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Sep 14, 2011 - Restructuring of Octanethiolate and Dialkyldithiocarbamate Monolayers in the Formation of Sequentially Adsorbed Mixed Monolayers...
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Restructuring of Octanethiolate and Dialkyldithiocarbamate Monolayers in the Formation of Sequentially Adsorbed Mixed Monolayers Annette F. Raigoza, George Kolettis, D. Andres Villalba, and S. Alex Kandel* University of Notre Dame, Department of Chemistry and Biochemistry, Notre Dame, Indiana 46556, United States ABSTRACT: Two-component octanethiolate-dialkyldithiocarbamate (DTC) monolayers were formed on Au(111) surfaces and studied using scanning tunneling microscopy (STM). Octanethiolate monolayers exposed to DTC in solution results in the erosion of octanethiolate domain boundaries and areas along terrace step edges and the insertion of DTC into these areas; this is consistent with the broad literature of substitution in alkanethiolate monolayers. Conversely, a DTC monolayer exposed to octanethiol results in displacement of DTC and the eventual formation of ordered octanethiolate domains. The effects of temperature, solution concentration, and deposition time are investigated.

’ INTRODUCTION Self-assembled monolayers (SAMs) are relatively simple to prepare, and the ability to tailor both the head and tail groups makes them ideal for creating interesting and technologically useful surface architectures. Interest in these systems has grown based on potential applications in lubrication, anticorrosion, molecular electronics, patterning, and biotechnology.1 10 The formation of multicomponent SAMs has been studied through various techniques that include coadsorption,11 13 sequential adsorption, 14 24 nanoscale “stamping”, 25 31 and scanningprobe-based lithography.25,32 44 The formation of these films is driven by the interplay between molecule surface and molecule molecule interactions, which typically create both thermodynamic and kinetic constraints. Through better understanding of this interplay, methods can be developed to control the resulting surface structures. Alkanethiolates on gold are the prototypical self-assembling system, first introduced by Nuzzo and Allara.45 The sulfur end group has a strong affinity for gold and forms a S Au bond, anchoring the molecule to the surface. Lateral van der Waals interactions along the methylene backbone order the monolayer to form 1,6,46 62 at high density, this includes a number √ of surface √ structures; the ( 3  3) R30° close-packed phase, along with several variants having c(4  2) superlattice symmetry.1,6,47,50,51,55 59,62 The alkanethiolate monolayer is also characterized by the presence of vacancy islands which arise from a restructuring of the underlying gold surface during monolayer formation.52,60,63,64 In various solvents, alkanethiolate SAMs spontaneously undergo exchange processes that are localized to defects.65 68 The presence of other molecular species gives rise to competition for available adsorption sites. The displacement of already-formed alkanethiolate monolayers is driven by an overabundance of a second molecular species. For this reason, short-chain alkanethiolate monolayers are easily displaced by long-chain alkanethiols in r 2011 American Chemical Society

solution66,69,70 and the same follows for adamantanethiolates by alkanethiolates71,72 and alkanethiolates by OH- and COOHterminated thiolates.73 76 Substitution occurs primarily at monolayer defects, including domain boundaries formed by a molecular tilt mismatch and the presence of terrace step defects from the underlying substrate. Additional substitution nucleates from these points and can result in the growth of domains and the complete removal of the original monolayer.72,77 81 Isolated- and bundled-molecule insertion at defects in an alkanethiolate matrix has allowed for the characterization of conductive properties.82 86 Dialkyldithiocarbamates (DTCs) bind strongly to the gold surface87 and form monolayers that are extremely robust and stable in a variety of environments.88 Consequently, dithiocarbamate functionalization has been incorporated in various fields that include biological studies, photolithography, and nanoparticle capping and anchoring on substrates.89 98 The S S spacing in the S C S bidentate structure is nearly the same as the spacing of the underlying gold.99 Diethyldithiocarbamate monolayers are able to adopt a hexagonal configuration on the Au(111) surface, and recently Morf et√al. observed √ a trimeric supramolecular structure exhibiting (2 3  2 3) R15° symmetry.100,101 In this arrangement, each imaged lobe corresponds to three diethyldithiocarbamate molecules. Our studies are designed to probe the extent to which thermodynamic and kinetic factors influence the formation of mixed monolayers. We approach the octanethiolate-DTC monolayer through two separate reactions involving the sequential adsorption of octanethiolate and DTC on the Au(111) surface. The exposure of DTC to octanethiol is an extension of our previous studies, which have focused on the restructuring of coronene and Received: July 21, 2011 Revised: September 5, 2011 Published: September 14, 2011 20274

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Figure 1. Octanethiol and three dithiocarbamate species used in mixed monolayers.

fullerene monolayers.102,103 Kinetic constraints during the vapor phase deposition of octanethiol result in the formation of mixed monolayer structures that depend on the arrangement and concentrations of the adsorbates in the initial monolayer. Mixed monolayers are also formed by coadsorption when a surface is exposed simultaneously to octanethiol and diothiocarbamate molecules in solution. The surface structure formed in these cases is complex and depends on the relative concentration of the two species as well as the DTC alkyl chain length. Coadsorbed monolayers are a subject of continuing study in our group; this manuscript describes only sequentially formed monolayers.

’ EXPERIMENTAL SECTION The preparation of single-component SAMs is similar to previously reported methods.88,103 Octanethiolate-on-gold samples are made by exposing a hydrogen-flame annealed Au(111)on-mica substrate (Agilent Technologies) to octanethiol vapor (Aldrich, 98.5%) at 70 °C for 17 24 h. DTC-on-gold samples also start with a clean gold substrate, which is immersed in dilute solutions of 1 10 mM DTC in ethanol or acetonitrile for up to 24 h at room temperature or 3 h at 70 °C. Here and subsequently, we use the term DTC to refer generically to dithiocarbamates, and DTC2, DTC4, and DTC10 to describe the particular molecules used. Figure 1 shows the molecular structures for octanethiol alongside all of the DTC variants used in these studies. Diethyldithiocarbamate (DTC2) and dibutyldithiocarbamate (DTC4) are commercially available as sodium or zinc salts, respectively (Acros Organics and MP Biomedicals). Didecyldithiocarbamate (DTC10) is synthesized through the equimolar addition of didecylamine (TCI America) to carbon disulfide (Acros Organics).88 Carbon disulfide is particularly reactive; therefore, all glassware used for the DTC synthesis is cleaned with piranha solution, a 3:1 solution of concentrated sulfuric acid, and 30% hydrogen peroxide. Piranha solution is extremely corrosive and must be used with caution. Octanethiol-followed-by-DTC and DTC-followed-by-octanethiol mixed monolayer samples are made by first adsorbing either octanethiolate or DTC onto a clean gold surface through the methods mentioned above. Subsequently, octanethiolate samples are placed in a solution of DTC at various concentrations, temperatures, and exposure times, and DTC samples are

exposed to octanethiol vapor for 17 24 h at 70 °C. Vapor deposition is a significantly cleaner method of preparing a monolayer and would have been preferable for DTC as well. However, we were not able to prepare monolayers in this fashion, most probably because of the lower vapor pressure of these compounds. Differences in deposition conditions (vapor vs solution) should be considered when comparing the two types of samples. These samples were characterized at ambient temperature and pressure. Images were acquired with a home-built scanning tunneling microscope (STM) operating at constant current using a tunneling bias of 0.5 V and a tunneling current of 10 20 pA. STM tips were made from mechanically cut Pt/Ir wire. Initial processing of the STM images consisted of removing noise along the fast-scan direction using a masked high-pass fitting procedure.104

’ RESULTS We create the mixed octanethiolate-DTC SAM by way of two separate reactions. In the first, an octanethiolate monolayer is formed on a Au(111) surface, and in a subsequent step, this same surface is immersed into solutions of DTC. We vary the DTC alkyl chain length, concentration, deposition temperature, and the amount of time the sample is placed in these solutions to observe the influence of these factors on the resulting surface. In the second reaction, we expose a DTC monolayer to octanethiol in the vapor phase. Only the relative amounts of octanethiol are varied; we use trace amounts of octanethiol and then increase to the typical amount used for a standard deposition. The octanethiol deposition temperature is not altered because the 70 °C temperature is ideal for monolayer formation at ambient pressures.105 In the formation of octanethiolate-DTC mixed SAMs, we observe the following: (1) removal of octanethiolate from within monolayer defects upon exposure to DTC, (2) displacement of DTC with exposure to octanethiol, and (3) removal of octanethiolate from mixed monolayer samples when annealed in neat ethanol. Octanethiol Followed by DTC. The solution-phase deposition of DTC causes significant changes to an octanethiolate-onAu(111) sample. The two prevalent features in our images are the erosion of defect areas across the monolayer and the presence of close-packed octanethiolate domains. The extent to which this 20275

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Figure 2. (a) Typical octanethiolate SAM on Au(111).(b) Mixed octanethiolate-DTC2 monolayer after an octanethiolate-on-gold substrate has come in contact with DTC. The monolayer is eroded primarily at defects in the film. Both images are 1730 Å  1670 Å.

Figure 3. Additional gold islands are formed when an octanethiolate monolayer is exposed to DTC2.

occurs depends on the deposition conditions. Figure 2 shows an image of a typical octanethiolate SAM (panel a), which can be compared with an octanethiolate sample that has been immersed in a 0.8 M DTC2 solution for 2 h at room temperature (b). In this image, disordered areas surround high-density octanethiolate regions and line terrace step edges. The exact structure of these disordered regions is difficult to resolve; this is often the case. In addition, we notice the appearance of bright patches within these disordered areas. When the height of these bright patches is compared with neighboring steps, demonstrated in Figure 3, they share the same height (2.35 Å),106 and we have determined these patches to be additional gold islands formed during the deposition. Figure 4 shows images of samples that have been immersed in 10 mM solutions of DTC2, DTC4, and DTC10 at room temperature for 24 h. In all three images, we find close-packed octanethiolate regions and vacancy islands typical of octanethiolate films. The amount of octanethiolate that is displaced from the Au(111) surface is inversely correlated with the length of the alkyl chains on the DTC; with increasing chain length, a smaller fraction of the surface is altered. In panel a, the domain boundaries of the octanethiolate appear rippled. We have seen these types of effects on occasion when an octanethiolate sample has been left in ethanol for long periods of time. We are able to resolve the formation of ordered DTC2 in some of our samples. Figure 5 is a higher resolution image of the left-hand side of Figure 5a. This image shows circular features spaced ∼10 Å apart, in contrast with thiolate that has a nearest-neighbor lattice spacing of 4.99 Å. These images are comparable to the closepacked structures previously observed for DTC2 on Au(111).99 Increasing the temperature extends and expands the regions of erosion within defect areas significantly. Figure 6 shows images of

Figure 4. Octanethiolate monolayer exposed to (a) DTC2 1368 Å  779 Å, (b) DTC4 1000 Å  1000 Å, and (c) DTC10 1000 Å  1000 Å. As the alkyl chain length is increased, less of the octanethiolate is replaced by DTC.

Figure 5. Areas of ordered DTC2 are resolved in mixed octanethiolateDTC monolayers, 418 Å  431 Å.

octanethiolate samples that have been exposed for 24 h to 10 mM DTC10 solutions: panel a at room temperature and panel b at 45 °C. In panel a, a large majority of the octanethiolate surface remains intact, and evidence of DTC10 exposure presents itself through the darkening and disordering of the areas surrounding the upper and lower terrace edges and within octanethiolate domain boundaries. A larger fraction of the surface is affected in panel b, and only a few domain boundaries remain intact. At higher temperatures, we observe these effects at even lower concentrations (1 mM) and much more quickly. Within one 20276

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The Journal of Physical Chemistry C hour in a DTC2 solution at 70 °C, domain boundaries have begun to darken, and we see the appearance of bright spots, shown in Figure 7a. In panel b, a large fraction of the octanethiolate monolayer is removed after 2 h. We find molecularly resolvable regions of DTC2 at the bottom left and right of panel b, which have been outlined in black, in addition to the closepacked octanethiolate domain at the top of the image. It is likely that DTC2 is present over much of the surface, albeit at lower concentrations and with less order; only the close-packed regions can be interpreted without ambiguity. At DTC exposure times above 2.5 h, we do not find any evidence of remaining octanethiolate on the surface, and we conclude that DTC2 has completely displaced all octanethiolate from the gold surface. From images in Figures 2, 4, 6, and 7, it seems that DTC is localized at domain boundaries and terrace step edges. By heating these mixed monolayers in neat ethanol, we can desorb thiolates; under these conditions, DTC remains on the surface.

Figure 6. Larger portions of the monolayer are eroded as the temperature is increased. (a) Room-temperature deposition of DTC10 imaged at 1000 Å  1000 Å and (b) 45 °C DTC10, 1000 Å  1000 Å.

Figure 7. Displacement of octanethiolate by DTC2 is rapid at 70 °C: (a) 1 h, 2000 Å  2000 Å, and (b) 2 h, 488 Å  421 Å. In the image in panel b, close-packed regions of both octanethiolate (at the top of the image) and DTC (outlined in black) coexist on the surface.

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After treating the surface in this way, we are able to image islands of densely packed DTC; in these cases, we also observe large areas that image noisily and show no clear structure, which can most likely be explained as loosely packed, disordered, and potentially mobile molecules on the surface. An image of a densely packed area is shown in Figure 8b. For comparison, we show a DTC2 monolayer on Au(111) in Figure 8a. We also see bright islands similar to those in Figures 2, 4a, 5, and 7. The contrast of the inset image in panel b has been adjusted to show the surface of the one of the bright features, which is covered with DTC2 as well. DTC Followed by Octanethiol. Displacement of DTC by octanethiol occurs much more readily, and exposing DTC monolayers to octanethiol vapor results in near-complete thiolate monolayers with few structural features indicating the presence of DTC. In Figure 9a,b, the result of exposing DTC2 and DTC10, respectively, to octanethiol is shown. The monolayers formed through these depositions appear to be those of typical octanethiolate surfaces with the occasional appearance of a bright spot in the middle of an ordered domain. These bright spots likely correspond to individual DTC molecules trapped within a close-packed octanethiolate domain, and we will examine this assignment and its implications in the Discussion. The vapor-phase deposition of octanethiol is performed in a sealed vial at 70 °C in the presence of excess liquid octanethiol (which is kept in a separate reservoir so it does not contact the sample). For nearly all of the process, then, the sample is exposed to the equilibrium partial pressure of 8.5 Torr of octanethiol. A first attempt to decrease the amount of octanethiol available limited the initial liquid volume to 1 μL, which after evaporation produces 6  10 3 Torr of alkanethiol in a 20 mL vial, assuming no loss. Even at these pressures, displacement of DTC by octanethiol was complete. Lower pressures were achieved by

Figure 9. DTC exposed to octanethiol results in a minimal presence of DTC in the mixed monolayer. (a) DTC2-octanethiolate mixed monolayer, 500 Å  500 Å. (b) DTC10-octanethiolate mixed monolayer, 500 Å  500 Å.

Figure 8. (a) DTC2 monolayer on Au(111), 500 Å  305 Å. (b) Octanethiolate-DTC2 mixed monolayer after octanethiolate has been desorbed by heating in neat ethanol 500 Å  305 Å. 20277

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The Journal of Physical Chemistry C introducing only trace amounts of octanethiol into the glass vial: no liquid thiol was visible, although the presence of vapor was confirmed by smell. (Assuming typical odor thresholds for thiols, this establishes a lower limit of ∼1  10 6 Torr.) DTC-covered Au(111) surfaces were exposed to these trace concentrations of octanethiol and produced mixed DTC-octanethiolate monolayers, shown in Figure 10 for DTC2 (left panel) and DTC4 (right panel). In both panels, areas between ordered domains appear unstructured and variegated and presumably contain a mixture of thiolate and dithiocarbamate molecules. These unstructured areas have an affinity for the tops of substrate steps, whereas regions of the surface near the bottoms of steps more often contain close-packed thiolates; this is especially evident in the right panel. We have not done a quantitative statistical analysis of this affinity, but we observe it frequently in images of mixed monolayers; it can also be seen in Figures 2b and 4c and most notably in Figure 6b. Characteristics of All Mixed DTC/Thiolate Monolayers. Regardless of the order in which the sample is exposed to reagents (octanethiol-DTC or DTC-octanethiol), close-packed alkanethiolate domains present in thiolate/DTC monolayers frequently contain point defects. These defects are imaged as bright features, and are encountered in many of our samples regardless of deposition conditions. Examples are shown in Figure 11 for DTC2, DTC4, and DTC10 in panels a c, respectively. The presence of these features does not disturb the surrounding octanethiolate, and as we will discuss in the following section, we believe these features likely correspond to individual DTC molecules trapped and stabilized against exchange by the surrounding alkanethiolate. An additional vaporphase deposition to an octanethiol-followed-by-DTC4 sample

Figure 10. Displacement of DTC by octanethiol is incomplete when very low pressures of octanethiol are used (see text). (a) DTC2octanethiolate mixed monolayer 1000 Å  1000 Å. (b) DTC4-octanethiolate mixed monolayer, 996 Å  932 Å.

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results in a very well-ordered octanethiolate monolayer, with very little of the domain-boundary disorder characteristic of octanethiolate-DTC or DTC-octanethiolate samples. However, point defects remain as isolated protrusions within ordered octanethiolate domains, as shown in Figure 12. We also observe more complex structures in mixed octanethiolate/ DTC monolayers, some examples of which are shown in Figure 13. These appear as ordered, linear features two or more molecules wide, either at domain boundaries or within octanethiolate domains. Somewhat similar features can sometimes be observed in single-component alkanethiolate monolayers as a result of lower-density molecular packing that allows some molecules to lie down on the surface, potentially overlapping or interdigitating with nearby molecules.105,107,108 The features in Figure 13 are structurally different, with different symmetry and less overall order present. We cannot say, on the basis of these images, whether the observed features result from DTC incorporation into ordered but not close-packed thiolate monolayer lattices or whether they are small, ordered areas of DTC molecules.

’ DISCUSSION There is an interplay between molecule molecule and molecule substrate interactions that drives the processes of monolayer formation and of molecular desorption, displacement, and exchange within formed monolayers. Octanethiolate monolayers are stabilized on the gold surface through a strong S Au bond

Figure 12. Point defects are scattered throughout the surface of an octanethiolate-DTC4 sample that has been exposed to a second deposition of octanethiol, 499 Å  435 Å.

Figure 11. We observe point defects in mixed monolayer samples for all of the DTC: (a) DTC2 332 Å  312 Å, (b) DTC4 484 Å  460 Å, (c) DTC10 500 Å  500 Å. 20278

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Figure 13. Bright features order in domain boundaries or form clusters in ordered octanethiolate domains. (a) DTC2 500 Å  500 Å, (b) DTC4 191 Å  182 Å, and (c) DTC4 250 Å  225 Å.

and van der Waals interactions between molecules. Placement in solution facilitates the desorption and subsequent readsorption of these molecules on the Au(111) surface.65 Defects in the monolayer result from the structure of the underlying gold substrate, that is, terrace steps, as well as the intersection of misaligned close-packed domains (domain boundaries). This is where exchange events primarily take place.66,67,72,75,76,78,109,110 Octanethiolate has a S Au bond strength of 1.30 eV and a packing density of√0.33 monolayers (MLs) on the Au(111) √ surface, assuming ( 3  3) R30° symmetry.1 For dithiocarbamates, X-ray photoelectron spectroscopy measures a net sulfur content for DTC2 that is 1.65 times that of octanethiolate monolayers on Au(111),99 corresponding to a packing density of 0.275 ML for DTC molecules. This measurement √ is consistent √ with the 0.25 ML coverage calculated for the (2 3  2 3) 100,101 The binding energy 3-DTC structure observed using STM. of DTC on gold is 1.50 eV, as measured through the thermal desorption of N-methyl,N-benzyl-dithiocarbamate on gold;87 values for DTC4 and DTC10 have not been reported in the literature. Because DTC monolayers are somewhat loosely packed, headgroup surface interactions are likely dominant, and we expect binding energies for DTC4 and DTC10 to be similar. However, even a larger-than-expected dependence on tailgroup is consistent with the qualitative nature of the argument below. For displacement experiments, the binding energy per surface site is most relevant for thermodynamic calculations. This energy is 0.43 eV per surface atom for octanethiolate monolayers and 0.38 eV per surface atom for DTC. As a result, thiolate monolayers are slightly preferred energetically, as a result of the higher packing densities compared with DTC. van der Waals interactions almost certainly favor the denser, more ordered thiolate monolayer as well. Our studies show that octanethiol and DTC can compete for adsorption sites on the gold surface. Molecular displacement—of thiolates by DTC or DTC by thiolates—is driven by the very large difference in concentration between the displacing molecules (in solution or the gas phase) and the initially adsorbed species. As an example, complete desorption of an octanethiolate monolayer into solution results in a 10 nM octanethiol concentration when given 20 mL of solvent and a sample size of ∼25 mm2. For displacement experiments under the conditions in our experiment, concentration provides up to a 10 20 kT driving force in favor of exchange. This is larger than the difference in bond strengths between DTC and octanethiolate and explains why DTC can insert into and eventually completely displace an octanethiolate monolayer. In addition, it explains why displacement of DTC by thiols proceeds so quickly and why very low

thiol concentrations are necessary for any appreciable amount of DTC to remain on the surface. Exposure of the DTC-covered surface to thiols occurs with thiols in the vapor phase, whereas displacement of thiols by DTC is performed in the presence of solvent. If anything, we would expect the solvent to stabilize desorbing species, and thus the slower process—the displacement of thiols by DTC—would be even less favored if it could be performed outside of solution. Our experiments also demonstrate mobility of DTC at elevated temperatures. We see evidence of this mobility when we heat mixed monolayers in neat ethanol. DTC2 remains on the surface while octanethiolate is removed. However, although DTC2 previously only decorated octanethiolate monolayer defects, it rearranges to form patches of close-packed molecules once the octanethiolate is removed. DTC monolayers are stabilized by strong S Au bonds and weak interactions between molecules. Upon octanethiol deposition, DTC and octanethiolate are both mobile on the gold surface because of the elevated temperatures. Upon cooling of the sample, octanethiolate forms close-packed, structured phases that originate at surface steps111 and expels DTC as the monolayer starts to crystallize. The preference for DTC aggregates for the tops of steps is similar to what we observed in our studies of C70/octanethiolate mixed monolayers102 and again suggests that nucleation of close-packed thiolate domains occurs preferentially at step bottoms. The difference in relative DTC DTC interactions and initial surface structure correlates with the structures we observe. DTC2 molecules interact more strongly and form dense monolayers, preventing the extended formation of closely packed octanethiolate domains. DTC4 is bulkier, interacts less strongly, and does not order well on the gold surface. It is forced to downward-going step edges and areas between ordered octanethiolate domains. This trend continues for DTC10, where we do not observe the presence of DTC aggregates. With increasing octanethiol concentrations, the overall quantities of DTCs are minimized to the extent that we mostly observe ordered octanethiolate monolayers. Likewise, we find evidence of point defects within ordered octanethiolate domains for all mixed octanethiolate-DTC samples. This is not the first time features of this type have been observed.103,112,113 STM does not provide chemical composition information, and so these could be explained by a variety of phenomena: gold adatoms, DTC, or changes in the surrounding octanethiolate structure. Many studies remark that dithiocarbamates are much more conductive than alkanethiolates;87,114 116 because STM images show a convolution of electronic structure with topographical features; this would explain the relative 20279

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The Journal of Physical Chemistry C brightness of these features in our images.117 In support of a tentative assignment of these point defects to isolated DTC molecules, we find a correlation between the scanned height and the length of the alkyl branches in the corresponding DTC; that is, a bright feature from an octanethiolate-DTC10 sample often appears to extend further out from the surface than one from an octanethiolate-DTC2 sample. However, this effect is difficult to quantify because it depends strongly on the nature of the tip while scanning. In conclusion, we find that in the formation of mixed octanethiolate-DTC monolayers, octanethiolate forms densely packed structures on the surface and DTC is present in smaller quantities within defects in the film. This resulting surface structure is thermodynamically controlled and accessible through two separate reactions. We find that DTC adsorption is more favorable on the gold surface than disordered octanethiolate; however, ordered octanethiolate is more favorable than DTC. We show that we are able to modify these structures kinetically by varying DTC chain lengths, concentrations, temperatures, and time.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]: 574-631-7837. Fax: 574-6316652.

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