Self-Assembly of Alkanethiol Molecules onto Platinum and Platinum

for identification and interpretation of XPS data; Physical Electronics: Eden Prairie, MN, 1995. Chart 1. Models of Alkanethiol Moleculesa Used for. S...
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Self-Assembly of Alkanethiol Molecules onto Platinum and Platinum Oxide Surfaces Zhiyong Li,* Shun-Chi Chang, and R. Stanley Williams Hewlett-Packard Laboratories, 1501 Page Mill Road, MS 1123, Palo Alto, California 94304 Received February 12, 2003. In Final Form: May 2, 2003 We studied the self-assembly of n-alkanethiols CH3(CH2)mSH (m ) 5, 7, 9, 11, 13, 15, 17, 19, 21) onto platinum thin films with two different surface properties: metallic and oxide. Water contact angle, ellipsometry, and reflection-absorption infrared spectroscopy (RAIRS) measurements suggested that wellorganized, methyl-terminated monolayer films formed on the metallic platinum surface. On the other hand, the molecular films formed with the alkanethiol molecules on platinum oxide surfaces were loosely packed and poorly ordered. For the monolayer films formed with alkanethiols of different chain lengths on metallic Pt surfaces, a transition from slightly disordered to crystalline-like structure was observed with increasing chain length. Based on the intensities of the methylene and methyl C-H vibration modes in the RAIRS, the tilt angle of the alkyl chain in the monolayers on Pt surfaces was estimated to be less than 15° with respect to the surface normal. X-ray photoelectron spectroscopy revealed the formation of metal-thiolate bonding of the thiol headgroups in the alkanethiol molecules with Pt atoms on the surfaces.

1. Introduction The formation of self-assembled monolayers (SAMs) of organosulfides or alkanethiols onto gold surfaces has inspired intensive studies during the past two decades in a variety of fields, such as catalysis, wetting, anticorrosion, sensing, and molecular electronics.1-5 More recently, in a general interest to extend the SAM process to more materials, other solid surfaces such as silver,6 copper,7 GaAs,8 platinum,9-11 palladium,12,13 and nickel14 have been examined. Previous studies have shown that the orientation and structure of alkanethiolate monolayers on coinage metal surfaces depend largely on the assembling conditions, such as solvent identity, concentration, temperature, and soaking time, and more importantly on the surface properties of the metal substrates. Clean, smooth, and often metallic surfaces are required for the formation of densely packed, well-ordered molecular monolayers. Both * To whom correspondence should be addressed. Phone: 650236-4393. Fax: 650-236-9885. E-mail: [email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. W.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (4) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (5) Reed, M. A.; Chen, J.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2001, 78, 3735. (6) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 7152-7167. (7) Lusk, A. T.; Jennings, G. K. Langmuir 2001, 17, 7830-7836. (8) Sheen, C. W.; Shi, J. X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514-1515. (9) Stern, D. A.; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885-4893. (10) Climent, V.; Rodes, A.; Albalat, R.; Claret, J.; Feliu, J. M.; Aldaz, A. Langmuir 2001, 17, 8260-8269. (11) Lang, P.; Mekhalif, Z.; B.;, R.; Garnier, F. J. Electroanal. Chem. 1998, 441, 83-93. (12) Carvalho, A.; Geissler, M.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2406-2412. (13) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 2003, 125, 25972609. (14) Mekhalif, Z.; Laffineur, F.; Couturier, N.; Delhalle, J. Langmuir 2003, 19, 637-645.

platinum single crystals and polycrystalline platinum thin films have been used as substrates for molecular assembly. For single-crystal platinum, which has been studied in relation to the fields of catalysis and electrochemistry, clean and well-ordered sample surfaces can be generated either by sputtering-annealing in ultrahigh vacuum or by flame-annealing-quenching in reducing gases such as hydrogen or carbon monoxide.15,16 For polycrystalline thin films, which can be readily deposited onto a substrate by evaporation or sputtering, those pretreatments developed for single crystals are no longer suitable. Particularly, for nano- or molecular electronics applications, polycrystalline films are usually patterned as wires or electrodes through multistep lithographic processes. A standard cleaning procedure finishing the lithographic processing is an oxygen plasma treatment to remove any remaining organic species on the film surface. In a previous study, we have observed that oxygen plasma generated with our RIE (reactive ion etching) system formed a thin layer of platinum oxide (2.4-2.7 nm) on the platinum film surface, while a subsequent “gentle” argon plasma removed this oxide layer and regenerated a smooth metallic surface.17 In this report, we will compare the self-assembly of a series of n-alkanethiols onto platinum surfaces of two different types, that is, metallic and oxide-covered. 2. Experimental Procedure 2.1. Materials. C6SH (1-hexanethiol, 95%, Aldrich), C8SH (1-octanethiol, 98.5%, Aldrich), C10SH (1-decanethiol, 96%, Aldrich), C12SH (1-dodecanethiol, 98%, Aldrich), C14SH (1tetradecanethiol, 98%, Fluka), C16SH (1-hexadecanethiol, 95%, Fluka), and C18SH (1-octadecanethiol, 98%, Fluka) were used as received. Absolute ethanol (Aldrich), anhydrous chloroform (Aldrich), and anhydrous THF (Aldrich) were used without further purification. C20SH (1-eicosanethiol) and C22SH (1docosanethiol) were synthesized according to literature procedures.2 2.2. Platinum Thin Film Preparation and Self-Assembly Procedure. Platinum substrates were prepared by RF sputtering (15) Hubbard, A. T. Chem. Rev. 1988, 88, 633-656. (16) Garwood, G. A. J.; Hubbard, A. T. Surf. Sci. 1982, 118, 223247. (17) Li, Z.; Beck, P.; Ohlberg, D. A. A.; Stewart, D. R.; Williams, R. S. Surf. Sci. 2003, 529, 410-418.

10.1021/la034245b CCC: $25.00 © 2003 American Chemical Society Published on Web 07/10/2003

Self-Assembly of Alkanethiols onto Pt Surfaces onto silicon wafers which were back-sputtered by argon plasma before the deposition of 100-150 nm films of Pt. The substrates were stored in Fluoroware wafer holders, and plasma-cleaning procedures were applied to the substrates immediately before use in experiments. Two different plasma conditions were used to generate platinum surfaces with different properties. Oxygen plasma (100 W, 100 mTorr, 5 min) alone formed a platinum surface with a layer of platinum oxide; oxygen plasma (100 W, 100 mTorr, 5 min) followed by a gentle argon plasma (15 W, 40 mTorr, 1 min) produced a surface chemically identical to a freshly deposited platinum thin film.17 Glass jars for alkanethiol selfassembly were cleaned with “piranha solutions” (1:1 concentrated H2SO4/30% H2O2) at 100 °C for half an hour, rinsed exhaustively with deionized water, and further dried overnight in an oven at ∼120 °C. Caution: piranha solution reacts violently with organic materials and should be handled with great care. The two types of platinum substrates were soaked in alkanethiol solutions (1 mM) in anhydrous ethanol inside a drybox under a nitrogen atmosphere for 18 h at room temperature unless otherwise specified. For comparison purposes, freshly deposited Au or Ag thin films (normally 100 nm by ion-beam evaporation) on clean silicon substrates were soaked in the same alkanethiol solutions for 18 h. The samples were rinsed with fresh solvents three times after being removed from the assembly solutions. After drying with flowing nitrogen, the samples were subjected to a battery of characterizations. 2.3. Water Contact Angle Measurements. The contact angles were recorded on a VCA 1000 video contact angle system (AST Products, Inc., Billerica, MA). A droplet of ∼2 µL of highpurity Milli-Q water (resistivity of >18 MΩ‚cm) generated by a NANOpure system (Barnstead, Dubuque, IA) was injected onto the sample surface from a syringe, and then the needle was retracted from the droplet. An image of the static water droplet was recorded by a digital camera and analyzed using the software provided by AST to provide a sessile contact angle. At least three readings from each sample were averaged to give the water contact angle data. 2.4. Ellipsometry Measurements. Ellipsometric measurements were performed with a Nanofilm BAM (Brewster angle microscopy) system (Accurion, Menlo Park, CA) using a laser wavelength of 532 nm and an incident angle of 58°. Readings from bare platinum substrates were collected to derive the complex reflective indexes of the substrates. The thickness of the SAMs was calculated from a three-phase model (airmonolayer-substrate) using a real reflective index of 1.50 for the monolayer and the previously measured complex reflective indexes for the substrates. The algorithms for these calculations were provided by Nanofilm. An average of three readings collected from different locations on a sample surface was reported as the SAM thickness result. For calibration purposes, the thickness of the SAMs formed from alkanethiols on Au surfaces was also measured by the same method. 2.5. Reflection-Absorption Infrared Spectroscopy (RAIRS). The RAIR spectra of the monolayers were acquired on a Nexus 870 FTIR spectrometer (Thermo-Nicolet, Madison, WI) with p-polarized incident light at an angle of 80° from the surface normal. Reflected light was detected with a narrow-band MCT (mercury-cadmium telluride) detector cooled with liquid nitrogen. Monolayer samples were normally subjected to 640 scans at a resolution of 2 cm-1 to obtain the IR spectra. All spectra of monolayer films formed from alkanethiols were referenced with respect to bare platinum metal or oxide films. To qualitatively compare the orientation of the alkyl chains in the monolayer films, we also collected the RAIR spectra of a monolayer formed from octadecanethiol on both Au and Ag thin film surfaces. 2.6. X-ray Photoelectron Spectroscopy (XPS). The XP spectra were acquired on a PHI Quantum 2000 spectrometer with monochromated Al KR 1486.6 eV X-ray radiation. The SAM samples were loaded into the vacuum chamber within 1 h after being prepared and were subjected to XPS analysis. The analysis area was 1400 µm × 300 µm on each sample. The takeoff angle in the instrument was set at 45°. Photoemission peak positions were corrected to C 1s at a binding energy of 284.8 eV.18

Langmuir, Vol. 19, No. 17, 2003 6745 Chart 1. Models of Alkanethiol Moleculesa Used for SAMs on Pt Substrates and Their Extended Lengths in All-trans Geometry Predicted with ChemBats3D Softwareb

b

a Sulfur in yellow, carbon in black, hydrogen in blue. Reference 20.

Figure 1. The thickness of an octadecanethiol SAM on a metallic Pt surface formed at different soaking times.

3. Results and Discussions 3.1. General SAM Formation and the Time Dependence of SAM Formation on Pt. As described previously, a platinum thin film surface can have two different optical characteristics depending upon the plasma treatment conditions.17 An oxygen plasma generated in our RIE system formed a thin oxide layer of 2.42.7 nm, while a brief argon plasma treatment recovered the metallic surface. In this study, we compared the selfassembly of a series of alkanethiols (Chart 1) onto the platinum films with metallic and oxide surfaces, respectively. To find the time requirement for a complete monolayer formation on platinum surfaces, we selected C18SH as the test molecule. Metallic platinum thin films were soaked in a 1 mM C18SH solution in ethanol for different periods of time. The SAMs formed on Pt surfaces were monitored with ellipsometric thickness measurements, and the results are shown in Figure 1. The film formed with only 1 min of soaking time had already reached a thickness of (18) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data; Physical Electronics: Eden Prairie, MN, 1995.

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Figure 2. The ellipsometric thickness of alkanethiol SAMs on (O) a metallic Pt surface and (+) a Pt surface with an oxide layer. The thickness of SAMs on Au surfaces (0) was plotted for comparison purposes.

2.4 nm. This suggests a fast chemical adsorption step in the initial stage of forming the self-assembled monolayers. After this initial adsorption step, the molecules reorganize slowly and the thickness of the C18SH SAM as measured by the ellipsometer increased slightly until it reached 2.9 nm after 18 h of soaking time. With even longer soaking time (up to 1 week), the thickness did not increase further, indicating the saturation of the surface molecular monolayer. This is also the main reason the soaking time for molecular assembly on platinum surfaces was kept at 18 h for most of the alkanethiolate SAMs throughout this paper. 3.2. Ellipsometric Thickness Measurements of Different SAMs. Figure 2 shows the ellipsometric thickness of the SAMs formed from alkanethiols on different platinum surfaces after a soaking time of 18 h. For comparison purposes, the thicknesses of monolayers formed on Au film surfaces were also recorded using the same method. The optical thickness of the monolayers formed on metallic platinum surfaces increased linearly as the number of carbons in the alkanethiols increased. A linear fit yielded a slope of 0.13 nm/CH2 and a y intercept of 0.63 nm. The thickness of the monolayers on Au surfaces also followed a linear relationship with the carbon numbers in the alkanethiols, and the slope of 0.11 nm/CH2 was in good agreement with the data reported by Allara and coworkers.19 Using an average C-C bond length of 0.154 nm and a C-C-C angle of 110.5° in the alkanethiol chain from the modeling shown in Chart 1, a slope of 0.127 nm/CH2 unit is predicted, assuming that the alkyl chains adopt a fully extended, all-trans configuration with the chain axis oriented normal to the surface. Comparing our experimental slope data between monolayer films on Pt (0.13 nm/CH2) and Au (0.11 nm/CH2), we believe that the alkanethiol molecules on the metallic Pt surfaces did arrange in a fully extended, all-trans geometry and oriented closer to the surface normal than those on Au surfaces. On the other hand, the optical thicknesses of octadecanethiol, eicosanethiol, and docosanethiol films on platinum oxide surfaces were ∼2.5 nm less than their (19) Collins, R. W.; Allara, D. L.; Kim, Y.-T.; Lu, Y.; Shi, J. In Characterization of Organic Thin Films; Ulman, A., Ed.; Manning: Boston, 1995; pp 35-55.

Li et al.

Figure 3. The contact angle of alkanethiol SAMs on (O) a metallic Pt surface and (+) a Pt surface with an oxide layer.

counterparts on metallic surfaces (Figure 2). For the film formed from hexadecanethiol and those alkanethiols with shorter chains, the thickness was unmeasurable at all. The large discrepancy between film thickness on metallic and oxide surfaces suggests that the films on platinum oxide surfaces were highly disordered and likely to be partially covered with alkanethiol molecules. 3.3. Contact Angle Measurements of Different SAMs. The sessile water contact angles of the SAMs formed from alkanethiols on different Pt substrates are compared in Figure 3. On metallic Pt surfaces, the water contact angle of the SAMs formed from alkanethiols increased with increasing alkyl chain length. The contact angle of the C6SH SAM was only 95°, whereas the contact angle reached 110° when the carbon number in the chain reached 16 and increased slightly for the alkanethiols with even longer chain lengths. This suggests that the longer the alkyl chain, the denser the packing and the more crystalline-like the ordering of the alkyl chains in the SAMs, mainly due to increased intermolecular van der Waals interactions. This observation is consistent with the alkanethiolate SAMs studied on gold surfaces.2 The contact angles of the SAMs on platinum oxide surfaces were 5-10° smaller than their counterparts on metallic platinum surfaces as shown in Figure 3, with the difference getting larger for shorter alkyl chains. This indicates poor ordering and low coverage of the molecules on the surfaces, which is consistent with the ellipsometric measurement discussed in the previous section. 3.4. XPS of C18SH SAMs. The XP spectra of SAMs formed from C18SH on both metallic and oxide-covered platinum films were selected to elucidate the molecular binding mode and the chemical composition at the interface between molecules and substrate surfaces. Figure 4a shows the survey spectra of the SAMs on metallic platinum and platinum oxide surfaces formed from the same ethanol solution of C18SH. All the binding energies were calibrated according to the C 1s peak at 284.8 eV to eliminate any possible shifts due to surface charge accumulation. Only C, S, and Pt were detected in the SAM formed from C18SH on the metallic Pt film surface, which corresponds to the composition of octadecanethiol and the platinum substrate. In addition to these three elements, a substantial amount of O, mainly due to the oxide of platinum on the substrate, was observed in the C18SH film on the platinum oxide surface. Trace amounts of Na and Cl were also detected in the film, which were observed

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Figure 4. XP spectra of the C18SH SAM on metallic Pt (dotted lines) and on a platinum oxide surface (solid lines): (a) survey scan, (b) S 2p high-resolution region, (c) O 1s high-resolution region, (d) Pt 4f high-resolution region, and (e) C 1s high-resolution region.

repeatedly. We do not believe this was due to contamination during preparation or handling of the sample, since all the samples for XPS analysis were treated identically. One possible explanation is that the platinum oxide on the surface catalyzed the oxidation of the surface-attached alkanethiol molecules to generate sulfonyl derivatives with strong ionic character, which could then attract the Na+ and Cl- ions from either the solvent or the air. The high-resolution XPS scans of the S 2p, O 1s, Pt 4f, and C 1s regions are shown in Figure 4b-e. The S 2p peak position of the C18SH film on the metallic platinum surface was located at 162.5 eV. Compared to surface-bound sulfur on Au systems, where S 2p3/2 is normally centered at 161.5-162.1 eV,21,22 the S 2p peak position for C18SH on the Pt surface is shifted slightly to higher binding energy (BE), but it is still lower than the S 2p3/2 at 163.5 eV for (20) Chem 3D, version 5.0; CambridgeSoft Corp.: Cambridge, MA, 1999. (21) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772-2774. (22) Li, Z.; Lieberman, M.; Hill, W. Langmuir 2001, 17, 4887-4894.

unbound sulfur on Au surfaces. Based on this observation, we believe that the thiol headgroups in the alkanethiol molecules formed covalent bonds between sulfur and Pt atoms on the surface. The chemisorption reactions of the alkanethiols on metallic platinum surfaces most likely generated platinum thiolate species on the surfaces with the evolution of hydrogen as suggested by eq 1. This mechanism is widely accepted for alkanethiol chemisorption on Au surfaces.23

R-SH + Pt f R-S-Pt + (1/2)H2

(1)

In the case of the C18SH film on the platinum oxide surface, a much weaker intensity of the S 2p peak was observed as shown in Figure 4b; in addition, a substantial amount of the S 2p peak appeared at a higher BE, at around 164 eV. This could be attributed to the alkanethiol molecules attached on the surface through noncovalent (23) Ulman, A. In Characterization of Organic Thin Films; Butterworth-Heinemann: Boston, 1995; pp 24, 75.

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Li et al.

Table 1. Relative Atomic Concentration (%) of Different Elements Detected within the Escape Depth of Photoelectrons samples

Pt

S

C

O

Na

Cl

bare Pta C18SH on Pt bare Pt-Oa C18SH on Pt-O

55.0 36.3 28.0 16.7

3.7 2.4

38.6 58.7 20.3 54.4

6.4 1.3 51.6 24.7

1.5

0.3

a

From ref 17; -, undetected.

bonding, such as hydrogen bonding and van der Waals interactions. The O 1s spectrum showed a barely discernible peak for the SAMs formed on the metallic platinum surface. This implies that (a) no oxygen-related adventitious species were present on the sample surface and (b) no oxidation of sulfur occurred during the XPS analysis. We also examined the XP spectra of the sample that was intentionally exposed in ambient for a week (spectra not shown); a substantial O 1s peak and sulfonyl S 2p peak appeared at the same time, indicating the oxidation of sulfur in the monolayer film over a long period of exposure to air. The O 1s peak in the film on the platinum oxide surface, on the other hand, was mainly due to the platinum oxide. XP spectra of metallic platinum surfaces typically have two well-separated peaks for Pt 4f7/2 and 4f5/2 at 71.2 and 74.5 eV BE, respectively,18,24 which is also the case for the spectrum of the C18SH SAM on the metallic platinum surface as shown in Figure 4d. The data indicate that the surface attachment of C18SH molecules did not change the metallic behavior of the platinum surface sufficiently to be detectable. In other words, this also excludes the formation of a platinum sulfide interfacial layer as Love and co-workers proposed for alkanethiolate SAMs on Pd surfaces recently.13 On the other hand, the Pt 4f peaks on the Pt oxide sample were dramatically different from their counterpart on the metallic Pt surface. Compared to the XP spectra of the bare substrate reported previously,17 the chemical states of Pt could be assigned as platinum monoxide, platinum dioxide, and platinum hydroxide with the coexistence of some metallic Pt. The atomic concentrations in Table 1 of the elements detected in C18SH SAMs on both metallic and oxidecovered Pt surfaces were determined from the peak intensity and known sensitivity factors.18 Compared to the corresponding bare substrate, the intensity of Pt 4f peaks in both samples decreased at least 40% after the attachment of octanethiol molecules. This is simply because the Pt photoelectrons were attenuated by the organic overlayer formed on the surface. The atomic ratio of S/C is about 1:16 for the C18SH SAM formed on the metallic Pt surface, which is reasonably close to the theoretical ratio of 1:18. This further implies that the binding of the alkanethiol molecules on Pt atoms was strong enough that any adventitious hydrocarbons initially present on the bare substrates were replaced. This was not the case for the C18SH film on the platinum oxide surface, where a low concentration of S was observed (the S/C ratio was approximate 1:23), indicating a lower coverage of alkanethiol molecules on the oxide surface than on the metallic Pt surface. 3.5. RAIRS of Different SAMs. The alkanethiol SAMs on metallic platinum surfaces were further studied by reflective absorption infrared spectroscopy. Figure 5 shows the C-H vibration region of the RAIR spectra of the SAMs.

As the figure shows, five marked peaks are assigned to the antisymmetric CH3 stretch (νa(CH3)) at 2966 cm-1, Fermi resonance symmetric CH3 stretch (νs(CH3, FR)) at 2938 cm-1, antisymmetric CH2 stretch (νa(CH2)) at 2918 cm-1, Fermi resonance symmetric CH3 stretch (νs(CH3, FR)) at 2897 cm-1, and symmetric CH2 stretch (νs(CH2)) at 2850 cm-1.25 The positions of the peak frequencies for the CH2 modes provide direct information about the intermolecular environment of the alkyl chains in these films and indicate the level of ordering of the molecules. In the SAMs of alkanethiols with shorter chain length, the CH2 peak positions, both νa(CH2) and νs(CH2), shifted to high wavenumbers, similar to that of the liquid phase or disordered solid phase, which indicates a loose packing of the alkyl chains in the monolayer films. As the chain length increases, the molecules pack more densely with less free volume between neighboring chains, and therefore, crystalline-like IR spectra were observed. The peak position of νs(CH2) for C18SH, C20SH, and C22SH SAMs is at 2850 cm-1, the same position observed for a crystalline sample of polymethylene, rather than that for a liquid sample of polymethylene at 2856 cm-1.25 Also the peak of νa(CH2) for the SAM formed with C22SH on the Pt surface approached 2918 cm-1, the value for a crystalline polymethylene. To compare qualitatively the chain tilting angle in the SAMs on the surface, we collected the RAIR spectra of SAMs formed with same molecule, C18SH, on Pt, Au, and Ag surfaces. The C-H region of the RAIR spectra is shown in Figure 6. In contrast to the similarities of the C-H peak positions on metallic Pt, Au, and Ag surfaces, the intensity ratio of the νa(CH3) to νa(CH2) modes was significantly different. The intensities of the C-H vibration modes in these alkanethiolate SAMs are directly related to the number m in the chain and the tilt angle of the chain with respect to the surface normal. In the alkanethiolate SAMs on Au surfaces, a ∼27° tilt angle of the chains is generally acknowledged, based on the experimental and theoretical intensities of the C-H vibration modes.23 On the other hand, the tilt angle was reported to be only ∼7-14° to the surface normal on Ag.26,27

(24) Hecq, M.; Hecq, A.; Delrue, I. P. J. Less-Common Met. 1979, 64, 25-37.

(25) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

Figure 5. The C-H region in the RAIR spectra of different alkanethiol SAMs on metallic Pt surfaces.

Self-Assembly of Alkanethiols onto Pt Surfaces

Figure 6. The C-H region of the RAIR spectra of SAMs formed with C18SH on Pt (solid line), on Au (dotted line), and on Ag (dashed line).

Our data showed that both νa(CH2) and νs(CH2) peaks in the SAMs on Pt were merely half of those on Au and were just slightly higher than those on Ag, while the ν(CH3) vibration modes on three different metal surfaces remained almost identical within the measurement noise level. Therefore, assuming the same coverage and same all-trans geometry of octadecanethiol molecules on Pt, Au, and Ag, a tilting angle of less than 15° was estimated with respect to the surface normal for the alkyl chains on metallic Pt surfaces. This result is consistent with the ellipsometric results discussed earlier, where a larger slope of the alkanethiolate SAMs on Pt surfaces (0.13 nm/CH2) was observed than for the same series of SAMs on Au surfaces (0.11 nm/CH2). The RAIR spectra of the films formed with alkanethiols on platinum oxide surfaces were also collected and referenced to the bare platinum oxide surface, as shown in Figure 7. In general, all five modes of C-H vibration were observed in the samples studied, demonstrating the attachment of alkanethiols to the oxide surfaces. However, the peaks were broader than the same monolayer films on metallic Pt surfaces. Especially for the νa(CH3) C-H vibration mode, a shoulder peak appeared at 2960 cm-1, indicating a nonuniform orientation of the methyl terminal groups on the surfaces. Comparing the relative intensities of the peaks shown in both Figure 7 and Figure 5, the CH3 vibration modes in the films on platinum oxide surfaces were noticeably weaker than those on metallic Pt surfaces, whereas the CH2 vibration modes were stronger. This could be attributed to the lack of ordering and large tilting of the alkyl chains on the platinum oxide surfaces, which is consistent with thickness measurement, contact angle, and XPS results discussed earlier. Therefore, based on this study, it is important to remove the platinum oxide (26) Ulman, A. In An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991; p 295. (27) Laibinis, P.; Whitesides, G. M.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167.

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Figure 7. The C-H region in the RAIR spectra of different alkanethiol SAMs on platinum oxide surfaces.

layer in order to form good-quality thiolate monolayers on platinum surfaces,. 4. Conclusions Based on ellipsometric thickness, wetting properties, XPS, and RAIR studies, excellent self-assembled monolayers from alkanethiols can be formed on metallic platinum surfaces. The assembly of the long alkanethiol molecules in ethanol solutions onto the Pt surface was found to be a fast absorption process followed by a slow reorganizing process. The attachment of the alkanethiol molecules on Pt surfaces was directed through the formation of Pt-thiolate covalent bonding between the thiol headgroup of the molecules and the Pt atom on the surfaces. The ordering of the alkanethiol molecules with different chain lengths on metallic platinum surfaces improves as the chain length increases. By comparison of the SAMs formed with the same molecule on Au, Ag, and Pt surfaces, the tilt angle of the alkyl chains in the SAMs formed with long-chain alkanethiols on the Pt surfaces studied here was estimated to be less than 15° from the surface normal, which was comparable with the similar results reported recently on Pd surfaces.13 On the other hand, the assembly of alkanethiols on a platinum surface with a thin platinum oxide overlayer produced a film with poor quality. The films were less ordered and more heterogeneous than those on metallic platinum surfaces. The alkanethiol molecules were likely attached to the surface through weak, noncovalent bonding. Therefore, clean, metallic surface conditions are extremely important for the formation of a well-ordered, fully covered monolayer of alkanethiols on Pt surfaces. Acknowledgment. We thank Dr. Greg Strawsman from Charles Evans & Associates for collecting XPS data. We also acknowledge Margie Flores for preparing platinum films. This research was supported in part by DARPA. LA034245B