The Role of Buried Hydrogen Bonds in Self-Assembled Mixed

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J. Phys. Chem. B 2001, 105, 10630-10636

The Role of Buried Hydrogen Bonds in Self-Assembled Mixed Composition Thiols on Au{111} Penelope A. Lewis,† Rachel K. Smith,† Kevin F. Kelly,† Lloyd A. Bumm,† Scott M. Reed,‡ Robert S. Clegg,‡,§ John D. Gunderson,‡ James E. Hutchison,*,‡ and Paul S. Weiss*,† Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802-6300, and Department of Chemistry and Materials Science Institute, UniVersity of Oregon, Eugene, Oregon 97403-1253 ReceiVed: March 6, 2001; In Final Form: May 14, 2001

We have investigated the role of internal functionality in self-assembled monolayers of a family of amidecontaining alkanethiol molecules on Au{111} using scanning tunneling microscopy. In addition to van der Waals interactions that are present within n-alkanethiol self-assembled monolayers, hydrogen bonding between adjacent buried amide groups contributes to the stability of the amide-containing molecules on the surface and causes spontaneous phase separation upon coadsorption with an n-alkanethiol. A deposition solution concentration dependence study reveals that this is an observed trend across a range of examined solution compositions. Additionally, hydrogen bonding affects the packing structure of the amide-containing alkanethiol self-assembled monolayers. Although they adopt the same (x3×x3)R30° base lattice as n-alkanethiolate self-assembled monolayers, the amide-containing molecules form superlattice structures that are more linear than n-alkanethiol monolayers due to the hydrogen bonds they form. The internal functionality of monolayers can be used to control their formation and stability.

Introduction Self-assembled monolayers (SAMs) have been extensively studied as model organic films from which information about wetting and adhesion processes and interfacial physical and chemical properties can be acquired.1-5 Self-assembly has been utilized as a facile means to create two- or three-dimensional networks with the potential for diverse chemical and physical properties. SAMs composed of alkanethiols on Au{111} are the most frequently studied systems as variation is easily achieved by altering the alkyl chain length or terminal functional group of the component molecules.3,6,7 The principal interactions in self-assembly of alkanethiols on Au{111} are the van der Waals interactions that exist between adjacent alkyl chains in the monolayer. These interactions stabilize the molecules on the surface and lead to the formation of densely-packed, wellordered monolayers. Binary component monolayers add to the versatility of SAMs and are considered a possible route to patterning at the molecular scale.8-15 The ability to control the spatial arrangement of atoms and molecules is a crucial factor in innovative technologies, such as molecular-scale electronics.16,17 Although LangmuirBlodgett films have been used for these purposes as well,1 SAMs are a more practical system to use in fabrication at the nanometer and micrometer scale because of their ease of preparation and stability. Recently, we and others have explored possible methods for patterning by capitalizing upon the inherent chemical and physical properties of SAMs. Patterning of features in multicomponent SAMs has been successfully demonstrated using methods such as “soft lithography”,18-20 selective de* To whom correspondence should be addressed: (J.E.H.) hutch@ oregon.uoregon.edu; (P.S.W.) [email protected]. † The Pennsylvania State University. ‡ University of Oregon. § Current address: IGEN International, Inc., Gaithersburg, MD, 20877.

sorption by electrochemical means,21-23 and scanning probe lithography.24-26 These processes often involve partial desorption of one component molecule with subsequent “back-filling” of the second molecule to create discrete homogeneous domains of a single component. Spontaneous phase separation of SAMs by coadsorption of two or more component molecules has been less widely studied and is not as well understood. It has been shown that monolayers adsorbed from miscible solutions of alkanethiols of similar alkyl chain lengths (differing by 2 methylene units) do not undergo phase separation unless postadsorption processing is performed (i.e., thermal annealing from solution).15 However, phase separation has recently been observed for a binary component SAM composed of alkanethiols in which the alkyl chain lengths differed by six or more carbons.27,28 Spontaneous phase separation of binary component SAMs consisting of molecules with differing terminal functional groups has also been observed where the separation is due to interactions between ω-functional groups.12,13 Since the functional groups have dissimilar chemical properties, these types of films have been proposed as useful systems where different reactivity is desired on the same surface. Recently, we demonstrated the spontaneous phase separation of a binary component SAM consisting of an alkanethiol (decanethiol) and an amide-containing alkanethiol (3-mercaptoN-nonylpropionamide, or 1ATC9) using scanning tunneling microscopy (STM).29 Here, we report a more detailed study of these molecules and their two- and three-amide counterparts (3-mercapto-N-(N′-n-hexylacetamido)propionamide, or 2ATC6, and 3-mercapto-N-(N′-(N′′-n-propylacetamido)acetamido)propionamide, or 3ATC3; Figure 1). This family of molecules is unique in that the amide functional group is buried within the monolayer, creating a system in which “layers” of interactions exist that provide increased stability of the monolayer (i.e., the van der Waals interactions of the alkyl chain layer and the hydrogen bond interactions in the amide layer).30-33 The stability

10.1021/jp010854l CCC: $20.00 © 2001 American Chemical Society Published on Web 10/05/2001

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Figure 1. Molecules 1ATC9, 2ATC6, and 3ATC3 that can form hydrogen bonds between the amino and carbonyl functional groups in adjacent molecules within monolayer films.

provided by the internal functionality can be exploited in lithographic techniques where binary-component monolayers possess the same chemical reactivity at the surface but differ in rates of desorption for the two molecules. First, we describe the results of a study of the phase behavior of 1ATC9 and decanethiol molecules as a function of relative concentration in which phase separation occurs at all concentrations investigated. We also found that phase separation occurs within SAMs composed of 2ATC6 and decanethiol, but we did not observe order in SAMs composed of 3ATC3 and decanethiol. We have also explored pure films of the amide-containing thiols and found that they adopt different structures from those of pure alkanethiol SAMs. Although the single-component SAMs of the amide-containing alkanethiol molecules adopt the same basic (x3 × x3)R30° overlayer geometry as alkanethiol films, the superlattices as well as the appearance of the domain boundaries that are observed in STM images for these films are markedly different. Experimental Section The 1ATC9, 2ATC6, and 3ATC3 molecules were prepared by previously described methods.31 Stock solutions of 1 mM 1ATC9, 1 mM 2ATC6, 1 mM 3ATC3, 1 mM octanethiol, and 1 mM decanethiol34 were made in degassed, absolute ethanol. All solutions were mixtures of these and 1 mM in total thiol. The lATC9:decanethiol solutions were mixed to the following proportions: 1:19, 1:3, 1:1, 3:1, 19:1. The 2ATC6:decanethiol and 3ATC3:decanethiol solutions were prepared as 1:1 solutions for coadsorption. A 1ATC9:octanethiol solution was also prepared as a 3:1 solution for coadsorption. Au-on-mica substrates34 were flame annealed to desorb any adventitious organic material. These substrates have predominantly Au{111} surfaces, and we select these crystalline regions for our STM measurements. The substrates were immersed in the deposition solutions, blanketed with N2, and stored in the dark. After 40 h, samples were removed, rinsed with ethanol, dried under a stream of N2, and stored in a humidity-controlled environment until imaged. (Note: Samples in which the substrates were immersed for 5 days were also prepared and showed no significant difference in the packing structure or defect density of the films.) STM images were obtained under ambient conditions using a home-built STM that has been described previously.15 Images were recorded in constant-current mode and at high tunneling gap impedances (∼1012 Ω) to ensure large tip-sample separation for minimal contact between the probe tip and the monolayer. Results and Discussion Figure 2 shows an example of the spontaneous phase separation that occurs upon coadsorption from a 1:1 solution

Figure 2. STM image of a SAM coadsorbed from a 1:1 mixture of 1ATC9 and decanethiol showing spontaneous phase separation. Arrows point to decanethiolate regions. Substrate defects appear as depressions (dark) in the image. Tunneling conditions: Vsample ) -1 V, I ) 2 pA, 400 Å × 400 Å.

of 1ATC9 and decanethiol. Islands of decanethiolate molecules are indicated in the image by white arrows. The darkest regions in the image are typical of STM images of alkanethiol SAMs and are attributed to deposition-induced defects in the underlying gold substrate.35 It is important to note that the areas of decanethiolate molecules are visually distinguishable from the substrate defects as the apparent height difference between the 1ATC9 and the decanethiol areas are due to three backbone atoms and appear ∼1.65 Å higher in the STM images,36 whereas the substrate defect sites are one gold atom deep (∼2.35 Å) and appear as the darkest areas in the image. This spontaneous phase separation is attributed to hydrogen bond interactions between adjacent amide-containing alkanethiol molecules on the surface.29 To ascertain the effect of the relative concentrations of the components in solution on the phase separation within the SAM, films coadsorbed from solutions of varying proportion were also imaged. Mixed SAMs coadsorbed from solutions containing higher 1ATC9 proportions (3:1 and 19:1 1ATC9:decanethiol) demonstrated spontaneous phase separation when adsorbed onto Au{111} (Figure 3a,b). As in the 1:1 mixture (Figure 2), decanethiol molecules form discrete islands in these SAMs, surrounded by 1ATC9 molecules. At these proportions, (g50% 1ATC9 molecules in solution) the surface has greater coverage of the 1ATC9 molecules compared to the solution. Selfassembly is understood as a dynamic process involving exchange of adsorbate molecules between the surface and solution. Longer-chain molecules tend to dominate these exchange processes by virtue of enhanced stability imparted by the greater numbers of interchain interactions and will be found in greater concentration on the surface compared to shorter chain molecules of similar structure.11,37 This coincides with the 1ATC9 molecules being longer than decanethiol by three backbone atoms. Additionally, after initial nucleation of the 1ATC9 molecules, remaining on the surface may be energetically advantageous over desorbing into the solvent, as the van der Waals interactions along with hydrogen bonding between

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Figure 3. STM images of SAMs coadsorbed from solutions containing a high relative concentration of 1ATC9 molecules. Phase-separated domains of decanethiolate are indicated by arrows and appear lower. The higher relative concentration 1ATC9 films exhibit less distinct domain boundaries and a different packing conformation than lower relative concentration 1ATC9 films. (a) SAM adsorbed from a 3:1 1ATC9:decanethiol solution. Tunneling conditions: Vsample ) -1 V, I ) 1 pA, 250 Å × 250 Å. (b) SAM adsorbed from a 19:1 1ATC9:decanethiol solution. Tunneling conditions: Vsample ) -1 V, I ) 2 pA, 300 Å × 300 Å.

Figure 4. STM images of SAMs coadsorbed from solutions containing a low relative concentration of 1ATC9 molecules (topographically higher areas) in which they are ordering at domain boundaries and at Au defects (white arrows). (a) SAM adsorbed from a 1:19 1ATC9:decanethiol solution. Tunneling conditions: Vsample ) -1 V, I ) 2 pA, 250 Å × 250 Å. (b) SAM adsorbed from a 1:3 1ATC9:decanethiol solution. Tunneling conditions: Vsample ) +1 V, I ) 2 pA, 250 Å × 250 Å.

adjacent amide alkanethiols would stabilize the molecules on the surface and be more favorable than exchange processes with the solvent. Parts a and b of Figure 4 show phase separation occurring for films made from solutions of low 1ATC9 proportion (1:19 and 1:3 1ATC9:decanethiol, respectively). The 1ATC9 molecules adsorb near Au substrate defects and the domain boundaries, since these areas are less stable and more accessible to exchange processes within the adsorption solution. We observed no instances of 1ATC9 molecules intermixing within the decanethiol domains. This differs from studies of binary

alkanethiol SAMs of comparable length (i.e., dodecanethiol and decanethiol), where coadsorption from a 19:1 decanethiol: dodecanethiol solution produced SAMs in which dodecanethiol molecules were randomly interspersed within the larger decanethiol matrix.15 From Figure 4a,b, it is apparent that the surface coverage of the 1ATC9 molecules does not simply mirror the solution composition. Since the 1ATC9 molecules are longer than the decanethiol molecules by three backbone atoms, the 1ATC9 molecules would be expected to adsorb more strongly on the surface based on long vs short chain behavior. In fact, for the

Buried Hydrogen Bonds in Self-Assembled Monolayers lower 1ATC9 proportion SAMs (50% in solution). Substrates were also prepared in which the 1ATC9 was coadsorbed with a shorter chain length n-alkanethiol (octanethiol), and for which the greater apparent height difference between the two molecules confirmed identification of the separate domains. Figure 5 is an STM image of a SAM adsorbed from a 1:3 solution of 1ATC9:octanethiol. In this image, the 1ATC9 molecules have adsorbed at higher coverage compared with SAMs adsorbed from 1ATC9:decanethiol solutions. This is consistent with long vs short chain coadsorption behavior since octanethiol is shorter than decanethiol molecules by two methylene units. The investigation of these phase-separated films led to the observation that 1ATC9 domains adopt a different superlattice structure from that of alkanethiolate domains. Figure 6a is a typical STM image of an n-alkanethiol SAM (dodecanethiol). Substrate defects (dark areas) and domain boundaries are clearly visible in the film. Alkanethiol SAMs form a (x3 × x3)R30° lattice commensurate with the underlying Au{111} substrate.3 The alkanethiol molecules adsorb on the Au surface with a nearest-neighbor spacing of ∼5.0 Å and a tilt angle of ∼30° from the normal. The amide-containing alkanethiol SAMs adopt the same basic (x3 × x3)R30° lattice. The spacing between

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Figure 5. STM image of a SAM coadsorbed from a 1:3 solution of 1ATC9:octanethiol. The greater topographic height difference highlights the location of the different molecules in these images compared to Figures 3 and 4. Note the greater surface coverage of 1ATC9 molecules compared to films adsorbed from comparable decanethiol solutions. Tunneling conditions: Vsample ) +1 V, I ) 2 pA, 250 Å × 250 Å.

nearest neighbors is also consistent with alkanethiol SAMs (∼5.0 Å), which was confirmed by Fourier analysis of the images. In addition to the (x3 × x3)R30° base lattice formed by the adsorbate overlayer, several superlattice conformations have been observed in alkanethiol SAMs.38 These are attributed to different orientations of the alkyl chains of the adsorbate molecules within the monolayers. The superlattice structures of a dodecanethiolate monolayer shown in Figure 6b,c are commonly found in alkanethiol SAMs and correspond to a c(4 × 2) structure with a rectangular unit cell defined by the alkyl chains in the corner sites twisting and extending out of the film, causing them to appear topographically higher (displayed as brighter) in the STM images. These superlattice structures contribute to the formation of structurally different domains in alkanethiol SAMs; however, the predominant packing structure remains the (x3 × x3)R30° lattice. In comparison to single component alkanethiol SAMs, Figure 7 shows typical conformations in films adsorbed from a single component 1ATC9 solution. Fourier analysis also revealed superlattice structures with spacings that are consistent with c(4 × 2) lattices. However, the superlattice structures in the realspace images differ visually from those typical of alkanethiol films. Parts b and c of Figure 7 show examples of the predominant conformations found for these films. The structure in Figure 7b appears more linear than structures typically found for alkanethiol SAMs. This could correspond to the hydrogen bonds in one amide-containing molecule aligning in an endto-end fashion with the hydrogen bonds in the adjacent molecule. In fact, the majority of the domains adopt these two conformations as superlattices of the (x3 × x3)R30° structure, indicating that the linear-based structures are energetically favorable. We anticipate that such linear structures can be used to advantage to form walls or lines within patterned SAMs. It is also apparent from comparisons of Figures 6 and 7 that differences in the structural domain boundaries exist between

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Figure 6. (a) STM image of a dodecanethiolate SAM containing numerous substrate defects and domain boundaries, both of which are typically found in images of alkanethiolate SAMs. Typical superlattices of n-dodecanethiolate SAMs are shown enlarged in (b) and (c). The conformations in (b) and (c) correspond to a c(4 × 2) structure. Tunneling conditions: Vsample ) -1 V, I ) 1 pA, (a) 500 Å × 500 Å, (b) 52 Å × 66 Å, (c) 60 Å × 48 Å.

single component 1ATC9 and single component alkanethiol SAMs. SAM growth nucleates at many different sites on the substrate, resulting in numerous combinations of headgroupsubstrate registry and chain tilt direction. Since alkanethiolate molecules create a stacked network of alkyl chains on the surface, convergence of two domains with interfering patterns will force conformational changes in the molecules at one of the domains.38 Thus, domain boundaries of alkanethiol SAMs occur when domains encounter one another on the surface during the growth process, causing distinct boundaries in the film due to chain twist or misalignment of the adsorbate molecules at the boundaries. Domain boundaries in single component 1ATC9 SAMs are not as distinct as in alkanethiol SAMs, although there are separate domains in the 1ATC9 film in which different conformations are present. This trend was observed across all the single component 1ATC9 SAMs, as well as the films that contained a higher proportion of 1ATC9 molecules (3:1 and 19:1 1ATC9:decanethiol). In contrast, the domain boundaries in the lower 1ATC9 concentration SAMs (1:19 and 1:3 1ATC9: decanethiol) resemble those in pure alkanethiol SAMs, leading us to the conclusion that formation of the domain boundaries in the lower proportion 1ATC9 films are dominated by the interactions of the decanethiol molecules. Since the domain boundaries in single component 1ATC9 SAMs are not prominent features, molecules at these domain interfaces are likely aligned such that no disruption of the chain tilt or chain twist is occurring at the boundary.39 This may be required to maintain

the hydrogen-bonding network of molecules within a single domain. Therefore, the variations between adjacent domains would involve differences in 1ATC9 orientation, as opposed to chain twist or chain tilt, as observed in n-alkanethiol SAMs. SAMs coadsorbed from solutions containing equimolar ratios of a multiple amide-containing alkanethiol (2ATC6 or 3ATC3) and decanethiol were also investigated to determine the effect of increasing hydrogen bonding on phase separation with a nonfunctionalized alkanethiol. The 2ATC6 and 3ATC3 molecules were chosen for these studies as their physical height is roughly the same as 1ATC9, since each acetamido subunit is approximately equal in height to three methylene units in the alkyl chain. The 2ATC6:decanethiol films adsorbed from a 1:1 solution appeared to exhibit behavior similar to 1ATC9: decanethiol films regarding surface coverage and phase separation. STM images were similar to images of SAMs coadsorbed from a 1:1 solution of 1ATC9:decanethiol, with the 2ATC6 domains appearing topographically higher (Figure 8). It was impractical to determine whether phase separation was occurring at the molecular level in SAMs coadsorbed from a 1:1 solution of 3ATC3:decanethiol, as we did not observe wellordered regions by STM. Although each 3ATC3 molecule contains six opportunities for hydrogen bonding (two per amide group) and would presumably form well-ordered SAMs, the alkyl chain is considerably shorter compared with the 1ATC9 and 2ATC6 molecule, decreasing the contribution of van der Waals interactions between adjacent chains. Since STM is a local probe designed to image the topmost layer of substrates,

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Figure 7. (a) Typical structures of amide-containing alkanethiolate (1ATC9) regions outlined in white, shown expanded in (b) and (c). The observed structures appear more linear than alkanethiol structures. Tunneling conditions: Vsample ) +1 V, I ) 3.0 pA, (a) 250 Å × 250 Å, (b) 88 Å × 50 Å, (c) 42 Å × 37 Å.

are necessary to form well-ordered SAMs along the entire length of the amide-containing alkanethiols32 that are measured by STM. The 3-amide alkanethiol SAMs containing a longer alkyl chain overlayer were not studied since these would not generate reproducible STM images because of the length of the insulating layer. Our assumption that the amide sublayer is ordered is based upon IR studies involving pure SAMs of similar 3-amide molecules that contain longer alkyl chains where these were shown to form well-ordered films.32,33 Conclusion

Figure 8. STM image of a SAM coadsorbed from a 1:1 solution of 2ATC6 and decanethiol. The 2ATC6 molecules are estimated to be the same height as the 1ATC9 molecules and show up as the brighter regions in the image surrounding islands of decanethiolate. SAMs coadsorbed from a 1:1 solution of 3ATC3 and decanethiol did not exhibit order and similar phase separation as for the molecules with fewer backbone amides. Tunneling conditions: Vsample ) +1 V, I ) 2 pA, 800 Å × 800 Å.

a system such as this containing a possibly disordered alkyl overlayer will appear disordered in the STM image, regardless of the packing of the amide underlayer. Therefore, along with the hydrogen bond interactions, the van der Waals interactions

We have studied SAMs of amide-containing alkanethiol molecules in which the formation and stability of mixed and pure films is strongly influenced by the hydrogen bond interactions of the internal functional group. In mixed SAMs of 1ATC9 or 2ATC6 and an n-alkanethiol, spontaneous phase separation occurs at all investigated concentrations. The stability gained by the hydrogen bonding amide groups in these molecules directly affects the ratio of the components within the SAM compared to the ratio in solution. The hydrogenbonding network within the SAM contributes to the formation of linear superlattice structures in single component amidealkanethiol SAMs. The results from this study demonstrate the possibility for tailoring the chemical and physical properties of SAMs based on their internal functionality. Acknowledgment. We thank NSF (all), ARO (PSU), ONR (PSU), DARPA (PSU), the Camille and Henry Dreyfus Foundation (J.E.H. is a Camille Dreyfus Teacher Scholar), the Alfred P. Sloan Foundation (J.E.H. is a Sloan Fellow), and a Department of Education GAANN fellowship (S.M.R.). The PSU authors would like to acknowledge Nancy Santagata for helpful discussions.

10636 J. Phys. Chem. B, Vol. 105, No. 43, 2001 References and Notes (1) Ulman, A. An Introduction To Ultrathin Organic Films: From Langmuir-Blodgett To Self-Assembly; Academic Press: San Diego, 1991. (2) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (3) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437-463. (4) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (5) Allara, D. L. Biosens. Bioelec. 1995, 10, 771-783. (6) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (7) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 71647175. (8) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563-571. (9) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 723-727. (10) Bain, C. D.; Whitesides, G.-M. Science 1988, 240, 62-63. (11) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097-5105. (12) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646. (13) Stranick, S. J.; Atre, S. V.; Parikh, A. N.; Wood, M. C.; Allara, D. L.; Winograd, N.; Weiss, P. S. Nanotechnology 1996, 7, 438-442. (14) Stranick, S. J.; Kamna, M. M.; Krom, K. R.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Vac. Sci. Technol. B 1994, 12, 2004-2007. (15) Bumm, L. A.; Arnold, J. J.; Charles, L. F.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017-8021. (16) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; II, L. J.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (17) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541548. (18) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (19) Zhao, X. M.; Xia, Y.; Whitesides, G. M. J. Mater. Chem. 1997, 7, 1069-1074. (20) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511.

Lewis et al. (21) Imabayashi, S. I.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502-4504. (22) Schoer, J. K.; Zamborini, F. P.; Crooks, R. M. J. Phys. Chem. 1996, 100, 11086-11091. (23) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073-5078. (24) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661-663. (25) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G. Y. Langmuir 1999, 15, 7244-7251. (26) Amro, N. A.; Xu, S.; Liu, G. Y. Langmuir 2000, 16, 3006-3009. (27) Chen, S. F.; Li, L. Y.; Boozer, C. L.; Jiang, S. Y. Langmuir 2000, 16, 9287-9293. (28) Cygan, M. T. Ph.D. Thesis, The Pennsylvania State University, 1997. (29) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119-1122. (30) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (31) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 53195327. (32) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876-8883. (33) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486-2487. (34) n-Octanethiol was used as received from Aldrich Chemical Co., Milwaukee, WI. n-Decanethiol was used as received from Lancaster Synthesis, Windham, NH. The gold substrates were purchased from Molecular Imaging, Inc., Phoenix, AZ. (35) Edinger, K.; Golzhauser, A.; Demota, K.; Woll, C.; Grunze, M. Langmuir 1993, 9, 4-8. (36) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 8122-8127. (37) Jung, L. S.; Campbell, C. T. J. Phys. Chem. B 2000, 104, 1116811178. (38) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853-2856. (39) Poirier, G. E. Chem. ReV. 1997, 97, 1117-1127.