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Coexistence of Different Structural Phases in Thioaromatic Monolayers on Au(111) Waleed Azzam,† Claus Fuxen,† Alexander Birkner,† Hai-Tao Rong,‡,§ Manfred Buck,*,§,| and Christof Wo¨ll*,† Lehrstuhl fu¨ r Physikalische Chemie I, Universita¨ tsstrasse 150, 44801 Bochum, Germany, Organisch Chemisches Institut, INF 270, 69120 Heidelberg, Germany, Lehrstuhl fu¨ r Angewandte Physikalische Chemie, INF 253, 69120 Heidelberg, Germany, and School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, U.K. Received October 21, 2002. In Final Form: February 21, 2003 Self-assembled monolayers formed by adsorption of 4-methyl-4′-mercaptobiphenyl, CH3-(C6H4)2-SH (BPT), on Au(111) have been studied using scanning tunneling microscopy. The results show that with increasing coverage (resulting from longer immersion times) BPT forms a series of different structural phases with different molecular arrangements, closely resembling the behavior reported previously for n-alkanethiolate adlayers. For short immersion times, striped structures are observed (R and β), where the molecules are orientated with their axes parallel to the surface. Longer immersion times yield additional phases, namely, an ordered χ-phase, where the molecules are proposed to be oriented with their molecular axis tilted away from the surface, a disordered δ phase, and, finally, a densely packed (2x3 × x3) phase where the molecular axes are orientated almost upright. Unexpectedly, for BPT after adsorption small islands are seen on the Au substrate instead of the etch pits commonly observed after formation of organothiolate adlayers.
Introduction After their discovery in 1983 by Nuzzo and Allara,1 selfassembled monolayers (SAMs) fabricated by adsorption of thiols or disulfides on gold surfaces have been intensively studied because of their potential applications in molecular devices, sensors, and surface engineering.2,3 The structures of n-alkanethiol SAMs have been studied in many previous works using a large number of different methods.4 With increasing coverage, the molecules form several ordered structures before a stable, densely packed c(4x3 × 2x3) structure is observed.5-7 For very long immersion times, recently a new phase has been reported.8 Whereas in earlier work the sulfur atoms were considered to bind covalently to the underlying Au(111) surface occupying the next-nearest-neighbor hexagonal close pack hollow sites,9 in more recent theoretical work, it has been suggested that the S atoms are located in bridge positions.10,11 There is general agreement that in the densely * To whom correspondence may be addressed: M.B., mb45@ st-and.ac.uk; C.W.,
[email protected]. † Lehrstuhl fu ¨ r Physikalische Chemie I. ‡ Organisch Chemisches Institut. § Lehrstuhl fu ¨ r Angewandte Physikalische Chemie. | University of St. Andrews. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (3) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (4) Ulman, A. Self-Assembled Monolayers of Thiols; Thin Films 24; Academic Press: Boston, 1998; p 279. (5) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (6) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503-3511. (7) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853-2856. (8) Noh, J.; Hara, M. Langmuir 2002, 18, 1953. (9) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401. (10) Hayashi, T.; Morikawa, Y.; Nozoye, H. J. Chem. Phys. 2001, 114, 7615. (11) Vargas, M. C.; Giannozzi, P.; Selloni, A.; Scoles, G. Phys. Chem. B 2001, 105, 9509-9513.
packed phases the alkyl chains are tilted against the surface normal by about 30°.12-14 Ordered low-density phases of the alkanethiolate films on Au(111) are usually obtained either by partial desorption from the (4x3 × 2x3) phase or by immersion into dilute solutions for short periods.15,16 These films are characterized by the formation of stripe patterns in which the molecular backbone is arranged with the molecular axis parallel to the surface. The stripe phases are described by (p × x3) structures, where p is referred to the stripe spacing and x3 is the periodicity within the stripes in units of the Au(111) substrate lattice constant. In particular, the low coverage phases of decanethiolate monolayers have been extensively investigated by numerous authors.17-24 It has been found that the structure of decanethiolate SAMs depends strongly on the surface coverage. Starting from a complete monolayer with (4x3 × 2x3) structure, where the area occupied by a single molecule was calculated to be 21.6 Å2, striped phases having (6 × x3), (7.5 × x3), (9 × x3), (12) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678-688. (13) Poirier, G. E.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (14) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-68. (15) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-3386. (16) Staub, R.; Toerker, M.; Fritz, T.; Schmitz-Hubsch, T.; Sellam, F.; Leo, K. Langmuir 1998, 14, 6693-6698. (17) Poirier, G. E.; Fitts, W. P.; White, J. M. Langmuir 2001, 17, 1176-1183. (18) Poirier, G. E. Langmuir 1999, 15, 1167-1175. (19) Toerker, M.; Staub, R.; Fritz, T.; Schmitz-Hu¨bsch, T.; Sellam, F.; Leo, K. Surf. Sci. 2000, 445, 100-108. (20) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855-861. (21) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (22) Gerlach, R.; Polanski, G.; Rubahn, H. G. Appl. Phys. A: Mater. Sci. Process. 1997, 65, 375-377. (23) Camillone, N.; Leung, T. Y. B.; PSchwartz, P. Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737-2746. (24) Schreiber, F.; Eberhardt, A.; Leung, T.; Schwartz, P.; Wetterer, S.; Lavrich, D.; Berman, L.; Fenter, P.; Eisenberger, P.; Scoles, G. Phys. Rev. B 1998, 57, 12476.
10.1021/la020868y CCC: $25.00 © 2003 American Chemical Society Published on Web 05/09/2003
Structural Phases in Thioaromatic Monolayers
and (11.5 × x3) structures have been discovered. Also arrangements with alternating row spacings have been reported.19 Compared to SAMs of n-alkanethiolates, the knowledge about thioaromatic monolayers is still rather scarce since they have moved into the focus of interest only more recently due to potential applications in molecule based electronics, lithography, and electrode modification.25,26 The oligophenylthiolate SAMs are expected to be different from films made from n-alkanethiols because of their more rigid backbones and because of the strong interactions between the adsorbate molecules.5 The high coverage phase of different aromatic thiolate SAMs has been investigated using various methods27,28 and was found to exhibit a hexagonal commensurate (x3 × x3)R30° structure. Frequently, also (2x3 × x3) structures26,29 as well as incommensurate structures30,31 have been reported. A low coverage phase of 4-mercaptopyridine SAMs has been studied in perchloric acid solution using in situ scanning tunneling microscopy (STM)32 and on the basis of the reported data an ordered striped structure with a rectangular (5 × x3) unit cell containing two molecules has been proposed. Recently, the high and low coverage phases of 4-methyl4′-mercaptobiphenyl (BPT) SAMs on Au(111) have been investigated using grazing incidence X-ray diffraction and low-energy He atom diffraction.33 The high coverage phase which was obtained by adsorption from solution was found to exhibit a (x3 × x3)R30° structure. A low coverage (8 × 2x3) striped phase was observed after deposition from the gas phase in an ultrahigh vacuum (UHV) apparatus, with a stripe spacing of 23.08 Å and a periodicity within the stripes of 10 Å. A centered rectangular unit cell containing four molecules was proposed, corresponding to a molecular packing density of 57.7 Å2/molecule where the molecules are arranged in a head-to-head orientation with their molecular axes tilted partly away from the surface. In contrast to this report of a commensurate structure of BPT on Au(111), in a recent STM study Kang et al.31 have found an incommensurate structure for a similar organothiolate where the CH3 group in the 4′position of BPT was replaced by chlorine. In the present publication, we report the results of a systematic investigation on the adsorption of 4-methyl-4′-mercaptobiphenyl (BPT) on Au(111) from solution using STM. Experimental Section BPT was synthesized using a previously described procedure.39 Ethanol (Baker), acetone (Baker), and chloroform (Baker) were used as received. High-quality Au films were deposited on mica using the following procedure. A freshly cleaved sheet of mica has been (25) 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, 9529-9534. (26) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320. (27) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101-111. (28) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974-2978. (29) Fuxen, C.; Azzam, W.; Arnold, R.; Terfort, A.; Witte, G.; Wo¨ll, C. Langmuir 2001, 17, 3689-3695. (30) Yang, G.; Quian, Y.; Engtrakul, C.; Sita, L. R.; Liu, G. Y. J. Phys. Chem. B 2000, 104, 9059-9062. (31) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G. Y. Langmuir 2001, 17, 95-106. (32) Wan, L.-J.; Hara, Y.; Noda, H.; Osawa, N. J. Phys. Chem. B 1998, 102, 5943-5946. (33) Leung, T.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34-52. (34) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058-4061.
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Figure 1. (a) Constant-current image for Au(111) substrate, which has been immersed for 4 min into a 0.1 µM solution of BPT in ethanol (U ) 550 mV; I ) 150 pA). The image shows the R phase. (b) Constant-current STM image (U ) 550 mV; I ) 220 pA), showing the R phase. The unit cell is marked by a rectangular box. (c and d) Cross-sectional height profiles along the lines labeled A and B in (a). (e) Small area from highresolution STM image of the R phase (Figure 1b), overlaid with the proposed positions of the S atoms. On the right, a structural model for the (8 × 2x3) R phase is presented. The unit cell is marked by a rectangular box in both parts. heated to 370 °C for about 48 h inside an evaporation apparatus (Leybold) to remove any residual water. Subsequently, 100 nm of Au (99.995%, Chempur) were deposited at the same temperature of the substrate and a pressure of approximately 10-7 mbar. The thickness and the deposition rate (20 Å s-1) were monitored using a crystal oscillator (Leybold Inficon). After deposition, the substrates were cooled and the vacuum chamber was filled with purified nitrogen. The substrates were stored under argon and flame-annealed in a butane/oxygen flame immediately before the adsorption experiments were carried out. This procedure yielded Au substrates with several 100 nm large terraces exhibiting a (111) surface orientation. BPT monolayers on Au(111) substrates were prepared by immersing the gold substrates into dilute ethanolic solutions at elevated temperatures (0.1 µM at 60 °C) of BPT. After immersion the substrate was removed from the solution and rinsed carefully with pure ethanol, acetone, chloroform, and finally again ethanol. Then the substrates were dried in a nitrogen stream. Selected samples were also prepared from room temperature solutions. The overall quality was, however, significantly better when higher temperatures were used (see below). The samples were imaged within 3 days after immersion. The stability of the films was
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Figure 2. (a) Constant-current image recorded for an Au(111) substrate, which has been immersed for 3 h into a 0.1 µM solution of BPT in ethanol (U ) 530 mV; I ) 146 pA). The image shows the phases R, β, χ, and δ. The relative orientation angle between the R and the β phase is estimated from (a) and amounts to 20 ( 2°. (b) Cross-sectional profiles along the line labeled A in (a), revealing the increased contrast of the χ phase relative to the R phase. (c) Constant-current STM image revealing the coexistence of different phases (U ) 530 mV; I ) 146 pA). rather high; no significant differences were seen between samples imaged directly after preparation and samples which had been stored for periods in time of up to 3 days. All STM measurements were carried out in air using a Digital Instruments (Nanoscope III) multimode microscope equipped with a scanning tunneling microscope head. Tips were prepared mechanically by cutting a 0.25 mm Pt/Ir alloy (8:2, Chempur) wire. The data have been collected in constant-current mode using tunneling currents between 120 and 500 pA and a sample bias between 100 mV and 1 V (tip positive).
Results Figure 1a shows a large-scale constant-current STM image recorded for an Au(111) substrate, which has been immersed for 4 min in a 0.1 µM solution of BPT in ethanol at 60 °C. The STM data have been recorded approximately 1 h after removing the sample from the thiol solution. No steps are visible in the image; the STM micrograph shows a single terrace of the Au(111) substrate. On the terrace, an ordered arrangement of protrusions is visible. Several rotational domains are seen. Within single domains the protrusions form lines (stripes). Figure 1b shows a high molecular resolution image of this striped phase. As demonstrated by the profile shown in parts c and d of Figure 1, the distance between the stripes amounts to 23.08 ( 1.0 Å; the distance of the protrusions along the rows amounts to 10 ( 0.3 Å. After this short immersion time, the size of the adlayer domains is fairly small
(3 h), characteristic changes in surface morphology caused by the appearance of numerous islands were seen; see Figures 6a, 6b, and 7. As demonstrated by the crosssectional height profile (Figure 7c) along the line labeled A in Figure 7b (the line A passes (from left to right) over one of the islands at the lower terrace through the step edge and then over an island located at the upper terrace), the height of these islands corresponds to the height of a single atomic step on the Au(111) surface (2.4 Å). Figure 5a shows a high-resolution constant current STM image recorded on top of one of these islands for a sample
prepared using an immersion time of 3 h. The data show bright protrusions which are assigned to individual adsorbate molecules forming regular spaced rows. This phase, which could only be resolved on the top of islands, corresponds to a new phase which is labeled . The data reveal the presence of three different rotational domains of the phase, in Figure 5a two orientations rotated by 120 ( 5° with respect to each other are clearly visible. A highresolution image recorded for a single domain is presented in Figure 5b. The dimensions of the unit cell were determined from the height profiles along the lines A and B labeled in Figure 5b and amount to a ) 4.8 ( 0.3 Å and b ) 10 ( 0.45 Å (see Figure 5c,d) corresponding to a (x3 × 2x3) structure. The high resolution achieved in this and other images clearly demonstrates the presence of two inequivalent molecules in the adsorbate overlayer unit cell. Considering the van der Waals dimensions of the cross section for a phenyl group (6.4 Å by 3.3 Å), a herringbone-like arrangement of the biphenyl backbones appears to be the most likely arrangement. Figure 5e shows a molecular model for this (2x3 × x3) structure assuming that the S atoms adopt a (x3 × x3) structure. The unit cell contains two (inequivalent) molecules yielding an area per molecule of 21.6 Å2. Parts a and b of Figure 6 show constant-current STM images recorded for samples prepared using immersion times of 6 and 12 h, respectively. These images reveal substantial changes in the Au-substrate morphology. The number of islands increases, while at the same time the average diameter is found to decrease. High-resolution images recorded on top of the islands for immerson times of 6 and 12 h revealed the presence of ordered biphenylthiolate adlayers with a (2x3 × x3) superlattic structure; see Figure 6c. At small immersion times (3 h), the
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Figure 7. (a) Constant-current image recorded for an Au(111) substrate which has been immersed for 17 h into a 0.1 µM solution of BPT in ethanol (U ) 1000 mV; I ) 150 pA). (b) Constant-current STM image (U ) 600 mV; I ) 180 pA), showing the phase. (c) Cross-sectional height profile along the line labeled A in (b). (d) A high-resolution constant current STM image (U ) 600 mV; I ) 150 pA) showing (x3 × x3) structure.
size of the islands covered by the phase amounts to 3580 nm (Figure 2c), which decreases for longer immersion times. After 17 h, the size of these domains is reduced to 5-35 nm (Figure 7a,b). In Figure 7d we display a STM micrograph which has been obtained from a sample which was prepared for long immersion time (17 h). The average distance between the spots amounts to 5 ( 0.2 Å (not shown), which is close to the distance in a x3 × x3 lattice. The molecules are therefore considered to form the (x3 × x3) structure. The observation that no (2x3 × x3) superlattic structure could
be seen on the islands formed by BPT molecules after long immersion times will be addressed in the discussion section. Discussion Our experiments reveal that immersion of Au substrates into diluted solutions of BPT leads to the formation of a number of different phases. A phase diagram describing these different structures is reproduced in Figure 8a; the corresponding structural models are provided in Figure 8b.
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Figure 8. (a) A schematic diagram for SAMs of BPT on Au(111) prepared by immersion into ethanolic solutions. (b) Unit cells of the of (8 × 2x3), (5x3 × 4), (2x3 × 2x3), (2x3 × x3), and (x3 × x3) structures.
If the Au substrates are immersed in diluted solutions of BPT in ethanol for times less than 4 min, the formation of a highly ordered striped phase, denoted R, is observed. The phase bears a strong similarity to the striped phases seen for alkanethiolates adsorbed on Au substrates.18-23 In the case of the alkanethiolates, the lowest density striped phase formed by adsorption from the gas phase in UHV has been attributed to alkanethiolate molecules orientated with their backbone mainly parallel to the substrate. The same phase has also been seen after adsorption from solution.16,20 In analogy with these previous results and the fact that our STM data indicate the presence of two molecules per unit cell, we propose that the low-density R phase for BPT on Au(111) consists of BPT molecules adsorbed with the molecular plane of the molecule parallel to the substrate. The proposed arrangement yields an average area per molecule of 115 Å2, clearly larger than the van der Waals (vdW) cross-sectional area 65 Å2 (the ring diameter is 6.4 Å and the length of BPT, from the S atom to the methyl C atom, is 10.37 Å) of the molecule. The size of the unit cell observed for the R phase in our data, 23.08 Å×10 Å, agrees very well with the results reported in a previous study by Leung et al.33 for the lowest ordered phase of BPT observed after gas-phase deposition under UHV conditions. Diffraction data obtained by grazing incidence X-ray diffraction (GIXD) and low-energy He atom diffraction yielded a centered 23.08 Å × 10 Å
rectangular unit cell. Since, among the number of different phases observed in this work, the R phase is the only one which matches the unit-cell parameters determined by Leung et al.,33 it is tempting to conclude that the two phases are identical. There is, however, a significant discrepancy between the structural model proposed by Leung et al. for the low-coverage phase and the molecular arrangement proposed here. Leung et al. proposed a structural model with four dimerized thiolate molecules per unit cell in a head-to-head configuration. The resulting S-S distance is 2.1 ( 0.2 Å. To fully accommodate the four molecules in this fairly small unit cell, it was suggested that the molecules are tilted away from the surface (at least 15°).33 Such an arrangement corresponds to an area of 57.7 Å2 per molecule. This area is very small, compared, e.g., to the van der Waals sizes of the molecule (see above) and the fact that for decanethiolate, an organothiolate with a similar (but smaller) vdW cross-sectional area, in the flatlying phase on Au substrates, an area of 82.8 Å2 has been reported.18 On the basis of our high-resolution STM results, we propose the presence of only two molecules per unit cell, yielding an average area per molecule of 115 Å2, in very reasonable agreement with the vdW cross-sectional area of the molecule. Since the present high-resolution STM data are not consistent with the previous GIXRD results by Leung et al.,33 we conclude that the phase observed by them after deposition from the gas phase does not occur when BPT molecules are adsorbed from solution.
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At larger coverages (resulting from longer immersion times), we observe a second striped phase with a slightly larger unit cell, 25 Å × 11.5 Å. On the basis of line profiles taken from the STM micrographs, we propose that also this phase consists of BPT molecules oriented with the backbone parallel to the Au substrate. In Figure 3d we present a structural model with four molecules per unit cell. The area per molecule amounts to 72 Å2 per molecule, significantly smaller than that seen for the R phase. For longer immersion times, the formation of a new phase, χ, is seen. STM data recorded for this phase reveal the presence of a hexagonal (2x3 × 2x3) unit cell. In this case we conclude from the difference in height between the β and χ phase as seen in the STM micrographs that the molecules are adsorbed with their main aixs tilted away from the surface. The next phase observed here is denoted δ. For this structure the STM resolution was not sufficiently high to obtain information about the arrangement of the molecules. This phase reveals the presence of small features which are in height of 2.4 Å. The highest coverage phase seen here after immersion times of 3 h and more is the phase. This phase exhibits a (2x3 × x3) unit cell; the STM micrographs clearly show the presence of two molecules per unit cell. The area per molecule amounts to 21.6 Å2, which is fully consistent with the van der Waals cross-sectional area for a BPT unit assuming a nearly perpendicular arrangement. The same phase, but without a superstructure (i.e., a x3 × x3 phase), has been seen also by Leung et al.; they propose a tilt angle of 19° with respect to the surface normal.33 In a recent detailed IR and near-edge X-ray absorption fine structure study on different organothiols containing biphenyl backbones,39 a very similar tilt angle of around 15° has been observed. An inspection of the STM micrographs recorded for this high-coverage phase reveals very interesting morphological features, namely, islands with a height corresponding to that of monatomic steps on Au(111) (2.4 Å) (see Figure 2b and Figure 7a,b). The origin of these islands might be related to the small features, with a height corresponding to that of a Au(111) monatomic step, which are observed within the δ phase around the holes and near the domain boundaries. We attribute these features to single gold thiolate moieties. Diffusion of these thiolate species then may lead to lateral growth and the formation of the islands seen in the phase. For long immersion time (17 h), the surface is almost completely covered with islands exhibiting diameters of 5-35 nm (see Figure 7a). The STM micrographs reveal that these islands are not randomly distributed on the surface but are aligned along rows, which possibly are related to the positions of the Au(111) substrate reconstruction lines present before the adsorption took place. The data reported here for biphenylthiolate SAMs demonstrate a significant difference to the morphology seen after the formation of alkanethiolates SAMs on Au substrates, namely, the appearance of islands instead of depressions.35,40 These differences in morphology must be due to the different molecular chain attached to the sulfhydryl group. This hypothesis is supported by experiments carried out for biphenylmethanethiol, BP1.41 In contrast to BPT, BP1 contains a methylene spacer between the thiol group and the biphenyl moiety (HS-CH2-
(C6H4)2-CH3). For BP1, no island formation was observed and the surface morphology was comparable to the morphology seen after the adsorption of n-alkanethiols. The same observation has also been made in a previous investigation of the p-terphenyl system; i.e., when we used HS-(C6H4)2-C6H5 (TPT), the islands appeared after adsorption, while they were absent after the adsorption of HS-CH2-(C6H4)2-C6H5 (TPMT).29 The presence of islands after adsorption of aromatic thiols has also been observed previously for 4-mercaptopyridine and 4-hydroxythiophenol,27 where the authors attributed this observation to gold atoms which have popped out of the surface due to stronger substrate-molecule binding compared to alkanethiols. The mechanism which for aromatic thiols leads to the formation of islands instead of the commonly observed depressions in the SAMs on Au substrates is currently under investigation and will be discussed in a future publication. The size of the islands exhibiting the phase is found to depend strongly on the immersion time. For short immersion times (3 h), these islands have sizes in the range of 35-80 nm (see Figure 2a). For long immersion times (17 h), the size of the islands is reduced significantly to 5-35 nm (see Figure 7a,b). Whereas for short immersion times the STM data clearly reveal the presence of a (2x3 × x3) superlattice structure on top of the large islands, after long immersion times only a low-quality (x3 × x3) structure could be imaged. We explain the absence of the superstructure on the small islands by the fact that the size of the islands is rather small. Apparently the (2x3 × x3) structure becomes unstable with regard to the formation of the (x3 × x3) structure if the corresponding domain size becomes smaller than 20 nm. In a recent publication by Leung et al.33 X-ray diffraction data (XRD) were reported for SAMs made by immersion into ethanolic solutions (0.01-1 mM) at room temperature, with immersion times ranging between 20 min and 7 days. The XRD data revealed the presence of BPT monolayers exhibiting a (x3 × x3) structure with poor structural order, the observed average domain size amounted to only about 6.5 nm. In this work the (2x3 × x3) superlattice spots were not checked. Thus, in principle, the present observation of a (2x3 × x3) structure is consistent with the previously published GIXRD data.33 We would like to note, however, that the conditions used in this previous work,33 namely, very long immersion times, rather concentrated solutions, and a preparation of the BPT films at room temperature, are very unfavorable for obtaining a high structural quality of the BPT adlayers. In reference experiments where the Au substrates were immersed in room-temperature solutions of BPT, the domains with (2x3 × x3) structure were observed to be considerably smaller (2-20 nm) than those obtained when using solutions at a temperature of 60 °C (35-80 nm). When using higher concentrations (>100 µM), we were unable to observe the striped phases (R and β) discussed above. The poor quality of our STM data recorded for films prepared under similar conditions as those used by Leung et al. is consistent with the observation of Leung et al. in that they were unable to obtain high-quality STM data for BPT adlayers prepared by immersion into solution.
(39) Rong, H.-T. Selbstorganisierende Monolagen aus Biphenylthiolen auf Gold und Silber; Ruprecht Karls Universita¨t: Heidelberg, 2001. (40) Edinger, K.; Grunze, M.; Wo¨ll, C. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1811-1815. (41) Azzam, W.; et al. To be submitted for publication.
In this work, we report the results of a systematic study on the formation of self-assembled monolayers of BPT on Au(111) prepared by immersion in solution using STM. Our results reveal the presence of several different
Conclusion
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structural phases thus yielding a scenario rather similar to that seen for n-alkanethiols on the same substrate. Large domains of a low-density striped phase have been observed, where the BPT molecules are orientated with their backbone parallel to the surface. For higher coverages, ordered and disordered intermediate phases are observed before densely packed films with the biphenyl units in almost upright adsorption geometry are seen. The BPT SAMs exhibit a characteristic morphological feature, namely, the presence of islands of monatomic
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height rather than the protrusions (“etch pits”) observed commonly for organothiolate adlayers on Au(111) substrates. Acknowledgment. We thank Dr. F. Schreiber, Oxford, for comments on the manuscript. Part of this work has been supported through the German DFG (WO 464/11-3). LA020868Y
