Langmuir 1997, 13, 4943-4947
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Ultrahigh Vacuum Study on the Reactivity of Organic Surfaces Terminated by OH and COOH Groups Prepared by Self-Assembly of Functionalized Alkanethiols on Au Substrates† H.-J. Himmel, K. Weiss, B. Ja¨ger, O. Dannenberger, M. Grunze, and Ch. Wo¨ll* Lehrstuhl fu¨ r Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Federal Republic of Germany Received February 6, 1997. In Final Form: July 21, 1997X Two different organic surfaces were created by self-assembly (SA) of bifunctionalized monomers using OH- and COOH-terminated alkanethiols. X-ray photoelectron (XP) and soft X-ray absorption (NEXAFS) spectroscopy have been used to determine the chemical composition of and the molecular orientation in the SA monolayers adsorbed on Au substrates. Subsequently the two OH- and COOH-terminated organic surfaces were exposed to phenyl isocyanate (C6H5NCO, PIC) in order to investigate their chemical reactivity. In both cases the reactivity to gas-phase PIC was very low for the sample at room temperature. Reaction yields of more than 80% could, however, be achieved by depositing multilayers on a sample cooled down to 120 K which was subsequently warmed to 290 K.
1. Introduction The formation of self-assembled monolayers of alkanethiols on gold and other metal surfaces has attracted considerable attention during the past decade.1-5 Oriented, densely packed organic layers with well defined, adjustable thicknesses in the nanometer region can be easily obtained by immersing a gold sample in an ethanolic solution of the corresponding thiol. Although regular, CH3-terminated thiols were first used as resist materials in nanolithography,6,7 the major interest stems from the possibility of creating organic surfaces with specific top layer functions by using end-functionalized alkanethiols. Organic monolayers obtained from alkanethiols with the terminating CH3 group replaced by several different functional groups have been investigated in a number of previous experiments,2,8,9 but detailed studies of the molecular orientation within the self-assembled monolayers (SAM’s) made from functionalized thiols are rather scarce. Generally, it is also unclear whether the chemical reactivity of the specific functional groups exposed at the organic surface is comparable to the corresponding data for molecules in solution or whether steric constraints strongly affect this property. In a series of previous experiments the reactivity of functionalized organic surfaces toward a number of different compounds was investigated by immersing them in solutions10-16 or by exposing them to different types of vapors at atmospheric † This manuscript is dedicated to Prof. R. Gleiter on the occasion of his 60th birthday. X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (4) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (5) Porter, M. D.; Bright, T. B.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 3559. (6) Calvert, J. M. J. Vac. Sci. Technol. B 1993, 11 (6), 2155. (7) David, C.; Mu¨ller, H. U.; Vo¨lkel, B.; Grunze, M. Microelectron. Eng. 1996, 30, 57. (8) Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116. (9) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D.; Siperko, L. M. Langmuir 1992, 8, 2707. (10) Wassermann, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (11) Li, D.; Ratner, M. A.; Marks, T. J. Am. Chem. Soc. 1990, 112, 7389.
S0743-7463(97)00121-2 CCC: $14.00
pressure.17-23 True ultrahigh vacuum (UHV) studies, which allow a detailed characterization of the substrates before and after reaction, are however rather scarce. UHV studies are well suited to study exclusively the reaction of interest and make it further possible to exclude contributions from the solvent. Additionally, the influence of water monolayers, which are generally assumed to be present on top of the hydrophilic COOH and OHterminated monolayers under atmospheric pressure,8 can be ruled out. In the present set of experiments we have used two end functions of rather small size, OH and COOH, and first investigated the molecular orientation within the (unreacted) monolayer. Subsequently the films were exposed to gaseous phenyl isocyanate at 10-7 mbar in order to gather information on their reactivity. Soft X-ray absorption spectroscopy, a technique which is particularly well suited for the orientational and chemical analysis of thin organic films,24 was used to investigate the molecular orientation and chemical structure. Additional quantitative information was obtained from X-ray photoelectron spectroscopy. 2. Experimental Section Gold substrates were prepared by evaporation of 100 nm of gold onto polished silicon single-crystal wafers which had been primed with a 1 nm layer of titanium in order to improve adhesion of the gold films. The self-assembly (SA) films were prepared by immersing Au samples into 0.1 mM solutions of 16-mercaptohexadecanol (HO(CH2)16SH) (MHO) and 16-mercaptohexade(12) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (13) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (14) Ulman, A.; Tillman, N. Langmuir 1989, 5, 1418. (15) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (16) Netzer, L.; Isovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. (17) Yang, H. C.; Dermody, D. L.; Xu, C.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 726. (18) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 2101. (19) Thomas, R. C.; Tangyunyong, P.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. J. Phys. Chem. 1994, 98, 4493. (20) Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1993, 9, 1775. (21) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830. (22) Matsuura, K.; Ebara, Y.; Okahata, Y. Langmuir 1997, 13, 814. (23) Sun, L.; Thomas, R. C.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1991, 113, 8550. (24) Kinzler, M.; Schertel, A.; Ha¨hner, G.; Wo¨ll, Ch.; Grunze, M.; Albrecht, H.; Holzhu¨ter, G.; Gerber, Th. J. Chem. Phys. 1994, 10, 7722.
© 1997 American Chemical Society
4944 Langmuir, Vol. 13, No. 19, 1997
Figure 1. NEXAFS spectra and difference spectra for CH3-, COOH-, and OH-terminated alkanethiolate films adsorbed on Au. canoic acid (HOOC(CH2)15SH) (MHC) in ethanol for 18 h. Phenyl isocyanate (C6H5NCO) (FLUKA, g99%) was cleaned by pump and freeze cycles and admitted to the UHV chamber through a leak valve. The adsorption of phenyl isocyanate (PIC) was carried out at a sample temperature of 120 K. At 300 K no significant adsorption was found for exposures up to 60 000 L (1 L ) 1 s × 10-6 mbar). Multilayers of phenyl isocyanate were deposited on the hydroxyl- and carboxyl-terminated thiolate films by cooling the samples to 120 K and then exposing them to 50 L of PIC at a pressure of 1 × 10-7 mbar. After X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (NEXAFS) measurements the samples were slowly heated to 290 K (in about 15 min). X-ray photoelectron spectra were recorded with Al KR light and a hemispherical electron energy analyzer. The NEXAFS spectra were recorded at the synchrotron radiation facility BESSY in Berlin (beamline HE-TGM 2) with a resolution of better than 0.8 eV at the C 1s edge using partial electron yield detection with a retarding voltage of -150 eV. The raw data were normalized to the incident photon flux by division through a spectrum recorded for a clean Au substrate. For energy calibration the spectra were referenced to a characteristic peak at 285.0 eV in the photocurrent spectra of the carbon-contaminated gold grid which was used as a photon flux monitor.
3. Results and Discussion 3.1 Unreacted Monolayers. In the left part of Figure 1 NEXAFS spectra obtained for monolayers of (from bottom to top) mercaptohexadecanoic acid (MHC), mercaptohexadecanol (MHO), and hexadecanethiol (HDT) are shown. The spectra for the unsubstituted thiols (top) show the features typical for alkyl chains in an all-trans conformation: an intense R* resonance at 287.7 eV (which actually consists of several components25 ), the absorption step at 288.1 eV, and a broader σ* resonance at around (25) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch. Chem. Phys. Lett. 1996, 248, 129.
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293.1 eV. The intensities of these transitions are governed by optical selection rules and the strong anisotropy observed for the hexadecanethiol films in the top of Figure 1 directly implies a high degree of molecular orientation within the films. The analysis of the NEXAFS spectra follows a scheme described previously,8 which is based on an analysis of difference spectra. In agreement with earlier results from our group26 and results obtained with different techniques by other groups,27 we obtain an average tilt angle of 38.7 ( 5° between the alkyl chain molecular axis and the surface normal in the case of hexadecanethiol. The NEXAFS spectra obtained for the OH-terminated alkanethiolate monolayers (center of Figure 1) closely resemble the data for the unsubstituted alkanethiols. As expected there are no new obvious features in the spectra below 292 eV. A weak resonance at around 289 eV is assigned to an excitation into the unoccupied C-O σ*orbital (see ref 28 for more details). A quantitative analysis using the same scheme as for the unfunctionalized thiols yields an alkyl chain tilt angle of 43.8 ( 5°. The NEXAFS data for the COOH-terminated alkanethiolate monolayers (Figure 1, bottom) show an additional sharp feature at 287.6 eV with a strong angular anisotropy, which is attributed to an excitation of a C 1s electron in the empty π*-orbital of the COOH group. This resonance overlaps with the R* resonance, and as a result the scheme used for the determination of the alkyl chain tilt angles of the unfunctionalized and OH-terminated thiolate films cannot be used. The orientation of the alkyl chain and the COOH end group was therefore determined by fitting the individual resonance intensities. The parameters for the fits were taken from previous work.9 Our analysis of the alkane chain R* resonance yields a tilt angle of 43.0 ( 5°. The anisotropy observed for the COOH π* resonance (288.6 eV) intensity indicates an average angle of 67° between the COOH plane and the surface normal. This value is in good agreement with the value obtained by Nuzzo et al. from analysis of the carbonyl streching mode of their infrared spectra (66°) recorded for the same chain length.2 The tilt angle determined from the angular anisotropy of resonances in NEXAFS spectroscopy is an average value, and there is no straightforward way to discriminate between the case of alkyl chains homogeneously tilted all by the same angle and the case of a disordered system with a broad distribution of tilt angles. In the case of alkyl chains, however, it was demonstrated that the large density of gauche defects accompanying high degrees of disorder in systems made up from alkanes gives rise to an additional feature in the C 1s NEXAFS spectra at 287 eV.29 A more elaborate analysis of the data shown in Figure 1 (details will be presented in a separate publication28) indeed reveals the presence of such a feature for the COOH-terminated film but not for the OH-terminated film. We conclude, therefore, that the OH-terminated film consists of alkyl chains mainly in the trans conformation with a homogenous tilt-angle of 44°, whereas the COOHterminated film is largely disordered and exhibits a high density of gauche defects. This conclusion is supported by data for SA films made up from OH- and COOHterminated alkanethiols with longer alkyl chains.28 (26) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol. A 1992, 10, 2758. (27) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (28) Dannenberger, O.; Weiss, K.; Schertel, A.; Himmel, H.-J.; Ja¨ger, B.; Buck, M.; Wo¨ll, Ch. Thin Solid Films, in press. (29) Schertel, A.; Ha¨hner, G.; Grunze, M.; Wo¨ll, Ch. J. Vac. Sci. Technol. A 1996, 14, 1801.
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Figure 2. Reaction equations for the formation of the mixed anhydride and the urethane out of COOH-/OH-terminated alkanethiolate films and PIC (the mixed anhydride should decompose at elevated temperatures).
3.2. Reaction of the Monolayers with Phenyl Isocyanate. In order to investigate the reactivity of the organic surfaces prepared by the self-assembly process, monolayers of 16-mercaptohexadecanoic acid (MHC) and 16-mercaptohexadecanol (MHO) were transferred into an UHV chamber and then exposed to gas-phase phenyl isocyanate (PIC). In solution the reaction of aliphatic alcohols and carboxylic acids with phenyl isocyanate proceeds with reaction yields of more than 90% according to the scheme depicted in Figure 2.30,31 But in our case even for exposures as high as 60 000 L, XP spectra showed no change in chemical composition, revealing a very low reactivity between the OH- and COOH-terminated organic surfaces and gas-phase PIC. It was found, however, that the reaction yield can be dramatically improved by first condensing a PIC multilayer on top of the SAM’s at low temperatures (130 K) and then warming the sample to 290 K. During heating the multilayer desorbs at 190 K, and XP spectra (Figure 3) recorded for both films after reaching 290 K reveal the presence of N atoms. Also the changes in shape and intensity of the O 1s and C 1s lines indicate that the deposited PIC has reacted with the organic surface. Note that the O 1s signal of the hydroxyl endgroups in the MHO film, located at 533.2 eV, shifts to a lower binding energy of 532.5 eV, typical for carboxylic O. The small reduction of the integrated O 1s intensity seen for the carboxyl-terminated film is due to a small amount of water present on the film before exposure. This residual water reacts with PIC and forms aniline, which then desorbs from the surface due to its high vapor pressure. In order to determine the reaction yield of the CVD reaction, the atomic O/N ratio was calculated from the intensity of the corresponding O 1s and N 1s signals in the XP spectra according to the formula
NN σO1s TE(O1s) IN1s ) NO σN1s TE(N1s) IO1s (30) Arnold, R. G.; Nelson, J. A.; Verbanc, J. J. Chem. Rev. 1957, 57, 47. (31) The mixed anhydrides formed from a weak carboxylic acid like hexadecanoic acid should be stable at room temperature.
Figure 3. XP spectra for the COOH-terminated film (MHC, left side) and the OH-terminated film (MHO, right side) before adsorption (dashed line) and after adsorption of 50 L of PCI at 120 K and subsequent heating to 300 K (solid line).
Here σ denotes the photoionization cross-section32 and TE denotes the transmission of the electron energy analyzer. Our analysis yields a O/N ratio of 2.4 for the COOHterminated film and one of 1.2 for the OH-terminated film. The percentage x of reacted carboxyl groups on the surface thus amounts to 83%, and that of reacted hydroxyl groups amounts to 87% ((5%). The reaction yield is thus (32) Scofield, J. H. J. Elecron Spectrocsc. Relat. Phenom. 1976, 8, 129.
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Figure 4. NEXAFS spectra for the COOH- (left side) and the OH- (right side) terminated films recorded after warming to 290 K. The inset shows a multilayer spectrum (solid line) together with the 90° spectrum of the OH-terminated film after reaction (dashed line).
nearly as high as observed in solution (g90%30). The C 1s/Au 4f peak intensity ratios are consistent with these values. The small difference in the reaction yield between the OH- and the COOH-terminated alkanethiolate films is surprising, since the high density of gauche defects within the COOH-terminated films implies that a significant amount of COOH groups are embedded in the alkylchains. Our results thus indicate that the affinity of the COOH groups toward phenyl isocyanate is sufficiently large to overcome this kinetic barrier so that the disorder does not influence the reaction yields. Our data shown here are consistent with results reported for the vapor phase reactivity of monolayers of HOOC(CH2)10SH toward alkaneamines by Yang et al.17 (reaction yield found therein: 75%). The NEXAFS spectra obtained for the two reacted monolayers are shown in Figure 4. The sharp phenyl π* resonance at 285.2 eV constitutes the main difference from the spectra of the unreacted films (Figure 1). For the OH-terminated film the anisotropy of the R* resonance at 287.7 eV is virtually unchanged by the reaction, revealing the absence of reaction-induced orientational changes of the hydrocarbon chains. In case of the COOHterminated film the reaction is accompanied by a slight but significant 5° increase of the hydrocarbon tilt angle. The average hydrocarbon chain orientation is thus only slightly influenced by the coupling of PIC to the end groups. This observation is consistent with the results of previous theoretical studies for thiolate films with phenyl sulfone units inside the hydrocarbon chains, where it was demonstrated that the space requirements of phenyl units are compatible with the packing of the alkyl chains in alkanethiolate monolayers.33 Noticeable chain distortions were only observed near the phenyl sulfone units, while the tilt angle of the complete hydrocarbon chains still amounts to 30°.33 From an analysis of the phenyl π* intensity variation an average tilt angle of 49° between the ring plane and (33) Shnidman, Y.; Ulman, A.; Eilers, J. E. Langmuir 1993, 9, 1071.
the surface normal is obtained for the COOH-terminated film and one of 43° is obtained for the OH-terminated film. Additional information is obtained from the NEXAFS spectrum for the PIC multilayer at 120 K, which is also shown in Figure 4. The sharp feature at 286.7 eV belongs to excitations of C 1s electrons in unoccupied CdN π* orbitals. During reaction with the OH and COOH end groups the CdN double bond is transformed into a single bond (see Figure 2), which does not give rise to sharp features in x-ray absorption spectra. The absence of a resonance at 286.7 eV in the spectra after heating to 290 K thus reveals the absence of unreacted PIC on the surface. In case of the OH-terminated monolayers the resulting urethane films were observed to be thermally unstable. Heating the samples slightly above room temperature caused a strong decrease of the phenyl π* resonance in the NEXAFS spectra and of the nitrogen XP signal. 4. Conclusions Reactive, OH and COOH-terminated organic surfaces were prepared by adsorption of HO(CH2)16SH and HOOC(CH2)15SH on Au substrates. The covalent anchoring of phenyl isocyanate onto these surfaces was studied with NEXAFS and XPS. The reactivity was observed to be strongly reduced with respect to that of corresponding functional groups in solutions. Exposing the organic surfaces to phenyl isocyanate at room temperature in an ultrahigh vacuum chamber did not result in a reaction for exposures up to 60 000 L. Only when multilayers were condensed at 130 K and subsequently annealed to 290 K was the formation of a mixed anhydride (C6H5(NH)(CO2)(CO)(CH2)15S-) (in the case of the COOH-terminated films) and of an urethane (C6H5(NH)(CO)O(CH2)15S-) (in the case of the OH-terminated films) observed. In both cases about 85% of the available active sites were found to react with the phenyl isocyanate. The small difference in reaction rate indicates that the urethane/mixed anhydride formation energy is sufficiently high to eliminate the steric effects expected from the different structures of the OH and COOH films.
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In case of the COOH-terminated film the reaction is accompanied by a slight but significant 5° increase of the hydrocarbon tilt angle. No orientational change of the hydrocarbon chains in the OH-terminated thiolate film was observed. Thus the newly (during the reaction) created end groups are nearly compatible with the structure of the alkanethiolate film.
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Acknowledgment. This work was funded in part by the Deutsche Forschungsgemeinschaft (SFB 247 E2 and Bu 820/4-1), the German BMBF (05625 VHA), and the German “Fonds der Chemischen Industrie”. We thank Dr. W. Unger (Berlin) for valuable discussions. LA970121L
