Langmuir 1999, 15, 6333-6342
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Preparation and Characterization of Ultrathin Layers of Substituted Oligo- and Poly(p-phenylene)s and Mixed Layers with Octadecanethiol on Gold and Copper Samuel Brunner, Walter R. Caseri,* and Ulrich W. Suter Department of Materials, Institute of Polymers, ETH, CH-8092 Zu¨ rich, Switzerland
Georg Ha¨hner, Dorothee Brovelli, and Nicholas D. Spencer Department of Materials, Laboratory for Surface Science and Technology, ETH, CH-8092 Zu¨ rich, Switzerland
Anja Vinckier Biochemistry II, ETH, CH-8092 Zu¨ rich, Switzerland
Iris U. Rau, Patrick Galda, and Matthias Rehahn Polymer-Institut der Universita¨ t Karlsruhe, D-76128 Karlsruhe, Germany Received June 24, 1998. In Final Form: February 23, 1999 Substituted poly(p-phenylene)s were adsorbed from solution onto gold and copper and oligo(p-phenylene)s onto gold. The layers were investigated with IR spectroscopy at grazing incidence reflection, XPS, NEXAFS, ToF-SIMS, surface profilometry, AFM, SEM, optical microscopy, ellipsometry, and contact angle measurements to examine their formation and structure. The structure and the properties of the investigated layers depend not only on the chemical structure of the polymer but also on the type of substrate. On gold, the polymers form layers of 15-25 Å in thickness and the oligomers of ca. 5 Å in thickness. On copper, “thick” layers of up to 900 Å were also observed. The oligomers have a lower affinity to gold than the polymers. Mixed octadecanethiol-polymer layers were prepared by immersion of polymer-coated substrates in an octadecanethiol solution or by exposure of self-assembled monolayers of octadecanethiol to polymer solutions. The structure of the mixed layers depends on the sequence of the exposure of the two components and on the chemical structure of the polymer. In the mixed layers, structures that protrude above the surroundings were frequently detected at the surface.
Introduction Monolayers of alkanethiols, dialkyl disulfides, and other sulfur-containing molecules prepared on gold and copper by adsorption from solution are described in numerous reports.1,2 It is well-established that these molecules interact with the metal surfaces via sulfur atoms. While low-molecular-weight compounds with sulfur functionalities have been studied extensively, less is known about larger molecules such as polymers on metal surfaces. Most of these studies deal with sulfur-functionalized polymers on gold.3-11 (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Bishop, A. R.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127. (3) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (4) Stouffer, J. M.; McCarthy, T. J. Macromolecules 1988, 21, 1204. (5) Stouffer, J. M.; McCarthy, T. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1986, 27 (2), 242. (6) Waldman, D. A.; Kolb, B. U.; McCarthy, T. J.; Hsu, S. L. Polym. Mater. Eng. Sci. 1988, 59, 326. (7) Lenk, T. J.; Hallmark, V. M.; Rabolt, J. F.; Ha¨ussling, L.; Ringsdorf, H. Macromolecules 1993, 26, 1230. (8) Enriquez, E. P.; Gray, K. H.; Guarisco, V. F.; Linton, R. W.; Mar, K. D.; Samulski, E. T. J. Vac. Sci. Technol. A 1992, 10, 2775. (9) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (10) Erdelen, C.; Ha¨ussling, L.; Naumann, R.; Ringsdorf, H.; Wolf, H.; Yang, J.; Liley, M.; Spinke, J.; Knoll, W. Langmuir 1994, 10, 1246. (11) Sun, F.; Castner, D. G.; Mao, G.; Wang, W.; McKeown, P.; Grainger, D. W. J. Am. Chem. Soc. 1996, 118, 1856
Recently, however, the adsorption of a number of substituted poly(p-phenylene)s and rigid-rod poly(imide)s with ter(p-phenylene) units was reported.12-15 These polymers form layers of typically 10-20-Å thickness upon adsorption from dichloromethane or chloroform onto gold and copper.12 The advancing contact angles of water on these layers are up to 90°, depending on the substrate and the chemical composition of the polymer. Occasionally, “thick” layers (200-700 Å) have been detected on copper.12 These layers were suggested to consist of organometallic complexes rather than organic multilayers. There are, however, a number of open questions, in particular with respect to the affinity of such compounds to the surfaces. On one hand, it has been found that side chains with oxygen groups promote the adsorption of aromatic units, while no significant adsorption was detected with pdidocylbenzene.13 On the other hand, poly(p-phenylene)s without heteroatoms also form strongly attached layers, obviously due to a cooperative effect of many phenylene units. No information is available at present about the affinity of related compounds of a molecular size between (12) Steiner, U. B.; Caseri, W. R.; Suter, U. W.; Rehahn, M.; Schmitz, L. Langmuir 1993, 9, 3245. (13) Steiner, U. B.; Caseri, W. R.; Suter, U. W.; Rehahn, M.; Rau, I. U. Langmuir 1994, 10, 1164. (14) Steiner, U. B.; Rehahn, M.; Caseri, W. R.; Suter, U. W. Langmuir 1995, 11, 3013. (15) Ha¨hner, G.; Marti, A.; Spencer, N. D.; Brunner, S.; Caseri, W. R.; Suter, U. W.; Rehahn, M. Langmuir 1996, 12, 719.
10.1021/la980748j CCC: $18.00 © 1999 American Chemical Society Published on Web 06/25/1999
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Figure 1. Chemical structures and acronyms of the rigid-rod polymers and oligomers studied here.
a single aromatic unit and polymers with a large number of aromatic units, in particular, of ter(p-phenylene) units that are common in the rigid-rod polymers mentioned above. Further, the poly(p-phenylene)s resist desorption in pure solvents once they are at the surface. However, it is not known if the polymers could be replaced by strongly adhering substances such as alkanethiols, or if mixed layers form. Such questions are addressed here with examples of the polymers, the structures and the acronyms of which are shown in Figure 1. Experimental Section The polymers16-20 and oligomers21 were synthesized according to the literature. The average number of skeletal phenylene rings, xn, derived from the vapor phase or membrane osmometry measurements, were 110 for PCA, 90 for PCCA, 25 for PCH, and 90 for PCCH (acronyms see Figure 1). Gold- and copper-coated silicon was prepared by evaporation of 2000 Å of gold or copper onto silicon wafers coated with 50 Å of chromium as the adhesion promoter. The slides were instantly immersed into the previously deoxygenated adsorption solutions that were kept under argon. After the time indicated in the text, the slides were removed from solution, rinsed with approximately 3 mL of solvent, dried in a nitrogen stream, and immediately used for the measurements. The poly(p-phenylene)s were adsorbed under argon from 0.1 mM solutions (the concentration refers to the constitutional repeat unit) from dichloromethane (UV quality, Fluka, Buchs, Switzerland). The adsorption time was 4-6 h, analogous to previous experiments.12-15 Immersion for longer periods (5-7 days) did not result in characteristic changes of the layers on gold. This is not always valid for the layers on copper (see below), most likely due to slow oxidation of the substrate. The oligomers were adsorbed on gold from 1 mM solutions (referred to as arene moieties) from toluene for 4 h. (16) Rehahn, M.; Schlu¨ter, A.-D.; Wegner, G.; Feast, W. Polymer 1989, 30, 1060. (17) Rehahn, M.; Schlu¨ter, A.-D.; Wegner, G. Makromol. Chem. 1990, 191, 1991. (18) Rau, I. U.; Rehahn, M. Makromol. Chem. 1993, 194, 2225. (19) Rau, I. U.; Rehahn, M. Polymer 1993, 34, 2889. (20) Rau, I. U.; Rehahn, M. Acta Polym. 1994, 45, 3. (21) Galda, P.; Rehahn, M. Synthesis 1996, 614.
Brunner et al. Mixed layers of poly(p-phenylene)s and octadecanethiol were prepared by immersion of polymer-modified slides into 1 mM solutions of octadecanethiol in ethanol for 5-7 days or by immersion of samples with octadecanethiol monolayers into 0.1 mM solutions of polymer in dichloromethane for 5-7 days. The octadecanethiol monolayers were prepared according to the literature22 by immersion of the metal substrates in 1 mM solutions of octadecanethiol in ethanol for 24 h, the polymer layers being deposited as described above (immersion time, 4-6 h). Reference experiments were performed with freshly prepared metal surfaces immersed in dichloromethane for 4-6 h before treatment with octadecanethiol solutions and with octadecanethiol monolayers prepared immediately after evaporation of the metals that were subsequently stored in dichloromethane for 5-7 days. Ellipsometric measurements were performed using a PLASMOS SD 2300 ellipsometer, equipped with a He-Ne laser (λ ) 632.8 nm), at an angle of incidence of 70°. The beam diameter was about 1 mm. For the layer thickness determination, slides were used that had been partially immersed in the reaction solution. The refractive index and the absorption coefficient were determined on the nonimmersed part of each substrate before the thickness measurements were performed. The layer thickness was measured at 39 spots on a straight line and separated by 1 mm. About half of the spots were located on the nonimmersed side of the sample. The layer thickness was calculated from the step height between the last 15 points on the nonimmersed and the first 15 points on the immersed part of the sample. For the thickness calculation, refractive indices of the polymers of 1.5 were used. These refractive indices were obtained from measurements on thick polymer films, prepared by evaporation of a polymer solution, on glass and gold surfaces. The determined refractive indices did not depend on the substrate (glass or gold, respectively). X-ray photoelectron spectra were measured on a Specs Sage 100 using Mg KR radiation at a pressure of ca. 10-9 mbar. The energy scale was calibrated with sputtered samples of copper, silver, and gold using the corresponding binding energies of 932.7 eV for Cu(2p3/2), 368.3 eV for Ag(3d5/2), and 84.0 eV for Au(4f7/2). This procedure resulted in a linear energy calibration with a deviation of (0.1 eV in the energy range of 100-900 eV. For the individual measurements, the signals of the elemental metal substrates were taken as an internal reference. The accuracy of the binding energies is estimated to be (0.2 eV. Details of the analyses with contact angle measurements,12 surface profilometry,12 and NEXAFS15 have been described previously. IR spectra were recorded at grazing incidence reflection on a Bruker IFS 66v spectrometer equipped with an MCT (mercurycadmium-telluride) detector. The measurements were performed at an incident angle of 80° with a fixed angle inset. To eliminate atmospheric bands of water and carbon dioxide, the pressure in the sample chamber was reduced below 1 mbar. Freshly prepared metal surfaces were used as references, i.e., the spectra of these surfaces were subtracted from the spectra of the polymer-modified substrates. ToF-SIMS measurements were performed with a PHI 7200 ToF-SIMS instrument (Physical Electronics, Eden Prairie, MN). This instrument is equipped with a Cs ion gun, which produces an 8-keV primary ion beam for high mass resolution spectra. Primary ion pulses on the order of 1 ns at the sample surface are achieved by a coaxial pulse bunching technique. The achieved mass resolution during our experiment was typically on the order of m/∆m ) 7500 at m/z ) 29 and of m/∆m ) 10 000 at m/z ) 119 with the time-of-flight mass analyzer used. Survey spectra in positive secondary ion mode were acquired over a mass range of m/z ) 5-420. The analysis area was on the order of 200 × 200 µm2, determined by scanning the beam with a diameter of approximately 100 µm over an area of 100 × 100 µm2. The primary ion dose was approximately 1011 ions cm-2. Masses were calibrated with the following mass peaks: CH3, C2H3, C3H5, Cs, and Au. (22) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
Oligo- and Poly(p-phenylene)s with Octadecanethiol
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Table 1. Advancing Contact Angles of Water (θa) and Layer Thicknesses (d) of PCH, PCA, PCCA, and PCCH on Gold and Coppera compound
substrate
d (Å)
θa (deg)
PCH
gold copper gold copper gold copper gold copper
17 ( 5 5(4 25 ( 4 7(2 22 ( 9 9(6 14 ( 3 9(2
84 ( 5 69 ( 15 90 ( 1 91 ( 4 77 ( 3 81 ( 2 81 ( 2 79 ( 4
PCCH PCA PCCA
a The indicated limits denote the error based on a 95% confidence level. In other series, “thick” layers were found on copper (see text).
Optical microscopy was performed in reflection with Leica DMRX equipment. The samples were observed under a magnification of 100-500×. AFM (atomic force microscope) experiments were performed in air on a multimode NanoScope III and a BioScope (Digital Instruments, Santa Barbara, CA) using standard silicon cantilevers holding a tip with a cone angle of 20° and a radius of curvature of 11 nm. The measurements were performed in Tapping mode at a frequency of 321.6 kHz and an amplitude of 100 mV. SEM (scanning electron microscope) images were recorded on a Hitachi S-900 in-lens field emission microscope with a Gatan cold stage. The samples were mounted with liquid carbon paste on the Gatan side entry cryoholder. The resolution was increased by means of a 1-nm tungsten coating. The pictures were taken with an acceleration voltage of 30 kV at a temperature of -90 °C.
Results Poly(p-phenylene)s. The investigated polymers (structures and acronyms, see Figure 1) consist of a poly(pphenylene) backbone. PCH and PCCH are substituted with 6-phenoxyhexyl groups. In addition, the phenyl rings in the side chains of PCA and PCCA contain methylester groups in p-position. Only PCH is a homopolymer; the other macromolecules are alternating copolymers also containing hexyl side chains. Layer thicknesses and advancing contact angles of water on the modified substrates are presented in Table 1. On gold, the thicknesses of the adsorbed layers (14-25 Å) are in the range of those expected for monolayers, and the surface roughnesses (5-20 Å), measured with a surface profilometer, did not change significantly upon adsorption. The highest contact angle was measured on the PCCH layer (90°). This value is near those observed on aliphatic hydrocarbon surfaces, e.g., 92-95° on polyethylene.23,24 The other contact angles on gold are below 90°, e.g., 84° on the PCH layer which is in the range of the values reported on aromatic hydrocarbons such as polystyrene (84°25). On pure gold substrates immersed in dichloromethane for 4-6 h, contact angles of 55-73° (average 60°) were found; i.e., the adsorption of the polymers results in an increase in contact angle on the gold surface. On the polymer-modified copper surfaces, the thicknesses, surface roughnesses, and contact angles seem to depend on parameters such as solvent quality and content of atmospheric impurities (e.g., oxygen and water). In a series of samples prepared on the same day and using the same bottle of solvent, however, only minor deviations in the characteristics of the layers were observed. The average of a number of such series resulted in the values presented in Table 1, where the highest contact angle is (23) El-Shimi, A.; Goddard, E. D. J. Colloid Interface Sci. 1974, 48, 242. (24) Henrici-Olive´, G.; Olive´, S. Adv. Polym. Sci. 1979, 32, 123. (25) Dann, J. R. J. Colloid Interface Sci. 1970, 32, 302.
Figure 2. IR spectra of PCCA in KBr and adsorbed on gold and copper.
91° for the PCCH-modified substrate, while the values on the other layers are between 69° and 81°. On blank copper substrates immersed in dichloromethane for 4-6 h, contact angles of 48°-68° (average 58°) were measured. The thicknesses of the PCH layers were small (
