J. Phys. Chem. C 2009, 113, 12395–12401
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Probing Structure and Molecular Conductance in Highly Ordered Benzyl Mercaptan Monolayers Florian von Wrochem,* Frank Scholz, Akio Yasuda,§ and Jurina M. Wessels†,* Sony Deutschland GmbH, Materials Science Laboratory, Hedelfinger Strasse 61, 70327 Stuttgart, Germany ReceiVed: February 27, 2009; ReVised Manuscript ReceiVed: May 18, 2009
The monolayer structure of alkanethiol and benzyl mercaptan (C6H5CH2SH) self-assembled monolayers on Au(111) is studied by scanning tunnelling microscopy in ultrahigh vacuum (UHV-STM) and photoelectron spectroscopy (XPS). Whereas alkanethiol monolayers exhibit the known c(4 × 2) overlayer structure, benzyl mercaptan (BM) monolayers show a novel reconstruction, resulting from thermal annealing at 368 K. Extended, striped phase domains having a commensurate, p(41/23 × 2) overlayer structure with an oblique unit cell are observed. The tunnelling decay constant β of the molecular layer and the molecular conductance at the probe-monolayer contact point are obtained from tunnelling current-distance characteristics in nanoscopic junctions. Data from alkanethiols with different chain-lengths (octanethiol and decanethiol) yield a value of β ) 0.95/Å, in good agreement with previous studies. BM monolayers show a lower decay constant (β ) 0.55/Å), attributed to the π-conjugation in the phenyl ring. The measured currents are about 2 orders of magnitude lower than those of analogue dithiol derivatives that are chemically linked to both electrodes. This is related to the higher impedance of the physical contact between monolayer and STM probe compared to the covalent metal-molecule contact. Introduction For the development of molecular electronics,1 a thorough understanding of the correlation between electrical properties and molecular structure is of fundamental importance.2,3 Charge transport behavior is strongly dependent on the local environment in a metal-molecule-metal junction,4 and uncontrolled parameters such as molecular conformation,5 bonding properties at the interface,6-8 and interaction with solvents or ions can complicate the interpretation of experimental results. A significant number of studies, mostly based on two terminal measurements of thiol- or amine-substituted alkanes,9,10 oligo-phenylenes,11 or oligo-phenylene-ethynilenes12-16 have addressed the issue of electron transport through single molecules. The contact to the metal is either realized by chemically connecting a single molecule to both electrodes, as in a mechanically controlled break junction,17,11 crossed-wire junction,18 nanopore junction,19,20 and in break junction scanning tunnelling microscopy (STM),21,9 or by linking the molecule covalently to one end and addressing it from the opposite end with a scanning probe (e.g., establishing a physical contact to an STM or AFM probe).12,22 By using STM, the tunnelling decay constant of the organic layer was determined by current-voltage spectroscopy,23 height profiles,24 and tunnelling current-distance (I-d) spectroscopy.25-27,14,28 The I-d characteristics of molecular layers can be interpreted with the two-layer tunnelling model,29 which allows the extraction of the molecular transconductance. In previous studies, the tunnelling decay constant β was measured using different approaches, and values within the range from 0.7/Å to 1.2/Å were found for alkanethiols,30,31 whereas values from 0.4/Å to 0.6/Å and from 0.53/Å to 0.8/Å were extracted § Present address: Sony Corporation Gotenyama Technology Center, Life Science Laboratory, 5-1-12 Kitashinagawa, Shinagawa-ku, Tokyo, 1410001, Japan. † Present address: Sony Deutschland GmbH, Kemperplatz 1, 10785 Berlin, Germany.
for oligophenylenes32,25,28,33 and oligophenylenevinylenes,16 respectively. STM offers the advantage that its nanometer-scale resolution in real space allows the selection of defined locations on a known two-dimensional surface topography. This provides the identification of molecules and their surface arrangement before electrical characterization, a valuable information for the interpretation of spectroscopic data. In this report, we compare two alkanethiol compounds, octanethiol and decanethiol, with the aromatic derivative benzyl mercaptan (BM), having a single phenyl ring connected to the thiol through a methylene spacer. Alkanethiols and BM both form crystalline and commensurate monolayers on Au(111);34,35 however, upon annealing, BM monolayers show a novel reconstruction that we relate to the π-interactions between neighboring aromatic rings. The monolayer structure differs from the (3 × 3)R30° lattice previously reported by Tao35 and Hallmann36 and from the c(53 × 3) structure found for thio-oligophenyls by Cyganik et al.37 Rather, we observe a lattice with a reduced packing density having an oblique p(41/23 × 2) unit cell. The decay constant β and the conductance of the molecular layers at the probe-molecule contact point are determined by I-d characteristics. A significantly reduced molecular conductance as compared to break junction results is found, what is discussed in terms of a different electrical coupling between molecule and probe. Methods Experimental Section. Octanethiol (SC8), decanethiol (SC10), and benzyl mercaptan (BM) were purchased from SigmaAldrich and used as received (for molecular structures, see the Supporting Information). Polycrystalline Au substrates were realized by thermal evaporation (pressure: 5 × 10-6 mbar) of 100 nm Au on freshly cleaved mica substrates. Atomically flat Au(111) surfaces were obtained by flame annealing of the
10.1021/jp901819z CCC: $40.75 2009 American Chemical Society Published on Web 06/18/2009
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substrates in a butane flame followed by quenching in methanol. The monolayers were prepared in argon environment with the immersion of the Au(111) surfaces into 1 mM solutions of SC8, SC10, or BM in ethanol for 24 h at room temperature. To avoid oxidation of the organic materials, methanol and ethanol were saturated with argon before usage. The quality of the resulting SAMs was verified by XPS analysis (Kratos Axis Ultra using Al KR emission (1486.6 eV))38 and by contact angle goniometry (KSV CAM 100 setup; see the Supporting Information). Upon preparation, the samples were immediately introduced into the UHV-STM chamber at a base pressure of 9 × 10-10 mbar. The instrument (Omicron VT-STM) was equipped with a low current IV-converter that allows scanning tunnelling spectroscopy in the femto-ampere range. Before imaging, the samples were thermally annealed at 368 ( 5 K for 10-15 min to allow the equilibration of the monolayer structure and to remove chemisorbed molecules from the surface (this allowed high resolution STM scans with a low frequency of tip changes). Subsequently, the STM measurements were performed at room temperature. For alkanethiols, the scanning tunnelling parameters were VGap ) 0.8 -1.0 V (both positive and negative values) and It ) 5-10 pA, corresponding to a tunnelling impedance of about 100-200 GΩ. BM monolayers were mostly scanned with VGap ) -0.5 V and It ) 50 pA, corresponding to an impedance of 10 GΩ. The lower impedance allowed an improved imaging of the more conductive BM molecules. For an accurate determination of the lattice constant, average lengths over an area of about 50 × 50 nm2 were extracted. Lateral drift was minimized by scanning the same sample area for several hours. For I-d characterization, a low bias voltage was selected (-0.2 V) in consideration of the higher polarizability of aromatic compounds, which might cause undesired forces between the tip and BM. As an STM probe, a Pt-Ir (80:20) tip was used. To record an I-d characteristic, the feedback loop was interrupted, and the probe was drawn back by 0.5 nm. After a short delay (1 ms), the acquisition started while the probe approached the surface at a rate of ∼100 Å/s. During the approach, the current was measured at 60 points with an acquisition time of 0.65 ms/point. With this short acquisition time, a negative impact of drift motion on the I-d characteristics was excluded. Each of the SC8, SC10, and BM monolayers was investigated with three STM probes. For each sample, 50-100 I-d characteristics were acquired in total, 10-20% of them showing two distinct linear slopes, which were related to tunnelling though the molecular medium and the vacuum layer. These curves were selected to obtain average values for βΜ, βv, and the molecular conductance G. Determination of Molecular Conductance from I-d Spectra. In scanning tunnelling spectroscopy, the current is recorded as a function of the vertical position of the STM probe over the sample.39 In the exponential electron tunnelling model,40 a coherent electron tunnelling process is assumed, and the dependence of the current on the distance d is expressed by
I ∼ G0 · exp(-βMdM - βvdv) where G0 is the contact conductance and βΜ and βv as well as dM and dv are the tunneling decay constant and the tunneling distance in the molecular medium and in the vacuum, respectively. If the probe is located in the vacuum region above the monolayer, the total conductance results from two contributions, that is from the molecular medium and from the tunneling gap between the molecular layer and the probe (two layer tunneling model29). By modulation of the probe-sample separation z, the
von Wrochem et al. tunnelling decay constant β of the vacuum or the molecular layer is obtained with
β)
d(ln I) dz
Depending whether the probe is located in the vacuum region or within the SAM, this expression allows the extraction of βΜ and βv. Finally, from the intercept of the two slopes identifying the tunnelling decay constant for the vacuum and the molecular medium, the contact conductance can be determined. Results and Discussion Monolayer Structure. In previous STM studies, the overlayer structure of thio-oligophenylenes on Au(111) has been resolved at the molecular level.41-43,37,44-46 Specifically, for BM monolayers, Tao and Chen determined a (3 × 3)R30° structure35 that was also observed for other thio-oligophenylene compounds with various molecular lengths (eventually including an alkyl spacer). In Figures 1 and 2, high resolution STM images of BM monolayers on Au(111) are presented. Upon thermal annealing at ∼368 K in ultrahigh vacuum (UHV), extended, crystalline BM domains evolve (40-50 nm in diameter; see Figure 1a and d), showing a striped phase pattern that is periodically repeated in the [121] direction of the substrate. A careful analysis of the present STM topographs shows a commensurate Au(111) p(41/23 × 2)BM structure with a primitive oblique unit cell (Figure 1c). The p(41/23 × 2) lattice is in good agreement with the experimentally determined lengths of the unit cell vectors (a ) 5.8 ( 0.1 Å, b ) 22.8 ( 0.2 Å), since they coincide with the theoretical values (a ) 5.77 Å, b ) 22.82 Å). The cell parameters yield a molecular area of 32.6 Å2, which is 50% higher than the area found in the (3 × 3)R30° phase (21.6 Å2), 25% higher than that of the c(53 × 3) phase (biphenyl and terphenyl alkanethiols),43,45 and comparable to that of the (63 × 23)R30° phase (terphenyl alkanethiols).45 The transition from the (3 × 3)R30° to the p(41/23 × 2) lattice is attributed in the first place to the thermal annealing procedure that progressively reduces the density of the BM monolayer by thermal desorption of molecules from the surface47 (see the thermal desorption data in the Supporting Information), subsequently to a reorganization of the whole monolayer, resulting in an energetically more favorable bond angle arrangement and phenyl ring stacking. On the basis of the experimental data, we suggest a structural model as depicted in Figure 1c. Within this model, hollow and bridge site coordination to the Au(111) substrate might alternate in each unit cell (Figure 1c). But, probably a tilt of the molecular backbone, supported by the flexibility of the methylene group, would also allow the thiolate of BM to bind at various possible binding sites that cannot be determined experimentally by STM. The crystallographic orientation of the unit cell relative to the surface is determined by reference to the Au(111) step edge direction (Figure 1d). The slightly oblique character of the unit cell (angle R ) 80°) is consistent with an offset of 11/2 aAu along the [101j] direction (aAu ) 2.884 Å), that is probably favored by an energetically optimized stacking geometry of adjacent phenyl rings of BM. The offset is also a direct consequence of the fact that the repeat unit in the [121] direction is not a multiple integer of 3 (see Figure 1c), which automatically implies an offset by multiples of 1/2 aAu. Whereas the STM micrographs clearly show that the offset is quite general (see unit cell in Figures 1b and 2), small variations in
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Figure 1. Constant current STM topographs of a crystalline BM monolayer on Au(111). (a) Ordered BM domain showing a striped phase with a periodicity of 22.4 Å (black lines), corresponding to 4 BM rows in parallel (scan range: 30 × 30 nm2). (b) High resolution STM image of the BM lattice, including the primitive oblique unit cell with the vectors a and b. The striped phase in only weakly visible (scan range: 17 × 17 nm2). (c) Schematic model of the BM monolayer showing the positions of BM molecules (gray circles) and Au atoms (white circles) defining a Au(111) p(41/23 × 2)BM overlattice. The unit cell vector a shows along the [101j] direction of the substrate, while b deviates from [121] due to an offset (doffset ) 11/2 aAu, with aAu ) 2.884 Å) in the position of the BM molecules. (d) Monocrystalline BM domain on Au(111) (scan range: 25 × 25 nm2). The rotation angle between the direction of the unit cell vector a and the Au(111) step edge is 60°. The black areas in (a) and (d) represent Au(111) terraces located one atomic layer below the main terrace. All topographs are recorded with UGap ) -0.5 V and Itunn ) 50 pA.
this offset are possible, both in magnitude and direction. Notably, in proximity to the Au(111) step edges, the BM stripes are parallel to the step edge direction (i.e., a // step edge direction, see black lines in Figure 5a). This is attributed to an enhancement of the phenyl-Au overlap at the Au(111) step edge and thus to the maximization of the metal-molecule interaction energy along the edge. The formation of low coverage phases has already been reported for alkanethiol48 and oligophenyl45 monolayers. The observation of the p(41/23 × 2) structure is inherently related to the reduced number of molecules/unit area and the selfassembly of BM molecules to form an energetically optimized staggering of phenyl rings. The proposed structure is shown in Figure 2. The striped phase character, in particular concerning the weakly pronounced fourth row of molecules along the [101j] direction (Figure 2), is most probably caused by different tilt angles of BM molecules within each unit cell, a consequence of the reduced packing density along the [121] direction. Even though an odd number of methylene groups in BM (in this case n ) 1) favors a rather perpendicular orientation of the phenyl ring,49 in the present case, we assume that the aromatic forces
between the rings drive the molecular tilt angle to values consistent with a close packed arrangement of these aromatic end groups. With the experimental molecular coverage, a geometric estimation based on the ratio of the van der Waals cross-section of BM (from DFT calculations) and the molecular area derived from STM topographs yields an average tilt angle of R ) arccos(20.5 Å/32.6 Å) ) 51° from the surface normal. Finally, a short comment on the role of the methylene spacer in the formation of ordered BM monolayers is appropriate. In previous studies, the insertion of a short alkyl-chain spacer in between the rigid molecular rod and the anchor group has already been shown to support the formation of densely packed thio-oligophenylene self-assembled monolayers.35,45 The present data reinforce these observations, since a crystalline lateral order and a standing up phase, as observed in BM monolayers, is not encountered in thiophenol monolayers prepared under similar conditions (Supporting Information).41 Figure 3 shows STM scans of SC8 and SC10 monolayers, respectively. These monolayers are ideal reference systems for transport studies,30,31 since their structures are well-known from extensive studies carried out over the last decades.50,34,51 Both
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Figure 2. STM scan showing the detailed structure within a p(41/23 × 2) BM monolayer (scan range: 11 × 11 nm2) The primitive oblique unit cell of the overlayer is shown in black. The inset displays a zoom image upon fast Fourier transform filtering. Each protrusion identifies a BM molecule. On the basis of the elliptical features, a parallel orientation of the phenyl rings seems likely, even though compelling evidence is hampered by the limited resolution of the scan. For comparison, a BM model (relaxed by DFT) is superimposed on the STM image (in scale). A lower STM topographic height is observed for the 4th row in the cell along the [101j] direction. Tunneling parameters: UGap ) -0.5 V; Itunn ) 50 pA.
SC8 and SC10 monolayers show the known (3 × 3)R30° overlayer structure52 and exhibit crystalline domains separated by well-resolved domain boundaries (dark trenches in Figure 3a). The ∼2.0 nm separation between SC8 domains results from thermal annealing, which initiates diffusion and partial desorption of alkanethiols from domain boundaries.53 In SC10 monolayers, changes in the monolayer structure are less pronounced as a consequence of the higher stability of alkanethiol monolayers with longer alkyl chains. Some domains show the wellknown c(4 × 2) reconstruction (Figure 3a and c).54 It is interesting to note that, with the given loop conditions (IT ) 5 pA, VG ) -1 V), the STM probe scans across the SC10 monolayer end groups during constant current imaging. Indeed, the origin of the distance axis in Figure 3d denotes the constant current condition during imaging, and the intersection of the two linear fits for βM and βv is found at ∼1 Å above this position. This would explain the limited lateral resolution often achieved when scanning long-chain alkanethiols (>SC10) with tunnelling impedances below 200 GΩ, caused by the perturbation of the monolayer by the probe. In Figure 4, XPS spectra acquired in the S 2p binding energy region for SC10 and BM monolayers are presented. The XPS signal can be accurately reproduced by a fit using two Voigt functions (S 2p1/2/S 2p3/2 doublet separated by 1.18 eV and with an intensity ratio of 1/2). All monolayers show a single signal with the S 2p3/2 peak centered at a binding energy of 162.1 eV. As known for alkanethiols on Au, this corresponds to the thiolate-Au bond formed upon chemisorption.55 The absence of additional signals in the S 2p spectra highlights the uniform chemisorption and the high degree of orientational order of BM monolayers on Au(111).
von Wrochem et al. Additional contact angle measurements, recording the surface energy of the monolayers, confirmed their high quality. We obtained water contact angles in the range of 85 ( 2° for BM and 108 ( 1° for SC10, which is in good agreement with accepted values for monolayers exposing phenyl41 and alkyl56 groups to the surface. The same conclusions as for SC10, regarding XPS and contact angle data, are drawn for SC8 (results not shown). I-d Spectroscopy. Figure 3 shows I-d characteristics acquired on molecularly resolved SC8 and SC10 domains. In the vacuum region (i.e., when the STM probe is translating above the sample), the tunnelling current shows a steep exponential decay (positive z range in Figure 3b and d). Upon contact, the recorded current reflects the decay constant β of the molecular medium, and the slope of the I-d characteristics in the semilogarithmic plot changes. Hence, if the decay constant β in the molecular medium is different from that in the vacuum, the intersection of these two slopes defines the gap impedance at the point contact of the probe with the monolayer (Supporting Information). For SC8 monolayers, a tunnelling decay constant of βv ) (2 ( 0.1)/Å is found in the vacuum region, in good agreement with reported STM data both from ambient24 and UHV26 measurements. Upon contact with the SC8 and SC10 monolayer, a value of β ) (0.95 ( 0.05)/Å is obtained, which agrees with the widely accepted value for electron transport through alkanethiols.30,31 The average molecular conductance at the point contact of the probe with the monolayer, as estimated from the intersection of the two linear fits in Figure 3b-d, is GSC8 ≈ 1.3 × 10-7 G0 and GSC10 ≈ 1.8 × 10-8 G0 for SC8 and SC10 monolayers, respectively. The ratio of these two values agree with the difference in chain length (i.e., in tunnelling barrier width) if β ) 0.95/Å is used within the exponential tunnelling model. However, the absolute conductance values determined with the STM junction clearly deviate from experiments in which the molecular end groups are chemically linked to both electrodes. For example, in studies performed on octanedithiols with the break junction technique, conductance values ranging from 1.3 × 10-5 G0 to 5 × 10-5 G0 were obtained,10,57,58 which is about 2 orders of magnitude higher compared to our data. To explain this difference, the presence or absence of a chemical bond at the molecule-electrode interface has to be considered.31 In our STM junction, no covalent bond between the STM probe and the monolayer end groups is formed, and the existence of a physical contact significantly affects the overall conductance across the junction. In previous studies, comparing physical and covalent contacts using conductive AFM,22,59,31 a 1-2 order of magnitude difference in the conductance of thiolates and dithiolates has been observed. Our findings, based on STM spectroscopy in UHV, are consistent with those results and emphasize the key role that interfacial bonding plays for the electrical conductance in metal-organic junctions. Given the consistency of SC10 and SC8 results, we will now turn our attention to BM. All I-d characteristics are taken on the brightest (most protruding) molecules of the molecular lattice to make sure that the orientation of the BM molecules is possibly standing up (i.e., max 45-50° from the surface normal) within the molecular junction. As expected, the point contact to the BM monolayer is established at significantly lower tunnelling impedance compared to the more insulating alkanethiol monolayers (Figure 5b). We find an average decay constant of β ) (1.9 ( 0.15)/Å for the vacuum region and β ) (0.55 ( 0.1)/Å upon contact with the monolayer. The decay constant
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Figure 3. STM scans of SC8 (a) and SC10 (c) monolayers on Au(111) (a: scan range: 54 × 54 nm2, tunneling parameters: UGap ) -0.8 V; Itunn ) 10 pA. c: scan range: 50 × 50 nm2, tunneling parameters: UGap ) -1 V; Itunn ) 5 pA). The domain boundaries of the crystalline films, in particular for SC8, are polygonal and aligned along preferential directions relative to the Au(111) lattice due to thermal annealing (at 368 K) before imaging. In some of the domains, the c(4 × 2) reconstruction is clearly visible. The black areas are monatomic vacancy islands. Semilogarithmic plots of representative I-d characteristics on SC8 (b) and SC10 (d) monolayers. The intercept of the two linear slopes in the monolayer and in the vacuum region (blue lines) identifies the point contact of probe and monolayer, where a tunneling resistance of 100 GΩ (SC8) and 700 GΩ (SC10) is determined (voltage during I-d acquisition: b: VG ) -0.8 V; d: VG ) -1 V). For the vacuum region, the linear fit to the I-d characteristics is obtained in the z range between the noise of the amplifier (∼0.3 pA) and the first change in slope. For the monolayer region, it is obtained in the z range within 2 Å below the change in slope. The two z ranges are indicated in gray. Data points between the two ranges deviating from the main slope are disregarded.
0.34-0.45/Å)14 compounds and is thus typical for the tunnelling probability in cycloaromatic systems. In such aromatic structures, a superposition of carbon pz orbitals perpendicular to the plane of phenyl ring gives rise to frontier orbitals representing a delocalized electronic π-system. Since charge transport is to a large extent determined by the energy and shape of the frontier molecular orbitals,61 the presence of aromatic structures effectively reduces the tunnelling barrier (as here in the low bias regime) for electron transport through the molecular junction. From the intersection of the two slopes (Figure 5b), we obtain the average molecular conductance for BM (GBM ≈ 2.6 × 10-6 G0). It is about 20 times higher than the conductance determined for SC8 monolayers, a direct effect of the structural differences existing between these two compounds. Conclusions Figure 4. XPS spectra of SC10 (black line) and BM (blue line) monolayers on Au(111) in the sulfur 2p region. The sulfur 2p3/2 (right peak) and 2p1/2 (left peak) components are fitted with a 2/1 peak area ratio (see red components), representative of a single sulfur species. An arbitrary offset between the plots has been introduced for clarity. Both SC10 and BM show sulfur 2p3/2 components at the characteristic energy of 162.1 eV, indicative of thiolates chemisorbed to Au.
determined for BM is very close to the values measured for oligophenylenes (β ) 0.47-0.53/Å),25,33,28,60 oligophenylenevinylene (β ) 0.53-0.8/Å),16 or oligophenylenethynylene (β )
High resolution STM scans have shown a novel p(41/23 × 2) reconstruction in BM monolayers on Au(111), which is attributed to the thermal annealing process with dense BM monolayers in UHV. In view of the electrical properties of these compounds, the tunnelling decay constant and the molecular conductance are obtained by current-distance tunnelling spectroscopy. The conductance of the aromatic derivative BM is found to exceed the conductance of SC8 by a factor of ∼20 and that of SC10 by ∼140, a direct consequence of the structural differences between BM and alkanethiols. The tunnelling decay
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von Wrochem et al. and decanethiol. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 5. (a) STM scan of an BM monolayer on Au(111) (scan range: 36 × 36 nm2, tunneling parameters: UGap ) -1 V; Itunn ) 50 pA). A striped phase with a uniform orientation extends over several Au(111) terraces. Close to the terrace boundaries, the unit cell vector a is parallel to the step edge direction (black lines). The plane of the image is tilted to enhance the contrast of molecular features. (b) Semilogarithmic plot of a representative I-d characteristic on BM. The intercept of the two linear slopes in the monolayer and the vacuum region (blue lines) identifies the point contact of probe and monolayer. Here, a tunneling resistance of 4 GΩ is determined (Voltage during I-d acquisition: VG ) -0.2 V). The linear fits to the I-d characteristics for the vacuum and the monolayer region are obtained from data points within the z range indicated by the two shaded areas. Data points between the two ranges, deviating from the main slope, are disregarded.
constant β for alkanethiols and BM is in good agreement with previous reports, and consistency in the relative conductance of alkanethiols with different chain lengths is achieved. We find that the conductance across a thiol-based, semicovalent junction in UHV-STM is low compared to an analogue junction with a covalent contact to both ends of the molecule (as for dithiols in a break junction or conductive AFM setup), which is attributed to the lower conductance of the physical contact between STM probe and self-assembled monolayer. Acknowledgment. The authors thank Prof. Christian Scho¨nenberger, Dr. Nikolaus Knorr, and Dr. Bjo¨rn Lu¨ssem for helpful discussions. Supporting Information Available: Detailed information on the relaxed structure of benzyl mercaptan, octanethiol, and decanethiol from DFT calculations. Photoelectron spectroscopy results from benzyl mercaptan and thiophenol monolayers on Au. Contact angle goniometry of benzyl mercaptan, octanethiol,
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