Resolving the 3D Orientation of Terphenylthiol Molecules on Noble

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Resolving the 3D Orientation of Terphenylthiol Molecules on Noble Metals with Kelvin Probe Force Microscopy Niklas Biere,*,† Sascha Koch,‡ Patrick Stohmann,‡ Volker Walhorn,† Armin Gölzhäuser,‡ and Dario Anselmetti*,† †

Experimental Biophysics & Applied Nanoscience, Faculty of Physics, and ‡Physics of Supramolecular Systems and Surfaces, Faculty of Physics, Bielefeld University, 33615 Bielefeld, Germany

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S Supporting Information *

ABSTRACT: The local work function is an invaluable feature for the specific analysis of the influences of atomic and molecular nanostructures on each other as well as the underlying surface. Adsorbate molecules can modify this parameter by introducing an electrical dipole moment, which affects the local contact potential. This can be accessed by Kelvin probe force microscopy (KPFM). In this paper, we demonstrate, by combining highly resolved topographic atomic force microscopy (AFM) data with the simultaneously acquired local work function signal, how each of these channels yield one angular coordinate, resulting in the three-dimensional determination of surface molecular dipole orientations. We studied the adsorption of terphenylthiol (TPT) self-assembled monolayers on Au(111) and Ag(111), as it is relevant in the light of electron radiation-induced transformation to carbon nanomembranes. We present noncontact AFM data combined with frequency-modulated KPFM in ultrahigh vacuum at room temperature without any kind of deliberate tip functionalization. Our results show a surface coverage-dependent Langmuir-like evolution of phases with domains of flat lying as well as with upright molecular arrangements. Whereas we found an almost complete vertical orientation on silver, the orientation on gold was found to be tilted, corresponding to sp- and sp3hybridized bond angles, respectively.

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INTRODUCTION

molecules and allows to be fully mapped within a single scan that does not require additional measurements. This additional information channel can be especially useful in the focus of molecular interactions, as such in a cross-linking process. Here, understanding of the involved moieties of the aromatic precursor molecules is crucial when it comes to tailoring surface properties. Therefore, self-assembled monolayers (SAMs) were chosen as a model system for producing artificial nanometer thin membranes. Specific aromatic SAMs can be turned into these so-called carbon nanomembranes (CNMs) by exposure to various types of radiation, for example, electrons or extreme ultraviolet light.11,12 Outstanding results were achieved by using approximately 1 nm thin CNMs made from SAMs of 1,1′:4′,1″-terphenyl-4-thiol (TPT) as molecular sieves for gas and water permeation.13 With regard to this application and beyond, the specific choice of the precursor molecules as well as the control of the epitaxial growth for the SAM formation process allows a subtle tuning of the intrinsic properties as, for instance, the selectivity and permeance of these CNM based nanofilters. Here, we show that KPFM can serve as a method to evaluate order and conformational changes during transition from SAMs to CNMs. It can also be of interest for applications and

When it comes to the multitude of surface analyzing techniques, atomic force microscopy (AFM) already offers access to a large variety of surface properties, such as friction, elasticity, or chemical reactivity.1,2 In combination with Kelvin probe force microscopy (KPFM), this palette can be even more broadened, giving access to properties like the local work function. This is based on the principle found by Kelvin,3 where an electrical connected tip of a dynamic force microscope and the sample form an ac-biased capacitor. Originated in different materials of the tip and sample, a contact potential difference (CPD) is caused, which reflects the work function difference between the tip and the underlying surface area.4 However, this technique was applied to gain information of monolayers not only on large scales5,6 but also at molecular resolution.7−9 Because local charge transfers contribute to the work function, it can be modified by introducing adsorbates that possess dipole moments.10 Moreover, the change in work function depends on the angular orientation and can thus be used to obtain in-depth information about molecular arrangements. Because only the perpendicular fraction of the surface dipole influences the work function, it can be used to determine their molecular inclination. In addition, from highresolution topographic images, the angular alignment parallel to the surface can be obtained. This set of two angular coordinates equals the three-dimensional orientation of the © XXXX American Chemical Society

Received: May 25, 2019 Revised: July 16, 2019

A

DOI: 10.1021/acs.jpcc.9b04982 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) Illustration showing the “Langmuir-like” evolution of molecular phases, with underlying topographic images of SAMs from TPT. (b) Dissolving the herringbone reconstruction on Au(111) by loosely lying packed phases (false colored in green, original in Supporting Information), scale bar = 50 nm (df = −15.0 Hz, A = 17.6 nm, f 0 = 286.17 Hz). (c) Nucleation of islands of closely packed phases (yellow) on Au, scale bar = 50 nm (df = −10.9 Hz, A = 18.6 nm, f 0 = 292.18 Hz). (d) Increasing island growth of closely packed phases on Ag(111), scale bar = 50 nm (df = −13.3 Hz, A = 17.7 nm, f 0 = 301.14 Hz). (e) Complete surface coverage on Au with closely packed domains, scale bar = 50 nm (df = −9.0 Hz, A = 17.7 nm, f 0 = 292.07 Hz). (f) Structure and dimension of 1,1′:4′,1″-terphenyl-4-thiol. (g) Close-up of both molecular phases on Au, shaded to emphasize corrugations, scale bar = 5 nm (df = −15.3 Hz, A = 17.6 nm, f 0 = 286.13 Hz). (h) Close-up of corresponding arrangements on Ag, shaded, scale bar = 5 nm (raw data available in the Supporting Information) (df = −19.0 Hz, A = 17.6 nm, f 0 = 301.13 Hz). (i) Associated line sections of (g,h) to indicate lattice parameters.

(RHK UHV 7500, RHK Technology, Troy, USA) in combination with dedicated R9 controller electronics. Images were acquired at room temperature in a UHV chamber with a base pressure of 5 × 10−11 mbar. The AFM was operated in the noncontact mode, while the KPFM data was recorded simultaneously in the frequency modulation mode.4 The ac bias frequency was 1.5 kHz with an oscillation amplitude of 1 V, applied to the sample while the tip was grounded. Single-crystalline silicon cantilevers of the type Tap300Al-G (Budget Sensors, Sofia, Bulgaria, ∼40 Nm−1, ∼280 kHz, Q ≈ 10 000, n-doped, 0.01−0.025 Ω cm−1) were sputtered with Ar+ ions (8 × 10−7 mbar) at 680 eV for 120 s before use. Postprocessing of the data, including drift correction following calculations from Rahe et al.,22 and quantitative analysis occurred in the open-source software package Gwyddion.23 Sample Preparation. Commercial gold and silver substrates (Georg Albert PVD, Silz, Germany) consist of a 300 nm layer of gold and silver, respectively, epitaxially grown onto sheets of mica, with the surface exposing the (111)crystallographic orientation. The Au/mica substrates were precleaned in ethanol and water and flame annealed in air. This procedure leads to appropriately large terraces (100−300 nm) of atomic order. In addition, Ag/mica was directly transferred from argon storage to UHV. Inside, both types of substrates were sputtered for 10 min with Ar+ ions at 8 × 10−7 mbar and subsequently annealed at 523 K for 2 h, which was repeated for a second cycle before SAM deposition, in order to avoid any contamination.

experiments where SAM precursors are specifically constructed with a desired dipole moment.14,15 A standard procedure to grow different SAMs for CNM formation on crystalline gold substrates was described by Angelova et al.,16,17 where SAM growth is induced by immersing a substrate into a solution of molecules with thiol moieties. The SAM preparation from solution is the prevalent procedure due to its ease and feasibility. As a complementary approach, the preparation from vapor deposition offers a much more defined control over the adsorbates and substrates.18 In particular, the behavior of terphenylthiol on Au(111) was thoroughly investigated and is well described in the literature,19−21 whereas the growth on other types of substrates is less common. In fact, we even found that the adequate choice of the substrate influences the self-assembly of our precursor molecules much more than expected in previous works, even with isoelectronic noble metal surfaces such as gold and silver. In this work, we present an approach of utilizing AFM as well as KPFM data to achieve information about the molecular arrangement of molecules in a SAM layer. We investigated by variable temperature UHV AFM the well-known system of TPT on Au(111) and extended our research then to TPT on Ag(111).

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EXPERIMENTAL SECTION Scanning Probe Imaging. For recording synchronously AFM and KPFM data, we utilized a commercial SPM system B


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Figure 2. TPT in the closely packed order (S1) on Au(111), upper row (a−c), scale bars 5 (a), 2 nm (b) (df = −16.7 Hz, A = 17.7 nm, f 0 = 292.06 Hz), marked with hole defects; S1 on Ag(111) (df = −28.6 Hz, A = 28.3 nm, f 0 = 282.71 Hz), middle row (d−f), scale bars 5 (d), 2 nm (e); and S2 on Ag(111) (df = −19.0 Hz, A = 17.6 nm, f 0 = 301.13 Hz), lower row (g−i), scale bars 10 (g), 4 nm (h). The dashed line indicates a step edge. Left: nc-AFM topography. Middle column: false color-marked domains of different orientations and unit cells labeled with measured distances. Right column: model of unit cells and theoretical lengths.

TPT molecules were purchased from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany) and were subsequently sublimated in order to maximize the purity before evaporation on the sample substrates using an effusion cell (MBE, Dr. Eberl Komponenten, Weil der Stadt, Germany). The evaporation procedure was carried out for 3 min at 343 K, which was controlled by a quartz microbalance. After successful deposition, the samples were directly transferred into the analysis chamber (5 × 10−11 mbar) without breach of vacuum.

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In the following, the observed structures are categorized into standing and lying phases, and their properties, superstructures, and equivalents on two different substrates (Ag and Au) will be discussed. Closely Packed Phases (S1) on Au and Ag. TPT on gold has been previously investigated, albeit mainly by scanning tunneling microscopy (STM).19−21,27 In accordance with these works, we found TPT molecules on gold forming a closely packed herringbone arrangement (0.26 nm2 per molecule), consisting of pairs of two molecules visibly rotated toward each other (Figure 2, upper row). All observed similar domains display prominent features, namely, corrugated triplet rows that run along the ⟨12̅1⟩-direction and its respective 60° equivalents of the underlying gold lattice, with a height difference of roughly 16 pm and line distances of 1.54 nm. Upon further investigation, rows (Figure 2a, green lines) and pair stacking directions (yellow lines) of different phases are misaligned by a deviation of 2° (Figure 2a), compared to pure hexagonal phases. The Moiré-like repeating corrugations lead to the notion of a ( 37 × 2 3 )R 30° unit cell (S1, Figure 2c). There, the top corrugations are attributed to molecules sitting on top of a bridge position between two gold atoms, while others reside in the hollow positions between three substrate atoms. This also explains well the angular discrepancies in stacking directions with an angle of 64.7° of the unit cell. These corrugated rows are well-known features with other aromatic thiols28,29 and have even been observed with TPT on Au(111). There have been attempts to interpret

RESULTS AND DISCUSSION

Utilizing high-resolution AFM noncontact topography imaging, several packing orders of different lateral dimensions and heights could be identified. The growth process on Au(111) starts with locally dissolving the herringbone reconstruction (Figure 1b),24 the formation of molecular phases with lower lattice constants (Figure 1c, depicted green), nucleation of taller islands with higher molecular density (Figure 1d, colored in yellow), and complete high-density surface coverage (Figure 1e). Often, these areas are in coexistence with each other even on smaller scales. A dependence of the island formation on the degree of coverage is implied, but a coherent correlation proves to be difficult due to inhomogeneities of the deposition profile. Such a relation has been observed several times, where the forming SAM obeys a “Langmuir-like evolution,”18,25 where the molecules undergo different phase stages, especially between “lying” and “upright” arrangements.26 C


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Figure 3. TPT in loosely packed order (L1) on Au(111), upper row (a−c) (df = −22.5 Hz, A = 19.1 nm, f 0 = 286.13 Hz), with defect row in the herringbone pattern (yellow line), and on Ag(111), lower row (d−f) (df = −20.7 Hz, A = 19.1 nm, f 0 = 301.14 Hz), with highlighted stacking mismatch (yellow lines). Left: nc-AFM topography. Middle: Contact potential difference. Right: Model of the stacking order in regard to the atomic order of the substrate. Scale bars = 5 nm.

(Figure 2g−i). These islands also displayed a difference in contact potential, which will be addressed in a later section. Obviously striking defects (Figure 2a, arrows), which appear here as holes or missing molecules, are a known characteristic. They are present in various published data from AFM and STM experiments performed on TPT on gold20,21 but are rarely discussed so far. In our data, they appear mobile (see the Supporting Information) with a hopping rate that seems to be strongly dependent on the local temperature, where measurements at 93 K exhibit freezing out of these dynamics. In previous works, they were explained as individual molecules with deviant spatial orientation.35 Because surface diffusion of the atoms in the outermost atomic layer of metals occurs constantly, incorporated sulfur atoms could block binding spots for further thiol molecules and thus create a vacancy in the SAM layer. The hopping behavior of these defects highly coincides with observations of others.36 Particularly, they do not occur in the system of TPT on silver. Nevertheless, it shall be noted that the islands with larger relative height can easily be mistaken for another emergence of thiolate adsorbates on gold. There, excess surface atoms allegedly get lifted out of the herringbone unit cell to diffuse and eventually be kinetically frozen by adsorbate growth into an additional gold surface layer.33 However, for our data, this seems to be very unlikely because the CPD signal differs significantly from the surrounding area. Also, the relative height of 0.64 nm with respect to the surrounding lying phase coverage does not match the height of a gold monolayer step (0.2 nm). In addition, the occurrence near step edges is not compatible with this model of gold island formation. Loosely Packed Phases (L1) on Au and Ag. In addition, we discovered areas with striped patterns of smaller lattice constants and herringbone-like arrangements, similar to systems such as PTCDA37 and PTCDI,38,39 as shown in Figure 3. The distances in lateral direction of 1.4 nm of these rows fit well to the TPT molecular length of 1.43 nm.21 Because of the lower relative height in regard to the islands of upright standing molecules [hS1 − hL1 = 0.64 nm on Au, 0.63

them with a plethora of different arrangements, including somewhat debatable concepts such as an incommensurable unit cell.20,30 It should be noted that a closely packed ( 3 × 2 3 )R 30°-structure is commonly found with numerous aromatic thiols25,31−33 and also with TPT.20,27 There, the findings suggested that this arrangement occurs exclusively with preparation from solvents and not when deposited from the gas phase.18,28,34 Upon using silver, exposing the (111)-surface, as a substrate for TPT adsorption, again, an intermediate coverage with domains of varying molecular densities was observed (Figure 2, lower row). We also found distinct rows, of different nature than on gold, following the direction in which pairs of rotated molecules are stacked (Figure 2d, yellow lines). They run along the ⟨11̅ 0⟩-direction and occasionally show slight irregularities, as they randomly vary in row distance and corrugation height around 5 pm. Similar to the molecular phases on gold, the assumed hexagonal stacking directions (Figure 2d, green lines) are not aligned in 120° to the respective counterparts in other domains. They comply with a (2 × 3 3 )-superstructure, as depicted in Figure 2f. The respective unit cell covers two similar molecular pairs, but is in itself not translational symmetric, due to a geometric offset between the hollow positions along lattice directions. The elongated unit cell forces the molecules to experience anisotropic attractions and to grow in the direction of the smaller periodicity.31 This sporadically causes individual wider grooves, which are randomly arranged. The molecular density on Ag (0.23 nm2 per molecule) differs slightly from the one on Au(111) (0.26 nm2 per molecule). In addition, domains of the closely packed order (S2) with a difference in corrugation height of approximately 270 pm relative to each other could be observed, where no step edge (Figure 2g, dashed line) was involved. This difference in height is accompanied by a more dense corrugation (0.22 nm2 per molecule) with a ( 3 × 21 )R 30° unit cell of almost rectangular symmetry D


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Figure 4. Comparison between islands of differently ordered phases on gold (a−c), scale bars = 20 nm (df = −10.9 Hz, A = 18.6 nm, f 0 = 292.18 Hz) and silver (d−f), scale bars = 50 nm (df = −8.2 Hz, A = 17.5 nm, f 0 = 286.13 Hz). Left column: topography. Middle column: CPD. Right column: line sections of the two previous data channels.

also been observed with 2D layer systems such as graphene. The magnitude of this effect can be as much as 0.12 nm.44 It has been shown that the mechanism behind this is indentation as well as pulling the layer away from the surface by the probing tip.45 Because the bonding mechanism of the adsorbate molecules here is covalent, there is much less freedom of movement in contrast to the weaker van der Waals bonded 2D layers and can thus be neglected. The CPD VCPD is defined as the difference in work function of the tip ϕtip and the sample ϕsample

nm on Ag, (see Figure 4)] and the accompanying area per molecule of 1.4 nm2 (on Au, 0.9 nm2 on Ag), it appears as if these phases consist of almost flat lying molecules. The large intermolecular distances and therefore low interaction, which correspond to a (4 × 3 3 ) unit cell on Ag and (10 × 2 3 ) on Au, make these arrangements susceptible for stacking faults, small domains with frequent symmetry changes and tilted unit cell variants (Supporting Information). This leads to deep streaks, also visible in CPD, where mismatches allow molecules to occupy more space and assume an even lower angle. Lying phases of TPT SAMs were already proposed,19 as well as coexistence of phases with differing angular arrangement with similar compounds.26,40,41 In addition, the standing and lying phases were shown to dynamically transition into each other, electrostatically induced by STM scans (Supporting Information). Our CPD maps exhibit molecular resolution, with an increase in CPD at the intermolecular spacings, where the bare substrate surface is visible without influence of the adsorbates. Nevertheless, the exact contrast mechanism for KPFM on molecular scales is still under discussion.42 Another central observation is displayed in Figure 4. Here, a significant difference in CPD between the lying (L1) and standing (S1) phases of ϕS1 − ϕL1 = −440 mV (on Au) to −490 mV (on Ag) is measured. Further, a small difference can also be made between two standing phases on Ag, with a difference in CPD of only ϕS1 − ϕS2 = +140 mV, which in turn differ slightly in height (0.27 nm). However, concerning height measurements in interaction with electrostatic potential changes involved, several effects have to be considered. First, tip contamination with an adsorbate molecule can lead to an erroneous reading of topographic features.43 Therefore, repeated experiments with different tips were conducted and all yielding similar height readouts. Second, small islands might cause a false reading of CPD values because a comparably large tip dominates the readout due to electrostatic interactions.42 To circumvent this averaging effect, the CPD values were only measured over locations significantly far away from step edges or if possible on samples with homogenous surface coverage. Actual mechanical deformations of the observed layers have

eVCPD = ϕsample − ϕtip

In order to determine the work function of the tip, it was calibrated against pristine silver (ϕAg = 4.5 eV, ϕAu = 5.25 eV),46 yielding ϕtip = 4.9 eV. In addition to the image measurements (Figure 4), subsequent CPD data were acquired using the very same tip and were double-checked on silver to exclude further influences from tip changes. Furthermore, the intrinsic local work function of a surface is strongly modulated by the presence of adsorbate molecules ϕsample = ϕSAM + ϕmetal

The absolute measured work functions are displayed in Figure 5a. Through the upright standing molecule domains, the overall work function is lowered (−0.4 to −0.6 eV), whereas the lying phases induce a slight increase (+0.01 to +0.07 eV) with respect of the pristine metal substrate. This implies a distinct correlation between layer height, angular orientation, and local work function, respectively. Determination of Inclination Angles of TPT Molecules. KPFM has been used for thickness determination before, but rather in the number of layers,47−49 as well as gauging of molecular dipole moments.50−52 A related correlation between the inclination angle of ordered molecules and work functions has only been proposed.53,54 In the following section, the angular tilt is determined from our CPD measurements. The dipole moment of a longitudinal extended molecule adsorbed to the surface, precisely the part in surface normal E


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These contributions can be treated separately because the rest of the molecule does not influence the charge transfer between the surface and the bonding moiety.55 The plane can be tilted by an angle φ from surface normal, connected to the sulfur atom as its joint10 μφ = μTPT ·cos φ

This is the same tilt angle φ as for the geometric height hφ = hTPT·cos φ. The work function of the whole SAM layer relates to the surface dipole also by packing density, or in other words, the area per molecule A (which gets determined by highresolution topographic images of the individual phases) ΔW = ϕSAM =

eΔμ ε0A

The effective dipole moment per molecule can then be calculated from the relative work function shift Δμ = ε0 ·A ·VCPD

The calculated effective vertical dipoles of both experiments are normalized by subtracting the theoretical sulfur−substrate dipole. When plotted against the geometric height hφ, the two data sets are shifted relative to each other to form linear dependence. A linear function is then fitted to all five data points because the angular dependence of the dipoles should be separate from influences of the substrate. After fitting, the functions are offset with the respective binding dipoles and can be extrapolated to determine the value of the total molecular dipole, as well as the involved angular inclinations. Taking into account the sole dipole of the sulfur−metal bond, which was determined by Rusu and Brocks to μchem = +0.46 D for Ag−S55 and μchem = +0.02 D for Au−S,56 results in a total molecular dipole moment of μTPT = −0.83 D for one unbound TPT molecule. There is also a residual dipole moment of roughly +0.02 D for completely flat lying adsorbates, possibly due to a quantum mechanical repulsive “push-back” effect of electrons in the interfacial metal substrate layers.57,58 The defined molecular area as well as all involved work functions can be found in Table 1. As for flat lying molecules, the contribution of the molecular dipole is negligible according to the angular model, so the CPD change for lying phases originates mostly from the sulfur-substrate dipole. Thus, it is oriented in such a way that it increases the work function, and even more for the stronger Ag−S dipole. The TPT molecular dipole moment, which is unrelated to the substrate binding, appears to have the opposite polarity of the sulfur−metal bond, so that the overall work function decreases. This corresponds to a higher electronegativity of the sulfur atom with respect to

Figure 5. (a) Recorded local work functions of different phase orders and substrates. (b) Simplified model of dipole moment contributions of an adsorbate molecule. (c) Calculated dipole moment per molecule plotted against the geometric molecule height (without sulfur− substrate bond length), caused by varying inclination angles of adsorbed molecules relative to the surface normal. The error is determined by the root-mean-square of irregularities and standard deviation over multiple values. For number values, see Table 1.

direction, depends on the inclination angle of the adsorbates. This in turn is determined by factors such as geometry and coverage density. A simplified model of a dipole used in the following calculations is depicted in Figure 5b, consistent with a commonly applied “two-layer dipole model”.6 The molecular dipole Δμ is split up into two components, the sulfur-substrate bond μchem and the effective aromatic moiety μφ, which is approximated as a flat plane, with the overall dipole moment μTPT Δμ = μφ + μchem

Table 1. Calculated Work Functions and Molecular Areas for Each Observed Phase number of molecules per unit cell

area per molecule, counted [nm2]

(4 × 3 3 )

2

0.88

0.27 ± 0.09

68.25 ± 1.77

−0.42 ± 0.05

(2 × 3 3 )

4

0.23

−0.26 ± 0.03

35.47 ± 5.36

−0.56 ± 0.05

( 3 ×

3

0.22

−0.36 ± 0.03

(10 × 2 3 )

2

1.40

0.04 ± 0.19

90.0 ± 1.06

( 37 × 2 3 )R 30°

6

0.26

−0.30 ± 0.03

63.43 ± 2.08

geometric layer height hφ [nm]

absolute work function [eV]

work function change [eV]

Ag L1

0.0 ± 0.04

4.57 ± 0.04

+0.07 ± 0.04

Ag S1

0.63 ± 0.08

4.08 ± 0.05

Ag S2

0.90 ± 0.10

3.94 ± 0.05

Au L1

0.0 ± 0.03

5.26 ± 0.05

+0.01 ± 0.05

Au S1

0.64 ± 0.05

4.95 ± 0.05

−0.43 ± 0.05

unit cell

21 )R 30°

F

dipole moment Δμ [D]

inclination angle φ [deg]

0.0 ± 21.25


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The Journal of Physical Chemistry C silver (ENS = 2.58, ENAu = 2.54, ENAg = 1.93, Pauling scale59), consistent with theoretical models.55 The results for the angular calculations are depicted in Figure 5c. The TPT molecules in the L1 phase on gold lie almost completely flat while standing nearly completely upright in the S2 phase on silver. Even though here the geometrical conversion causes small height irregularities to lead to a large error in angle. According to this model, the lying phase L1 on silver is characterized by molecules tilted by 68° from surface normal, which is close to the angle where the bonds of the sulfur atom were completely sp3-hybridized. A phase with sp3hybrid angles has been proposed for TPT on gold,19 and is generally accepted as the prevalent bond configuration for sulfur on gold.31,60−63 Intriguingly, there are indications that sulfur on silver energetically prefers a more sp-hybridized bonding scheme,60 which would account to straight upward standing molecules, in excellent agreement with our findings. With respect to an energetically unfavorable bonding angle of flat lying molecules, a commonly mentioned binding motif is the lifting of singular atoms from the substrate, which in turn offer two bonding sites.64−66 Several other groups have tried to determine tilting angles of TPT molecules with different methods. The most common scheme is the geometrical estimation from projected area in STM images. This yields angles from straight upward (0°−5° and 10−20° from surface normal-tilted) molecules21 to 12° (close-packed phase), 33−49° (incommensurate unit cell),20 and 51−54°.30 There is a broad variation in these results, which is especially striking because all of these have explicitly been conducted on Au(111). Hence, this is a fairly imprecise approach because it assumes an arbitrary effective density of the layer and neglects influences of intermolecular interactions and bond properties. Measurements with NEXAFS yielded 28° tilting angle27 and molecular simulations lead to angles from

Resolving the 3D Orientation of Terphenylthiol Molecules on Noble

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