OddâEven Effect in Molecular Packing of Self-Assembled Monolayers

Article pubs.acs.org/JPCC

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

Odd−Even Effect in Molecular Packing of Self-Assembled Monolayers of Biphenyl-Substituted Fatty Acid on Ag(111) Anna Krzykawska, Monika Szwed, Jakub Ossowski, and Piotr Cyganik* Smoluchowski Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland ABSTRACT: Self-assembled monolayers (SAMs) of the homologous series of biphenyl-substituted fatty acids on Ag(111) in the form of (C6H4)2−(CH2)n−COO/Ag (BPnCOO/Ag, n = 1−4) were studied using infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). The combination of spectroscopic (IRRAS and XPS) and microscopic analyses (STM) revealed that depending on the parity of the parameter n, which defines the length of the short aliphatic linker, two types of structures are formed by the BPnCOO/Ag. For n = even, highly ordered and stable SAMs are formed in a very short time. For n = odd, the respective monolayers have a disordered liquidlike structure with more canted orientation of the molecular backbone and the anchoring carboxylic group, which results in lower packing density and film thickness compared to the even-numbered SAMs. By comparing obtained results with former odd−even effects reported for analogous SAMs based on thiols and selenols, a common, qualitative model relating the odd−even effect to the monolayer stability and structure is discussed. Our results demonstrate that for BPnCOO/Ag, this odd−even effect is particularly strong and fully controls the ability of molecules to form highly ordered structures. This observation seems to be of key importance for the design of SAMs based on the carboxylic group, which, for correctly designed molecules, forms much better two-dimensionally ordered structures compared to commonly used thiols.

1. INTRODUCTION Aromatic self-assembled monolayers (SAMs)1 offer the potential to assemble molecular devices with defined electronic properties, thus providing a very promising prototype system for molecular electronics studies.2−4 However, real application of SAMs requires a high level of control over their structure and properties to reduce the concentration of defects and to optimize functionality and stability in the final electronic device. The structure and properties of SAMs are the result of a complicated interplay of intermolecular interactions, molecule−substrate bonding, and lattice mismatch between the most favorable molecular lattice and the inorganic substrate. The complexity of the organic−inorganic interface structure created upon monolayer formation is the main obstacle in predicting the structure and properties of SAMs using theoretical calculations. Therefore, the general strategy for the rational design of SAMs must be based mainly on an experimental approach that includes a systematic modification of the chemical composition of the SAM constituents and detailed investigation of the structure and properties of the resulting monolayer. This strategy has been realized in recent years using a model system based on a homologous series of biphenyl-substituted alkanethiol or alkaneselenol SAMs on the Au(111) and Ag(111) substrates in the form of CH3−(C6H4)2−(CH2)n− S(Se)/Au(Ag) (n = 1−6) described further by the acronym BPnS(Se)/Au(Ag). These SAMs formed on two different substrates (Au or Ag) combine the biphenyl moiety and an aliphatic linker of a variable length inserted between this © XXXX American Chemical Society

moiety and two different binding groups (S or Se) to probe the influence of the intermolecular interactions, the binding group atom, and the type of substrate on the final structure of the monolayer. The series of spectroscopic5−8 and microscopic9−11 experiments demonstrated that, for both types of substrates and the binding atoms, the BPnS(Se)/Au(Ag) exhibits a pronounced structural odd−even effect as a function of the parameter n that defines the length of the short aliphatic link. For the Au(111) substrate, this odd−even effect results in a higher packing density of more upright-oriented molecules for odd-numbered members of the BPnS(Se)/Au series. In contrast, for the Ag(111) substrate, this odd−even effect for the BPnS(Se)/Ag series is reversed as a consequence of a different value of the substrate−S(Se)−C bond angle that is preferred for the Au(111) and Ag(111) substrates and amounts to ∼104° and ∼180°, respectively. Moreover, further analysis revealed that these structural odd−even effects are accompanied by the respective variation of the film stability toward electrochemical desorption,12,13 exchange by other molecules capable of forming SAMs on the same substrate,14−16 electron irradiation,17 ion induced desorption,16,18 and thermally induced phase transitions.19−21 As a result of these findings, a qualitative model has been proposed20 in which the relationship between the SAM structure and its stability is determined Received: November 1, 2017 Revised: November 29, 2017 Published: December 11, 2017 A

DOI: 10.1021/acs.jpcc.7b10806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

were used as received without further purification. The BPnCOOH molecules (C6H5−C6H4−(CH2)n−COOH) with n = 1−2 and 3−4 were purchased from Alfa Aesar and Wako, respectively, and used without further purification. For the scanning tunneling microscopic (STM) measurements, the Ag(111) substrates were prepared by evaporation of ∼100 nm of silver (rate = 0.7 nm/s, temperature ∼ 260 °C) on mica substrates. For the X-ray photoelectron spectroscopic (XPS) and the infrared reflection absorption spectroscopic (IRRAS) analyses, the Ag(111) substrates were prepared by evaporation of approximately 100 nm of silver (rate = 0.7 nm/ s, room temperature) onto polished single-crystal silicon (100) wafers (ITME, Warsaw) primed with 3 nm of chromium to improve the adhesion of the silver layer. For the BPnCOO/Ag formation, the Ag(111) substrate was immersed for 5 min at room temperature in 1 mM nhexadecane and tetrahydrofuran (1:1) solution of the respective compound. After incubation samples were removed from the solution, rinsed with pure solvent, and dried under nitrogen. Although our former STM analysis demonstrated22 that structures of these types of SAMs remain stable even after exposure to the ambient conditions for 3 months, all spectroscopic and microscopic analysis for the current studies was performed immediately after film formation. 2.2. STM. STM measurements were carried out in air at room temperature using a low current NanoScope MultiMode 8 microscope from Bruker. In all cases, the tips were prepared mechanically by cutting a 0.25 mm Pt/Ir alloy (8:2, Goodfellow) wire. The data were collected in constant current mode using tunneling currents in the range 2−35 pA and a sample bias between 0.2 and 1 V (tip positive). 2.3. IRRAS. IR reflection absorption spectroscopy measurements were performed with a dry-air-purged Thermo Scientific Fourier transform infrared (FTIR) spectrometer model Nicolet 6700 equipped with a liquid-nitrogen-cooled MCT detector. All spectra were taken using p-polarized light incident at a fixed angle of 80° with respect to the surface normal. Spectra were measured at a resolution of 2 cm−1 and are reported in absorbance units A = −log(R/R0), where R is the reflectivity of the substrate with the monolayer and R0 is the reflectivity of the reference. Substrates covered with a perdeuterated hexadecanethiolate SAM were used as a reference. 2.4. XPS. X-ray photoelectron spectroscopy (XPS) measurements were performed with a photoelectron spectrometer equipped with a hemispherical analyzer VG SCIENTA R3000. The spectra were taken using a monochromatized aluminum source Al Kα (E = 1486.6 eV), MX650 VG Scienta. The base pressure in the analytical chamber was 5 × 10−9 mbar. The spectral acquisition was carried out in normal emission geometry with an analyzer energy step of 0.15 eV. The overall resolution of the XP spectra (based on the Au 4f7/2 peak analysis) was ca. 1.14 eV. The inelastic background was subtracted using the Shirley method, and the photoemission peaks were fit using convolution of the Gaussian and Lorenzian functions with adjustable weights.

by either the cooperative or competitive way in which factors determining the energetics of a SAM enter into the energy balance. The key factors determining the energy of the system in this model are the Au(Ag)−S(Se)−C bending potential, the density of the S(Se)−Au(Ag) bonds (surface coverage), and the intermolecular interactions. Whereas for the oddnumbered BPnS(Se) SAMs on the Au(111) substrate, all of these factors act cooperatively and form a stable structure with higher packing density, for the even-numbered members of the series, the Au−S(Se)−C bending potential opposes the other two factors and results in the formation of a less stable film with a lower packing density.11,20 For the Ag(111) substrate, this odd−even effect in film stability is reversed following the reverse of the odd−even effect in the film structure upon Au → Ag substrate modification.16 The reverse of the odd−even effects in SAMs upon substrate modification indicates that the exact bonding configuration of the molecule with the substrate, i.e., the Au(Ag)−S(Se)−C bending potential, remains the key element of this phenomenon that has a significant contribution to the overall energetics of the BPnS(Se)/Au(Ag) monolayers. In the current paper, we would like to further test this hypothesis by investigating the impact of substitution from the S or Se binding groups, investigated so far, into the carboxylic group. This idea is based on our recent microscopic and spectroscopic analysis22 of BP2S/Ag and BP2COO/Ag SAMs, which show the formation of very similar structures with superior two-dimensional (2D) ordering in the case of the BP2COO/Ag system. These data indicate that, for SAM formation on the Ag(111) substrate, the carboxylic binding group could be a direct alternative for the commonly used thiol group.22 Following these observations, in the current paper, we conduct systematic spectroscopic and microscopic analysis of the BPnCOO/Ag series (C6H5−C6H4−(CH2)n−COO/Ag, with n = 1−4), schematically presented in Figure 1. To keep

Figure 1. Schematic composition of SAMs used in this study.

the overall length of the chemisorbed BPnCOO molecules comparable with the thiol (BPnS) and selenol (BPnSe) analogues analyzed earlier, the terminal CH3 group in the biphenyl moiety has been omitted.

3. SPECTROSCOPIC ANALYSIS (IRRAS AND XPS) The summary of the IRRAS data obtained for the BPnCOO/ Ag series is presented in Figure 2. As a representative example of this analysis, the spectrum obtained for BP3COO/Ag is shown (Figure 2a), where all characteristic stretching vibrational bands are indicated. The band at ∼1400 cm−1 corresponds to the symmetrical carboxylate stretching

2. EXPERIMENTAL METHODS 2.1. SAM Preparation. All solvents were obtained from commercial sources. n-Hexadecane (99.9%), tetrahydrofuran, and absolute ethanol (99.8%) were obtained from Acros Organics, Sigma-Aldrich, and POCH, respectively. Reagents B


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Figure 3. Overview of XPS spectra of (a) Ag 3d, (b) C 1s, and (c) O 1s signals for the BPnCOO/Ag series.

Figure 2. (a) IRRAS spectrum of BP3COO/Ag with characteristic absorption bands. (b−e) Intensities of bands at ∼3060, ∼1490, ∼1400, and ∼1007 cm−1 presented as a function of the parameter n (see text for more details).

(νs(COO−)), which, according to previous studies,23,24 indicates bidentate bonding of the molecules to the Ag(111) substrate. The aromatic moieties of the BP2COO molecule can be identified by the bands visible at ∼3060, ∼3035, ∼1490, ∼1040, and ∼1007 cm−1.25 The intensity of the C−H stretching vibrational bands characteristic of the short aliphatic chain is very low in most cases (see the band at ∼2918 cm−1 which can be assigned to νa(CH2)), as it was reported in our recent experiments22 for the BP2COO/Ag monolayer. The weak band at ∼1224 cm−1 can be assigned to the progression of methylene wagging modes (ω(CH2)) indicating all-trans conformation of the alkyl chain.24 In Figure 2b−e, the intensities of the most intense bands at ∼3060, ∼1400, ∼1490, and ∼1007 cm−1, respectively, are presented as a function of the parameter n = 1−4 for the BPnCOO/Ag series. All analyzed signals exhibit pronounced odd−even oscillations. The phase of these oscillations is the same for all signals with higher values obtained for the even-numbered monolayers of the BPnCOO/Ag series. We note, at this point, the odd−even change in the intensity of the absorption bands analyzed by IRRAS indicates not only the possible modification of the packing density of molecules on the surface but also change in their orientation relative to the metal substrate due to the surface selection rules.26 The XPS data are summarized in Figure 3 and analyzed in Figures 4 and 5. The Ag 3d signals are presented in Figure 3a for all members of the BPnCOO/Ag series. The calculated

Figure 4. Intensities of (a) Ag 3d, (b) C 1s, and (c) O 1s XPS signals presented as a function of the number n of methylene groups for the BPnCOO/Ag series.

intensities of these signals are shown in Figure 4a and exhibit general decay with an increasing number n of methylene units, which is, however, overlaid with some odd−even modulation of signal intensities. Considering the attenuation of the Ag 3d photoelectron signal by the monolayer, this observation indicates the odd−even effect in the film thickness (vide infra). The analysis of the C 1s signals is presented in Figure 3b and consists of two characteristic components. The first, and dominant, component at the binding energy (BE) of ∼284 eV can be assigned to the aromatic part of the molecule, as it was C


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where d2 − d1 can be taken directly as a difference in the effective film thickness shown in Figure 5a. Taking the BP2COO/Ag as a reference system with the area per molecule determined by the STM analysis as s2 ∼ 0.283 nm2/molecule (vide infra), the relative area per molecule for all members of the BPnCOO/Ag series could be calculated. As documented in Figure 5b, the calculated area per molecule also exhibits an odd−even effect that has a reversed phase compared to the odd−even effect in the effective film thickness.

4. MICROSCOPIC ANALYSIS (STM) The STM data for the BP2COO/Ag sample are summarized in Figure 6. Large area analysis shown in Figure 6a reveals formation of depressions with diameters of ∼1−10 nm. As documented by the respective height profile C presented in Figure 6b, the depth of these depressions corresponds to the

Figure 5. Film thickness (a) and area per molecule (b) calculated from XPS data as a function of the number n of methylene groups for the BPnCOO/Ag series (see text for more details).

reported earlier6,8 for the BPnS(Se)/Ag(Au) SAMs. The second, high energy component at BE ∼ 287.3 eV, which has much lower intensity, can be assigned27 to the emission from the COO− group confirming, in accordance with the IRRAS data,23,28 the formation of the bidentate carboxylate−metal bond. The total intensity of the C 1s signals, presented in Figure 4b, exhibits pronounced odd−even changes with the parameter n with an appositive phase to the odd−even modulation of the Ag 3d signal. Such anticorrelation of the Ag 3d and the C 1s signals with the parameter n would be consistent with the odd−even modulation in the film thickness. The standard analysis of the effective film thickness based on the C 1s/Ag 3d intensity ratios, assuming the exponential attenuation of the photoelectron signal29 and using attenuation lengths reported earlier,30 fully confirms such a prediction, as documented by the data presented in Figure 5a. Higher values of the film thickness in Figure 5a are observed for the even-numbered members of the BPnCOO/Ag series. The O 1s signal is shown in Figure 3c, and for all members of the BPnCOO/Ag series, it reveals a single, and symmetric, component at 530.5 eV consistent27 with the emission from the COO− group confirming, again, formation of the bidentate carboxylate−metal bond. The intensity of this signal for all analyzed SAMs is presented in Figure 4c. The O 1s signal intensity I is exponentially attenuated by the hydrocarbon film with thickness d and proportional to the concentration of molecules on the surface in the following way: A exp( −d /λ) I= (1) s Figure 6. Overview of STM data for BP2COO/Ag. White arrow in (a) illustrates the size of the rotational domain. Height profile presented in (b) is taken along the yellow line C depicted in (a). The yellow and blue arrows in (c) mark two sets of 3-fold rotational domains rotated by ±10° with respect to the high symmetry ⟨11̅0⟩ directions on the Ag(111) substrate, indicated by white arrows. The cross sections A and B indicated in (d) are presented in (e). The yellow unit cell marked in (d) has dimensions of a ≈ 0.59 nm and b ≈ 1.10 nm, indicated in cross sections A and B, respectively (see text for details).

where the parameter λ ∼ 22 Å31 describes the attenuation length for the O 1s photoelectrons, parameter s is the area per molecule (inversely proportional to the concentration of molecules on the surface), and parameter A is an experimental constant. From eq 1, the ratio of the area per molecule (s1/s2) for any two members of the BPnCOO/Ag series can be calculated in the form s1/s2 = I2/I1 exp[(d 2 − d1)/λ]

(2) D


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The Journal of Physical Chemistry C height of the single step of the Ag(111) substrate, i.e., ∼0.24 nm. Generally, for the Au(111) substrate, such monatomic depressions are characteristic of the process of SAM formation based on thiols,10,32−35 selenols,11,36−38 or even alkynes.39 For the Ag(111) substrate, few existing publications also show formation of such monatomic depressions in the case of SAMs based on thiols22,40,41 and selenols.37 Interestingly, in contrast to the current results, formation of such monatomic depressions was not reported in the recent study of SAMs based on oligophenylenecarboxylic,27 biphenyl-3,4′,5-tricarboxylic,42 trimesic,28 and isophthalic28 acids, where, similarly to the current study, the carboxylic group acts as a binding group with the Ag(111) substrate. We note, however, that, in contrast to the solid Ag(111) substrate used in the current experiments, the Ag(111) substrate was formed via underpotential deposition of only two atomic layers of silver on the Au(111) substrate in other studies, which most probably strongly affects transport of surface atoms associated with the formation of the monatomic depressions. The analysis of data obtained for BP2COO/Ag on a larger scale also enables identification of molecular domains (vide infra) with dimensions up to ∼100 nm as indicated in Figure 6a. Higher resolution data presented in Figure 6c reveal two types of STM contrast. The first type is related to periodic rows of molecules exhibiting an alternating intensity (i.e., every second row of molecules has higher intensity) and running along the directions indicated by the yellow and blue arrows in Figure 6c. Importantly, these directions deviate by ∼10° from the high symmetry ⟨11̅0⟩ directions of the Ag(111) substrate indicated by the white arrows in Figure 6c. Such misalignment of the azimuthal orientation of the molecular structure with respect to the Ag(111) substrate results in formation of mirror domains that are rotated by approximately +10 or −10° with respect to the ⟨11̅0⟩ direction. Thus, a total of two sets of 3fold symmetric domains rotated with respect to each other by ∼20° (six different orientations) are observed, as indicated by the blue and yellow arrows in Figure 6c. The second type of contrast visible in Figure 6a,c is related to a much less regular pattern of dark stripes that is overlaid with a periodic pattern of alternating rows. The orientation of these dark stripes is, to some extent, correlated with the orientation of rotational domains and thus enables identification of their size (as indicated in Figure 6a) in images of a larger area where the periodic pattern of alternated rows cannot be identified. As documented in our previous publication,22 this dark stripe pattern does not correspond to any visible defects but appears only as an additional contrast modulation superimposed on the undisturbed molecular lattice. Such a phenomenon was also reported earlier for the BPnS(Se)/Au (n = 2−6) SAMs and has been attributed to the domain boundaries at the molecule− substrate interface (solitons) that are formed as an additional form of stress relaxation in the hybrid aliphatic−aromatic SAMs in the case where large rotational domains are formed.11,20,21,43 The oblique unit cell of the molecular structure formed by the BP2COO/Ag is indicated in high-resolution data presented in Figure 6d with dimensions a = 0.58 ± 0.03 nm and b = 1.11 ± 0.05 nm measured from the corresponding cross sections A and B presented in Figure 6e, respectively. The angle between cross sections A and B is close to ∼60°. Considering two molecules per unit cell, the area per molecule in this structure amounts to ∼0.279 nm2.

Figure 7. Overview of STM data for BP4COO/Ag. Black arrow in (a) marks the size of the rotational domain. Height profile presented in (b) is taken along the yellow line C depicted in (a). The yellow and blue arrows in (c) mark two sets of 3-fold rotational domains rotated by ±5° with respect to the high symmetry ⟨11̅0⟩ directions on the Ag(111) substrate, indicated by the white arrows. The cross sections A and B indicated in (d) are presented in (e). The yellow unit cell marked in (d) has dimensions of a ≈ 0.57 nm and b ≈ 1.07 nm, indicated in cross sections A and B, respectively (see text for details).

The corresponding STM data obtained for another evennumbered system, i.e., BP4COO/Ag, are presented in Figure 7. Compared to the data obtained for the BP2COO/Ag, the analysis on a larger scale presented in Figure 7a shows an additional feature of the STM contrast, i.e., formation of islands usually just a few nanometers in diameter. As documented by the height profile C presented in Figure 7b, the height of these islands corresponds to the height of the step edges of the Ag(111) substrate and can therefore be attributed to the islands formed by the substrate atoms. Formation of such islands on the Ag(111) substrate during SAM formation was also reported in prior studies of aliphatic36,40 and aromatic37,44 thiols and selenols. In the case of the Au(111) E


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The Journal of Physical Chemistry C substrate, formation of such islands was observed exclusively for aromatic thiols34,35,45,46 and selenols.37 The molecular domains identified in large-scale images of BP4COO/Ag have a size up to ∼50 nm. As documented by the data presented in Figure 7c, the orientation of the rotational domains for BP4COO/Ag (indicated by blue and yellow arrows) is off from the high symmetry ⟨11̅0⟩ directions of the Ag(111) substrate (indicated by white arrows), as for BP2COO/Ag. However, in this case, this misalignment is only approximately ±5° and results in two sets of mirror domains (six different rotational domains) rotated with respect to each other by ∼10°, i.e., approximately half of the rotation angle observed for BP2COO/Ag. The oblique unit cell formed by BP4COO/Ag is marked in the molecularly resolved data presented in Figure 7d. As documented by the respective cross sections A and B presented in Figure 7e, the dimensions of this oblique unit cell are a = 0.57 ± 0.03 nm and b = 1.07 ± 0.05 nm. Considering two molecules per unit cell and an angle of ∼60° between cross sections A and B, these dimensions give an area per molecule of ∼0.264 nm2. The STM analysis for the odd-numbered sample of BP1COO/Ag is summarized in Figure 8. The data obtained on a larger scale are presented in Figure 8a and indicate the coexistence of two different structures assigned as phases α and β. Most of the sample area is covered by the 2D disordered phase α, which, as documented by the higher resolution data presented in Figure 8b, exhibits characteristic “scratching” along the fast scanning direction of the STM. The observation of such a feature in the STM image indicates high mobility of molecules in this, apparently, liquidlike structure. Only a minor portion of the BP1COO/Ag surface is covered by the quasiordered phase β, which coexists with the disordered phase α and forms small islands just 5−15 nm in diameter. As documented by the series of STM images presented in Figure 8c−f, the stability of phase β is also very limited, and repeated scanning of the same area results in its gradual “melting” into disordered liquidlike phase α. Considering high density of molecular-scale defects within very small domains of phase β and limited stability of these structures during STM imaging, we were unable to perform a more detailed analysis of phase β to identify the unit cell associated with this unstable structure. The final step in the STM analysis was another oddnumbered sample, i.e., BP3COO/Ag. In contrast to all other BPnCOO/Ag samples, data obtained in this case (Figure 9a) reveal a very irregular structure of substrate step edges, which apparently has been significantly modified from the original regular structure of facets characteristic of the native Ag(111) substrate. The repeated STM scanning of the same area of the sample, presented in the sequence of images in Figure 9b−d, shows STM-induced modification of the substrate surface additionally documented in Figure 9e. These observations demonstrate a very high mobility of molecules and the substrate atoms in the case of BP3COO/Ag. Due to this effect, higher resolution STM analysis for BP3COO/Ag was extremely difficult. However, occasionally, as documented in Figure 9f, we could observe the coexistence of disordered (α) and ordered (β) phases as for the BP1COO/Ag system.

Figure 8. Overview of STM data for BP1COO/Ag. Images at different scale (a, b) present coexistence of the liquidlike phase α (exhibiting characteristic “scratching” due to the high mobility of the molecules) and quasi-ordered phase β. The (c−f) series of subsequent STM images of the same sample area demonstrate gradual STMinduced “melting” of phase β (indicated by the white loop) into disordered liquidlike phase α (see text for details).

odd−even oscillation of the characteristic absorption bands related either to the biphenyl moiety or directly to the COO− binding group. The odd−even effect related to the biphenyl moiety has the same phase and similar amplitude as reported earlier for BPnS/Ag6 and BPnSe/Ag8 SAMs, i.e., for analogous monolayers where a S or Se atom was used as a binding group. As pointed out in the Introduction, the substrate−S(Se)−C bond angle in these SAMs is close to 180° and, therefore, the S(Se)−C bond between the aliphatic chain and the anchoring group is perpendicular to the substrate plane. Considering the perpendicular orientation of the COO− anchoring group to the Ag substrate, the C−C bond between the aliphatic linker of BPnCOO and the COO− group is also perpendicular to the Ag substrate plane, as it was also concluded in former studies of n-alkanoic acids on Ag(111).24 Therefore, the same phase of the odd−even reorientation of the biphenyl group for

5. DISCUSSION We start the discussion with the analysis of the IRRAS data, which, for all members of the BPnCOO/Ag series, indicates not only formation of the monolayer via bidentate bonding of the COO− group to the Ag(111) substrate but also systematic F


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Another odd−even change in the orientation of molecules in BPnCOO/Ag can be inferred from the odd−even variation of the ∼1400 cm−1 band, which is related to the COO− binding group. Following a previous study,24 we assume that the TDM associated with this symmetric oscillation is parallel with the C−COO− bond. Considering surface selection rules, the odd−even modulation of the COO− band intensity reflects an odd−even change in the tilt of the COO− plane relative to the Ag(111) surface with a more upright orientation for the evennumbered members of the BPnCOO/Ag series. From IRRAS analysis, we therefore conclude an odd−even structural effect in which, for odd-numbered BPnCOO/Ag SAMs, a more canted orientation of the biphenyl moiety is correlated with the tilting of the COO− plane of the binding group. Such a correlation seems to be well justified sterically considering that the more canted orientation of the biphenyl group will increase the distance between neighboring molecules, which can be to a certain extent compensated (to optimize the intermolecular interactions and to keep the area per molecule as small as possible) by tilting the plane of the COO− bonding group toward the substrate, as schematically presented in Figure 10. The odd−even modification in the film thickness calculated from the XPS data is correlated with the odd−even orientation of molecules on the surface inferred from the IRRAS and shows lower film thickness for odd-numbered members of the series for which the biphenyl group has a more canted orientation toward the Ag(111) substrate. Another important parameter that should be affected by the odd−even reorientation of molecules is the area per molecule. Our STM data show that a well-ordered structure, for which the area per molecule can be measured directly, is formed only for even-numbered members of the series. For odd-numbered members, the film structure is dominated by formation of the liquidlike phase α, which precludes such analysis by the STM. However, as presented in Figure 5, the area per molecule can be approximated for all members of the BPnCOO/Ag series by the analysis of the O 1s signal. Such analysis demonstrates the odd−even modulation of this parameter that is fully correlated both with the odd−even effect in the film thickness and the odd−even effects in the tilt of the molecule backbone and the COO− headgroup. Namely, odd-numbered systems that have more canted orientation of the biphenyl backbone as well as the plane of the COO− headgroup exhibit a higher area per molecule (lower surface density) and consequently lower film thickness, as schematically illustrated in Figure 10. The STM analysis of the BPnCOO/Ag series revealed that this odd−even effect has a profound impact on the 2D organization of these monolayers. Our results show that, for even-numbered members of the series (n = 2 and 4), a highly ordered and stable structure is formed in a very short time. The model of this structure is schematically presented in Figure 10c. The dimensions of the oblique unit cell (marked in red in Figure 10c) identified for BP2COO/Ag and BP4COO/Ag are, within the precision of the STM measurements, similar and close to the dimensions of the commensurate (4 × 2) structure (marked in green in Figure 10c). Considering, however, rotation of the observed structure by the angle ϕ, which is different for BP2COO/Ag and BP4COO/Ag (ϕ ∼ 10° for n = 2 and ϕ ∼ 5° for n = 4), with respect to the ⟨11̅0⟩ direction of the Ag(111) substrate, we conclude that this structure is in fact incommensurate with the Ag(111) substrate and has slightly different dimensions for BP2COO/Ag and BP4COO/Ag (particular value of the rotation angle of the molecular

Figure 9. Overview of STM data for BP3COO/Ag. In (a), the large area image reveals irregular orientation of the substrate step edges (no high symmetry orientation of step edges characteristic for the bare Ag(111) substrate) induced by the adsorption of the BP3COO monolayer. (b−d) Series of STM images of the same sample area taken as a first (b), third (c), and eight (d) scan demonstrates gradual STM-induced reorganization of step edges. The yellow rectangle in (e) marks the area scanned in (b)−(d). In (f), the high-resolution image indicates the coexistence of liquidlike phase α and quasiordered phase β (see text for details).

BPnCOO/Ag and BPnS(Se)/Ag SAMs is not surprising and results from similar binding geometry of the aliphatic linker to the anchoring group. We note that the transition dipole moments (TDMs) corresponding to the bands at ∼1500 and ∼1005 cm−1 are oriented along the 4,4′-axis of the biphenyl moiety.6 Therefore, considering the surface selection rules, the odd−even variation of these signals as a function of parameter n indicates an odd−even reorientation of the biphenyl moiety relative to the Ag(111) substrate with a more upright orientation in the case of the even-numbered members of the BPnCOO/Ag series, as it was concluded earlier for the BPnS/Ag and BPnSe/Ag series.6,8 G


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The strong impact of the aliphatic linker on the 2D structure of these SAMs is also visible within the odd and even groups of molecules. For even-numbered structures, we note the following: (i) a change in the angle ϕ by which these structures are rotated relative to the Ag(111) substrate (vide supra), and (ii) a reduction (roughly by half) of the size of the domain structures of BP4COO/Ag compared to BP2COO/ Ag. The latter effect is expected, assuming an increase in the intermolecular interactions due to the increased length of the aliphatic linker that reduces the diffusion efficiency of molecules on the substrate and, thus, the maximum size of the structural domains that can be formed. The former effect indicates that dimensions of the unit cells associated with structures of BP4COO/Ag and BP2COO/Ag are slightly different (vide supra). For odd-numbered structures, the length of the aliphatic linker also strongly affects the mobility of the molecules and, additionally, substrate atoms in the surface layer. For BP1COO/Ag only the molecular structure is unstable during STM scanning. For BP3COO/Ag, additionally, high mobility of substrate atoms is observed during STM scanning. We note at this point that, since such tip-induced sample modifications are not observed for the longest of the analyzed systems, i.e., for BP4COO/Ag, they cannot simply result from an increased resistance of the tunneling junction but must be related to the molecular structure of the film and the molecule−metal interface. Observation of the liquid phase formation in STM only for odd-numbered SAMs indicates higher area per molecule compared to the even-numbered systems. This conclusion is fully supported by the spectroscopic analysis discussed above, and quite naturally explains the increased mobility in the case of odd-numbered SAMs. However, one could expect that the increased length of the aliphatic linker, when comparing BP1COO/Ag and BP3COO/ Ag, will reduce this mobility for the longer system of BP3COO/Ag. Instead, an increased mobility of molecules and, additionally, substrate atoms is observed for BP3COO/ Ag. This behavior can be understood considering the simple, qualitative model that explains the relationship between the odd−even effect and the energetics of BPnCOO/Ag SAMs. As schematically presented in Figure 10, for even-numbered systems, the high packing density (i.e., density of the COO− Ag bonds), the Ag−O−C bending potential, and the intermolecular interactions enter into the energetics of the system in a cooperative way with all three factors being satisfied by the same configuration and thus forming a welldefined and very stable monolayer. However, for oddnumbered systems, the more canted orientation of the biphenyl group toward the substrate results in a reduced packing density and nonoptimal intermolecular interactions (the optimal overlap of the biphenyl units and thus the π−π interaction is affected). The reduced contribution of these two factors building the energetics of the system can be improved at the price of changing the third factor from its optimal configuration, i.e., the Ag−O−C bending potential. Thus, the competitive, instead of cooperative, relationship between these factors results in the lower stability of odd-numbered compared to even-numbered BPnCOO/Ag. Considering the above model, we may therefore expect that for odd-numbered systems, by increasing the length of the aliphatic linker, the competition between opposing factors increases (the increasing distance between molecules, due to the longer aliphatic chain, is partially compensated by further bending of the Ag−

Figure 10. Schematic presentation of the odd−even structural effect in BPnCOO/Ag SAMs. (a) Ordered “even” structure with (i) upright orientation of the biphenyl unit, (ii) distance a between neighboring molecules, and (iii) upright orientation of the COO− headgroup plane (angle ω of the Ag−O−C bond close to 180°). (b) Disordered “odd” structure with (i) canted orientation of the biphenyl unit, (ii) an increased distance between molecules that is not well-defined (a1 ≠ a2 ≠ a3 ≠ a4), and (iii) canted and not well-defined orientation of the COO− headgroup plane (ω′ < ω). (c) The unit cell of the incommensurate “even” structure (marked in red) is rotated by the angle ϕ (ϕ ∼ 10° and 5° for n = 2 and n = 4, respectively) with respect to the ⟨11̅0⟩ direction of the Ag(111) substrate. The dimensions of this incommensurate structure (a and b) as well as the angle θ (between directions of a and b) are close to the values for the commensurate (4 × 2) structure indicated in green. The herringbone arrangement of aromatic parts of adsorbed molecules in (c) is marked by the orientation of phenyl rings. The adsorption sites in (c) are taken arbitrarily (see text for details). The molecule−substrate bonding configuration in (a) and (b) is shown in a “side view”; i.e., only one out of two O−Ag bonds is visible to show tilting of the COO− plane.

structure relative to the substrate results from the respective misfit of this structure with the Ag(111) lattice). As schematically shown in Figure 10c, we also assume that the alternating row structure observed in the STM data results from the herringbone arrangement of phenyl rings, characteristic of aromatic SAMs.9−11,20−22,33 We note at this point that although the molecular footprint obtained for BP2COO/Ag and BP4COO/Ag SAMs (0.264−0.279 nm2) is much bigger as compared to the value (0.189 nm2) known for alkanethiols on Ag(111),44 it remains practically the same as for the recently analyzed22 thiol analogue system of BP2S/Ag (0.288 nm2) which exhibits also a very similar type of surface structure. In contrast, for odd-numbered members (n = 1 and 3) of the series, the majority of the surface is covered with the disordered liquidlike phase α. Thus, the odd−even modification of the parameter n is not only changing the type of the ordered structure of BPnCOO/Ag, as observed earlier for the BPnS(Se)/Au series,10,11 but the odd or even value of this parameter defines formation of either ordered or disordered 2D structure, respectively. H



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O−C bond out of the optimal configuration), which results in even deeper destabilization of the system and thus possibly higher mobility of molecules in the liquidlike structure of BP3COO/Ag. At this point, we note that our simple qualitative model does not consider the energetics of the Ag(111) substrate, and in particular, the mobility of the top layer of the substrate atoms that is, apparently, enhanced upon increasing the number n from 1 to 3. However, it is wellestablished experimentally32,34,47−49 that the mobility of the top layer substrate atoms in SAMs is directly correlated with the mobility of adsorbed molecules and becomes most efficient when the monolayer exhibits a liquidlike structure. Consistently, with this general observation, a higher mobility of molecules in BP3COO/Ag compared to BP1COO/Ag results in enhanced mobility of the top layer substrate atoms and leads to the observed complete reorganization of substrate terraces for the former system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Piotr Cyganik: 0000-0001-6131-4618 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors would like to thank Dr. Kung-Ching Liao (Department of Chemistry and Chemical Biology, Harvard University) for his advice on the sample preparation protocol and Mr. Marek Drozdek (Department of Chemistry, Jagiellonian University) for his assistance in collecting the XPS data. This work was supported financially by the National Science Centre Poland (Grant UMO-2015/19/B/ST5/ 01636). The XPS was purchased with the financial support of the European Regional Development Fund (Grant POIG.02.02.00-12-023/08).

6. SUMMARY AND CONCLUSIONS In summary, by the combination of spectroscopic and microscopic methods, we have demonstrated a structural odd−even effect for homologous series of biphenyl-substituted carboxylic acid monolayers on the Ag(111) substrate (BPnCOO/Ag). This odd−even effect, induced by changing the number of the CH2 groups in the aliphatic linker between the top biphenyl group and the anchoring carboxylic group, is visible in all aspects of the monolayer structure, i.e., in the packing density, in the orientation of the molecules relative to the substrate, and in the 2D order of the monolayer. This last aspect of the observed odd−even effect is particularly interesting because it shows that seemingly minor modification of molecules decides whether monolayers formed by the molecules are perfectly ordered and stable, or liquidlike and unstable. A similar odd−even effect, however, less pronounced, was reported earlier for BPnS(Se)/Au(Ag) structures, where the S or Se atom was used as an anchoring group. This similarity observed for different anchoring groups and substrates indicates a common mechanism that is based on the odd−even modification of the cooperative or competitive way by which three main factors such as surface coverage, intermolecular interactions, and configuration of the anchoring group enter the energetics of the monolayer. The stable structures are formed when all three factors work in a cooperative way, i.e., when they are all satisfied with the same configuration of the film structure. On the contrary, the unstable systems result from a competitive relationship when the gain in the first two factors (surface coverage and intermolecular interactions) is achieved only at the cost of the third factor (configuration of the anchoring group) and vice versa. Our results demonstrate that, for BPnCOO/Ag SAMs, this odd−even stability effect is particularly strong and determines the ability of molecules to form highly ordered structures. This observation seems to be of key importance for different applications of SAMs based on the carboxylic group that, in a very short time, for correctly designed molecules, forms much better ordered structures compared to the commonly used thiols. Considering the high degree of 2D ordering and stability combined with simple and very fast formation procedures as well as the high conductance of aromatic molecules and silver substrate, this type of SAM seems to be particularly interesting for molecular electronics applications.

(1) 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, 1103−1170. (2) Casalini, S.; Bortolotti, C. A.; Leonardi, F.; Biscarini, F. SelfAssembled Monolayers in Organic Electronics. Chem. Soc. Rev. 2017, 46, 40−71. (3) Xiang, D.; Wang, X.; Jia, C.; Lee, T.; Guo, X. Molecular-Scale Electronics: From Concept to Function. Chem. Rev. 2016, 116, 4318− 4440. (4) Thompson, D.; Nijhuis, C. A. Even the Odd Numbers Help: Failure Modes of SAM-Based Tunnel Junctions Probed via Odd-Even Effects Revealed in Synchrotrons and Supercomputers. Acc. Chem. Res. 2016, 49, 2061−2069. (5) Zharnikov, M.; Frey, S.; Rong, H.-T.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. The Effect of the Sulfur-Metal Bond on the Structure of Self-Assembled Monolayers. Phys. Chem. Chem. Phys. 2000, 2, 3359−3362. (6) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wühn, M.; Wöll, C.; Helmchen, G. On the Importance of the Headgroup Substrate Bond in Thiol Monolayers: A Study of Biphenyl-Based Thiols on Gold and Silver. Langmuir 2001, 17, 1582−1593. (7) Heister, K.; Rong, H. T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. Odd-Even Effects at the S-Metal Interface and in the Aromatic Matrix of Biphenyl-Substituted Alkanethiol SelfAssembled Monolayers. J. Phys. Chem. B 2001, 105, 6888−6894. (8) Weidner, T.; Shaporenko, A.; Müller, J.; Schmid, T.; Cyganik, P.; Terfort, A.; Zharnikov, M. The Effect of the Bending Potential on Molecular Arrangement in Alkaneselenolate Self-Assembled Monolayers. J. Phys. Chem. C 2008, 112, 12495−12506. (9) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wöll, C. Pronounced Odd-Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Substituted Alkanethiol SAMs. Langmuir 2003, 19, 8262−8270. (10) Cyganik, P.; Buck, M.; Azzam, W.; Wöll, C. Self-Assembled Monolayers of ω-Biphenyl-Alkane Thiols on Au(111): Influence of Spacer Chain on Molecular Packing. J. Phys. Chem. B 2004, 108, 4989−4969. (11) Cyganik, P.; Szelagowska-Kunstman, K.; Terfort, A.; Zharnikov, M. Odd-Even Effect in Molecular Packing of Biphenyl-Substituted Alkaneselenol Self-Assembled Monolayers on Au(111): Scanning Tunneling Microscopy Study. J. Phys. Chem. C 2008, 112, 15466− 15473. I


Article

The Journal of Physical Chemistry C (12) Long, Y. T.; Rong, H. T.; Buck, M.; Grunze, M. Odd-Even Effects in the Cyclic Voltammetry of Self-Assembled Monolayers of Biphenyl Based Thiols. J. Electroanal. Chem. 2002, 524−525, 62−67. (13) Thom, I.; Buck, M. Electrochemical Stability of Self-Assembled Monolayers of Biphenyl Based Thiols Studied by Cyclic Voltammetry and Second Harmonic Generation. Surf. Sci. 2005, 581, 33−46. (14) Felgenhauer, T.; Rong, H. T.; Buck, M. Electrochemical and Exchange Studies of Self-Assembled Monolayers of Biphenyl Based Thiols on Gold. J. Electroanal. Chem. 2003, 550−551, 309−319. (15) Szelagowska-Kunstman, K.; Cyganik, P.; Schüpbach, B.; Terfort, A. Relative Stability of Thiol and Selenol Based SAMs on Au(111) - Exchange Experiments. Phys. Chem. Chem. Phys. 2010, 12, 4400−4406. (16) Ossowski, J.; Rysz, J.; Terfort, A.; Cyganik, P. Relative Stability of Thiolate and Selenolate SAMs on Ag(111) Substrate Studied by Static SIMS. Oscillation in Stability of Consecutive Chemical Bonds. J. Phys. Chem. C 2017, 121, 459−470. (17) Frey, S.; Rong, H. T.; Heister, K.; Yang, Y. J.; Buck, M.; Zharnikov, M. Response of Biphenyl-Substituted Alkanethiol SelfAssembled Monolayers to Electron Irradiation: Damage Suppression and Odd-Even Effects. Langmuir 2002, 18, 3142−3150. (18) Vervaecke, F.; Wyczawska, S.; Cyganik, P.; Bastiaansen, J.; Postawa, Z.; Silverans, R. E.; Vandeweert, E.; Lievens, P. Odd-Even Effects in Ion-Beam-Induced Desorption of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers. ChemPhysChem 2011, 12, 140−144. (19) Cyganik, P.; Buck, M. Polymorphism in Biphenyl-Based SelfAssembled Monolayers of Thiols. J. Am. Chem. Soc. 2004, 126, 5960− 5961. (20) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; WiltonEly, J. D. E. T.; Zharnikov, M.; Wöll, C. Competition as a Design Concept: Polymorphism in Self-Assembled Monolayers of BiphenylBased Thiols. J. Am. Chem. Soc. 2006, 128, 13868−13878. (21) Dendzik, M.; Terfort, A.; Cyganik, P. Odd-even Effect in the Polymorphism of Self-Assembled Monolayers of Biphenyl-Substituted Alkaneselenolates on Au(111). J. Phys. Chem. C 2012, 116, 19535− 19542. (22) Krzykawska, A.; Ossowski, J.; Zaba, T.; Cyganik, P. Carboxylic Acid Headgroups for the Formation of Highly Ordered SAMs on Ag(111). Chem. Commun. 2017, 53, 5748−5751. (23) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Formation and Structure of a Spontanously Adsorbed Monolayer of Arachidic on Silver. Chem. Phys. Lett. 1986, 132, 93−98. (24) Smith, E. L.; Porter, M. D. Structure of Monolayers of Short Chain n-Alkanoic Acids (CH3(CH2)nCOOH, n = 0−9) Spontaneously Adsorbed from the Gas Phase at Silver As Probed by Infrared Reflection Spectroscopy. J. Phys. Chem. 1993, 97, 8032−8038. (25) Gardner, A. M.; Wright, T. G. Consistent Assignment of the Vibrations of Monosubstituted Benzenes. J. Chem. Phys. 2011, 135, 114305. (26) Tolstoy, V. P.; Chernyshova, I. V.; Skryshevsky, V. A. Handbook of Infrared Spectroscopy of Ultrathin Films; John Wiley & Sons: Hoboken, NJ, 2003. (27) Aitchison, H.; Lu, H.; Hogan, S. W. L.; Früchtl, H.; Cebula, I.; Zharnikov, M.; Buck, M. Self-Assembled Monolayers of Oligophenylenecarboxylic Acids on Silver Formed at the Liquid−Solid Interface. Langmuir 2016, 32, 9397−9409. (28) Cebula, I.; Lu, H.; Zharnikov, M.; Buck, M. Monolayers of Trimesic and Isophthalic Acid on Cu and Ag: The Influence of Coordination Strength on Adsorption Geometry. Chem. Sci. 2013, 4, 4455−4464. (29) Dannenberger, O.; Weiss, K.; Himmel, H. J.; Jager, B.; Buck, M.; Woll, C. An Orientation Analysis of Differently EndgroupFunctionalised Alkanethiols Adsorbed on Au Substrates. Thin Solid Films 1997, 307, 183−191. (30) Lamont, C. L. A.; Wilkes, J. Attenuation Length of Electrons in Self-Assembled Monolayers of n-Alkanethiols on Gold. Langmuir 1999, 15, 2037−2042.

(31) Lamont, C. L. A.; Wilkes, J. An Orientation Analysis of Differently Endgroup-Functionalised Alkanethiols Adsorbed on Au Substrates. Langmuir 1999, 15, 2037−2042. (32) Poirier, G. E. Characterization of Organosulfur Molecular Monolayers on Au(111) Using Scanning Tunneling Microscopy. Chem. Rev. 1997, 97, 1117−1127. (33) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Witte, G.; Zharnikov, M.; Wöll, C. Influence of Molecular Structure on Phase Transitions: A Study of Self-Assembled Monolayers of 2(Aryl)-Ethane Thiols. J. Phys. Chem. C 2007, 111, 16909−16919. (34) Yang, G. H.; Liu, G. Y. New Insights for Self-Assembled Monolayers of Organothiols on Au(111) Revealed by Scanning Tunneling Microscopy. J. Phys. Chem. B 2003, 107, 8746−8759. (35) Käfer, D.; Witte, G.; Cyganik, P.; Terfort, A.; Wöll, C. A Comprehesive Study of Self-Assembled Monolayers of Anthracene Thiol on Gold: Solvents Effects, Structure and Stability. J. Am. Chem. Soc. 2006, 128, 1723−1732. (36) Shaporenko, A.; Cyganik, P.; Buck, M.; Ulman, A.; Zharnikov, M. Self-Assembled Monolayers of Semifluorinated Alkaneselenolates on Noble Metal Substrates. Langmuir 2005, 21, 8204−8213. (37) Shaporenko, A.; Cyganik, P.; Buck, M.; Terfort, A.; Zharnikov, M. Self-Assembled Monolayers of Aromatic Selenolates on Noble Metal Substrates. J. Phys. Chem. B 2005, 109, 13630−13638. (38) Monnell, J. D.; Stapleton, J. J.; Jackiw, J. J.; Dunbar, T. D.; Reinerth, W. A.; Dirk, S. M.; Tour, J. M.; Allara, D. L.; Weiss, P. S. Ordered Local Domain Structure of Docosaneselenol Monolayers on Au(111). J. Phys. Chem. B 2004, 108, 9834−9841. (39) Zaba, T.; Noworolska, A.; Bowers, C. M.; Breiten, B.; Whitesides, G. M.; Cyganik, P. Formation of Highly Ordered SelfAssembled Monolayers of Alkynes on Au(111) Substrate. J. Am. Chem. Soc. 2014, 136, 11918−11921. (40) Dhirani, A.; Hines, M. A.; Fisher, A. J.; Ismail, O.; GuyotSionnest, P. Structure of Self-Assembled Decanethiol on Ag(11 1): A Molecular Resolution Scanning Tunneling Microscopy Study. Langmuir 1995, 11, 2609−2614. (41) Azzaroni, O.; Vela, M. E.; Andreasen, G.; Carro, P.; Salvarezza, R. C. Electrodesorption Potentials of Self-Assembled Alkanethiolate Monolayers on Ag(111) and Au(111). An Electrochemical, Scanning Tunneling Microscopy and Density Functional Theory Study. J. Phys. Chem. B 2002, 106, 12267−12273. (42) Aitchison, H.; Lu, H.; Zharnikov, M.; Buck, M. Monolayers of Biphenyl-3,4′,5-tricarboxylic Acid Formed on Cu and Ag from Solution. J. Phys. Chem. C 2015, 119, 14114−14125. (43) Cyganik, P.; Buck, M.; Wilton-Ely, J. D.; Wöll, C. Stress in SelfAssembled Monolayers: ω-biphenyl Alkane Thiols on Au(111). J. Phys. Chem. B 2005, 109, 10902−10908. (44) Yu, M.; Woodruff, D. P.; Satterley, C. J.; Jones, R. G.; Dhanak, V. R. Structure of the Pentylthiolate Self-Assembled Monolayer on Ag(111). J. Phys. Chem. C 2007, 111, 10040−10048. (45) Duan, L.; Garrett, S. J. An Investigation of Rigid PMethylterphenyl Thiol Self- Assembled Monolayers on Au(111) Using Reflection-Absorption Infrared Spectroscopy and Scanning Tunneling Microscopy. J. Phys. Chem. B 2001, 105, 9812−9816. (46) Azzam, W.; Fuxen, C.; Birkner, A.; Rong, H. T.; Buck, M.; Wöll, C. Coexistence of Different Structural Phases in Thioaromatic Monolayers on Au(111). Langmuir 2003, 19, 4958−4968. (47) Poirier, G. E.; Tarlov, M. J. Molecular Ordering and Gold Migration Observed in Butanethiol Self-Assembled Monolayers Using Scanning-Tunneling-Microscopy. J. Phys. Chem. 1995, 99, 10966− 10970. (48) Poirier, G. E. Mechanism of Formation of Au Vacancy Islands in Alkanethiol Monolayers on Au(111). Langmuir 1997, 13, 2019− 2026. (49) McDermott, C. A.; McDermott, M. T.; Green, J. B.; Porter, M. D. Structural Origins of the Surface Depressions at Alkanethiolate Monolayers on Au(111): A Scanning Tunnelling and Atomic Force Microscopic Investigation. J. Phys. Chem. 1995, 99, 13257−13267.

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