Lattice Structure of Self-Assembled Monolayers of Dialkyl Sulfides and

University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands. Received December 31, 1998. In Final Form: May 11, 1999. The lattice structure ...
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Langmuir 1999, 15, 5541-5546

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Lattice Structure of Self-Assembled Monolayers of Dialkyl Sulfides and Calix[4]arene Sulfide Adsorbates on Au(111) Revealed by Atomic Force Microscopy Holger Scho¨nherr and G. Julius Vancso*.† Materials Science and Technology of Polymers, MESAplus Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

Bart-Hendrik Huisman, Frank C. J. M. van Veggel, and David N. Reinhoudt*.‡ Supramolecular Chemistry and Technology, MESAplus Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received December 31, 1998. In Final Form: May 11, 1999 The lattice structure of a variety of self-assembled monolayers (SAMs) on Au(111), derived from dialkyl sulfides and calix[4]arene-based tetrasulfide adsorbates, was elucidated by atomic force microscopy (AFM). SAMs of n-alkanethiols and a fluorinated alkanethiol were imaged as reference layers. The sulfide-based SAMs were annealed at elevated temperatures in order to improve the monolayer quality. AFM revealed depressions with depths of a single or multiple steps in the Au(111) for SAMs of thiols, as well as sulfides. The tail group lattice of dialkyl sulfides has a (x3 × x3)R30° structure identical to that of n-alkanethiols. The results are indicative for the dominating role of intermolecular van der Waals interactions for determining the structure of the dialkyl sulfide-based SAMs. For the calix[4]arene adsorbates the packing of the alkyl chains between the cavity and the substrate could be imaged, which indicates a considerable “information depth” of the AFM measurements. The tilt angle of the alkane segment was found to depend on the rigidity of the cavity.

Introduction Self-assembled monolayers (SAMs) of organic molecules on solid substrates are valuable model systems that provide an advanced understanding of surface modifications and interfacial properties.1 In addition to fundamental aspects on, for example, wetting,1 adhesion,2 and tribology,3 SAMs are used as sensors, such as electrochemical sensors,4 quartz microbalance sensors,5,6 or surface plasmon resonance spectroscopy based sensing devices.7 SAMs will also find industrial applications in soft lithography, e.g. microcontact printing, for the production of submicrometer-sized integrated circuits.8 * Corresponding authors. † Tel: ++31 53 489-2967. Fax: ++31 53 489-3823. E-mail: [email protected]. ‡ Tel: ++31 53 489-2981. Fax: ++31 53 489-4645. E-mail: [email protected]. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Bishop, A. R.; Nuzzo, R. G. Curr. Opinion Colloid Interface Sci. 1996, 1, 127. (c) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (d) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (b) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381. (3) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (4) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1989, 337, 217. (c) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1989, 337, 514. (d) Turyan, I.; Mandler, D. Anal. Chem. 1994, 66, 58. (e) Gafni, Y.; Weizman, H.; Libman, J.; Shanzer, A.; Rubinstein, I. Eur. J. Chem. 1996, 2, 759. (5) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191. (6) Schierbaum, K.-D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Go¨pel, W. Science 1994, 265, 1413. (7) (a) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009. (b) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 561. (c) Huisman, B.-H.; Kooyman, R. P. H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 1996, 8, 561. (d) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Langmuir 1997, 13, 561.

Stable SAMs are generally derived from alkanethiols or disulfides. The stability of the monolayers is partly due to the strong interaction of sulfur with gold with binding energies on the order of 120 kJ/mol.9 Additional enhancement of the mechanical or thermal stability is achieved by polymerizing functional groups of the assembled molecules, e.g. diacetylene units.10 The use of dialkyl sulfide adsorbates seems less attractive compared to the corresponding thiols or disulfides owing to the conceptually “weaker“ interaction of the sulfide sulfur atoms with gold. The binding of sulfides is considered to occur through physisorption.11 Recent desorption studies support this conclusion.12 However, an early investigation of Troughton et al. showed that dialkyl sulfides form ordered SAMs on gold.13 We have previously shown that high structural order and low defect density are general prerequisites for the good performance of SAMs in sensor applications.14 For receptor adsorbates which possess resorcin[4]arene head(8) (a) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (b) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (9) (a) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (b) Nuzzo, R. G.; Zegarsky, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (10) (a) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (b) Chan, K.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (11) Allara, D. L. Biosensors Bioelectronics 1995, 10, 771. (12) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456. (13) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (14) (a) Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1994, 116, 3597. (b) Thoden van Velzen, E. U.; Engbersen, J. F. J.; de Lange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 6853. (c) Huisman, B.-H.; Rudkevich, D. M.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1996, 118, 3523.

10.1021/la981787y CCC: $18.00 © 1999 American Chemical Society Published on Web 07/16/1999

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Chart 1. Adsorbate Molecules Studies on Au(111)

Figure 1. AFM image of Au(111) prior to assembly (z-scale 3 nm).

groups, the heterogeneous electron transfer is reduced significantly only in SAMs formed from tetra(dialkyl sulfide)-substituted molecules. This is attributed to the filling of the void space underneath the resorcin[4]arene cavity by the eight alkyl segments of the sulfide moieties.14 Assembly at elevated temperatures (annealing) is crucial for good monolayer quality. The SAMs formed by a number of these derivatives are similarly stable and as highly ordered as alkanethiol SAMs. The binding of all four sulfide groups per molecule to the gold is proven by X-ray photoelectron spectroscopy (XPS).15 SIMS experiments proved that the sulfur-carbon bond in sulfides is not cleaved during the adsorption.16,17 The high degree of order in SAMs of resorcin[4]arene sulfide 1 (Chart 1) on Au(111) could be revealed by atomic force microscopy (AFM).18 The cavity headgroups form a hexagonal lattice of 11.6 Å, while the additionally observed periodicity of 4.2 Å is attributed to the packing of the alkyl chains underneath the cavities. The lattice structure of the alkane segments indicated a perpendicular orientation of the chains. While normal dialkyl sulfides form SAMs with a (x3 × x3)R30° lattice structure (vide infra) with significantly tilted alkane chains, the rigid resorcin[4]arene cavity seems to force the alkane segments into a nearly perpendicular orientation. These are strong indications for a direct tail group influence on the structure of the monolayer. Recently, scanning tunneling microscopy (STM) studies of monolayers of hydrogen sulfide (H2S),19 thiophene,20 and hexaalkyl sulfides of tricycloquinazoline (TCQ)21 on gold were reported. The interaction of these molecules (15) Huisman, B.-H. Ph.D. Thesis, University of Twente, 1998. (16) Beulen, M. W. J.; Huisman, B.-H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170. (17) A recent paper by Porter et al. (Zhong, C. J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518) showed that impurities were indeed the origin of the observations initially attributed to the carbon-sulfur bond cleavage. (18) Scho¨nherr, H.; Vancso, G. J.; Huisman, B.-H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1997, 13, 1567. (19) Touzov, I.; Gorman, C. B. Langmuir 1997, 13, 4850. (20) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1996, 12, 6176. (21) Hiesgen, R.; Scho¨nherr, H.; Kumar, S.; Ringsdorf, H.; Meissner, D. Submitted.

with gold results in a substrate morphology very similar to those of alkanethiols and disulfides on gold. Depressions in the gold corresponding to one monolayer of Au were detected by STM. For the TCQ SAMs, gold was detected spectroscopically in the assembly solutions. In contrast to other reports,11,12 these observations seem to indicate that sulfides interact quite strongly with gold surfaces. To clarify these issues, we investigated the possible corrosion of the substrate and the tail group lattice structure of SAMs of highly pure, thiol-free16,22 dialkyl sulfides on Au(111). In this paper we present our results of a comparative AFM study of SAMs of conventional n-alkanethiols (3a-d), a fluorinated alkanethiol (4), dialkyl sulfides (5a, 5b), and the receptor adsorbates 2a and 2b on Au(111) (Chart 1). Experimental Section Substrate and SAM Preparation. Evaporated gold substrates (borosilicate glass, 2 nm Cr, 250 nm Au) were purchased from Metallhandel Schro¨er (Lienen, Germany). Au(111) samples were prepared by annealing in a highpurity hydrogen flame for 10 min. The substrates showed numerous Au(111) terraces of several µm2 lateral size. On the annealed substrates equilateral triangular terraces were frequently found (Figure 1). On these terraces the Au(111) lattice could be imaged by AFM (lattice spacing d ) 2.8 Å).23 SAMs were formed on annealed Au(111) by assembly from dilute (1 mM) solution of the corresponding adsorbate in ethanol or chloroform/ethanol (40/60, v/v) for 16 h. Monolayers of the sulfide derivatives were prepared at 60 °C. After the assembly, the samples were rinsed with a large amount of chloroform, ethanol, and water (Millipore), respectively. The samples prepared at 60 °C were allowed to cool to room temperature over a period of 3 h before taking them out of the solution and subsequent rinsing. The properties of these SAMs were investigated thoroughly by contact angle measurements, FT-IR, XPS, electrochemistry, and TOF-SIMS.14-16 The sulfide compounds were purified by column chromatog(22) Trevor, J. L.; Lykke, K. R.; Pellin, M. J.; Hanley, L. Langmuir 1998, 14, 1664. (23) Manne, S.; Butt, H.-J.; Gould, C. A. C.; Hansma, P. K. Appl. Phys. Lett. 1990, 56, 1758.

Lattice Structure of SAMs of Dialkyl Sulfides

Figure 2. AFM image of hexadecanethiol SAM on Au(111) (z-scale 7.5 nm).

raphy followed by repeated recrystallization from acetone/ diisopropyl ether. NMR and gas chromatography showed that thiol impurities were absent. n-Alkanethiols (Aldrich) were used without further purification, while the fluorinated thiol was synthesized and purified according to the literature procedure.24 Atomic Force Microscopy (AFM). AFM images were acquired on a NanoScope II and a NanoScope III multimode AFM (Digital Instruments (DI), Santa Barbara, CA) operated in contact mode. Silicon nitride cantilever-tip assemblies (DI) with a nominal spring constant between 0.06 and 0.38 N/m were used. The images were obtained in air, as well as in ethanol or in water (using a DI liquid cell). Prior to experiments the AFM setup was equilibrated until the instrumental drift was eliminated. The calibration in the lateral direction was based on the results obtained on n-alkanethiol SAMs, as well as on Au(111). This calibration eliminates any effect of specimen height.25 The vertical scale was calibrated by imaging single steps in the Au(111) terraces. We have evaluated the data as reported previously.18,26 Results and Discussion Substrate Corrosion. The Au(111) substrates used for self-assembly frequently showed triangular terraces (Figure 1). The terrace edges correspond to the next neighbor directions of the Au(111) lattice. By imaging first the terrace edge orientations and then the tail group lattice of the corresponding SAM on top of the selected terrace, the mutual orientation of the tail group lattice and Au(111) can easily be determined (vide infra).27 After assembly of a SAM of, for example, hexadecanethiol, the initially atomically smooth surface of the terraces is corroded as shown by AFM (Figure 2). The depressions observed have a depth of 2.4 ( 0.2 Å or multiples of this value. The depth corresponds to one (or multiple) atomic layers of Au(111) which is known to be 2.36 Å thick.28 The possible origins of these depressions (24) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (25) Sne´tivy, D.; Vancso, G. J. Langmuir 1993, 9, 2253. (26) Scho¨nherr, H.; Vancso, G. J. Langmuir 1997, 13, 3769. (27) Jaschke, M.; Scho¨nherr, H.; Wolf, H.; Butt, H.-J.; Bamberg, E.; Besocke, M. K.; Ringsdorf, H. J. Phys. Chem. 1996, 100, 2290. (28) Larsen, N. B.; Biebuyck, H. A.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017.

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have been discussed in the literature.29 Corroded Au(111) terraces were also observed on SAMs of didecyl sulfide and dihexadecyl sulfide (Figure 3). The depth of the depressions is the same as it was found for the thiols. The observation of depressions in the Au(111) substrate can be attributed to the interaction of the sulfur atoms with the Au(111).30 The presence of alkanethiol impurities can be excluded in the sulfides used in this study because gas chromatography proved the absence of traces of alkanethiol.15,16 Therefore, it is likely that part of the gold is dissolved in the assembly solution as observed for SAMs of TCQ sulfides assembled on Au(111).21 Lattice Structure of Reference SAMs. To accurately calibrate the AFM for the measurement of the lattice constants of the sulfide derivatives, we measured the tail group lattice structure of four n-alkanethiols on Au(111) (perdeuterated decanethiol 3a, dodecanethiol 3b, hexadecanethiol 3c, and octadecanethiol 3d), as well as a fluorinated alkanethiol (4). The tail group lattice structures of dodecanethiol and the fluorinated alkanethiol 4 are clearly visible in Figures 4 and 5. The experimental lattice parameters are given in Table 1. The values agree very well with data obtained by other groups using AFM or other methods.27,31-33 The orientation of the next neighbor direction of the tail group lattices are in all cases rotated by 30° with respect to the underlying Au(111). The (x3 × x3)R30° structure of n-alkanethiols and the higher order commensurate c (7 × 7) structure for the fluorinated alkanethiol are fully consistent with the literature.32,33 The tail group lattice structures of thiol- and disulfidebased SAMs, i.e. the (x3 × x3)R30° of n-alkanethiols, the higher order commensurate c (7 × 7) reported for fluorinated alkanethiols and disulfides, and the p (2 × 2) for novel fluorinated alkanethiols and disulfides,34 are generally interpreted as a consequence of the presence of preferred binding sites of the sulfur atoms on the Au(111) lattice and the maximization of chain-chain interactions (van der Waals forces). For n-alkanethiols on Au(111), binding of the sulfur to the 3-fold hollow site of the Au(111) is assumed.1,9b,35 The molecules have an average tilt angle of ca. 30°.36 The tilt angle is responsible for maximized intermolecular van (29) (a) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. Langmuir 1994, 10, 611. (b) Poirier, G. E. Langmuir 1997, 13, 2019. (c) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1997, 13, 2318. (d) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (30) In the XPS analyses of samples prepared at 60 °C the amount of sulfur bound to gold was found to increase significantly as compared to samples prepared at room temperature, e.g. for SAMs of adsorbate 1, 100% of the sulfur atoms bound (60 °C) vs 86% bound (RT) (further details can be found: Huisman, B.-H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Pure Appl. Chem. 1998, 70, 1985). In addition, the number of defects in the alkyl portion of the molecules can be expected to be significantly reduced for samples prepared at elevated temperatures. Assembly at elevated temperatures was previously shown to improve the quality of n-alkanethiol SAMs: Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (31) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (32) (a) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (b) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (c) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556. (d) Delamarche, E.; Michel, B.; Kang, H.; Gerber, Ch. Langmuir 1994, 10, 4103. (e) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (f) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (g) Touzov, I.; Gorman, C. B. J. Phys. Chem. 1997, 101, 5263. (33) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (34) (a) Scho¨nherr, H.; Vancso, G. J. Polym. Prepr. 1998, 39 (2), 904. (b) Scho¨nherr, H.; Vancso, G. J. ACS Symp. Ser. Submitted. (35) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.

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Figure 3. AFM images of dihexadecyl sulfide 5b SAM on Au(111) (z-scale 7.5 nm).

Figure 4. Unprocessed AFM image of dodecanethiol on Au(111) (insets: autocovariance pattern, 2-D fast Fourier transform).

Figure 5. Unprocessed AFM image of fluorinated alkanethiol 4 on Au(111) (insets: autocovariance pattern, 2-D fast Fourier transform).

der Waals interactions, as the diameter of an alkane chain (4.2 Å)1c is significantly smaller than the sulfur-sulfur distance (4.97 Å). Calculations of the van der Waals energy in SAMs showed local minima for interchain distances of 4.2 and 5.0 Å, respectively, which correspond to 0° or ca. 30° tilt angles of the molecules.37 Our AFM measurements are clearly capable of distinguishing different lattices, i.e. lattice constants of 4.2 and 5.0 Å. Thus, indirect information about the average tilt angle of the alkane chain in the adsorbates can be obtained from a correlation with the lattice distances. Lattice Structure of Dialkyl Sulfide SAMs. The high-resolution AFM images shown in Figures 6 and 7 reveal the tail group lattice of didecyl sulfide 5a and dihexadecyl sulfide 5b SAMs on Au(111), respectively. The hexagonal lattice constant are identical to the lattice constants observed for n-alkanethiols (Table 1). For

Table 1. Lattice Constants of Investigated SAMs

(36) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys. 1997, 106, 1600. (37) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147.

adsorbate

lattice constant d (Å) (hexagonal lattice)

lattice structure

resorcin[4]arene (1)a calix[4]arene (2a) calix[4]arene (2b) decanethiols-d21 (3a) dodecanethiol (3b) hexadecanethiol (3c) octadecanethiol (3d) fluorinated thiol (4) didecyl sulfide (5a) dihexadecyl sulfide (5b)

11.6 ( 0.4,a 4.2 ( 0.2a 4.8 ( 0.1 4.9 ( 0.2 5.0 ( 0.1 5.0 ( 0.1 5.0 ( 0.1 5.0 ( 0.2 5.8 ( 0.3 4.9 ( 0.3 4.9 ( 0.2

b b b (x3 × x3)R30° (x3 × x3)R30° (x3 × x3)R30° (x3 × x3)R30° c(7 × 7) b (x3 × x3)R30°

a For details see ref 15. b In several cases a determination of the mutual orientation of the adsorbate lattice with respect to the underlying Au(111) lattice was not achieved. Hence, in these cases no lattice structure could be determined.

dihexadecyl sulfide SAMs the relative orientation of the tail group lattice with respect to the underlying Au(111) could also be determined. The mutual rotation of the next

Lattice Structure of SAMs of Dialkyl Sulfides

Figure 6. Unprocessed AFM image of didecyl sulfide 5a on Au(111) (insets: autocovariance pattern, 2-D fast Fourier transform).

Figure 7. Unprocessed AFM image of dihexadecyl sulfide 5b on Au(111) (insets: autocovariance pattern, 2-D fast Fourier transform).

neighbor direction of 30° proves a (x3 × x3)R30° lattice structure. This structure is identical to the structure of dialkyl sulfides on Au(111) as determined by Strong and Whitesides with diffraction techniques.38 However, these authors expressed concern about the purity of their sulfides. In several reports11,12,39 the interaction between the sulfur atom of dialkyl sulfides and gold was assumed as rather weak. Disordered layers with low surface coverage were reported. However, in these studies the assembly process was carried out at room temperature. The assembly at elevated temperatures (in our case 60 °C) is beneficial in order to obtain high-quality SAMs.15,30 SAMs of 5a and 5b which were prepared at room temperature (38) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (39) Jung, C.; Dannenberger, O.; Xu, Y.; Buck, M.; Grunze, M. Langmuir 1998, 14, 1103.

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Figure8. UnprocessedAFMimageofcalix[4]arenetetrasulfideadsorbate 2a on Au(111) (insets: autocovariance pattern, 2-D fast Fourier transform).

showed lower water contact angles and significantly lower proportion of sulfur bound to the gold. In addition, it was not possible to image a tail group lattice structure by AFM. The fact that n-alkanethiols and dialkyl sulfides form the same tail group lattice structure on Au(111) suggests that the van der Waals interactions between neighboring molecules are the dominant driving forces for the formation of the lattice structure. Lattice Structure of Receptor Adsorbate SAMs. For the two receptor adsorbates 2a and 2b it was not possible to image any periodic structure that can be attributed to the lattice of the cavity headgroups. Even at imaging forces of 1 nN and weaker, no periodical array could be visualized. Instead a fuzzy structure became visible. For imaging forces of 1 nN and higher, a periodicity of 4.9 Å could be resolved for both compounds (Table 1). This nearest neighbor distance is attributed to the packing of the alkyl chains underneath the cavities. However, compared to the chain-chain distance of 4.2 Å found for SAMs of 1,18 these values indicate that the alkyl chains underneath the cavities in SAMs of 2a and 2b are tilted similar to n-alkanethiols and n-alkyl sulfides. The hexagonal lattices of 2a and 2b are shown in Figures 8 and 9, respectively. The more flexible character of the calix[4]arene cavity40 apparently prevents the imaging of a cavity headgroup lattice. In addition, the constraint of the cavities on the alkane segments underneath seems to be negligible as compared to the receptor adsorbate 1. The alkane segments in 2a and 2b are oriented similar to normal dialkyl sulfides. The AFM effectively images a portion of the monolayers that is not directly located at the surface, i.e. the packing of the alkane segments in 1, 2a, and 2b. In view of the size of the tip and the size of the contact area between the tip and the sample surface,41 it is unreasonable to assume that the tip penetrates physically into the SAMs of the receptor adsorbates. We have previously shown that there is a significant “information depth“ in AFM measurements (40) (a) Gutsche, C. D. Calixarenes; Royal Society of Chemistry: Cambridge, 1989. (b) Calixarenes: A Versatile Class of Compounds; Vicens, J.; Bo¨hmer, V., Eds.; Kluwer Academic Publishers: Dordrecht, 1991. (c) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713.

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the interaction between the tip and contributions of the alkane segments. Thus, it is reasonable to state that the “information depth” of the AFM experiment at a given normal force can depend on the structure and the nature of the SAM, and more specifically on the structure and the nature of the cavity headgroups of SAMs of 1, 2a, and 2b. Conclusions

Figure 9. Unprocessed AFM image of tert-butylcalix[4]arenetetrasulfide-adsorbate 2b on Au(111) (insets: autocovariance pattern, 2-D fast Fourier transform).

on SAMs of unsymmetrical disulfides (R-S-S-R′), which can also be considered a “penetration depth“.41 In the case of the adsorbates 2a and 2b, the complex multibody interactions between the tip and the many atoms in the molecules near the surface of the SAM are dominated by (41) (a) Nelles, G.; Scho¨nherr, H.; Jaschke, M.; Wolf, H.; Schaub, M.; Ku¨ther, J.; Tremel, W.; Bamberg, E.; Ringsdorf, H.; Butt, H.-J. Langmuir 1998, 14, 808. (b) Nelles, G.; Scho¨nherr, H.; Vancso, G. J.; Butt, H.-J. Appl. Phys. A 1998, 66, S1261.

The interaction of sulfide-based adsorbates with Au(111) during self-assembly was shown to alter the substrate morphology similar to that of n-alkanethiols. Typical depressions with depths of single or multiple monolayers of Au were observed by AFM. In addition, AFM unveiled the lattice structure of a variety of self-assembled monolayers (SAMs) on Au(111) derived from n-alkanethiols, a fluorinated alkanethiol, dialkyl sulfides, and calix[4]arenebased tetrasulfide receptor adsorbates. For dialkyl sulfides the (x3 × x3)R30° structure is indistinguishable from the tail group lattice formed by n-alkanethiols. Therefore, intermolecular van der Waals interactions seem to dominate the structure of these SAMs. For the calix[4]arene-based tetrasulfide-derivatives the packing of the alkyl chains below the receptor cavity could be imaged, which suggests a considerable “information depth” in AFM experiments on SAMs. Furthermore, the tilt angle of the alkane segment was shown to depend on the rigidity of the cavity. Acknowledgment. This research has partly been supported by the Council for Chemical Sciences of The Netherlands Organization for Scientific Research (CWNWO) in the priority program materials (PPM). LA981787Y