experimental Section). The N 1s core-level signal in Figure 4b confirms the presence of coupled aniline on the GPS-glass surface to give rise to the An-GPSglass surface. The extent of the coupling reaction can be expressed as the [N]:[C] molar ratio and determined from the corrected N 1s and C 1s core-level spectral area ratio. The [N]:[C] molar ratio for the present An-GPS-glass surface is about 0.07, in comparison to the [N]:[C] molar ratio of about 0.17 for aniline. This lower [N]:[C] ratio is (41) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang, E. T.; Neoh, K. G. Macromolecules 1993, 29, 2832. (42) Rozenberg, B. A. Adv. Polym. Sci. 1986, 75, 73.

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Figure 1. Schematic representations of the process of silanization, coupling reaction, oxidative graft polymerization, and electroless plating of Pd and Cu on the glass surface.

consistent with the presence of additional contribution to the C 1s signal from the underlying GPS structure which is still within the probing depth of the XPS technique. In fact, the [N]:[C] ratio is fairly close to the theoretical ratio of 0.08 when each GPS unit couples with one aniline unit. The coupling of aniline on the GPS-glass surface leads to a decrease in the relative signal intensity of the CO component, as shown in Figure 4a, in comparison to that of the original GPS-glass surface in Figure 2d. Moreover, the N 1s core-level line shape in Figure 4b suggests that the reaction between the epoxide groups and aniline involves only the primary amine. The coupled amine functional groups probably do not undergo further reaction with the epoxide groups, as the N 1s core-level spectrum is dominated by the -NH peak component at the BE of about 399.4 eV.43 The phenomenon probably has resulted

from the presence of a high concentration of aniline, which contains primary amine, in the reaction mixture and the fact that the reactivity of the primary amine toward epoxide is higher than that of the secondary amine. The morphology of the An-GPS-glass surface is shown in Figure 3c. The root-mean-square surface roughness (Ra) is about 4.5 nm. The increase in Ra value over that of the GPS-glass is consistent with the presence of the coupled aniline units on the GPS-glass surface. The above An-GPS-glass surface with a [N]:[C] ratio of 0.07 is used in the subsequent oxidative graft polymerization of aniline. The pendant aniline groups on the An-GPS-glass surface can serve as sites for the covalent anchoring of the PANI (43) Kang, E. T.; Neoh, K. G.; Tan, T. C.; Khor, S. H.; Tan K. L. Macromolecules 1990, 23, 2918.

Electroless Metallization of Glass Surfaces

Figure 2. Wide-scan and C 1s core-level spectra of the pristine glass surface (a, b) and the GPS-deposited glass surface from immersion into 1 vol % GPS solution for 10 min, followed by curing at 60 °C for 10 min (c, d).

Figure 3. Three-dimensional AFM images of (a) a pristine glass surface, (b) a GPS-glass surface, (c) an An-GPS-glass surface with a [N]:[C] ratio of 0.07, and (d) a PANI-An-GPSglass surface with a [N]:[C] ratio of 0.13.

Figure 4. C 1s and N 1s core-level spectra of the GPS-glass surface after reaction with aniline at 60 °C for 6 h in 60 vol % ethanol solution of aniline.

chains onto the glass substrate during the subsequent oxidative graft polymerization of aniline. Oxidative Graft Polymerization of Aniline on the An-GPS-Glass Surface: PANI-An-GPS-Glass Surface. Oxidative graft polymerization of aniline was carried out on the An-GPS-glass surface which had a [N]:[C] ratio of 0.07. The high surface concentration of the immobilized aniline units will help to promote the subsequent oxidative graft polymerization of aniline. Figure 5a,b shows the respective C 1s and N 1s core-level spectra of the An-GPS-glass surface after the oxidative graft polymerization in 0.5 M H2SO4 containing 0.05 M

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Figure 5. C 1s and N 1s core-level spectra of the NMP-washed An-GPS-glass surface ([N]:[C] ) 0.07) from oxidative graft polymerization in 0.05 M aniline solution, followed by deprotonation with distilled water (a, b) and reprotonation by 1 M HClO4 (c, d).

aniline. The grafted polyaniline (PANI) has been deprotonated by equilibrating in copious amounts of doubly distilled water to give rise to the neutral emeraldine (EM) base form. The resulting PANI-An-GPS-glass surface has also been extracted with NMP (a good solvent for EM base) to remove the physically adsorbed homopolymer of PANI. The corresponding spectra for the 1 M HClO4 reprotonated PANI-An-GPS-glass surface are shown in Figure 5c,d. The PANI graft concentration within the probing depth of the XPS technique is still defined as the [N]:[C] molar ratio, as each aniline repeat unit of PANI contains one nitrogen. The N 1s core-level spectrum of the deprotonated PANI-An-GPS-glass surface in Figure 5b shows predominantly the presence of the quinonoid imine (dN-) structure and the benzenoid amine (-NH-) structure. The two species correspond to peak components with BE’s at about 398.2 and 399.4 eV, respectively.37,43 The presence of about equal proportion of the imine and amine nitrogen in the N 1s core-level spectrum of the deprotonated surface is consistent with the intrinsic redox state of the EM base form ([)N-]:[-NH-] ratio ∼ 1) of PANI.37,43 The residual high BE tail in the N 1s spectrum probably has originated from the surface oxidation products or weakly charge-transfer complex oxygen.44 Comparison of the N 1s core-level spectrum in Figure 5b to that of the reprotonated surface in Figure 5d suggests that the grafted EM base on the glass surface can be effectively reprotonated by a protonic acid, such as HClO4. This fact is indicated by the disappearance of the -Nd component and the appearance of the corresponding proportion of the positively charge nitrogen. For the EM base form of the aniline homopolymer, protonation occurs preferentially at the imine units.36 Thus, the deprotonation-reprotonation behavior of the grafted PANI chains on the glass surface is not unlike that of the aniline homopolymer. The XPS results of Figure 5 indicate that the graft concentration of PANI (expressed as the [N[:[C] molar ratio) on the PANI-An-GPS-glass surface for oxidative graft polymerization carried out in 0.05 M aniline solution can reach about 0.13, in comparison to the [N]:[C] ratio of 0.07 for the original An-GPS-glass surface or 0.17 for the PANI homopolymer. The substantially enhanced (44) Kang, E. T.; Neoh, K. G.; Khor, S. H.; Tan, K. L.; Tan, B. T. G. J. Chem. Soc., Chem. Commun. 1989, 696.

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Figure 6. Effect of aniline monomer concentration used during the oxidative graft polymerization on the PANI graft concentration, expressed as the [N]:[C] ratio, for the An-GPS-glass surface.

[N]:[C] ratio of the PANI-An-GPS-glass surface over that of the precursor An-GPS-glass surface confirms the occurrence of oxidative graft polymerization of aniline on the An-GPS-glass surface. The process is shown schematically in Figure 1. Figure 6 shows the effect of aniline monomer concentration used for the oxidative graft polymerization on the graft concentration of PANI. Thus, the PANI graft concentration increases sharply with increasing aniline monomer concentration but levels off at aniline monomer concentration greater than 0.05 M. All the samples in Figure 6 have been deprotonated in copious amounts of doubly distilled water and extracted with NMP to remove the physically adsorbed PANI. Complete deprotonation can be achieved with repeated soaking in distilled water, as protonation and deprotonation in PANI is an equilibrium process.36 The use of ammonium hydroxide for rapid deprotonation was avoided, as the base was found to destabilize the coupled silane layer. The amount of physically adsorbed PANI can be estimated from the changes in [N]:[C] ratio before and after the NMP extraction. The C 1s and N 1s core-level line shapes of the An-GPS-glass surface after the oxidative graft polymerization in 0.05 M aniline solution, followed by deprotonation in copious amounts of doubly distilled water, but in the absence of NMP extraction, are similar to those shown in Figure 5 for the NMP-extracted sample. The [N]:[C] ratio of the surface is about 0.15, which is slightly higher than that of 0.13 for the PANI-AnGPS-glass washed by NMP. It is also closer to that of 0.17 for the PANI homopolymer. The small decrease in [N]:[C] ratio after the NMP extraction suggests that most of the PANI chains on the glass surface are in the grafted form. The successful grafting of the PANI chains is further confirmed by the changes in electrical conductivity of the glass surface. For the present study, the surface resistance for the protonated PANI-An-GPS-glass is about 107 Ω/square. The bulk conductivity of the protonated PANI films, on the other hand, is in the order of 1-10 S/cm.45 The relation between surface resistance (Rs) and bulk resistivity (F) is given by Rs ) F/t, where t is the thickness of the grafted PANI film. Thus, using 7.5 nm (the probing depth of the XPS technique in an organic matrix41) as the approximate thickness of the grafted aniline polymer layer, the bulk resistivity is in the order of 10 Ω‚cm, which is equivalent to a bulk conductivity value (σ) of the order of 0.1 S/cm, as σ ) 1/F. The fact that the σ value of the grafted (45) Trivedi, D. C. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons: Chichester, U.K., 1997; Vol. 2, p 505.

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Figure 7. UV-visible absorption spectra of the NMP-washed PANI-An-GPS-glass surface (with the PANI in the EM base from) before (trace a) and after (trace b) reprotonation. The corresponding spectra for the PANI-An-GPS-glass surface in the absence of NMP washing are represented by traces c and d.

PANI layer is lower than that of the PANI homopolymer suggests the presence of a “dilution effect” from the underlying silane layer, as the [N]:[C] ratio of the surface is somewhat lower than that of 0.17 for the PANI homopolymer. To further characterize the grafted PANI chains on the glass surface, UV-visible absorption spectroscopy was used to study the protonation effect. Trace a of Figure 7 is the UV-visible absorption spectrum of the PANI-AnGPS-glass with the PANI in its neutral EM base form and after extraction by NMP. The corresponding spectrum for the PANI-An-GPS-glass after reprotonation by 1 M HClO4 is shown in trace b. The corresponding spectra of the deprotonated and reprotonated PANI-An-GPSglass surfaces in the absence of NMP extraction, on the other hand, are shown in traces c and d, respectively.The EM base has two broad absorption peaks at about 330 and 648 nm. The former is associated with the π∠π* transition of the benzenoid rings, while the latter is attributable to the π∠π* transition of the quinonoid rings of the EM chains.46 For the spectra of the reprotonated surfaces (traces b and d), the appearance of the polaron absorption band at about 440 nm and the enhanced absorption after 800 nm suggest that the grafted PANI chains exist in the protonated state and protonation occurs preferentially at the imine sites of EM.36 Furthermore, the absorbances of the deprotonated and reprotonated PANI-An-GPS-glass after extraction by NMP are correspondingly lower than those of the deprotonated and reprotonated PANI-An-GPS-glass in the absence of NMP extraction (compare trace a to trace c for the deprotonated state and trace b to trace d for the reprotonated state). The higher absorbance for the PANI-AnGPS-glass surface in the absence of NMP abstraction is consistent with the presence of physically adsorbed PANI. The surface morphology of the deprotonated PANIAn-GPS-glass surface ([N]:[C] ) 0.13) is revealed by the AFM image shown in Figure 3d. The root-mean-square surface roughness (Ra) is about 9.7 nm, which is much higher than that of the precursor An-GPS-glass surface shown in Figure 3c. This increase in Ra value is consistent with the presence of the oxidatively graft polymerized aniline on the surface of the An-GPS-glass. Polyaniline Reduction, Reduction of Palladium Ions, and Electroless Plating of Copper. The EM base grafted on the glass surface can also be reduced to the leucoemeraldine (LM) state. Figure 8a,b shows respectively the C 1s and N 1s core-level spectra of a deprotonated PANI-An-GPS-glass surface ([N]:[C] ) 0.13) after reduction by hydrazine. The N 1s core-level spectrum of the reduced PANI-An-GPS-glass surface is dominated (46) Fu, Y. P.; Weiss, R. A. Synth. Met. 1997, 84, 103.

Electroless Metallization of Glass Surfaces

Figure 8. C 1s and N 1s core-level spectra of the LM state of the PANI-An-GPS-glass surface from immersing in NH2NH2 for 1 h (a, b) ([N]:[C] ) 0.13), the Pd 3d and N 1s core-level spectra of the corresponding surface after equilibrating in 100 mg dm-3 Pd(NO3)2 nitric acid solution for 10 min (c, d) ([Pd]:[N] ) 0.8, [Pd(0)]:[Pd(II)] ) 5.1), and the wide-scan and Cu 2p core-level spectrum of the above Pd-laden glass surface after electroless plating of copper (e, f).

by the -NH- peak component at the BE of about 399.4 eV,43 as shown in Figure 8b. The high-BE component above 400 eV, on the other hand, may have resulted, at least in part, from surface oxidation products or weakly chargetransfer complexed oxygen and is consistent with the reactive nature of most conjugated polymer surfaces.43,47 The fact that PANI can exist in a large number of interconvertible intrinsic oxidation states suggests that by coupling the metal reduction process in acid solution to an increase in the intrinsic oxidation state of the polymer, and the subsequent reprotonation and reduction of the oxidized PANI in the acid medium, spontaneous and sustained reduction of certain metal ions to their elemental form can be achieved.47-49 Figure 8c,d shows the respective Pd 3d and N 1s XPS core-level spectra for the fully reduced PANI-An-GPS-glass surface after equilibrating for about 10 min in a palladium nitrate acid solution containing initially 100 mg dm-3 of Pd(II) ions. Comparison of the N 1s core-level spectrum in Figure 8d to that in Figure 8b suggests that there is an increase in the proportion of imine nitrogen after the metal uptake. The result is consistent with an increase in the intrinsic oxidation state of the grafted PANI (in its LM state) upon metal reduction. The proportion of imine nitrogen, however, remains below 25%, as the imine nitrogen atoms are readily reprotonated in the palladium nitrate acid solution.49 The Pd 3d core-level spectrum in Figure 8c can be curve-fitted with a major and two minor spin-orbit-split (47) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277. (48) Ting, Y. P.; Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Chem. Technol. Biotechnol. 1994, 59, 31. (49) Kang, E. T.; Ting, Y. P.; Neoh, K. G.; Tan, K. L. In The Polymeric Materials Encyclopedia: Synthesis, Properties and Applications; Salamone, J. C., Ed.; CRC Press Inc.: Boca Raton, FL, 1996; p 5496.

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doublets. The major doublet with the BE’s for the Pd 3d5/2 and Pd 3d3/2 peak components lying at about 335 and 340 eV, respectively, are assigned to the Pd(0) species.51 The minor doublet with the BE’s for the Pd 3d5/2 and Pd 3d3/2 peak components lying at about 338 and 343 eV, respectively, are assigned to the Pd2+ ion.50 The doublet with intermediate BE’s for the respective Pd 3d5/2 and Pd 3d3/2 component is associated with the formation of the Pd-N complex.51,52 Thus, the ability of the reduced PANI-AnGPS-glass surface to become coupled with the metal reduction process is similar to that of the fully reduced LM state of the aniline homopolymer.47-49 The Pd-laden PANI surface can be used to catalyze the subsequent electroless plating of copper.52 The above Pdladen glass (Pd-PANI-An-GPS-glass) with a surface [Pd]:[N] ratio of about 0.8 was used for the electroless plating of copper at the temperature of 80 °C for 30 min. The wide scan spectrum in Figure 8e shows that the glass surface has been fully covered by copper to a thickness beyond the probing depth of the XPS technique and Pd can no longer be detected. The thickness of the copper layer was estimated gravimetrically to be more than 7.5 µm. Figure 8f shows the Cu 2p3/2 core-level spectrum and the corresponding Cu (LMM) Auger signal. The Cu 2p3/2 core-level spectrum is dominated by a major peak component at the BE of about 932.7 eV. The corresponding modified Auger parameter is about 1850.6 eV. Thus, the major peak component can be assigned to metallic copper.50 The copper-laden Pd-PANI-An-GPS-glass (Cu-PdPANI-An-GPS-glass) was subjected to the 180°-peel adhesion strength test. Adhesion Strength of the Electroless Deposited Copper to the PANI-An-GPS-Glass Surface. To investigate the nature of the linkages among the various layers on the glass surface, a Cu foil backing and an epoxy adhesive were applied and cured on the Cu-Pd-PANIAn-GPS-glass surface in an attempt to peel off the surface layers from the glass sunstrate. The compositions of the delaminated surfaces were analyzed by XPS. The 180°-peel adhesion strength of the Cu foil/epoxy/Cu-PdPANI-An-GPS-glass laminate exceeds 8 N/cm. Figure 9a-d shows the respective wide scan and C 1s core-level spectra of the two delaminated surfaces of the Cu foil/ epoxy/Cu-Pd-PANI-An-GPS-glass assemble. The presence of strong copper signals and a weak C 1s signal, together with the absence of any N 1s signal, from the delaminated copper surface (Figure 9a) suggests that the adhesion failure has occurred at the interface between the copper foil and the epoxy adhesive. Furthermore, the wide scan and C 1s core-level spectra of the delaminated glass surface (Figure 9c,d) are similar to those of the cured epoxy adhesive (Figure 9e,f). The residual N 1s signal in the wide scan spectra of Figure 9c,e is consistent with the presence of the amine curing agent in the epoxy adhesive. Thus, the composition of the delaminated glass surface further confirms that the adhesion failure has occurred at the interface between the copper foil and the epoxy adhesive. The peel adhesion test results are also consistent with the presence of a strong spatial interaction between the electrolessly deposited metals (Cu and Pd) and the grafted PANI chains. The extent of this interaction is greater than that between the copper foil and the epoxy (50) Moulder, J. F.; Stickler, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992; pp 86, 119. (51) Yang, G. H.; Kang, E. T.; Neoh, K. G.; Zhang, Y.; Tan, K. L.; Langmuir 2001, 17, 211. (52) Ma, Z. H.; Tan, K L.; Kang, E. T. Synth. Met. 2000, 114, 17.

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strengths of the electrolessly deposited copper on the glass and the GPS-glass surfaces activated by the “two-step” process are only in the order of 0.1 N/cm (or negligible) and 1-2 N/cm, respectively. The lack of surface immobilized long-chain molecules on these two substrates for spatial interaction with the electrolessly deposited metals (Pd and Cu) has also resulted in the nonuniform surface coverage of the electrolessly deposited metals. Conclusion

Figure 9. Wide-scan and C 1s core-level spectra of the two delaminated surfaces from an epoxy/Cu-Pd-PANI-An-GPSglass laminate and the pure epoxy adhesive.

adhesive, since the adhesion failure occurred at the interface between the copper foil and the epoxy adhesive. Finally, it is appropriate to point out that the present approach to the molecular design of glass surface by silanization, coupling reaction, oxidative graft polymerization, reduction of palladium, and electroless plating of copper, as shown schematically in Figure 1b, should be readily applicable to other inorganic substrates. It is also appropriate to point out that the electroless plating of copper proceeds via a “Sn-free” process on the PANIgrafted substrate surface. In the absence of PANI graft, the substrate surfaces have to be activated by the conventional “two-step” process,53,54 which involves first sensitization by SnCl2 and then activation by PdCl2, prior to the electroless plating of copper. The 180°-peel adhesion (53) Sard, S. J. Electrochem. Soc. 1970, 117, 864.

Surface modification of glass substrates was carried out via silanization with GPS (the GPS-glass surface), coupling of the epoxide group of GPS with aniline (the An-GPS-glass surface), oxidative graft polymerization of aniline (the PANI-An-GPS-glass surface), and finally electroless plating of palladium and copper. The composition and microstructure of the modified glass surfaces were characterized by X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectroscopy, and atomic force microscopy (AFM). The intrinsic oxidation states, protonation-deprotonation behavior, electrical conductivity, and metal reduction ability of the aniline homopolymer were preserved in the PANI chains tethered on the glass surface as a result of the oxidative graft polymerization process. The electroless reduction of Pd ion by the nitrogen moieties on the grafted PANI chains and the spatial distribution of the grafted PANI chains into the matrix of the electrolessly deposited copper had given rise to strong adhesion of copper to the glass surface. The 180°-peel adhesion strength of the electrolessly deposited copper on the PANI-An-GPS-glass surface exceeded 8 N/cm. The present work provided an effective approach to the functionalization of glass surfaces via molecular design. The technique should be readily applicable to the surface modification and functionalization of other inorganic substrates of importance to the electronic industries, such as the silicon wafers. The ability of PANI in complexing and reducing a palladium salt, in the absence of prior sensitization by SnCl2, might be of importance to the electroless plating industry. LA010866Y (54) Muller, G. Boudrand, D. W. Plating on Plastics: A Practical Handbook; Robert Drapper Ltd.: Teddington, UK, 1971.