Langmuir 1998, 14, 4679-4682
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Solvent Dependence of the Self-Assembly Process of an Endgroup-Modified Alkanethiol Oliver Dannenberger,†,‡ J. Jens Wolff,§ and Manfred Buck*,† Lehrstuhl fu¨ r Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany, and Organisch-Chemisches Institut, Universita¨ t Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany Received May 6, 1998 Adsorption of 12-(4-nitroanilino)dodecanethiol from ethanol and n-hexane onto polycrystalline gold substrates was studied in situ and in real time by second-harmonic generation. Depending on the fundamental wavelength used, the attachment of the thiol to the substrate or the behavior of the p-nitroaniline (pNA) endgroup can selectively be monitored. Striking solvent effects are shown for the process of film formation. The time scales on which changes related to the coverage and the orientation of the pNA moiety occur vary widely. In hexane the thiol coverage increases about four times faster than that in ethanol. The rearrangement of the pNA endgroups in ethanol occurs on a time scale that is larger by a factor of 2 to three compared to the adsorption. In contrast, hexane extends the process of reorientation by a factor of more than 30 relative to the time scale of adsorption. A qualitative model based on the differences in the solvent-thiol interactions is suggested.
The possibility to tailor surface properties, in combination with a simple preparation procedure, has recently led to a mushrooming interest in self-assembled monolayers (SAMs) formed from pure and functionalized n-alkanethiols.1-3 However, the ease of preparation is contrasted by the complexity of the film formation process that involves conformational changes of the molecules as well as interactions of the molecules with each other, with the substrate, and with the solvent. So far detailed experimental investigations of the process of film formation have basically been limited to n-alkanethiols.4 Already for these simple systems a rather complex behavior was observed. Microscopic5,6 and spectroscopic techniques4,7-10 reveal a multistep process with rather different time scales for the major part of the adsorption and ordering of the thiol molecules.10 Additionally, corrosion of the gold substrate takes place.11 In almost all of these studies the influence of the solvent was of minor concern, even though an influence of the solvent on the stability of the layer,12 on the electrochemical properties,13 on the corrosion process of the substrate during adsorption,14 and on the kinetics8 has been reported. The * Author to whom correspondence may be addressed. † Lehrstuhl fu ¨ r Angewandte Physikalische Chemie. ‡ Current address: Department of Bioengineering, University of Washington, Box 357962, Seattle, WA 98195. § Organisch-Chemisches Institut. (1) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (2) Wink, T.; van Zuilen, A.; Bult, A.; van Bennekom, W. P. Analyst 1997, 122, R43-R50. (3) Finklea, H. O. In Electroanalytical Chemistry; Marcel Dekker: New York, 1996; Vol. 19, pp 105-335. (4) For a recent compilation of kinetic studies performed to date see: Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H.; Kruus, E. Langmuir 1997, 13, 5335-5340. (5) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (6) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218-5221. (7) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825-1831. (8) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740. (9) DeBono, R. F.; Loucks, G., D.; Della Manna, D.; Krull, U. J. Can. J. Chem. 1996, 74, 677-688. (10) Ha¨hner, G.; Wo¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955-1958. (11) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4-8. (12) Schlenoff, J. B.; Li?M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-12536. (13) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973.
experiments presented below give clear evidence of the drastic influence solvents can exert on the kinetics of SAM formation. Adsorption of thiols on polycrystalline gold substrates was monitured in situ and in real time by second harmonic generation (SHG), a nonlinear-optical technique wellsuited to study processes at buried interfaces, for example, liquid/solid interfaces.15,16 Use of 12-(4-nitroanilino)dodecanethiol (NAT), allowed submolecular parts of the system to be selectively addressed by wavelength-dependent SHG. The intensity of the SHG signal from a SAM of NAT on a gold substrate is given by
I2ω ∝ |χsub(ω) + χint(ω,θ) + χpNA(ω,θ,ϑ,solvent)|2 Iω2 (1) where χsub, χint, and χpNA represent the susceptibilities of the bare gold substrate, of the Au-S bond, and of the p-nitroaniline (pNA) endgroup. The hydrocarbon chains do not contribute due to their low hyperpolarizability.17 ω and θ denote the laser frequency and the surface coverage, respectively. Whereas, at fixed frequency, χint is only a function of the thiol coverage, χpNA is additionally dependent on the orientation of the pNA moiety, that is, the tilt angle ϑ, and on the environment, that is, the solvent, due to solvatochromic effects of the p-nitroaniline group.18 At a harmonic wavelength of λ2ω ) 532 nm the pNA endgroup contributes only weakly and the signal is determined by χsub and χint. In this case the change of the SHG signal due to thiol adsorption directly reflects the coverage. In contrast, an SHG wavelength of λ2ω ) 350 nm is in the range of the charge-transfer transition of the p-nitroaniline endgroup19 and, therefore, the resonantly (14) Edinger, K.; Grunze, M.; Wo¨ll, Ch. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1811-1815. (15) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107-125. (16) Buck, M.; Dannenberger, O.; Wolff, J. J. Thin Solid Films 1996, 284/285, 396-399. (17) Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, FL, 1987; Vol. 1. (18) Sta¨helin, M.; Burland, D. M.; Rice, J. E. Chem. Phys. Lett. 1992, 191, 245-250. (19) Dannenberger, O. PhD Thesis, University of Heidelberg, 1996.
S0743-7463(98)00532-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/18/1998
4680 Langmuir, Vol. 14, No. 17, 1998
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Figure 1. SHG signal at 532 nm recorded in pp-polarization during the adsorption of NAT on polycrystalline gold from a 2 µm solution in (a) ethanol (9) and (b) hexane (b). Solid lines represent fits according to eq 2. Signals are normalized to the bare gold substrate. Fit parameters are ApNAθ)1 ) 0.28 (0.37), φint ) 180° (180°),28 and kint ) 3400 L mol-1 s-1 (15380 Lmol-1 s-1) for ethanol (hexane).
enhanced susceptibility χpNA dominates the signal. Consequently, proper choice of the wavelength allows to switch between χint and χpNA, and thus the coverage-dependent behavior of the endgroup can be studied. A detailed analysis shows that χint is nonnegligible and has to be separated by comparative measurements with n-alkanethiols.20,21 However, for the present discussion the contribution of χint at λ2ω ) 350 nm can be ignored. We note at this point that the SHG experiments reported here deal with the relatively fast steps of film formation and do not include the subsequent slow steps that yield the structure of the final monolayer and last hours at the thiol concentrations used.10 Figure 1 shows the adsorption of NAT on a polycrystalline gold substrate from a 2 µM solution measured at λ2ω ) 532 nm.22 The squares and circles represent the adsorption from ethanol and hexane, respectively. Qualitatively, the curves exhibit the same shape as found previously for n-alkanethiols and are approximated by the phenomenological equation24
I2ω ∝ |1 + Aintθ)1 eiφint (1 - e -ckintt)|2
(2)
where Aintθ)1 is the ratio of χint to χsub at final coverage. The phase factor φint takes into account that, in general, the susceptibilities are complex quantities. The term in brackets reflects a Langmuir adsorption model with c as the concentration of NAT in the solution, kint as the rate constant of adsorption, and t as the adsorption time. Note, (20) Dannenberger, O.; Buck, M. In preparation. (21) Eisert, F.; Dannenberger, O.; Buck, M. Submitted for publication in Phys. Rev. B. (22) The in situ experiments were performed in a liquid cell with the substrate (about 10 × 10 mm2 in size) parallel and in close distance (∼3 mm) to the cell window. Adsorption was started by replacing the solvent with the thiol-containing solution. All signals were recorded with both the incident fundamental and analyzed second-harmonic light ppolarized. Substrates were prepared by evaporating a 100 nm Au layer onto Si(100) wafers with a thin (5 nm) layer of Cr acting as an adhesion promoter. To minimize surface contamination, the substrates were stored under argon. Ethanol (Riedel-de Hae¨n, 99.8%) and hexane (Riedelde Hae¨n, g 99%) were used without further purification. NAT was prepared according to the literature.23 (23) Wesch, A.; Dannenberger, O.; Wo¨ll, Ch.; Wolff, J. J.; Buck, M. Langmuir 1996, 12, 2, 5330-5337. (24) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F. J. Vac. Sci. Technol., A 1992, 10, 926-929. Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F., Appl. Phys. A 1991, 53, 552-556.
Figure 2. Same experiment as in Figure 1 but recorded at a second harmonic wavelength of 350 nm. The insets show the first part of the adsorption process on an expanded scale. Parameters for the fits (solid lines) according to eq 2 were chosen to match the minima of the curves. Fit parameters are ApNAθ)1 ) 2.0 (1.5), φint ) 195° (153°), and kpNA ) kint for ethanol (hexane). Note, that the signal levels off after about 1200 s for ethanol (a, 9) whereas for hexane (b, b) signal changes occur on a much longer time scale, in contrast to Figure 1 where the time scales are reversed.
that the larger change of the SHG signal for hexane compared to ethanol does not indicate that the final thiol coverage is higher for hexane but, due to different refraction indices of the solvents, the angles of incidence of the laser radiation are different and cause Aintθ)1 to change. A least-squares fit for both curves yields kNAT ) 3400 L mol-1 s-1 for ethanol and kNAT ) 15380 L mol-1 s-1 for hexane. As a rough measure the signals level off at about 500 s for EtOH and 125 s for hexane. The situation changes when the pNA group is addressed (Figure 2). For both solvents the intensity strongly decreases first before rising to a level much higher than that of the signal from the bare substrate. For ethanol (Figure 2a) the intensity minimum almost reaches the noise level. This indicates that at this early stage of adsorption the susceptibility of the pNA moiety interferes destructively with that of the substrate. A more detailed phase-sensitive analysis reveals φpNA to vary between 170° and 210° for θ < 0.7 (t < 180 s).16,20 For hexane (Figure 2b) the substrate signal is incompletely compensated and the minimum amounts to about 20% of the starting signal. Attempts to describe the behavior of the pNA moiety by the Langmuir-type eq 2 (subscript int replaced by pNA) fail for both solvents. Since the rate constant of adsorption
Letters
is given by the measurement at 532 nm, ApNAθ)1 and φpNA are the only adjustable parameters. In the case of EtOH the minimum of the experimental curve can only be reproduced by a fit if the magnitude of ApNAθ)1 equals 2.0, a value by far too small to describe the final signal level (|ApNAθ)1| ) 4.28). ApNAθ)1 is dependent on θ and must significantly increase with increasing coverage, in contrast to the coverage-independent Aintθ)1 at λ2ω ) 532 nm. Reorientation and solvatochromism of the endgroup cause ApNAθ)1 to change with coverage.20 Comparison of the measurements at both wavelengths shows that the times where the signals have reached a constant level do not differ by more than a factor of 2 to 3 if EtOH serves as solvent. For adsorption in hexane at λ2ω ) 350 nm (Figure 2b) a fit matching the minimum yields a value of 1.5 for the magnitude of ApNAθ)1. Again ApNAθ)1 is too small compared to the final value of |ApNAθ)1| ) 3.57. However, in contrast to ethanol, the time scales of the measurements at 532 and 350 nm exhibit large differences. The former yields a constant signal after about 125 s whereas the latter levels off beyond 6000 s. Whereas for NAT in EtOH adsorption and ordering take place on similar time scales, adsorption from hexane proceeds faster by a factor of about 4 compared to EtOH and changes related to the pNA moiety take place on a time scale extended by a factor of 30; that is, in hexane reorientation of the endgroups is much more decoupled from the adsorption process. This is opposite to the experiments reported by Peterlinz and Georgiadis.8 Investigating n-alkanethiols, that is, a type of thiol very different from ours, they observed as well pronounced solvent effects but no distinct second step for hexane. However, this is not a contradiction but rather suggests that there is not a single mechanism of film formation. Even though we have to refer to refs 20 and 21 for more details we note that the solvent not only affects the kinetics of film formation but affects as well the structure of a completed monolayer, that is, causes the pNA group to change its orientation. For the interpretation of the present experiments we differentiate between the anchoring of the molecules (monitored at λ2ω ) 532 nm) and the reorientation of the pNA groups (monitored at λ2ω ) 350 nm). NAT shows an adsorption behavior similar to n-alkanethiols for which anchoring to the substrate also proceeds significantly faster in hexane.8,19,25 Systematic kinetic studies of n-alkanethiols and di-n-alkyl disulfides suggest that displacement of solvent molecules in the interfacial region is rate-limiting.19,26,27 Since EtOH has an intermolecular interaction stronger than hexane, adsorption is expected to proceed faster in hexane. The large alteration of the time scales of adsorption and reorientation upon changing the solvent requires a different explanation. As known from n-alkane thiols the formation of SAMs proceeds in several steps.5,6,8-10 For NAT, the extent to which the steps are separated in time is determined by the solvent. Hexane and EtOH are different in their molecular size, polarity, and their ability to form hydrogen bonds. Since NAT consists of a polar (25) Eisert, F. Ph.D. Thesis, University of Heidelberg, 1993. (26) Dannenberger, O.; Grunze, M.; Buck, M. In preparation. (27) Jung, C.; Dannenberger, O.; Yue, X., Y.; Buck, M.; Grunze, M. Langmuir 1998, 40, 1103-1107. (28) The value of 180° for the phase factor φint was determined experimentally by phase-sensitive detection.25 This coverage independent value does hold only for λ2ω ) 532 nm. At different wavelengths the situation can be dramatically different.29 (29) Buck, M.; Eisert, F.; Grunze, M.; Tra¨ger, F. Appl. Phys. A 1995, 60, 1-12.
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Figure 3. Sketch of the different stages of NAT film formation: a, low coverage; b and c, higher coverages. The time scale on which the transition from b to c occurs is determined by the solvent and is associated with a slight increase in coverage, reorientation, and domain formation.
endgroup and a hydrophobic alkyl chain, a preferential interaction of ethanol with the pNA group and of hexane with the hydrocarbon chain is expected. At low coverages NAT is likely to behave similar in ethanol and in hexane. Assuming a uniform adsorption, the NAT molecules are well separated and, thus, are completely solvated (Figure 1a). A highly disordered structure is expected with a slight preference of the pNA moiety to align toward the surface normal due to the anchoring of the molecules to the gold substrate. At this stage the rate of the anchoring of NAT should determine the change of the SHG signal at λ2ω ) 350 nm. That the minimum of the curves occurs much earlier for hexane than for ethanol is in agreement with this interpretation. At higher coverage, however, pNA moieties will preferentially interact with each other rather in hexane. This causes the formation of a polar sheet of pNA moieties confined by a nonpolar layer of the alkyl chains of NAT and the solvent (Figure 3). To minimize the free enthalpy of the system the NAT molecules have to undergo conformational and orientational changes (Figure 3c). During this reorientational process the interactions between the pNA groups are disrupted and they are exposed to the solvent. A higher activation energy for this reorientation process is thus expected for hexane compared to ethanol. If this process is rate-determining, a delayed reorientation of the pNA groups in hexane should result. At present we do not have information about the nature of the reorientation process. This can include narrowing of the distribution of tilt angles, an overall change of the average tilt angle, and the change of the domain size of ordered regions. Further studies are needed to unravel the mechanistic details. In conclusion, the wavelength-dependent in situ SHG experiments revealed that the solvent can decisively alter the kinetics of film formation. Whereas the adsorption proceeds significantly faster in hexane compared to ethanol, the time scales of the conformational changes are reversed. With respect to a fundamental understanding of the mechanisms of film formation and the optimi-
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zation of the film properties, the example presented here clearly demonstrates the necessity for a more detailed understanding of the role of solvents. Acknowledgment. The authors are indebted to M. Grunze for his continuous support. We gratefully thank
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W. Schrepp and H. Kullmann, BASF AG, and H. Schwenk, Wacker Siltronic, for providing the substrates. This work was funded by the Deutsche Forschungsgemeinschaft and in part by the Fonds der Chemischen Industrie. LA980532H
