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Compression-Induced Conformation and Orientation Changes in an n-Alkane Monolayer on a Au (111) Surface Osamu Endo, Masashi Nakamura, Kenta Amemiya, and Hiroyuki Ozaki Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04259 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Compression-Induced Conformation and Orientation Changes in an n-Alkane Monolayer on a Au (111) Surface Osamu Endo*,†, Masashi Nakamura‡, Kenta Amemiya¶, and Hiroyuki Ozaki† †

Department of Organic and Polymer Materials Chemistry, Faculty of Engineering, Tokyo

University of Agriculture and Technology, Tokyo 184-8588, Japan ‡

Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba

University, Chiba 263-8522, Japan ¶

Photon Factory, High Energy Accelerator Research Organization (KEK-PF), Institute of

Materials Structure Science (IMSS), Tsukuba 305-0801, Japan Author information *

Corresponding Author. Email: [email protected]

ACS Paragon Plus Environment

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Abstract The influence of the preparation method and adsorbed amount of n-tetratetracontane (n-C44H90) on its orientation in a monolayer on the Au (111) surface is studied by near carbon K-edge X-ray absorption fine structure spectroscopy (C K-NEXAFS), scanning tunneling microscopy (STM) under ultrahigh vacuum, and infrared reflection-absorption spectroscopy (IRAS) at the electrochemical interface in sulfuric acid solution. The n-C44H90 molecules form self-assembled lamellar structures with the chain axis parallel to the surface, as observed by STM. For small amounts adsorbed, the carbon plane is parallel to the surface (flat-on orientation). An increase of the adsorbed amount by ~10–20% induces compression of the lamellar structure either along the lamellar axis or alkyl chain axis. The compressed molecular arrangement is observed by STM and induced conformation and orientation changes is confirmed by in situ IRAS and C KNEXAFS.

Keywords: n-alkane, Au (111), C K-NEXAFS, IRAS, STM


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1. Introduction The self-assembly of organic molecules on solid surfaces is an intriguing phenomenon due to its numerous applications in surface chemistry, including surface modification, functionalization, and corrosion protection.1 Self-assembled monolayers (SAMs) also attract interest in nanomaterial science, because the 2D molecular patterns of a certain class of SAMs can be used as building blocks for nanostructures.2 Chain molecules lying flat on an inert surface, such as the graphite basal plane or the Au (111) surface, are characterized by a close-packed lamellar structure.3–8 The lamellar structure of n-alkane molecules on the Au (111) surface has been used as a starting structure to obtain graphene nanoribbons (GNRs) by thermal dehydrogenation.9 The n-alkane monolayer can be used as a metal growth template to promote anisotropic growth of palladium.10 Dehydrogenation is strongly affected by the type of methylene group contact with the Au (111) surface and the related intermolecular distance. The metal structure and growth modes are influenced by the smoothness of the outermost surface of the n-alkane template. In this respect, it is important to elucidate the inner lamellar structure and the carbon plane orientation of flat-lying n-alkane molecules on the Au (111) surface from various points of view. Diverse orientations and molecular packing patterns have been reported for lamellar n-alkane structures on the Au (111) surface, depending on the chain length, number of carbons (odd or even), and substrate temperature. The structure of the lamellae is affected by their environment, depending on whether the molecules are present at a solid-liquid interface or under vacuum. The orientation of the carbon plane with respect to the surface is controversial between parallel (flaton)3,7,8 and perpendicular (edge-on)4,6 orientations. The intermolecular distance is also variable, with reported values lying between 0.43 and 0.5 nm.3,4,6–8 Most of these studies were carried out by scanning tunneling microscopy (STM) at the solid-liquid interface. Conversely, few studies of


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n-alkanes on the Au (111) surface under vacuum have been reported. An infrared reflectionabsorption spectroscopy (IRAS) study on an n-C44H90 monolayer on the Au (111) surface under ultrahigh vacuum (UHV) has shown coverage-dependent molecular orientation.5 It was concluded that the monolayer comprises molecules with a tilted-on orientation, that is, the carbon plane is inclined with respect to the surface, and molecules with a flat-on orientation. The molecules with a tilted-on orientation exhibit a gauche conformation, while the molecules with a flat-on orientation possess all-trans conformations. This result suggests that the monolayer is heterogeneous and possibly exhibits a phase differing from the uniform lamellar structure, as observed by STM at the liquid-Au (111) interfaces. At present, no microscopic investigation concerning this issue is available. In this study, monolayers of n-C44H90 prepared on the Au (111) surface by vapor deposition and adsorption from n-hexane solution were investigated by several surface analysis techniques. The orientation of the carbon plane is discussed based on the results of near carbon K-edge X-ray absorption fine structure spectroscopy (C K-NEXAFS) and those of STM under UHV. In addition, IRA spectroscopy characterization, using the monolayer on Au (111) as an electrode, was performed in sulfuric acid solution. Neutral n-alkane molecules are preferentially adsorbed on the Au (111) electrode surface at the potential of zero charge (pzc).11 Therefore, their CH stretching vibrations are detected in the subtractively normalized spectra at anodic potentials that induce the electrochemical exchange reaction between n-alkane molecules and sulfate anions.12


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2. Experimental A Au (111) single crystal was cleaned by flame-annealing in air to an orange-hot state or by Ar+ sputtering (1.5 keV) and annealing at ~800 K under UHV. The n-C44H90 molecules from an nhexane solution were adsorbed on the surface prepared in air and rinsed with pure n-hexane. Vapor deposition was conducted on the surface prepared under UHV, with the amount of adsorbed molecules observed using a thickness monitor. The thickness monitor was calibrated by electrochemically measuring the stripping charge of the deposited silver. The monolayer nominally corresponded to 5 Å. The surface carbon residue after cleaning under UHV was checked before vapor deposition via photoelectron intensity at the carbon K-edge (Figure S1a). The sample was heated at 400 K to remove the residual solvent and at 460 K under UHV to change the amount of adsorbed molecules. C K-NEXAFS experiments were performed on the BL-7A at the Photon Factory of the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-PF). The energy resolution of the photon is approximately 0.2 eV at 285 eV in this beamline.13 Spectra were recorded by collecting electrons with kinetic energies of ~250 eV (carbon KVV Auger electrons) using either a microchannel plate or a hemispherical electron analyzer. The incidence angle of the linearly polarized X-rays was 90° (normal incidence, NI) or 15° (grazing incidence, GI) with respect to the surface. The photon intensity was measured by the photocurrent of an Au mesh in an upper stream chamber. The linear background was subtracted and the spectra were normalized with the intensity at a photon energy 310 eV. The photon energy was calibrated by referring to the π* resonance located at 285.5 eV in the spectrum of the highly oriented pyrolytic graphite.14 STM measurements were performed at room temperature under UHV. In situ IRAS analyses were conducted using an FTIR 4700 instrument (JASCO International Co., Ltd.) equipped with a mercury cadmium telluride


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(MCT) detector. A CaF2 crystal was placed on the window of a Teflon-body electrochemical cell, which was equipped with a Pt counter electrode and a Hg|Hg2SO4 reference electrode. The incidence angle of the IR radiation polarized along the surface normal direction was ~70°. A 0.5 M sulfuric acid solution was prepared from sulfuric acid (Super Special Grade) and Milli-Q water (18.2 MΩ).

3. Results and discussion 3.1 C K-NEXAFS spectroscopy Figure 1a shows the C K-NEXAFS spectra of the n-C44H90 monolayer on the Au (111) surface, prepared using a solution in n-hexane and rinsed with n-hexane. The broad band at ~292 eV in the NI spectrum is assigned to the 1s → σ*CC resonance (σ*CC resonance),15 and the corresponding band is not observed in the GI spectrum. Because the transition dipole moment of the σ*CC resonance is directed along the molecular chain, this suggests that the molecular chain lies flat on the surface. In fact, the n-hexane molecules in the monolayer on the Au(111) surface grown on mica, which lie flat, exhibit similar polarization-dependent spectra.16 The average inclination angle of the molecular chain of n-hexane is 75±5° with respect to the surface normal, as estimated from the polarization dependence of the σ*CC resonance intensity.16 A value smaller than 90° is ascribed to the inhomogeneity of the substrate surface, where n-hexane molecules are adsorbed with the chain axis not parallel to the surface.16 The σ*CC resonance observed in the GI spectrum for the n-hexane monolayer is attributed to these molecules. In contrast, σ*CC resonance is not observed in the GI spectrum shown in Figure 1a for the n-C44H90 monolayer on the Au (111) single crystal surface and, therefore, the average inclination angle is larger than 75±5°.


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This suggests that the majority of the n-C44H90 chain lies flat. The flat-lying alkyl chain is common on solid surfaces and similar spectral features are reported for the SAMs of alkanethiols at low coverages.17 The small absorption structures around 287–288 eV observed in both NI and GI spectra are attributed to the σ*CH/R resonance (R denotes the Rydberg contribution).

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The σ*CH/R

resonance for an isolated n-alkane molecule with the transition dipole moment parallel to the carbon plane (σ*CH/R||) is located ~0.4−0.7 eV lower than that with the transition dipole moment perpendicular to the carbon plane (σ*CH/R ⊥ ).18,19 The polarization dependence of these resonances is useful in determining the orientation of the carbon plane with respect to the surface. The σ*CH/R|| and σ*CH/R⊥ resonances would be predominantly observed for molecules with the flat-on orientation in the NI and GI conditions, respectively. However, this is not the case for the molecules in the first layer on the solid surface because of the symmetry change and the interaction with the substrate. The final state orbital is strongly modified upon flat-on adsorption; therefore, the energy and direction of the transition dipole moment—and hence the resonance intensity—change considerably. Since the transition dipole moment of the modified σ*CH/R resonances is directed along the CH bond, corresponding resonances are observed in both NI and GI spectra at 286.9 eV and 287.9 eV (Figure 1a). These spectral features are in accordance with the spectra of n-octane molecules adsorbed with the flat-on orientation on a Cu (110) surface.19,20 Although the assignment of the resonance at 289 eV in the GI spectrum is unclear, a similar resonance was also observed in the GI spectrum of n-octane on the Cu (110) surface. 19,20 The σ*CC resonance is observed only along the surface parallel direction for both nC44H90 on Au (111) and n-octane on Cu (110).19,20 Therefore, it is concluded that the n-C44H90 molecules adopted the flat-on orientation in the solution-deposited monolayer on the Au (111)


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surface. The resonance located at 285 eV is attributed to the M* resonance, which appears as a result of the electronic interactions between the σ*CH orbital of the flat-on molecules and the metal substrate.19–22 This spectral feature is another characteristic of flat-on adsorption on the metallic substrate. On the other hand, the spectra of the n-C44H90 monolayer on the Au (111) surface prepared by vapor deposition exhibit different features, as shown in Figure 1b. The σ*CC resonance is enhanced in the NI spectrum, reflecting the flat-laid molecular chain on the surface. The σ*CH/R resonances are, however, significantly enhanced in both spectra and indicate a different orientation of the carbon plane. When the carbon plane is inclined with respect to the surface, every second methylene group is detached from the surface. As a result, the interaction between the detached methylene group and the substrate becomes ineffective and the final state orbital is not modified much. The calculated σ*CH/R|| and σ*CH /R⊥ resonances for the methylene in the edge-on molecules on a graphite basal plane appear at 287.2 eV and 288.5 eV, respectively, with higher intensity than that of the modified σ*CH/R resonance for the flat-on adsorption.23 Therefore, the enhanced σ*CH/R resonances are attributed to the inclined carbon plane. The relatively broad shape of the observed bands indicates contribution of both the σ*CH/R|| resonance at lower energy and the σ*CH/R ⊥ resonance at higher energy in all spectra. A dip in the subtractive spectra at 287 eV suggests that the σ*CH/R|| resonance dominates the GI spectrum. This polarization dependence suggests that the carbon plane is slightly inclined from the edge-on orientation, adopting the tilted-on orientation on the average. It should be noted that the possible gauche conformation of the alkyl chains is not detected by the present C K-NEXAFS technique.


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The monolayer obtained by vapor deposition and heated at 460 K shows spectra similar to those displayed in Figure 1a (shown in the Supporting Information as Figure S1b). The spectral change is caused by the desorption of some molecules at this temperature. The corresponding difference of the amount of adsorbed molecules estimated from the absorption intensities before normalization is ~10–20% (shown in the Supporting Information as Figure S1c). These results suggest that the molecular orientation does not depend on the preparation method, but on the amount of molecules in the monolayer. The results of in situ IRAS and STM under UHV are discussed in the following sections.

3.2 In situ IRA spectroscopy Figure 2 shows the cyclic voltammograms (CVs) and in situ IRA spectra obtained in 0.5 M sulfuric acid for the Au (111) electrode covered with the n-C44H90 monolayer. The upper panel (Figures 2a–2d) shows the CV and IRA spectra for the solution-deposited monolayer, while the lower panel (Figures 2e–2h) shows the corresponding spectra for the monolayer obtained by vapor deposition. The anodic current observed at 0.0−0.2 V (vs. Hg|Hg2SO4) in both CVs (Figures 2a and 2e) corresponds to the lifting of the reconstruction and adsorption of (bi)sulfate anions on the Au (111) surface.24 The sharp peaks at 0.4 V reflect the phase transition of (bi)sulfate anions between a disordered phase at lower potentials and the ordered √3 × √7 phase at higher potentials.25 The shapes of these CVs are identical to those reported previously, suggesting the exchange of n-C44H90 molecules with (bi)sulfate anions in the anodic potential region.12


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The IRA spectra shown in Figures 2b–2d and 2f–2h were subtractively normalized using the reference spectra obtained at −0.25 V vs. Hg|Hg2SO4, which is close to the pzc.11 The absorbance increase corresponds to the appearance of species at the electrode interface, while the corresponding decrease reflects their disappearance. Because of the IRAS selection rule, only the transition dipole moment along the surface normal is detected. Figure 2b shows the CH stretching region for the solution-deposited monolayer. The upward bands appear at 2922 and 2850 cm–1 at −0.05 V and grow in intensity up to a potential of +0.15 V. These bands are assigned to the antisymmetric and symmetric stretching vibrations of methylene (CH2) bonds, respectively, 12,26 and attributed to the desorbed alkyl chains with a tilted carbon plane in solution near the Au (111) surface.12 Since the all-trans alkyl chains exhibit the corresponding stretch at ~2917 cm–1,5,26 the slightly higher wavenumber of 2922 cm–1 for the antisymmetric stretch indicates the gauche conformation.5,12,26 On the other hand, the downward bands observed at 2904 cm–1 for the same potentials are assigned to the CH stretch of the CH bond directed toward the solution when the molecules adopt the flat-on orientation (CHdistal mode).5,12,27,28 Since the stretching of methylene (CH2) bonds is decoupled due to flat-on adsorption, the CH bond directed toward the metal surface (CHproximal) exhibits a peak at smaller wavenumbers (~2810 cm–1, soft mode) due to an electronic or dipole interaction with the metal substrate.27,28 The softmode band is observed as a small downward hump at ~2810 cm–1 in Figure 2b. These spectral features indicate that the methylene sequences of the n-C44H90 molecules adopt the flat-on orientation on the Au (111) surface at potentials around the reference potential (−0.25 V) and change their orientation upon desorption at anodic potentials. The flat-on orientation of molecules in the solution-deposited monolayer is consistent with the C K-NEXAFS results. At positive potentials, the (bi)sulfate anions are adsorbed on the Au (111) surface, as confirmed by


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the SO stretch shown in Figure 2c. The symmetric SO stretch is observed at ~1170 cm–1 as an upward band at potentials above +0.05 V, undergoing a blue shift with increasing potential. This behavior is caused by the direct adsorption of (bi)sulfate anions on the Au (111) surface, with the electron density in the antibonding orbitals of the SO bonds decreasing as the potential becomes more positive. These results agree with those obtained for a monolayer of n-C36H74 on the Au (111) electrode surface.12 Figure 2d shows the CH stretch region of the spectra obtained for the solution-deposited monolayer at cathodic potentials, showing no detectable bands at −0.35 and −0.45 V. The upward bands at 2924 and 2850 cm–1 are observed together with the downward band at 2906 cm–1 at –0.55 and –0.65 V. Therefore, the molecules do not change their orientation until –0.45 V and start to tilt at –0.55 V. This orientation change may be accompanied by the adsorption of cations, possibly hydronium ions, which is difficult to detect. Figure 2f shows the CH stretch region of the IRA spectra for the vapor-deposited monolayer of n-C44H90 on the Au (111) surface. The downward band at 2906 cm–1 starts to appear at –0.05 V and grows in intensity until +0.15 V. The upward bands are observed at 2918 and 2850 cm–1 at the same potentials. In addition, the downward band at 2927 cm–1 appears at +0.15 V. The CH stretching bands in the downward direction in Figure 2f are similar to those observed in the IRA spectra obtained under UHV for a vapor-deposited monolayer of n-C44H90 molecules on the Au (111) surface.5 The n-C44H90 monolayer deposited on Au (111) under UHV exhibits prominent absorption bands at 2924 and 2910 cm–1, as well as a smaller intensity band at 2853 cm–1. The bands at 2924 and 2853 cm–1 are assigned to the antisymmetric and symmetric stretches of the alkyl chains in the gauche conformation, respectively. Since both antisymmetric and symmetric stretches are observed, the carbon plane of the alkyl chains adopts the tilted-on orientation. On the other hand, the band at 2910 cm–1 is attributed to the CHdistal stretch of the flat-on oriented


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molecules. Based on these observations, the monolayer contains both molecules having the alltrans conformation with a flat-on orientation and the ones having the gauche conformation with a tilted-on orientation. The similar spectral features of the downward bands shown in Figure 2f suggest that the molecules in the vapor-deposited monolayer adopt an orientation similar to that of the molecules in the monolayer deposited under UHV. 5 The symmetric stretching mode at 2853 cm–1 is not observed in the downward direction in Figure 2f, probably because it is obstructed by the upward band of the desorbed molecules at the same wavenumber. The tilted-on orientation is consistent with the C K-NEXAFS spectra. The adsorption of (bi)sulfate anions is observed at positive potentials, as shown in the SO stretch region in Figure 2g. Therefore, the potential-induced exchange of n-C44H90 molecules and (bi)sulfate anions occurs in the same way for both monolayers. The upward bands at 1387 cm–1 observed at potentials above +0.15 V (Figure 2g) are assigned to the CH3 bending vibration. The transition dipole moment of this vibration is directed along the chain axis; therefore, at least one end of the desorbed alkyl chain is standing upright with respect to the Au (111) surface. All these exchange processes are completely reversible; therefore, the molecules in the vapor-deposited monolayer are re-adsorbed to construct the same structure at potentials around the pzc. Figure 2h shows the CH stretch region in the IRA spectra of the vapor-deposited monolayer at cathodic potentials. Almost no distinguishable bands are observed for all spectra, and hence, the molecules in the vapor-deposited monolayer are stable at negative potentials and are not easily exchanged with cations. The exchange with (bi)sulfate anions at positive potentials occurs because the n-C44H90 molecules become unstable on the Au (111) surface, and (bi)sulfate anions are attracted to the positively charged surface. Since the n-C44H90 molecules are stable on the reconstructed Au (111) surface,3 their destabilization is enhanced by the lifting of the


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reconstruction at a positive potential. This destabilization does not seem to occur at negative potentials, where the reconstructed surface is preserved.24 Therefore, the attack of cations on the n-C44H90 monolayer is an important factor for the described exchange, probably being ineffective for the vapor-deposited monolayer as compared to the solution-deposited monolayer. The solution-deposited monolayer without an n-hexane rinse exhibits IRA spectra similar to the spectra of the vapor-deposited monolayer (shown in Figure S2a and S2b in the Supporting Information). This result also suggests that the molecular orientation depends on the amount of molecules in the monolayer, which is deduced from the temperature-dependent C K-NEXAFS of the vapor-deposited monolayer. Moreover, the rinse can remove molecules effectively and as conveniently as heating under vacuum. Therefore, the monolayers corresponding to the NEXAFS, CV, and IRA spectra shown in Figure 1a and Figures 2a-d are regarded as lowerdensity phases in the following discussion. On the other hand, the monolayers corresponding to the NEXAFS, CV, and IRA spectra shown in Figure 1b and Figures 2e-h are regarded as higherdensity phases. The IRAS results suggest the absence of domain separation in the higher density phase, i.e., no exposed domains of flat-on orientated molecules having an all-trans conformation, as in the lower density phase. This is rationalized by the fact that these domains would respond to the electrode potential in the same manner as the monolayer of the lower density phase, and this response should be detected by IRAS at cathodic potentials. The minute picture of the higher density phase is discussed based on the results of STM observations in the following section.

3.3 STM characterization


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Figure 3 shows the STM images of the n-C44H90 monolayer on the Au (111) surface in the lower density phase (a) and in the higher density phase (b–e). These images were obtained at room temperature under UHV. The vertical lamellar boundaries (dark troughs) are observed with an interval of ~6 nm in Figure 3a. This interval corresponds to the chain length of n-C44H90 molecules in the all-trans conformation, and hence, the molecular chains are orthogonal to the lamellar axis. The lateral herringbone pattern is characteristic of the reconstructed bare Au (111) surface,29,30 with the n-alkane lamellar structure grown to maintain this structure.3,4,6–8 This pattern is useful for deducing the molecular arrangement with respect to the lattice vector of the substrate, although the inner lamellar structure in Figure 3a is obscure. The herringbone ridges are a result of uniaxial compression along one atomic row, with this row being orthogonal to the herringbone ridge. Therefore, the lamellar axis, which makes an angle of 60° with the herringbone ridges, indicates that the molecular chains are aligned along another atomic row of the Au (111) surface, in agreement with previous results.3,4,6–10 The lack of the molecular image is reasonable because the molecules are mobile in the lower-density phase as in the smectic phase on the graphite basal plane.23 STM images of the higher density phase monolayer also exhibit lamellar structures (Figures 3b– 3e). The corresponding lamellar axes are not straight in some places, as indicated by the white arrow in Figure 3b. Figures 3c–3e show magnified STM images of the higher density phase monolayer. The width of the lamellae shown in Figure 3c is ~6 nm, identical to that of the lamellae in the solution-deposited monolayer (Figure 3a), indicating that the molecular chains are straight. Line patterns directed perpendicular to the lamellar axis are observed in Figure 3c. This pattern is ascribable to the molecules, and is observed because the mobility of the molecules is reduced. This is in accordance with the increase of density in the vapor-deposited monolayer.


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The molecular interval is computed to be 0.44 ± 0.05 nm from the periodicity analysis using the 2D Fast Fourier Transform (FFT) images. The error of ± 0.05 was calculated from the peak widths of the FFT spots. The interval of 0.44 nm is slightly smaller than the value advocated for the n-alkane lamellae observed at the liquid-solid interface (~0.48 nm), which corresponds to a lattice commensurate with the reconstructed Au (111) surface.7,8 The interval of 0.48 nm is consistent with the flat-on orientation because the minimal intermolecular distance for the flat-on orientation is estimated at ~0.46 nm due to van der Waals repulsion.7 On the other hand, the smaller value of 0.44 nm is consistent with the tilted-on orientation because the molecules can be packed more closely with the inclined carbon plane till the minimal value of ~0.43 nm for the edge-on orientation.31 The interval in the higher density phase with the tilted-on orientation— which is approximately 10% shorter than that in the lower-density phase with the flat-on orientation—is in agreement with the 10% coverage increase. The compression along the lamellar axis, which occurs in response to the molecular density increase of the monolayer, induces the orientation change from flat-on to tilted-on and the shrinkage of the intermolecular distance. The partial orientation change observed for molecules at the lamellae bending points is shown in a magnified image in Figure 3d. In this image, the molecules in the white circle seem to be twisted, and the lower portion of the chains shows enhanced contrast for the right half of the molecules, reflecting the tilted-on orientation. The left half of the molecules seems to retain the flat-on orientation, and therefore, the twisted chains should include the gauche conformation. Regularly aligned twisted chains form thinner lamellae, as observed in Figure 3e. The twisted chain is bent along its axis, as indicated by the white circle in Figure 3e. As a result, the width of these lamellae shrinks to ~5.4 nm, which is approximately 10% shorter than that of the lamellae


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composed of n-C44H90 molecules with the all-trans conformation. Therefore, these lamellae are compressed along the alkyl chain axis. The structural model for the two kinds of compressed lamellae is shown in Figure 3f. The above STM images provide a reasonable interpretation of the monolayer structure in the higher density phase, which is consistent with the C K-NEXAFS and IRA spectra discussed in the previous sections. The picture of molecules with the tilted-on orientation in the lamellae compressed along the lamellar axis (as shown in Figure 3c) is consistent with the C K-NEXAFS spectra, where the σ*CH/R|| and σ*CH/R⊥ resonances are detected under both NI and GI conditions. This rationalization is also in line with the IRA spectra, which show antisymmetric CH2 stretches. The symmetric CH2 stretches might be obscured by the upward band of the desorbed molecules. Compression along the chain axis causes an incomplete orientation change from flat-on to tiltedon, or twisting of the chains, as shown in Figures 3d and 3e; the gauche conformation in such chains exhibits higher-energy CH2 stretches. The segments with the flat-on orientation in these molecules show the CHdistal stretch in their IRA spectra. It should be noted that the spectral features corresponding to the flat-on orientation might be obscured by those of the tilted-on orientation in the C K-NEXAFS spectra of the higher density phase. In contrast to the lower density phase, no domains comprising all-trans molecules with the flat-on orientation exist in the higher density phase, since the omnipresent pressure in the monolayer due to the compression induced by the increased amount of adsorbed molecules causes orientation and conformation changes throughout the layer. Thus, the higher density phase monolayer responds to the electrode cathodic potentials differently than the lower density phase one, as shown in Figures 2d and 2h.


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For n-alkanes, the monolayer compression along the lamellar axis occurs on a graphite basal plane, causing a change of the intermolecular distance from 0.464 to 0.426 nm.31 Upon this compression, the carbon plane orientation for at least half of the molecules is changed from flaton to edge-on. This compressed phase is observed at low temperatures and is crystalline, with a lattice commensurate with that of graphite. In contrast, the higher density phase on Au (111) is observed at room temperature, with its intermolecular spacing being incommensurate with the substrate lattice; hence, this compression is different from that of the crystalline phase. Since compression to 0.426 nm with the edge-on orientation cannot provide an energy gain on the Au (111) surface, the molecules adopt the tilted-on orientation with an intermolecular spacing of 0.44 ± 0.05 nm. Moreover, the gauche conformation, which is energetically less stable, is populated at room temperature, and hence, the compression along the chain axis is allowed for the monolayer on Au (111). Further studies on the temperature dependence of the structure are necessary for a detailed discussion.

4. Conclusion The structure, orientation, and conformation of n-C44H90 molecules in a monolayer on the Au (111) surface were studied by C K-NEXAFS, STM, and in situ IRAS in sulfuric acid solutions as a function of the adsorbate amount. The molecules in the lower density phase adopt the flat-on orientation with the all-trans conformation and construct a lamellar assembly with a width of 6.0 nm. Conversely, the molecules in the higher density phase exhibit various lamellar structures due to compression. The compression along the lamellar axis induces the shortening of the intermolecular distance to 0.44 ± 0.05 nm, accompanied by the tilted-on molecular orientation.


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The compression along the molecular chain axis shows a smaller lamellar width of 5.4 nm, with the bent chains having gauche conformations. The compression is induced by the ~10–20% increase of the amount of molecules in the monolayer.

ASSOCIATED CONTENT Supporting Information Photoelectron intensity of the clean Au (111) surface, C K-NEXAFS spectra of the heated monolayer, absorption intensity changes upon heating, and the in situ IRA spectra of the solution-deposited monolayer not rinsed by n-hexane. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements C K-NEXAFS experiments were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2013G689 and 2015G562). This work was supported by JSPS KAKENHI Grant Number 26390061.

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(31) Diama, A.; Matthies, B.; Herwig, K. W.; Hansen, F. Y.; Criswell, L.; Mo, H.; Bai, M.; Taub, H. Structure and Phase transitions of Monolayers of Intermediate-length n-Alkanes on Graphite Studied by Neutron Diffraction and Molecular Dynamics Simulation. J. Chem. Phys. 2009, 131, 084707.


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Figure 1. C K-NEXAFS spectra of n-C44H90 monolayer on Au (111). (a) Lower density phase obtained from the corresponding solution. (b) Higher density phase obtained by vapor deposition. Black lines: normal incidence (NI). Red lines: grazing incidence (GI). Blue lines: NI–GI.


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Figure 2. (a) CV of the Au (111) electrode covered with the solution-deposited n-C44H90 monolayer in 0.5 M H2SO4. (b) CH stretch region and (c) CH bend and SO stretch regions of in situ IRA spectra for the Au (111) electrode covered with the solution-deposited nC44H90 monolayer in 0.5 M H2SO4 at anodic potentials. (d) CH stretch region of in situ IRA spectra for the Au (111) electrode covered with the solution-deposited n-C44H90 monolayer in 0.5 M H2SO4 at cathodic potentials. (e) CV of the Au (111) electrode covered with the vapor-deposited n-C44H90 monolayer in 0.5 M H2SO4. (f) CH stretch region and (g) CH bend and SO stretch regions of in situ IRA spectra for the Au (111) electrode covered with the vapor-deposited n-C44H90 monolayer in 0.5 M H2SO4 solution at anodic potentials. (h) CH stretch region of in situ IRA spectra for the Au (111) electrode covered with the vapordeposited n-C44H90 monolayer in 0.5 M H2SO4 at cathodic potentials


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Figure 3. (a) STM image of the lower density phase n-C44H90 monolayer on Au (111). Sample bias V = 2.0 V, tunneling current I = 30 pA. Image size = 55 × 55 nm2. (b) STM image of the higher density phase n-C44H90 monolayer on Au (111). V = 2.0 V, I = 200 pA, 55 × 55 nm2. (c) Magnified STM image of the higher density phase n-C44H90 monolayer on Au (111) with the tilted-on orientation. V = 2.0 V, I = 200 pA, 13 × 13 nm2. (d) Magnified STM image of the higher density phase n-C44H90 monolayer on Au (111) indicating orientation change accompanied by conformation change. V = 2.0 V, I = 300 pA, 13 × 13 nm2. (e) Magnified STM image of the higher density phase n-C44H90 monolayer on Au (111) including the gauche conformation. V = 2.0 V, I = 200 pA, 13 × 13 nm2. (f) Structural model for the higher density phase n-C44H90 monolayer on Au (111).


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TOC GRAPHIC


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Figure 1. C K-NEXAFS spectra of n-C44H90 monolayer on Au (111). (a) Lower density phase obtained from the corresponding solution. (b) Higher density phase obtained by vapor deposition. Black lines: normal incidence (NI). Red lines: grazing incidence (GI). Blue lines: NI-GI. 257x89mm (150 x 150 DPI)


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Figure 3. (a) STM image of the lower density phase n-C44H90 monolayer on Au (111). Sample bias V = 2.0 V, tunneling current I = 30 pA. Image size = 55 × 55 nm2. (b) STM image of the higher density phase n-C44H90 monolayer on Au (111). V = 2.0 V, I = 200 pA, 55 × 55 nm2. (c) Magnified STM image of the higher density phase n-C44H90 monolayer on Au (111) with the tilted-on orientation. V = 2.0 V, I = 200 pA, 13 × 13 nm2. (d) Magnified STM image of the higher density phase n-C44H90 monolayer on Au (111) indicating orientation change accompanied by conformation change. V = 2.0 V, I = 300 pA, 13 × 13 nm2. (e) Magnified STM image of the higher density phase n-C44H90 monolayer on Au (111) including the gauche conformation. V = 2.0 V, I = 200 pA, 13 × 13 nm2. (f) Structural model for the higher density phase n-C44H90 monolayer on Au (111). 79x120mm (150 x 150 DPI)


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