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Polyaniline Monolayer Self-Assembled on Hydroxyl-Terminated Surfaces Ruthy Sfez, Liu De-Zhong, Iva Turyan, Daniel Mandler, and Shlomo Yitzchaik* Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received September 18, 2000. In Final Form: January 3, 2001 A monolayer of conducting polymer, polyaniline (PAN), was assembled on hydroxyl-terminated surfaces, such as quartz, glass, indium-tin oxide, and native oxide on Si. The approach for assembling two-dimensional monolayers of PAN is based on a chemical or electrochemical surface polymerization of electrostatically bound anilinium. The latter was bound to the negatively charged sulfonate group of mercaptoethanesulfonate that was previously coupled by an SN2 reaction to an iodopropyl self-assembled monolayer. The consecutive assembling steps were followed by X-ray photoelectron spectroscopy, UV-vis-NIR spectroscopy, variable angle spectroscopic ellipsometry, contact angle, and atomic force microscopy measurements. The characteristic electronic properties of the PAN monolayers were studied by UV-vis-NIR spectroscopy and cyclic voltametry.
Introduction Since their discovery, conducting organic polymers1 provided wide expectation to replace metals and semiconductors in different devices, their advantage being the ability to coat a large variety of surfaces, including large screens. Polyaniline (PAN), which is characterized by its versatile redox behavior2 and proton-doping effect,3 has been among the most studied polymers. Changes in its conductivity can easily be obtained by redox reactions, which transform it from one redox state to another, as well as by proton doping or humidity,4 which change the conducting properties while maintaining the redox state. However, solubility and processability limitations have restricted wide application of PAN films in devices. The ability to form monolayers of conducting polymers has scientific as well as applied implications in organic light emitting diodes (OLED), as charge transport layer materials. PAN has already been used as an electrode (anode) in devices5 and recently as a hole-transport material. A few years ago SCALE6 (symmetrically configured alternating current light emitting) devices have been introduced, in which PAN was used as an interface between the electrode and the emissive material, greatly improving charge injection at the electrode interface. Another PANbased interface was demonstrated recently,7 taking advantage of soluble polyaniline8 as a charge transport interlayer in a LED device assembled on ITO. The assembling method of the polymer involved the layerby-layer electrostatic self-assembly technique,9 which resulted in an improved charge injection. Recently, Turyan * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. Engl. 1998, 37, 402. (2) MacDiarmid, A. G.; Chiang, J.; Halpern, M.; Huang, W.; Mu, S.; Somasiri, N. L. D.; Wu, W.; Yaniger, S. I. Mol. Cryst. Liq. Cryst. 1985, 121, 173. (3) Chiang, J. C.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193. (4) Angelopoulos, M.; Anjan, R.; MacDiarmid, A. G. Synth. Met. 1987, 21, 21. (5) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477. (6) Wang, Y. Z.; Gebler, D. D.; Lin, L. B.; Blatchford, J. W.; Jessen, S. W.; Wang, H. L.; Epstein, A. J. Appl. Phys. Lett. 1996, 68, 894. (7) Ho, P. K. H.; Granstrom, M.; Friend, R. H.; Greenham, N. C. Adv. Mater. 1998, 10, 769. (8) Jiang, Y.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800. (9) Decher, G.; Hong, J. D. Macromol. Chem. Macromol. Symp. 1991, 46, 321.
and Mandler10 presented an approach of a surface polymerization of anilinium electrostatically bound to a negatively charged sulfonate monolayer on Au. In this paper we report on the formation and characterization of a PAN monolayer formed as a result of a surface polymerization of anilinium on modified hydroxylterminated surfaces, such as quartz, silicon dioxide, and indium-tin oxide (ITO). The ability of making layers of PAN on hydroxyl-terminated surfaces is of great importance in sensors11 or luminescent devices in which transparent ITO is used. The approach is based on the electrostatic extraction of anilinium onto a sulfonateterminated monolayer. The formation of the latter was achieved by two synthetic steps as illustrated in Figure 1. In the first step a short alkyl chain, which is terminated with a superior leaving group, is attached to the hydroxylated surface. The second step involves an in situ SN2 reaction in which mercaptoethanesulfonate (MES) is anchored through a thioether bridge. The electrostatic adhesion in our case differs from the polyanion layer-bylayer assembly technique due to the fact that molecules (monomers) and not polymers are attached to the surface. Furthermore, one can have a better control on the thickness and redox state of the PAN layer. Experimental Section Substrates and Cleaning. Glass, quartz (Chemglass) and n-Si 〈100〉 (Virginia Semiconductors) substrates were cleaned and activated by the following cleaning procedure in order to increase the density of hydroxyl groups on the surface. The substrates were cleaned by sonicating in soapy water at 60 °C for 30 min, washed three times with deionized water, and then immersed in piranha solution (H2SO4/H2O2, 70:30 v/v) at 90 °C for 1 h. Caution: The piranha solution should be handled with extreme care! After the substrates were washed three times with deionized water, the RCA12 cleaning procedure was employed. This involved sonicating the substrates in ammonium hydroxide solution (H2O/H2O2/NH3, 5:1:1 v/v/v) for 30 min followed by washing three times with deionized water, rinsing with acetone, and drying for 10 min at 130 °C. ITO substrates (Delta Technologies) were treated similarly, however omitting cleaning with piranha solution to prevent etching of the ITO. Chemicals. Mercaptoethanesulfonic acid sodium salt (MES, Aldrich), iodopropyltrimethoxysilane (IPM, Gelest), and methanol (10) Turyan, I.; Mandler, D. J. Am. Chem. Soc. 1998, 120, 10733. (11) Sangodkar, H.; Sukeerthi, S.; Srinivasa, R. S.; Lal, R.; Contractor, A. Q. Anal. Chem. 1996, 68, 779. (12) RCA Engineering 1983, p 28.
10.1021/la001343d CCC: $20.00 © 2001 American Chemical Society Published on Web 04/05/2001
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Figure 1. Scheme of the synthetic route developed for the self-assembly of a 2D monolayer of PAN. (Baker) were used without further purification. Aniline (Aldrich) was distilled under reduced pressure while toluene (Baker) was distilled and dried over Na under nitrogen before use. Instrumentation. Electrochemical measurements were carried out with a BAS100B electrochemical analyzer using a conventional three-electrode cell. All potentials are quoted vs Ag/AgCl. Atomic force microscopy (AFM) measurements were carried out with a Nanoscope II in the contact mode using a cantilever with a 0.58 N/m force constant. Si, ITO, and glass substrates were imaged in air. The best AFM images were obtained on Si due to its smoothness. The surface roughness (root mean square) was calculated from 100 × 100 nm2 areas. UV-vis-NIR spectra were acquired with a Shimadzu spectrophotometer UV-3101PC on ITO or glass substrates. Contact angle measurements were performed using glass or Si with a Rame´Hart (model 100) goniometer. X-ray photoelectron spectroscopy (XPS) spectra were recorded using an Al KR X-ray source on a Kratos axis-HS instrument. Ellipsometry measurements were carried out on a VB-200 ellipsometer (Woolam Co.) Procedure. A clean ITO, silicon, or glass substrate was immersed in a 1% (vol) solution of IPM in dry toluene with 3 ppm of glacial acetic acid. The sample was allowed to react for 16 h at 80 °C, under nitrogen atmosphere, to form a self-assembled monolayer (SAM) while avoiding polysiloxane formation. Upon completion of reaction the substrates were washed three times with toluene and sonicated for 1 min in acetone in order to remove any excess of IPM. The iodopropyl-functionalized monolayer was cured for 10 min at 130 °C to produce siloxane network by condensation of the residual methoxysilane groups. The coated substrates were then immersed in a solution of 0.5 g of MES in 60 mL of methanol for 3 days at 60 °C. After the substrates were washed with 2-propanol and acetone, the negatively charged sulfonated substrates were immersed in 0.01 M anilinium in HCl solution (pH 2.5) allowing the electrostatic exchange to proceed for 30 min. For cyclic voltammetry (CV) measurements, 0.1 M phthalate buffer with 0.01 M anilinium was used (pH 2.5). The pH was chosen in order to maintain the negative charge of the sulfonate (pKa ) 0.7) and the positive charge of anilinium
(pKa ) 4.5). Excess of anilinium was removed by washing with HCl solution (pH 2). Finally, surface polymerization was carried out either chemically (with 5 × 10-4 M of ammonium persulfate in 0.1 M HCl) or electrochemically by CV. The potentials were scanned several times between -0.2 and 1.1 V vs Ag/AgCl in 0.1 M H2SO4, 0.5 M Na2SO4 solution.
Results and Discussion Coupling Layer Deposition. The formation of an iodoterminated SAM (step i, Figure 1) was verified by contact angle measurements, XPS, and AFM. Specifically, the contact angle of the clean substrate increased from 15° to 80° upon coupling. The XPS provides additional evidence for coupling. Namely, the characteristic XPS signals for alkyl iodide functionality, i.e., I 3d5/2 at 621 eV and I 3d3/2 at 633 eV, appeared (Figure 2a). The AFM measurements (Figure 3b) showed a decrease of surface roughness (root mean square) from 0.38 to 0.09 nm as a result of the coupling reaction. Ellipsometric measurements clearly indicated the formation of a monolayer with a thickness of 8.3 Å obtained for the coupling layer. Surface Sulfonatation. Step ii introduces alkylsulfonate functionality by the formation of a thioether bridge with the underlying monolayer. As expected, the contact angle of a water drop decreased from 80° on an iodoterminated SAM to 34° on the sulfonated monolayer. At the same time, the XPS signal characteristic of the S was not detected due to noisy measurement and a broad signal of Si at the same region, but evidence for the substitution reaction was provided by two features. First, the iodide signals vanished completely, suggesting a complete substitution of iodide to thioether bridge. Second, the Na 1s signal appeared (Figure 2b), indicating the MES negatively charged monolayer formation and its sodium counterion. The AFM image (Figure 3c) shows a distinct increase of
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Figure 2. XPS spectra of (a) iodide (I 3d signal, Figure 1, step i); (b) sodium (Na 1s signal, Figure 1, step ii); (c) quaternary amine (N 1s signal for anilinium Figure 1, step iii); (d) tertiary and quaternary amine (N 1s signal for PAN monolayer Figure 1, step iv).
the surface roughness (from 0.09 to 0.13 nm) as a result of attaching the MES. Ellipsometric measurements proved the layer to be 4.9 Å thick, which agrees with a monolayer formation of MES. Electrostatic Assembly of Anilinium Monomer. Soaking the MES-terminated SAM in anilinium solution resulted in exchanging the sodium ions by anilinium, (step iii, Figure 1). This fact is shown by XPS measurements in which the Na 1s signal disappeared and the N 1s signal appeared at 399 eV (Figure 2c). This signal corresponds to the quartenary nitrogen of the anilinium. An UV-visNIR spectrum of a modified quartz substrate was recorded after preconcentration of anilinium. The two new peaks that are detected at 200 and 230 nm are identical to those found in dissolved anilinium and therefore verify its adsorption onto the surface. Ellipsometric measurements proved the layer to be 4.8 Å thick, which agrees with an anilinium monolayer formation. Polyaniline Formation. The polymerization step (Figure 1, step iv (chemical), or step v (electrochemical) increased the contact angle to 62° from a substantial smaller value with either the sulfonate or the anilinium monomer. Analysis of the XPS data reveals a broad signal
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from 399 to 402 eV (Figure 2d) which corresponds to the quaternary and tertiary amines (N 1s signal at 399.5 and 401.5 eV, respectively). The signals appear to be of the same height suggesting a 1:1 ratio between tertiary and quaternary amines in the polymerized PAN monolayer. This fact can account for the hydrophobic nature of PAN, since 1/2 of the monolayer is composed of neutral (nonionic) domains. The mean surface roughness of the chemically formed PAN layer was 0.05 nm (Figure 3d), which indicates the extremely smooth surface. Ellipsometry measurements proved the polymer monolayer to be 2 Å thick. This value is smaller with respect to a monomer’s monolayer thickness comparing the monomer’s monolayer and can indicate a different orientation of the anilinium in the monomeric and polymeric films. AFM measurements, which were carried out on Si after each step, showed a trend of roughness diminishing that has already been observed in previous studies.13 The mean surface roughness of the Si wafer diminished from 0.38 nm for a bare Si substrate to 0.05 nm for the polymer layer. Interestingly, a correlation appears to exist between the measured roughness and the hydrophilicity of the surfaces. It is evident that the bare Si and the sulfonateterminated surfaces are substantially rougher than the other more hydrophobic substrates. This might be due to tip-surface interactions suggesting that care must be taken in interpretation of the AFM images. UV-vis-NIR measurements were conducted on an ITO substrate after the last step of anilinium electropolymerization. The 2D-PAN polymer spectrum was recorded using a sulfonate monolayer as reference. The anilinium spectrum was acquired on modified quartz and had no absorbance in the visible region. The PAN spectrum showed the polaron band transition in the range of 8001000 nm (Figure 4), which is in agreement with that reported in the literature14 for emeraldine films, and is different from anilinium spectrum with its characteristic peak at 251 nm.15 The second band (at 400-600 nm) is probably due to an undoped form and corresponds to the emeraldine-base form. CV measurements (Figure 5) were conducted with ITOmodified substrates. The electrochemical oxidation of anilinium does not show a clear oxidation wave at positive potentials; however, the next cycles clearly show a doping and undoping wave at 0.38V that is well characteristic of PAN.16 From this wave it is possible to roughly estimate the surface coverage of aniline. The value, which is obtained by taking into account that the ratio between an excess charge of oxidized PAN (emeraldine) and aniline units is 0.52, is ca. 3.4 × 10-10 mol‚cm-2. It indicates that the excess of surface coverage of the PAN is relatively low. This can be explained mainly by the low hydroxyl group density on ITO. Conclusion We presented a new approach for obtaining a SAM of PAN on hydroxylated surfaces using electrostatic interaction between anilinium ions and a negatively charged sulfonated surface. The latter was obtained by an in situ SN2 reaction, taking advantage of a mercapto group as a nucleophile and an iodo moiety as a superior leaving group. (13) Lin, W. B.; Lin, W. P.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034. (14) Stafstrom, S.; Breda J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; MacDiarmid, A. J. Phys. Rev. Lett. 1987, 1464. (15) Robinson, J. W. Handbook of Spectroscopy; CRC Press: Cleveland, OH, 1974; Vol. 2, p 43. (16) Bhadani, S. N.; Gupta, M. K.; Sengupta, S. K. J. Appl. Polym. Sci. 1993, 49, 397.
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Figure 3. AFM images of (a) bare Si wafer; (b) after IPM coupling; (c) after forming a sulfonated monolayer; (d) after chemical polymerization of attached anilinium.
Figure 4. UV-vis-NIR absorption spectra of 2D-PAN monolayer on ITO substrate after electropolymerization of anilinium.
The ability of controlling the redox state17 combined with the change in properties due to protonation, provides PAN an important role in tuning the work function of conducting surfaces and OLED charge injection efficiency. Significant changes in bulk properties due to monolayer functionalization are known, which suggest that the approach presented here could be used further by employing derivatives of anilinium for this purposes. Moreover, it (17) Ping, Z.; Nauer, G. E.; Neugebauer, H.; Theiner, J.; Neckel, A. J. Chem. Soc., Faraday Trans. 1997, 93, 121.
Figure 5. Cyclic voltammetry of modified ITO electrode after preconcetration of anilinium. The voltammogram was recorded in 0.1 M H2SO4, 0.5 M Na2SO4, scan rate was 100 mV/s.
allows diminishing the dimensions of molecular electronic and optoelectronic devices while improving their efficiency. Acknowledgment. S.Y. thanks the US-Israel Binational Science Foundation for funding under Grant No. 95000-85. LA001343D
