Surface Structure and Composition of Thiophene-Bearing Monolayers

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Surface Structure and Composition of Thiophene-Bearing Monolayers Katherine E. Harrison,† Jung F. Kang,† Richard T. Haasch,‡ and S. Michael Kilbey II*,† Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634-0909, and Center for Microanalysis of Materials, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received April 14, 2001. In Final Form: July 30, 2001 We have used X-ray photoelectron spectroscopy and external reflectance Fourier transform infrared spectroscopy (ER-FTIR) to investigate the structure and composition of mixed monolayers made from n-alkyl chains and ω-(3-thienyl)alkyltrichlorosilanes. Monolayers made from 11-(3-thienyl)undecyltrichlorosilane (3TUTS) and undecyltrichlorosilane (UTS) and also from 16-(3-thienyl)hexadecyltrichlorosilane (3THTS) and hexadecyltrichlorosilane (HTS) were examined. ER-FTIR indicates that mixed monolayers made from 3TUTS/UTS and 3THTS/HTS form nearly liquidlike monolayers on indium tin oxide (ITO) substrates. The pendant thiophene group is located at the periphery of the self-assembled monolayer (SAM), and it appears to be oriented along the SAM. Both spectroscopic techniques suggest that during assembly of mixed monolayers, the ω-(3-thienyl)alkyltrichlorosilane species preferentially adsorbs, leading to a monolayer enriched in the thiophene-capped component. The information resulting from these studies will be generally useful for tailoring the structure and properties of electroactive monolayers on ITO substrates.

Introduction Devising methods to prepare surfaces with precise control over the molecular-level structure and properties is of central interest in surface engineering. The ability to manipulate the structure of a surface layer or the manner in which a functional moiety is presented at an interface opens possibilities for mimicking biological systems, developing devices and nanostructures with features approaching molecular sizes, or creating novel chromatographic separation agents. In pursuit of these technologies, monolayers assembled on silicon and goldcoated substrates are frequently used as model systems. Self-assembled monolayers (SAMs) provide uniformly structured surfaces, and incorporating chemical functionality permits manipulation of, for example, the wetting,1-15 frictional,16-20 or adhesive21 properties of the * Corresponding author phone, 864-656-5423; e-mail, mkilbey@ clemson.edu. † Clemson University. ‡ University of Illinois at Urbana-Champaign. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Chem. 1992, 43, 437-463. (2) Wasserman, S. R.; Tau, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (3) Allara, D. L.; Parikh, A. N.; Judge, E. J. Chem. Phys. 1994, 100, 1761-1764. (4) Kang, J. F.; Liao, S. L.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662-9667. (5) Graupe, M.; Takenaga, M.; Koini, T.; Colorado, R., Jr.; Lee, T. R. J. Am. Chem. Soc. 1999, 121, 3222-3223. (6) Bain, C. D.; Beibuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (7) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996-10008. (8) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577-7590. (9) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Macromolecules 1993, 26, 5870-5875. (10) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (11) Evans, S. D.; Uranker, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121-4131. (12) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.

surface layer. In many cases, creating a mixed monolayer by coadsorption allows the physicochemical properties of the SAM to be varied in a controllable fashion. Of recent interest are SAMs that incorporate monomers or oligomers of conductive polymers. These systems are viewed as potential platforms for creating chemical and biological sensors22-24 and optoelectronic devices25-47 due (13) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 100, 6136-6144. (14) Yam, C. M.; Tong, S. S. Y.; Kakkar, A. K. Langmuir 1998, 14, 6941-6947. (15) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274. (16) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Langmuir 1997, 13, 7192-7196. (17) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Immaduddin, S.; Lee, T. R.; Perry, S. S. Langmuir 1999, 15, 3179-3185. (18) Peach, S.; Polack, R. D.; Franck, C. Langmuir 1996, 12, 60536058. (19) Clear, S. C.; Nealy, P. F. J. Colloid Interface Sci. 1999, 213, 238-250. (20) Brewer, N. J.; Beake, B. D.; Leggett, G. J. Langmuir 2001, 17, 1970-1974. (21) Quon, R. A.; Ulman, A.; Vanderlick, T. K. Langmuir 2000, 16, 3797-3802. (22) Guiseppi-Elie, A.; Wilson, A. M.; Tour, J. M.; Brockmann, T. W.; Zhang, P.; Allara, D. L. Langmuir 1995, 11, 1768-1776. (23) Guiseppi-Elie, A.; Tour, J. M.; Allara, D. L.; Sheppard, N. F., Jr. In Electrical, Optical, and Magnetic Properties of Organic Solid-State Materials; Jen, A. K-Y., Lee, C. Y-C., Dalton, L. R., Rubner, M. F., Wnek, G. E., Chiang, L. Y., Eds.; Materials Research Society Symposium Proceedings 413; Materials Research Society: Pittsburgh, 1996; pp 439444. (24) Kumpumbu-Kalemba, L.; Leclerc, M. Chem. Commun. (Cambridge) 2000, 19, 1847-1848. (25) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031-2034. (26) Collard, D. M.; Sayre, C. N. J. Electroanal. Chem. 1994, 375, 367-370. (27) Inaoka, S.; Collard, D. M. Langmuir 1999, 15, 3752-3758. (28) Smela, E.; Kariis, H.; Yang, Z.; Uvdal, K.; Zuccarello, G.; Liedberg, B. Langmuir 1998, 14, 2976-2983. (29) Smela, E.; Kariss, H.; Yang, Z.; Mecklenberg, M.; Liedberg, B. Langmuir 1998, 14, 2984-2995. (30) Appelhans, D.; Ferse, D.; Adler, H.-J. P.; Plieth, W.; Fikus, A.; Grundke, K.; Schmitt, F.-J.; Bayer, T.; Adolphi, B. Colloids Surf. 2000, 161, 203-212.

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Thiophene-Bearing Monolayers

to their ability to accept and eject charge and to compatibilize interfaces. SAMs can also be imprinted on surfaces so that conducting polymer films can be grown atop the patterned SAM.48-53 While most of the work in this arena has been focused on single component SAMs that are capped with a monomeric species of a conducting polymer, we have been interested in mixed monolayers comprised of n-alkyltrichlorosilanes and ω-(3-thienyl)alkyltrichlorosilanes. In a recent paper, we discussed the wetting and electrochemical behavior of pure and mixed SAMs made with tethered thiophenes.31 From the contact angle and electrochemical data, we inferred that the SAMs were not phase separated. The evidence for this was that the contact angles and oxidation potential of the SAMs changed in a continuous fashion as the composition of the deposition solution used to make the SAM was altered. However, as is the case with most mixed monolayer systems, the composition of the deposition solution is not the same as the surface composition, and the surface composition cannot be predicted a priori from a given set of deposition conditions. Therefore, to have precise control over the surface structure or properties, it is important to investigate the relationship between the composition of the deposition solution and the surface composition and the impact that mixing two distinct species has on the structure and properties of the SAM. This subject is routinely investigated and has been extensively reported on for SAMs made from mixtures of n-alkyl chains of different lengths54-57 and mixtures of n-alkyl chains and alkyl chains that are (31) Sullivan, J. T.; Harrison, K. E.; Mizzell, J. P., III; Kilbey, S. M., II. Langmuir 2000, 16, 9797-9803. (32) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296-301. (33) McCarley, R. L.; Willicut, R. J. J. Am. Chem. Soc. 1998, 120, 9296-9304. (34) Willicut, R. J.; McCarley, R. L. Anal. Chim. Acta 1995, 307, 269-276. (35) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823-10824. (36) Schomburg, K. C.; McCarley, R. L. Langmuir 2001, 17, 19831992. (37) Schomburg, K. C.; McCarley, R. L. Langmuir 2001, 17, 19931998. (38) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821-3827. (39) Liedberg, B.; Yang, Z.; Engquist, I.; Wirde, M.; Gelius, U.; Go¨tz, G.; Ba¨uerle, P.; Rummel, R.-M.; Ziegler, Ch.; Go¨pel, W. J. Phys. Chem. B 1997, 101, 5951-5962. (40) Kim, Y.-H.; Kim, Y.-T. Langmuir 1999, 15, 1876-1878. (41) Wurm, D. B.; Kim, Y.-T. Langmuir 2000, 16, 4533-4538. (42) Michalitsch, R.; El Kassmi, A.; Yassar, A.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1998, 457, 129-139. (43) Michalitsch, R.; Nogues, C.; Najari, A.; El Kassmi, A.; Yassar, A.; Lang, P.; Garnier, F. Synth. Met. 1999, 101, 5-6. (44) Kuwabata, S.; Fukuzaki, R.; Nishizawa, M.; Martin, C. R.; Yoneyama, H. Langmuir 1999, 15, 6807-6812. (45) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688-3694. (46) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511-2518. (47) Berlin, A.; Zotti, G. Macromol. Rapid Commun. 2000, 21, 301318. (48) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 526-529. (49) Sayre, C. N.; Collard, D. M. Langmuir 1997, 13, 714-722. (50) Nishizawa, M.; Miwa, Y.; Matsue, T.; Uchida, I. J. Electrochem. Soc. 1993, 140, 1650-1655. (51) Collard, D. M.; Sayre, C. N. Synth. Met. 1997, 84, 329-332. (52) Rozsnyai, L. F.; Wrighton, M. S. Chem. Mater. 1996, 8, 309311. (53) Rozsnyai, L. F.; Wrighton, M. S. Langmuir 1995, 11, 39133920. (54) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. (55) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (56) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097-5105. (57) Bumm, L. A.; Arnold, J. J.; Charles, L. F.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017-8021.

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capped with hydroxyl, carboxylic, and halogen groups.58-65 It appears that only Willicut and McCarley have previously investigated the relationship between deposition solution and surface concentration for mixed SAMs where one component was capped with the monomer of a conducting polymer. Their electrochemical measurements on ω-(Npyrroyl)hexanethiol and n-hexanethiol SAMs that were formed on polished Au showed that the pyrrole-capped component was preferentially adsorbed.34 Although the molecular architecture of the molecules and the solvent condition from which a multicomponent SAM is deposited strongly influence monolayer structure, a mixed monolayer is, in general, enriched in one component. This information is usually discerned from the wetting behavior (contact angle measurements), and more quantitative information may be obtained from spectroscopic investigations. In this paper, we report the results from both X-ray photoelectron spectroscopy (XPS) and external reflection Fourier transform infrared (ER-FTIR) experiments on mixed monolayers made from binary mixtures of ω-(3thienyl)alkyltrichlorosilanes and n-alkyltrichlorosilaness we report on systems with two different chain lengths. We had two main goals when conducting this work: first, because we were creating mixed monolayers, we wanted to know the relationship between the composition of the deposition solution and the composition of the SAM; second, because we were using indium tin oxide (ITO)coated glass slides, which are inherently rough, we wanted to know whether it is possible to create “good” monolayers of ω-functionalized, trichlorosilane-anchored chains on this surface. ITO is an important substrate for electrochemical measurements, and although this substrate has been frequently used with ω-functionalized, electroactive monolayer-forming molecules,31,66-71 the complementary spectroscopic studies of the organization of those monolayers are scarce. Thus, the compositional and structural information resulting from our studies will help provide a more complete picture of the structure-property relationships of this particular system and also yield general insight into the surface organization of monolayers made from n-alkyl and ω-functionalized silanes on ITO. Experimental Section Materials. Monolayers were made from ω-(3-thienyl)alkyltrichlorosilanes and n-alkyltrichlorosilanes. The particular systems explored include mixtures of 11-(3-thienyl)undecyltri(58) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990-1995. (59) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665-3666. (60) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560-6561. (61) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 38823893. (62) Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438, 91-97. (63) Heise, A.; Stamm, M.; Rauscher, M.; Duschner, H.; Menzel, H. Thin Solid Films 1998, 327-329, 199-203. (64) Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.-I.; Niki, K. Langmuir 2000, 17, 1599-1603. (65) Hobara, D.; Ota, M.; Imabayshi, S.-I.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. (66) Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A.; Pagani, G.; Canavesi, A. Langmuir 1997, 13, 2694-2698. (67) Zotti, G.; Berlin, A.; Schiavon, G.; Zecchin, S. Synth. Met. 1999, 101, 622-623. (68) Berlin, A.; Zotti, G.; Schiavon, G.; Zecchin, S. J. Am. Chem. Soc. 1998, 120, 13453-13460. (69) Back, R.; Lennox, R. B. Langmuir 1992, 8, 959-964. (70) Appleyard, S. F. J.; Day, S. R.; Pickford, R. D.; Willis, M. R. J. Mater. Chem. 2000, 10, 169-173. (71) Markovich, I.; Mandler, D. J. Electroanal. Chem. 2000, 484, 194-202.

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chlorosilane (3TUTS) and undecyltrichlorosilane (UTS) and 16(3-thienyl)hexadecyltrichlorosilane (3THTS) and hexadecyltrichlorosilane (HTS). The synthesis, purification, and analysis of 3TUTS, UTS, and 3THTS used to make the monolayers for the studies described in this paper have been reported in detail elsewhere.31 Preparation of Samples. Two different substratesssilicon wafers and ITO-coated glass slidesswere used in these studies. Silicon wafers were cut into rectangular pieces of approximately 25 mm × 7 mm. The silicon substrates were cleaned by stirring them in a solution of concentrated H2SO4 and 30% H2O2 (70:30 v/v). Caution! Piranha solution should be handled with extreme care, as it reacts violently with most organic materials. Do not store piranha solution in a closed vessel. The mixture was heated to 90 °C for approximately 45 min and then allowed to cool. Sample substrates of ITO-coated glass slides (Delta Technologies, 0.7 mm thickness) were cut into pieces of appropriate size (25 mm × 7 mm). The ITO-coated glass slides were then cleaned by sonicating in acetone, drying with a stream of nitrogen gas, and then sonicating in a 20 wt % ethanolamine solution that was heated to 90 °C. After being cleaned and cooled to room temperature, the silicon and ITO substrates were rinsed with copious amounts of distilled water, blown dry with a stream of nitrogen, and then dried in a vacuum oven at 110 °C for 3 h. Immediately prior to use, the ω-(3-thienyl)alkyltrichlorosilanes were vacuum distilled. Deposition solutions consisting of ω-(3thienyl)alkyltrichlorosilane and n-alkyltrichlorosilane (in the proper proportion) in freshly distilled and dried methylene chloride were made. The concentration of the deposition solutions was 1 mM. The freshly cleaned substrates were immersed in the deposition solution for 24 h. We also determined that a 10 mM deposition solution could be used, and the monolayer formed within 6 h. Deposition was carried out in flame-dried, cleaned test tubes, and a slow nitrogen purge was used during monolayer assembly. After the deposition period, the SAM-covered substrates were rinsed with and sonicated in methylene chloride and then dried with nitrogen. Prior to analysis, the samples were sonicated in ethanol and dried with nitrogen. In reporting results on mixed monolayer systems, the deposition solution compositions are expressed on a molar basis. XPS. A Physical Electronics PHI 5400 XPS equipped with a spherical capacitor analyzer and dual channel plate detector was used to provide information about the surface composition of the monolayers. A monochromatic Al KR (1486.6 eV) X-ray source at a power of 400 W was used, and the operating pressure was 10-9 Torr. The pressure was monitored throughout the experiments because a drastic change in pressure would be indicative of contamination in the chamber. Binding energies were calibrated with Au 4f7/2 at 84.0 eV and C 1s at 285.0 eV. A survey spectrum (10 sweeps, 179 eV pass energy) and high-resolution spectra of C 1s, S 2s, and Si 2p regions (5-10 scans, 36 eV pass energy) were taken for each sample. Depth-profile studies were collected by rotating the sample about an axis perpendicular to the analyzer lens axis and in the plane of the X-rays. The data treatment used to calculate the atomic compositions is described in ref 72. FTIR. The ER-FTIR experiments were performed with a Nicolet Nexus 870 FTIR spectrometer using a liquid nitrogen cooled MCT-A detector. A Whatman laboratory gas generator (model 75-45) was used to purge the sample compartment with dry, CO2-free air. The Spectra-Tech FT-80 Horizontal Grazing Angle accessory was used to collect the spectra with an angle of incidence of 80°. A total of 2000 scans were collected for each spectrum with a resolution of 8 cm-1. The reflection experiments utilized p-polarized light, which increases the sensitivity to thin films because the absorbances from transition dipole moments that are oriented perpendicular to the surface are enhanced.73 A spectrum of the neat, viscous liquid 11-(3-thienyl)undecene spread on a zinc selenide crystal was recorded in transmission mode. A total of 32 scans at a resolution of 4 cm-1 were acquired. The transmission IR spectrum of a dilute solution (85 mM) of the (72) Moulder, J. F., Stickle, W. F., Sobal, P. E., Bomben, K. D., Eds. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, MN; 1995. (73) Greenler, R. G. J. J. Chem. Phys. 1966, 44, 310-315.

Harrison et al. same required the use of a KBr liquid cell with a path length of 0.1 mm and the DTGS detector. For this spectrum, 64 scans were collected with a resolution of 4 cm-1.

Results and Discussion Because a significant portion of our efforts involve mixed SAMssSAMs made from both thiophene-capped and n-alkyl monolayer constituentssit is useful to know the relationship between the concentration of thiopheneterminated molecules in solution and the concentration of thiophene-terminated molecules in the monolayer. Monolayers made from 3TUTS and UTS on silicon were studied with XPS to determine the composition and structure of the monolayer. We found that quantitative analysis of the XPS results from SAMs on ITO was problematic because of the low intensity of the sulfur signal as compared to the indium, tin, and oxygen signals. FTIR was used to investigate the order, composition, and orientation of thienyl groups of the SAMs. For these experiments, SAMs of 3TUTS and UTS and 3THTS and HTS were made on ITO substrates. XPS. The XPS survey spectra shown in Figure 1 indicate that the 3TUTS monolayer-covered surfaces were composed of oxygen (1s, 2s, Auger-KLL), silicon (2s, 2p), carbon (1s), and sulfur (2s, 2p). Close examination of the spectra reveals that the sulfur 2p peak overlaps with the silicon 2s peak that results from energy loss to the bulk plasmons of the substrate. Therefore, any numeric analysis of thiophene/sulfur content of the SAMs will have to be deduced from the sulfur 2s signal. Peaks indicating carbon oxidation normally appear at 287-290 eV; however, these were absent in all spectra. Chlorine was not detected, suggesting that the trichlorosilane tethering groups of the 3TUTS and UTS molecules had reacted completely. Varying the angle between the sample surface and the input lens of the analyzer can provide depth-profiling information by effectively varying the photoelectron sampling depth. Assuming only inelastic scattering, 95% of the photoelectron signal will come from a depth of up to d ) λ sin θ, where λ is the inelastic mean-free path for an electron and θ is the electron takeoff angle relative to the sample surface.74 Therefore, at lower takeoff angles, information about the upper strata of the monolayer is revealed. As shown in Figure 1, as the takeoff angle is decreased, the oxygen and silicon intensities decrease relative to the carbon intensity. This is the appropriate pattern of behavior for hydrocarbon monolayers that are bonded to the oxide layer of the silicon substrate.2 Also, Figure 2 shows that at takeoff angles of 45 and 90°, the Si peak intensity is greater than that of SiO2; however, at 15°, the SiO2 peak intensity is greater than the Si peak intensity. Quantitative verification of these trends can be obtained from the ratio of areas of the C 1s, silicon 2p, and silicon dioxide peaks, and the results are shown in the first three columns of Table 1. The sulfur 2s peak intensity also changes with takeoff angle. As the takeoff angle is decreased, the sulfur peaks become more intense relative to the other photoelectron lines, indicating that the thienyl groups are at the top of the monolayer. This finding reinforces results from contact angle experiments.31 The ratio of the C 1s to S 2s peak areas at various takeoff angles is given in the rightmost column of Table 1. Our data show that for 3TUTS monolayers, as the take angle is decreased, the ratios of the C/S peak areas decrease. (74) Briggs, D.; Rivie`re, J. C. In Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: Chichester, 1996; Vol. 1, p 134.


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Figure 1. XPS survey spectra of monolayers made from 3TUTS at takeoff angles of 15, 45, and 90°. The experiment is more sensitive to atoms in the topmost layer of the SAM at lower takeoff angles.

Figure 2. High-resolution spectra of SiO2 2p and Si 2p at the three different takeoff angles. As the takeoff angle is decreased, more SiO2 is detected, reflecting the siloxane network that tethers the SAM. Table 1. Peak Ratios for 3TUTS Monolayers on Silicon at Different Takeoff Anglesa angle (deg)

[Si/C]

[SiO2/C]

[SiO2/Si]

[C/S]

90 45 15

1.41 0.51 0.08

0.69 0.37 0.19

0.49 0.73 2.4

15.1 13.9 12.3

a The results are consistent with an upright, monolayer structure that is bonded to the SiO2 layer with the sulfur-containing heterocyclic rings at the periphery.

This pattern of behavior differs from the XPS results reported by Applehans et al.30 On the basis of the C 1s and S 2p peak areas, they reported a lower C/S ratio at 85° rather than at 5° for 3TUTS on silicon (with greater angles said to be probing greater depths) and suggested that this was because some thiophene groups were adsorbed at the

silicon/SAM interface. Because they used the S 2p to extract quantitative information, it may also be possible that plasmon loss from the Si 2s peak (and not an increase in the sulfur signal) was being detected as the takeoff angle was increased. High-resolution C 1s spectra for the different takeoff angles are shown in Figure 3. The symmetric carbon peaks indicate that the carbon atoms in the alkyl tail of the SAM have the same binding energy as the carbon bonded to the silicon atom.2 The slight shift to higher binding energies of the C 1s peaks as the takeoff angle decreases is most likely due to charging. Perhaps most importantly, we note that the spectra do not show any evidence of C-O bonds, which would appear at approximately 289 eV.72 This indicates that the SAM/thiophene ring has not oxidized.

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Figure 3. High-resolution C 1s spectra at the three different takeoff angles. The dotted line at 285.0 eV marks the binding energy for the C-H bond in hydrocarbons. Although there is a slight shift with decreasing takeoff angle, most likely due to charging, the spectra show no evidence of C-O bonds, which normally appear at ∼289 eV.

Figure 4. XPS survey spectra of mixed monolayers (with the composition expressed on a molar basis) made from 3TUTS and UTS. The takeoff angle for these spectra is 15°. As the relative amount of 3TUTS in the deposition solution decreases, the sulfur 2s signal attenuates.

Thus far, we have used monolayers composed only of 3TUTS and examined the spectra at various takeoff angles to show that the thiophene rings are at the periphery of the monolayer. To investigate the SAM composition, monolayers prepared from various solution concentrations of 3TUTS and UTS were analyzed. XPS survey spectra of monolayer-modified surfaces made from 3TUTS and UTS are shown in Figure 4. The takeoff angle used in these experiments was 15° because at this angle, the experiment is more sensitive to the atoms in the topmost layer of the SAM. As the amount of 3TUTS in the deposition solution

(from which the monolayer was made) increases, the intensity of the sulfur 2s signal increases. The atomic compositions determined at two different takeoff angles from the ratio of carbon 1s and sulfur 2s peaks for the different amounts of 3TUTS and UTS in solution are presented in Table 2 along with the expected atomic compositions. The sulfur content in the SAMs for all concentrations of 3TUTS in the deposition solutions is higher at 15° than at 45°, again suggesting that the thiophene rings are at the periphery of the SAM. For a monolayer comprised of solely 3TUTS chains, the sulfur


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Table 2. Atomic Compositions (in Percent, Expressed as S/[C + S]) for SAMs Deposited from Mixtures of 3TUTS and UTSa % 3TUTS in solution

expected

XPS at 45°

XPS at 15°

100 75 50 33 25 0

6.25 5.1 3.7 2.6 2.0 0.0

6.7 5.9 4.6 3.9 3.3 0.0

7.5 6.3 4.9 4.3 3.8 0.0

a The expected compositions are calculated based on molecular stoichiometery, and the compositions at 45 and 15° are obtained from the areas of the C 1s and S 2s peaks.

Figure 5. XPS data for the percent of sulfur 2s in the monolayer as a function of the molar composition of the deposition solution. Takeoff angles of 45 and 15° are shown as well as the expected sulfur compositions. Trends show a decrease in the percent of sulfur in the monolayer as the percent of thiophene-terminated molecules in solution decreases.

atomic composition was found to be 7.5% at 15° and 6.7% at 45°. This is in contrast to the 6.25% expected for the sulfur composition. For both takeoff angles investigated, the sulfur content in the SAM decreases as the concentration of 3TUTS in the deposition solution decreases, and this trend can be seen in Figure 5. Although the values for the sulfur composition obtained from XPS measurements are consistently higher than the composition expected from molecular stoichiometry, the agreement seems quite good given that resolution becomes an issue with the small S 2s peaks. Also, in every signal, there is energy loss associated with surface plasmons. Nevertheless, we believe that the data suggest that mixed SAMs of 3TUTS and UTS are enriched in 3TUTS, particularly at lower compositions. We have also used ER-FTIR to buttress this assertion. FTIR. External specular reflection IR spectroscopy is a useful technique for investigating the molecular orientation and chemical composition of SAMs. The peak areas and relative intensities of the symmetric and antisymmetric methylene- and the methyl-group stretching modessνs(CH2), νa(CH2), and νa(CH3), respectivelys are indicative of the composition and orientation of the chains in the monolayer. Information about the intermolecular environment and lateral interactions can also be inferred from the peak positions.75-77 (75) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

Figure 6. External reflectance IR spectra showing the absorbance of the symmetric CH2, antisymmetric CH2, and antisymmetric CH3 bands as a function of relative amounts of (a) 3TUTS and UTS and (b) 3THTS and HTS in the deposition solutions. (Note that the scaling in a and b are different.) As expected, the antisymmetric CH3 band increases with the decrease in the amount of thiophene-terminated sites in the monolayer.

As shown in Figure 6a, monolayers made from 3TUTS/ UTS showed νa(CH2) at 2927 cm-1 and νs(CH2) at 2855 cm-1, and as shown in Figure 6b, the 3THTS/HTS monolayers exhibited νa(CH2) at 2926 cm-1 and νs(CH2) at 2854 cm-1. While the wavenumbers of these bands are higher than those for a highly ordered crystalline system, they are marginally lower than the stretching modes seen for liquidlike monolayers made from n-alkyl chains; a wellpacked SAM has a νa(CH2) stretching band at 2920 cm-1 and a νs(CH2) stretching band at 2850 cm-1, and a liquidlike SAM shows νa(CH2) and νs(CH2) at 2928 and 2856 cm-1, respectively.1-3 Even the SAMs composed of only n-alkyl chains are not highly orderedsthey too remain liquidlike. The positions and widths of the methylene stretching modes for 3TUTS/UTS and 3THTS/HTS systems clearly suggest that the alkyl chains are in a disordered environment. Markovich and Mandler have similarly reported that monolayers made from octadecyltrichlorosilane on ITO are disorganized.71 IR spectroscopy experiments on monolayers made from ω-(N-pyrrole)alkanethiols on gold and aniline-terminated alkanethiols on gold also showed that SAMs capped with a single (76) Synder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (77) Synder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623.

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monomer of a conducting polymer do not assemble into a dense, crystalline monolayer structure.28,33,36,37 (Although in some of the short-chain systems, the interchain interactions themselves are not strong enough to create a highly packed SAM1-3 even without the heterocyclic endgroups.) Liedberg and co-workers, however, demonstrated that undecanethiol molecules that are capped with a more conjugated species, terthiophene, for example, can assemble into crystalline monolayers, presumably because the terthiophene groups help drive the packing of the alkyl chains.39 In our system, the shift to higher wavenumbers in the 3TUTS and 3THTS monolayers is thought to be a result of surface roughness of the ITO, which results in decreased order. The roughness also influenced the contact angle measurements, which was discussed in our previous paper. An atomic force microscopy image showed that the ITO surface is comprised of grains of various sizes with edges of different heights,31 and the root mean square (rms) roughness of the ITO-coated glass surfaces is typically between 5 and 7 nm.78 We had expected that the presence of the bulky thiophene rings would disrupt the packing of the SAMs,80 but this does not appear to be the case since the νa(CH2) and νs(CH2) peak positions do not change as the composition of the deposition solution (and the SAM) is changed. Presumably, this is because the thiophene groups are stratified at the periphery of the SAM and the monolayers are not well-packed to begin with. The consistency of these peak positions also indicates that packing of the alkyl chains over all compositions is similar, even as a higher percentage of monolayer chains capped with the thienyl group is incorporated into the SAM. Also, the ratio of the intensities of the νa(CH2) and νs(CH2) stretching modes remains constant as the monolayer composition is varied, suggesting that the chain tilt is not changing as the relative number of thienyl groups is changed. The absorption band at 2965 cm-1, which is assigned to the CH3 asymmetric in-plane CH stretching mode, is clearly visible for SAMs made from UTS or HTS, but this peak attenuates as either 3TUTS or 3THTS, respectively, is incorporated into the monolayers. For all of the SAMs containing thienyl-terminated molecules, the only mode attributable to the thiophene rings that was visible with p-polarized radiation in the ER-FTIR experiments was the out-of-plane C-H bending mode, νoop(CH), between 770 and 775 cm-1.81 Figure 7 presents a comparison between a transmission spectrum of the viscous liquid 11-(3-thienyl)undecene and an ERFTIR spectrum of a monolayer of 3TUTS on ITO. (The alkene was used in the transmission experiment to circumvent problems with reactivity of the silane end group of 3TUTS.) The spectrum for 11-(3-thienyl)undecene shows stretches at 3076 and 1641 cm-1 due to the vinyl group82 and ring modes,81,83 νs(ring), at 1440 and 1537 cm-1. Also seen are bands at 1464 and 1411 cm-1, which have been assigned82 to the in-plane deformation of the methylene groups, νd(CH2), and the vinyl group, νd(CHd (78) Measurements were made in contact mode. In comparison, thermally evaporated gold coatings have a rms roughness on the order of 0.5-1 nm, according to data published in refs 12 and 79. (79) Goss, C. A.; Brumfield, J. C.; Irene, E. A.; Murray, R. W. Langmuir 1993, 9, 2986-2994. (80) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; Mo¨hwald, H. Langmuir 1998, 14, 6485-6492. (81) Gronowitz, S.; Katritzky, A. R.; Reavill, R. E. J. Chem. Soc. 1963, 3881-3882. (82) Bellamy, L. J. The Infrared Spectra of Complex Molecules; John Wiley & Sons: New York, 1975. (83) Katritzky, A. R. Physical Methods in Heterocyclic Chemistry; Academic Press: New York, 1971.

Harrison et al.

Figure 7. Comparison of a transmission spectrum of the neat liquid, 11-(3-thienyl)undecene, and a reflectance spectrum of a monolayer of 3TUTS on ITO. The band associated with outof-plane C-H bend, νoop(CH), is seen in both spectra, while the in-plane ring modes (at 1537 and 1440 cm-1) are visible only in the transmission spectrum. Note that the spectra are scaled so that the νoop(CH) modes are of comparable size.

CH2), respectively. The two spectra in Figure 7 are scaled so that the νoop(CH) are of comparable size, and it is evident that the νs(ring) modes are absent in the 3TUTS sample. For the neat liquid, there are also small features at 3064 and 3106 cm-1, which have been assigned to the in-plane C-Hβ and C-HR stretches of the thiophene ring, respectively.84 In the SAM systems, the absence of the in-plane stretching modes in the 1400-1600 cm-1 region and the presence of the out-of-plane bending mode suggest that the thiophene rings are oriented nearly parallel to the surface. To establish this more firmly, an 85 mM solution of 11-(3-thienyl)undecene in methylene chloride was prepared and subjected to a transmission IR experiment so that the molar absorptivities could be determined. The spectrum of this solution was the same as the neat liquid except that the C-Hβ and C-HR stretches seen from the solution were shifted ∼3 cm-1 higher. Using the BeerLambert Law and the measured absorbance, cell path length, and solution concentration gave molar absorptivities (expressed as extinction coefficients)85 of 49 and 12 for the νs(ring) bands at 1440 and 1537 cm-1, respectively. Although affected by the substitution,83 the molar absorptivity of the ring mode at 1440 cm-1 is of moderate strength, and because this signal was not detected in the ER-FTIR experiments, this supports our contention that the thiophene rings are oriented nearly parallel to the surface. In a similar study, McCarley and Willicut observed33 both in-plane and out-of-plane modes for a SAM composed of only pyrrole-capped alkanethiols on gold and concluded that the pyrrole groups adopted no particular orientation. With IR spectroscopy and contact angle measurements on SAMs composed solely of aniline-capped alkanethiols, Schomburg and McCarley showed that the aniline group is tilted slightly away from the surface normal.36 At this juncture, the differences in the observed orientation of the heterocyclic groups in the different systems (pyrrole-, aniline-, and thiophene-capped SAMs) are not completely clear but may be related to the substrate. One might speculate that when the number density of the heterocyclic (84) Delabouglise, D.; Garreau, R.; Lemaire, M. New J. Chem. 1988, 12, 155-161. (85) Rao, C. N. R. Ultra-Violet and Visable Spectroscopy: Chemical Applications, 3rd ed.; Butterworth: London, 1974.


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Figure 9. Comparison of deposition solution and SAM composition for SAMs made from mixtures of 3TUTS and UTS and 3THTS and HTS. As discussed in the text, χ is the (fractional) molar composition of the solution and SAM, with the latter determined from the peak areas of the νoop(CH) mode. The data indicate that the thiophene-terminated molecules preferentially adsorb on the surface.

Figure 8. Absorbance of the out-of-plane C-H bend as a function of deposition solution concentration. The intensity of this bend increases with the increase in the relative amount of thiophene in the monolayers. (a) Spectra for monolayers composed of 3TUTS and UTS. (b) Spectra for monolayers composed of 3THTS and HTS.

groups decorating the SAM is sufficiently small so that neighboring groups do not interact, the pendant heterocyclic rings would prefer to lie along the monolayer rather than protrude into the ambient. At higher surface densities, however, it would seem reasonable to expect that the crowding of the adjacent groups would cause these rings to be more upright. With this view, the fact that only the out-of-plane mode is observed in the 3TUTS/UTS and 3THTS/HTS systems on ITO may be because these SAMs are not highly ordered due to the roughness of the ITO surface. Nevertheless, for the 3TUTS and UTS monolayers, the CH out-of-plane mode appears at 771 cm-1 (Figure 8a). Monolayers composed of 3THTS and HTS exhibit the CH out-of-plane bending mode at 775 cm-1 (Figure 8b). The intensity of the νoop(CH) mode in the monolayers decreases as the amount of thiophene-capped chains in the monolayers decreases. For the two chain lengths investigated, this peak position remains fixed as the relative amount of thiophene-bearing chains in the monolayer varies, which suggests that the packing/tilting of the thienyl groups does not change. It is the openness of the liquidlike SAM structure along with substrate roughness that probably gives rise to the result that the orientation of the terminal thienyl rings does not seem to depend on the number of methylene groups in the alkyl tail of the two systems tested. This inference needs to be tested more thoroughly;

however, quantitative information can still be extracted from the IR data. Although a horizontal baseline in this region with an ITO substrate is difficult to obtain, the baselines (obtained from 0% 3TUTS and 0% 3THTS) are at least flat in the region of interest. By integrating the area beneath the νoop(CH) peaks, a relationship between the amount of thiophene-capped molecules in the monolayer and the deposition solution can be obtained, and these results are presented in Figure 9. In this figure, χ denotes the composition relative to a SAM made from only thiophenethiophene was calculated by normalbearing moleculessχsurface izing the peak area of the νoop(CH) mode (at each concentration investigated) by the peak area of this mode for a monolayer made only from the thienyl-bearing chains thiophene is on a molar basis). The data suggest that (and χsolution the thiophene-terminated molecules are more favorably adsorbed on the surface than the n-alkyl chains, particularly when the relative amount of 3TUTS or 3THTS in the deposition solutions is low. Although we are dealing with small areas and inferring baselines, this observation agrees with the XPS results presented in this article and also the findings of Willicut and McCarley that were based on electrochemical measurements.34 As Willicut and McCarley pointed out,34 the preferential adsorption of the electroactive-capped component could be due to interactions between the heterocyclic headgroups, which help stabilize the monolayer. Slight differences in the polarity of the n-alkyltrichlorosilanes and ω-(3-thienyl)alkyltrichlorosilanes may also promote the adsorption of the thiophene-capped component, since the depositions were done from methylene chloride. While our previous contact angle and electrochemical measurements on these systems suggest that there is no phase separation of the monolayer components on the micrometer scale,31 it remains an open question as to whether there is phase separation on length scales approaching the nanoscale. Conclusions We have shown that ω-(3-thienyl)alkyltrichlorosilanes and n-alkyl chains can be used to form upright monolayers (not necessarily parallel to the surface normal) on ITO surfaces. The results clearly show that the SAMs are more liquidlike than crystalline, ostensibly due to the inherent

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roughness of the substrate. The thiophene ring, which is at the periphery of the monolayer, does not appear to be the origin of the loose packing of the SAM; as evident from ER-FTIR studies on 3TUTS/UTS and 3THTS/HTS monolayers, the wavenumbers at which the symmetric and asymmetric methylene stretches are seen remain constant as the relative amount of the two components used to make the mixed SAM is changed. Also, there is no change in the relative intensities of these two modes, suggesting that the tilt angle of the SAMs is not changing as the composition of the SAM changes. Results from both XPS and ER-FTIR experiments suggested that during assembly of mixed monolayers, the thiophene-terminated molecules adsorb more favorably, creating a SAM that is enriched (relative to the deposition solution concentration) in the ω-(3-thienyl)alkyltrichlorosilane molecule.

Harrison et al.

Acknowledgment. We gratefully acknowledge Prof. Ralph Nuzzo of the Department of Chemistry and Prof. Ivan Petrov of the Center for Microanalysis of Materials at the University of Illinois at Urbana-Champaign for facilitating the interaction between our groups. This work was supported in part by the ERC Program of the National Science Foundation under award number EEC-9731680; funding from the 3M through an Untenured Faculty Grant is also gratefully acknowledged. The XPS experiments were carried out through the Outreach Program of the Center for Microanalysis of Materials, Frederick Seitz Materials Research Laboratory, which is partially supported by the U.S. Department of Energy under award number DEFG02-96-ER45439. LA010546E

Surface Structure and Composition of Thiophene-Bearing Monolayers

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