Alkanethiol Monolayers Contain Gold Adatoms, and Adatom

Oct 9, 2009 - The nature of the sulfur-gold bond in alkanethiol films self-assembled on Au(111) surfaces is widely contested, despite significant inte...
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Alkanethiol Monolayers Contain Gold Adatoms, and Adatom Coverage Is Independent of Chain Length Natalie A. Kautz and S. Alex Kandel* UniVersity of Notre Dame, Department of Chemistry and Biochemistry, Notre Dame, Indiana 46556 ReceiVed: August 13, 2009; ReVised Manuscript ReceiVed: September 22, 2009

The nature of the sulfur-gold bond in alkanethiol films self-assembled on Au(111) surfaces is widely contested, despite significant interest in these systems over the last 25 years. Recent theoretical and experimental studies have suggested gold adatoms are incorporated into the alkanethiol-gold interface. We have exposed alkanethiol self-assembled monolayers (SAMs) to gas-phase hydrogen atoms to remove the monolayer; the gold adatoms remain on the surface and form features that we observe using scanning tunneling microscopy (STM). The features include the formation of single-atom-thick gold islands, decreasing size of surface vacancy pits, and faceting of terrace step edges; compared to the alkanethiol-terminated surface, these features indicate a net increase in the amount of gold present on the surface, compared to the alkanethiol-terminated bulk structure. Varying the length of the alkane chain does not affect the total adatom coverage, as the adatom coverages for ethanethiol (0.172 ( 0.039), octanethiol (0.143 ( 0.033), and dodecanethiol (0.154 ( 0.024) SAMs are within experimental error of one another. This corresponds to one gold adatom for every six atoms in the bulk-terminated surface, and thus one gold adatom for every two alkanethiol molecules. Introduction 1-9

3-5,10-16

Recent experimental and theoretical studies have shown that the formation of self-assembled alkanethiol monolayers on gold is accompanied by a significant reconstruction of the gold surface. Various models of this reconstruction have been proposed; many are “adatom” models, where additional gold atoms are incorporated into the monolayer above an approximately bulk-terminated gold surface structure. Other suggested structures include mixtures of adatoms and gold vacancies, as well as more extensive rearrangements involving both surface and subsurface atomic layers. As of this time, there is no general agreement as to the exact nature of the Au(111) reconstruction, and the diversity of experimental and theoretical results is reviewed thoroughly in ref 17. Our laboratory has completed a set of experiments in which alkanethiol molecules are removed from the surface so that the underlying gold surface can be observed and characterized. This approach is indirect in that Au atomic positions are not measured while the monolayer is present. However, it provides an unambiguous and quantitative measurement of one parameter: the difference in occupancy between the reconstructed and unreconstructed gold surfaces. Previous work in our laboratory measured this difference in occupancy for octanethiol monolayers,18 and this result is extended in the current manuscript to ethanethiol and dodecanethiol. We observe the same adatom coverage for monolayers of all three molecules; this strongly suggests that the result is general for alkanethiols (possibly excepting methanethiol), and it allows us to compare much more directly with theoretical and other experimental studies which have been done with a wide range of chain lengths. X-ray photoelectron spectroscopy (XPS)19 and scanning tunneling microscopy (STM)18 studies have shown that alkanethiol self-assembled monolayers (SAMs) can be removed from the Au(111) surface through reaction with gas-phase * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 574-631-7837. Fax: 574-631-6652.

hydrogen atoms. Using XPS, Fairbrother and co-workers observed two pathways for this reaction to occur, and showed that the chain length of the alkanethiol determines the importance of each reaction path.19 The first pathway, hydrogenation of the sulfur-gold bond, was seen primarily for medium-length alkanethiol chains (9-12 carbon atoms), where the incident hydrogen atom was able to permeate through the monolayer and attack the sulfur-gold bond directly, resulting in desorption of the entire alkanethiol molecule. In longer alkane chains (16-18 carbon atoms), desorption of complete alkanethiol molecules was more difficult, as hydrogen abstraction predominated. Removal of a hydrogen from the alkane chain resulted in a surface radical, which would then interact with a neighboring alkane chain, forming a cross-linked carbon film and preventing desorption of the molecule. Over extended exposure, the cross-linked monolayer was slowly eroded away. For all alkanethiol chain lengths studied, XPS measurements showed that prolonged H-atom exposure removed the monolayer, resulting in a clean gold surface. Our experiments are designed to image the same area of a surface before, during, and after hydrogen-atom exposure. This provides the unique opportunity to study the reconstruction of the gold surface caused by alkanethiol SAM formation; as the SAM is removed, one can monitor the resulting changes in the surface structure. As significant portions of the monolayer are removed, the underlying gold surface is exposed, and we have observed that additional gold atoms are freed up by the removal of alkanethiols. These gold atoms add to alreadypresent step defects, causing both an advance and faceting of upper terraces over lower. Atoms also fill in the gold vacancy islands present at full monolayer coverage, which then shrink in size and sometimes disappear completely. Finally, gold atoms nucleate and grow to form new single-atom-high gold islands on existing terraces.18 When alkanethiols are removed by H-atom reaction, the clean gold surface left behind relaxes to a hexagonally close-packed arrangement, either with a bulk fcc(111) termination or the 23

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Alkanethiol Monolayers Contain Gold Adatoms × 3 “herringbone” reconstruction typical of Au(111) in a vacuum. The additional gold atoms that we observe must be adatoms incorporated in the alkanethiol-gold interface. Our previous experiments involving octanethiol SAMs removed by hydrogen atoms revealed a net gold increase of 0.143 ( 0.033 monolayers, that is, approximately one additional gold atom for every six to eight atoms on a bulk-terminated surface.18 Because removal of the alkanethiol monolayer allows for rearrangement of surface atoms, this result does not provide any detail as to the exact arrangement of gold atoms in an octanethiol/Au(111) monolayer. However, we can conclude that (1) within experimental error, it is consistent with adatom-based models where each gold adatom bridges two surface-bound sulfurs; (2) it is inconsistent with adatom models that assign one adatom to every adsorbed alkanethiol (unless there are additional surface vacancies as well); and (3) it is inconsistent with a bulk gold termination. The formation of the large bright islands has also been observed upon removing medium-length alkanethiol SAMs using hyperthermal protons20,21 and in electrochemical STM studies.22-24 The apparent similarity in the STM images suggests that in these experiments, too, gold islands are formed after alkanethiol removal and gold adatom diffusion. The chemical identity of the islands in these other studies, however, remains an open question, as it has not been verified by experimental evidence from XPS, STM, or other techniques. Indeed, several of these experiments propose a model where the islands are composed of cross-linked alkanethiol chains.20-23 We are interested to see if further investigations with proton bombardment and electrochemical etching result in gold adatom coverage measurements similar to those that we report in this manuscript.

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Figure 1. The blue square in part a indicates an area of the surface (1900 Å × 1960 Å, dodecanethiol) that is scanned only twice: once before H-atom exposure (b) and once after complete monolayer removal (c). The area used for monitoring monolayer removal is shown in red in part a and is scanned multiple times (d-g) at increasing H-atom exposure. The use of these two separate areas minimizes any disturbance of the surface by the STM tip of images b and c, which are the only ones used to calculate adatom coverage. Additionally, during hydrogenatom exposure, the tip was moved to the black “X” in part a to avoid tip shadowing of the surface.

Experimental Section The scanning tunneling microscope and hydrogen-atom source have been previously described.18,25 An Oxford Scientific OS-Crack Thermal Gas Cracker replaced the skimmer and molecular beam source used in previous experiments. Hydrogen atoms were created by flowing H2 gas through a tungsten capillary heated to 1800-2000 K. With an H2 back pressure of 1 × 10-6 Torr, we estimate that 6-27% of the hydrogen molecules dissociate into hydrogen atoms.26 Sample exposure to hydrogen atoms was controlled by a shutter incorporated into the hydrogen-atom source. A second shutter was placed downstream between the source and the sample in a differentially pumped chamber (Chamber II of ref 25). This second shutter was used to minimize any chance that hydrogen atoms or other reactive species could reach the sample after one or more gas-phase collisions. All alkanethiol self-assembled monolayers were deposited on a flame-annealed Au(111) surface, rinsed with ethanol, and placed in the UHV STM chamber for imaging. Vapor deposition was used for the formation of ethanethiol and octanethiol monolayers, while the increased alkane chain length of dodecanethiol required solution-phase deposition. Ethanethiol monolayers were prepared at room temperature for 24 h. Octanethiol monolayers were vapor deposited at 70 °C for 24 h. Solution deposition of dodecanethiol was achieved by heating the gold substrate at 70° in a 1 mmol ethanol solution for 2 h. For octanethiol and dodecanethiol, STM images of the monolayers showed large, ordered, close-packed domains separated by domain boundaries, indicating complete coverage of the surface by a high-density alkanethiol monolayer. For ethanethiol, monolayer structure cannot be easily resolved; however, the homogeneous appearance of the surface again indicated saturated coverage.

Figure 2. Series of 1550 Å × 1780 Å images monitoring the erosion of an octanethiol monolayer using hydrogen atoms. Panel a shows the surface prior to any hydrogen-atom exposure. Upon dosing (b-d), the monolayer is removed and additional bright features (resulting from the rearrangement of gold adatoms) are formed.

Tunneling conditions varied slightly for each monolayer. Unless otherwise noted, all monolayers were scanned with an STM bias voltage of 0.5 V. Ethanethiol and octanethiol monolayers were imaged with a 10 pA current. The longer alkane chain of dodecanethiol required scanning at 5 pA to avoid tip interactions with the surface. All images acquired were processed using a masked high-pass filter in the fast-scan direction;27 this ensured that terrace-by-terrace area analysis could be performed accurately. Figures 1, 2, and 6 were additionally filtered in Fourier space to remove high-frequency noise resulting from external vibrations, and Figures 1 and 6 were median filtered to reduce speckle noise. The presence of the STM tip close to the sample creates the possibility of interference with the delivery of hydrogen atoms to the surface or the free motion of gold atoms on the surface. We refer to two potential problems caused by the proximity of the scanning tip as “shadowing” and “sweeping” problems.

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Tip shadowing occurs when the STM tip blocks the line-ofsight from the hydrogen-atom source to the area of the surface being imaged, resulting in decreased hydrogen-atom exposure in the scanning area. We have attempted to minimize the shadowing effect by moving the tip laterally ∼2000-3000 Å away from the scanning area during dosing (shown as the black “X” in Figure 1a), and we verify that we observe an even distribution of newly formed gold islands. Once gold adatoms are freed up by the removal of alkanethiols, they can diffuse readily until they nucleate at step edges or islands. During this time, they can be easily moved out of the imaging area by the tip during scanning. Small gold islands are also prone to disruption by the tip during scanning.28 After repeated imaging under some conditions, gold islands disappear from the scan area, and upon zooming out are observed to pack densely in the immediately surrounding area. Continuously scanning the same area risks “sweeping” the adatoms out of the imaging area, causing the total measured adatom coverage to be systematically lower than the actual value. We minimized the influence of the STM tip on adatom coverage results by the use of a monitoring area and a measurement area for each sample, shown in Figure 1. The measurement area (shown in blue) was a (2000 Å)2 or (3000 Å)2 region of the surface chosen to be typical of its surroundings, and this area was scanned multiple times to verify that there were no changes occurring on the surface prior to hydrogenatom exposure. The tip was then moved laterally by ∼2000 Å to acquire images of the monitoring area (shown in red). The surface was then exposed to the hydrogen-atom source at an ∼45° angle for 2-10 min by opening the mechanical shutters. The monitoring area was immediately imaged with the STM to track changes in the surface structure, and the process repeated until the monolayer was removed and no additional surface changes occurred in the monitoring area. We then moved back to the original measurement area and imaged the surface. Total adatom coverage was only analyzed from the measurement area, which was scanned by the STM at most 1 or 2 times. Results Erosion of an octanethiol monolayer upon exposure to hydrogen atoms is observed in Figure 2. Prior to hydrogenatom exposure in Figure 2a, features typical of octanethiol SAMs are observed, including areas of close-packed octanethiol molecules separated by domain boundaries, and single-atomdeep vacancy islands in the underlying gold terrace. After 2 min of hydrogen-atom exposure, Figure 2b shows dark features have developed along domain boundaries and in close-packed regions where the monolayer has been removed. In these areas, small bright islands have formed. The monolayer has further eroded in Figure 2c, after 4 min of hydrogen-atom exposure. The bright islands have increased in number, as well as grown in size and shape. After dosing the surface with hydrogen atoms for a total of 8 min in Figure 2d, the monolayer has been completely removed and new features have resulted: large bright islands have formed, gold vacancy islands have decreased in size, and the edges of terrace steps have changed in shape. XPS results19 show that sulfur and carbon are removed completely from the surface after extensive hydrogen-atom exposure. We confirm this result in Figure 3. A cross section of a triangularly shaped island formed after monolayer removal has the same apparent height as a neighboring gold terrace present prior to dosing with hydrogen atoms (Figure 3a,b). The apparent height of features in STM images is dependent upon the local density of surface electronic states, and changing the

Kautz and Kandel

Figure 3. The 1340 Å × 1340 Å image (a) was formed after an octanethiol monolayer was exposed to 253 min of hydrogen atoms. A topographic cross section of the island and neighboring gold terrace is shown in part b. Panel c shows cross sections of an island taken at STM bias voltages from -1.0 to 2.0 V. The apparent height of the triangular islands is identical to that of gold terraces, and does not vary with tunneling conditions, proving that these islands are composed of gold.

STM bias voltage changes the range of electronic state energies accessed. If the bright features observed in Figure 3 were due to alkane reaction products (cross-linked alkane chains being one possibility), we would expect to see observable changes in the island height or structure upon varying the tip-sample bias voltage. However, residuals of topographic cross sections taken at STM bias voltages ranging from -1.0 to 2.0 V in Figure 3c show no measurable changes, indicating that these features are indistinguishable from the gold surface underneath. Analysis of sequences of images such as the ones shown in Figures 1 and 2 allows us to determine quantitatively the number of gold adatoms incorporated within an alkanethiol monolayer. While exposing the monolayer to hydrogen atoms is a clean way of removing the alkanethiols and leaving the gold adatoms on the surface, the adatoms, once free of the alkanethiols, can diffuse on the surface. We must measure (1) the total area of new gold islands formed, (2) the changes in shape and location of terrace step edges, and (3) the decrease in size of vacancy islands. All together, these provide an accurate count of the number of gold adatoms. Figure 4 helps demonstrate how we determine the net increase in gold. In Figure 4a, an area of an octanethiol monolayer is shown prior to any hydrogen-atom exposure. The surface contains two gold terraces: the primary terrace, comprising 95.9% of the image, and a surface vacancy defect one atomicgold step below the primary terrace that comprises the remaining 4.1% of the image. After the monolayer has been removed in Figure 4b, the surface area of each terrace has changed. The surface vacancy defect has decreased in size and now accounts for only 2.3% of the total image. The primary terrace comprises 84.4%; while the step edge along the surface vacancy defect has grown, a new terrace (due to the formation of the adatom islands) has formed on top. This new terrace now represents 13.3% of the image. The changes in surface structure caused by hydrogen-atom exposure are shown graphically in Figure 4c; to form this composite image, the two individual images are color masked and added numerically. Blue-tinted features in Figure 4c result from increases in the apparent topographic height as a result of hydrogen exposure. The new terraces formed by nucleation and growth of adatoms can be seen in this fashion, as well as the shrinking of the vacancy island as adatoms deposit at the edge

Alkanethiol Monolayers Contain Gold Adatoms

Figure 4. The same area of an octanethiol monolayer before (a) and after (b) 576 s of exposure to hydrogen atoms. In panel c, terraces existing prior to hydrogen-atom dosing are masked in orange and terraces observed after the monolayer is removed are shown in blue. A simplified version, removing the features but retaining the masks, is shown in part d, where areas shown in blue indicate features derived from gold adatoms and areas in gray or black appear in both images.

of its surrounding step. Mathematical thresholding results in Figure 4d, from which the above numerical area measurements can be extracted. The area of new gold islands observed (13.3%) is added to the decrease in vacancy island size (4.1% - 2.3% ) 1.8%), resulting in a 15.1% increase in the apparent number of gold atoms between the octanethiol monolayer in Figure 4a and the clean gold surface in Figure 4b. Consequently, there must be 0.151 monolayers of gold atoms present as adatoms in the monolayer, beyond those needed to form the clean, unreconstructed gold surface. This measurement was repeated for octanethiol four times. For each measurement, the STM tip was replaced and an entirely new octanethiol sample used. We measure a net increase of 0.143 ( 0.033 monolayers for gold adatoms from octanethiol samples.18 A series of four ethanethiol SAMs exposed to hydrogen atoms produced an adatom coverage of 0.172 ( 0.039 monolayers; three dodecanethiol samples resulted in a net adatom coverage of 0.154 ( 0.024 monolayers. Sample beforeand-after images of ethanethiol monolayers (Figure 5) and dodecanethiol SAMs (Figure 6) are shown. We note that, for all three alkane chain lengths, the adatom coverages are within mutual 1σ error bars and are consistent with an overall chainlength-independent value of 0.16 monolayers. Despite similar adatom coverages for ethanethiol, octanethiol, and dodecanethiol monolayers, the gold adatom islands vary in size and distribution. Removal of ethanethiol SAMs tends to result in smaller-sized islands than does removal of dodecanethiol (though as adatom coverages are approximately the

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Figure 5. 1200 Å × 1000 Å images of ethanethiol SAMs before (a) and after (b) dosing 176 min with hydrogen atoms.

same, there are a greater number of these small islands.) This indicates that the monolayer can greatly influence the formation of these adatom islands, and is consistent with a model of nucleation and growth for these islands. As incident hydrogen atoms remove the alkanethiol molecules, the gold adatom that remains on the surface is free to move around. Gold adatoms can find each other more quickly along domain boundaries and regions where alkanethiol molecules have been removed than in close-packed areas of the surface. The longer the monolayer remains on the surface, the more influence it will have on island size and growth. Ethanethiol SAMs have shorter alkane chains than dodecanethiol monolayers, resulting in faster monolayer erosion, and favoring island nucleation over island growth. The longer alkane chain of the dodecanethiol SAMs results in monolayer erosion over a longer time scale, as the sulfur-gold bond is not as accessible to the incidental hydrogen atom. Free adatoms will be channeled by the remaining monolayer to favor island growth over nucleation. We have observed similar behavior varying H-atom flux, where lower fluxes and longer etching times result in fewer and larger gold islands. Slower monolayer removal also favors addition of adatoms onto preexistent steps. Discussion The traditional (and simplest) model of the alkanethiol-gold interface assumes that alkanethiol molecules are bound to a bulkterminated gold surface. Under this assumption, removal of the alkanethiol SAM would result in a gold surface with features similar to the alkanethiol-terminated surface: either there would be no change in surface morphology at all, or the appearance of new gold features would be balanced exactly by the creation of vacancies. Clean gold surfaces in a vacuum typically reconstruct into a 23 × 3 “herringbone” pattern; however, relaxation of the bulk-terminated surface to the reconstructed one would require creation of even more vacancies, with a net

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Kautz and Kandel TABLE 1: Published Models That Include Gold Adatoms Incorporated into the Alkanethiol-Gold Interface, Listed According to Alkanethiol(S)-to-Adatom Ratio S:Au adatom ratio

techniquea

alkane chain length

ref

1:(-1)b 1:(-0.4)b,c 1:0c 1:0 1:1 1:1 1:1 1:1 1:1 1:1 2:1 2:1 2:1 2:1

DFT GXRD, DFT PED, GXRD, DFT DFT NIXSW CLS LEED, SXRD DFT NIXSW, PhD NEXAFS, NIXSW STM, DFT DFT DFT STM/H-atom

1 6 1 1-8 1, 4, 6, 8 1 4 1, 6 1 1 1, 3, dimethyldisulfide 1 2 2, 8, 12

11, 16 5 4 13 1, 2 6 7 10 8 9 3 12, 14 15 18, this study

a NIXSW (normal-incidence X-ray standing waves), PhD (photoelectron diffraction), DFT (density functional theory), LEED (low-energy electron diffraction), NEXAFS (near-edge X-ray absorption fine structure), STM (scanning tunneling microscopy), SXRD (surface X-ray diffraction), GXRD (grazing-incidence X-ray diffraction), CLS (photoelectron core-level shifts). b Negative numbers of adatoms indicate models with surface vacancies. c Adatoms and vacancies are in dynamic equilibrium, with coverage either balanced (ref 4) or with excess vacancies (ref 5).

Figure 6. 1900 Å × 1960 Å images of a dodecanethiol monolayer before (a) and after (b) 127 min of hydrogen-atom exposure. These images are from the same data set, as shown in Figure 1b,c.

-0.043 (i.e., vacancy) coverage. The observation of a +0.16 increase in adatom-related coverage islands, step growth, and vacancy removal proves that the traditional model is incorrect. Close-packed alkanethiol monolayers have a surface packing density of 0.33 monolayers, that is, one thiol sulfur for every three surface gold atoms. Considering our gold adatom coverage of 0.16 monolayers, this indicates that, within error, there is one gold adatom for every two sulfur atoms that comprise the monolayer. One complication of these experiments is that the reconstructed herringbone pattern is not observed. We suspect that the hydrogen-exposed samples lack the surface cleanliness and time to reach this preferred state. However, we will not discount the possibility that partial or complete reconstruction could be missed in the noise floor of our images. If, despite its lack of appearance in our images, the herringbone reconstruction forms, our values for the adatom coverage would include a systematic error of up to 0.043 monolayers. Adatom coverages would then be closer to 0.215 ( 0.039 monolayers for ethanethiol, 0.186 ( 0.033 monolayers for octanethiol, and 0.197 ( 0.024 monolayers for dodecanethiol SAMs. Despite an increase in the adatom coverage, these results are still best fit to a model where every two sulfur atoms share one additional gold adatom. A recent paper by Li et al. reports a similar adatom coverage for an analogous experiment, in which octanethiol monolayers are removed through STM voltage-induced desorption under ambient conditions, where monolayer removal is assisted by electrolysis within a water droplet at the tip-sample junction.29 Their result for the area of gold islands is 0.15 monolayers, to

which they add filling in of vacancy islands (0.02 monolayers, estimated but not measured) and herringbone reconstruction (0.043 monolayers, assumed but not observed). These experimental results are very close to the observations we have reported previously18 and in this manuscript; we do not believe there is a substantive difference between our results and those obtained in ref 29 using electrochemical desorption. However, we believe Li et al. interpret this result erroneously, as they state that the inferred 0.22 monolayer increase is best explained by a one-to-one sulfur-to-gold-adatom ratio. We do not agree with this conclusion: a two-to-one sulfur-to-gold ratio is within the bounds of error estimates for our measurements, while a one-to-one ratio is well outside. (Error bounds are not reported by ref 29.) This remains true even when including the 0.043 adjustment for herringbone reconstruction (which, as detailed above, we would argue against). Because of the diffusion and rearrangement of gold adatoms during the removal of an alkanethiol monolayer, we can only report the net increase in gold adatoms. It is important to note that this could encompass a number of different surface structures: two alkanethiol molecules bonded to one gold adatom, sitting on a bulk-terminated gold surface, one alkanethiol for every gold adatom, with a surface vacancy for every two adatoms, or any number of more complicated reconstructions that involve both the gold surface and subsurface layers. Many recent experimental and theoretical studies have suggested the incorporation of gold adatoms into alkanethiol SAMs, and an overview is given in Table 1. Each model proposes several different features: the location of alkanethioladatom binding sites, the number of adatoms and single-atom surface vacancies, and how the length of the alkane chain influences the interfacial structure. Table 1 evaluates each of these models with respect to the quantity observed in our experiments: the net coverage of gold adatoms remaining on the surface after the monolayer is removed. Our results are consistent only with models that propose a 2:1 ratio of alkanethiol molecules to gold adatoms.3,12,14,15

Alkanethiol Monolayers Contain Gold Adatoms Conclusion We have shown that removal of alkanethiol SAMs reveals additional gold adatoms that are incorporated into the original monolayer structure. Within experimental error, the same number of gold adatoms is contained in monolayers of alkanethiols with 2, 8, and 12 carbons. As a further extension of the alkyl backbone is unlikely to affect the sulfur-gold interface, we propose that these results are independent of chain length, with the possibility of an exception for the structure of methanethiol monolayers, which were not studied here. Diffusion of gold during monolayer removal means that we do not observe the exact structure of the adatom layer, which will have to be resolved using other experimental or theoretical techniques. The advantage of our approach is in its clarity of interpretation, as our quantitative result depends only on straightforward data analysis, and neither complex deconvolution nor comparison with theory are necessary. We conclude that monolayers of alkanethiols on Au(111), independent of chain length, incorporate one additional gold adatom for every two alkanethiol molecules. Acknowledgment. This work was supported by the National Science Foundation (NSF Grants CHE-034857 and CHE0848415). References and Notes (1) Woodruff, D. P. Appl. Surf. Sci. 2007, 254, 76–81. (2) Yu, M.; Bovet, N.; Satterley, C. J.; Bengio, S.; Lovelock, K. R. J.; Milligan, P. K.; Jones, R. G.; Woodruff, D. P.; Dhanak, V. Phys. ReV. Lett. 2006, 97, 166102. (3) Maksymovych, P.; Sorescu, D. C.; Yates, J. T. Phys. ReV. Lett. 2006, 97, 146103. (4) Mazzarello, R.; Cossaro, A.; Verdini, A.; Rousseau, R.; Casalis, L.; Danisman, M. F.; Floreano, L.; Scandolo, S.; Morgante, A.; Scoles, G. Phys. ReV. Lett. 2007, 98, 016102. (5) Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M. L.; Scoles, G. Science 2008, 321, 943–946.

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19291 (6) Chaudhuri, A.; Lerotholi, T. J.; Jackson, D. C.; Woodruff, D. P.; Dhanak, V. Phys. ReV. Lett. 2009, 102, 126101. (7) Chaudhuri, A.; Lerotholi, T. J.; Jackson, D. C.; Woodruff, D. P.; Jones, R. G. Phys. ReV. B 2009, 79, 195439. (8) Jackson, D. C.; Chaudhuri, A.; Lerotholi, T. J.; Woodruff, D. P.; Jones, R. G.; Dhanak, V. R. Surf. Sci. 2009, 603, 807–813. (9) Chaudhuri, A.; Odelius, M.; Jones, R. G.; Lee, T. L.; Detlefs, B.; Woodruff, D. P. J. Chem. Phys. 2009, 130, 124708. (10) Gronbeck, H.; Hakkinen, H. J. Phys. Chem. B 2007, 111, 3325– 3327. (11) Molina, L. M.; Hammer, B. Chem. Phys. Lett. 2002, 360, 264– 271. (12) Nagoya, A.; Morikawa, Y. J. Phys.: Condens. Matter 2007, 19, 365245. (13) Wang, J. G.; Selloni, A. J. Phys. Chem. C 2007, 111, 12149–12151. (14) Gronbeck, H.; Hakkinen, H.; Whetten, R. L. J. Phys. Chem. C 2008, 112, 15940–15942. (15) Torres, E.; Blumenau, A. T.; Biedermann, P. U. Phys. ReV. B 2009, 79, 075440. (16) Carro, P.; Salvarezza, R.; Torres, D.; Illas, F. J. Phys. Chem. C 2008, 112, 19121–19124. (17) Woodruff, D. P. Phys. Chem. Chem. Phys. 2008, 10, 7211–7221. (18) Kautz, N. A.; Kandel, S. A. J. Am. Chem. Soc. 2008, 130, 6908– 6909. (19) Gorham, J.; Smith, B.; Fairbrother, D. H. J. Phys. Chem. C 2007, 111, 374–382. (20) Xi, L.; Zheng, Z.; Lam, N. S.; Grizzi, O.; Lau, W. M. Appl. Surf. Sci. 2007, 254, 113–115. (21) Xi, L.; Zheng, Z.; Lam, N. S.; Nie, H. Y.; Grizzi, O.; Lau, W. M. J. Phys. Chem. C 2008, 112, 12111–12115. (22) Hobara, D.; Yamamoto, M.; Kakiuchi, T. Chem. Lett. 2001, 374– 375. (23) Wano, H.; Uosaki, K. Langmuir 2005, 21, 4024–4033. (24) Esplandiu, M. J.; Carot, M. L.; Cometto, F. P.; Macagno, V. A.; Patrito, E. M. Surf. Sci. 2006, 600, 155–172. (25) Fogarty, D. P.; Kandel, S. A. ReV. Sci. Instrum. 2005, 76, 083708. (26) Eibl, C.; Lackner, G.; Winkler, A. J. Vac. Sci. Technol., A 1998, 16, 2979–2989. (27) Fogarty, D. P.; Deering, A. L.; Guo, S.; Zhongqing, W.; Kautz, N. A.; Kandel, S. A. ReV. Sci. Instrum. 2006, 77, 126104. (28) Guo, Q. M.; Yin, F.; Palmer, R. E. Small 2005, 1, 76–79. (29) Li, F. S.; Zhou, W. C.; Guo, Q. M. Phys. ReV. B 2009, 79, 113412.

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