Coadsorption of Octanethiol and Dialkyldithiocarbamate on Au(111)

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Coadsorption of Octanethiol and Dialkyldithiocarbamate on Au(111) Annette F. Raigoza,† George Kolettis,† T. E. Sharon Brandt,‡ Guido Caponigri-Guerra,§ Christopher Agostino,§ and S. Alex Kandel*,† †

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States LaSalle Intermediate Academy, South Bend, Indiana 46628, United States § St. Joseph’s High School, South Bend, Indiana 46617, United States ‡

ABSTRACT: Au(111) surfaces are exposed to solutions containing both octanethiol and dithiocarbamate (DTC) molecules, and the resulting surface composition and structure are studied using scanning tunneling microscopy (STM). DTC adsorption and monolayer formation are favored when present at the same concentration as octanethiol in solution. Higher octanethiol concentration in solution results in the incorporation of thiol into the resulting monolayer, with a strong dependence on the chain length of the DTC molecules. For diethyldithiocarbamate, thiol adsorption is limited and close-packed thiolate monolayers are not formed even at 100:1 excesses of the thiol in solution. For didecyldithiocarbamate, higher thiol concentrations lead to the formation of full thiolate mononlayers and the complete displacement of DTC.

I. INTRODUCTION Multicomponent self-assembled monolayers have been in development for applications where controlled and deliberate surface structures are desired.1 7 The approaches toward the creation of mixed SAMs are extensive and include coadsorption,8 13 adsorption of asymmetric sulfides or disulfides,14 17 or sequential adsorption.18 25 Other approaches incorporate further manipulation of a preformed monolayer through molecular printing26 31 or probe-based lithography.32 39 Alkanethiolate on gold is one of the most well-studied selfassembling systems, first introduced in 1983 by Nuzzo and Allara.40 Monolayer formation occurs via a two-step process; a fast adsorption step and a much slower reorganization step.41 48 Alkanethiols form a strong S Au (1.3 eV) bond that anchors the molecule to the surface.1 Initially, molecules lie parallel to the gold surface but as thiol coverage increases, molecules stand up to maximize S Au bond density on the surface while van√der Waals √ interactions along neighboring alkyl chains form the ( 3  3) R30° close-packed phase, c(4  2) superlattice, and other ordered structures.1,4,47,49 63 Dithiocarbamate (R2NCS2) monolayer formation is comparable to alkanethiolates on Au(111) in the strong binding of the sulfur headgroup to the substrate.64 Similar to alkanethiols, ordering within the monolayer is dependent on the length of the alkyl group (R) attached to the nitrogen; dioctadecyldithiocarbamate (DTC18) is able to form monolayers with similar crystallinity found in octadecanethiol monolayers.65 However, due to the geometric constraints caused by the headgoup geometry, monolayers of DTCs with shorter alkyl chains are less ordered. Of the shorter-chain dithiocarbamates, only diethyldithiocarbamate (DTC2) has shown to form a well-ordered monolayer; this is possible in part to the intertwining of ethyl chains to form a trimeric supramolecular structure.64,66 DTC monolayer stability67 r 2011 American Chemical Society

and strong coupling to the gold surface68 70 have led to their use in a number of fields including molecular electronics, biological studies, photolithography, and nanoparticle capping and anchoring on substrates.71 82 Our lab has focused on the sequentially deposited selfassembled monolayers of octanethiol with C70-fullerenes,20 coronene,22 and DTC.23 We observed that exposing a monolayer of any of these adsorbates to excess thiol in the vapor phase results in a drastic restructuring of the initial surface. This is primarily driven by the kinetics of the octanethiolate monolayer formation process but the extent to which this happens is dependent on the molecule molecule and molecule surface interactions of the adsorbate due to the initial coverage and order of the monolayer. The octanethiolate monolayer is also substantially modified when immersed in a solution containing DTC.23 When an alkanethiol SAM is in solution, spontaneous exchange events will occur in defect areas across the monolayer where molecules are weakly bound or poorly stabilized;83 86 DTC in solution competes for these available adsorption sites. In our investigations, we observed monolayer erosion and DTC substitution that was driven by the surplus of DTC in solution. We noted that DTC mostly displaced low density or weakly bound thiolate within defects and radiated from these areas. The degree to which displacement occurred depended on temperature, DTC concentration, and the choice of DTC in solution. In this manuscript we examine the coadsorption of octanethiol and DTC on the Au(111) surface. We expose a gold substrate to varied ratios of octanethiol and DTC in solution and we Received: November 16, 2011 Revised: December 20, 2011 Published: December 20, 2011 1930

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The Journal of Physical Chemistry C characterize these surfaces using scanning tunneling microscopy to observe how the differing molecule molecule and molecule surface interactions, adsorption rates and molecular exchange affect the monolayer formation process.

II. EXPERIMENTAL SECTION Single-component octanethiol- and DTC-on-gold SAMs are prepared by immersing a hydrogen-flame annealed Au(111)-onmica substrate (Agilent Technologies) in a dilute (0.1 mM) solution of the octanethiol (Aldrich, 98.5%) or the DTC in ethanol or acetonitrile for 2.5 h at 70 °C. Coadsorbed octanethiol-DTC SAMs are prepared similarly. A clean gold substrate is exposed to a solution containing octanethiol and DTC for 2.5 h at 70 °C. DTC concentration remains fixed at 0.1 mM for every coadsorbed sample, while the concentration of octanethiol in solution is varied between 0.1 mM to 10 mM in a series of experiments. Each set of experiments is repeated for the three DTCs used in this study: diethyldithiocarbamate (DTC2), dibutyldithiocarbamate (DTC4), and didecyldithiocarbamate (DTC10). DTC2 and DTC4 are commercially available as sodium or zinc salts, respectively (Acros Organics and MP Biomedical).

Figure 1. 970 Å  945 Å image of an octanethiolate SAM formed by exposing a clean gold substrate to a 0.1 mM octanethiol solution. Domains contain well-ordered molecules and several single-thiol vacancies. Areas between domains are often disordered.

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DTC10 is not, therefore it is produced through the addition of didecylamine (TCI America) to carbon disulfide (Acros Organics and MP Biomedicals) in acetonitrile.67 Piranha solution is used to clean all glassware because of the reactive nature of carbon disulfide. Caution! Piranha solution is extremely corrosive and must be used with caution. Samples are characterized in a home-built scanning tunneling microscope operating at ambient temperature and pressure. Images are acquired at constant current using a tunneling bias of 0.5 V and a tunneling current of 20 pA. The image of DTC2 on Au(111) was taken using a Nanosurf easyScan 2 STM (nanoScience Instruments) with a tunneling bias of 0.05 V and 1 nA tunneling current. STM tips are made from mechanically cut Pt/Ir wire. STM images are processed to remove noise along the fast-scan direction using a masked procedure.87

III. RESULTS A. Single-Component Monolayers. Figure 1 shows a 970 Å  945 Å image of an octanethiolate SAM formed in solution. Close-packed domains are speckled with single thiol vacancies and larger vacancy islands cover each terrace. Though many of the domain boundaries are straight, we observe that several are jagged. The surface is mostly well-ordered, with some disorder in the vicinity of surface and monolayer defects such as step edges, vacancy islands, and domain boundaries. DTC2, DTC4, and DTC10 monolayers are shown in Figure 2 in panels a c, respectively. DTC2 forms a well-ordered SAM on the Au(111) surface; the surface is hexagonally packed with a lobe lobe spacing of 10 Å.88 DTC4 and DTC10 are much more disordered. In panel b, DTC4 significantly restructures the gold surface to produce several gold islands across each terrace. There are occasional groups of circular-shaped features in images of the DTC10 Au(111) surface that are comparable in size to molecular features in images of DTC2; however, no real short or longrange order is evident. B. Coadsorbed Monolayers. Coadsorbed SAMs are formed by exposing a clean gold substrate to octanethiol and DTC simultaneously in solution. The concentration of DTC does not change (0.1 mM) while the amount of thiol is varied (0.1 mM, 1 mM, and 10 mM). Therefore, the ratio of octanethiol to each DTC is 1:1, 10:1, or 100:1. Figure 3 shows images of the monolayers that are formed when a gold substrate is exposed to DTC2 and octanethiol in solution. Panels a c show 1:1, 10:1, and 100:1 octanethiol to DTC2 ratios, respectively. A solution with a 1:1 octanethiolDTC2 ratio forms a well-ordered DTC2 monolayer. The surface is hexagonally packed, with a lobe lobe distance of 10 Å. We also observe the presence of brighter, less distinct features embedded in the monolayer. Because these features are not typical of images

Figure 2. DTC-on-Au(111) monolayers prepared by immersing a clean gold substrate in a 0.1 mM solution of the corresponding DTC. (a) DTC2 230 Å  230 Å (b) DTC4 1450 Å  984 Å (c) DTC10 470 Å  400 Å DTC2 is hexagonally packed while DTC4 and DTC10 form a disordered surface. 1931

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The Journal of Physical Chemistry C

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Figure 3. Octanethiol DTC2 mixed monolayer formed by immersing a gold substrate in solutions containing 1:1, 10:1, or 100:1 thiol to DTC. Initially, the surface is mostly covered with DTC2 but as the amount of thiol is increased, more thiol is incorporated into the monolayer. (a) 0.1 mM/ 0.1 mM 350 Å  350 Å, (b) 1 mM/0.1 mM 920 Å  920 Å, and (c) 10 mM/0.1 mM 420 Å  420 Å.

Figure 4. DTC4 octanethiolate mixed monolayer formed by immersing a gold substrate in varied ratios of DTC4 and octanethiol. (a) 0.1 mM/ 0.1 mM 920 Å  920 Å, (b) 0.1 mM/1 mM 890 Å  890 Å, and (c) 0.1 mM/10 mM 960 Å  960 Å.

Figure 5. DTC10-octanethiolate mixed monolayer formed by immersing a gold substrate in varied ratios of DTC10 and octanethiol. (a) 0.1 mM/ 0.1 mM 1500 Å  1500 Å, (b) 0.1 mM/1 mM 1000 Å  1000 Å, and (c) 0.1 mM/10 mM 500 Å  500 Å.

of neat DTC2 monolayers, it is likely that the brighter features in panel a correspond to single or few octanethiol molecules that are confined between ordered DTC2 molecules. Fuzziness in the image could result from interactions with the scanning tip, as octanethiolates would not be stabilized by neighboring DTC2 in the way they would be in close-packed thiolate monolayers. In panels b and c, the DTC2 lattice has disappeared, and features typical of either DTC or thiolate monolayers are not observed; indeed, it is difficult to assign molecular features to either DTC2 or thiol. Our speculation is that these images are of surfaces with a higher concentration of thiol mixed with DTC2. Figure 4 shows images of the monolayers that are formed when a gold substrate is exposed to DTC4 and octanethiol; panels a c show the same DTC-octanethiol ratios as in Figure 3. At equal thiol and DTC4 ratios, the monolayer is similar to that of the single component DTC4 self-assembled monolayer. An increase in the amount of thiol in solution relative to DTC4 results in the segregation of thiol from DTC4 and the growth of octanethiol domains. We observe small octanethiol domains in the middle of panel b and larger areas surrounded by DTC4 in

panel c. Higher-resolution images of these features show the characteristic 5 Å lattice spacing of close-packed thiolates. Figure 5 corresponds to the octanethiol-DTC10 mixed monolayers with panels a c showing 1:1, 10:1, and 100:1 ratios, respectively. At equal thiol and DTC10 ratios, the monolayer is similar to the DTC10 on Au(111) surface although we observe the appearance of several small vacancy islands. As the amount of thiol in the deposition solution is increased, we observe striping in the 10:1 sample and the formation of high-density octanethiol features in both the 10:1 and 100:1 samples. Disordered features are also intermixed with the striped features in panel b indicating the possible presence of DTC10 with the lower density thiol features. In panel c, this is not the case. Ordered octanethiol domains cover the surface and the only possible indication of any remaining DTC10 on the surface is the occasional bright spots in the monolayer.

IV. DISCUSSION When a gold substrate is exposed simultaneously to octanethiol and dithiocarbamate at equal molar ratios in solution, we 1932

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The Journal of Physical Chemistry C observe the formation of DTC monolayers with little or no adsorption of thiolates. Higher octanethiol-to-DTC ratios result in the incorporation of increasing amounts of thiolate into the monolayer, and this effect is more pronounced for longer-chain DTCs; that is, shorter-chain DTCs are more likely to remain in the mixed monolayer even at high solution concentrations of octanethiol. Octanethiol and DTC compete for available adsorption sites on the Au(111) surface. In considering the energetics of this process, DTC has a higher binding energy on gold than thiol, 1.5 eV compared to 1.3 eV.1,70 On the basis of an energy-per-area calculation, however, thiol adsorption is thermodynamically preferred, as four alkanethiol molecules occupy the same amount of area as three DTCs. In previous work, we have studied the exchange of DTC into already formed alkanethiolate monolayers, as well as the exchange of thiols into already-formed DTC surfaces.23 We observed that when a complete, close-packed thiolate monolayer is exposed to DTC, the resulting surface structure consists of closepacked octanethiolate domains, with DTC present only along surface defects. Because we observe far more DTC adsorption in the current study, DTC adsorption must occur (or begin to occur) before thiolates have adsorbed at high density and reached a closepacked structure. However, ref 64 shows that thiols do adsorb approximately 4 times faster than DTC. Tying these results together, we conclude that initial exposure of a surface to equimolar thiol and DTC in solution must result in the formation of disordered or low-density thiolate phases, which are then displaced by DTC. Our previous study also showed that complete monolayers of DTC were subject to exchange when exposed to thiols.23 This is consistent with our current results, in which high thiol concentrations do produce surfaces with greater thiolate coverage. This observation can be explained, however, either as displacement of DTC by thiols or by the formation of some close-packed thiolate domains before significant DTC adsorption. In either case, though, thiolates at surface defects should remain vulnerable to attack and replacement by DTC, so the fact that significant surface coverage of thiolates remains strongly suggests that there is also active displacement of DTC by thiols. We next consider the effect of DTC alkyl chain length on thiol adsorption at high solution thiol concentrations. Under these conditions, while the DTC2 thiolate surface likely contains a partially ordered mixture of DTC and octanethiolate, the DTC4 thiolate surface has several large close-packed thiolate domains, and experiments with DTC10 yield a surface that mostly consists of small, ordered thiolate domains. We posit that van der Waals interactions between DTC10 and octanethiolates could be quite favorable, and could result in the ready nucleation of thiolate domains around adsorbed DTC10 molecules. The large number of nucleation sites would then produce the small closepacked domains observed in Figure 5c. These interactions will not be nearly as strong for DTC4 or DTC2. In conclusion, we find that the coadsorption of octanethiol and DTC on the gold surface results in a range of compositions, from DTC monolayers to near-complete thiolate monolayers. These surface structures are accessible by the choice of DTC and the concentration of thiol in solution relative to the DTC.

’ ACKNOWLEDGMENT This work was supported by the National Science Foundation (NSF Grant No. CHE-03448577).

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