Preparation, Modification, and Crystallinity of Aliphatic and Aromatic

3980

Langmuir 2002, 18, 3980-3992

Preparation, Modification, and Crystallinity of Aliphatic and Aromatic Carboxylic Acid Terminated Self-Assembled Monolayers Ralf Arnold,† Waleed Azzam,† Andreas Terfort,‡ and Christof Wo¨ll*,† Ruhr-Universita¨ t Bochum, Physikalische Chemie I, Universita¨ tsstr. 150, D-44801 Bochum, Germany, and Universita¨ t Hamburg, Institut fu¨ r Anorganische und Angewandte Chemie, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany Received November 26, 2001. In Final Form: February 4, 2002 COOH-terminated organic surfaces were prepared by self-assembly of different organothiols, mercaptohexadecanoic acid (MHDA) and mercaptomethylterphenylcarboxylic acid (MMTA), on Au substrates. The SAMs were explored by infrared reflection-absorption spectroscopy and X-ray photoelectron spectroscopy. A rather strong dependence on preparation conditions is observed which explains inconsistencies between previous studies reported for these systems. Using optimized preparation conditions, films with a high degree of molecular orientation could be prepared. A significant amount of disorder, however, is induced by hydrogen bonds between the terminating -COOH groups. An organothiol with a more rigid backbone, MMTA, is demonstrated to yield carboxy-terminated SAMs which are more straightforward to prepare and which show a significantly higher degree of order.

Introduction Presently, there is a pronounced interest in the preparation of structurally well-defined organic surfaces for a number of different purposes, ranging from the control of physical surface properties, for example, wettability, to the creation of highly specific biocompatible surfaces, for example, for biochemical sensor devices. Today, the most straightforward approach to fabricate such layers is to adsorb appropriate organothiols onto a gold substrate. Such self-assembled monolayers (SAMs) prepared by chemisorption of thiols can be prepared in a rather straightforward fashion and are fairly robust. The chemical composition of the surface formed by the adsorption process is controlled by the termination of the organothiols. In case of the alkanethiols (CH3 termination), the adsorption process leads to the formation of a highly ordered, crystalline organothiolate adlayer. It is not certain a priori whether replacing the terminal -CH3 group by other functions is compatible with this high degree of order or whether possible interactions between the functionalities added to the chain termini lead to a different ordering pattern or even to disorder. One of the most interesting functions in this context is a carboxylic acid group, since -COOH-terminated surfaces are of interest for a wide range of applications in surface science,1-7 electrochemistry,8,9 biology,10-14 biomineral* To whom correspondence should be addressed. † Ruhr-Universita ¨ t Bochum. ‡ Universita ¨ t Hamburg. (1) Yang, H. C.; Dermody, D. L.; Xu, C. J.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 726-735. (2) Smith, D. A.; Wallwork, M. L.; Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; Wong, M. J. Phys. Chem. B 2000, 104, 8862-8870. (3) Kokkoli, E.; Zukoski, C. F. J. Colloid Interface Sci. 2000, 230, 176-180. (4) Kokkoli, E.; Zukoski, C. F. Langmuir 2001, 17, 369-376. (5) Ashby, P. D.; Chen, L. W.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 9467-9472. (6) Fisher, G. L.; Hooper, A. E.; Opila, R. L.; Allara, D. L.; Winograd, N. J. Phys. Chem. B 2000, 104, 3267-3273. (7) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704-6712. (8) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222-4228.

ization,15,16 surface engineering,17-20 sensor development,21 and nanoparticles.22,23 Regarding the structural properties of such films made from carboxylated organothiols, however, there is conflicting evidence in the literature. In the first paper describing this particular type of organothiol, Nuzzo et al.24 concluded from an IR investigation of SAMs made from mercaptohexadecanoic acid (MHDA) that the films exhibit a high degree of molecular orientation and that there is only a small number of carboxylic acid groups linked together by hydrogen bonds (Figure 1, structure A). The absence of such head-to-head dimers (which in the unconstrained case exhibit a binding energy of 50-60 kJ mol-1 25,26) was explained by the high degree of orientation within the MHDA SAM, which (9) Sugihara, K.; Shimazu, K.; Uosaki, K. Langmuir 2000, 16, 71017105. (10) Franco, M.; Nealey, P. F.; Campbell, S.; Teixeira, A. I.; Murphy, C. J. J. Biomed. Mater. Res. 2000, 52, 261-269. (11) Chapman, R. G.; Ostuni, E.; Yan, L.; Whitesides, G. M. Langmuir 2000, 16, 6927-6936. (12) Lahiri, J.; Kalal, P.; Frutos, A. G.; Jonas, S. T.; Schaeffler, R. Langmuir 2000, 16, 7805-7810. (13) Mirsky, V. M.; Riepl, M.; Wolfbeis, O. S. Biosens. Bioelectron. 1997, 12, 977-989. (14) Lestelius, M.; Liedberg, B.; Tengvall, P. Langmuir 1997, 13, 5900-5908. (15) Ku¨ther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641-650. (16) Ku¨ther, J.; Tremel, W. Thin Solid Films 1998, 329, 554-558. (17) Lee, S. W.; Laibinis, P. E. J. Am. Chem. Soc. 2000, 122, 53955396. (18) Lee, S. W.; Laibinis, P. E. Isr. J. Chem. 2000, 40, 99-106. (19) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G. Y. Langmuir 1999, 15, 7244-7251. (20) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862-6867. (21) Bertilsson, L.; Potje-Kamloth, K.; Liess, H. D.; Liedberg, B. Langmuir 1999, 15, 1128-1135. (22) Patil, V.; Mayya, K. S.; Sastry, M. Thin Solid Films 1997, 307, 280-282. (23) Auer, F.; Scotti, M.; Ulman, A.; Jordan, R.; Sellergren, B.; Garno, J.; Liu, G. Y. Langmuir 2000, 16, 7554-7557. (24) Nuzzo, R. G.; Dubois, L. H.; Allara, D. A. J. Am. Chem. Soc. 1990, 112, 558-569. (25) Kutzelnigg, W. Einfu¨ hrung in die Theoretische Chemie; VCH: Weinheim, 1994. (26) Taylor, M. D.; Bruton, J. J. Am. Chem. Soc. 1952, 74, 41514152.

10.1021/la0117000 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/20/2002

Aliphatic and Aromatic COOH-Terminated SAMs

Langmuir, Vol. 18, No. 10, 2002 3981

Figure 1. Structure of a MHDA SAM without (A) and with (B,C) hydrogen bonds between the terminal carboxylic acid groups. See the text for a more detailed discussion.

excludes the formation of -COOH‚‚‚HOOC- dimers for steric reasons (see Figure 1). Instead of this head-to-head interaction, Nuzzo et al.24 proposed that about 50% of neighboring -COOH groups are bound by single hydrogen bonds thus forming linear chains of hydrogen bonds (Figure 1, structure B, Figure 2). Li et al.27 performed grazing incidence X-ray diffraction measurements of MHDA SAMs in an aqueous environment and in ultrahigh vacuum and found that the domain sizes range between 50% and 100% of the corresponding values of dodecanethiol.28 In a later IR study, Smith et al.29 also found a highly oriented structure of a MHDA SAM. In contrast to Nuzzo et al.,24 they considered the presence of slightly distorted head-to-head bound -COOH‚‚‚HOOC- dimers consistent with the presence of a highly oriented SAM. In contrast to these investigations, the near-edge X-ray absorption (27) Li, J.; Liang, K. S.; Scoles, G.; Ulman, A. Langmuir 1995, 11, 4418-4427. (28) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447-2450. (29) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D. Langmuir 1992, 8, 2707-2714.

fine structure results reported by Dannenberger et al.30 and Himmel et al.31-33 revealed a highly disordered structure of the MHDA SAM (Figure 1, structure C). It was proposed that the disorder is linked to the high flexibility of the alkyl chain anchoring the carboxylic acid to the substrate or to hydrogen-bond formation between neighboring carboxylic groups. When using a more rigid backbone, for example, a terphenyl group, the formation of highly oriented films was observed.34 To resolve these inconsistencies, we have carried out a systematic study of MHDA SAMs assembled onto the Au(111) surface with the goal to find conditions under which films with a reproducible high quality can be formed. In addition, SAMs made from the above-mentioned organ(30) Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Ja¨ger, B.; Buck, M.; Wo¨ll, C. Thin Solid Films 1997, 307, 183-191. (31) Himmel, H.-J. Strukturen und Reaktivita¨ten von selbstorganisierenden Du¨nnstschichten aus Organothiolen. Dissertation, RuhrUniversta¨t Bochum, Bochum, Germany, 1998. (32) Himmel, H.-J.; Weiss, K.; Jager, B.; Dannenberger, O.; Grunze, M.; Wo¨ll, C. Langmuir 1997, 13, 4943-4947. (33) Himmel, H.-J.; Wo¨ll, C. Chem. Unserer Zeit 1998, 6, 294-301. (34) Himmel, H.-J.; Terfort, A.; Wo¨ll, C. J. Am. Chem. Soc. 1998, 120, 12069-12074.

3982

Langmuir, Vol. 18, No. 10, 2002

othiol with a rigid backbone, MMTA (mercaptomethylp-terphenylcarboxylic acid), were investigated. Experimental Section Infrared spectra were acquired using a Bio-Rad Excalibur FTS3000 Fourier transform infrared spectrometer. The monolayer spectra were recorded by infrared reflection-absorption spectroscopy (IRRAS) using a Bio-Rad “Uniflex” universal sampling accessory and p-polarized light incident at 80° relative to the surface normal. The spectra are reported in absorbance units [AU] which are equal to log(I0/I) where I0 is the reflected intensity of the reference sample and I is the measured intensity. A perdeuterated docosanethiolate film on Au was used as a reference sample. The IR spectrometer was purged by dry air delivered from a commercial purge gas generator. The reflected light was detected by a liquid nitrogen cooled MCT narrow band detector. A small aperture of 10 mm diameter located on the entrance of the sample compartment was used to protect the detector against overload, to avoid nonlinear effects of the detector, and to reduce the f-number of the incident light bundle.35,36 The measurements were started 5 min after mounting a new sample. For all measured spectra, the resolution was set to 2 cm-1 and 2000 scans were accumulated, averaged, and transformed by using a triangular apodization. All IR spectra from bulk samples were recorded using a DTGS detector, averaged over 100 scans, and also transformed by using a triangular apodization. The IR spectra of solutions were taken using a 1 mm ZnSe cuvette. All spectra were baseline corrected by subtracting spline functions fitted to the nonabsorbing regions beside the bands using a commercial software package.37 X-ray photoelectron (XP) spectra were recorded for normal electron exit angles using a Al KR source and a Leybold MAX200 hemispherical electron energy analyzer operated at a constant pass energy of 117 eV.

Sample Preparation Chemicals. Hexadecanethiol (HDT, 99%, Aldrich), potassium bromide (KBr, Aldrich, optical grade), ethanol (EtOH, 99.8%, Baker, reagent grade), tetrachloromethane (CCl4, 99.9%+, Aldrich, HPLC grade), hydrochloric acid (HCl, 37%, Baker), trifluoroacetic acid (TFA, 99%+, Aldrich, optical grade), sodium hydroxide (NaOH, 98.4%+, Baker, reagent grade), and acetic acid (AA, 99%+, Baker, reagent grade) were used as received. Deionized water was further cleaned by a Millipore four-stage filter system delivering water of 18 MΩ/cm resistance. Mercaptohexadecanoic acid (MHDA, 90%, Aldrich) was purified by recrystallizing twice before use. p-Mercaptomethyl-p-terphenylcarboxylic acid (MMTA)34 and p-terphenyl-bis(methylthiol) (TPBMT)38 were synthesized as described elsewhere. Laboratory Equipment. All items used in the laboratory were precleaned in a bath of KOH/H2O/2-propanol (15/15/1), fine cleaned in a second identical bath, and finalized in a H2O/HCl (50/1) bath. The items were rinsed between the immersion steps by deionized water and after the last step by water provided from the filter system. Then the equipment was stored in an oven at 60 °C until used. Immediately before use, the bottles were flushed by acetone, dichloromethane, and the solvent used for preparing the samples. Monolayers. In most cases, the adsorption of the organothiolate monolayers was carried out by immersing the substrates for typically 24 h into ethanolic solutions of the thiol compounds with or without additional acids as specified in the description of the individual preparation. In a few cases, tetrahydrofuran (THF) was used as a solvent. After removal from the solution, the samples were rinsed thoroughly with ethanol and dried in a flow of nitrogen gas. In the case of MMTA, the solubility in (35) Arnold, R. Struktur und Ordnung selbstordnender Monolagen aliphatischer und aromatischer Thiole auf Goldoberfla¨chen. Dissertation, Ruhr-Universta¨t Bochum, Bochum, Germany, 2001. (36) Arnold, R.; Terfort, A.; Wo¨ll, C. Langmuir 2001, 17, 4980-4989; see footnote 25. (37) Commercial software: Win-IR-Pro; Bio-Rad Laboratories: Cambridge, 1998. (38) Terfort, A. Manuscript to be published.

Arnold et al. ethanol and other solvents is very small.39 To obtain an unsaturated solution with a reproducible concentration, an amount of MMTA corresponding to a 0.1 mM solution was weighed into the solvent and sonicated for 5 min at room temperature. Then the turbid solution was filtered. In a last step, the solution was diluted by addition of 10% of the pure solvent. Pellets. MHDA pellets were produced by pregrinding40 4 mg of MHDA into 400 mg of KBr for 2 min using a hand mortar in order to obtain a homogeneous dispersion. Subsequently, smaller portions of 0.3 g were further ground in a vibrating mill for times between 1 and 10 min. The milled mixtures were pressed to pellets in the evacuated die for 3 min and installed in the IR spectrometer without further delay. The concentration of MHDA was chosen so as to obtain a maximum peak height of 0.5 AU41 in order to get a minimum of measuring inaccuracy. The MMTA pellet was made by grinding approximately 0.1 mg of pterphenylthiol (TPT) into 0.3 g of KBr using the vibrating mill. The substrates were prepared by thermal evaporation of 5 nm of Ti onto Si(100) prior to the deposition of 100 nm of Au using a Leybold Univac 300 system at a pressure of 10-7 mbar. Previous studies have shown that the Au films deposited are predominantly (111) textured.42 For all substances used in the present work, a possible dependence of IR band intensities and position on grinding time was studied, since there are several cases where these dependencies are known to be strong.43

Results Pellets. In Figure 3A, we present the IR spectrum of a MHDA/KBr pellet which was prepared by milling for 10 min. The appearance of strong CH2 bands is typical for all MHDA spectra in pellets. The bands are assigned in Table 1. The shape of the CH2 asym band is asymmetric because of the blue-shifted R- and ω-methylene units adjacent to the thiol and the carboxylic acid group.44-46 Pronounced variations in the IR spectra were observed for milling times below 4 min. This is demonstrated in Figure 3B where the peak heights of the CH2 asym vibration of the IR spectra taken from MHDA/KBr pellets are shown as a function of the grinding time. The peak heights were normalized to the MHDA content of each pellet to obtain comparable intensities. This investigation on the importance of the grinding effects has been carried out in order to obtain reliable values for the absorbance and to minimize contributions from optical scattering effects in the pellet.43 As can be seen in Figure 3, the intensities depend on the milling time43 and converge to a saturation value for longer milling times.47 An identical series taken for pellets of MHDA in potassium chloride (not shown) yields the same result. To calculate the spectrum of the complex part k of the refractive index of the CH stretching vibration region, (39) The saturation concentration of MMTA in ethanol was estimated by first preparing a 0.1 mM solution and then diluting it until any turbidity disappeared. A ultrasonic bath was used to accelerate the solution process. The values determined this way amount to about 0.003 mM for ethanol and to about 0.05 mM for THF. Note that these values are significantly lower than those provided in earlier work (ref 34). (40) An ultrahigh precision balance (Sartorius 2104) with a resolution of 1 µg and a reproducibility of better than 5 µg was used. (41) Absorbance units: 0 AU ) 100% transmission, 1 AU ) 10% transmission, 2 AU ) 1% transmission (logarithm scale). (42) Hallmark, V. M.; Chiang, S.; Rabolt, J. F.; Swalen, J. D.; Wilson, R. J. Phys. Rev. Lett. 1987, 59, 2879-2882. (43) Arnold, R.; Terfort, A.; Wo¨ll, C. Langmuir 2001, 17, 4980-4989. (44) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (45) Tao, Y.-T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724-6735. (46) Parikh, A. N.; Gillmor, S. D.; Beers, J. D.; Beardmore, K. M.; Cutts, R. W.; Swanson, B. I. J. Phys. Chem. B 1999, 103, 2850-2861. (47) The saturation value was calculated by averaging the normalized peak heights for all pellets milled 4 min and longer, and the result is 3.76 ( 0.08 AU/mg for the CH2 asym. vibration.


Langmuir, Vol. 18, No. 10, 2002 3983

Table 1. Assignment of the IR Bands of MDHA Based on the Positions Given in the Literaturea and Fit Results of the k-Spectrum (Figure 3C)b maximum [cm-1] band OH str ω-CH2 asym R-CH2 asym CH2 asym ω-CH2 sym FR R-CH2 sym FR CH2 sym FR ω-CH2 sym R-CH2 sym CH2 sym CdO str, monomer CdO str, acycl dimer CdO str cycl dimer liq water COO- asym COO- sym CH2 def C-O-H def ω-CH2 def a

literature

pellet

fit of k-spectrum of the MHDA pellet (Figure 3C) SAM

position [cm-1] fwhh [cm-1] amplitude intensity

∼3050 ∼3042 ∼2930 2930 ∼2925 2927 2919 2918 2919-2928 ∼2900 2901 ∼2890 2893 ∼2890 2883 ∼2860 2869 2862 2856 2850 2851 2850-2854 1741, 1767 1743 1740-1744 1718, 1745 1716 1715-1718 1699-1728 1699 ∼1630 1628 ∼1630 1550-1555 1568-1579 1423-1430 1437-1466 1473 1473 ∼1473 1463 1463 ∼1466 1430 1430 ∼1430 1411 1410 ∼1410

2930.2 2927.1 2918.5 2901.1 2892.8 2883.4 2869.4 2856.3 2850.7

29.8 18.0 9.0 7.7 7.5 11.1 11.2 9.9 8.0

0.016 0.044 0.231 0.012 0.013 0.009 0.010 0.025 0.136

0.597 1.015 3.703 0.107 0.112 0.108 0.143 0.337 1.409

remarks adj COOH adj SH adj COOH adj SH adj COOH adj SH 2nd literature value from formic acid in Ar matrix incorporated

References 24, 29, 45, 46, 49, and 50. b The fit was done using Pearson-VII functions.

Figure 2. Variation of the carboxylic acid carbonyl band frequency with different types of interactions. See the text for further details.

which is necessary to determine the orientation,43,48 all IR spectra from the pellets prepared by milling for more than 3 min were averaged. The resulting k-spectrum is shown in Figure 3C (CH stretch region). The result of the corresponding fit is provided in Table 1. In Figure 3D, we provide the corresponding averaged (as described above) and normalized IR spectrum of the carbonyl stretch region from the series of pellets. The spectrum shown as the solid line (equivalent to the data point marked by the star in part B) was obtained from the pellet prepared by a milling time of 3 min. This spectrum shows remarkable differences (see discussion) as compared to the data for the other pellets and allows identification of the carbonyl band positions of the different hydrogenbonded MHDA species (Figure 2). The peak assignments were taken from the literature24,29,45,46,49,50 and are provided in Table 1 together with the fit result of the k-spectrum. For comparison, in Figure 4 we present normalized IR spectra recorded for MHDA dissolved in CCl4. Spectrum (48) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-944. (49) Gantenberg, M.; Halupka, M.; Sander, W. Chem.sEur. J. 2000, 6, 1865-1869. (50) Halupka, M. Entwicklung neuer Techniken zur Untersuchung reaktiver Moleku¨le. Dissertation, Ruhr-Universita¨t Bochum, Bochum, Germany, 2000.

Figure 3. IR bulk spectra recorded for MHDA in KBr pellets. (A) shows the spectrum obtained for a grinding time of 10 min (see text). (B) depicts the height of the CH2 asym band of a series of pellets milled for different times (see ref 43) and normalized to the substance content. The line is provided to guide the eye. Note the data point marked by a star (see text). (C) presents the isotropic k-spectrum (imaginary part k of the complex index of refraction) of the bulk polycrystalline MHDA as obtained from the averaged pellet spectrum (averaged for all pellets with milling times longer than 3 min). The fit of the spectrum was carried out using nine Pearson-VII functions. The vibrations and peak positions are assigned in Table 1. (D) shows the carbonyl region of the averaged spectrum (dashed line) and of the spectrum of a pellet obtained for a milling time of 3 min (solid line).

A was recorded for a solution of ∼90% saturation concentration, whereas for spectrum E the solution was further diluted to 20%. Spectrum B was taken from a solution of TFA in CCl4 (about 1 mM), and spectrum C shows a mixture of MHDA and TFA with an excess (approximately 5 times) of TFA. In the MHDA spectrum on the left (part 1) of Figure 4, two different carbonyl stretch frequencies at 1759 and 1711 cm-1 are seen. In accordance with previous work,50 the vibration at 1759 cm-1 is assigned to the carbonyl

3984


Arnold et al.

Figure 4. IR bulk spectra of MHDA and TFA in CCl4 solution. (1) shows the IR carbonyl bands of MHDA (A), TFA (B), and a mixture of MHDA and TFA with an excess of TFA (C). (2) demonstrates the different IR band positions of the CH2 sym and asym vibrations of the polymethylene chain of MHDA in solution (A) as well as in a KBr pellet (D). (3) reveals the dependence of the relative intensities of the two IR carbonyl bands of MHDA on the concentration of the MHDA in the CCl4 solution. Spectrum (A) was taken near saturation concentration of the solution, whereas for spectrum (E) the concentration of the solution was 5 times smaller.

stretch vibration in a single MHDA molecule, whereas the vibration at 1711 cm-1 is assigned to the carbonyl stretch vibration in a MHDA cyclic dimer, where the two carboxylic acids form hydrogen bonds (Figure 2, conformation 3). The TFA spectrum also shown in the left of Figure 4 again reveals two carbonyl bands at 1810 and 1779 cm-1 (B), which in an analogous way are assigned to the monomer (1810 cm-1) and the cyclic dimer (1779 cm-1). The spectrum of the mixture of both substances (C) shows only a very weak intensity of the MHDA cyclic dimer peak (1711 cm-1). The peak attributed to the TFA monomer (1810 cm-1) is somewhat stronger. The two most intense bands at 1759 and 1790 cm-1 are attributed to a mixture of TFA-MHDA cyclic dimers and TFA cyclic dimers as well as MHDA monomers. Part 2 of Figure 4 compares the positions of the symmetric and the asymmetric CH2 vibrations of MHDA in the pellet (D) and in the solution (A). For MHDA in solution, the bands are found to be blue-shifted by 9.6 cm-1 relative to the crystalline MHDA. The spectrum shown in part 3 of Figure 4 reveals that the relative intensities of the carbonyl bands related to the monomer to the cyclic dimer depend on the concentration of the solution. As expected, the vibration at 1759 cm-1 assigned to the monomer is stronger in the diluted than in the more concentrated solution. (Note that both spectra are normalized to the intensity of the band of the cyclic dimer.) Figure 5 shows the IR spectra recorded for several MHDA/Au SAMs which were prepared using different procedures. All SAMs were made from ethanolic solutions containing lower (0.02 mM, samples 1 and 2) and higher (0.5 mM, samples 3 and 4) concentrations of MHDA. To the solutions used for samples 2 and 4, a small amount

(0.5%, 5%) of an organic acid (TFA, AA) was added. The frequencies of all relevant bands are provided in Table 2. Of special interest is the band at ∼1450 cm-1, which was not observed either in the pellets or in solution. This band is assigned to the C-O symmetric stretch vibration in a carboxylate group (Figure 2, conformation 4), which most likely results from a small amount of salt formation. This assignment is corroborated by XP spectra (see Figure 6), which reveal the presence of Na+ cations within the SAM (O/Na ∼ 2-4). Sample 5 was prepared as sample 2, but in this case an inorganic acid (1% HCl) was used to acidify the solution. Figure 7 shows a series of IR spectra recorded for MHDA sample 7. Going from bottom to top, the first spectrum was taken for a freshly prepared SAM. Before recording the other spectra, the sample was immersed in different solutions for 24 h. Note the appearance of the carboxylate bands after immersing into pure ethanol. The intensity of these carboxylate bands did not increase after immersion of the sample into a solution of NaOH in ethanol.51 Note also that immersion of this sample into a 10% solution of AA in ethanol restored the spectrum to its original form. Figure 8 provides a comparison of the CH2 bands as recorded for a MHDA SAM (sample 7) and a HDT SAM (sample 6). The CH2 bands at 2919 and 2850 cm-1 exhibit comparable intensities, and their positions are found to differ only very slightly (0.7 cm-1 for CH2 asym). The two bands at 2965 and 2878 cm-1 seen for HDT but not for MHDA correspond to the symmetric and asymmetric CH3 vibrations, respectively, which are absent for MHDA. (51) NaOH is not soluble in pure ethanol, but ethanol regularly contains small amounts of water which are sufficient to solve small amounts of NaOH.


Langmuir, Vol. 18, No. 10, 2002 3985

Figure 7. IRRAS spectra of a MHDA SAM (sample 7) demonstrating the deprotonation and reprotonation of the carboxylic acid groups. The SAM was prepared from an acidified solution (bottom) and then immersed in pure ethanol (second), in a saturated solution of NaOH in ethanol51 (third), and finally in ethanol containing 10% of acetic acid. Table 2. Positions of the IR Bands Presented in Figure 5 maximum [cm-1] band CH2 asym CH2 sym CdO str, monomer CdO str, acyclic dimer COO-/CH2 def

sample 1 sample 2 sample 3 sample 4 sample 5 2920.7 2850.8

1443

2919.9 2851.6 1744

2927.9 2854.3 1727

2926.0 2854.3 1744

1718

1727

1717

1466

1466

2918.5 2850.5 1740

1466

Figure 5. IRRAS spectra of MHDA/Au SAMs prepared by immersion in different ethanolic solutions. Details are provided in the text; the band positions are listed in Table 2.

Figure 8. IRRAS spectra recorded for a HDT/Au SAM (sample 6) and a MHDA/Au SAM (sample 7).

Figure 6. XP spectrum of a MHDA/Au SAM (sample 1) revealing the presence of Na+ ions.

In Figure 9, we present the IR spectrum recorded for a MMTA/KBr pellet. In this case, no variations of the band intensities as a function of milling time could be detected. The band assignments (except for the carboxylate band) are provided in Table 3 using the Wilson notation of normal modes of benzene.52,53 The presence of bands at 1748, 1725, and 1710 cm-1 indicates that the three differently bound monomers/dimers (see Figure 2) must be present in the pellet. Some of the bands were fitted by

Gaussian functions; the results of the fit are shown in the lower part of the figure and are listed in Table 4.54 In Figure 10, we display the IRRAS spectra recorded for two differently prepared MMTA SAMs. Both samples were prepared from a solution of MMTA in ethanol (sample 8) and THF (sample 9). In both cases, the concentration corresponded to 90% of the saturation value.39 The differences between these spectra and the pellet spectra are attributed to the so-called surface selection rule in IR spectroscopy, which implies that only bands with a transition dipole moment orientated normal to the surface can be seen.43 The intensities of the IR bands recorded for sample 9 are about twice as high as those measured for sample 8. The carbonyl vibration of sample 8 is located (52) Wilson, G.; Bloor, J. E. Phys. Rev. 1934, 45, 706-714. (53) Varsanyi, G. Vibrational Spectra of Benzene Derivates; Academic Press: New York, 1969. (54) The bands were fitted using a set of more than 30 Pearson-VII functions to take overlapping band intensities into account.

3986


Arnold et al.

and indicate that the Au/S concentration in sample 9 is about twice as high as for sample 8 whereas the C/O and C/S ratios remain approximately constant.

Figure 9. IR bulk spectrum recorded for MMTA in a KBr pellet (top) and a fit of the most relevant bands (bottom). The bands are assigned in Table 3; the results of the fit are provided in Table 4.

at 1748 cm-1, and the corresponding value for sample 9 amounts to 1710 cm-1. Note also the band at 1430 cm-1 which is not seen for the pellets. In accordance with the discussion presented above, this band is assigned to the carboxylate symmetric stretch vibration (Figure 2, conformation 4). Figure 11 shows XP spectra recorded for the two samples discussed in connection with Figure 10. The spectra were fitted with Gaussian functions after subtraction of a Shirley background55 (solid lines). The energy differences for the S 2p3/2 to 2p1/2 were set to 1.18 eV,56 and those for the two different O 1s species (COOH and COOH) were set to 1.5 eV.57 In the fitting process, the intensity ratio of the S 2p3/2 to 2p1/2 peaks was fixed to 2:1 and that for Au 4f7/2 to 4f5/2 was fixed to 5:4. The results of the fits are provided in Table 5. As can be seen, sample 9 shows a significantly stronger intensity of the C 1s peak when compared to sample 8. Both samples reveal the presence of a sulfur species with a S 2p binding energy of ∼162 eV. Sample 9 shows a second sulfur species with a S 2p binding energy of 163.2 eV. We compared these two measurements of the sulfur regions to the sulfur XP spectrum of the TPBMT/Au SAM, sample 10, which is used here as a reference system.58 The width and shape of the S 2p signal in the spectra of samples 9 and 10 are identical, while the width of sample 8 is found to be significantly reduced. The layer thicknesses as presented in Table 5 were calculated from the intensity ratios of the Au 4f to the C 1s excitations and are referenced to those of octadecanethiol.59 All element ratios were obtained from the ratio of the peak intensities, each normalized to its excitation probability.60 In the case of the Au 4f and the S 2p peaks from X-ray photoelectron spectroscopy (XPS), this ratio cannot be related directly to the relative S/Au concentration. But the ratios can be compared between the samples (55) Shirley, D. A. Phys. Rev. B 1972, 5, 4709-4714. (56) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corp., Physical Electronics Division: Eden Prairie, MN, 1992. (57) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 2740-2748. (58) Wehner, B.; Fuxen, C.; Arnold, R.; Azzam, W.; Terfort, A.; Fischer, R. A.; Wo¨ll, C. Manuscript to be published. (59) For photoelectrons with a kinetic energy of 1202 eV (C 1s excited by Al KR), a mean free path of 35 Å was used in the case of the aliphatic octadecanethiol whereas 0.78 × 35 Å was used in the case of the aromatic MMTA. This reduced mean free path reflects the somewhat higher electron absorption/scattering of the aromatic species. For 1400 eV electrons (Au 4f), a mean free path of 45 Å was used in all cases. (60) Scofield, J. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129137.

Quantitative Orientation Determination of Molecular Tilt Angles from IR Data Each molecular vibration is characterized by the corresponding transition dipole moment (TDM). For a given molecular orientation, the TDM can be decomposed into components orientated normal and parallel to the surface. If the two components were both known, the tilt angle of the TDM could be obtained directly. Unfortunately, at a metal surface the field component parallel to the surface of the incident electromagnetic wave (IR light) is almost completely screened by the electrons of the metal substrate. As a consequence, the component of the TDM parallel to the surface does not contribute. Therefore, for a single vibrational mode, the tilt angle can only be determined if the absolute magnitude of the transition dipole moment, |TDM|, is known. Parikh and Allara48 have proposed a method which is based on the determination of the absolute excitation probability (corresponding to |TDM|) of a given mode in the bulk. If this information is available, the molecular tilt angle of a TDM relative to the surface normal and therefore the orientation of the molecule can be calculated from the intensity in the SAM IR spectra. See Arnold et al.43 for details. Since in the case of MHDA all vibrations with a TDM orientated along the chain axis of the molecule are very low in intensity, only the CH2 modes (sym and asym, TDM orientated perpendicular to the alkyl chain) can be used to determine the tilt angle by employing the absolute method of Parikh and Allara.48 For the calculation, the k-spectrum of Figure 3 and the fit results as presented in Table 1 were used. In this analysis, first the spectrum of sample 7 was approximated by the same fit set as used for the k-spectrum to determine the positions of the appropriate IR bands of the SAM. As discussed in the literature,61-63 there is a deshielding of the R-methylene which can cause a change of the band positions.64 Furthermore, the positions of the CH2 bands are sensitive to the order of the SAM as discussed above. Therefore, the positions of the single bands in the k-spectrum were numerically shifted to the corresponding positions of the SAM before performing the calculations. The results of the analysis are presented in Figure 12. The simulation shown as a dashed line was performed using orientation parameters of Θ ) 23° (tilt angle of the carbon chain to the surface normal) and Ψ ) 47° (twist angle of the chain around the chain axis, see ref 43 for the definition of the angles). As can be seen, the band heights of the calculated spectrum agree very well with the band heights of the spectrum measured for sample 7. The bandwidths of the calculated spectrum, however, are significantly smaller. In a different calculation, the band areas instead of the band heights were fitted, yielding at orientation parameters of Θ ) 24° and Ψ ) 47°. Considering the day by day variations of the preparation of the SAMs and the error related to fitting the k-spectrum, the error bars of both Θ and Ψ are (2°. The values of Θ ) (24 ( 2)° and Ψ ) (47 ( 2)° are slightly below the values reported in the literature24,27,29 previously. (61) Badia, A.; Demers, L.; Dickinson, L.; Lennox, F. G. M. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104-11105. (62) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cucia, L.; Reven, L. Langmuir 1996, 12, 1262-1269. (63) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643-10646. (64) A possible change of the excitation probabilities, which may also be caused by the deshielding, will be neglected.


Langmuir, Vol. 18, No. 10, 2002 3987

Table 3. Assignment of the IR Bands of MMTA Based on the Positions Given in the Literature for p-Terphenylthiola and MHDA (See Above) maximum [cm-1]

assignment TDMb

band CdO str, monomer CdO str, acyclic dimer CdO str, cyclic dimer benzene ring 1 COO- asym benzene ring 2 COO- sym C-O-H def C-O str benzene ring 3 benzene ring 4 benzene ring 5 O-H‚‚‚H def benzene ring 6

ip perp ip par ip par op op

benzene ring 7

op

Wilsonc

ip par ip perp ip par ip par op

8a

literature 1741, 1767 1718, 1745 1699, 1728 1593 1550-1555

19a

15 18a 18a 10a 11 4

pellet

SAM

1718 1688 1608 1492

1423-1430 1395-1440 1280-1325 1117 1014 1002 875-960 825 828 723

remarks

1748 ∼1725 1710 1610 1491 1424

∼1400 ∼1280 1112 1018 1003 943 823

1018 1003 821

different rings different rings not resolved, but both op

742

a

Reference 43. b The “TDM” column contains the direction of the transition dipole moment of each vibration: bands labeled “ip par” have their TDM orientated parallel to the phenyl ring plane and along the chain axis, the label “ip perp” denotes an orientation in the phenyl ring plane but perpendicular to the chain axis, and “op” vibrations are orientated perpendicular to the plane of the phenyl rings of the planar MMTA molecule. cThe “Wilson” row assigns the bands according to Wilson’s notation of the benzene normal vibrations (refs 52 and 53). Table 4. Fit Results for the IR Spectra of the MMTA Pellet and for the MMTA SAMs, Samples 8 and 9a pellet band 10a, 11, 17b op 18a (1) ip par 18a (2) ip par 19a 8a ip par CdO str, cyclic dimer CdO str, acycl dimer CdO str, monomer a

pos fwhh [cm-1] [cm-1] 823 1003 1018 1492 1609 1686 1701 1720

10.1 4.1 5.1 7.4 8.5 16.8 16.8 16.8

amplb [AU] 0.191 0.041 0.015 0.042 0.098 0.167 0.136 0.120

SAM sample 8 intensityc

pos fwhh [AU cm-1] [cm-1] [cm-1] 3.313 0.319 0.091 0.349 0.897 3.007 2.442 2.161

818 1003 1018 1491 1608 1709 1732 1749

12.1 4.6 5.2 7.1 12.2 18.7 18.7

ampl [AU] 0.00052 0.00067 0.00028 0.00076 0.00098 0.00000 0.00045 0.00085

SAM sample 9 intensity pos fwhh [AU cm-1] [cm-1] [cm-1] 0.00669 0.00324 0.00153 0.00571 0.01263 0.00000 0.00894 0.01701

819 1003 1018 1491 1609 1709 1734 1742

13.1 5.4 6.0 8.3 12.6 17.1 8.6 10.6

ampl [AU]

intensity [AU cm-1]

0.00063 0.00109 0.00046 0.00086 0.00202 0.00462 0.00019 0.00027

0.00878 0.00632 0.00297 0.00760 0.02716 0.09920 0.00169 0.00306

The fits were done using Pearson-VII functions. b Height of band. c Area of band.

Figure 10. IRRAS spectra recorded for MMTA/Au SAMs prepared from different solutions. See the text for details. The bands are assigned in Table 3.

In the case of MMTA, where molecular vibrations with TDMs orientated both along and perpendicular to the molecular axis with sufficient intensities are present in the experimental data, the so-called relative method65 can be used to determine the molecular orientation. The intensities of these bands were determined using a fitting scheme (see Table 4 for the results) and then allowed an orientation determination to be performed; see Arnold et (65) Debe, M. K. J. Appl. Phys. 1984, 55, 3354-3366.

Figure 11. XP spectra recorded for the MMTA samples 8 and 9. The experimental data are shown as dots. The corresponding fits are shown as solid lines, as well as the Shirley background. For comparison, the S 2p region of a p-terphenyl-bis(methanethiol) SAM (Au-S-CH2-Ph3-CH2-SH, sample 10) is shown as a dashed line.

al.35,43 for details. The analysis yields a tilt angle of Θ ) (28 ( 4)° for sample 8 and of Θ ) (24 ( 4)° for sample 9. Discussion Figure 3D shows the carbonyl region of the averaged IR spectra recorded for a MHDA pellet obtained using a

3988


Arnold et al.

Table 5. Fit Results (Positions and Intensities) for the XP Spectra Recorded from the MMTA SAMsa position [eV]

intensity [cps eV]

element ratios

sample

Au 4f7/2

C 1s

S 2p3/2

O 1s

Au 4f7/2

C 1s

S 2p

O 1s

layer thickness [Å]

C/O

C/S

Au/S

8 9

84.0 84.0

284.5 284.7

162.2 161.8/163.2

532.8 532.5

70240 69001

7958 14370

499 267/637

3068 3805

19.6 32.9

8 11

21 20

32 17

a Gaussian functions were used for the fit. The intensities of the O and S excitations are the sum of all approximated bands; the position of the O 1s excitation corresponds to the average of the two bands of each sample. The results calculated from the intensities are presented in the right part of the table. The estimated overall error amounts to about (10%.

Figure 12. Measured and calculated IRRAS spectra of MHDA/ Au. The calculations were performed on the basis of the averaged bulk spectrum obtained for the pellets using the k-spectrum of Figure 3 for a tilt angle Θ ) 23° relative to the surface normal and a twist angle Ψ ) 47° around the molecular axis (dashed line). Results for Θ ) 24° and Ψ ) 47° (dotted line) are also shown.

milling time of 3 min. The spectral feature at 1699 cm-1 is assigned to the carbonyl CdO vibration in a cyclic carboxylic acid dimer. Hayashi et al.66 attributed the rather large width of this band to the overlap of two bands located at 1710 and 1680 cm-1 which are assigned to the cis/trans conformation of the CdO group relative to the n-alkyl chain. This explanation is supported by the observation of Higashi et al.67 who demonstrated that the shape of this band depends on the preparation (crystal bulk, pellet, thick film) of the sample. Occasionally, spectra of pellets produced from MHDA revealed significant differences when compared to the spectra discussed above. One example is the spectrum obtained using a milling time of 3 min as displayed as a solid line in Figure 3D (as well as a star in Figure 3B). In this spectrum, three different bands (∼1743, 1716, and ∼1699 cm-1) are observed in the carbonyl stretch region. This sample was prepared in the same way as the other samples; we tentatively attribute this observation to the formation of a different crystal phase of MHDA. This hypothesis is based on the analogy to hexadecanoic acid, for which six different crystal phases are known.68,69 Not all of these phases have yet been studied using IR spectroscopy, but the positions of the CdO bands differ significantly between the different phases. One phase shows CdO bands at 1672 and 1718 cm-1, the intensities of which depend on the polarization of the incident IR light. (66) Hayashi, S.; Umemura, J. J. Chem. Phys. 1975, 63, 1732-1740. (67) Higashi, A.; Czarnecki, M. A.; Ozaki, Y. Thin Solid Films 1993, 230, 203-208. (68) Kobayashi, T.; Kobayashi, M.; Tadokoro, H. Mol. Cryst. Liq. Cryst. 1984, 104, 193-206. (69) Kaneko, F.; Shirai, O.; Miyamoto, H.; Kobayashi, M.; Suzuki, M. J. Phys. Chem. 1994, 98, 2185-2191.

Previous work has demonstrated that the carbonyl stretch frequency in carboxylic acids is very sensitive to the type of hydrogen bonds formed between the individual molecules. Gantenberg et al.49 and Halupka et al.50 have studied formic acid (HCOOH) in an argon matrix and found several different bands. A band located at 1767 cm-1 was assigned to the monomer, a second band at 1745 cm-1 to acyclic dimers, and a third band at 1728 cm-1 to cyclic dimers (see Figure 2). On the basis of these results, it is straightforward to assign the carbonyl bands observed for the MDHA samples (see Figure 3D) to a monomer (1743 cm-1), an acyclic dimer (1718 cm-1), and a cyclic dimer (1699 cm-1). The band at ∼1630 cm-1 is attributed to the incorporation of a small amount of water into the pellet. In contrast to the spectra measured for the MHDA/KBr pellets, the spectrum obtained for MHDA in solution (Figure 4, part 1, spectrum A) reveals only two carbonyl bands located at 1759 and 1711 cm-1. This observation is consistent with the presence of monomers and cyclic dimers and the absence of acyclic dimers. While AA carbonyl bands (1758 and 1718 cm-1 70) are almost identical in frequency to those observed for MHDA, the carbonyl bands of TFA (spectrum B) are shifted by more than 50 cm-1 toward higher frequencies and allow one to distinguish between TFA and MHDA. As indicated in spectrum C, TFA (as well as AA, not shown) can be used to reduce the concentration of MHDA dimers as evidenced by the very weak band of the cyclic dimer (1711 cm-1).71 This observation is attributed to the formation of mixed dimers. Also, the variation of the intensities of the two carbonyl bands as shown in part 3 of Figure 4 fully supports the assignment of the two bands to MHDA monomers and dimers. Part 2 of Figure 4 compares the positions of the MHDA CH2 sym and asym stretching vibrations. In spectrum D, obtained from MHDA in a KBr pellet, the two CH2 bands are located at 2918.3 and 2850.8 cm-1. For MHDA in solution, the bands are shifted to 2927.9 and 2855.6 cm-1. In previous work, it has been demonstrated that this blue shift of the methylene bands is related to the crystallinity of the sample and can be used to infer on the amount of gauche defects.45,46,72-75 For the present case, the positions of the bands indicate a high crystallinity for the MHDA pellets but the presence of a significant amount of gauche defects for MHDA in solution. (70) DMS Raman/IR Atlas of Organic Compounds; Schrader, B., Meier, W., Eds.; VCH: Weinheim, 1977. (71) The band at 1810 cm-1 is related to the TFA monomer, and the band at 1790 cm-1 is related to an overlap of the band of the TFA cyclic dimer and the two bands of the TFA-MHDA cyclic dimer. These bands are not relevant for the present investigations. (72) Snyder, R. G.; Strauss, H. L.; Ellinger, C. A. J. Phys. Chem. 1982, 86. (73) Maroncelli, M.; Qi., S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6237-6247. (74) Touwslager, F. J.; Sondag, A. H. M. Langmuir 1994, 10, 10281033. (75) Smith, E. L.; Porter, M. D. J. Chem. Phys. 1993, 97, 8032-8038.


Crystallinity of the MHDA SAMs. Also in the case of the SAMs, the position of the CH2 bands can be used to obtain information on the degree of crystallinity. The samples prepared from solutions of high concentrations (0.5 mM) exhibit a CH2 asym vibration at 2927.9 and 2926.0 cm-1, which differs strongly from the position seen for the corresponding vibration in the pellet (2918.3 cm-1) but which is very similar to the frequency seen for MHDA in solution (2729.9 cm-1). This comparison indicates the presence of a high degree of disorder in MHDA SAMs prepared from highly concentrated solutions. This result is in agreement with the findings of Dannenberger et al. and Himmel et al.30,31,33 We speculate that the formation of the disordered MHDA SAMs when prepared from highly concentrated solutions is related to the higher number of dimeric MHDA molecules present in solution, which is evident from the IR data recorded for a concentrated solution (Figure 1C). When the MHDA SAMs are prepared from solutions with low concentrations (0.02 mM), the positions of the CH2 asym band (2920.7 and 2919.9 cm-1) agree very well with the corresponding frequencies seen for the MHDA pellet. This strongly indicates a high degree of crystallinity and, accordingly, a high degree of orientation in these SAMs. The remaining difference of 2.4 and 1.6 cm-1 is attributed to a small number of gauche defects which are mostly located at the end of the molecules of the SAM.76 This result for the MHDA SAMs prepared from solutions with low MHDA concentrations is consistent with the earlier results presented by Nuzzo et al.24 and Smith et al.29 The data reported by Himmel et al.32 show CH2 band positions indicating a degree of order (and therefore crystallinity) between the two cases discussed above. This is consistent with the fact that these data have been obtained from solutions with an intermediate concentration (0.1 mM), thus supporting the inverse correlation between crystallinity and concentration proposed above. XP Data for MHDA. The XP spectra recorded immediately after the IR measurements (not shown) for the first three MHDA samples reveal the presence of gold thiolates (S 2p peak located at 162 eV). The thicknesses of the SAMs were determined from the Au 4f and C 1s excitations relative to those of an octadecanethiolate SAM and amount to 25.1 Å for the sample prepared from the low-concentration nonacidified solution (sample 1), 21.8 Å for the sample prepared from the low-concentration acidified solution (sample 2), and 31.2 Å for the sample prepared from the high-concentration nonacidified solution (sample 3). The thickness of the organothiolate adlayer of the sample from the low-concentration acidified solution (sample 2) agrees well with the thickness of a HDT monolayer of 22 Å.77 The sample from the high-concentration nonacidified solution (sample 3) shows a width of the S 2p doublet which is 40% larger than for the corresponding signals recorded for samples immersed in the low-concentration solution. This observation reveals the presence of a second sulfur species in this film. Since the sample prepared from the highly concentrated solution exhibits a very low degree of crystallinity, this observation is explained by a large number of MHDA dimers present in the film: some of the dimers are not bound with both sulfur atoms to the Au substrate, thus giving rise to two (76) Laibinis, P. E.; W. H. G. M; Allara, D. L.; T. Y.-T., Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (77) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

Langmuir, Vol. 18, No. 10, 2002 3989

signals in the S2p XPS data, a thiolate sulfur species at 162 eV78-80 and a thiol sulfur species at 163.1 eV.56 In contrast, the samples obtained from the low-concentration solutions reveal a high degree of crystallinity, which is consistent with the XPS results revealing the presence of one sulfur species (thiolate) only. In other words, all MDHA molecules are directly bound to the Au substrate through thiolate bonds. Furthermore, the rather large thickness of the film prepared from the highly concentrated solution indicates the formation of an incomplete second layer. We propose that the second-layer molecules are bound to the first layer by hydrogen bonds, in agreement with the conclusions drawn from the corresponding IR data. The C/O ratios of the samples were calculated from the C 1s and O 1s band intensities. Values of (10 ( 2):1 were obtained for the samples prepared in the low-concentration acidified solution. The value obtained for the highly concentrated nonacidified solution agrees within the error bars with the stoichiometric value (8:1). In contrast, the C/O ratio of (5 ( 2):1 for the sample prepared from the low-concentration nonacidified solution is significantly smaller. The most likely explanation for the deviation seems to be the presence of contaminations bound to the surface of the MHDA SAM. From the fact that the corresponding IR data show an additional band at 1214 cm-1 (C-O stretch vibration) and an increased intensity at ∼2960 cm-1 (CH3 asym), we propose that oxygen-containing organic contaminations from the environment were adsorbed. Note that for this sample the XPS data indicated the presence of Na cations (see below), which may stabilize the adsorbed contaminations. Deprotonation and Reprotonation of the MHDA SAMs. All IR spectra recorded for MHDA SAMs which were prepared by immersion in nonacidified ethanolic solutions of MHDA show only very weak carbonyl bands at ∼1700 cm-1. This is a surprising observation, since it directly implies that fully protonated COOH units are largely absent in these monomolecular films. The presence of a strong carboxylate band at ∼1450 cm-1 (COO- sym) reveals that the MHDA COOH units were transformed into the corresponding carboxylates, that is, they were deprotonated, either before or during the process of SAM formation. Since the XPS data demonstrate the presence of Na cations on these films, we propose that Na contaminations present in the ethanol adsorb to the MHDA SAM surface and form sodium carboxylates. The most likely source of Na ions seems to be the wall of the glass containers used to store the ethanol and to prepare the SAMs. Attempts to reduce the amount of Na+ contaminations present on the MHDA SAMs by using ultrapure ethanol purchased and subsequently stored in plastic containers and Teflon bottles resulted in a reduction but not in a complete suppression of the carboxylate signal. A complete removal of Na could not be achieved.81 The upper limit of Na concentrations present in the ethanol used (reagent grade) would be consistent with this hypothesis, since only about 1 × 1014 Na+ ions are needed for formation of a complete monolayer. Further, we note that the crystallinity of the samples which have been prepared from MHDA/ethanol solutions is inferior (78) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092-2096. (79) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408-2415. (80) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wuhn, M.; Wo¨ll, C.; Helmchen, G. Langmuir 2001, 17, 1582-1593. (81) The amount of cations required to completely deprotonate the SAM is only about 50 nmol per cm2 of the SAM surface. This amount is clearly below the maximum amount of metal ion content as provided by the supplier (typical value of 0.02 ppm per species of metal ions).

3990


to that of the other samples (as judged from the position of the CH2 asym vibration, see above) made from acidified solutions (see below) suggesting that the formation of sodium carboxylates is accompanied by a small increase of disorder within the MHDA SAMs. The deprotonation resulting from the carboxylate formation can be prevented by acidifying the MHDA/ ethanol by adding a few percent of an organic acid like AA or TFA. The samples that were immersed in such a solution where 0.5% TFA or 5% AA was added show no carboxylate bands at 1450 cm-1. The band at 1466 cm-1 visible in the IR spectrum recorded for the sample immersed in the higher concentrated acidified solution (sample 4, Figure 5) is assigned to the CH2 deformation vibration and does not indicate the presence of carboxylates. XPS measurements revealed a concentration of Na ions below the detection limit (ratio of Na to MHDA of less than 5%) for all samples which we prepared from acidified solutions. These results suggest that the presence of small amounts of an organic acid in solution effectively excludes the formation of sodium carboxylates at the surface of the MHDA SAMs. An interesting question is whether the deprotonation and reprotonation is possible on an already formed, protonated SAM. Indeed, the IR data shown in Figure 7 reveal the presence of carboxylates, indicating that the deprotonation can be carried out by bringing fully protonated MHDA SAMs into contact with Na cations.51 It was also found that these samples could be reprotonated by immersion into acidified ethanol (topmost spectrum). This reprotonation could also be achieved for MHDA SAMs prepared from nonacidified solutions. With regard to reprotonation of the MHDA SAMs, no differences between AA or TFA were observed as long as the concentration of the acids was kept below 10%. When using higher concentrated solutions, however, significant differences were noted. The differences are demonstrated best by using the following procedure. First, highly orientated, protonated SAMs prepared as described above were immersed into pure AA or TFA for 24 h. IR spectra recorded after removal from the acid (data not shown, see ref 35) reveal strongly blue-shifted CH2 asym bands for both samples, revealing a significant reduction of crystallinity. This observation thus indicates the presence of a high degree of disorder. After deprotonating the SAMs by immersion into a solution of NaOH in ethanol and reprotonating by immersion into a solution of 5% of AA in ethanol, the CH2 vibrations shifted back to the position of those in the highly oriented SAM in the case of the sample first immersed into pure AA. In contrast, for the sample immersed into the pure TFA, the CH2 remained unshifted at the positions revealing a rather low crystallinity, indicating that the process was not fully reversible and a significant amount of disorder remained. At the same time, the intensity of the CH2 vibration was found to be reduced compared to that seen in the spectra before the process. In addition, a new band at 2960 cm-1 indicates the presence of methyl (CH3) groups. From these observations, we conclude that MHDA SAMs are not stable with respect to concentrated solutions of TFA and are chemically modified upon prolonged exposure to the acid. The Influence of Inorganic Acids. In a different set of experiments, we have investigated whether the reprotonation of the MHDA COOH groups could also be achieved by an inorganic acid. Whereas the IR spectra of a sample immersed into an ethanolic MHDA solution containing 1% HCl (see Figure 5) at first sight seem to indicate the presence of a highly oriented, fully protonated MHDA SAM (as judged from the absence of carboxylate

Arnold et al.

bands and the position of the carbonyl band indicating monomers) with a high degree of crystallinity (as judged from the positions of the CH2 asym band, see above), a closer inspection of the IR data reveals the presence of new bands (1200-1400 cm-1), which strongly indicate the presence of ester groups. Since it is known that inorganic acids such as HCl or H2SO4 catalyze the formation of esters from organic acids and ethanol (esterification of fatty acids82), we propose that in the presence of HCl in the solution the carboxylic end groups of the solved MHDA react with ethanol to form mercaptohexadecanoic acid ethyl ester. Subsequent experiments revealed that the esterification also takes place after the formation of SAMs. In these experiments, a highly oriented MHDA SAM was immersed into a 5% solution of HCl in ethanol for 24 h. The subsequently recorded IR spectrum (not shown) shows the same bands as discussed above.83 Determination of the Orientation of MHDA SAMs. The highest degree of crystallinity (as judged from the position of the CH2 asym band, see above) was observed for SAMs prepared from a 0.02 mM ethanolic solution acidified with 10% AA. A typical IR spectrum recorded for this MHDA SAM is presented in Figure 8 (sample 7). A comparison to corresponding data for a HDT SAM reveals very similar positions and intensities of the CH2 bands. This observation already indicates that the molecular orientation within the MHDA SAM must be very similar to that of the HDT SAM. Indeed, a quantitative determination of the molecular orientation within the MHDA SAM from the intensities of the two CH2 bands using the absolute method revealed an average tilt angle of 24°, very similar to the corresponding value for HDT.76 Formation of the MMTA SAMs. The aromatic bands in the IRRAS data obtained for MMTA SAMs (Figure 10) show very similar positions and intensities as compared to those reported previously for TPT SAMs43 and thus reveal the presence of highly oriented SAMs. The strong vibrations at ∼820 cm-1 which have TDMs oriented out of the plane of the phenyl units are very weak in the IRRAS spectra, whereas all aromatic vibrations with TDMs orientated along the chain axis of the molecule carry significant intensity. A quantitative analysis (see above) yields average tilt angles of Θ ) (24 ( 4)° for sample 8 and of Θ ) (28 ( 4)° for sample 9. Samples which were immersed in a more concentrated solution (∼0.05 mM) of MMTA in THF show the same band positions as samples immersed in more diluted ethanolic solutions, but all bands are found to carry about twice the intensity. This observation corroborates the finding reported earlier34 that under these conditions double layers of MMTA are formed, where the second layer is bound to the first layer by hydrogen bonds (see Figure 13). Similar to the case of MHDA discussed above, also in this case the concentration of the organothiol strongly influences the formation of the SAM. Samples prepared from dilute solutions of MMTA (∼0.003 mM) in ethanol40 show the formation of only a single layer of MHDA. For MMTA SAMs prepared from saturated solutions, the carbonyl bands are observed at 1710 cm-1, shifted by 30 cm-1 with regard to those of the MMTA monolayer (1748 cm-1). Since a frequency of 1710 cm-1 is typical for cyclic dimers of carboxylic acids, this observation provides (82) Morrison, R. T.; Boyd, R. N. Lehrbuch der Organischen Chemie; VCH: Weinheim, 1986. (83) The esterification was also checked by immersing the MHDA SAM into trifluoroethanol using the same procedure. In this case, CF3 vibrations could be detected in the corresponding IR spectrum.


Langmuir, Vol. 18, No. 10, 2002 3991

contrast to the previous work by Himmel et al.,84 where MMTA in pure TFA was used as the solution to obtain the monolayer, we found that on a substrate immersed into a solution of MMTA in pure TFA a SAM will not be formed. Further, MMTA single-layer SAMs immersed into concentrated solutions of TFA for 24 h were characterized by a complete loss of the phenyl IR bands (data not shown, see ref 35). In the corresponding XPS data, the S-signal could be seen, indicating a complete removal of the MMTA molecules except the sulfur from the surfaces after contact with concentrated TFA. We therefore conclude that MMTA monolayers are not chemically stable against concentrated TFA. Preparation Recommendation. Our experiments suggest that the best way to obtain highly oriented MHDA samples with a high degree of crystallinity is to immerse the Au substrates into dilute (0.02 mM) ethanolic solutions of MHDA with an additional content of 10% of acetic acid. The highest quality of MMTA monolayers was obtained when immersing the Au substrates into 0.003 mM solutions of MMTA in ethanol with an additional content of 10% of acetic acid. This MMTA concentration corresponds to about 90% of the saturation concentration. The best way to produce this solution is by sonication of ∼0.1 mM MMTA in ethanol, filtration of the sonicated solution, and further dilution by adding 10% of ethanol. Finally, 10% of AA should be added. We recommend immersion times of 24 h in both cases. Conclusions Figure 13. Schematic model of the MMTA/Au SAM.

independent support for the presence of a second layer of MMTA hydrogen-bonded to the first layer. Also, the XP spectra recorded for both samples (Figure 11, Table 5) confirm the conclusion presented above. For MMTA samples prepared from saturated solutions, the adsorbed molecular films are consistently about twice as thick as those observed for the samples made from diluted solutions. The Au/S ratios also exhibit values about twice as large as those for the same bilayer. In the case of the SAMs made from more concentrated solutions, two different types of S-species characterized by peak positions of 161.8 and 163.2 eV were observed, revealing the presence of two different sulfur species for the bilayer samples but of only one species for the single-layer SAMs. This observation very nicely corroborates the structural model for the bilayer (see Figure 13), which implies the existence of two sulfur species (thiol and thiolate). Deprotonation and Reprotonation of the MMTA SAMs. The IR data recorded for MMTA SAMs prepared from a diluted solution of MMTA in ethanol reveal the presence of protonated MMTA (CdO stretch vibration at 1748 cm-1) as well as deprotonated MMTA (carboxylate band at 1430 cm-1). As in the case of MHDA described above, the carboxylate bands could be removed by either immersion into slightly acidified solutions of MMTA in ethanol or by a subsequent immersion into a dilute solution of AA or TFA in ethanol. In the case of the bilayer MMTA (see above), Na+ cations incorporated into the SAM84 were detected by XPS. Attempts to reprotonate the bilayer samples by immersion into acidified ethanol lead to reprotonation but also to a partial desorption of the second layer (not shown). In (84) Himmel, H.-J.; Terfort, A.; Arnold, R.; Wo¨ll, C. Mater. Sci. Eng., C 1999, 8-9, 431-435.

The structural quality and chemical composition of SAMs produced by immersing Au substrates into solutions of MHDA are found to depend critically on the preparation conditions. Even with the highest purity ethanol commercially available, the SAMs were found to contain a significant amount of deprotonated acid groups yielding a characteristic carboxylate vibration in the IR spectra. Films prepared from diluted solutions were found to exhibit a significantly higher degree of crystallinity and molecular orientation than films produced from concentrated solutions. An orientation determination yields values very similar to those seen for the corresponding unfunctionalized alkanethiol (hexadecanethiol76,77). Apparent inconsistencies between previous reports on the structural quality of MHDA SAMs24,27,29,31-33,84 can thus be explained by different concentrations used in these previous experiments. Films obtained from concentrated solutions reveal a high concentration of carboxylic acid cyclic dimers, indicating that the dimers already present in solution are not dissociated upon adsorption onto the Au substrate. Fully protonated MHDA SAMs could only be obtained either by adsorption from acidified solutions or by a subsequent immersion into a solution of an organic acid in ethanol. The position of the CH2 asym stretch vibration for the MHDA films with the highest crystallinity is located at 2919 cm-1. The carbonyl band of such films is mainly present at (1720 ( 3) cm-1 and is thus compatible with the presence of a mixture of monomers (minority) and acyclic dimers (majority), that is, the presence of single hydrogen bonds between adjacent carboxylic acid groups (see Figure 2, conformation 2b). The amount of monomeric carboxylic acids as characterized by a carbonyl band at (1742 ( 2) cm-1 is below 40%. Preparation of organic surfaces terminated by carboxylic acid functions is somewhat more straightforward from an

3992


organothiol with a more rigid backbone, MMTA. For all concentrations investigated, the formation of highly oriented SAMs was observed; the orientation and structural quality were found to be comparable to those of the corresponding unfunctionalized organothiol, p-terphenylthiol.35,43 Again, fully protonated surfaces could only be obtained either by use of acidified thiol solutions or by subsequent immersion into a solution of AA in ethanol. For immersion into a highly concentrated (90% of saturation) solution of MMTA in ethanol, the formation of highly oriented double layers was observed, with the second layer

Arnold et al.

bound to the first layers by hydrogen bonds, that is, the formation of cyclic carboxylic acid dimers. Acknowledgment. A part of this work has been funded by the German BMBF (05625VHA3), the DFG (Te247/2-1), and the German “Fonds der Chemischen Industrie”. We acknowledge fruitful discussions with Professor R. Nuzzo (Urbana/Champaign) and Dr. H.-J. Himmel (Karlsruhe) and a careful reading of the manuscript by Dr. T. Strunskus (Bochum). LA0117000

Preparation, Modification, and Crystallinity of Aliphatic and Aromatic

Recommend Documents