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Chem. Mater. 1996, 8, 26-31
Articles Gold-Decorated Poly(3-alkylthiophenes) Mohamed S. A. Abdou† and Steven Holdcroft* Department of Chemistry, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6 Received June 2, 1995. Revised Manuscript Received October 3, 1995X
Films of poly(3-alkylthiophenes) on glass substrates were oxidized with acetonitrile solutions of AuCl3 to yield electronically conductive materials containing the AuCl4counterion, whereas oxidation with nitromethane solutions resulted in both AuCl4--doped polymer and electroless deposition of metallic gold. X-ray photoelectron spectroscopy (XPS) has been used to investigate the chemical nature of the counterion and the chemical evolution of interfaces during oxidation. XPS confirms AuCl4- as being the sole gold species present in films oxidized in acetonitrile. XPS of films oxidized using nitromethane solutions indicate that AuCl4- and Au(0) are present in the bulk and at the polymer/solution and polymer/ substrate interfaces immediately following oxidative doping. However, metallic gold continues to deposit at the polymer/solution interface well beyond the period required for complete oxidative doping. The localization of metallic Au at this interface was confirmed by XPS and depth profile Auger electron spectroscopic analysis. The electroless deposition process is autocatalytic in nature and leads to the formation of a homogeneous metallic layer which increases the conductivity of the film to values in excess of 10 000 Ω-1 cm-1. The high conductivity is solely due to the presence of the metallic layer, the underlying polymer film possessed a conductivity 15 Ω-1 cm-1. The solvent dependency of the electroless deposition process is due to the solvent’s ability to coordinate with Au(I) intermediates, formed during oxidative doping, prior to disproportionation into Au(0).
Introduction Since their discovery in the late 1970s, electronically conducting polymers have shown promise as next generation materials for application in the electronic sector. Such applications were foreseen to include electrostatic dissipation materials, “polymer wires”, thin-film conductors, electrochromic displays, and others.1 However progress in integrating conducting polymers into current technology has been hampered by their relative instability. The reason for their instability lies in the fact that conjugated polymers must be “doped” (i.e., oxidized or reduced) in order to show appreciable conductivity. Unfortunately, the polymers are much less stable in their oxidized or reduced state. Of the various class of conducting polymer, polythiophenes have attracted significant attention because derivatization of the 3-position of the thienyl ring (poly(3-alkylthiophenes), P3AT) with long alkyl chains yields polymers that are processable.2 The thermal stability of doped P3ATs has been studied.3 There appears to † On leave from the Department of Chemistry, University of Cairo, Egypt. * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 1, 1995. (1) (a) Reynolds, J. R. J. Mol. Electron. 1986, 2, 1. (b) Kanizidis, M. Chem. Eng. News 1990, Dec, 36. (2) (a) Elsenbaumer, R. L., Jen, K. Y., Oobodi, R. Synth. Met. 1986, 15, 169. (b) Sato, M., Tanaka, S., Kaeriyama, K. J. Chem. Soc., Chem. Commun. 1986, 873. (3) (a) Wang, Y.; Rubner, M. F. Macromolecules 1992, 25, 3284. (b) Gustafsson, G.; Inganas, O.; Nilsson, J. O.; Liedberg, B. Synth. Met. 1988, 26, 297. (c) Osterholm, J. E.; Passiniemi, P.; Isotalo, H.; Stubb, H. Synth. Met. 1987, 18, 213.
0897-4756/96/2808-0026$12.00/0
be a consensus concerning the origin of the process that reverts the oxidized, conducting polymer to its neutral, semiconducting state (a process coined dedoping). Of the many types of counterion used to stabilize the positive charge carriers in P3ATs, FeCl4- has been shown to yield relatively stable P3AT-based conducting polymers. However, even these materials are thermally unstable over periods of days and photochemically unstable over a period of minutes.4 Previous reports suggest thermal conductivity degradation is due to thermal disordering in the alkyl side chains. This apparently leads to a less coplanar conformation of the polymer backbone, a higher oxidation state of the polymer, and localization of the polaron/bipolaron. As a result of the instability of the localized charge the polymer undergoes chemical reaction and loses conductivity. Our research group has been investigating the stability of electronically conducting polymers with the view to understanding mechanisms of chemical degradation of these systems. A motivation for this work is improve upon the stability of electronically conducting polymers for microelectronic applications.5 Recently, we discovered that AuCl4--containing conducting polymers are orders of magnitude more stable than FeCl4- ana(4) (a) Sandberg, M.; Tanaka, S.; Kaeriyama, K. Synth. Met. 1993, 55-57, 3587. (b) Abdou, M. S. A., Holdcroft, S. Chem. Mater. 1994, 6, 962. (5) Abdou, M. S. A., Xie, Z. W., Leung, A., Holdcroft, S. Synth. Met. 1992, 52, 159.
© 1996 American Chemical Society
Gold-Decorated Poly(3-alkylthiophenes)
logues.6 Half-lifetime values for the rate of decrease of conductivity of P3HT-AuCl4- and P3HT-FeCl4- conducting polymer thin films are 5 months and 8 h, respectively. These results support the fact that the stability of the conducting polymers is strongly dependent on the nature of the counterion. In addition, they show that FeCl4- is not necessarily the most suitable counterion. In addition to this we reported that oxidative doping of thin π-conjugated polymer films using solutions of AuCl3 in nitromethane results in both the formation of AuCl4--doped polymer and electroless deposition of zerovalent gold. Under controlled conditions these processes afford the growth of a homogeneous and continuous metallic layer on top of the polymer film resulting in metal/polymer bilayers with conductivities in excess of 11 000 Ω-1 cm-1. In this report, we describe the results of further studies into the mechanisms of oxidative doping of P3ATs and electroless deposition of metallic gold. Particular use is made of X-ray photoelectron spectroscopy to determine the nature and evolution of gold species at the polymer/substrate and polymer/solution interfaces.
Chem. Mater., Vol. 8, No. 1, 1996 27
Figure 1. UV-vis spectra of neutral (solid line) and oxidized (dotted line) P3HT films. Oxidizing solution: AuCl3 (0.01 M)/ acetonitrile.
Experimental Section Chemicals. Anhydrous ferric chloride (BDH Chemicals), gold trichloride (Aldrich Chemicals) and ammonia solution (BDH) were used as received. Chloroform (Caledon, spectroscopic grade) and acetonitrile (BDH, spectroscopic grade) were distilled over P2O5 and CaH2, respectively. Nitromethane (BDH, Spec. grade) was dried over molecular sieves (type 4A). Poly(3-hexylthiophene) was prepared via oxidative coupling according to the procedure previously described.6 The molecular weight, Mn, molecular weight distribution, head-to-tail: head-to-head dyad ratio, and λmax have been previously reported.6 Instrumentation and Procedures. Conductivities of doped polymer films were measured by collinear four-point probe method. XPS spectra were obtained using an AEI ES200B spectrometer with a Mg KR X-ray excitation source (1253.6 eV line). Oxidized polymer samples were held under vacuum (10-10 Torr) for 2 h before measurements in order to remove surface contamination. The operating pressure was also 10-10 Torr. Data were collected using IBM software. Curve fitting and deconvolution were performed using a nonlinear least-squares curve-fitting program with a Gaussian function. Auger electron spectroscopy (AES) was performed using a Perkin-Elmer 595 scanning Auger microprobe. This technique was employed to investigate the concentration profile of gold species perpendicular to the plane of the film (depth profile analysis). XPS of the polymer/substrate interface was achieved by peeling the polymer film from the substrate and analyzing the exposed polymer surface. AES spectra were obtained using a primary Auger current density of 35 mA/cm2 and a 3 keV electron energy. For depth-profile analysis, samples were sputtered with Ar+ (beam energy, 1 keV). During the measurements, the operating pressure was 10-9 Torr. Elements were identified from their Auger electron energies.7 Thin films of P3HT (0.25 µm), spin-cast onto quartz slides from chloroform solutions (2000 rpm, 30 s), were oxidatively doped by immersion into nitromethane or acetonitrile solutions of AuCl3 (0.01 M). Doped samples were rinsed with the solvent and dried under vacuum.
Figure 2. Kinetic plots of the change in optical density of P3HT upon exposure to AuCl3/acetonitrile.
potential of Au3+/Au+ is 1.16 V (SCE),8 and the oxidation potential of P3HT is 0.97 V (SCE). Red films of P3HT turn blue upon contact with acetonitrile solutions of AuCl3. The UV-vis absorption spectra of neutral and oxidized films are shown in Figure 1. Following doping, two broad absorption bands developed at λmax 765 and 1690 nm consistent with the formation of chargecarrying bipolaronic states.9 In addition, oxidized films show transitions at 232 and 324 nm. These have been previously assigned to the ligand-to-metal chargetransfer bands of AuCl4-.10 The electronic conductivity of AuCl3-oxidized films was 13-15 S cm-1 as determined by the four-point probe technique. Kinetic analysis of the doping process was studied using optical spectroscopy. A plot of the optical density (OD) at 500 nm (π-π* transition of P3HT) and 230 nm (AuCl4- ligand-to-metal charge transfer band) versus time is shown in Figure 2. Using a 0.01 M AuCl3 solution in acetonitrile and 0.25 µm thick films, the doping process was 95% complete within 80 s. XPS spectra of oxidized polymer show the presence of a single gold species at both the polymer/solution and polymer/substrate interfaces. Binding energies associated with these species are 86.3 and 89.9 eV (Figure 3),
Results and Discussion We have previously shown that AuCl3 oxidizes P3ATs by virtue of the fact that the electrochemical redox (6) Abdou, M. S. A.; Holdcroft, S. Synth. Met. 1993, 60, 93. (7) Muilenberg, G. E. Handbook of Auger Electron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1979.
(8) Bard, A. J., Lund, H., Eds. The Encyclopedia of the Electrochemistry of the Elements; Marcel Dekker: New York, 1973. (9) (a) Harbeke, G.; Meier, E.; Kobel, W.; Egli, M.; Kiess, H.; Tosatti, E. Solid State Commun. 1985, 55, 419. (b) Nowak, M. J., Spiegel, D., Hotta, S., Heeger, A. J.; Pincus, P. A. Macromolecules 1989, 22, 2917. (10) (a) Mason, W. R., Gray, H. B. Inorg. Chem. 1968, 7, 55. (b) Gangopadhayay, A. K., Chakravorty, A. J. Chem. Phys. 1961, 35, 2206.
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the Au2+ species,13 but Au2+ is highly unstable and disproportionates to Au3+ and Au+. The rate constant, k, of disproportionation at [Au2+] ) 10-6 M is ∼1010 M-1 s-1.13c Au+ ions, however, form stable and soluble complexes in acetonitrile (Au(CH3CN)nCl).14 In the presence of Cl-, the formation constant of AuCl4- (β4 ) 1026) is much larger than that of Au(CH3CN)nCl2- (β2 ) 1017), indicating that AuCl4- is the dominant species.15 AuCl4- thus serves as the counterion for oxidized polymer. This was previously postulated by optical measurements6 and is now confirmed by XPS measurements since AuCl4- is the only gold species identified in the polymer (at both interfaces) following oxidation. Accordingly, the following mechanism for oxidation of the polymer in acetonitrile is proposed.
P + AuCl3(CH3CN) f P•+ + Au(CH3CN)nCl2 + ClFigure 3. Au 4f XPS spectrum of the polymer/solution interface for P3HT films, on glass, exposed to AuCl3/acetonitrile. Points represent experimental data, solid and dotted lines represent fitted curves.
(1) k
2Au(CH3CN)nCl2 98 Au(CH3CN)Cl3 + Au(CH3CN)nCl (2) Cl- + AuCl3(CH3CN) f AuCl4-
(3)
2P•+ f BP2+ + P
(4)
P + 3AuCl3(CH3CN) f BP2+ + 2AuCl4- + Au(CH3CN)nCl
(5)
Figure 4. Cl 2p XPS spectrum of the polymer/solution interface for P3HT films, on glass, exposed to AuCl3/acetonitrile. Points represent experimental data, solid and dotted lines represent fitted curves.
and are assigned to the 4f7/2 and 4f5/2 levels of Au(III).11 The characteristic XPS spectra of Cl 2p at 197.7 and 199.6 eV were observed at both interfaces (Figure 4) in addition to S 2p and C 1s (spectra not shown).11 Cl is associated with the dopant ion, while C and S are due to the polymer. No Au(0) was detected using the AuCl3/ acetonitrile system. XPS spectra of the polymer/ substrate interface were identical to those of the polymer/ solution interface indicating a homogeneous film. The results are consistent with AuCl4- being the counterion for the electronically conducting polymer. Acetonitrile and organic nitriles in general coordinate AuCl3, forming square-planar complexes exhibiting ligand-to-metal charge transfer absorption bands at 227 and 319 nm.12 Au3+ compounds are readily reduced to (11) (a) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Wiley: New York, 1983. (b) Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer, Eden Prairie, MN, 1979. (12) Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978.
where P represents neutral polymer, and P•+ and BP2+ represent polarons and bipolarons, respectively. We have previously reported that thin films of P3HT oxidized with nitromethane solutions of AuCl3 acquire a metallic gold overlayer. The quality of the gold layer improved with exposure time to AuCl3/nitromethane. For polymer films exposed for 600 s, the quality/ homogeneity of the gold layer was visually indistinguishable from films obtained by metal evaporation. The electronic conductivity of the material increased with increasing doping time as shown in Figure 5. Conductivities as high as 1.1 × 104 S cm-1 were obtained for these polymer/metal bilayers. The high conductivities are attributed to the metallic gold layer. This was confirmed by dedoping the polymer with triethylamine, whereupon the high conductivities remained, while the underlying polymer turned red, indicating dedoping. Oxidation of P3HT films (0.25 µm) with nitromethane solutions of AuCl3 was also monitored by optical spectroscopy. Oxidation of films for 100 s was insufficient time for formation of a homogeneous gold layer, however, the films appeared to be completely oxidized and UV-vis absorption spectra of the film showed two clear absorption bands at 232 and 324 nm confirming the presence of AuCl4-. In addition, a weak and broad characteristic absorption band due to gold(0) was ob(13) (a) Rich, R. L.; Taube, H. J. Phys. Chem. 1954, 58, 6. (b) Mazumder, A. s.; Hart, E. J. Adv. Chem. Ser. 1968, 81, 193. (c) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (14) (a) Goolsby, A. D.; Sawyer, D. T. Anal. Chem. 1968, 40, 1978. (b) Roulet, R., Nguyen, Q. L.; Mason, W. R.; Fenske, G. P., Jr. Helv. Chim. Acta. 1973, 56, 2405. (15) Sillen, L. G., Martell, A. E., Eds. Stability Constants of MetalIon Complexes; The Chemical Society, London, 1964; No. 17.
Gold-Decorated Poly(3-alkylthiophenes)
Figure 5. Change in conductivity of P3HT films upon exposure to AuCl3/nitromethane.
Chem. Mater., Vol. 8, No. 1, 1996 29
Figure 7. Au 4f XPS spectrum of the polymer/solution interface for P3HT films, on glass, exposed for 600 s to AuCl3/ nitromethane. Points represent experimental data, solid and dotted lines represent fitted curves.
Figure 6. Au 4f XPS spectrum of the polymer/solution interface for P3HT films, on glass, exposed for 100 s to AuCl3/ nitromethane. Points represent experimental data, solid and dotted lines represent fitted curves.
Figure 8. AES depth profile of P3HT/Au bilayer prepared by exposure of P3HT films to AuCl3/nitromethane for 600 s. The dotted curve is the derivative of the Au signal.
served at ∼520 nm,16 indicating the formation of aggregates of metallic gold. The conductivities of polymer/ metal composites after 100 s doping were 10-15 S cm-1, which is consistent with those doped with other oxidizing agents. From the increase in conductivity at times >100 s (Figure 5), it appears that gold deposition is largely autocatalytic in nature since the deposition process continued even after the polymer was fully oxidized. Figures 6 and 7 show XPS spectra of the polymer/ solution interface corresponding to the Au 4f region after exposure to AuCl3/nitromethane for 100 and 600 s, respectively. XPS spectra of films exposed for 100 s were fitted to three sets of doublets (Figure 6). The binding energies of 83.6 and 87.3 eV are typical of Au(0),11 whereas the chemical shifts at 86.3 (shoulder) and 89.9 eV are due to Au(III). The chemical shifts corresponding to the fitted curves (Figure 6) at 84.8 and 88.2 eV were assigned to the 4f7/2 and 4f5/2 levels of Au(I) species. Elements S, C, and Cl were also detected at the interfaces (spectra not shown). XPS spectra of the polymer/solution interface of polymer films exposed for 600 s were fitted to two set
of doublets (Figure 7). The 83.6 and 87.3 eV species are due to Au(0). The higher binding energy species at 86.4 and 88.9 eV (fitted curves) are due to Au(III) but their energies are shifted with respect to the AuCl4species identified in Figures 3 and 6. The signals 86.4 and 88.9 eV are relatively weak and are assigned to Au(III) species in the form of adsorbed AuCl3. Signals for carbon and sulfur at this interface due to the polymer were completely absent, indicating that the polymer film is completely covered with gold metal. XPS spectra of the polymer/substrate interface after 100 s exposure indicated only the presence of AuCl4-. Unlike the case of the polymer/solution interface, no Au(0) was detected. The polymer/substrate interface also showed an absence of Au(0) after 600 s exposure. Figure 8 shows the results of an Auger electron spectroscopic depth-profile analysis of a polymer film exposed for 600 s to AuCl3/nitromethane. The depth profile plot illustrates that Au0 is confined to the surface. Elements other than gold could not be detected until the metal had been extensively sputtered. The rate of sputtering, z (nm/s), of an element is given by11a
(16) Dutton, T.; VanWonterghem, B.; Saltiel, S.; Chestnoy, N. V.; Rentzeepis, P. M.; Shen, T. P.; Rogovin, D. J. Phys. Chem. 1990, 94, 1100.
z ) (M/ρNAe)Syip
(6)
where M is the atomic mass of the element, F is the
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density, NA is Avogadro’s number, e is the electron charge, Sy is the sputtering yield, and ip is the primary ion current density. The literature value of Sy for polycrystalline gold using normal incidence of Ar+ gas ion at a bombarding energy of 1 keV is 3.08 atoms/ion.17 Since we employed similar conditions, an approximate rate of sputtering for gold was calculated to be 1.1 nm/ s. The depth resolution function, shown as the dotted curve on Figure 9, indicates that 13 min of sputtering was required to penetrate the polymer layer. Thus the thickness of the gold layer deposited on the polymer is estimated to be ∼890 nm. It is evident that the nature of the solvent plays a significant role in the mechanism of oxidation. In contrast to the AuCl3/acetonitrile doping system, oxidation of P3HT using solutions of AuCl3 in nitromethane results in deposition of metallic gold, in addition to formation of AuCl4--doped polymers. We have previously stated that nitromethane is a relatively poor coordinating solvent for gold trichloride.6 It does not form a square-planar complex with AuCl3,18 as evidenced by the single absorption band of the AuCl3/CH3NO2 solution at 340 nm. Furthermore, the stability of Au(I) complexes is highly dependent on the nature of the solvent. Au+ is stable if coordinated with solvents such as acetonitrile but uncoordinated Au+ is reported to disproportionate forming Au3+ and Au0.19 Accordingly, the following mechanism is proposed for oxidation of P3HT with nitromethane solutions of AuCl3:
P + AuCl3 f P•+ + AuCl2 + Clk
(7)
2AuCl2 98 AuCl3 + AuCl
(8)
Cl- + AuCl3 f AuCl4-
(9)
3AuCl f AuCl3 + 2Au0V
(10)
2P•+ f BP2+ + P
(11)
3P + 8AuCl3 f 3(BP2+2AuCl4-) + 2Au0
(12)
The formation of AuCl and AuCl4- during the course of the oxidation reaction was confirmed using optical and XPS spectroscopies (see Figure 6). Deposition of Au0 was confirmed by XPS analyses and could be inferred from the exceptionally high electronic conductivity of the oxidized materials. The equilibrium constant for reaction 10 is 107.74 in aqueous solution.19b This disproportionation constant is anticipated to be even greater in nitromethane due to its relatively poor coordinative properties. Indeed, when the oxidation of polymer films in nitromethane/AuCl3 was performed in the presence of acetonitrile (5 vol %), electroless deposition of Au0 was completely suppressed since acetonitrile complexes with Au+ to form a stable species. According to eq 12, the following expression (eq 13) relates the reducing capacity of the polymer films to the (17) Behrisch, R. Sputtering by Particle Bombardment I; Sringer-Verlag: Berlin, 1981. (18) Waddington, T. C. Non-aqueous Sovent Systems; Academic Press: New York, 1965. (19) (a) Bravo, O.; Iwamoto, R. T. Inorg. Chem. Acta 1969, 3-4, 663. (b) Moodley, K. G.; Nicol, M. J. J. Chem. Soc., Dalton Trans. 1977, 993.
theoretical thickness of the gold layer for a stoichiometric redox reaction between the polymer and AuCl3:
ρP3HT AWAu0 3 lAu0 ) lP3HT 2 ρAu0 MWP3HT
(13)
where l and F are the film thickness and density. MWP3HT is the molecular weight of P3HT, and AWAu is the atomic weight of gold. The factor 3/2 represents the stoichiomety of the reaction whereby 3 mol of polymer yields 2 mo of Au(0). The molecular weight of the reducing agent in this system was calculated based on 4 thiophene units which is the number of rings found per AuCl4- dopant ion for oxidized polymers.6 For a 250 nm thick polymer film, the estimated thickness of Au(0) layer was 6.3 nm. However, the experimental value determined by AES was ∼890 nm. This value is much greater than the stoichiometric value and thus the mechanism of deposition of gold on P3HT must possess a significant autocatalytic component. Autocatalysis requires a sacrificial reducing agent, presumably moisture. This was confirmed by the observation that electroless deposition of gold was suppressed by performing the oxidation under ultradry nitrogen conditions and using thoroughly dried solvents. The process of electroless deposition of gold on solid polymer films may be regarded as beginning with a nucleation step whereby Au0 particulates are formed on the polymer surface via redox reactions with the polymer. This is followed by an autocatalytic electroless deposition step on metallic nuclei. The fact that no Au(0) was detected at the polymer/ substrate interface indicates that the initial redox reaction which deposits Au(0) takes place at the polymer/ solution interface. Hence oxidation of the polymer occurs primarily at this interface. This occurs when both the rate of electron transfer between the polymer and oxidant and the rate of charge transfer through the polymer film to the film’s surface are much faster than diffusion of oxidant through the bulk of the polymer film. In these cases, the overall rate of oxidation of the polymer film is determined by the rate of diffusion of the counterion.20 Formation of Au(0) particulates at the polymer/solution interface, together with poor diffusion of oxidant through the film, ensures that subsequent electroless deposition onto nucleated particles occurs at the polymer/solution interface. Conclusions XPS analysis supports the original report that oxidative doping of thin π-conjugated polymer films using acetonitrile solutions of gold trichloride yields polymer films containing the AuCl4- counterion, whereas oxidation of polymer films using solutions of AuCl3 in nitromethane result in electroless deposition of zerovalent gold in addition to formation of AuCl4--doped polymer. For short exposure times to AuCl3/nitromethane solutions the presence of both Au(III) and Au(0) on the polymer/solution interface are detected. Longer periods of exposure results in a homogeneous layer of gold for which Au(0) was deposited largely at the polymer/ (20) (a) Andrieux, C. P.; Saveant, J. M. J. Electroanal. Chem. 1982, 142, 1. (b) Murray, R. W. Philos. Trans. R. Soc. London, A 1981, 302, 253.
Gold-Decorated Poly(3-alkylthiophenes)
solution interface. This was supported by depth profile Auger electron analysis. The electroless deposition of gold may prove useful in the microelectronics sector wherein the conductivity of conducting polymers wires can be enhanced by several order of magnitude.21 Golddecorated films may also prove useful in the design of integrated sensors since the polymer/gold bilayer can be functionalized by the self-assembly of organic thiols onto the gold surface.22 (21) Bartels, C., Abdou, M. S. A.; Holdcroft, S., manuscript in preparation.
Chem. Mater., Vol. 8, No. 1, 1996 31
Acknowledgment. Financial support of this work by the Natural Sciences and Engineering Research Council of Canada, and the National Research Council, Canada, is gratefully acknowledged. XPS and Auger measurements were carried out with the assistance of K. Myrtle of the Department of Physics, Simon Fraser University. CM950241J (22) (a) Bain, C. D.; Whitesides, G. M. Adv. Mater. 1989, 28, 506. (b) Dressick, W. J.; Dulcey, C. S.; Georger, J. H., Jr. Chem. Mater. 1993, 5, 148.
