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Ordered Self-Assembled Monolayers of 4,4’-Biphenyldithiol on Polycrystalline Silver: Suppression of Multilayer Formation by Addition of Tri-n-butylphosphine Ulrike Weckenmann,† Silvia Mittler,‡ Kai Naumann,§ and Roland A. Fischer*,† Lehrstuhl fu¨ r Anorganische Chemie II, Ruhr-Universita¨ t Bochum, Universita¨ tsstrasse 150, 44780 Bochum, Germany, Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and Organische Chemie I, Ruhr-Universita¨ t Bochum, Universita¨ tsstrasse 150, 44780 Bochum, Germany Received December 28, 2001. In Final Form: April 2, 2002 Self-assembled monolayers (SAMs) of 4-biphenylthiol (BT) and 4,4’-biphenyldithiol (BDT) formed on polycrystalline silver substrates were characterized by surface plasmon resonance spectroscopy (SPR), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). The RAIRS measurements reveal that the molecules in a BT-SAM are oriented perpendicular to the surface whereas the adsorption of BDT results in the formation of multilayers due to the oxidative coupling of the terminal thiol groups forming disulfide-bridged species. The process of multilayer formation was monitored by SPR and RAIRS indicating that BDT initially forms a monolayer which exhibits a structure similar to the one found for BT and that disorder is introduced by the rapid oxidation of the exposed thiol groups resulting in the formation of a multilayer. The addition of tri-n-butylphosphine to the self-assembly solution of 4,4’-biphenyldithiol prevents the formation of multilayers and allows the generation of well-ordered monolayers of the dithiol in which the molecules adopt a standing-up orientation as observed for 4-biphenylthiol. The existence of free thiol groups at the surface of the dithiol SAM was proven by quantitative XPS measurements.
Introduction Self-assembled monolayers (SAMs) have attracted considerable interest over the past 15 years because of their potential applications in corrosion prevention,1 lubrication,2 wetting control,3 and molecular recognition.4 Due to their ease of preparation and well-defined order, thiols on gold (111) and to a lesser extent thiols on silver (111) have become model systems for SAMs. Most investigations of these systems have focused on alkane based thiols of which the structure and adsorption kinetics have been thoroughly studied.5-8 In contrast, the structure of SAMs prepared from aromatic thiols has been investigated relatively little even though thiols possessing a rigid-rod backbone like oligophenylthiols have recently attracted considerable attention for the following reasons: First, it is expected that the rigidity of the molecular backbone and the strong π-πinteractions will lead to the formation of monolayers which exhibit an increased stability against thermally induced disorder which has been found to be a problem if alkyl derivatives are used.9,10 This is an especially noteworthy * To whom correspondence should be addressed. E-mail: roland.
[email protected]. † Lehrstuhl fu ¨ r Anorganische Chemie II, Ruhr-Universita¨t Bochum. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. §Lehrstuhl fu ¨ r Organische Chemie I, Ruhr-Universita¨t Bochum. (1) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (2) Berman, A.; Steinberg, S.; Campbell, S.; Ulman, A.; Israelachvili, J. Tribol. Lett. 1998, 43. (3) Bain, C. D.; Whitesides, G. M. Adv. Mater. 1989, 4, 110. (4) Spinke, J.; Liley, M.; Guder, H.-J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821. (5) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (6) Fenter, P.; Eisenberger, P.; Camillone, N.; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013. (7) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (8) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Wo¨ll, C.; Grunze, M. J. Electron Spectrosc. 1998, 92, 139.
process for chains containing polar end groups, such as OH or COOH,11 where the introduced disorder is not confined to the surface but affects the alkyl chains as well. Thus, the generation of stable surfaces cannot be based on flexible alkyl SAMs. Second, SAMs formed of aromatic thiols are currently investigated with respect to their electronic properties and their potential as building blocks in future microelectronics.12-14 Associated with their delocalized π-systems, conjugated polymers such as oligophenyls have a small band gap (2-4 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and thus behave like ordinary semiconductors.15 Furthermore, the electronic properties of oligophenyls can be easily modified by introducing eligible groups at the phenyl rings, e.g., by the introduction of a redox center.16 These features make oligophenyls promising candidates for an application in the field of molecular electronics. Indeed, several groups have demonstrated the applicability of oligophenyls as molecular wires12,15 and reconfigurable switches.13 SAMs of oligophenyldithiols are particularly interesting since they adsorb easily on gold and silver surfaces ands assuming they bind to the surface with only one thiol groupsthe unreacted thiol group can serve as anchor to selectively tether further gold particles resulting in a (9) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (10) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (11) Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Ja¨ger, B.; Buck, M.; Wo¨ll, C. Thin Solid Films 1997, 307, 183. (12) Bumm, L. A.; Arnold, J. J.; C. M. T., Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (13) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (14) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones, L., II; Tour, J. M. Appl. Phys. Lett. 1997, 71, 611. (15) Tian, W.; Datta, S.; Hong, S.; Reifenberger, R.; Henderson, J. I.; Kubiak, C. P. J. Chem. Phys. 1998, 109, 2874. (16) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550.
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layered metal-SAM-metal structure. Such architectures will play an important role in future microelectronics and several groups have investigated the use of oligophenyldithiols sandwiched between metal layers as functional electronic devices. Recently, very promising results on this topic have been presented by Scho¨n et al. who have demonstrated the possibility to engineer a field-effective transistor on the molecular level using a SAM formed of 4,4’-biphenyldithiol between two layers of gold.17 We have been active in the field of selective metalization of dithiol SAMs ourselves18,19 and have been able to show that it is possible to selectively deposit gold via metal organic chemical vapor deposition (MOCVD) onto self-assembled monolayers formed of octanedithiol and 4,4’-biphenyldithiol. Since the surface analytical methods we have applied (i.e., X-ray photoelectron spectroscopy (XPS)) required the use of a substrate other than gold in order to distinguish between the substrate and deposited gold particles on top of the SAM, we have chosen silver as underlying substrate which has led us to an investigation of the self-assembly process of dithiols on silver. Detailed studies on the structural characteristics of SAMs formed of oligophenyldithiols on gold as well as on silver are very scarce, and to our knowledge, a detailed investigation on the molecular orientation and adsorption kinetics has not been reported yet. Several groups have reported that one of the major problems in the generation of well-ordered monolayers formed of aromatic dithiols originates from the formation of multilayers which occurs in the course of the adsorption of dithiols on gold andsto a much greater extentson silver.20-22 The results indicate that one thiol group adsorbs to the surface while the other group is projected away from the surface and becomes available for oxidative S-S coupling. Most likely, traces of oxygen and moisture which are always present in the self-assembly solution act as oxidizing agent. Obviously, the susceptibility of aromatic dithiols to oxidation impedes the formation of ordered monolayers and promotes the assembly of ill-defined multilayers. The most straightforward solution to prevent the undesired oxidative S-S coupling is the preparation of dithiol SAMs under strictly oxygen-free conditions. However, due to the very small number of adsorbed molecules on the substrate surface (∼10-15 atoms/cm2) even the minimal amounts of oxygen still present under so-called inert conditions in the selfassembly solution (routinely dried and argon-saturated solvents contain about 1 ppm oxygen) are likely to be sufficient to initiate the oxidation of the terminal thiol groups. Since the complete exclusion of oxygen in wet chemistry requires a considerable effort and is very difficult to control, we searched for a more practicable method to reliably produce monolayers of aromatic dithiols. It is well-known from organic chemistry that aromatic as well as aliphatic disulfides are readily reduced by trialkylphosphines to give the corresponding thiols and phosphineoxides in the presence of water (eq 1).23,24 The strength of the phosphorus-oxygen bond renders the reduction irreversible. Some reports show that adding a phosphine to a reaction mixture containing thiols inhibits the oxidation of the thiol (17) Scho¨n, J. H.; Meng, H.; Bao, Z. Nature 2001, 413, 713. (18) Winter, C.; Weckenmann, U.; Fischer, R. A.; Ka¨shammer, J.; Scheumann, V.; Mittler, S. Chem. Vap. Deposition 2000, 6, 199. (19) Fischer, R. A.; Weckenmann, U.; Winter, C.; Ka¨shammer, J.; Scheumann, V.; Mittler, S. J. Phys. IV 2001, 11, 1183. (20) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G. Langmuir 2001, 17, 95. (21) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529.
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PR3 + R′-S-S-R′ + H2O f OdPR3 + 2R′-SH (1) groups by the generation a reducing environment.25 Therefore, it is imaginable that the simple addition of a trialkylphosphine to the regular self-assembly solution of a dithiol prevents the oxidative coupling of the thiol groups and favors the formation of a dithiol monolayer. The major advantage of this strategy would be that the preparation of the SAM can be routinely conducted under ambient conditions. Since the reduction of disulfides by phosphines necessitates the presence of stoichiometric amounts of water, it becomes redundant to carry out the self-assembly of dithiols under strictly controlled inert conditions. Here, we present the results of a study on organic films formed of 4-biphenylthiol (BT) and 4,4′-biphenyldithiol (BDT) on silver. In addition we describe a new approach toward the preparation of ordered monolayers of rigidrod dithiols taking advantage of the reducing properties of tri-n-butylphosphine (TBP). Experimental Section Materials. 4,4’-Biphenyldithiol (BDT) and 4-biphenylthiol (BT) were synthesized from the corresponding sulfonyl chlorides (Aldrich) by reduction with lithium aluminum hydride (LiAlH4). The crude products were purified by sublimation.26 Tri-nbutylphosphine (95%) (TBP) was purchased from Acros and purified by distillation. Ethanol (Riedel de Haen) and dichloromethane (J.T. Baker) were used without any further purification (i.e., without degassing and drying). All chemical manipulations were carried out in open glass vessels under ambient conditions. Sample Preparation. The silver substrates for XPS and reflection absorption infrared spectroscopy (RAIRS) measurements were prepared by evaporation of 5 nm of titanium and 100 nm of silver onto 100-silicon wafers (Wacker silicone) using a commercial evaporator (Leybold Univex 300) at a base pressure of 10-6 mbar. The wafers were cut into 10 × 13 mm2 pieces for XPS measurements and 15 × 20 mm2 pieces for RAIRS measurements prior to the self-assembly process. For SPR measurements, LaSFN9-glass substrates (Berliner Glas) were cleaned by ultrasonic treatment in Hellmanex solution (Helma, Mu¨hlheim), water, and ethanol. The deposition of silver was conducted using a commercial evaporator (Balzers, BAE 250) at a pressure of 5 × 10-6 mbar. It has been found that the thermal evaporation of silver onto Si(100) yields polycrystalline silver films which exhibit predominantly a (111) texture. Furthermore, it has been shown that the structures of alkanethiol SAMs determined by diffraction studies on single-crystal silver substrates have agreed with the structures determined by RAIRS on polycrystalline silver substrates.27,28 This suggests that the surface roughness of polycrystalline silver substrates does not influence the orientational analysis of the films by RAIRS. Self-assembled monolayers were prepared by immersion of the silver substrates in a 1 mM solution of the aromatic thiols in dichloromethane. For the experiments with TBP, a 1 mM solution with respect to BDT and TBP was used. After removal from the solution, the substrates were thoroughly rinsed with the dichloromethane and dried in a stream of argon. For the kinetic measurements by SPR, the self-assembly solution was prepared using ethanol due to its lower refractive index compared to dichloromethane. Analytical Methods. The SPR measurements were performed on a home-built spectrometer. Angular reflection scans were collected by varying the angle of incidence and detecting (22) Wehner, B. I.; Arnold, R.; Fuxen, C.; Azzam, W.; Terfort, A.; Weckenmann, U.; Spo¨llmann, S.; Fischer, R. A.; Wo¨ll, C., unpublished results. (23) Scho¨nberg, A. Chem. Ber. 1935, 68, 163. (24) Ayers, J. T.; Anderson, S. R. Synth. Commun. 1999, 29 (3), 351. (25) Tam, J. P.; Lu, Y.-A.; Yu, Q. J. Am. Chem. Soc. 1999, 121, 4316. (26) Himmel, H. J.; Terfort, A.; Wo¨ll, C. J. Am. Chem. Soc. 1998, 120, 12069. (27) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (28) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663.
SAMs of 4,4′-Biphenyldithiol on Silver
Figure 1. Adsorption isotherms of (a) BT, (b) BDT, and (c) a mixture of BDT and TBP on silver measured by SPR at an incident angle of 55.3°. The arrows indicate the end of the adsorption process. the reflected light with a phase-sensitive diode detector. All reflectivity measurements were carried out using p-polarized light from a HeNe laser (632.8 nm). The samples were mounted in the Kretschmann configuration.29,30 A Teflon cell with an inlet and outlet port was used as a sample holder. Kinetic measurements were carried out by detecting the intensity of the reflected light at a fixed angle of incidence (0.5° lower than the surface plasmon resonance) over a period of 3 h. The reflected intensity was monitored as a function of time. However, it was not possible to determine the absolute film thicknesses of the various adsorbate layers. The obtained values were well outside the expected range of thickness. It has been reported by Nuzzo et al. that the adsorption of thiols on silver involves some rather complex processes which render the determination of film thicknesses by SPR impossible since the calculation relies on a detailed knowledge of the exact composition of the system and of the refractive indices of the individual layers.27 Although the absolute thicknesses can therefore not be determined, the comparison of the angular shifts of the SPR curves provides some information on the relative film thicknesses which will be discussed for the various adsorbate layers. XPS experiments were carried out with a modified Fisons X-ray photoelectron spectrometer equipped with a Al KR X-ray source and a CLAM2 electron energy analyzer. The pass energy was set to 50 eV. The typical operating pressure was less than 10-8 mbar. All binding energies were referenced to the substrate signal, i.e., the Ag3d5/2 peak at 368.3 eV. RAIR spectra were recorded using a Biorad Excalibur Fourier transform infrared spectrometer (FTS 3000) equipped with a grazing incidence reflection unit (Biorad Uniflex) and a narrow band MCT detector. All spectra were taken with 2 cm-1 resolution at an angle of incidence of 80° relative to the surface normal. Bulk spectra were obtained by transmission infrared spectroscopy of a KBr pellet containing the corresponding thiol.
Results and Discussion Surface Plasmon Resonance Spectroscopy. SPR spectroscopy was used to qualitatively monitor the changes in film thickness in dependence of the immersion time and to compare the resulting film thicknesses. Figure 1 shows the development of the film thickness during the course of the adsorption on a silver surface from a solution of BT, BDT, and BDT/TBP. All curves show an abrupt increase in intensity upon addition of the self-assembly solution which can be related to the immediate adsorption (29) Knoll, W. MRS Bull. 1991, 16, 29. (30) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Springer-Verlag: Berlin, 1998; Vol. 111.
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of thiol molecules from the solution on the silver surface. In the case of BT, the curve exhibits a sharp bend after approximately 5 min and does not show any further increase after 20 min indicating that the growth of film thickness proceeds much slower after the initial adsorption step and finally stops. In contrast, if BDT is adsorbed on the surface, a constant increase of the film thickness can be observed after the first step. Even after 210 min, the curve has still not reached a plateau indicating further film growth. SPR is very sensitive to small changes in the film thickness, but if the thickness exceeds a critical value, it is not possible to monitor the further growth of the adsorbate layer by the method presented here. Since the reflectivity due to the adsorption of BDT has reached a value of 75% after 210 min which is close to the critical value of 80% and therefore the further growth of the BDT layer cannot be observed by a change in the reflectivity, the kinetic measurement of the adsorption of BDT was stopped. However, if the self-assembly solution contains BDT together with an equimolar amount of TBP, the adsorption curve exhibits features similar to the kinetic measurement of the BT adsorption process: After the first adsorption step the gradient decreases significantly and the curve finally reaches a plateau after 210 min marking the end of the adsorption process. All substrates were rinsed with ethanol after the kinetic measurements to remove physisorbed material from the surface and SPR spectra were recorded as a function of the angle of incidence. Due to the adsorption of organic material, the minimum of the curve which denotes the excitation of the surface plasmon has shifted to a higher angle in all cases (Figure 2). In Table 1, the differences between the minima of the resonance angles before and after the adsorption process are listed. However, the thicknesses of the adsorbate layers which were calculated on the basis of the SPR curves amount to 30, 39, and 140 Å for BT, BDT with addition of TBP, and BDT without TBP, respectively. All values are well outside the expected range for a single molecular layer since the length of a BT and a BDT molecule was calculated to be 12.4 and 13.4 Å, respectively. As mentioned earlier (see Experimental Section), it is suspected that complex surface processes in the course of the adsorption of thiols on silver substrates are responsible for the large deviations. This assumption was further supported by our observation that the SPR measurement of a SAM of decanethiol on silver which is known to form a well-ordered monolayer provided a value of 25 Å instead of 11 Å. Obviously, there is a systematic error in the determination of the thickness by SPR. But although the absolute thickness of the adsorbate layers cannot be determined, the relative differences in film thickness provide some valuable qualitative information: The BDT layer prepared by immersion in a solution of pure BDT is much thicker than the BDT layer formed from a mixed BDT/TBP solution. The thickness of the latter is comparable to the thickness of the SAM formed from BT which is known to form a monolayer on silver surfaces.20 We therefore conclude that without the addition of TBP, BDT forms a multilayer on the surface of the silver substrate whereas the addition of TBP to the selfassembly solution of BDT yields a monolayer the thickness of which is in the range of the thickness of a BT monolayer. In summary, it can be inferred from the results of the SPR measurements that BT forms a monolayer upon adsorption on a silver surface within minutes. The slight increase in film thickness over a period of 30 min after the first adsorption step is most likely related to an ordering process as reported for the self-assembly of alkanethiols.31
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Weckenmann et al. Table 2. Vibrational Mode Assignments for BT in the Solid State (KBr)a band position (cm-1)
transition dipole momentb
Wilson notation
1479 1403 1105 1004 826 757
ip par ip perp ip perp ip par op ip par
19a 19b 15 18a 10a 12
a Only the vibrations relevant for the determination of the molecular orientation are listed. b Key: ip perp ) in-plane perpendicular, ip par ) in-plane parallel, op ) out-of-plane.
Table 3. Vibrational Mode Assignments for BDT in the Solid State (KBr)a band position (cm-1) 1592 1475 1394 1105 999 809
transition dipole momentb ip par ip par ip perp ip perp ip par op
Wilson notation 8a 19a 19b 15 18a 10a,b
a Only the vibrations relevant for the determination of the molecular orientation are listed. b Key: ip perp ) in-plane perpendicular, ip par ) in-plane parallel, op ) out-of-plane.
Figure 2. (a) SPR spectra obtained before and after the adsorption of BT, BDT, and BDT/TBP. The shift of the minimum indicates the thickness of the adsorbed layer. (b) A magnified view of the minima. Table 1. Differences between Surface Plasmon Resonance Angles before and after the Adsorption of BT, BDT, and BDT/TBP self-assembly solution BT BDT/TBP BDT
shift of resonance angle (deg) 0.565 0.715 1.571
On the other hand, if BDT is adsorbed on a silver surface, the adsorption process does not come to a standstill after the formation of a monolayer. The formation of a multilayer is probably caused by the oxidative coupling of the terminal thiol groups exposed to the surface and BDT molecules from the solution resulting in a disulfide bridged multilayer of BDT molecules. The SPR data indicate that the multilayer formation can be suppressed by adding TBP to the solution as reducing agent. In this case the adsorption process is completed after 3 h and the resulting film thickness is comparable to the thickness of a BT monolayer. Infrared Spectroscopy. RAIRS spectra have been recorded in order to obtain information on the molecular orientation of the adsorbed thiols and on the degree of order of the SAMs. The band positions and the mode assignments of the relevant vibrations in the bulk state (KBr pellet) of BT and BDT are summarized in Tables 2 and 3. (31) Himmelhaus, M.; Eisert, F.; Buck, M.; Grunze, M. J. Phys. Chem. B 2000, 104, 576.
Figure 3. Comparison of IR spectra obtained from bulk BT in KBr and a SAM of BT on silver prepared by immersion of the silver substrate in a 1 mM solution in dichloromethane for 15 h.
(a) Adsorption of BT. Figure 3 presents the lowfrequency region of the bulk spectrum of BT and the RAIR spectrum of a BT SAM prepared by immersion of the silver substrates in a 1 mM solution in dichloromethane for 15 h. Compared to the bulk spectrum which exhibits a multitude of bands, the RAIR spectrum of a BT SAM on silver exhibits only two strong bands at 1477 and 1003 cm-1 and a weak band at 756 cm-1. These vibrational modes possess a transition dipole moment (TDM) oriented parallel to the 4,4′-axis of the molecule. The most striking feature of the spectrum is the disappearance of the outof-plane band located at 826 cm-1 of the bulk spectrum. According to the surface selection rule only those vibrations are observable in the surface spectrum which possess a TDM with a component perpendicular to the surface plane. Since the RAIR spectrum of the BT SAM only exhibits those vibrations which have a TDM parallel to the 4,4′axis whereas the modes with TDMs perpendicular to the 4,4′-axis have completely disappeared, it can be concluded that the BT molecules adopt a standing-up conformation with their aromatic backbone oriented perpendicular to
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Figure 4. IR spectra of BDT in KBr and in a film on silver prepared by immersion of the silver substrate in a 1 mM solution in dichloromethane for 15 h.
the silver surface. Taking a measurement uncertainty of 10° into account,32 the average tilt angle of BT on silver can be determined to be within 0° and 10°. This result is in agreement with the studies of Ulman et al. who have found an almost perpendicular orientation of the biphenyl moiety for various 4′-substituted 4-biphenylthiols on silver substrates with tilt angles varying between 8° and 21°.20 However, the relative band intensities of the three bands observable in the RAIR spectrum significantly deviate from the band intensities in the bulk spectrum. Whereas in the bulk spectrum the band at 1479 cm-1 is of similar intensity as the band located at 756 cm-1, the latter is hardly visible in the RAIR spectrum. This phenomenon has been observed before and can be ascribed to the fact that the magnitude of a TDM is dependent on its environment, which is different for the bulk state and the SAM.32 (b) Adsorption of BDT from a Pure BDT Solution. Turning to the IR spectra of BDT (Figure 4), it can be seen that the most dominant bands of the bulk spectrum are still observable in the RAIR spectrum of a BDT film prepared by immersion in a 1 mM BDT solution for 15 h without TBP. The bands in the RAIR spectrum located at 1589, 1472, and 999 cm-1 and can be assigned to in-plane parallel (ip-par) vibrations. Additionally, bands at 1386 and 1096 cm-1, which originate from in-plane perpendicular (ip-perp) vibrations and an intensive out-of-plane vibration at 805 cm-1 are visible in the spectrum. The similarity of the bulk and the surface spectrum of BDT indicates that the order of the BDT film on silver is very poor. Furthermore, the very high intensities of the bands observable in the RAIR spectrum of BDT indicate that a multilayer of BDT molecules has formed which agrees with our interpretation of the results obtained from the SPR measurements. The additional layers cannot be removed by thoroughly rinsing the sample with dichloromethane indicating that the BDT layers are linked by strong interactions which can be ascribed to the formation of disulfide bridges by oxidative coupling of the thiol groups. However, it is not possible to verify the existence of disulfide bonds in the multilayer by RAIRS since the infrared active vibration of disulfides appears at around 400-600 cm-1 and is beyond the detection range of a MCT detector.33,34 To study the adsorption process and obtain a more detailed insight in the mechanism of the multilayer (32) Rong, H.-T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, C.; Helmchen, G. Langmuir 2001, 17, 1582.
Figure 5. RAIR spectra of BDT on silver after immersion of the silver substrate for (a) 1 h, (b) 4 h, and (c) 12 h.
formation, a silver substrate was immersed in a 1 mM solution of BDT in dichloromethane, removed after 1 h (a), 4 h (b), and 12 h (c), and rinsed with dichloromethane and then a RAIR spectrum was recorded. After the measurement, the substrate was reimmersed in the selfassembly solution. The obtained spectra are shown in Figure 5. In the spectrum taken after immersion for 1 h (Figure 5a), three bands are clearly visible at 1594, 1473, and 1001 cm-1, which can be assigned to vibrations with a TDM parallel to the 4,4’-axis. Two further bands with weak intensity can be observed at 809 and 1089 cm-1, which possess a TDM oriented orthogonal to the 4,4’-axis. The spectrum is very similar to the spectrum taken of a BT monolayer indicating that in the first step a monolayer of BDT molecules adsorbs on the surface with the 4,4’axis oriented perpendicular to the surface analogous to the adsorption of BT. However, it is not possible to determine the packing density of the monolayer by the RAIRS measurements, and thus, it is not certain that the surface is completely covered with BDT molecules before the multilayer formation sets in. After a total immersion time of 4 h the intensity of the bands located at 809 and 1089 cm-1 has significantly increased whereas the intensity of the bands at 1594, 1473, and 1001 cm-1 has increased very little (Figure 5b). The strong increase of the bands originating from out-of-plane and in-plane perp vibrations can be explained by the adsorption of an additional layer of BDT molecules in which the molecules are oriented parallel to the surface. The spectrum taken after a total immersion time of 12 h shows that the intensity of all bands has strongly increased (Figure 5c). Altogether the spectrum is virtually identical to the bulk spectrum indicating that a multilayer has formed in which the molecules are randomly oriented. Since the extent of multilayer formation depends on the immersion time, one could argue that well-ordered SAMs of BDT can be easily obtained by decreasing the exposition time of the substrate to the self-assembly (33) Devlin, M. T.; Kalasinsky, V. F.; Levin, I. W. J. Mol. Struct. 1989, 213, 35. (34) Kluth, G. J.; Carraro, C.; Maboudian, R. Phys. Rev. B 1999, 59, R10449.
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Figure 6. IR spectra of BDT in KBr and in a SAM on silver prepared by immersion of the silver substrate in a solution of BDT/TBP (1 mM with respect to each component) in dichloromethane for 15 h. The arrows mark the missing out-of-plane and ip-perp vibrations in the RAIR spectrum.
solution. However, this approach exhibits two major drawbacks: First, upon immersion of the silver substrate in the solution both the adsorption of the thiols to the silver surface and the oxidation of unreacted thiol groups take place in parallel. Although the oxidation proceeds at a considerably slower reaction rate than the adsorption, it is impossible to completely exclude the oxidation of terminal thiol groups at the SAM surface. Second, studies on the adsorption of BT on gold have shown that immediately after the initial chemisorption of BT the lateral order of the SAM is very poor and that the following surface diffusion process which eventually leads to the formation of well-ordered and densely packed monolayers takes at least 12 h.35 It is reasonable to assume that a similar process takes place during the adsorption of BT and BDT on silver substrates and therefore immersion times of at least 12 h are advisable in order to ensure lateral order and a dense packing of the SAM, but then multilayer formation is inevitable. (c) Adsorption of BDT from a Mixed BDT/TBP Solution. If a silver substrate is immersed in a solution containing equimolar amounts of TBP and BDT for 15 h, the low-frequency region of the resulting RAIR spectrum shown in Figure 6 is dominated by three bands at 1594, 1473, and 1001 cm-1, all of which can be assigned to ippar vibrations. The presence of the ip-par bands in conjunction with the complete absence of bands originating from ip-perp and out-of-plane vibrations indicates that the molecules are oriented with their 4,4′-axis perpendicular to the surface as it has been observed for a SAM formed of BT; i.e., the tilt angle is between 0° and 10°. Since it is well-known that tertiary phosphines adsorb on a silver surface and form stable monolayers under ambient conditions,36 it is possible that the addition of TBP to the self-assembly solution of BDT results in the adsorption of BDT as well as TPB on the silver substrate. However, the interaction of a thiol group with a silver surface is stronger than the corresponding interaction of a tertiary phosphine and the adsorption of a BDT is therefore expected to be thermodynamically favored over the adsorption of TBP. The presence of TBP on the surface would lead to the appearance of the C-H stretching (35) Azzam, W.; Wo¨ll, C. Unpublished results. (36) Westermark, G.; Kariis, H.; Persson, I.; Liedberg, B. Colloid Surf., A 1999, 150(1-3), 31.
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vibrations in the range from approximately 2800 to 3000 cm-1. As anticipated, no bands at all can be observed in this region, which reveals that TBP is finally not adsorbed on the silver substrate. We conclude that the presence of TBP in the self-assembly solution completely prevents the formation of multilayers and most likely allows the preparation of a well-ordered monolayer but has no influence on the overall kinetics of the chemisorption of the dithiol on the silver surface. The standing-up configuration of the BDT SAM should yield a thiol-terminated surface, and the S-H stretching vibration of the thiol groups exposed to the SAM-air interface should be observable in the RAIR spectrum. However, the band located at 2550 cm-1 in the bulk spectrum which can be assigned to the S-H stretching vibration is not visible in the RAIR spectrum of a BDT monolayer (Figure 6). The absence of the S-H stretching vibration is not necessarily caused by the absence of thiol groups on the surface for the following reasons: In the case of a standing-up orientation of the BDT molecules on the surface, the angle between the surface normal and the TDM of the S-H stretching vibration is close to 90°; i.e., the component of the TDM perpendicular to the surface is very small and therefore the intensity in the RAIR spectrum is much weaker than that in the bulk spectrum. Considering the poor signal-to-noise ratio of RAIR spectra and the fact that the S-H vibration only appears as a weak band in the bulk spectrum anyway, it is possible that the band originating from the S-H vibration gets lost in the background signal. However, it was possible to prove the existence of free thiol groups at the surface of the SAM by XPS measurements as will be discussed in another section. (d) Adsorption of BDT from a Pure BDT Solution and Subsequent Immersion into a TBP Solution. The RAIR study on the mechanism of the multilayer formation has revealed that in the first step of the adsorption process, a monolayer of BDT molecules adsorbs on the surface with their 4,4′-axes oriented perpendicular to the surface and that disorder is introduced by the formation of disulfides and the subsequent adsorption of additional layers of BDT. Since TBP effectively reduces disulfide bonds, it should be possible to convert a disordered BDT film consisting of multilayers into an ordered monolayer by exposure to TBP. To verify the validity of this concept, the BDT multilayer which was prepared by immersion of a silver substrate in a solution of pure BDT for 12 h (Figure 7a) was immersed in a 1 mM solution of TBP for 2 h. The resulting RAIR spectrum (Figure 7b) clearly exhibits the same features as the RAIR spectrum taken from a BDT monolayer prepared by immersion of a silver substrate in a mixed BDT/TBP solution. This result indicates that the additional layers of BDT linked to the first monolayer by disulfide bonds have been removed by reduction with TBP indeed. X-ray Photoelectron Spectroscopy. The photoelectron spectra of the SAMs prepared by immersion of a silver substrate for 15 h in a solution of BT and BDT/TBP support the data obtained from SPR and RAIRS measurements. Figure 8 shows the X-ray photoelectron spectra of the S2p region. For the peak fit, the energy difference between the S2p1/2 and S2p3/2 components was set to 1.2 eV, their intensity ratio was set to 1:2.37 In the spectrum of a BT SAM, a single S2p doublet with the S2p3/2 component located at 162.0 eV can be observed, which agrees well with the reported shift of the S2p3/2 (37) Scofield, J. H. J. Electron Spectrosc. 1976, 8, 129.
SAMs of 4,4′-Biphenyldithiol on Silver
Figure 7. RAIR spectra (a) of a BDT multilayer prepared by immersion of a silver substrate in a 1 mM solution of BDT for 12 h and of the same sample after immersion in a solution containing TBP for 2 h.
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Figure 9. Illustration of the suggested adsorption process of BT, BDT, and BDT/TBP on silver. In the case of BDT, the formation of disulfide-linked multilayers sets in at an early stage of the adsorption process. The presence of TBP in the solution effectively prevents this process.
can be assigned to sulfur atoms in free thiol groups (SH),38,39 indicating that a certain amount of thiol groups does not react with the silver surface to form S-Ag bonds. The intensity of the S2p signal at 163.6 eV (SH) in the spectrum from a BDT SAM is slightly larger than the intensity of the 162.0 eV S2p component (S-Ag) with a ratio of I(163.6 eV)/I(162.0 eV) ) 1.3 ( 0.1. A value >1 is expected if the BDT molecules bind with only one thiol group per molecule to the surface and adopt a standingup orientation with the molecular backbone oriented perpendicular to the surface. In this case, electrons emerging from the sulfur atoms bound to the surface are attenuated by the overlying organic monolayer, whereas electrons from sulfur atoms in thiol groups exposed to the surface of the SAM do not experience such attenuation. The expected ratio I(163.6 eV)/I(162.0 eV) for a SAM of BDT, in which the molecules form a densely packed monolayer with the molecular axis oriented perpendicular to the surface, can be calculated on the basis of LambertBeer’s law
I(162.0 eV) ) I(163.6 eV) exp(-d/λ)
Figure 8. X-ray photoelectron spectra of the S2p region from SAMs prepared from (a) BT and (b) BDT/TBP on silver. The signals at 162 and 163.6 eV originate from sulfur atoms in a thiolate and a thiol species, respectively.
signal for sulfur atoms in thiolate-silver (S-Ag) bonds.38 This confirms that a monolayer of BT has formed in which the molecules are bound to the silver surface via thiolatemetal bonds. In contrast, the spectrum of a BDT SAM reveals the presence of a second sulfur species giving rise to a signal with the S2p3/2 component at 163.6 eV in addition to the signal at 162.0 eV. The signal at 163.6 eV (38) Bensebaa, F.; Zhou, Y.; Deslandes, Y.; Kruus, E.; Ellis, T. H. Surf. Sci. 1998, 405, L472.
(2)
where d is the length of the biphenyl unit with one thiol group, which was calculated to be 9.5 ( 1 Å, and λ is the mean free path of the S2p photoelectrons through the hydrocarbon film. The value of λ was estimated to be 41 ( 2 Å based on the reported attenuation lengths for C1s and Au4f photoelectrons, excited by Al KR radiation, in hydrocarbon films.40 Equation 2 provides a value of 1.26 ( 0.05 for the ratio I(163.6 eV)/I(162.0 eV), which is consistent with the measured value of 1.3 ( 0.1. In Figure 9, a schematic illustration summarizes the proceedings of mono- and multilayer formation which have been inferred from the data obtained by SPR , RAIRS, and XPS measurements. (39) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (40) Hansen, H. S.; Tougaard, S.; Biebuyck, H. J. Electron Spectrosc. Relat. Phenom. 1992, 58, 159.
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Conclusions The reliable preparation of ordered dithiol monolayers presents an important step in the fabrication of molecular devices based on oligophenyldithiols. Therefore, we have investigated the adsorption of 4-biphenylthiol and 4,4′biphenyldithiol on polycrystalline silver by surface plasmon resonance spectroscopy, reflection absorption infrared spectroscopy, and X-ray photoelectron spectroscopy. It has been found that in both cases the thiols adsorb on the surface within minutes forming a monolayer in which the aromatic planes of the molecules exhibit a tilt angle between 0° and 10° away from the surface normal. In contrast to the adsorption of the monothiol which leads to well-ordered monolayers, the dithiol shows a pronounced tendency to the formation of multilayers in which the molecules are most likely linked to each other via disulfide bonds generated by oxidation of the thiol groups. Strictly speaking, there is no direct evidence for the presence of disulfide bonds in the multilayers formed by BDT. But since we found that the addition of tri-nbutylphosphine to the self-assembly solution of BDT
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effectively prevents the multilayer formation and since it is well-known that TBP reduces disulfides to give the corresponding thiols, it is reasonable to assume that indeed the oxidative coupling of the thiol groups is responsible for the formation of multilayers. The structural investigation of a SAM prepared from BDT and TBP has shown that a well-ordered BDT monolayer has formed in which the molecules adopt a standing-up orientation yielding a thiol-terminated surface. The addition of TBP to the selfassembly solution of aromatic dithiols effectively prevents the formation of multilayers. Whether this approach is applicable to the preparation of ordered monolayers of aromatic dithiols on gold (111) surfaces is currently under investigation. Acknowledgment. We thank Dr. R. Arnold for the introduction in the field of RAIR spectroscopy and his constant helpfulness. U. Weckenmann thanks the Graduierten Kolleg “Dynamische Prozesse an Festko¨rperoberfla¨chen” for a fellowship. LA011857S
