Complete Structural Phases for Self-Assembled Methylthiolate

Sep 24, 2013 - The structure of self-assembled methylthiolate monolayers on Au(111) as a function of surface coverage has been investigated using scan...
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Complete Structural Phases for Self-Assembled Methylthiolate Monolayers on Au(111) L. Tang,†,‡ F. S. Li,†,‡ and Q. Guo*,† †

School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, U.K. State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China

‡

ABSTRACT: The structure of self-assembled methylthiolate monolayers on Au(111) as a function of surface coverage has been investigated using scanning tunneling microscopic imaging, and complete structural phases are obtained. A full layer (1/3 ML) of methylthiolate organizes into a long-range ordered (3 × 4) phase with chiral domains. The domain boundary has a local (3 × 4√3)-rect structure. Gradual removal of methylthiolate by partial thermal desorption leads first to the formation of a local (7 × √3)-rect phase with coverage equal to 0.286 ML. This (7 × √3)-rect phase is found to coexist with the (3 × 4) phase and a disordered phase. As the coverage is reduced to below 0.28 ML, stripes parallel to the ⟨112̅⟩ directions appear. All the phases are made from the basic, CH3S−Au− SCH3, structural motif. Cis−trans transformation of the CH3S−Au−SCH3 unit is observed. For an isolated CH3S−Au−SCH3 unit, both methyl groups have equal probability of contributing to the cis−trans transformation at 77 K. Within the stripe phase, only one of the methyl groups actively participates in cis−trans transformation while the other group remains as a spectator.

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INTRODUCTION Self-assembled monolayers (SAMs) of alkanethiols on the Au(111) surface are the most studied chemisorption systems1−6 since they were discovered in 1983.7 Apart from pure scientific interest, alkylthiolate SAMs have found applications in many technologically important fields ranging from the passivation of gold nanoparticles8−10 to prototype molecular electronics.11,12 Despite the extensive amount of effort from the scientific community devoted to the understanding and application of alkylthiolate monolayers,13−43 the detailed atomic structure at the thiolate−Au(111) interface remains a debated issue.15,36,44−48 The first structures found for alkanethiol SAMs adsorbed on Au(111) are (√3 × √3)R30°49 and the associated (3 × 2√3)-rect/c(4 × 2).50,51 These structures, found after the formation of a complete monolayer at room temperature, have a coverage of 1/3 monolayer (ML) when normalized to the surface atomic density of Au. The (√3 × √3)-R30° and (3 × 2√3)-rect phases were initially found with SAMs of nonbranched alkanethiol molecules containing more than four carbon atoms.49−51 Subsequent diffraction studies on SAMs of shorter alkane chains provided some evidence suggesting that these structures are common for SAMs of all nonbranched alkanethiols. However, an increasing number of investigations have began to cast some doubt on the physical existence of the (√3 × √3)-R30° and (3 × 2√3)-rect phases for alkanethiols with short alkane chains. For example, a 3 × 4 phase has been observed using different techniques for ethylthiolate and methylthiolate monolayers.52−60 This 3 × 4 structure, at times, has been assumed to be a metastable structure alongside the more accepted and “stable” (√3 × √3)-R30° phase. A recent investigation using high-resolution © 2013 American Chemical Society

scanning tunneling microscopic imaging performed in our laboratory demonstrates that, contrary to general belief, the familiar (√3 × √3)-R30° phase does not exist for either methylthiolate (C1) or ethylthiolate (C2) monolayers at any temperature.48 The 3 × 4 phase consisting of the basic CH3S− Au−SCH3 units is the only stable structure for both C1 and C2 SAMs with a saturation coverage of 1/3 ML, and it is stable up to the point of partial thermal desorption which occurs at ∼325 K. A complete phase diagram for methylthiolate monolayers on Au(111) is lacking at the present time, despite the groundbreaking contribution from Voznyy et al.15 with their discovery of the 3 × 4, the (3 × 4√3)-rect, and the striped phases for C1 monolayers. Here we conduct a detailed analysis of STM images obtained from a methylthiolate layer. We follow the structural transformation of the C1 SAM as the surface coverage is gradually reduced by thermal desorption. A new (7 × √3)rect phase is found during the transition from the 3 × 4 to the striped phase. By examining the detailed structure around three types of domain boundaries, a connection between various phases is established. For the low coverage striped phase, cis− trans transformation within each CH3S−Au−SCH3 unit has been observed. The cis−trans transformation for CH3S−Au− SCH3 units within a one-dimensional stripe shows an interesting behavior that the two methyl groups in each CH3S−Au−SCH3 unit have rather different activities. Received: April 24, 2013 Revised: August 28, 2013 Published: September 24, 2013 21234

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resolution, and the corresponding structural model for the 3 × 4 phase is shown in the inset near the top left corner. In each inset, the 3 × 4 unit cell is illustrated. The physical dimensions of the 3 × 4 unit cell are 0.86 nm × 1.15 nm with unit vectors along the close-packed directions of the Au atoms in the substrate. There are two types of domain boundaries in this image. Type I boundary, highlighted by dotted blue lines, is frequently found between two adjacent 3 × 4 domains. This type of boundary is straight and parallel to the ×3 direction. The 3 × 4 domains on the opposite sides of a type I boundary look like mirror images of each other. They are not actually mirror images due to a translational shift of one domain relative to the other along the direction of the boundary. Type II boundary shown in Figure 1a is highlighted by the dotted white lines. Both type I and II boundaries are observed also for C2 monolayers as shown in Figure 1b. The image in Figure 1b is acquired from a 3 × 4 ethylthiolate monolayer. Also shown in Figure 1b is another type of boundary, type III, which is found in between a 3 × 4 domain and a 4 × 3 domain. STM images taken from C2 monolayers with the VT-STM suffer from some scanner drift, causing a slight distortion to the 3 × 4 lattice. The images shown here have not been subject to any digital correction unless stated. Before analyzing the detailed structure of these three types of boundaries, we briefly describe the characteristics of the 3 × 4 phase. High-resolution STM images of the 3 × 4 phase of C1 monolayers have been published previously,48 but we include one such image here in Figure 2a for convenience of discussion.

EXPERIMENTAL METHODS We conducted experiments in an ultrahigh vacuum (UHV) chamber with a base pressure of 6 × 10−11 mbar using an Omicron low-temperature STM (LT-STM). The gold sample is a (111)-oriented single crystal which is cleaned using cycles of Ar+ ion sputtering and thermal annealing. The methylthiolate monolayer was prepared by exposing the gold single crystal in a preparation chamber to ∼10−8 mbar of dimethyl disulfide (DMDS), CH3S−SCH3, vapor at room temperature (RT) until saturation coverage is reached within ∼15 min. During deposition, the vapor was introduced to the vacuum chamber using a leak valve, and the whole chamber containing the sample was backfilled with the vapor. The sample was then transferred under UHV conditions to the STM chamber where it was imaged at 77 K. After imaging the full layer, the sample was moved to the preparation chamber where it was heated step-by-step to desorb the thiolate. STM images were acquired after each heating step. For the purpose of clarity, some data from C2 monolayers are also included in the discussion. C2 monolayers were produced by exposing a (111)-oriented Au film to ethanethiol vapor at RT. To make a full C2 layer, the sample needs to be exposed to >10−5 mbar of ethanethiol vapor for more than 2 h. Imaging of the C2 monolayer was performed using an Omicron variable temperature STM (VT-STM) at RT. Electrochemically etched tungsten tips were used in both the LT-STM and the VT-STM. dI/dV curves are obtained from first derivatives of the I−V data. Annealing temperature of the Au sample was measured with a thermocouple attached to the sample holder. The molecular coverage is obtained by finding the density of the thiolate on the surface and then normalize this with the density of surface gold atoms.

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RESULTS AND DISCUSSION Structural Phases at Near Saturation Coverage. Figure 1a shows an STM image of a C1 monolayer in the 3 × 4 phase. The 3 × 4 phase consists of a layer of most densely packed thiolate at a coverage of 1/3 ML.48 Inset in the upper righthand corner shows a small number of thiolate species with high

Figure 1. (a) STM image, 35 nm × 35 nm, obtained using 2.0 V sample bias and 0.3 nA tunneling current from Au(111) with a saturation layer (1/3 ML) of methylthiolate. Dashed blue lines and dashed white lines mark the locations of type I and type II boundaries, respectively. Inset near the upper right-hand corner shows a portion of a high-resolution image where the internal structure of individual Au adatom−dithiolate (AAD) is resolved. Inset near the upper left corner is a structural model of the 3 × 4 phase for direct comparison with the STM image. Red spheres represent the Au adatoms. (b) STM image, 33 nm × 33 nm, from a saturated ethylthiolate layer, obtained using −0.6 V sample bias and 0.05 nA tunneling current. A third type of boundary, found between a 3 × 4 and a 4 × 3 domain, is highlighted with dashed white line.

Figure 2. (a) High-resolution STM image of a 3 × 4 methylthiolate layer, obtained using 0.09 V sample bias and 1.0 nA tunnelling current. The basic constituents of an AAD are illustrated with three circles: purple circle representing the Au adatom; green circles representing the two methyl groups. (b) dI/dV data acquired from an AAD unit. Dashed arrows indicate locations of the filled states, and solid arrows indicate that of the empty states. (c) Height profiles. The topmost two traces are profiles across lines A−B and C−D. The bottom four traces are profiles across C−D under different sample bias. 21235

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The fundamental building block for the 3 × 4 phase is the Au adatom−dithiolate (AAD): CH3S−Au−SCH3.15,48 One such AAD unit in Figure 2a is shown with three overlaid circles: the circle in the middle represents the Au adatom, and the other two circles represent the two methyl groups. The sulfur atoms are invisible in the image. Differential conductance curves, dI/ dV, taken above the AAD unit from −3.0 to +3.0 V show the presence of two empty states at +1 and +1.5 V and two filled states at −1 and −1.8 V (Figure 2b). The dI/dV spectrum in Figure 2b shares some similar features with that obtained from an octanethiol monolayer,61 but with shifted peak positions. We collected dI/dV data at locations directly above the Au adatom as well as above the CH3 groups, with a view to identifying spatial charge distribution variations around the AAD. dI/dV curves from different sites are almost identical. This suggests that within the energy range probed densities of states are rather delocalized over the entire CH3S−Au−SCH3 unit. Figure 2c shows height profiles taken at various bias voltages across the CH3S−Au−SCH3 unit. For each CH3S−Au−SCH3, the two methyl groups appear with a ∼30 pm height difference which is insensitive to the applied bias voltage. Thus, one of the CH3 groups is physically taller than the other.15 The widths of the three peaks in Figure 2c change with bias voltage. This is because of better spatial resolution at lower bias voltages. STM images show that every two AAD units appear to join to form a tetramer.15 Each tetramer shows six bright spots, of which two are from the Au adatoms and the remaining four from the methyl groups, hence the name “tetramer”. Alternatively, we can call them AAD dimers. The tetramer unit is similar to that found by Kondoh et al.52 and by Voznyy et al. for C1 layers at less than saturation coverage.15 The distance between the two Au adatoms, 1 and 2 in Figure 2a, for example, within one tetramer is 0.5 nm (√3a). This is the same distance between the Au adatom in one tetramer, 2 e.g., and its nearest neighbor in the adjacent tetramer, 3. With the fundamental features of the 3 × 4 phase summarized above, we now focus on the structure of the domain boundaries. Based on high-resolution images such as that shown in Figure 3a, a structural model for type I boundary is proposed and shown in Figure 3b. The tetramer has a chiral structure;15 type I boundary is hence formed between two chiral domains. In terms of thiolate coverage, type I boundary can be regarded as a defect-free boundary because the introduction of such boundaries does not change the coverage at all. The distance between methyl groups along the domain boundary becomes more regular than that inside the 3 × 4 domains. The dashed green line in Figure 3b marks the boundary. Methyl groups in the immediate vicinity of the green line are separated with a uniform spacing. Figure 3c shows an STM image of a C2 monolayer where several type I boundaries between chiral domains are observed. For C2 monolayers, the same kind of AAD tetramer arrangement exists, but the Au adatom is not visible in the STM images under normal imaging conditions, probably because the ethyl tails extend much more beyond the Au adatom. The C2 monolayer was imaged using a VT-STM at RT, and thermal drift prevented us from obtaining reproducible dI/ dV data. Inset in Figure 3c shows the positional relationship between the ethyl groups near the boundary. Each AAD unit is represented by two circles linked by a stick. The dark “voids” in the image correspond to locations between two tetramers. Each chiral domain has a rectangular shape which is long in the ×3 direction but narrow in the ×4 direction. The minimum width,

Figure 3. (a) STM image, 4.5 nm × 7 nm, obtained using 0.09 V sample bias and 1.0 nA tunneling current. A type I domain boundary is marked by two white arrows. (b) Structural model of the type I boundary. Dashed green line is used to separate the AAD units from the two neighboring 3 × 4 domains. (c) STM image, 20 nm × 14 nm, of an ethylthiolate layer showing several type I domain boundaries. Ball and stick models are used in the inset to illustrate the positional relationship between AAD at one side of the domain boundary and those at the other side. Circles represent the methyl groups, but Au adatoms are not shown. Dashed black line marks the boundary. A 3 × 4 unit cell is marked in the inset for comparison with the ball model in (b).

as seen in Figure 1b, spans only two tetramer distances. If the width of every chiral domain is reduced to just a single row of tetramers, a (3 × 4√3)-rect phase with 1/3 ML coverage should emerge. Such a (3 × 4√3)-rect phase is shown schematically in Figure 4a and has been observed previously for C1 monolayers, albeit as a rather localized structure.15 For C1 monolayers with 1/3 ML coverage, only the 3 × 4 phase is observed in our experiment, suggesting that the (3 × 4√3)-rect phase is less stable. The abundance of type I domain boundaries, however, suggests that the energy difference between the (3 × 4√3)-rect phase and the (3 × 4) phase is relatively small. DFT calculations show a 0.12 eV per AAD energy difference between these two phases.15 We did, however, observe a localized (3 × 4√3)-rect phase from a C2 monolayer with an overall coverage less than 1/3 ML as shown in Figure 4b. Here the C2 monolayer consists of a mixture of 3 × 4 and (5√3 × √3)-R30° phases. The (5√3 × √3)-R30° phase, with AAD stripes parallel to the [112̅] direction, has a lower coverage of 0.27 ML.48 In Figure 4b, a small area in between the 3 × 4 and (5√3 × √3)-R30° phase, enclosed by a blue colored rectangle, has a local (3 × 4√3)rect structure. A magnified view of this patch of C2 with the (3× 4√3)-rect structure is shown in Figure 4c. As can be seen in Figure 4d,e, both the (3 × 4) and the (3 × 4√3)-rect phases consist of zigzag rows of AAD. In fact, at the single zigzag AAD row level, the two phases are identical. However, adjacent AAD rows are positioned in different ways in the two phases. For the (3 × 4√3)-rect phase, all S atoms are ∼ √3a distance from one another. For the (3 × 4) phase, there is a rather short distance, between S1 and S2 for example in (e), for half of the S atoms. Considering that S is electronegative, a shorter S−S distance suggests a stronger repulsive force between them. Based on this argument, the (3 × 4) phase is expected to be less stable. STM images, however, clearly demonstrate that the saturated methylthiolate layer at 21236

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Figure 4. (a) Ball model showing the possible (3 × 4√3)-rect phase of a C1 layer. Red lines in the figure are used to highlight the orientation of the AAD tetramers. (b) STM image from a C2 layer where the (3 × 4) phase coexists with a lower coverage (5 √3 × √3)-R30° phase. A small patch showing the (3 × 4√3)-rect structure is found inside the blue rectangle. (c) Magnified view of the (3 × 4√3)-rect structure where the unit cell is shown with the white rectangle. (d) The (3 × 4√3)-rect phase represented with a larger (3 × 12) cell. Dotted black lines highlight the relationship between two neighboring zigzag AAD rows. (e) Ball model of the (3 × 4) phase for comparison with (d).

RT is dominated by the (3 × 4) phase. In order to compare the (3 × 4) and the (3 × 4√3)-rect phases in a more direct manner, an alternative surface cell for the (3 × 4√3)-rect phase is illustrated in Figure 4d. This alternative cell is a (3 × 12) cell which is 3 times as large as the (3 × 4√3)-rect unit cell, and it has the same principal axes as that of the (3 × 4) cell. By comparing the (3 × 4) and the (3 × 12) cells, one immediately finds that the distinctive difference between the two phases lies in the different ways the AAD units are organized along the ×4/or ×12 direction. For the (3 × 4) phase, the AAD superlattice has a period of 4a, while this has increased to 12a for the (3 × 4√3)-rect phase. This shows that the (3 × 4) phase is better lattice-matched to the Au(111) substrate, since along the ×3 direction the AAD units are organized in the same way in both phases. The (3 × 4√3)-rect phase with a poorer lattice match would introduce a greater level of strain into the interface. Although strain usually appears as some kind of mechanical phenomenon, the underlying cause of strain is electron charge density mismatch. For the thiolate monolayer, the electron charge density at the interface is controlled by both the intrinsic surface potential of the Au(111) substrate and the extra potential coming from the AAD lattice. The above analysis explains why long-range ordered (3 × 4) phase is preferred over the (3 × 4√3)-rect phase. When the physical dimensions of the phases are reduced to approximately unit cell distances, the effect of lattice mismatch becomes less significant. Under such situations, the (3 × 4√3)-rect phase may become as stable as or even more stable than the (3 × 4) phase because of the more regular S−S spacing. The above analysis ignores the presence of the methyl/ethyl groups. For SAMs of relatively long alkanethiols, the contribution from the alkane chains to the stability of the adsorbate overlayer can no longer be

ignored. The (3 × 2√3)-rect phase, similar to (3 × 4√3)-rect with alkane chains in the cis configuration, observed for butylthiolate monolayer for example is likely due to an increased level of interaction among the alkane chains. We now examine the structure of type II boundary. Type II boundary is found between two (3 × 4) domains with a relative translational shift between them. Figure 5a,b shows the

Figure 5. Ball models of the type II boundary. (a) The boundary is formed by one row of AAD along the [43̅1]̅ direction. Dashed black line illustrates the translational shift in position of the AAD units acrosss such a boundary. (b) A wider type II boundary consisting two rows of AAD units along the [431̅ ̅] direction.

structural models. In Figure 5a, the methyl groups of the AAD units within the boundary are labeled with orange color. All AAD units above the boundary are shifted, relative to those below the boundary, by a distance 1a in the ×4 direction, where a is the nearest-neighbor distance of surface gold atoms. Figure 5b shows a wider type II boundary. In this case, thiolate units above the boundary are shifted by 2a in the ×4 direction relative to those below. A domain can be shifted relative to its 21237

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II boundary can thus be regarded as a precursor of the (7 × √3)-rect phase. Figure 6c shows an image where a type III boundary is found in a C1 layer. In contrast to type I and type II boundaries, type III boundary does not have any particular ordered structure. This type of boundary is formed when an expanding (3 × 4) domain collides with an expanding (4 × 3) domain. Such a boundary contains a small region occupied by randomly oriented AAD units and will not be examined in any further detail. Among the ordered phases, only the 3 × 4 phase has longrange order. Both the (3 × 4√3)-rect and the (7 × √3)-rect phases possess good order within only a few unit cell distances, and they are in competition for existence with a disordered phase. Figure 7a shows an image of the disordered phase at a

neighbor by 1a or 2a. A shift of 3a in one direction is equivalent to a shift of 1a in the opposite direction. For a shift of 1a, there is a single row of AAD units sandwiched between the two neighboring 3 × 4 domains (Figure 5a). For a shift of 2a, there are two rows of AAD units in the boundary (Figure 5b). In general, the difference is between an even number of rows and an odd number of rows. A characteristic of type II boundary is that inside the boundary AAD units do not follow the tetramer arrangement. This is highlighted by the dotted black lines in Figure 5a,b. Type II boundary is parallel to the [43̅1̅] direction, which is the same direction as the short diagonal of the 3 × 4 unit cell. Comparing with type I boundary, the creation of type II boundary requires a reduction in thiolate coverage. Thus, the number of type II boundaries increases as the coverage is reduced. Figure 6a shows an STM image after the sample was

Figure 7. (a) STM image, 30 nm × 30 nm, obtained using −0.6 V sample bias and 0.3 nA tunneling curent. Areas marked with rectangles and squares show local (3 × 4√3)-rect structure. (b, c) STM images showing that structural disorder occurs preferentially on narrow terraces.

Figure 6. (a) STM image, 17 nm × 17 nm, obtained using −0.1 V sample bias and 1.0 nA tuneling current. Areas inside the rectangles have the (7 × √3)-rect structure. Solid ovals and dotted ovals mark the individual AAD tetramers and single AAD units, respectively, within the disordered phase. Three type II boudaries inside the 3 × 4 phase are indicated with letters A−B, A′−B′, and C−D. (b) Ball model for the (7 × √3)-rect phase. (c) STM image showing details around a type III boundary between a (3 × 4) and a (4 × 3) domain.

nominal coverage of ∼0.28 ML. Rectangles and squares drawn into the image show the local (3 × 4√3)-rect domains. Most part of the image shows AAD stripes without long-rang order. 0.28 ML is only a fraction lower than 0.286 ML which is associated with the pure (7 × √3)-rect phase. This suggests that there is a strong tendency for the AAD units to stay away from each other. The long-range ordered (3 × 4) phase is associated with the maximum surface coverage, so the ordering can be understood as a result of “jamming”. STM images in Figure 7b,c show that the best-ordered (3 × 4) phase is found on large terraces. On narrow terraces, the growth of the (3 × 4) phase is confined by the step edges acting as natural boundaries. In rare situations, it is possible for a narrow terrace to have a perfect width which can support an integral number of tetramer units. But in general, the width of a terrace is either too wide or too narrow to accommodate an integral number of AAD tetramers. With the understanding that coverage cannot exceed 1/3 ML, this means that the coverage on a narrow terrace would always be less than 1/3 ML, and thus the structure on such terraces tends to be less ordered. In the extreme case when a terrace is narrower than the (3 × 4) unit cell dimension, the (3 × 4) phase will not form at all. This may

heated to 330 K for 30 min. This has caused a small reduction in surface coverage, resulting in a significant change in structure. The left half of the image shows evidence of complete destruction of the 3 × 4 phase, while the right half shows a reasonably ordered 3 × 4 domain with a number of type II boundaries. Boundaries A−B and A′−B′ have the same structure as that shown in Figure 5a; boundary C−D has the same structure as that illustrated in Figure 5b. If the width of the type II boundary keeps increasing, then the boundary itself would gradually develop into a new phase comprising AAD stripes aligned along the [112̅] direction. In the orthogonal [11̅0] direction, the distance between adjacent stripes according to Figure 5b is 3.5a. Several patches of this striped phase can be seen in the STM image in Figure 6a. The measured distance between adjacent stripes is 1 nm, which can be confidently taken as 3.5a. This new phase can thus be described as a (7 × √3)-rect phase with a coverage equal to 0.286 ML. A structural model of this (7 × √3)-rect phase is shown in Figure 6b. Type 21238

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aligned in the same direction, the distance between the two neighboring stripes is not regular. In Figure 8b, double-headed arrows are used to mark out the stripe spacing. The minimum spacing is 3.5a, which is the same at that in the (7 × √3)-rect phase, and the largest spacing is 9a. In between, all other spacing with increment equal to 0.5a is observed. The surface coverage corresponding the image shown in Figure 8a is 0.23 ML. There are some isolated AAD units, marked with circles. These isolated AAD units are probably trapped as the stripes in their neighborhood are completed, leaving no chance for them to be added to the ends of any stripes. Many isolated AAD units in Figure 8c are seen to consist of five spots instead of the expected three. This is due to the cis−trans transformation under the imaging process. Cis−trans transformation can be caused by thermal effect or by the STM tip. If thermal effect dominates, the methyl group would appear to flip between two stable configurations and producing a kind of fuzziness in the image. Here all the spots in the images appear well-defined, suggesting that the cis−trans transformation is induced by the STM tip. Tip-induced cis−trans transformation has the following feature which is best described using the bistable model. A methyl group can switch between two stable states. If the tip moves above a methyl group which is already in one of the two stable states, it simply appears as a bright spot. As the tip moves on, there is a moment when the methyl group would switch to the other stable configuration under the influence of the tip. This causes the same methyl group imaged twice in a single scan. Cis−trans transformation also takes place within the stripes. However, the two methyl groups at the opposite ends of the AAD behave differently. One of them is active in the cis−trans transformation while the other does not participate in this process. This will be referred to as the hindered cis−trans transformation. In Figure 8d, for those stripes inside the dotted rectangles, the methyl group at the right-hand side of the AAD is seen to undergo cis−trans tranformation. For stripes inside the solid reactangles, it is the methyl group on the left that flips between the cis and trans configurations. To understand why there exists such a difference, we performed triangulation on the STM data. In Figure 9a, there are three stripes: L−M, N−P, and Q−R. AAD units in stripe N−P have the same orientation as those in stripe Q−R. In stripe L−M, AAD units take the alternative orientation. Although the two orientations appear to be identical, there are subtle differences between them as shown schematically in Figure 9b. The Au adatom occupies the

have some implications on the formation of SAMs on highly stepped vicinal surfaces or on the surfaces of Au nanoparticles. The bottom two terraces in Figure 7b are less than 10 nm wide, and they already show a noticeable terrace-width-dependent ordering effect. Lower Coverage Striped Phases. With a further reduction of surface coverage after heating the sample to 351 K, the C1 layer changes to a pure striped phase with no tetramer units left. However, the striped phase is not a twodimensionally ordered phase such as the (7 × √3)-rect. Instead, neighboring stripes tend to have different orientations as shown in Figure 8a. In cases where adjacent stripes are

Figure 8. STM images of the C1 striped phase. (a) Image, 35 nm × 35 nm, obtained using −1.7 V sample bias and 1.0 nA tunneling current. (b) 20 nm × 20 nm, −0.6 V; 1.0 nA. Double-headed arrows are used to indicate the spacing between neighboring parallel stripes. (c) 20 nm × 20 nm, −0.05 V; 1.0 nA. Circles mark the individual AAD units in both the cis and trans configurations. (d) STM image, 10 nm × 10 nm, −0.01 V; 1.0 nA, showing evidence of surface-controlled cis−trans isomerization involving the flippping of methyl groups at just one end of the AAD unit.

Figure 9. (a) Triangulation used to identify the relative orientation of AAD units in the stripes. In the image, AAD inside stripes NP and QR take the same orientation. AAD inside stripe LM takes the alternative orientation. (b) Ball model illustrating the local structure around isolated AAD and around AAD stripes. 21239

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Figure 10. (a) STM image, 22 nm × 22 nm, obtained using −0.01 V sample bias and 1.0 nA tunneling curent at 77 K, following a thermal annealing of the sample to 371 K. (b) STM image, 33 nm × 33 nm, obtained using −0.6 V sample bias and 1.0 nA tuneling current, after the desorption of the methylthiolate.

bridge site regardless the orientation of the AAD. S atoms sit above surface Au atoms, but not directly above. The S−Au adatom bond distance is 0.23 nm,62 which is shorter than 0.25 nm, which is the distance between an atop site and its nearest bridge site. The hollow site between the S atom and the Au adatom is a fcc hollow on one side of the Au adatom, and it is a hcp hollow on the other side. There is consequently an asymmetry for the two methyl groups: one of the methyl groups leans toward an hcp hollow and the other leans toward an fcc hollow. So the energy barrier for cis−trans transformation is not the same for the two methyl groups. This energy difference could be small because for an isolated AAD, both methyl groups participate in cis−trans transformation. For AAD in a stripe, methyl groups at one side of the stripe are spectators as illustrated in Figure 9b. Cis−trans transformation takes place on one side only. It is not known if the methyl group next to an hcp hollow is the spectator or the other way around. It is likely that cis−trans transformation takes place collectively within one stripe. If this is the case, then during STM scanning this kind of collective switching process occurs many times. The coverage associated with the striped phase shown in Figure 8a is 0.23 ML. As the coverage is reduced from 0.23 ML, this striped phase is retained, but the average distance between neighboring stripes become longer. Figure 10a shows an STM image after the coverage is reduced to 0.11 ML by heating the sample to 371 K. If the sample is heated to high enough temperature, complete desorption of methylthiolate occurs accompanied by the return of the herringbone reconstruction of the Au(111) surface (Figure 10b). What is surprising is that after desorption the elbow sites are observed to have bright spots. Some of the bright spots appear in pairs, and their height measured from the Au(111) substrate is about 0.2 nm. The paired bright spots are hence likely to be residual AAD units. The individual bright spots are possibly monothiolate species. There are also ∼10−4 ML of isolated, less bright, spots in Figure 10b. They could be due to S atoms as a result of thermal decomposition of thiolate although we do not have spectroscopic data proving this or otherwise. We next turn our attention to the structure of atomic steps of Au(111) in the presence of the AAD. Figure 11a shows an STM image from an area containing four atomic steps, and a filtered version of this image is shown in Figure 11b. The steps are roughly along the [11̅0] direction, but they are not straight at the atomic scale. The terraces between the steps are about 2

Figure 11. Structures around steps. (a) STM image showing four atomic steps. These steps are approximately parallel to the [11̅0] direction. (b) Filtered image of (a) with enhanced contrast around the steps. (c) STM image showing the structure of [112̅] oriented steps. Here the steps are straight and aligned exactly with the [112]̅ direction.

nm wide. On the wider terraces, there are AAD stripes of various length. Along the step edges, we observe individual spots separated by 0.56 nm, which is twice of the nearestneighbor distance of Au atoms. It appears that the spots are not in pairs as that in AAD. It is possible that what we see here are individual monothiolate species attached to every other Au atom along the step. Figure 11c shows an STM image from a region containing [112̅] oriented steps. These steps are straight at the atomic scale. Moreover, it seems that the steps are covered by rows of AAD units. The distance between adjacent spots along the steps here is 0.5 nm, which is the √3a distance. Therefore, AAD stripes are formed along the [112̅] oriented steps in the same way as they do on the flat terraces. Without more detailed information, we cannot form a conclusive view on the nature of thiolate attachment to steps. However, Figure 11 demonstrates that the interaction between thiolate and steps on Au(111) has some degree of step-specificity. The possible 21240

dx.doi.org/10.1021/jp4058127 | J. Phys. Chem. C 2013, 117, 21234−21244

The Journal of Physical Chemistry C

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Table 1. List of Structural Phases for C1 Monolayer on Au(111)