Langmuir 2006, 22, 3647-3655
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Combined STM and FTIR Characterization of Terphenylalkanethiol Monolayers on Au(111): Effect of Alkyl Chain Length and Deposition Temperature W. Azzam,† A. Bashir,‡ A. Terfort,§ T. Strunskus,‡ and Ch. Wo¨ll*,‡ Department of Chemistry, Tafila Technical UniVersity, P.O.Box 179, Tafila, Jordan, and Lehrstuhl fu¨r Physikalische Chemie I, Ruhr-UniVersita¨t Bochum, D-44780 Bochum, Germany, and Institut fu¨r Anorganische und Angewandte Chemie, UniVersita¨t Hamburg, D-20146 Hamburg, Germany ReceiVed NoVember 14, 2005. In Final Form: February 16, 2006 Self-assembled monolayers (SAMs) of 4,4′-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)n-SH (TPn, n ) 1-6) on Au (111) substrates were studied using scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRRAS). When the SAMs were prepared at room temperature (RT, 298 K), TPn films (except TP2) exhibit an odd-even effect regarding both molecular orientation and packing density. For all investigated films, STM data reveals the presence of a large degree of lateral order. In the case of odd-numbered TPns, the films revealed a (2x3 × x3)R30° molecular arrangement. For the even-numbered TP4 and TP6 SAMs, a c(5x3 × 3) rectangular unit cell was found. The packing density for the even-numbered TPn SAMs is 25% lower than that for the oddnumbered TPn SAMs. When the SAMs were prepared at 333 K, the even-numbered SAMs were found to form structures with a significantly lower packing density. In the case of TP2, instead of the (2x3 × x3)R30° structure formed at room temperature, a c(5x3 × 3) structure was observed. For TP6 SAMs, the room-temperature c(5x3 × 3) structure was replaced by a (6x3 × 2x3)R30° structure.
1. Introduction During the last two decades, self-assembled monolayers (SAMs) of organic molecules on metal surfaces, in particular on Au(111), have attracted significant attention because of their applications in molecular technologies.1,2 In most cases, the preparation of SAMs leads to the formation of high-quality and well-defined organic surfaces with a homogeneous composition, structure, and thickness. Therefore, SAMs can serve as an ideal model system in understanding various interfacial phenomena, such as wetting,3,4 adhesion,5,6 and catalysis.7 Many applications have emerged as a result of the robust, closely packed, and highly ordered structures of these monolayers.8-13 In the case of n-alkanethiolate films on Au(111), it was reported that the alkane chains are tilted by about 35° with respect to the surface normal. The distance between adjacent sulfur atoms was found to be 5 Å. The latter value is larger than the bulk value for the distance between closely packed alkane chains (4.2-4.4 Å). As a result, the alkane chains tilt away from the surface normal in order to achieve a close-packed structure. * To whom correspondence should be addressed. E-mail: woell@ pc.rub.de. † Tafila Technical University. ‡ Ruhr-Universita ¨ t Bochum. § Universita ¨ t Hamburg. (1) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (2) Ulman, A. An Introduction to Ultrathin Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (3) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (4) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (5) Kidoaki, S.; Matsuda, T. Langmuir 1999, 15, 7639. (6) Kim, S.; Choi, G. Y.; Ulman, A.; Fleischer, C. Langmuir 1997, 13, 6650. (7) Berman, A.; Steinberg, S.; Campbell, S.; Ulman, A.; Israelachvili, J. Tribol. Lett. 1998, 43. (8) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (9) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (10) Poirier, G. E. Langmuir 1999, 15, 1167. (11) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (12) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (13) Poirier, G. E.; Fitts, W. P.; White, J. M. Langmuir 2001, 17, 1176.
There is now a general agreement that n-alkanethiols on Au(111) form a c(4 × 2) superstructure with regard to a (x3 × x3)R30° basic lattice. Recently it has been realized that the lateral chain-chain interaction plays an important role in both the process of SAM formation and in the stabilization of the SAMs.14,15 Alkanethiolatebased SAMs with short alkyl chains, for example, exhibit film properties which are quite different from those found for long alkyl chains. Therefore, it has to be expected that replacing the alkyl molecular backbone within the alkanethiols by chains of different composition will lead to significant effects on the molecular packing and on the properties of the SAM. In the past years, the interest has been shifted to organothiol molecules with aromatic backbones. In a number of publications the monolayers of aromatic oligophenyl thiols have been studied.16-28 In some cases, it has been found that the structural quality of oligophenyl thiols is superior to that of alkanethiolate (14) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (15) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G. Y.; Jennings, G. K.; Yong, T. H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002. (16) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (17) Buckel, F.; Effenberger, F.; Yan, C.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 901. (18) Kang, J. F.; Ulman, A.; Jordan, R.; Kurth, D. G. Langmuir 1999, 15, 5555. (19) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 805. (20) Tour, J. M.; Jones, L.; 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. (21) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 1999, 103, 1686. (22) Al-Rawashdeh, N. A. F.; Azzam, W.; Wo¨ll, C. in preparation. (23) Fuxen, C.; Azzam, W.; Arnold, R.; Terfort, A.; Witte, G.; Wo¨ll, C. Langmuir 2001, 17, 3689. (24) Arnold, R.; Terfort, A.; Wo¨ll, C. Langmuir 2001, 17, 4980. (25) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 10, 3980. (26) Niklewski, A.; Azzam, W.; Strunskus, T.; Fischer, R. A.; Wo¨ll, C. Langmuir 2004, 20, 8620. (27) Azzam, W.; Fuxen, C.; Birkner, A.; Rong, H.-T.; Buck, M.; Wo¨ll, C. Langmuir 2003, 19, 4958.
10.1021/la053065u CCC: $33.50 © 2006 American Chemical Society Published on Web 03/17/2006
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SAMs. The latter finding is probably due to the presence of fairly strong interactions between the aromatic moieties within the SAM.29,30 Moreover, the aromatic thiol molecules are more rigid than n-alkanethiols, and this property provides a greater stability for the molecules within the SAMs.20,23,24 Last but not least, due to their extended π-systems, aromatic thiols are very promising with regard to applications in the field of molecular electronics. The oligophenyl based rigid-rod thiols, which were previously studied, include phenyl (Ph),31 biphenyl, 32-39 and terphenyl21,23 as well as oligophenyl systems linked by acetylenic units.40 In these oligophenyl-based organothiols, the sulfur is either connected directly to the aromatic system of the last phenyl unit or linked via an alkyl unit (i.e., Ph-(CH2)n-S).32,38 The previous studies revealed that the inclusion of an alkyl chain between the sulfur atom and the first phenyl unit significantly affects the structure and in particular the surface morphology of the SAMs. In the presence of a short alkyl chain spacer, STM investigations have revealed a morphology similar to that obtained for n-alkanethiols on gold formed at room temperature, namely the presence of typically roundish holes in the top layer of the gold substrate.41 For oligophenylthiols without an alkane chain spacer, islands instead of the roundish holes were observed. Highresolution STM images obtained for such systems revealed that the islands are also covered with thiolates and exhibit the same (2x3xx3)R30° molecular arrangement as observed between the islands.33 In general, the insertion of methylene units into aromatic thiols between the sulfur headgroup and the last phenyl moiety yields films with a morphology and structure comparable to those of n-alkanethiols.33 To carry out a systematic investigation of the different phenomena occurring at the S-Au organic interface, several groups have started to investigate homologous series of organothiols which consist of a biphenyl moiety and an alkane spacer of varying length between the aromatic system and the thiol unit (CH3-(C6H4)2-(CH2)n-SH, BPn, n ) 1-6).33-35,38,39,42 The previous microscopic and spectroscopic studies of this thiol molecules series on Au and Ag surfaces have revealed that the S-Au organic interface, i.e., the presence and the precise length of the alkyl chain, has a significant influence on the structure of the adsorbate molecules within the SAM. It was found that the orientation and the molecular packing density of the biphenyl moieties are affected by the length of the spacer between the sulfur atom and the benzene ring (i.e., by the number of methylene (28) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18 (21), 7766. (29) Cyganik, P.; Buck, M. J. Am. Chem. Soc. 2004, 126, 5960. (30) Cyganik, P.; Buck, M.; Wilton-Ely, J. D. E. T.; Wo¨ll, C. J. Phys. Chem. B 2005, 109, 10902. (31) Wan, L.-J.; Hara, Y.; Noda, H.; Osawa, N. J. Phys. Chem. B 1998, 102, 5943. (32) Leung, T.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34. (33) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wo¨ll, C. Langmuir 2003, 19, 8262. (34) M. Zharnikov; S. Frey; H. Rong; Y.-J. Yang; K. Heister; M. Buck; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (35) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, C.; Helmchen, G. Langmuir 2001, 17, 1582. (36) Heister, K.; Rong, H. T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 6888. (37) Felgenhauer, T.; Rong, H. T.; Buck, M. Electroanal. Chem. 2003, 550, 309. (38) Long, Y. T.; Rong, H. T.; Buck, M.; Grunze, M. Electroanal. Chem. 2002, 524-525, 62. (39) Frey, S.; Rong, H. T.; Heister, K.; Yang, Y. J.; Buck, M.; Zharnikov, M. Langmuir 2002, 18, 3142. (40) Yang, G.; Quian, Y.; C.;, E.; Sita, L. R.; Liu, G. Y. J. Phys. Chem. B 2000, 104, 9059. (41) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, C.; Grunze, M. Langmuir 1993, 9, 4.
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groups in the aliphatic part); that is, a pronounced odd-even effect has been reported.43 On the basis of a systematic analysis of biphenyl films grown at higher temperature, it has been suggested that the presence of the alkyl spacer can reduce the stress within the films and thus can lead to an improved molecular packing.30 Furthermore, on silver, the orientational odd-even effect is opposite to that on gold.35 Thus, for odd numbers of CH2 units in the BPn SAMs on Au or for an even number on Ag, a denser molecular packing and a less tilted orientation of the biphenyl moieties occur. For an even number of the CH2 units in the BPn SAMs on Au or for an odd number on silver, on the other hand, the opposite behavior at reduced density and a larger tilt of the BP moieties is found. These observations were explained by a simple model considering the intermolecular interactions and the C-S-Au bending potential.35 Very recently, oligophenyl thiols containing a terphenyl backbone together with an alkyl spacer, i.e., the series C6H5(C6H4)2(CH2)n-SH (TPn, n ) 1-6), have been investigated using different spectroscopic techniques on Au and Ag surfaces.44 These thiols are different from BPn thiols in that they contain an additional benzene ring in the molecular backbone. Since the interaction between the terphenyl units is significantly larger than for the biphenyls (sublimation temperature of 649 K for p-terphenyl vs 529.3 K for biphenyl)45 one might expect a significantly smaller influence of the length of the alkyl spacer. The spectroscopic results of TPn SAMs revealed, however, an odd-even behavior with regard to the number of methylene chains in the alkyl spacer similar to that observed for the BPn SAMs.44 In the present study, previous work providing information about the orientation of the single molecules within the terphenyl SAMs is extended by using high resolution scanning tunneling microscopy (STM). To support these investigations, a detailed spectroscopic analysis has been carried out using infrared spectroscopy. Since it has been shown in previous studies of the BPn system that the structure of the monolayers not only depends on the length of the alkyl chains but also on the deposition temperature, we have also investigated the effect of deposition temperature on the structure of the TPn SAMs. 2. Experimental Section 2.1. Chemicals. TPns (n ) 1-6) were synthesized and purified using previously described procedures.46 Ethanol (Baker), acetone (Baker), and chloroform (Baker) were used as received. 2.2. Sample Preparation. 2.2.1. Substrates. For the IRRAS investigations, polycrystalline Au substrates were prepared by evaporating 5 nm of titanium (99.8%, Chempur) and subsequently 100 nm of gold (99.995%, Chempur) onto polished silicon wafers (Wacker) in an evaporation chamber (Leybold) operated at a base pressure of about 10-7 mbar. The thickness and the deposition rate (20 Å s-1) were monitored using a quartz crystal oscillator (Leybold Inficon). These substrates were stored in a vacuum desiccator until the adsorption experiments were carried out. For the STM investigations, first a freshly cleaved sheet of mica was heated to 370 °C for about 48 h inside the evaporation apparatus to remove residual water contained between the mica sheets. Subsequently, 100 nm of Au were deposited at a substrate temperature of 370 °C and a pressure of approximately 10-7 mbar. After (42) Cyganik, P.; Buck, M.; Azzam, W.; Wo¨ll, C. J. Phys. Chem. B 2004, 108, 4989. (43) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018. (44) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnicov, M. J. Phys. Chem. B 2004, 108, 14462. (45) David, R. L. Handbook of Chemistry and Physics, 79 ed.; CRC Press: Boca Raton, FL, 1998. (46) Mu¨ller, J.; Brunnbauer, M.; Schmidt, M.; Zimmermann, A.; Terfort, A. Synthesis 2005, 998.
Characterization of TPn Monolayers
Langmuir, Vol. 22, No. 8, 2006 3649 Table 1. Vibrational Mode Assignment for TP1 in the Solid State (KBr) and for SAMs on Au following the Notation of Varsanyi et al.56 a band position of TP1 [cm -1] direction of transition dipole ⊥ 4,4′-axis ⊥ 4,4′-axis | 4,4′-axis ⊥ 4,4′-axis | 4,4′-axis
Figure 1. Comparison of the IR spectrum of bulk TP1 (KBr pellet) with the spectrum of the SAM of TP1 on Au(111) prepared at 298 K. deposition, the substrates were allowed to cool, and the vacuum chamber was vented with purified nitrogen. Between substrate preparation and SAM formation, the substrates were stored in an argon atmosphere. Immediately before the SAM formation, the substrates were flame-annealed using a butane-oxygen flame. This procedure yielded Au substrates with well-defined terraces exhibiting a (111)-oriented surface. The terraces are separated by steps of monatomic height and have sizes in excess of 100 nm. 2.2.2. Preparation of the Terphenylthiolate SAMs. TPn (n ) 1-6) monolayers on Au(111) substrates were prepared by immersing the gold substrates into dilute ethanolic solutions (2.5 µM) of the thiols for 24 h. Different solution temperatures were used (see text). The substrate was removed from the solution and rinsed carefully with pure ethanol, acetone, chloroform and again ethanol. Finally the substrates were dried in a stream of dry nitrogen. 2.3. Structural Investigations. IRRAS spectra were recorded using a Biorad Excalibur FTIR spectrometer (FTS 3000) equipped with a grazing incidence reflection unit (Biorad Uniflex) and a narrow band MCT detector. All spectra were recorded using a resolution of 2 cm-1 and an angle of incidence of 80° relative to the surface normal and further processed by using boxcar apodization. Baseline correction was done using spline functions. STM data were obtained using a commercial Nanoscope IIIa Multimode microscope (Digital Instruments, Santa Barbara, CA) equipped with a type “E” scanner. The tips were prepared by cutting a Pt/Ir (80:20, Chempur) wire mechanically. All STM micrographs were recorded in air at room temperature in constant current mode. The STM data presented here and in the Supporting Information correspond to unfiltered (except where explicitely stated), uncorrected raw data. As a result, thermal drifts are not compensated for and lead to a distortion of, e.g., the rectangular unit cells of the SAM superlattices. In all cases, we have carefully checked (by a detailed analysis of the backscans, etc.) that the structural models presented below are consistent with the STM data.
3. Results 3.1. IR. In Figure 1, the low-frequency region of a TP1 SAM fabricated at 298 K is shown together with the corresponding bulk spectrum recorded using KBr pellets. A comparison of the band positions for bulk samples embedded in KBr and SAM spectra is provided in Table 1 together with an assignment of the band positions. Three different labels will be used to assign the IR-bands: op for out-of-plane modes, i.e., where the transition dipole moment (TDM) is orientated perpendicular to both the phenyl ring plane and the molecular axis; ip-par for the case when the TDM is parallel to both the phenyl-ring-plane and the terphenyl chain axis; and ip-perp for the case when the TDM is orientated parallel to the phenyl ring plane but perpendicular to the terphenyl 4,4′ axis.
| 4,4′-axis
assignment
KBr
ring, C-H op, 11, r1* ring, C-H, op, 17b, r2 ring, C-H, ip-par, 18a, r2 and r3 ring,C-H str ring, C-C ip-par, 19a ring, C-C str
761
SAM (298 K)
SAM (333 K)
824
814
1002
1002
1002
1385 1485
1485
1487
1438
a
r1 is the top monosubstituted phenyl ring, r2 is the middle ring, and r3 is the sulfur substituted ring.
The most intense absorption peaks in the KBr spectrum are located at 761, 824, 1002, and 1485 cm-1. The low-frequency vibrations located at 761 and 824 cm-1 are assigned to op ring modes of the mono- and para-substituted phenyl units, respectively. The bands located at 1002 and 1485 cm-1 are assigned to C-H bending and C-C ip-par ring modes, respectively. The IRRAS data recorded for TP1 SAMs on Au(111) reveals significant differences relative to the corresponding KBr spectrum. Below 1600 cm-1, only two bands with high intensity can be detected for the SAM located at 1002 and 1487 cm-1. In addition to these bands, a weak band is seen at 814 cm-1. The strong reduction in intensity of some of the bands results from the so-called surface selection rule governing vibrational spectroscopy at metal surfaces, which states that only those vibrations can be seen which have a component of their TDM orientated perpendicular to the surface of the metal substrate.24,47 The disappearance and reduction of the bands at 761 and 814 cm-1, respectively, is explained by their TDM being oriented almost parallel to the surface. From the large reduction of the relative intensities of the op peaks (761 and 814 cm-1), we conclude that the average orientation of the TP1 molecular axis is very close to the surface normal. The IRRAS spectra recorded for the complete series of TPn SAMs on gold obtained for samples prepared at 298 K are shown in Figure 2. In the odd-numbered case (TPn with n ) 1, 3, and 5), the peak at 761 cm-1 is almost completely absent in all the spectra. The peak at 814 cm-1 exhibits a very low intensity only. Since the relative intensity of the op-band to the ip-par bands is very low, it can be concluded that the TPn (n ) 1, 3, and 5) molecules adopt a standing-up conformation with their aromatic backbone orientated almost perpendicular to the gold surface. The IR spectra obtained for TP2 are quite similar to those of the odd-numbered SAMs. On the basis of this observation, we conclude that the orientation of the terphenyl backbone must be similar. Compared to TPn with n ) 1, 2, 3, and 5, the IRRAS spectra of TP4 and TP6 SAMs on Au(111) collected at 298 K exhibit significant differences. The op bands at 761 and 825 cm-1 appear in the TP4 and TP6 spectra with high intensity. The relative intensities of the op bands to the ip-par bands are significantly increased, which strongly indicates that the molecules are significantly tilted away from the surface normal. From a comparison to previous IR data for terphenyls,23 we yield an estimate of this tilt angle of about 40°. (47) Chabal, Y. J. Surf. Sci. 1988, 8, 211.
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Figure 3. Low-frequency region of the IR spectra of bulk TPns (KBr pellet) as well as that of the corresponding SAMs on Au(111) prepared at 333 K with n ranging from 1 to 6.
(333 K) results in a molecular orientation in which the aromatic backbone is significantly tilted away from the surface normal. 3.2. STM. 3.2.1. Common Features for All TPn SAMs. For all TPn SAMs, the well-known depressions (or pit holes) are observed. These depressions were attributed to vacancy islands in the top Au layer.41,48 Their depth amounts to the height of a step on a Au(111) surface, i.e., 2.4 Å. A comparison of SAMs prepared at 298 K with those prepared at 333 K reveals a pronounced increase in the size and a corresponding decrease in the density of the depressions which has been explained by an Ostwald ripening process.49 Moreover, the size of the ordered domains is found to increase with preparation temperature. At 298 K, the size of the ordered domains is in the range of 5-30 nm. Depending on the length of the alkane spacer, this size increases to 40-70 nm at 333 K. The clean Au(111) surface exhibits a well-known herringbone reconstruction50,51 which locally reduces the surface symmetry from 3-fold to 2-fold. It has been observed in many cases that this reconstruction is lifted upon chemisorption of reactive species.52 There is no indication in the present STM data that the herringbone reconstruction of the clean Au(111) substrate affects the formation and, in particular, the orientation of the domains in the TPn SAMs studied here. 3.2.2. Odd-Numbered TPn SAMs. Among the TPn monolayers with odd-numbered n, the TP1 samples prepared at 298 K created the most difficulties with regard to achieving high resolution STM images. From eight samples prepared under the same conditions, molecular resolution could be obtained only in one case. Even in this favorable case, the quality of the STM topography is rather poor. We attribute these problems to the presence of a high density of defects such as the depressions and domain boundaries. When the preparation was carried out at 333 K, high-resolution STM images could be obtained in a much more straightforward fashion (see Figures S2 and S3 of the Supporting Information). A detailed analysis of all available data revealed, however, that the only difference was a lower defect density for the SAMs prepared at higher temperature; the molecular arrangement was, within the experimental resolution, the same. In the cases of TP3 and TP5 SAMs, high-resolution STM data could be obtained in a straightforward fashion. The corresponding data revealed the presence of structural features quite similar to those observed for TP1. The structural quality, however, was found to be superior with regard to the TP1 case (see Figures S4-S6 in the Supporting Information). In the following, we will use the high-resolution STM images of a TP5 monolayer prepared at 333 K (Figure 4) for a detailed discussion of the structural features common to the odd-numbered TPn SAMs. This micrograph shows domains with diameters of about 40 nm which have a row structure that can adopt three different orientations. A detailed analysis of the structures in high-resolution STM data using line profiles reveals that TP5 SAMs, as all of the other TPn SAMs with odd n, exhibit a periodic structure with a (2x3 × x3)R30° unit cell. The unit cell contains two inequivalent molecules, corresponding to an area of 21.6 Å2 per molecule. No significant changes with variation of the solvent temperature were observed. The data are consistent with the sulfur atoms
Figure 3 shows IRRAS spectra recorded for TPn (n ) 1-6) SAMs prepared at 333 K. Close inspection of the data reveals that with the exception of the TP2 spectrum no significant changes are present when compared with the SAMs prepared at room temperature (298 K). Only for TP2 SAMs, the intensity of the op-band located at 761 cm-1 is found to be significantly increased. This observation suggests that adsorption of TP2 from hot solution
(48) Edinger, K.; Grunze, M.; Wo¨ll, C. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1811. (49) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (50) Harten, U.; Lahee, A. M.; Toennies, J. P.; Wo¨ll, C. Phys. ReV. Lett. 1985, 54, 2619. (51) Wo¨ll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. ReV. B 1989, 39, 7988. (52) Huang, L.; Zeppenfeld, P.; Chevrier, J.; Comsa, G. Surf. Sci. 1996, 352, 285.
Figure 2. Low-frequency region of the IR spectra of bulk TPns (KBr pellet) as well as that of the corresponding SAMs on Au(111) prepared at 298 K with n ranging from 1 to 6.
Characterization of TPn Monolayers
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Figure 4. Constant-current STM micrographs showing the gold substrate after immersion into a 2.5 µM ethanolic solution of TP5 at 333 K for 24 h. In (C), the unit cell of the (2x3 × x3)R30° structure is marked by the oblique. Tunneling parameters: (A) Ut ) 531 mV, It ) 153 pA; (B) Ut ) 392 mV, It ) 150 pA; and Ut ) 530 mV, It ) 159 pA.
forming a (x3 × x3)R30° lattice, as is the case for the n-alkanethiolate SAMs. Considering the van der Waals dimensions of the phenyl rings (6.4 Å by 3.3 Å)23 with a cross-sectional area of 21.1 Å2 for the phenyl rings and the area per molecule in the (2x3 × x3)R30° structure (21.6 Å2), we expect a tilt angle of about arccos (21.1/21.6) ) 12.5° with respect to the surface normal. 3.2.3. Even-Numbered TPn SAMs. Figure 5 shows representative STM data recorded for TP2 SAMs prepared at 298 K. The high-resolution STM image reveals that the TP2 molecules adopt the same molecular arrangement as observed for the oddnumbered TPn, i.e., a (2x3 × x3)R30° structure. Figure 6 shows STM micrographs recorded for TP2-SAMs prepared from a 2.5 µM solution at 333 K. At this higher preparation temperature, a quite different structure is seen. Figure 6B shows a well-defined periodic arrangement of TP2 molecules exhibiting a unit cell which is significantly larger than that of the (2x3 × x3)R30° structure seen for TP2 SAMs prepared at room temperature. The same structure is found in TP4 SAMs, prepared at either 298 or 333 K. For the samples prepared at 333 K, the typical size of an ordered domain amounts to about 50 nm (Figure 7), whereas for the preparation temperature of 298 K, significantly smaller domain sizes are observed (see Figure S7 of the Supporting Information). These high-resolution STM data show a highly ordered adlayer structure characterized by a rectangular primitive unit cell with dimensions of 8.65Å × 25Å. The unit cell consists of eight molecules, corresponding to an area of 27 Å2 per molecule. The simplest commensurate structure consistent with these lateral dimensions is a c(5x3 × 3) structure. Since the spacing between the molecules in this structure is not compatible with the S atoms adopting a x3 × x3 sublattice, the sulfur headgroups must adopt different adsorption sites, such as 3-fold-hollow, bridge, and on-top sites. Every fourth TP4 molecule along the molecular rows is positioned on an equivalent site. Considering the van der Waals dimensions of the molecule (21.1 Å2 for the phenyl rings, see above) and the area per molecule
Figure 5. Constant-current STM micrographs showing the gold surface after immersion into a 2.5 µM ethanolic solution of TP2 at 298 K for 24 h. In (C), the unit cell of the (2x3 × x3)R30° structure is marked by the oblique box. Tunneling parameters: (A) Ut ) 530 mV, It ) 150 pA; (B) Ut ) 341 mV, It ) 252 pA; and (C) Ut ) 341 mV, It ) 252 pA.
(27 Å2), we expect a tilt angle of about arccos (21.1/27) ) 39° with respect to the surface normal. Figure 8 shows a representative high-resolution STM micrograph recorded for a TP6 SAM at 298 K. The large image shown in Figure 8B reveals, as for TP4, well-defined rows of molecules. Again, three different domains can be observed, which are rotated by 120° with respect to each other. The high-resolution STM
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Figure 7. Constant-current STM images showing the gold surface after immersing into 2.5 µM ethanolic solution of TP4 at 333 K for 24 h. In (D), the unit cell of the c(5x3 × 3) structure is marked by the rectangle. (E) and (F): Cross-sectional height profiles along the lines (a) and (b) labeled in (D), respectively. Tunneling parameters: (A) Ut ) 575 mV, It ) 150 pA; (B) Ut ) 581 mV, It ) 200 pA; (C) Ut ) 541 mV, It ) 158 pA; and (D) Ut ) 528 mV, It ) 115 pA.
Figure 6. Constant-current STM images showing the gold surface after immersion into a 2.5 µM ethanolic solution of TP2 at 333 K for 24 h. The arrows in (B) show the direction of the c(5x3 × 3) domains. In (C), the unit cell of the c(5x3 × 3) structure is marked by the rectangle. Tunneling parameters: (A) Ut ) 1000 mV, It ) 360 pA; (B) Ut ) 520 mV, It ) 158 pA; and (C) Ut ) 536 mV, It ) 189 pA.
data shown in Figure 8C are similar to that obtained for TP4 and indicate the presence of a c(5x3 × 3) structure containing eight molecules per unit cell. In Figure 9, we present high-resolution STM data recorded for TP6 SAMs prepared at 333 K. The large-scale STM micrographs do not reveal significant differences to films prepared at 298 K,
only the size of vacancy islands is found to be slightly increased. The small-scale STM images shown in Figure 9B reveal the presence of six different rotational domains with relative orientations of 60°. This observation is in contrast to that seen for samples prepared at 298 K, where only three different rotational domains rotated by 120° with respect to each other have been seen. Close inspection of the high-resolution data in Figure 9C reveals the coexistence of three different structures labeled R, β, and γ. The R structure exhibits a rectangular unit cell with the same dimensions and molecular arrangements of the adsorbate as found for the c(5x3 × 3) structure. The unit cell of the β phase was determined from the height profiles along the lines A and B within the single domain displayed in Figure 9E. The unit cell is oblique and is consistent with the presence of a commensurate (6x3 × 2x3)R30° structure containing eight molecules per unit cell. This result yields an area per molecule of 32.4 Å2 corresponding to a tilt angle of about arccos (21.1/32.4)) 49° with respect to the surface normal. A schematic model for this structure is shown in Figure 10C. The variation in the topographic heights of the protrusions (TP6 thiolate species) is assumed to arise from the multiple adsorption sites and from the herringbone arrangement of the terphenyl units. The γ phase was commonly found to be located in the areas between the R and β phases. All attempts to obtain molecular resolution within areas belonging to the γ phase were unsuccessful.
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Figure 9. Constant-current STM images showing the gold surface after immersion into a 2.5 µM ethanolic solution of TP6 at333 K for 24 h. The arrows in (B) and (C) show the six directions of the ordered domains. The rectangular box depicted in (D) shows the c(5x3 × 3) structure. The oblique box depicted in (E) marks the (6x3 × 2x3)R30° oblique unit cell. (F) and (G) represent the height profiles along the lines (a) and (b), respectively. Tunneling parameters: (A) Ut ) 510 mV, It ) 361 pA; (B) Ut ) 523 mV, It ) 130 pA; (C) Ut ) 682 mV, It ) 132 pA; (D) Ut ) 530 mV, It ) 170 pA; and (E) Ut ) 580 mV, It ) 130 pA.
Figure 8. Constant-current STM images showing the gold surface after immersing into 2.5 µM ethanolic solution of TP6 at 298 K for 24 h. In (C), the unit cell of the c(5x3 × 3) structure is marked by the rectangular box. Tunneling parameters: (A) Ut ) 550 mV, It ) 150 pA; (B) Ut ) 581 mV, It ) 115 pA; and (C) Ut ) 531 mV, It ) 150 pA.
4. Discussion Odd-Numbered TPn Monolayers. The IR data of oddnumbered TPn SAMs prepared at 298 and 333 K demonstrate that the molecules are oriented within the SAM with their molecular axis almost perpendicular to the Au surface. The corresponding STM results show the presence of a high degree of lateral order which can be described by an oblique unit cell with dimensions of 10.0 Å × 5.0 Å corresponding to a
commensurate (2x3 × x3)R30° structure. The unit cell contains two inequivalent terphenyl molecules, with the sulfur atoms being located at a (x3 × x3)R30° sublattice. The area occupied by a single molecule amounts to 21.6 Å2, which is similar to the packing density within alkanethiols SAMs on Au substrates. The structural model shown in Figure 10A, which is derived from the terphenyl bulk structure,53 proposes a herringbone like arrangement of the terphenyl backbones in the (2x3 × x3)R30° unit cell. A rough estimate based on the van der Waals dimensions of the terphenyl backbone yields a tilt angle of 12.5° of the terphenyl backbone with respect to the surface normal. This value is consistent with the IR data and with the angle determined in a previous NEXAFS investigation, where a tilt angle of 15 ( 8° was reported for TP1 SAMs.23 In case of the oddnumbered TPn SAMs, no significant changes were observed when SAMs were prepared at elevated temperatures (333 K). The structure adopted by the odd-numbered TPn SAMs is thus analogous to that of the corresponding BPn SAMs studied in previous work.33 In previous work by Ishida et al.,21 a smaller (x3 × x3)R30° unit cell was observed for TP1 SAMs instead of the larger (53) Baudour, J. L.; Yelon, W. B. Acta Crystal. 1977, B33, 1773.
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Figure 10. (A) Top view of a (2x3 × x3)R30° model for oddnumbered TPn (n) 1-6) SAMs on Au(111). (B) Top view of a structural model for the c(5x3 × 3) rectangular overlayer of TP4. (C) Top view of a (6x3 × 2x3)R30° model for TP6 SAMs on Au(111) at 333 K.
(2x3 × x3)R30° unit cell reported here. When discussing this apparent discrepancy, one has to note first that the data presented here are generally of higher quality than those presented in this previous paper, possibly due to the use of a different solvent (ethanol instead of chloroform). Although we are unable at present to fully explain these differences to previous work, we would like to point out that a (x3 × x3)R30° implies the presence of only one molecule per unit cell, which is not compatible with the herringbone like packing motif in TP bulk crystals. TP2 Monolayers. For TP2, the IR data for samples prepared at room temperature (RT) are comparable to those observed for
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the odd-numbered TPn SAMs, thus also indicating a near-normal orientation of the terphenyl backbones. The STM data revealed a (2x3 × x3)R30° structure for RT samples. The structure is therefore analogous to that found for the odd-numbered case, see above. For TP2 SAMs prepared at higher temperatures (333 K), the intensity ratio in the IR data of the op-bands to the ip-par bands is comparable to that seen in their bulk spectra. This result reveal the presence of a large tilt angle between the molecular axis and surface normal. At the same time, the STM data demonstrate the presence of a different lateral packing; for the higher temperature, a less densely packed c(5x3 × 3) structure is observed. High-resolution STM data for this phase reveal the presence of eight molecules in each unit cell, yielding an area of 27 Å2 per molecule. Compared to the total area of 21.6 Å2 for the (2x3 × x3)R30° phase seen for the odd-numbered TPn SAMs, this corresponds to a 25% lower packing density. The rather strong differences in height between the different molecules in the unit cell are attributed to different conductivities of the molecules resulting from different S-atom adsorption sites, as has been discussed in a previous publication.42 On the basis of the vdW dimensions of the TPn molecules a tilt angle of 39° is estimated, which is consistent with the IR data. The same results are found for BP2 at 298 K and will be published in a forthcoming article.54 TP4 and TP6 Monolayers. For SAMs made from the evennumbered TP4 and TP6, the IR results collected for samples prepared at 298 K reveal a significant tilt angle of the TP backbones away from the surface normal, as seen for TP2 prepared at 333 K. For both TP4 and TP6, the STM data demonstrate the presence of a c(5x3 × 3) structure with eight molecules per unit cell, as found for the even-numbered TPns. As for TP2, we propose a tilt angle of 39°. A structural model for this phase, again derived from the corresponding bulk data and previously published results for the BPn series33 is shown in Figure 10B. In the case of TP6, preparation of the SAMs at elevated temperatures (333 K) resulted in the formation of a new (6x3 × 2x3)R30° structure characterized by an oblique unit cell with dimensions of 29.9 Å × 10.0 Å. The size of this unit cell is about 20% larger compared to the unit cell of the c(5x3 × 3) phase and indicates a smaller packing density with a larger tilt angle of the molecular axis with respect to the surface normal. The area occupied by a single molecule amounts to 32.4 Å2. The intermolecular spacing of about 7.5 Å is significantly larger that of a x3 × x3 lattice (5 Å). We propose that the c(5x3 × 3) structure observed for films prepared at lower temperatures transforms to the high temperature (6x3 × 2x3)R30° structure through a decrease of packing density. A similar c(5x3 × 3) f (6x3 × 2x3)R30° phase transition has been observed for the biphenyl-based BP6 SAMs on Au(111) when the samples were annealed at 373 K,55 Figure 10C. It is interesting to note that for organothiolate films containing only 2 phenyl units the temperature had to be raised to 373 K to observe the c(5x3 × 3) f (6x3 × 2x3)R30° phase transition. Therefore, increasing the length of the molecular structure of the adsorbate by an additional phenyl ring such as in the TP6 molecules apparently leads to a significant reduction of the threshold temperature needed for the formation of the (6x3 × 2x3)R30° phase. One may speculate that this effect is due to the larger oligophenyl-oligophenyl interaction in the TP6 SAM, which stabilizes the (6x3 × 2x3)R30° phase. (54) Azzam, W.; Bashir, A.; Wo¨ll, C. to be published 2005. (55) Cyganik, P.; Buck, M. J. Am. Chem. Soc. 2004, 126, 5960. (56) Varsanyi, G. Assignments for Vibrational spectra of seVen hundred benzene deriVatiVes; Wiley: New York, 1974.
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Table 2. Structures Adopted by the TPn SAMs as a Function of Alkyl Chain Length and Deposition Temperature thiols
depos. @ 298 K
depos. @ 333 K
TP1 TP2 TP3 TP4 TP5 TP6
(2x3 × x3)R30° (2x3 × x3)R30° (2x3 × x3)R30° c(5x3 × 3) (2x3 × x3)R30° c(5x3 × 3)
(2x3 × x3)R30° c(5x3 × 3) (2x3 × x3)R30° c(5x3 × 3) (2x3 × x3)R30° c(5x3 × 3) (R) (6x3 × 2x3)R30° (β) amorphous (γ)
5. Conclusions In this work, SAMs on Au substrates made from a homologous series of terphenyl-containing organothiols, C6H5(C6H4)2(CH2)n-SH (TPn, n ) 1-6) have been studied using STM and IR spectroscopy. The importance of solvent temperature was investigated by using two different preparation temperatures, 298 and 333 K. The different SAMs were found to adopt a number of different lateral structures, which are tabulated in Table 2. For odd-numbered SAMs, well-ordered and highly oriented SAMs were obtained at 298 and 333 K. For both temperatures, only one structure, (2x3 × x3)R30°, was seen
in which the molecular axis of the TP backbones are orientated almost perpendicular to the surface. Even-numbered SAMs were found to behave quite differently. TP2 SAMs showed a (2x3 × x3)R30° structure at low temperature (analogous to the odd-numbered TPns) and a c(5x3 × 3) lower density phase with significantly tilted molecules at 333 K. This low-density c(5x3 × 3) phase is also seen for TP4 and TP6 SAMs prepared at room temperature. A third phase with even lower density, (6x3 × 2x3)R30°, was observed for TP6 SAMs prepared at 333 K. For TP4, increasing the solvent temperature did not lead to the formation of new structures. With the exception of TP2, TPn SAMs exhibited a pronounced odd-even effect on the molecular orientation and packing density. A higher packing density and a smaller inclination of the terphenyl moieties were observed for an odd number of methylene units in the aliphatic part compared to those TPn with an even number of methylene units. Supporting Information Available: STM data (Figures S1S7). This material is available free of charge via the Internet at http://pubs.acs.org. LA053065U
