Nanostructuring of Au(111) during the Adsorption of an Aromatic

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Nanostructuring of Au(111) during Adsorption of an Aromatic Isocyanide from Solution Ahmed Ghalgaoui, Nassar Doudin, and Martin Sterrer Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03759 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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Nanostructuring of Au(111) during Adsorption of an Aromatic Isocyanide from Solution Ahmed Ghalgaoui,1,2 Nassar Doudin,1 Martin Sterrer1,2,* 1

Institute of Physics, University of Graz, Universitätsplatz 5, A-8010 Graz, Austria.

2

Department of Chemical Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft,

Faradayweg 4-6, D-14195 Berlin, Germany.

Abstract

We present a combined vibrational and morphological characterization of the self-assembly of 1,4-phenylene-diisocyanide (PDI) on Au(111) from methanol solution. Vibrational sum frequency generation (vSFG) and scanning tunneling microscopy (STM) have been applied to determine the adsorption geometry of the PDI-Au adatom complexes as well as the morphological transformations of the Au(111) substrate upon SAM formation from solutions with PDI concentrations in the µM to mM range. At low concentration/coverage, PDI adsorbs in a flat adsorption geometry, with both isocyanide groups attached to Au adatoms on the Au(111) surface.

Transformation

to

a

standing-up

phase

is

observed

with

increasing

concentration/coverage. In contrast to findings for PDI adsorbed in ultrahigh vacuum, PDI does not form a long-range ordered monolayer phase when adsorbed from solution. In addition, the

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Au(111) surface is subjected to structural modifications. Au vacancy islands and ad-islands, which are typical substrate defects formed during self-assembly of aromatic thiols on Au(111), are also created during PDI adsorption from solution. At low PDI concentration, the Au vacancy islands and ad-islands are found at specific sites mediated by the herringbone reconstruction of the Au(111) surface, giving rise to long-range ordered structures. These structures do not form during UHV adsorption of PDI on Au(111), nor has a similar ordering effect been observed for any related thiol-SAM system investigated so far.


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Introduction Self-assembled monolayers on metal surfaces have received great attention in the past decades due to their potential in various applications ranging from electronic devices, corrosion protection to catalysis.1-3 The usefulness of a particular SAM is strongly dependent on the order as well the stability in various environments, which were shown to depend, among others, on the type of the headgroup, the nature of the organic backbone, and the applied preparation conditions (solvent, temperature, time). Among possible candidates for functionalized organic molecules for SAMs, those with thiol end groups have received most attention because of their relatively straightforward preparation, well-defined surface structures, and reasonable stability. A large body of work has gone into studying the formation of ordered phases as a function of SAM coverage, both in ultrahigh vacuum (UHV) environment during adsorption of thiols from the gas-phase, and during deposition from solutions.4-6 Those studies revealed that in the lowcoverage structures the molecules lie flat on the surface (either physisorbed or chemisorbed), which are transformed into a standing-up phase at increasing coverage. But also the substrate structural modifications during SAM formation, in particular of the herringbone reconstructed Au(111) surface as the most-widely studied substrate for thiol-based SAMs, have been in the focus.7-10 It is generally accepted that chemisorption of thiols on gold involves the dissociation of the thiol bond and formation of a thiolate-Au adatom adsorption complex. As a consequence of Au atom extraction from the Au terraces or from step edges, the Au(111) substrate undergoes structural modifications in the form of lifting of the herringbone reconstruction, erosion of step edges, and formation of Au vacancy-islands and/or Au ad-islands. The latter in particular has been shown to depend on the nature of the organic backbone, with purely aromatic systems


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leading to Au ad-island formation sometimes in combination with Au vacancy islands,9,10 whereas with aliphatic thiols the main structural modification is Au vacancy island formation.7,8 While lifting of the herringbone reconstruction during SAM formation can be rationalized on the grounds of surface stress,6 it is less clear how the differences in Au vacancy and Au ad-island formation observed for aliphatic and aromatic thiols come about. It has been proposed that the diffusion of Au adatoms/vacancies is limited on Au(111) covered with aromatic thiols, which hinders them to merge with step edges (adatoms), or to form larger vacancy islands by Ostwald ripening (vacancies).10 In most systems investigated so far, the Au vacancy islands and Au adislands created during SAM formation were found to be randomly distributed on the Au(111) terraces. Only in few cases experimental evidence was provided for initial vacancy formation at specific surface sites (the elbows of the herringbone reconstructed Au(111) surface),8 or partial alignment of Au ad-islands along particular directions mediated by the herringbone reconstruction of the Au(111) surface.11,12 In the present work, we show that an ordered Au vacancy island / ad-island structure, which replicates the herringbone pattern of the reconstructed Au(111) surface, is created during adsorption of 1,4-phenylene-diisocyanide (PDI, CN-C6H4NC), a molecule that forms molecule-Au adatom complexes on Au(111) similar as thiols do, from solution. PDI is considered to self-assemble from solution on gold surfaces in upright geometry similar as the analogous thiol-compound, 1,4-benzene-dithiol, with one functional group coordinated to the metal surface via an Au ad-atom (Figure 1d).13,14 Evidence for this binding mode has been obtained by vibrational spectroscopy on the basis of the differences in stretching frequencies of metal-coordinated and free isocyanide groups.13,14 By contrast, PDI self-assembles in UHV on Au(111) into 1-dimensional (PDI-Au)n oligomer chains with both isocyanide groups coordinated


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to Au adatoms (Figure 1d), as proven by scanning tunneling microscopy (STM) and vibrational spectroscoy.15 Similar flat adsorption complexes have recently been found for BDT adsorption on Au(111) in UHV.16 There is substantial charge transfer from the PDI molecule into Au,17 which opens the possibility to use this substrate for CO2 reduction.18 Interestingly, the flat PDI adsorption geometry is maintained up to saturation coverage in UHV,19 but co-adsorption of CO does lead to de-coordination of one isocyanide end-group, resulting in a phase with upright PDIAu adatom complexes.20 The assembly process of PDI-Au adsorption complexes to linear chains has recently been modeled by DFT calculations and is suggested to be initiated by the bonding of a vertical PDI-Au adatom complex, which experiences small diffusion barriers on Au(111), to a PDI molecule adsorbed on a step edge. Further chain growth may then proceed by attachment of another mobile PDI-Au adatom complex to the terminus of a growing chain.21 The erosion of step edges observed in UHV suggests that the Au adatoms are taken from the low-coordinated step sites rather than from terraces. This is further supported by the observation that the herringbone reconstruction gets only weakly distorted and no Au vacancy islands are formed upon assembly of PDI-Au adatom chains in UHV.19 The apparent discrepancy between the adsorption structures found with UHV and solution deposition protocols has partly been resolved in a very recent study and ascribed to a concentration effect. By decreasing the PDI concentration in the deposition solution to a sufficiently low level, Tysoe and coworkers were able to isolate the flat adsorption state, as suggested by the analysis of the corresponding vibrational spectrum, which revealed the presence of metal-coordinated –NC vibrations without accompanying free -NC.22 While the vibrational data support the interpretation of a flat adsorption configuration, the structural properties of the SAM formed from solution deposition remain unclear. In particular, from comparison of the


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vibrational data it cannot straightforwardly be concluded that similar long-range ordered PDI oligomer chains are formed, or that the Au substrate remains undistorted as found in UHV studies. Herein, we present a combined vibrational spectroscopic and morphological characterization of the adsorption structures formed by PDI on Au(111) during adsorption from methanol solution. Vibrational sum frequency generation (vSFG) and scanning tunneling microscopy (STM) operated both, in ultrahigh vacuum and in air have been used (i) to determine the adsorption geometry of the PDI-Au adatom complexes, and (ii) to study the structural transformations of the Au(111) substrate upon SAM formation from PDI/methanol solutions. Our study reveals differences in PDI adsorption structure following UHV and solution deposition and furthermore provides indication for structural modifications of the Au(111) surface during exposure to PDI/methanol solutions. In particular at low PDI concentration the structural modifications result in long-range ordered nanostructures (Au vacancies and Au ad-islands). The results will be discussed in comparison with previous findings for PDI adsorption in UHV and Au(111) surface modification during formation of thiol-based SAMs.

Experimental Methods The experiments were carried out in two ultrahigh-vacuum (UHV) chambers in combination with a UHV-STM, a STM operated in air, and a set-up for broadband vibrational SFG. The UHV chambers are equipped with standard equipment for single-crystal sample cleaning and preparation (sample heating, ion bombardment, low energy electron diffraction, LEED) and a load-lock for fast sample transfer between UHV and ambient environment. One of the chambers was primarily used for the preparation of Au(111) samples for subsequent air-STM and SFG


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studies of PDI-Au(111), whereas the other one was used for morphological characterization of PDI-Au(111) with STM in UHV. The Au(111) single-crystal was mounted on a molybdenum sample plate (Omicron). Sample heating was performed by electron bombardment heating from the backside using a tungsten filament. The sample temperature was monitored using a type-K thermocouple spot-welded directly onto the single-crystal. The Au(111) surface was cleaned within UHV environment by cycles of ion bombardment (Ar+, 1 keV, 30 min.) and annealing (900 K, 5 min., and 600 K, 30 min.) until a well-defined herringbone reconstruction pattern was observed with LEED or STM. The clean Au(111) samples were transferred out of the UHV chamber into air and PDI (SigmaAldrich, 99%) was adsorbed by soaking the Au(111) surface in PDI/methanol solution (PDI concentration in the range of 5 µM to 10 mM) for 60 minutes. After thorough rinsing with methanol and drying under a stream of He gas, the PDI-Au(111) samples were investigated with UHV-STM, air-STM and vSFG. Sum Frequency Generation Spectroscopy (SFG)23 was used to characterize PDI adsorption on the Au(111) surface as a function of PDI concentration by probing the –NC stretching mode in the 2100-2200 cm−1 spectral region. vSFG spectra were collected from the PDI-Au(111) samples in air with a home-built SFG set-up described elsewhere.19 In brief, our laser system (Coherent Vitesse, Legend Elite Duo and OPerA Solo) provides two p-polarized laser beams (near-infrared pulse with ωVIS= 12500 cm−1, 5 cm−1 FWHM, 4 ps; infrared pulse with ωIR=2160 cm−1, 150 cm−1 FWHM, 120 fs) that are spatially and temporally overlapped on the sample surface, where a signal is generated at the sum frequency of the two incoming beams (ωSF =ωIR + ωVIS). The SF signal is detected with a CCD camera after spatial separation and spectral filtering. The total


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SFG signal is the sum of the resonant vibrational contribution and a non-resonant substrate response and is modeled with equation 1:

∝ + = ∑ +

(1)

where and are the resonant and non-resonant second-order non-linear susceptibility, and

AR, ωR and ΓR refer to the amplitude, resonance frequency and damping constant (homogeneous linewidth). ANR is the amplitude of the non-resonant contribution and φ is its phase relative to the resonant term. Morphological characterization of the PDI-Au(111) samples was performed in air using a Wandelt-type STM,24 and in UHV with a Omicron variable-temperature STM. The tips used in the STM studies were obtained from 0.25 mm W wire (for UHV-STM) and 0.25 mm Pt/Ir wire (for air-STM) by electrochemical etching. In both STMs, the bias voltage is applied to the tip and the sample is grounded. Image plane correction, flattening, and noise filtering (necessary for better visualization of the STM images presented in Figures 1a, 1c, 3b, 4a,b, 6b, 7a,b) has been performed using the WSXM6.2 software.25

Results Herringbone reconstruction of Au(111) and PDI adsorption on Au(111) in UHV. For the sake of comparison of PDI-Au(111) samples obtained by evaporation of PDI in UHV and by deposition from a PDI/methanol solution, and to understand the origin of the nanostructure formation on the Au(111) surface during PDI adsorption from solution, we begin by a brief description of the Au(111) surface reconstruction and PDI adsorption structures on Au(111) obtained in UHV.


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In Figure 1a an STM image of the clean Au(111) surface is shown. The observed features are due to the well-known (22×√3) reconstruction of the Au(111) surface, which forms as a result of the uniaxial compression of the surface Au atoms along one of the close-packed 〈110〉 directions.26,27 The compression results in the formation of regions where the surface atoms occupy fcc and hcp sites with respect to the bulk atomic arrangement, which are separated by discommensuration lines (DL), where the surface atoms are in bridge position. To relief the stress more isotropically, the reconstruction forms along all three close-packed directions, leading to a characteristic zig-zag pattern of the discommensuration lines with 120° bends (elbows) between alternating domains. A pair of discommensuration lines consists of an unfaulted and a faulted discommensuration line, where the latter contains a point dislocation at every elbow site, giving rise to two alternating elbow configurations (“pinched” and “bulged”, Figure 1a).

Figure 1. a) UHV-STM image of the Au(111) surface showing the typical herringbone reconstruction (50 nm × 33 nm, Vtip = −0.75 V, it = 5 pA). Indicated are the principal crystal directions, regions of hcp and fcc stacking, discommensuration lines (DL) and elbow sites (EB). b,c) STM images (23 nm × 23 nm, Vtip = −2.0 V, it = 5 pA) of the 1-dimensional (PDI-Au)n chain structures formed in UHV. The white arrow indicates the chain direction. The image in b)


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was obtained for low PDI coverage (0.37 ML PDI; Au adatom coverage: 0.05 ML), where the interchain distance is 1.3 nm, whereas the image in c) is from a medium coverage PDI-Au(111) sample (0.75 ML PDI; Au adatom coverage: 0.10 ML), which exhibits regions of ordered chains with an interchain separation of 0.6 nm, and regions with disordered (PDI-Au) adsorption complexes formed in the hcp regions of the herringbone reconstruction19; the insert in c) shows a zoom of the ordered chain structure formed at 0.75 ML PDI. d) Model of upright (left) and flat (right) PDI-Au adatom complexes on Au(111). Note that in UHV, PDI adsorbs predominantly in the flat adsorption state.

Upon deposition of PDI on Au(111) under UHV conditions, 1-dimensional (PDI-Au)n oligomer chains spontaneously form at room temperature (Figure 1b,c). According to the accepted adsorption model, the chains are aligned along a close-packed substrate direction with PDI molecules bridging two Au adatoms (Auad-PDI-Auad), which are separated by four latticetranslation distances (Figure 1d).28 Inspection of previously published STM images of (PDI-Au)n oligomer chains shows a clear preference for one particular chain direction within each of the three rotational domains of the herringbone reconstruction. The preferred direction is identified as the close-packed direction with the smoothest variation of Au-Au interatomic distances on the reconstructed Au(111) surface. For an orientation of the herringbone reconstruction as shown in Figure 1a, this means that the chains grow along the 011! direction, as observed experimentally (Figure 1b and 1c), and not along the other close-packed 110! and 101! directions. Note that the periodicity of the herringbone reconstruction is hardly affected by adsorption of PDI. Because of the repulsive interaction between individual chains, the interchain separation varies continuously with PDI coverage down to a distance of around 1 nm (Figure 1b). With increasing coverage, the repulsive interaction breaks down and the chain separation reduces to 0.5 – 0.6 nm


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(Figure 1c).19,28 With STM both, the Au adatoms and the phenyl ring of the PDI molecules can simultaneously be resolved (Figure 1c),18,19,28 but more frequently only one of them is imaged, leading to a periodicity of 1.14 nm as observed in Figure 1b.15,19 Adsorption States of Liquid Deposited PDI. To reproduce the results of previous work regarding the adsorption structure of PDI deposited from solution on Au,22 we have studied the PDI concentration-dependent evolution of metal-coordinated and non-coordinated (free) –NC groups by means of vibrational spectroscopic detection using vibrational sum frequency generation (vSFG) spectroscopy. Figure 2a shows vSFG spectra (black traces) recorded in the – NC stretching region as a function of PDI concentration. Assuming a Lorentzian line shape, all spectra can be fitted by using equation (1) (Figure 2a, red traces). Au(111) gives a significant non-resonant response, which can be fitted by a Gaussian line shape (Figure 2a, gray traces). The contribution of the –NC resonant vibrational signals to the spectra is separately presented in Figure 2b.


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Figure 2. a) SFG spectra recorded in the –NC stretching region and corresponding fits of Au(111) exposed for 60 min. to PDI/methanol solution with PDI concentration ranging from 0.005 mM (bottom) to 10 mM (top). Black: raw SFG data; grey: non-resonant background; red: fit result using equation 1. b) Corresponding resonant vibrational contributions to the SFG spectra shown in a). The peaks at higher and lower frequency, respectively, are attributed to the stretch vibration of metal-coordinated (green) and free (blue) –NC groups. Note that the resonant susceptibilities have opposite sign.

The spectrum obtained from a PDI-Au(111) sample exposed to the lowest PDI concentration (0.005 mM, lowest trace) can be fitted with a single resonant component at 2192 cm−1, which is attributed to metal-coordinated –NC groups. The frequency of this vibration is consistent with previous data for PDI adsorbed from liquid,29 but is higher than that obtained under UHV


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condition.30 Upon increasing the concentration the signal of metal-coordinated –NC shifts to lower frequency and reaches 2175 cm−1 at 10 mM PDI, while a second signal at around 2130 cm−1, which is attributed to uncoordinated –NC,13,14 emerges and becomes pronounced at the highest concentration. Note that our vSFG spectrum for 10 mM PDI from methanol solution is very similar to that obtained by Uosaki et al. for 10 mM PDI adsorbed on an Au film from THF solution.29 In addition, the concentration-dependent trend seen here nicely resembles that of PDI adsorption from benzene solution on Au films recently reported by Tysoe et al., which is interpreted in terms of a transition from an adsorption state mainly composed of flat (PDI-Au)n oligomer chains at low concentration, where both –NC groups of the PDI molecule are coordinated to Au ad-atoms on the Au surface, to mainly vertical PDI-Au ad-atom adsorption complexes at high concentration, where each PDI molecule has one metal-coordinated and one pendant –NC group.22 This result suggests, because of the similarity of the vibrational data, that PDI-Au adsorption states similar to those observed in UHV may form upon adsorption of PDI on Au(111) from solution.22 To test this hypothesis, the PDI-Au(111) samples were transferred into a UHV chamber and analyzed with UHV-STM.

Figure 3. a) UHV-STM image (22 nm × 22 nm, Vtip = −1.3 V, it = 5 pA) of Au(111) exposed for 60 min. to 0.005 mM PDI/methanol solution. The alignment of small PDI-Au chain segments


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along the close-packed substrate directions is indicated by dashed lines. The depressions correspond to regions of missing Au atoms (Au vacancy islands). b,c) Higher resolution STM image (5.3 nm × 2.6 nm) of a chain segment and corresponding line profile.

When exposed to 0.005 mM PDI solution, the surface appears to be homogeneously decorated with adsorbates (Figure 3a); however, no long-range adsorbate order, as found for UHVdeposited PDI (Figure 1), is directly evident. The herringbone reconstruction is not visible any more after exposure to PDI from solution, in contrast to results of UHV-deposited PDI at similar coverage.19 Moreover, regularly distributed one atomic layer deep vacancy islands are visible on the surface (large depressions in Figure 3a). Bright protrusions, which are attributed to Au adislands, often appear in close neighborhood of the vacancy islands. Before commenting in more detail on the vacancy islands, we take a closer look at the structure of the adsorbed PDI. The small protrusions visible on the terraces in Figure 3a are attributed to adsorbed PDI. Close inspection of this image reveals that the protrusions appear in pairs or as short chain segments aligned along specific substrate directions and with a certain separation between the protrusions. Several of the pairs and short chain segments seen in Figure 3a have a separation, which is in the order of 1.1-1.2 nm. Occasionally, chain segments with a separation of 0.55-0.6 nm between the protrusions were resolved (Figure 3b). These distances compare well with the separation of individual protrusions observed for (PDI-Au)n oligomer chains formed in UHV (Figure 1). With the additional information gained from vibrational spectroscopy demonstrating that the majority of –NC groups is metal-coordinated under these deposition conditions (0.005 mM PDI, Figure 2), this suggests that the observed protrusions can be attributed to flat (Au-PDI-Au) adsorption complexes. However, it is clear that deposition from methanol solution leads to a significantly more disordered arrangement of (Au-PDI-Au) adsorption complexes and much shorter chain


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segments compared to PDI adsorption in UHV, where on extended Au(111) terraces the (PDIAu)n oligomer chains are strictly 1-dimensional and extend over tens of nanometers (Figure 1). Moreover, we find chain segments aligned along all three close-packed Au atomic rows (dashed lines in Figure 3a), albeit some preference for the 011! direction still exists. This result implies that the unidirectional character of PDI chains, which is a direct consequence of the atomic arrangement of the reconstructed Au(111) surface in UHV, is lost after deposition from solution. This would be consistent with the assumption that the herringbone reconstruction partially relaxes during PDI adsorption from solution. The observation of vacancy island formation would be an additional support for this assumption. In summary, our SFG measurements confirm the results of previous vibrational spectroscopic investigations, suggesting the formation of flat (Au-PDI-Au) oligomer chains during adsorption of PDI on Au(111) from solutions with low PDI concentration. Support comes from UHV-STM images taken from the same PDI-Au(111) samples, where short chain segments could be identified. Long-range ordered molecular structures as seen for UHV-deposited PDI-Au(111) are, however, completely absent. Moreover, even at the lowest PDI concentration used in the present study (0.005 mM, 60 min.), PDI adsorption induces structural modification of the Au(111) substrate in the form of Au vacancy / ad-island formation. Below, we show that the herringbone reconstruction of Au(111) strongly influences the distribution of Au vacancy islands and ad-islands. In particular at low PDI concentration, this influence results in the formation of long-range ordered Au vacancy island and ad-island structures. Ordered Au vacancy island and Au ad-island structures. Figure 4a shows again an UHVSTM image of Au(111) exposed to 0.005 mM PDI/methanol solution. The surface is homogeneously covered with adsorbates, and contains Au vacancy and ad-islands. Most notable


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is the regular arrangement of Au vacancy islands, which are aligned along the 211! substrate direction with a separation of about 15 nm between rows. Within the rows, the vacancy islands have a periodicity of 10 nm. Comparison with the STM image of clean Au(111) (Figure 1a) suggests that the vacancy islands are formed at the elbow sites of the herringbone reconstruction. To better illustrate this, we have overlaid onto the STM image in Figure 4a a schematic of a pair of discommensuration lines with alternating 120° turns as found on clean Au(111) (Figure 1a) and fixed the position of one of the elbows at a vacancy island. The agreement between the lateral periodicities of the created vacancy islands and the positions of the elbow sites is obvious. Note that the vacancy islands formed at bulged elbows are morphologically better defined than those at pinched elbows, which sometimes appear elongated (top left) or are missing (bottom right). When viewed with enhanced contrast, additional dark features can be found in the areas between the vacancy islands (Figure 4b), which we also assign to Au vacancies. The larger protrusions, which are frequently located in close vicinity of the larger vacancy islands, are attributed to Au ad-islands formed by the extracted Au atoms.

Figure 4. a) UHV-STM image (50 nm × 50 nm, Vtip = −1.3 V, it = 5 pA) of the Au(111) surface exposed for 60 min. to 0.005 mM PDI/methanol solution. Au vacancy islands and Au ad-islands appear as depressions and protrusions, respectively. The Au ad-islands are found in close neighborhood of the vacancy islands and the vacancy islands appear ordered. Comparison with


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the structure of the clean, herringbone reconstructed Au(111) surface indicates that the vacancy islands are primarily formed at the elbow sites. (For better visualization, a schematic of a pair of discommensuration lines is overlaid on the STM image). b) UHV-STM image (22 nm × 22 nm, Vtip = −1.3 V, it = 5 pA) of the same sample preparation presented with strongly enhanced contrast. In addition to the vacancy island small depressions (encircled areas) are found at the Au(111) terrace, which are tentatively assigned to single Au vacancies.

Because of the increasing difficulty to obtain stable UHV-STM images for samples prepared with higher PDI concentration, all subsequent STM investigations were performed in air. We found similar surface morphologies for samples prepared with PDI concentrations varying between 0.005 mM and 0.1 mM and present representative STM images of those samples in Figure 5. The disordered structure of adsorbed PDI molecules is not resolved in air-STM. Therefore, we focus on the vacancy and ad-island structures.


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Figure 5. Air-STM image (450 nm × 300 nm, Vtip = −0.3 V, it = 0.1 nA) of Au(111) exposed to 0.005 mM PDI/methanol solution. b) Enlarged area (100 nm × 100 nm) of the same preparation. A schematic of the herringbone reconstruction is overlaid to highlight the ordering of the vacancy islands (depressions) and ad-islands (protrusions). The Fast Fourier Transform (FFT) of the STM image b) shown in c), and the corresponding FFT-filtered image shown in d) provide additional support for the ordering of vacancy islands and ad-islands.

The morphological modifications induced by PDI adsorption result in a long-range ordered vacancy/ad-island structure as shown by the STM image in Figure 5a, which presents a large terrace (450 nm x 300 nm) of the PDI-exposed Au(111) surface. In this large-scale image we identify two kinds of ordering: The first is generated by the trenches running along the 211!


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direction, which have a periodic separation of about 28 nm in 011! (indicated by white arrows at the top of the image), and the second is due to short rows of Au ad-islands nucleated in the areas between the trenches (see white arrows in the encircled area in Figure 5a), which are separated by 8 – 9 nm in 211! direction. By zooming in (Figure 5b), we can also identify the vacancy islands, which appear as dark depressions, albeit not as well resolved as in UHV-STM. As observed in UHV, the vacancy islands are aligned in rows running parallel to the 211! direction. The separation of the rows is 14 nm in 011! direction, which is half that of the trenches seen in the large scale image. The mono-disperse Au ad-islands formed on this surface have a diameter of ~ 2 nm and are single-layer high. They are not randomly distributed over the terrace, but show preferred nucleation in specific regions. From Figure 5b it is apparent that the 28 nm periodicity of the trenches is a consequence of the lower density of Au ad-islands around every other row of Au vacancy islands. In addition, the periodic 9 nm separation between Au ad-island rows (see Figure 5a) points to some influence of the herringbone reconstruction on Au ad-island nucleation. To confirm this, we present in Figure 5c the Fast-Fourier-Transform (FFT) of the STM image in Figure 5b, and in Figure 5d the corresponding FFT-filtered image, which reveals the presence of zig-zag features similar to the characteristic herringbone pattern of the reconstructed Au(111) surface. These features arise from Au ad-islands nucleated either in the fcc or the hcp regions of the herringbone reconstruction. Overlaying a herringbone pattern onto the STM image in Figure 5b and aligning the elbows at the depressions attributed to the vacancy islands suggests that the ad-islands nucleate preferentially in the hcp regions. To further confirm the assumption of area-selective island nucleation, we present in Figure 6a an STM image of a surface region, which exhibits large single-domain (22×√3) reconstructed


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areas (i.e. no zig-zag herringbone pattern). In this case, the Au ad-islands are aligned in long rows, which are separated by multiples of 8 nm, i.e, in the range of the separation of individual hcp (or fcc) regions of the reconstructed surface. An enlarged detail of a row of Au ad-islands and the corresponding line profile are presented in Figure 6b and Figure 6c, confirming the narrow size-distribution of the islands (island diameter in the range of 2 nm) and the single layer height (note that because the islands are rather small, their apparent height is 1.5 Å, instead of 2.4 Å expected for a monoatomic step).19

Figure 6. Air-STM image (150 nm × 150 nm, Vtip = −0.3 V, it = 0.1 nA) of the same preparation as shown in Figure 5, but taken in a region with large, single-domain (22×√3) reconstructed areas of the Au(111) substrate. b) Enlarged view of a chain of Au ad-islands. The line profile shown in c) indicates that the Au ad-islands are nearly monodisperse (~1.5-2 nm diameter) and equally spaced. According to the vibrational data (Figure 2), the morphological characteristics of PDI-Au(111) at low PDI concentration are related with the presence of PDI molecules in the flat adsorption state. Further morphological changes are expected upon transformation from the flat to the upright phase, which occurs at high PDI concentration. Indeed, our STM data of an Au(111) surface exposed to 10 mM PDI/methanol solution reveals the formation of large monolayer high Au ad-islands (Figure 7a) and interconnected ad-island structures (Figure 7b). Because of the


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presence of mobile surface species, the quality of the STM images obtained from samples exposed to high PDI concentration is poor. Nevertheless, it is clear that ordered structures, which typically form at low PDI concentration, are absent at higher concentration. In addition, the area of ad-islands exceeds by far the area of vacancy islands. The growth of Au ad-islands can, therefore, be related to an increased mobility of upright PDI/Au ad-atom complexes and a preferential removal of Au ad-atoms from step sites.

Figure 7. Air-STM images (50 nm × 50 nm, Vtip = −0.3 V, it = 0.1 nA) of Au(111) exposed to 10 mM PDI.

Discussion The self-assembly of PDI deposited on Au(111) under UHV conditions has thoroughly been studied using STM and vibrational spectroscopies.15,19,28,30 In the present work, we have extended these studies to PDI adsorption from solution and found significant differences in molecular arrangement and surface morphologies between UHV and solution deposition. In agreement with previous studies,22 our SFG results show the transition from a flat PDI adsorption geometry upon adsorption from solutions with low PDI concentration, to an upright adsorption state at high concentration. For samples with mainly flat PDI molecules, STM images reveal the presence of protrusions that are partly assembled to linear structures with


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characteristic distances of 1.15 nm (or 0.6 nm), which is consistent with the repeat distances of PDI-Au adatom structures in the 1-dimensional chains created in UHV. However, long-range ordered molecular structures as found in UHV do not form from solution deposition, suggesting that the assembly process is influenced by the presence of the solvent (methanol). In this regard it should be noted that the frequency of the metal-coordinated isocyanide vibration for PDI adsorbed on Au from benzene solution reported by Tysoe and coworkers22 is closer to the value obtained for UHV-deposited PDI than ours, which might be taken as an indication that longer chain structures form when PDI is deposited from benzene solution, suggesting that the selfassembly process is to some extent influenced by the solvents polarity. Another difference between UHV and solution deposition is the formation of Au vacancy islands and Au ad-islands on Au(111) following exposure to PDI/methanol solutions, which is not observed during PDI adsorption in UHV. The transformations in adsorbate and substrate structure observed for PDI-Au(111), including the formation of molecule/Au ad-atom complexes, the transformation from flat to upright binding geometry, and the restructuring of the Au(111) surface, are in many respects similar to those commonly encountered during the formation of thiol-SAMs on Au(111). However, the appearance of ordered vacancy / ad-island structures, which are formed at low PDI concentrations, is unique to this system. In the following, we compare our findings with those of other SAM-Au(111) systems to work out the differences that give rise to the specific ordering. The formation of thiolate-Au adatom complexes during self-assembly of thiols on Au(111) requires a source of Au ad-atoms. Lifting of the herringbone reconstruction provides about 4 % of ad-atoms, and the additional ad-atoms required to establish a full SAM monolayer are taken from terraces sites, thus forming vacancies, which are mobile and aggregate to larger vacancy


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islands. On average, there is a good correlation between the number of Au atoms extracted from the surface during lifting of the herringbone reconstruction and vacancy island formation, and the number of Au ad-atoms incorporated in alkylthiol-SAMs.4 Less clear is the relation between selfassembly of aromatic thiols and the corresponding Au vacancy and Au ad-island formation. It has been proposed that Au adatoms and Au vacancies are significantly less mobile on arylthiolcovered Au surfaces, which helps stabilizing the vacancy island and ad-island structures on the terraces.10 On the time scales of hours, ad-islands have been shown to change their size and partly or completely merge with step edges of the Au substrate, suggesting slow ad-atom dynamics during the assembly process.9,11 Other proposals put forth to explain differences in adisland / vacancy formation observed for particular self-assembled systems argue with differences in molecular packing density,31 or differences in the flexibility within the SAM.32 Since the formation of thiol SAMs on Au(111) is accompanied by distortion and lifting of the herringbone reconstruction,8 specific adsorption and nucleation sites (e.g. elbows, dislocation lines, hcp region) are lost. Thus, vacancy islands and ad-islands are generally not ordered on most SAMcovered Au(111) surfaces. The differences between the present system and the thiol-based SAMs may therefore be found in the different interactions between the corresponding head-groups (-NC vs. –SH) and the Au surface, and in particular their influence on the herringbone reconstruction. Changes to the periodicity of the herringbone reconstruction have been shown to be a good indicator for the strength of the adsorbate-substrate interaction on Au(111).33 The herringbone reconstruction forms as a consequence of the tensile stress experienced by the Au atoms on the unreconstructed Au(111) surface. Because of its uniaxial nature, the stress compensation by formation of the reconstruction is anisotropic. As shown by theoretical work, the surface stress is nearly perfectly compensated only in the hcp regions, while the fcc regions remain under tensile


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stress, and the dislocation lines and the elbow sites are under compressive stress.34 Binding of adsorbates adds another stress contribution, which follows the direction of charge transfer during adsorbate binding and correlates with the electron acceptor/donation properties of the molecule.35 In general, adsorbate-induced stress is compressive when adsorbate binding involves charge transfer from the surface into the surface-adsorbate bond and tensile when charge flows into the surface. The consequences of adsorbate-induced stress are easily detected by changes of the herringbone periodicity, which decreases upon alkali metal adsorption on Au(111) (charge transfer into the surface),36 but increases and ultimately leads to lifting of the herringbone reconstruction upon adsorption of electron-acceptor atoms or molecules (O, S, Cl, NO2).37-40 Lifting of the herringbone reconstruction during SAM formation suggests that thiols induce compressive stress to Au(111).6 On the contrary, the herringbone reconstruction remains to a large extent intact during PDI adsorption in UHV. The calculated binding energy of flat PDI-Au oligomer chains (~177 kJ/mol)21 is only slightly smaller than typical binding energies of thiols on Au(111). However, the maximum coverage of PDI molecules in dense (PDI-Au)n chain structures is significantly smaller (0.13 for PDI19 vs. 0.33 for thiols adsorbing in √3×√3 structure). Moreover, for conditions where ordered vacancy / ad-island structures have been observed upon PDI adsorption from solution, the coverage is even smaller and estimated to be in the range of 0.05 for the state shown in Figure 3. Taking the low surface density into account, the adsorption enthalpy is calculated to be in the order of 0.2 J/m², which is in the same range as reported for a full monolayer of PPh3 on Au(111), which was shown to only moderately increase the periodicity of the herringbone reconstruction.31 The small adsorption enthalpy could in part explain the apparent stability of the herringbone reconstruction at low PDI coverage. In addition, electronic


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effects due to charge localization around the Au adatoms, which gives rise to repulsion between individual PDI-Au chains and is responsible for the charge transfer onto co-adsorbed CO2,18 could also play a role. Opposite to thiols, this charge localization may induce a small tensile adsorbate stress and therefore lead to stabilization of the herringbone reconstruction. In the same spirit, the absence of Au(111) surface restructuring upon adsorption of 6-mercaptopurin has been explained by a discharging of the Au(111) surface and small repulsive forces on the Au atoms.12 The persistence of the herringbone reconstruction during PDI adsorption in UHV can therefore be explained by the relatively small area density and the sign of adsorbate-induced surface stress. In agreement with this assumption, the Au adatoms required to form PDI-Au oligomer chains in UHV are primarily provided by the low-coordinated step sites.15,19 As for the adsorption of PDI from solution, the specific formation of vacancy islands at the elbow sites of the Au(111) herringbone reconstruction, and the nucleation of the ad-islands in regions that can also be related to the herringbone reconstruction, is in apparent contradiction with the assertion that the formation of vacancy islands (in particular for alkyl-thiol SAMs) and of Au ad-islands (for aryl-thiol SAMs) follows lifting of the herringbone reconstruction. On the other hand, the alignment of PDI-Au adsorption complexes along all three close-packed directions (Figure 3) suggests that the herringbone reconstruction relaxes slightly upon interaction of Au with PDI/methanol solutions. Therefore, we propose that by some additional effect induced by the co-presence of PDI and the solvent, the extraction of Au atoms from specific, reactive surface sites of the herringbone reconstructed Au(111) surface is energetically more favored in solution environment compared to UHV. From these considerations the following picture emerges for the formation of ordered Au vacancy / ad-islands structures on PDI-Au(111): The elbow sites on the herringbone


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reconstructed Au(111) surfaces are the most reactive sites because of under-coordinated Au atoms present around the dislocation, and Au atoms are extracted from these sites in the early stages of PDI adsorption. The resulting vacancies act as nucleation centers for further Au atom extraction and the formation of Au vacancy islands. The extracted Au atoms agglomerate on the Au(111) terrace and form Au ad-islands. For the low-coverage case presented in Figure 5, we determined the area of Au ad-islands to be similar to the area of Au vacancy islands (~ 10% of the total area), suggesting that the Au vacancies are the source of Au ad-atoms, which then agglomerate to form the Au ad-islands. Whether the Au atom extraction from elbow sites is the initial process during interaction of Au(111) with the PDI/methanol solution, or triggered by a slight relaxation of the herringbone reconstruction induced by PDI adsorption on the terraces, cannot be answered. In either case, limited vacancy / ad-atom mobility is required to prevent the Au vacancy islands and ad-islands from aggregating. The mobility of vacancies is small because of the overall weak influence of PDI adsorption on the herringbone reconstruction, whereas both, stabilization by adsorbed molecules19,31 or the presence of specific nucleation centers on the herringbone reconstructed Au(111)41 surface might contribute to the stabilization of the Au adislands. Because PDI adsorption from solution does not give rise to long-range ordered molecular structures (Figure 3 and Figure 4), we consider that nucleation of Au ad-islands in the hcp regions of the herringbone reconstruction is more likely to be responsible for the observed specific alignment of Au ad-islands. With increasing PDI concentration (> 1mM), the adsorption geometry of PDI changes from flat to upright (Figure 2) and the number of Au ad-atoms increases, which leads to the formation of large Au ad-islands or interconnected ad-island structures (Figure 7). Because our STM data do not reveal a likewise increase in the number of vacancies, the main source for the additional


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ad-atoms are the low-coordinated step-sites. In line with previous proposals put forth to explain the formation of large Au islands on Au(111) during self-assembly of aromatic thiols (for example, of pentafluorobenzenethiol)11 and because of the small diffusion barriers experienced by upright PDI-Au ad-atom complexes,21 the formation of large Au ad-islands at high PDI concentration can be explained by the increased mobility of upright PDI-Au adatom complexes and removal of Au ad-atoms from the step sites, which finally leads to Au island growth by attachment of Au ad-atoms to already existing Au ad-islands.

Conclusions In summary, the structure of adsorbed PDI and morphological modifications of the Au(111) substrate during adsorption of PDI on Au(111) from methanol solutions have been studied by vibrational spectroscopy and STM. At low PDI concentration, where PDI is predominantly in a flat adsorption state according to the vibrational data, the STM results indicate the formation of molecule-Au adatom complexes, which consist of a PDI molecule bridging two Au adatoms. In contrast with results obtained for UHV-deposited PDI, no long-range 1-dimensional (PDI-Au)n oligomer chains could be identified after solution deposition, suggesting that the self-assembly process into chain structures is significantly disturbed in solution environment. Furthermore, exposure to the PDI/methanol solution leads to restructuring of the Au(111) surface and creation of Au vacancy islands and Au ad-islands. The vacancy islands and ad-islands formed after exposure to solutions with low PDI concentration (< 0.1 mM) are not randomly distributed, but arranged in a way that replicates the structure of the herringbone reconstruction of Au(111). Vacancy islands are created preferentially at the elbow sites, and the extracted Au ad-atoms assemble to small particles, which nucleate most likely in the hcp regions of the herringbone reconstruction, giving rise to


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long-range ordered nanostructures. In comparison with self-assembly of thiols on Au(111), the unique ordering of vacancy islands and ad-islands on PDI-Au(111) can be explained on the basis of adsorbate-induced surface stress: Whereas thiol adsorption on Au(111) promotes lifting of the herringbone reconstruction because of the electron acceptor character of the thiol group, which helps to reduce the tensile surface stress of the reconstructed Au(111) surface, the bonding of isocyanide leaves the reconstruction almost unaffected. At high PDI concentration, the extraction of Au ad-atoms from step sites is more dominant and the formation of large Au ad-islands is promoted because of the increased mobility of upright PDI-Au adatom complexes. Partial substrate restructuring is frequently seen to accompany self-assembly, in particular of thiols, on Au(111), and the behavior observed for PDI on Au(111) fits into the general picture that adsorption of functionalized organic molecules (thiols, selenols) with purely aromatic backbone gives rise to the formation of both, Au vacancy islands and Au ad-islands on the Au(111) surface. Our study shows that, in addition to the nature of the organic backbone, the properties of the head group of the organic molecule and the type of interaction with the substrate influence the way the Au(111) surface is structurally modified during adsorption from solution. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENT


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This work was supported by the European Union through the ERC Starting Grant agreement No. 280070. We are grateful to Svetlozar Surnev and Michael G. Ramsey for fruitful discussions. REFERENCES (1) Schreiber, F. Self-Assembled Monolayers: From 'Simple' Model Systems to Biofunctionalized Interfaces. J. Phys.: Condens. Matter 2004, 16, R881-R900. (2) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. Molecular Self-Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin-Film Transistor Applications. Adv. Mater. 2009, 21, 1407-1433. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 11031169. (4) Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Self-Assembled Monolayers of Thiols and Dithiols on Gold: New Challenges for a Well-Known System. Chem. Soc. Rev. 2010, 39, 1805-1834. (5) Schreiber, F. Structure and Growth of Self-Assembling Monolayers. Prog. Surf. Sci. 2000, 65, 151-256. (6) Maksymovych, P.; Voznyy, O.; Dougherty, D. B.; Sorescu, D. C.; Yates, J. T. Gold Adatom as a Key Structural Component in Self-Assembled Monolayers of Organosulfur Molecules on Au(111). Prog. Surf. Sci. 2010, 85, 206-240.


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(29) Ito, M.; Noguchi, H.; Ikeda, K.; Uosaki, K. Substrate Dependent Structure of Adsorbed Aryl Isocyanides Studied by Sum Frequency Generation (SFG) Spectroscopy. PCCP 2010, 12, 3156-3163. (30) Boscoboinik, J.; Kestell, J.; Garvey, M.; Weinert, M.; Tysoe, W. T. Creation of LowCoordination Gold Sites on Au(111) Surface by 1,4-Phenylene Diisocyanide Adsorption. Top. Catal. 2011, 54, 20-25. (31) Jewell, A. D.; Sykes, E. C. H.; Kyriakou, G. Molecular-Scale Surface Chemistry of a Common Metal Nanoparticle Capping Agent: Triphenylphosphine on Au(111). ACS Nano 2012, 6, 3545-3552. (32) Käfer, D.; Bashir, A.; Witte, G. Interplay of Anchoring and Ordering in Aromatic SelfAssembled Monolayers. J. Phys. Chem. C 2007, 111, 10546-10551. (33) Jewell, A. D.; Kyran, S. J.; Rabinovich, D.; Sykes, E. C. H. Effect of Head-Group Chemistry on Surface-Mediated Molecular Self-Assembly. Chem. Eur. J. 2012, 18, 7169-7178. (34) Bulou, H.; Goyhenex, C. Local Strain Analysis of the Herringbone Reconstruction of Au(111) through Atomistic Simulations. Phys. Rev. B 2002, 65, 045407. (35) Ibach, H. Adsorbate-induced Surface Stress. J. Vac. Sci. Technol., A 1994, 12, 2240-2243. (36) Barth, J. V.; Behm, R. J.; Ertl, G. Mesoscopic Structural Transformations of the Au(111) Surface Induced by Alkali Metal Adsorption. Surf. Sci. 1994, 302, L319-L324. (37) Biener, M. M.; Biener, J.; Friend, C. M. Revisiting the S-Au(111) Interaction: Static or Dynamic? Langmuir 2005, 21, 1668-1671.


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(38) Gao, W. W.; Baker, T. A.; Zhou, L.; Pinnaduwage, D. S.; Kaxiras, E.; Friend, C. M. Chlorine Adsorption on Au(111): Chlorine Overlayer or Surface Chloride? J. Am. Chem. Soc. 2008, 130, 3560-3565. (39) Baker, T. A.; Kaxiras, E.; Friend, C. M. Insights from Theory on the Relationship Between Surface Reactivity and Gold Atom Release. Top. Catal. 2010, 53, 365-377. (40) Driver, S. M.; Zhang, T. F.; King, D. A. Massively Cooperative Adsorbate-Induced Surface Restructuring and Nanocluster Formation. Angew. Chem. Int. Ed. 2007, 46, 700-703. (41) Dretschkow, T.; Dakkouri, A. S.; Wandlowski, T. In-situ Scanning Tunneling Microscopy Study of Uracil on Au(111) and Au(100). Langmuir 1997, 13, 2843-2856.


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Langmuir

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Figure 1. a) UHV-STM image of the Au(111) surface showing the typical herringbone reconstruction (50 nm × 33 nm, Vtip = −0.75 V, it = 5 pA). Indicated are the principal crystal directions, regions of hcp and fcc stacking, discommensuration lines (DL) and elbow sites (EB). b,c) STM images (23 nm × 23 nm, Vtip = −2.0 V, it = 5 pA) of the 1-dimensional (PDI-Au)n chain structures formed in UHV. The white arrow indicates the chain direction. The image in b) was obtained for low PDI coverage (0.37 ML PDI; Au adatom coverage: 0.05 ML), where the interchain distance is 1.3 nm, whereas the image in c) is from a medium coverage PDIAu(111) sample (0.75 ML PDI; Au adatom coverage: 0.10 ML), which exhibits regions of ordered chains with an interchain separation of 0.6 nm, and regions with disordered (PDI-Au) adsorption complexes formed in the hcp regions of the herringbone reconstruction19; the insert in c) shows a zoom of the ordered chain structure formed at 0.75 ML PDI. d) Model of upright (left) and flat (right) PDI-Au adatom complexes on Au(111). Note that in UHV, PDI adsorbs predominantly in the flat adsorption state. Figure 1 85x57mm (300 x 300 DPI)


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Langmuir

Figure 2. a) SFG spectra recorded in the –NC stretching region and corresponding fits of Au(111) exposed for 60 min. to PDI/methanol solution with PDI concentration ranging from 0.005 mM (bottom) to 10 mM (top). Black: raw SFG data; grey: non-resonant background; red: fit result using equation 1. b) Corresponding resonant vibrational contributions to the SFG spectra shown in a). The peaks at higher and lower frequency, respectively, are attributed to the stretch vibration of metal-coordinated (green) and free (blue) –NC groups. Note that the resonant susceptibilities have opposite sign. Figure 2 84x107mm (300 x 300 DPI)


Langmuir

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Figure 3. a) UHV-STM image (22 nm × 22 nm, Vtip = −1.3 V, it = 5 pA) of Au(111) exposed for 60 min. to 0.005 mM PDI/methanol solution. The alignment of small PDI-Au chain segments along the close-packed substrate directions is indicated by dashed lines. The depressions correspond to regions of missing Au atoms (Au vacancy islands). b,c) Higher resolution STM image (5.3 nm × 2.6 nm) of a chain segment and corresponding line profile. Figure 3 85x42mm (300 x 300 DPI)


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Langmuir

Figure 4. a) UHV-STM image (50 nm × 50 nm, Vtip = −1.3 V, it = 5 pA) of the Au(111) surface exposed for 60 min. to 0.005 mM PDI/methanol solution. Au vacancy islands and Au ad-islands appear as depressions and protrusions, respectively. The Au ad-islands are found in close neighborhood of the vacancy islands and the vacancy islands appear ordered. Comparison with the structure of the clean, herringbone reconstructed Au(111) surface indicates that the vacancy islands are primarily formed at the elbow sites. (For better visualization, a schematic of a pair of discommensuration lines is overlaid on the STM image). b) UHV-STM image (22 nm × 22 nm, Vtip = −1.3 V, it = 5 pA) of the same sample preparation presented with strongly enhanced contrast. In addition to the vacancy island small depressions (encircled areas) are found at the Au(111) terrace, which are tentatively assigned to single Au vacancies. Figure 4 86x42mm (300 x 300 DPI)


Langmuir

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Figure 5. Air-STM image (450 nm × 300 nm, Vtip = −0.3 V, it = 0.1 nA) of Au(111) exposed to 0.005 mM PDI/methanol solution. b) Enlarged area (100 nm × 100 nm) of the same preparation. A schematic of the herringbone reconstruction is overlaid to highlight the ordering of the vacancy islands (depressions) and adislands (protrusions). The Fast Fourier Transform (FFT) of the STM image b) shown in c), and the corresponding FFT-filtered image shown in d) provide additional support for the ordering of vacancy islands and ad-islands. Figure 5 85x115mm (300 x 300 DPI)


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Langmuir

Figure 6. Air-STM image (150 nm × 150 nm, Vtip = −0.3 V, it = 0.1 nA) of the same preparation as shown in Figure 5, but taken in a region with large, single-domain (22×√3) reconstructed areas of the Au(111) substrate. b) Enlarged view of a chain of Au ad-islands. The line profile shown in c) indicates that the Au adislands are nearly monodisperse (~1.5-2 nm diameter) and equally spaced. Figure 6 83x45mm (300 x 300 DPI)


Langmuir

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Figure 7. Air-STM images (50 nm × 50 nm, Vtip = −0.3 V, it = 0.1 nA) of Au(111) exposed to 10 mM PDI. Figure 7 85x41mm (300 x 300 DPI)


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Nanostructuring of Au(111) during the Adsorption of an Aromatic

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