Enhanced Conductivity of Thin Film Polyaniline by Self-Assembled

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Langmuir 2007, 23, 879-884

879

Enhanced Conductivity of Thin Film Polyaniline by Self-Assembled Transition Metal Complexes David M. Sarno,† Justin J. Martin,† Steven M. Hira,‡ Cliff J. Timpson,‡ Jean P. Gaffney,† and Wayne E. Jones, Jr. *,† Chemistry Department and Institute for Materials Research, Binghamton UniVersity, Binghamton, New York 13902, and Department of Chemistry, Roger Williams UniVersity, Bristol, Rhode Island 02809 ReceiVed August 15, 2006 In a recent study, the transition metal complex, cis-dichlorobis(2-,2′-dipyridyl)ruthenium (II) (Ru(bpy)2Cl2), and the macrocycle Ru(TPP)CO (TPP:- tetraphenylporphine) were bound to pyridine terminated self-assembled monolayers on quartz. Following modification of the quartz surface with metal complexes, the conducting polymer polyaniline was deposited via in situ polymerization. The sheet conductivity (as measured by the four-probe method) of the resulting polyaniline films deposited onto Ru(bpy)2Cl2 and Ru(TPP)CO surfaces was significantly enhanced relative to films deposited onto unmodified quartz. It is postulated that either the macrocycle or the transition metal complexmodified surface interacts with the conducting polymer as it is forming, resulting in a more ordered expanded coil conformation for the polymer. The net result of such an interaction is a thin film possessing significantly greater electrical conductivity.

1. Introduction In the study of surfaces, the chemical and physical properties of the outermost few angstro¨ms of a material govern the interactions that occur at the solid-solid, solid-liquid, and solidvapor interfaces. The interfacial properties of a substrate, such as wetting, biocompatibility, adhesion, diffusion, conductivity, and charge transfer can be altered or tuned for specific applications via chemisorption or physisorption of nanoscale thin films.1-7 Advances in the processability of conducting polymers such as polyaniline (PANI), polypyrrole (PPy), poly(phenylene vinylene) (PPV), and poly-3,4-ethylenedioxythiophene (PEDOT) allow the deposition of thin films on a variety of substrates, including glass, polymers (e.g., polyethylenes, polypropylenes, and other conventional plastics), textiles, and micro- and nanostructured metal oxides.2-13 As a result, conducting polymers have demonstrated much promise for the development of novel batteries, antistatic and anticorrosive coatings, sensors, liquid crystal displays, LEDs, and transistors, as well as replacements or alternatives to metallic wires and connectors in conventional electronic devices.8-13 Flexible throwaway applications, such as * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (607) 777-2421. Fax: (607) 777-4478. † Binghamton University. ‡ Roger Williams University. (1) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (2) Malinauskas, A. Polymer 2001, 42, 3957-3972. (3) Wu, C.-G.; Chen, J.-Y. Chem. Mater. 1997, 9, 399-402. (4) Wu, C.-G.; Yeh, Y.-R.; Chen, J.-Y.; Chiou, Y.-H. Polymer 2001, 42, 28772885. (5) Kang, E. T.; Neoh, K. G.; Pun, M. Y.; Tan, K. L.; Loh, F. C. Synth. Met. 1995, 69, 105-108. (6) Neoh, K. G.; Teo, H. W.; Kang, E. T.; Tan, K. L. Langmuir 1998, 14, 2820-2826. (7) Zhao, L.; Neoh, K. G.; Kang, E. T. Chem. Mater. 2002, 14, 1098-1106. (8) MacDiarmid, A. G. Synth. Met. 1997, 84, 27-34. (9) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480-6484. (10) Huang, Z.; Wang, P.-C.; Feng, J.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Synth. Met. 1997, 85, 1375-1376. (11) Stejskal, J.; Sapurina, I.; Prokesˇ, J.; Zemek, J. Synth. Met. 1999, 105, 195-202. (12) Sapurina, I.; Riede, A.; Stejskal, J. Synth. Met. 2001, 123, 503-507. (13) Fedorova, S.; Stejskal, J. Langmuir 2002, 18, 5630-5632.

electronic paper and other display devices, have also been of recent interest.14 In situ polymerization methods have been widely exploited for the deposition of conducting polymers on nearly any substrate and are especially useful for coating insulating materials.2-13,15-18 In contrast to solution casting of polymers, the in situ method takes advantage of the insolubility normally associated with many high molecular weight polymers to deposit the polymeric material on a substrate. For example, in acidic aqueous solution, a strong oxidant such as (NH4)2S2O8 converts a variety of monomers to the radical cation, which is adsorbed onto a suitable substrate with subsequent polymerization outward from its surface11-13,15-19 This method can produce high quality, continuous, transparent films that range from 50 to 500 nm thick. The thickness and the morphology of the deposited films can be controlled in a predictable and reproducible way by varying experimental parameters such as substrate, temperature, and concentration.8-13, 20 Polyaniline (PANI) is among the most widely studied conducting polymers for reasons that include its ease of synthesis and chemical versatility. It is unique in its ability to undergo reversible switching between insulating and conducting forms. The insulating emeraldine base form of PANI can be acid-doped with 1 M HCl to obtain the conducting emeraldine salt in which the conductivity is increased by 9-10 orders of magnitude. Sheet conductivity measurements as high as 2-6 S/cm have been recorded for acid-doped PANI films on clean hydrophilic glass (14) Sirringhaus, H.; Kawase, T.; Friend, R. H. MRS Bull. 2001, 26, 539-543. (15) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss., Chem. Soc. 1988, 88, 317-332. (16) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103-116. (17) Avlyanov, J. K.; Min, Y.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995, 72, 65-71. (18) Zheng, W.; Min, Y.; MacDiarmid, A. G.; Angelopoulos, M.; Liao, Y.-H.; Epstein, A. J. Synth. Met. 1997, 84, 63-64. (19) Stejskal, J.; Kratchovı´l, P.; Jenkins, A. D. Polymer 1996, 37, 367-369. (20) Avlyanov, J. K.; Josefowicz, J. Y.; MacDiarmid, A. G. Synth. Met. 1995, 73, 205-208.

10.1021/la0624135 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/01/2006

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depending on the reaction conditions.9,15-18,20 In comparison, the sheet conductivity of copper metal is on the order of ∼105 S/cm. The film properties can be modulated by the properties of the surface onto which the conducting polymer is deposited. For example, the sheet resistivity of PANI and PPy on hydrophilic surfaces is 3 orders of magnitude greater than on hydrophobic surfaces.8-10 The differences in the electrical properties have been attributed to the intermolecular conformation of the polymer chains of the film.15-18 For example, it is known that the conductivity is increased if the interaction between the surface of the substrate and the conducting polymer favors an expanded coil structure. In contrast, when a compact coil structure dominates, defects due to ring twisting result in disruption of the extended π-conjugation in the polymer backbone, which attenuates the conductivity.16-18 When a more expanded-coil structure is achieved, the conductivity is increased through two effects. First, conjugation defects are reduced as planarity between neighboring rings is enhanced, thus resulting in an increase in the intramolecular component of conductivity. Further, the more open conformation encourages a greater degree of polymer crystallinity, which enhances the intermolecular component of the bulk conductivity.8-10,16-18 Several reports have explored the use of self-assembled monolayers to control interfacial thin film growth.8-10 In addition to imparting changes in surface hydrophobicity, this method can be used to introduce reactive transition metal complexes, organic functional groups, or stable molecular species to the substrate surface. In this study, we have prepared monolayers of transition metal complexes by coordination to a self-assembled pyridine terminated alkylsilane coupling layer on fused quartz and investigated the effects of this chemically-modified surface on the in situ polymerization of polyaniline thin films. Previous studies have suggested a stabilizing effect from transition metal complexes on the overall conductivity of conducting polymers.21,22 In our study, surface conductivity measurements and surface structural characterization were used to identify the benefits associated with this approach to substrate modification. 2. Experimental Procedures Aniline (>99% purity) was purchased from Aldrich and distilled prior to use. Ammonium persulfate (98%) was purchased from Aldrich and stored under nitrogen at 5 °C. Concentrated hydrochloric acid, concentrated sulfuric acid, and hydrogen peroxide (30% w/w) were purchased from Fisher and used as received. Deionized water showing greater than 18 MΩ resistance was obtained using a Barnstead Nanopure filtration system. The silane coupling agent, 11-bromoundecyltrichlorosilane (11-BrUTS), was purchased from Gelest and used as received. The porphyrin complex, ruthenium (II) tetraphenylporphine carbonyl (Ru(TPP)CO), was purchased from Frontier Scientific and used as received. Ru(bpy)2Cl2 was synthesized from RuCl3‚nH2O according to previously published procedures.23 All other reagents were purchased from Aldrich and used as received. All substrates were Corning 7980 grade, high-quality, fused quartz obtained from Technical Glass Products (Painesville, OH). In most cases the 3 in. × 3 in. × 1/16 in. purchased slides were cut into 1 cm2 squares with a diamond saw. To prepare the fused quartz substrates for surface modification, the substrates were cleaned and hydroxylated by immersing them in a 7:3 (v/v) mixture of concentrated sulfuric acid and 30% hydrogen peroxide. This (21) Jiang, B.; Yang, S.; Bailey, S.; Hermans, L.; Niver, R.; Bolcar, M.; Jones, W. E., Jr. Coord. Chem. ReV. 1998, 171, 365. (22) Multimetallic and Macromolecular Inorganic Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 1999; Vol. 4, Ch. 1. (23) Jones, W. E., Jr.; Smith, R. A.; Abramo, M. T.; Williams, M. D.; Van Houten, J. Inorg. Chem. 1989, 28, 2281.

Sarno et al. H2SO4/H2O2 solution, commonly referred to as a piranha solution is highly corrosive and strongly oxidizing. All suitable precautions to guard against personal injury should be observed. The mixture containing the slides was heated to 90 °C for 1 h after which the quartz slides were removed, rinsed thoroughly with distilled water, and dried under vacuum at 140 °C for 1 h. Deposition of the alkyltrichlorosilane coupling layer was carried out under an inert atmosphere in a Kontes 100 mL cylindrical reaction flask equipped with a detachable three-neck, flanged head. The detachable head allowed insertion of a custom fabricated aluminum disk that contained shallow (3 mm) grooves to hold 14 quartz slides upright in the flask. The flask containing the slides was purged with dry N2, and then freshly distilled toluene (100 mL) was transferred into the flask through a cannula. Exactly 0.5 mL of 11-BrUTS was added to the flask via a syringe. After 24 h of stirring at room temperature under a constant flow of nitrogen, the entire sealed reaction assembly was sonicated for 20 min to remove material that was physically adsorbed on the slides. The reaction solution was then removed through a cannula and replaced with enough fresh distilled toluene to completely submerge the samples. The system was again sonicated for 20 min, after which the flask was opened. The bromine terminated silanated (Br-sil) samples were then sonicated for 20 min in methanol. Finally, each slide was rinsed with fresh methanol, dried under a stream of N2, and then heated in an oven at ∼135 °C for 20 min to promote cross-linking and surface bonding.24,25 Samples were stored under vacuum in an ambient temperature desiccator. A pyridine terminated surface was prepared from Br-sil derivatized quartz slides by modification of a previously published method.26 The Br-sil slides were placed in the sample holder and sealed in the reaction flask equipped with a condenser. The system was purged with dry N2 before transferring 100 mL of freshly distilled toluene into the flask through a cannula. A 10 mL aliquot of 4-picolyllithium in THF was added to the flask via a syringe, causing the solution to darken significantly. The system was refluxed for 3 h during which the brown precipitate, lithium bromide, was generated. After a brief cooling period, the reaction was quenched by the addition of 10 mL of anhydrous EtOH, which produced vigorous bubbling that quickly subsided. The solution immediately became a transparent pale yellow as the precipitate dissolved. The flask was opened, and the slides were immersed in anhydrous EtOH to remove most of the physically adsorbed material. Samples were then subjected to two 15 min sonications, each in fresh EtOH, followed by an EtOH rinse and drying under a stream of nitrogen. To further ensure cleaning of the samples, they were refluxed overnight in ethanol under a N2 atmosphere, then sonicated for 15 min in EtOH, rinsed again with fresh EtOH, and finally dried under a constant stream of nitrogen. The clear, colorless, streak-free slides were stored under vacuum in a desiccator. This procedure is outlined in Scheme 1. The porphyrin complex RuTPP(CO)(EtOH) and the complex Ru(bpy)2Cl2 shown in Figure 1 were covalently coordinated to the pyridine terminated surfaces by placing the pyr-sil derivatized slides into refluxing solutions of each complex. In a general preparation, approximately 0.037 g of Ru(TPP)CO or 0.024 g of Ru(bpy)2Cl2 was added to the reaction flask, followed by the sample holder containing the pyr-sil derivatized fused quartz slides. The system was thoroughly purged with dry N2 before adding 50 mL of solvent via cannula (toluene for RuTPP(CO)(EtOH); 1:1 anhydrous EtOH/water for Ru(bpy)2Cl2) to the reaction flask. The resulting 1 mM solution was bubbled with N2 for approximately 30 min before bringing the solution to reflux under a N2 atmosphere. Although Ru(TPP)CO was initially only slightly soluble, upon reflux, it dissolved to yield a deep red solution. The Ru(bpy)2Cl2 solution was first deep purple but became deep orange-red as the reflux progressed. (24) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 43674373. (25) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236-2242. (26) Paulson, S.; Morris, K.; Sullivan, B. P. J. Chem. Soc., Chem. Commun. 1992, 1615-1617.

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Scheme 1. Covalent Attachment of 11-Bromotrichlorosilane to the Hydroxlyated Surface of Fused Quartz Followed by Reaction of the n-Terminal Bromide with 4-Picolyllithium to Yield a Pyridine Terminated Alkyl Silane Derivatized Surface

After ∼72 h of reflux, the reaction flask was opened to the atmosphere, and the aluminum slide holder with the slides intact was removed and immersed in fresh solvent to remove any physically adsorbed material (toluene for RuTPP(CO) and absolute EtOH for Ru(bpy)2Cl2). The samples, which had a dull orange film, were then sonicated in fresh solvent for 15 min, after which they appeared clean and virtually colorless. After rinsing each slide with solvent, all slides were dried under a stream of dry N2 and stored under vacuum in a desiccator until needed. To deposit polymeric material onto the metal-modified slides, separate solutions of the monomer and oxidant were prepared in 100 mL volumetric flasks. A typical preparation utilized a 0.2 M distilled aniline solution in 1 M HCl and a 0.25 M (NH4)2S2O8 solution in 1 M HCl. Exactly 50 mL of the aniline solution was added to a 100 mL beaker equipped with a magnetic stir bar. The metal-modified quartz slides were completely submerged within the aniline solution by clamping each slide in a plastic forcep and suspending the forcep from a rack. Exactly 50 mL of the (NH4)2S2O8 solution was added to the aniline solution to initiate the polymerization. Slides underwent deposition for exactly 10 min, after which they were removed from the stirring solution and placed in separate vials of aniline/1 M HCl to quench the polymerization and wash away loosely adsorbed material. UV-vis absorption spectra were obtained on a Hewlett-Packard 8452A diode array spectrometer. The slides were immobilized in the path of the interrogating beam by clamping them to the sample holder. In some instances, multiple slides were stacked to enhance absorbance features. In general, the instrument was blanked using either bare quartz or the pyr-sil coated slides. Conductivity measurements were made using a custom fabricated four-probe cell equipped with four spring loaded copper contacts spaced 1 mm apart. The tips were engaged with the substrate using either a pair of threaded contact screws or a spring loaded lever to deliver a steady contact pressure. Conductivity data were generated using a Keithley 182 voltmeter interfaced to a Keithley programmable current source set to provide 1.0 µA of current. A custom data acquisition program written in BASIC was used to control both devices, and all data were acquired via a standard GPIB interface. Atomic force microscopy (AFM) data were collected using a Digital Instruments Multimode scanning probe microscope set to tapping mode, equipped with a single crystal silicon probe tip. The scan rate was generally set to 0.6 Hz with a resolution of 512 points per line. The surface was sampled over a 0.2-0.7 µm2 region. Polyaniline film thickness was measured as the height difference between the film surface and the bare substrate, exposed by scoring the substrate with a razor blade. Advancing contact angles were measured using a KSV contact angle system equipped with a digital camera interfaced to a PC. A droplet of distilled water was formed at the tip of an airtight syringe

to which the sample was brought into contact. The stage was retracted from the droplet, and a picture was taken of the droplet. The advancing contact angle was determined from this picture based on computer best-fit calculations.

3. Results and Discussion The UV-vis absorption of the pyr-sil-modified surfaces prior to the deposition of PANI films reveal an absorbance feature at 256 nm that can be attributed to the n f π* transition of the terminal pyridine of the silane layer.27,28 The RuTPP(CO)modified samples also exhibit this 256 nm band as well as a new feature at 412 nm. This new feature has been assigned to the well-characterized Soret band of the porphyrin. Several absorptions are evident on the Ru(bpy)2Cl2-modified slides. The most intense absorptions are below ∼300 nm, and a weaker, broad absorbance band is observed centered at ∼350 nm. The highenergy features can be assigned to π f π* transitions of the coordinated bipyridine ligand and to the n f π* transitions arising from the uncoordinated n-terminus pyridine of the silyl monolayer. The broad, lower energy absorption centered around 350 nm corresponds to the well-characterized dπ f π* MLCT transitions evident in polypyridyl complexes of d6 metal complexes.29,30 Representative spectra are shown Figure 2.

Figure 1.

In addition to demonstrating the presence of the porphyrin on the surface of the substrate, the Soret band can be used to estimate the surface coverage using the Beer-Lambert law (A ) lc). (27) Chupa, J. A.; Xu, S.; Fischetti, R. F.; Strongin, R. M.; McCauley, J. P., Jr.; Smith, A. B., III; Blasie, J. K.; Peticolas, L. J.; Bean, J. C. J. Am. Chem. Soc. 1993, 115, 4383-4384. (28) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: San Diego, 1992; Ch. 6. (29) Bryant, G. M.; Fergusson, J. E.; Powell, H. K. J. Aust. J. Chem. 1971, 24, 257-273. (30) Li, D.; Moore, L. W.; Swanson, B. I. Langmuir 1994, 10, 1177-1185.

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Sarno et al. Table 1. Contact Angle Measurements for Quartz and SAMs surface

contact angle (deg)

H2SO4/H2O2 treated quartz quartz-Si(CH2)11-Br quartz-Si(CH2)12-pyr quartz-Si(CH2)12-pyr-Ru(TPP)CO quartz-Si(CH2)12-pyr-Ru(bpy)2Cl2

75 ( 1 88 ( 1 54 ( 1 47 to 57 ( 2 46 to 64 ( 1

of ∼6 × 10-11 mol/cm2 of RuTPP(CO) is being deposited on the pyr-sil derivatized surfaces. Contact angle data for each of the metal surfaces as compared to those of the pyridine and bromine terminated self-assembled monolayers and quartz are shown in Table 1. Atomic force microscopy (AFM) has made it possible to examine and image the surface morphology of the thin films with nanometer resolution. The RMS roughness and average surface feature dimensions have been measured for quartz, Brsil, pyr-sil, Ru(TPP)CO, and Ru(bpy)2Cl2. AFM images of select surfaces are presented Figure 3. AFM studies reveal no significant difference in RMS roughness between the two ruthenium-modified surfaces. However, RMS roughness varies from that determined for pure quartz by ∼40 nm for the ruthenium surface. This can serve as a qualitative determination of a change in the surface morphology. AFM analysis of polyaniline films on the thin layer film of Ru(TPP)CO shows a thickness of approximately 47 ( 4 nm. The thickness of polyaniline deposited onto the Ru(bpy)2Cl2 film is approximately 43 ( 4 nm. Images of polyaniline on the ruthenium surfaces are presented Figure 4. Once a reasonable estimate for the PANI film thickness is obtained from AFM measurements, the sheet conductivity is determined by first calculating F, the sheet resistance of the thin film. This is done using eq 2, in which the t is the thickness and V and I are the voltage and current, respectively.

F)

Figure 2. UV-vis absorbance of self-assembled thin films of pyrsil (A), Ru(bpy)2Cl2 (B), and Ru(TPP)CO (C) on fused quartz slides.

For thin films, the equation is modified as shown in eq 1

Γfilm )

A 2film

(1)

where Γfilm is the coverage in mol/cm2 and the absorbance is halved as a result of the beam passing through two sides of the glass substrate. The extinction coefficient () is approximated to be the same as in solution.31 An extinction coefficient of 2.3 × 108 cm2/mol, based on RuTPP(CO) in CHCl3,32 yields a reproducible average surface coverage, Γfilm, of 2.8 ((0.4) × 10-11 mol/cm2 or ∼591 Å2 per porphyrin molecule. This simplified approach to estimating surface coverages suggests that approximately one-half of the theoretical maximum amount (31) Bonnet, J. J.; Eaton, S. S.; Eaton, G. R.; Holm, R. H.; Ibers, J. A. J. Am. Chem. Soc. 1973, 95, 2141-2149. (32) Stejskal, J.; Sapurina, I. Pure Appl. Chem. 2005, 77 (5), 815-826.

(lnπt2) (VI)

(2)

The sheet conductivity is the inverse of the sheet resistance, F. The measured sheet conductivity of PANI deposited onto fused quartz slides, Ru(bpy)2Cl2, and Ru(TPP)CO surfaces is shown Table 2. Generally, the conductivity of PANI films in situ deposited onto H2SO4/H2O2, Br-sil, and Pyr-sil treated quartz is under 0.5 S/cm for the reaction conditions used in this experiment. This conductivity is similar to other values reported for polyaniline on glass.9,15-18,20 An alternative preparation, not used in this study, uses aniline hydrochloride instead of freshly distilled aniline. Typical conductivities for such aniline hydrochloride prepared films tend to be 18.8 ( 7.1 S/cm.32 The increased conductivity for the aniline hydrochloride preparation relative to a distilled aniline preparation is likely due to the increased concentration of chloride counterions. For the slides that were modified by binding either the polypyridyl ruthenium complex or the ruthenium porphyrin complex prior to the in situ deposition of the PANI film, we observe a significant and reproducible increase in conductivity of nearly 2 orders of magnitude relative to the conductivity of polyaniline on quartz. It is important to note that this enhancement is observed without any additional secondary chemical or physical modification of the polymers. MacDiarmid et al. have reported a large increase in the conductivity of PANI films deposited on octadecyltrichlorosilane (OTS) patterned glass.8-10 They postulated that the hydrophobicity of the OTS surface, as demonstrated by a contact angle

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Figure 4. AFM images of PANI films deposited on thin films of RuTPP(CO) (top) and Ru(bpy)2Cl2 (bottom). Table 2. Conductivity Data for PANI Films Deposited on Bare Quartz and on Each of the Indicated Thin Films

Figure 3. AFM images of H2SO4/H2O2 treated quartz slides (A), slides treated to yield the pyridine terminated silane layer (B), and pyridine treated slides exposed to refluxing Ru(bpy)2Cl2 solution for 72 h (C).

of 110°, promotes and stabilizes an expanded coil structure responsible for the observed increase in conductivity. In our study, however, contact angle data indicate that our surfaces, after pyridine termination and modification with transition metal complexes, are not substantially hydrophobic. Although this does not rule out the possibility of the enhanced conductivity arising from similar, expanded PANI coil structures, a less hydrophobic surface typically results in a more compact polymer conformation and would be expected to diminish electronic communication and thus reduce conductivity.

surface

conductivity (S/cm)

H2SO4/H2O2 treated quartz quartz-Si(CH2)11-Br quartz-Si(CH2)12-pyr quartz-Si(CH2)12-pyr-Ru(TPP)CO quartz-Si(CH2)12-pyr-Ru(bpy)2Cl2

0.3 ( 0.8 ∼0.3 ( 0.8 ∼0.3 ( 0.8 35.3 ( 4 14.5 ( 4

If surface hydrophobicity is not primarily responsible for conductivity enhancement in our present systems, it is reasonable to conclude that it must arise from some other interaction of the growing PANI film with the metal-modified surfaces. Other groups have demonstrated surface specific interactions with polyanilines. In fact, significantly different chain lengths were discovered in the bulk as compared to a conducting surface.33-35 We propose that the aromatic π systems present in the surface coordinated species encourage π type interactions with the π systems in the conjugated polymer that substantially promote and/or stabilize an expanded coil conformation within the in situ deposited film. Furthermore, it seems reasonable to postulate that covalently attaching additional aromatic systems to the quartz surface would promote additional π interactions in the polymer that are not present in the bare quartz systems. If the phenomenon of enhanced conductivity is a result of surface directed growth, then as films become thicker, the ordering as a result of the surface would decrease. Consequently, thicker polyaniline films should demonstrate decreased conductivity. This effect is observable in a study by Wu et al. where polyaniline (33) Mazur, M., Krysinski, P. Electrochim. Acta 2001, 46, 3963. (34) Mazur, M.; Krysinski, P. Langmuir 2001, 17, 7093. (35) Mazur, M.; Blanchard, G. J. Langmuir 2004, 20, 3471.

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Sarno et al.

4. Conclusion

Table 3. Film Thickness and Conductivity at Various Deposition Times deposition time (min)

film thickness

conductivity (S/cm)

8 16

56 ( 6 91 ( 12

47 ( 6 36 ( 1

was deposited on aniline terminated silanes. While they initially observed an increase in conductivity with increasing thickness, the conductivity of the resulting polymer thin film significantly decreased when the thickness exceeded 2000 Å.36 A similar effect was observed for polymerization on our transition metal derivitized surfaces in a separate study. Slides were pulled from solution and gently rinsed at 8 and 16 min. Eight minutes was chosen as it has been reported that polyaniline polymerization at room temperature is generally complete at this time.37 After this time, the thickness of the films generally increased with time until the rapid stirring removed weakly adsorbed material from the surface. These results of the thickness dependence study are tabulated in Table 3. From the data presented in Table 3, one can see that a reproducible decrease in conductivity is observed for the thicker films generated from the 16 min polymerization. This result would be consistent with a surface phenomenon as the transition metal complexes are only able to direct the growth of the first several nanometers of the thin film. Preliminary modeling suggests that the RuTPP macrocycle may be immobilized such that the porphyrin ring is oriented reasonably parallel with the substrate. In this conformation, it seems feasible that the π systems present in the PANI polymer may interact in such a fashion with the macrocycle during deposition to stabilize local polymeric structures/geometries that are favorable for enhancing bulk conductivities. The fact that the Ru(TPP) system appears to exhibit greater enhancement relative to the nonplanar, cis-Ru-bipyridyl geometry may indicate that the orientation and position of the aromatic rings may play an important role in the observed conductivity enhancement. These hypotheses are currently being tested by modifying quartz substrates with additional covalently attached porphyrins and metal complexes. We expect to observe conductivity enhancement in these systems as well. A continued observation of orders of magnitude conductivity enhancement in ruthenium complexes possessing aromatic ligands strongly suggests that novel π interactions at the deposition interface play a role in the phenomenon. (36) Wu, C. G.; Yeh, Y. R.; Chen, J. Y.; Chiou, Y. H. Polymer 2001, 42, 2877-2885. (37) Stejskal, J.; Gilbert, R. G. Pure Appl. Chem. 2002, 74 (5), 857-867.

The results of this study demonstrate that it is possible to enhance, by nearly up to 2 orders of magnitude, the thin film sheet conductivity of polyaniline films on quartz by modifying the substrate with covalently anchored metal complexes. Previous studies have established that hydrophobic surfaces favor the formation of the expanded coil conformation of polyaniline during thin film depositions, which promotes both inter- and intrapolymeric electronic communication, resulting in enhanced conductivity of the resulting thin film. If in fact a similar mechanism of conductivity enhancement is operating in our systems, our results would suggest that metal-modified surfaces also promote and/or stabilize an expanded coil polymer conformation of the deposited polymer even though our modified surfaces are significantly less hydrophobic. Assuming that the enhanced conductivity in our systems is arising directly from the metal complex influencing the polymer morphology at the metalcomplex/polymer interfacial boundary, we would expect the conductivity enhancement to decrease with increasing film thickness. This is precisely what is observed. In addition to the net effect of enhanced thin film conductivity, it is interesting to note the significantly enhanced conductivity of the thin PANI films deposited on the Ru porphyrin macrocycle (∼118 times that observed on H2SO4/H2O2 treated quartz) relative to the PANI films deposited on the Ru polypyridyl complex (∼48 times that observed on H2SO4/H2O2 treated quartz). It seems reasonable to speculate that the conductivity enhancement in the porphyrin system may be arising from either a size or geometric effect of the macrocyclic ligand, an electronic effect arising from the π interaction of the phenyl substitutents of the macrocycle and the polymer π systems, or some combination of size and electronic effects. Additional experiments are currently underway to examine these possibilities and to test the generality of this approach using additional metal complexes and other polymeric systems. In any event, our results open the door to a new and convenient method of tuning thin film polymer conductivities that does not require chemically altering the polymer. Acknowledgment. We thank Drs. Stanley Madan and Brendan Flynn for their assistance in preparing this manuscript. We would also like to thank the Binghamton University IEEC (Integrated Electronics Engineering Center) for financial support of this project. LA0624135