Promoting Effect of Protecting Group on the Structure and Morphology

Article pubs.acs.org/JPCC

Promoting Effect of Protecting Group on the Structure and Morphology of Self-Assembled Monolayers: Terphenylylethanethioactate on Au(111) Asif Bashir,*,† Danish Iqbal,† Sagar Motilal Jain,‡,∥ Kathrin Barbe,§ Tarek Abu-Husein,§ Michael Rohwerder,† Andreas Terfort,§ and Michael Zharnikov*,‡ †

Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany § Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany ‡

ABSTRACT: Taking self-assembled monolayers (SAMs) of 4,4′terphenylyl-substituted ethanethioacetate (TP2-SAc) on Au(111) as a test system, we studied the effect of the protecting group on the structure and morphology of this monolayer. The films were prepared at both room (298 K) and elevated (333 K) temperature, at either the presence or absence of a deprotecting agent, viz., triethylamine. The presence of the protecting group resulted in distinctly different crystallographic structure, described by the (2√3 × 4) rect unit cell, in all SAMs studied as compared to the case of the nonprotected analogue. The molecules within this unit cell were arranged in a herringbone fashion as could be observed by variation of the scan direction during the image acquisition by scanning tunneling microscopy. Most important and in contrast to previous studies of similar systems, the presence of the protecting group led to significant improvement of the SAM morphology in the case of preparation at 333 K, resulting in formation of comparably large domains with dimensions exceeding 100 nm. The effect of the deprotecting agent was found to be small when preparing at 298 K and hardly perceptible at 333 K. Determination of the reaction kinetics gave evidence of a completely different reaction mechanism for the thioacetate as compared to the thiol, which presumably is responsible for the observed differences.

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INTRODUCTION Self-assembled monolayers (SAMs) attract considerable attention from both scientific and industrial communities because of their flexibility in tailoring surface properties such as wetting behavior1,2 and affinity to biomolecules3−5 as well as potential applications including, in particular, SAM-based lithography,6,7 corrosion inhibition,8−11 and sensor fabrication.12 While the early work was mainly focused on alkanethiols,13,14 aromatic monolayers are particularly favorable as basic systems for nanolithography and nanofabrication15−18 but also in view of their superior electronic transport properties19,20 as well as potential applications as molecular transistors,21 switches,22−25 and intermediate layers in organic electronics assemblies.26,27 A prerequisite of such applications is, in most cases, a precise control over molecular structure and morphology. Such a control is, however, a difficult task for aromatic monolayers on coinage metal substrates, particularly in view of a misfit between the optimal crystalline lattice of the aromatic moieties and the structural template provided by the substrate which is, most frequently, Au(111). This results in significant stress which is released by introducing defects, domain boundaries, and stacking faults.28,29 Several routes have been suggested to improve the structural perfection to yield © XXXX American Chemical Society

defect-free, long-range ordered aromatic SAMs, viz., (i) preparation of monolayers at elevated temperature, (ii) postdeposition annealing, (iii) introduction of flexible aliphatic spacer between the docking group and aromatic ring,30−34 (iv) the use of selenium as docking group, as an alternative to sulfur, which up to now has been most frequently utilized for this purpose.29,35−39 Another factor that can be of importance for structure and morphology of SAMs is the exact chemical composition of the precursor, particularly the presence of a protecting group at the docking moiety. Without such a protecting group, complex preparation protocols have to be applied to avoid oxidation of the docking group, e.g., of thiols to disulfides40,41 or sulfonates, which otherwise would result in a limited quality of the resulting SAMs.42,43 To overcome this problem, various protected precursors were employed such as thiosulfates,44,45 thiocyanates,46,47 and thioacetates.20,48−50 However, the effect of protecting group was not always positive only. In particular, several studies clearly demonstrated that SAMs obtained from Received: July 15, 2015 Revised: October 20, 2015

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DOI: 10.1021/acs.jpcc.5b06813 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C thioacetates51,52 and disulfides53 are of inferior quality as compared to the monolayers derived from their thiols analogues. Such films exhibited poor structural order and inhomogeneous surface morphology, while analogous thiols formed homogeneous SAMs with well-defined crystalline domains.54 In particular the introduction of the protecting acetyl group to the ferrocene-substituted biphenylthiols caused a noticeable (by 20%) lowering of the packing density and a decrease in the orientational order of the resulted films as compared to the SAM prepared from the nonprotected molecules.55 At the same time, no negative effect of the acetyl protecting group on the structure and morphology of SAMs was observed in some other cases, e.g., for monolayer of ruthenocene-substituted biphenylthiols on gold.56 Thus, the results on the effect of protecting group are controversial and further studies to clarify specific aspects of this phenomenon are necessary. In this context, we studied here the effect of the protecting acetyl group on the structure and morphology of a terphenylsubstituted alkanethiolate SAM on Au(111), taking 4,4′terphenyl-substituted ethanethiol (TP2-SH) and the respective thioacetate (TP2-SAc) as test systems. The first reason for this choice was that the TP2-SH derived SAMs are well studied and can therefore be taken as a reliable reference.31,33 The second reason was that the above SAMs are most likely prone to polymorphism, similar to the monolayers formed by the analogous biphenyl-substituted compounds.57−59 Accordingly, the structure and morphology of the TP2-based SAMs are strongly affected by the preparation temperature but may also be dependent on the presence of the protecting group. In view of the strong effect of the preparation temperature, we varied this parameter in the present study as well. Another factor, considered at the same time, was the use of a deprotecting agent. Such agents are frequently applied upon the preparation of SAMs from protected precursors but their effect is controversial so that the use or omission of a deprotecting agent is optional and, probably, specific for a particular system. In some cases, such an agent has no influence on the SAM quality, e.g., as happens for ruthenocenesubstituted biphenylthiols56 or acetyl protected hexabenzocorone (HBC) thiols20,60 on Au(111). On the other hand, other studies documented that SAMs obtained from thioacetates can only achieve high structural quality upon the use of a deprotecting agent.61

(d,3JH−H = 8.2 Hz, 2H, CH), 7.48−7.45 (m, 2H, CH), 7.38− 7.35 (m, 1H, CH), 7.32 (d,3JH−H = 8.2 Hz, 2H, CH), 3.19− 3.16 (m, 2H, CH2), 2.95−2.92 (m, 2H, CH2), 2.36 (s, 3H, CH3). 13C NMR (125 MHz, CDCl3): δ (ppm) = 195.9 (CO); 140.8, 140.1, 139.9, 139.3, 139.1 (C-1, C-4, C-1′, C-4′, C-1″); 129.2, 128.9, 127.6, 127.5, 127.5, 127.3, 127.2 (C-2, C-3, C-5, C-6, C-2′, C-3′, C-5′, C-6′, C-2″ to C-6″); 35.6 (CH2); 30.9 (CH3); 30.6 (CH2). Triethylamine (TEA) was purchased from ABCR. Ethanol and acetone were purchased from Sigma-Aldrich. All chemicals were used as received. For the kinetic measurements, ethanol was ultrapurified following a literature protocol.63 Preparation of the Substrates. Two kinds of gold substrates were used. The substrates for the spectroscopic measurements were prepared by subsequent evaporation of 5 nm of titanium and 200 nm of gold onto Si(100). STM measurements were carried out on substrates that have been prepared by evaporating 200 nm of Au onto freshly cleaved mica, which had been preliminarily annealed at ∼600 K for 3 days in the evaporation chamber. After the Au evaporation, the substrates were cooled and the chamber was backfilled with nitrogen. The substrates were stored under argon and flameannealed in a butane/oxygen flame immediately before the adsorption experiments were carried out. This procedure yields Au substrates with large terraces (several hundreds of nanometres, as evidenced by STM) exhibiting a (111) surface. Similar orientation is assumed to dominate for the Au/Ti/ Si(100) substrate, even though with a smaller size of individual domains.64 Preparation of SAMs. The monolayers were prepared by immersing the substrates into the dilute ethanolic solutions (0.1 mM) of TP2-SAc or TP2-SH with/without addition of TEA (0.7 mM). The solution temperature was kept at either 298 K, denoted as RT, or 333 K, denoted as HT. The immersion time was 24−35 h. Afterward, the samples were taken out from the solution and rinsed carefully with pure ethanol, acetone, and once more with ethanol. Finally, the samples were dried in a nitrogen stream. Characterization: General Comments. The TP2-SAc derived SAMs were characterized by scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), highresolution XPS (HRXPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. All measurements were performed at room temperature. The XPS, HRXPS, and NEXAFS spectroscopy measurements were carried out under ultrahigh vacuum (UHV) conditions at a base pressure better than 1.5 × 10−9 mbar. In addition, the kinetics of the SAM formation was studied using electrical resistance measurements. Scanning Tunneling Microscopy. STM measurements were carried out in air, using an Agilent Technologies 5500 STM device which had been cross-calibrated by imaging HOPG with atomic resolution. The tips were prepared mechanically by cutting a 0.25 mm Pt0.8Ir0.2 wire (Chempur). All data were collected in a constant-current mode with typical tunneling currents of 50−300 pA and a sample bias of 0.5−0.7 V (tip positive). Infrared Reflection Absorption Spectroscopy. The spectra were recorded at a Biorad Excalibur Fourier transform infrared spectrometer (FTS 3000) equipped with a grazing incidence reflection unit (Biorad Uniflex) and a narrow band MCT detector.

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EXPERIMENTAL SECTION Chemicals. 2-[1.1′;4′.1″]Terphenyl-4-ylethan-1-thiol (C6H5(C6H4)2(CH2)2SH; TP2-SH) was synthesized according to the literature.62 2-[1.1′;4′.1″]Terphenyl-4-ylethan-1-thioacetate (C6H5(C6H4)2(CH2)2S-CO-CH3; TP2-SAc) was synthesized as follows: A stirred mixture of terphenylylethanthiol (0.58 g, 2 mmol), acetic anhydride (0.3 mL, 3 mmol), and silver triflate (5 mg, 1 mol %) was heated to 60 °C for 90 min. After completion, the reaction mixture was washed with sat. aq NaHCO3 (5 mL) and extracted with CH2Cl2 (3 × 10 mL). Purification of the crude product by column chromatography on silica gel with CH2Cl2/n-hexane (1:1) gave a white solid (Rf = 0.3). To obtain the colorless product with high purity, gradient sublimation in high vacuum is necessary. Isolated yield: 0.60 g (90%). Mp 172 °C. HR-MS (MALDI): C22H22OS [M]+ calcd 332.12294; found 332.12297. EA: calcd C 79.48, H 6.06, S 9.64; found C 79.20, H 6.03, S 10.02. 1H NMR (500 MHz, CDCl3): δ (ppm) = 7.68−7.64 (m, 6H, CH), 7.60 B


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XPS. The measurements were performed using a laboratory spectrometer equipped with a Mg Kα X-ray source and a hemispherical electron energy analyzer. The X-ray source was operated at 260 W power and positioned ∼1 cm away from the samples. The spectra were acquired in normal emission geometry with an energy resolution of ∼0.9 eV. The binding energy (BE) scale was referenced to the Au 4f7/2 peak at a BE of 84.0 eV.65 Since the quality of the XP spectra was inferior to the high resolution ones, they were only used to calculate the effective thickness of the TP2-SAc derived monolayers. The latter parameters were calculated by evaluating the intensity ratios of the C 1s and Au 4f emissions66 and using a monolayer of well-defined thickness, ODT/Au (21.3 Å),67,68 as a reference. A standard, exponential attenuation of the photoemission signal was assumed; attenuation lengths reported for a series of nonsubstituted alkanethiolate SAMs were used.69 The C 1s and Au 4f spectra were fitted by symmetric Voigt functions and either Shirley-type or linear backgrounds. HRXPS. The measurements were conducted at the bending magnet D1011 beamline at the MAX-IV synchrotron radiation facility in Lund, Sweden. The spectra acquisition was performed in the normal emission geometry and at photon energies (PEs) of 350−580 eV. The BE scale of every spectrum was individually calibrated to the Au 4f7/2 emission of the gold substrate at 84.0 eV.65 The energy resolution was better than 100 meV. NEXAFS Spectroscopy. The spectra were recorded at the C K-edge, at the same beamline as the HRXP ones. The acquisition was performed in the partial electron yield acquisition mode with a retarding voltage of −150 V. Linearpolarized synchrotron light with a polarization factor of 95% was used as the primary X-ray source. The energy resolution was better than 100 meV. The incidence angle of the X-rays was varied from 90° (electric field vector in surface plane) to 20° (electric field vector nearly parallel to surface normal) in steps of 10−20° to monitor the orientational order in the SAMs. This approach is based on the dependence of the cross section of the resonant photoexcitation process on the orientation of the electric field vector of the synchrotron light with respect to the molecular orbital of interest (so-called linear dichroism in X-ray absorption).70 The PE scale was referenced to a pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV.71 Determination of Adsorption Kinetics. For the determination of the SAM formation kinetics, gold films of 25 nm thickness with an underlying chromium layer of 2 nm (as adhesion promotor) were utilized. The resistivity of these films was determined in a time-resolved manner by a two-point measurement using a 3607 digital multimeter (Keithley), which was connected to a computer using custom-made software. To avoid contaminations, the glassware was cleaned using 10% KOH/H2O containing 10 mM H2O2, followed by rinsing with hot water, deionized water, and ultrapure water (Millipore, 18.2 MΩ cm) before each experiment. The sensors were calibrated with HDT and cleaned by hydrogen plasma immediately before the measurements. All kinetic experiments were performed at 293.1 ± 0.1 K, with the temperature maintained constant by a FP 40 thermostat (Julabo). The thiols and the thioacetate were each dissolved in ultrapure ethanol at the desired concentrations (1−1000 μM) and thermally equilibrated before the measurements started.

RESULTS STM. STM data for the TP2-SAc derived SAMs are presented in Figures 1−4 and summarized in Table 1. The Table 1. Overview of the STM Results (Figures 1−4) for the TP2-SAc Derived SAMs system

structure

molecular footprint (Å2)

domain size (nm)

RT RT + TEA HT HT + TEA

β α + β (main) β β

28 21.6/28 28 28

10−15 10−15 >100 >100

data for the SAM prepared at RT in the presence of TEA are compiled in Figure 1. The STM image in Figure 1a shows

Figure 1. Summary of the STM data obtained for TP2-SAc derived SAM prepared at RT in the presence of TEA: (a−d) representative STM images taken at different magnifications. A coexistence of two phases labeled as α and β is illustrated in (b). The structure of these phases is well visible in (c) and (d). Selected areas of the images in (c) and (d) are magnified to show the respective unit cells, which are marked. The height profiles of α and β phase along the black lines in (c) and (d) are presented in (e) and (f), respectively. Orientation of the underlying Au(111) substrate is indicated in (c) and (d).

atomically flat terraces of Au(111) decorated with the adsorbed molecules forming numerous defects such as depressions and domain boundaries. The depression can be attributed to the vacancy islands in the topmost layer of the substrate, since their depth corresponds exactly to the step height on the Au(111) surface, i.e., 2.4 Å.72,73 The STM image presented in Figure 1b, corresponding to a higher magnification, exhibits a coexistence of two different structures that are denoted as the α and β phases. The β phase was found to be the dominating one, but the α phase could also be clearly recognized in some areas. A C


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The Journal of Physical Chemistry C close inspection of the respective high resolution STM image in Figure 1c, along with the height profiles presented in Figure 1e, reveals that the α phase has an oblique unit cell defined by the lattice vectors a = 0.51 ± 0.05 nm and b = 0.98 ± 0.05 nm. This unit cell corresponds to the (2√3 × √3)R30° structure characteristic also of the TP2-SH derived SAMs prepared at RT.33 This structure, shown schematically in Figure 2a, has four

Figure 2. Proposed structural models for the TP2-SAc derived SAMs on Au(111): (a) α-phase with the (2√3 × √3)R30° unit cell; (b) βphase with the rectangular (2√3 × 4) unit cell. The molecules are drawn tilted, according to the NEXAFS spectroscopy data (see below). The adsorption sites are selected arbitrarily, since they are not exactly known.

molecules in the unit cell corresponding to a molecular footprint of 21.6 Å2. A close inspection of the high resolution STM image of the β phase in Figure 1d, along with the height profiles presented in Figure 1f, reveals that this phase has a rectangular unit cell defined by the lattice vectors a = 0.98 ± 0.1 nm and b = 1.12 ± 0.05 nm. Such an arrangement corresponds to the (2√3 × 4) rect structure, which should have a size of 0.99 nm × 1.15 nm and is shown schematically in Figure 2b. The respective unit cell accommodates four molecules with a molecular footprint of ∼28 Å2. Representative STM data for the TP2-SAc derived SAM prepared at RT without TEA are shown in Figure 3. Similar to the case of TEA (Figure 1a), large scale STM images in Figure 3a and Figure 3b exhibit high density of vacancy islands and small rotational domains with a typical size of 10−15 nm. These domains are rotated by 60° mimicking the symmetry of the underlying gold substrate, as marked in Figure 3b by black arrows. A representative high resolution STM image of a single rotational domain displayed in Figure 3c, along with the respective height profiles in Figure 3d, suggests that the structure of these domains corresponds to the β phase. The observed unit cell with the lattice vectors a = 0.97 ± 0.1 nm and b = 1.13 ± 0.05 nm corresponds indeed to the (2√3 × 4) rect structure with four molecules per unit cell and a molecular footprint of ∼28 Å2. This is the only molecular arrangement observed at the given preparation conditions. Figure 4 summarizes STM data obtained for the TP2-SAc derived SAM prepared at HT without TEA. On large scale (Figure 4b), remarkable changes in the surface morphology were observed by comparing to the TP2-SH derived monolayer (Figure 4a) fabricated at the same preparation conditions. First, no vacancy islands were found in the case of TP2-SAc, which is also in contrast to the typical morphology of other biphenyl and terphenyl based thiol monolayers.33,34,57,74 Second, the TP2-SAc derived SAM exhibits large (>100 nm), defect-free crystalline domains (Figure 4b) with bright and dark periodic contrast forming stripes running preferentially along ⟨110̅⟩ direction of Au(111) (Figure 4e). In the vicinity of the step edges, a certain evolution of these stripes occurred, which eventually led to the dense network of bright rows close to the grain boundaries. The separation between the stripes varies to

Figure 3. Summary of the STM data obtained for TP2-SAc derived SAM prepared at RT in the absence of TEA: (a−c) representative STM images taken at different magnifications. The unit cell is marked in (c). The height profiles along the black lines in (c) are presented in (d). Orientation of the underlying Au(111) substrate is indicated in (c).

some extent, whereas the height difference between the bright and dark stripes is around 3−4 Å. In addition, a close inspection of the STM images reveals the presence of bright spots which, presumably, also contain adsorbed molecules; these spots are marked by white dashed circles in Figure 4d and Figure 4f. For a detailed structural analysis, we turn to the high resolution STM image displayed in Figure 4g, along with the respective height profiles shown in Figure 4h. The measured lattice constants are a = 0.86 ± 0.2 nm and b = 1.14 ± 0.05 nm corresponding to the (2√3 × 4) rect unit cell, characteristic of the β phase and observed previously for the TP2-SAc derived SAMs prepared at RT. Representative STM data for the TP2-SAc derived SAM prepared at HT in the presence of TEA are depicted in Figure 5. Once again we observe large, defect-free crystalline domains. Interestingly, the boundaries of these domains are decorated by small (10−15 nm) vacancy islands with a depth corresponding to 2−4 atomic layers of gold. The domains exhibit a characteristic stripe superstructure with an average distance between the bright or dark stripes of ∼7 nm. According to the high resolution STM images in Figure 5d and Figure 5e, the molecular structure is identical within both bright and dark stripes. A close inspection of these images, along with the relevant height profiles in Figure 5f, suggests once again the formation of the β phase. Accordingly, the unit cell vectors a = 0.97 ± 0.05 nm and b = 1.12 ± 0.05 nm correspond to (2√3 × 4) rect structure with four molecules per unit cell and a molecular footprint of 28 Å2. As to the origin of the observed stripe superstructure, it is presumably attributed to solitons (or domain walls), resulting from structural mismatch between the most favorable arrangement of the molecular adlayer and the “template” provided by the underlying gold substrate.28 Apart from the identification of the SAM structure, a clear effect of the scan direction during the acquisition of the STM D


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Figure 4. STM image of a TP2-SH derived SAM prepared at HT (a), along with a summary of the STM data obtained for TP2-SAc derived SAM prepared at HT in the absence of TEA (b−h). (b−g) Representative STM images taken at different magnifications. The unit cell is marked in (g). The height profiles along the white lines in (g) are presented in (h). Orientation of the underlying Au(111) substrate is indicated in some cases (see text for details).

Figure 5. Summary of STM data obtained for TP2-SAc derived SAM prepared at 333 K in the presence of TEA: (a−e) representative STM images taken at different magnifications. The arrows in (a) show the three different orientational domains rotated by 60°. The periodic contrast in (b) is highlighted by a line scan across the rows (black line). The unit cell is marked in an inset in (c) and, additionally, in (d) and (e) . This unit cell can be found within both bright and dark rows as shown in (d). The height profiles along the white lines in (e) are presented in (f). Orientation of the underlying Au(111) substrate is indicated in some panels.

images was observed. Comparison of high resolution images acquired from the same area at different scan directions reveals remarkable variation in the appearance of the molecular structure with the characteristic (2√3 × 4) rect unit cell as shown in Figure 6. Whereas the basic parameters of this structure were persistent upon the variation of the scan direction, the contrast and shape of particular features, associated with individual molecules, varied significantly. In particular, a transition of bright circular protrusions into elliptical shape was clearly observed at a certain scan direction (Figure 6c), mimicking the specific geometry of the oligophenyl

chains and highlighting a herringbone-like arrangement of the neighboring molecules with a twist angle of 90°. This is an additional, valuable information about the molecular arrangement in the TP2-SAc derived SAMs, supporting and verifying our tentative model in Figure 2b. This observation also highlights the importance of the scan direction for the recognition of subtle details of molecular structure. Such details were observed in few selected cases only,36,75 but probably, this is also possible for a broad variety of monomolecular films if the scan direction is varied. E


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a significant improvement in the structural ordering upon increase in the preparation temperature. XPS and HRXPS. The S 2p and C 1s HRXP spectra of the TP2-SAc derived SAMs are presented in Figure 8a and Figure

Figure 6. Series of STM images recorded for a TP2-SAc derived SAM (333 K, TEA) at varying scan direction, marked by a double-headed white arrow. The unit cell is marked. Herringbone-like arrangement of individual molecules (blue ellipses) is perceptible in (c).

IRRAS. Figure 7 shows comparison of low-frequency region of the IRRAS spectra of bulk TP2-SH and of the TP2-SAc

Figure 8. S 2p (a) and C 1s (b) HRXP spectra of the TP2-SAc derived SAMs. The spectra were acquired at a photon energy of 350 eV.

8b, respectively. The S 2p spectra in Figure 8a are dominated by the characteristic S 2p3/2,1/2 doublet at a BE position of ∼161.9 eV (S 2p3/2), for all TP2-SAc derived monolayers, with only a weak trace of atomic sulfur or differently bound molecules (a BE position of ∼161.0 eV for S 2p3/2).77 The BE value ∼161.9 eV corresponds to the thiolate species bound to noble metal surfaces,77,78 suggesting that almost all molecules in the films were bound to the substrate via a thiolate−gold bond, as it should be in well-defined SAMs. No traces of other sulfurderived species such as physisorbed sulfur, disulfide, SAc, or sulfonate were observed. The C 1s HRXP spectra of all TP2SAc derived SAMs in Figure 8b exhibit a single emission at a BE of 284.1−284.2 eV, typical of the films with terphenyl backbone,77 with no trace of oxidative species, including acetate. These spectra mimic the analogous spectrum of the TP2-SH derived SAM on Au.31 The lack of residual acetate species and other oxygen contamination is supported by the O 1s HRXP spectra (not shown); occasional traces of O were observed only for the TP2-SAc derived SAMs prepared at RT in the presence of TEA. The effective thicknesses of the TP2-SAc derived SAMs, calculated on the basis of the XPS data, are compiled in Table 2. The thicknesses are similar to or slightly smaller than the

Figure 7. Comparison of IRRAS spectra of the bulk TP2-SH and TP2SAc derived SAMs.

derived SAMs prepared at different conditions. The spectra are dominated by the characteristic absorption modes at 761 cm−1 (1), 1002 cm−1 (2), and 1486 cm−1 (3), which can be assigned to out-of-plane perpendicular (op-perp; 761 cm−1) and in-plane perpendicular (ip-perp; 1002, and 1486 cm−1) vibrational bands of the terphenyl backbone.31,33,76 These spectra are similar to the analogous spectra of the TP2-SH derived SAMs on Au(111).31,33 For the case of upright standing aromatic molecules, the intensity of the op-perp bands should be lower than that of the ip-perp bands, which is indeed the case for all TP2-SAc derived SAMs of this study. This follows from comparison of the spectra in Figure 7 to the spectrum of bulk TP2-SH where the op-perp bands have almost the same or even higher intensity than the ip-perp bands. Beyond this general statement, relative intensities of the op-perp and ip-perp bands can be compared to each other as a fingerprint of the orientational order and molecular orientation. In this context, the effect of the deprotecting agent on the above parameters seems to be minor, since no pronounced difference between the spectra of the monolayers prepared in the presence or absence of TEA could be observed at both RT and HT. At the same time, the intensity of the 1486 cm−1 mode (ip-perp) is noticeably higher for the films prepared at HT as compared to those fabricated at RT, whereas the intensity of the 761 cm−1 mode does not change visibly. This suggests a low molecular inclination and/or

Table 2. Effective Thickness of the TP2-SAc Derived SAMsa

a

system

RT

RT + TEA

HT

HT + TEA

effective thickness (nm)

1.70

1.83

1.93

1.87

The error bars can be estimated as ±5°.

sum of the molecular length (16.2 Å) and the length of the S− Au bond (2.4−2.5 Å),79,80 which suggests upright molecular orientation in the SAMs studied. The effective thicknesses of the films prepared at HT is higher than those of the films fabricated at RT, which suggests a promoting effect of the elevated temperature on the film quality, in accordance with the IRRAS data (see the respective section). Another interesting feature is a higher effective thickness for the films prepared at room temperature with TEA as compared to those fabricated without the deprotecting agent. This can be related to the F


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The Journal of Physical Chemistry C presence of the high density α phase in the latter film (see the STM section and Table 1, in particular). NEXAFS Spectroscopy. The C K-edge NEXAFS spectra of the TP2-SAc derived SAMs are presented in Figure 9. The

Table 3. Derived Average Tilt Angle of the π1* Orbital for the TP2-SAc Derived SAMs, along with the Average Tilt Angle of the Terphenyl Backbonea system

tilt, π1* (deg)

tilt, backbone (deg)

RT RT + TEA HT HT + TEA

65 67 70 69

30 28 24 25

a A planar conformation and a twist angle of 32° were assumed. The error bars can be estimated as ±3°.

TEA, it is hardly perceptible at HT but is pronounced at RT. Indeed, the SAMs prepared with TEA exhibit a smaller molecular inclination as compared to that fabricated without. Note that because of the integrating character of NEXAFS spectroscopy, it cannot distinguish between a smaller molecular inclination and a better orientational order of a molecular 2D assembly. Therefore, the average angles compiled in Table 3 can also be considered as fingerprints of orientational order, according to the principle the smaller the angle, the higher the order. Note also that the relations between the orientational order in the films studied could be visualized by an alternative evaluation procedure of the NEXAFS data, viz., consideration of the intensities of the difference peaks at the position of the π1* resonance.70,82 A suitable plot of such intensity difference as a function of cos2 θ − cos2 θd, where θ and θd are variable and fixed angles of X-ray incidence, should represent a straight line with a slope proportional to the average tilt angle of the respective molecular orbital.70 As seen in Figure 10, the slope of

Figure 9. C K-edge NEXAFS spectra of the TP2-SAc derived SAMs acquired at an X-ray incident angle of 55° (a), along with the respective difference between the spectra collected under the normal (90°) and grazing (20°) incidence geometry (b). Individual absorption resonances are marked by numbers (see text for the assignments). The horizontal dashed lines in panel b correspond to zero.

spectra depicted in Figure 9a are most representative of the electronic structure of the studied systems, since they were acquired at the so-called “magic” angle of X-ray incidence (55°) and are, therefore, not affected by orientational effects.70 The difference spectra shown in Figure 9b are fingerprints of molecular orientation, relying on the linear dichroism effect in X-ray absorption (see Experimental Section).70 The spectra in Figure 9a exhibit characteristic absorption structure of phenyl rings70,81 and mimic the analogous spectrum of the TP2-SH derived SAM on Au.31 The spectra are dominated by a pronounced absorption resonance at ∼285.0 eV (π1*; 1) accompanied by several weaker and broader resonances at ∼287.6 eV (mixture of R* and C−H*; 2), ∼289.0 eV (π2*; 3), ∼293.0 eV (σC−C*; 4), ∼297.0 eV (σC−C*; 5), and ∼305.0 eV (σC−C*; 5).70,81 The transition dipole moments (TDMs) of the molecular orbitals related to the π1*, R*, and π2* resonances are oriented perpendicular to the phenyl rings, while TDMs of the σC−C* resonances are directed along the molecular axis.70,81 Accordingly, positive anisotropy peaks at the positions of the π1*, R*, and π2* resonances and negative anisotropy peaks at the position of the σC−C* resonances in the 90°−20° curves in Figure 9b suggest an upright molecular orientation in all SAMs studied. A quantitative evaluation of the π1* resonance intensity for the entire sets of the NEXAFS spectra, performed within the standard procedure for a vector-type orbital,70,82 resulted in the average tilt angles of the π1* orbital compiled in Table 3. Assuming a planar molecular conformation, typical of bulk terphenyls83 and densely packed 2D assemblies,55 and a reasonable twist angle of 32°,36,83,84 we calculated the average tilt angles of the terphenyl backbones in the SAMs studied. The respective values are compiled in Table 3. According to this table, the films prepared at HT exhibit a smaller molecular inclination as compared to the monolayers fabricated at RT, which is in full agreement with both IRRAS and XPS data (see the respective sections). As to the effect of

Figure 10. Plots of the intensities of the difference peaks at the position of the π1* resonance (see Figure 9b) for the TP2-SAc derived SAMs versus cos2 θ − cos2 θd (θd = 20°) with the respective linear fits (the solid lines) using least-squares analysis.

the lines for the films prepared at HT is larger than that for the SAMs fabricated at RT, suggesting, in agreement with Table 3,a smaller molecular inclination in the former case. The positive effect of TEA at RT is perceptible as well, again in agreement with Table 3. Indeed, the slope of the line for the SAM prepared with TEA is larger than that for the film fabricated without. SAM Formation Kinetics. To understand the influence of the protective group on the SAM formation kinetics, we used a sensory principle developed by Bohn et al.,85,86 which is based on the change in the electrical resistance of a very thin metal film upon adsorption as described by the Fuchs−Sondheimer theory.87,88 Shortly said, these sensors detect the increase in G


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The Journal of Physical Chemistry C electrical resistance of such a film (here, gold) upon chemisorption of an adsorbate. For a significant contribution of this interfacial resistance to the total resistance of the system, the surface/bulk ratio needs to be maximized, which can be most easily attained by employing thin films. The effect becomes significant when the thickness of the films is in the same order of magnitude as the free mean path of the electrons in the bulk, thus typically film thicknesses below 50 nm are employed. Since the resistivity of these films also changes with temperature, this parameter has to be carefully controlled. For technical reasons, measurements at 60 °C and in the presence of (TEA) could not be performed, so this investigation was limited to the deposition of the materials at 20 °C. Figure 11 depicts two representative isothermal adsorption curves for TP2-SH and TP2-SAc from 400 μM solutions in

which permits the determination of ka and n from a doublelogarithmic plot of the initial rate as a function of precursor concentration. Such plots for TP2-SH, TP2-SAc as well as for HDT, for comparison, are shown in Figure 12. The respective

Figure 12. Initial rates of the SAM formation for TP2-SH and TP2SAc. The data for HDT are presented for comparison. From the data, the reaction constants and the reaction order can be extracted as described in text.

Table 4. Kinetic Data Derived from the Plots in Figure 12

Figure 11. Isothermal adsorption kinetics for the formation of SAMs from TP2-SH (blue) and TP2-SAc (black) by resistivity measurements. The data are normalized to the complete coverage theoretically attainable by TP2-SH after infinite time. Left: Overview for 29 h. Right: Close-up of the first 30 min. The noise of the data is within the width of the lines except for the very early recording time for TP2-SH, where a higher sampling rate (and thus bigger noise) was chosen for a better temporal resolution.

SAM precursor

reaction constant ka, Ln/(moln·s)

reaction order n

HDT TP2-SH TP2-SAc

(2 ± 1) × 105 (3 ± 2) × 104 (8 ± 3) × 10−2

1.22 ± 0.06 1.14 ± 0.07 0.26 ± 0.04

kinetic data are compiled in Table 4. As can be seen in Figure 12, the behavior of TP2-SH and HDT is very similar, with both reaction orders of around 1 but a somewhat lower reaction constant for TP2-SH. This could be expected due to the identical surface chemistry taking place in both cases. In contrast, the behavior of TP2-SAc is significantly different: With an even slower reaction kinetics and a reaction order of about 0.25, a significantly more complex surface reaction has to take place. While within this project a complete elucidation of the mechanism cannot be provided, reaction orders below unity are typical for reactions in which dissociations have to occur.89 Tentatively, the thioacetate dissociates at the surface to form a thiolate species and an acetyl group bound to one or more Au atoms. The latter has to be removed to permit the adsorption of another TP2-SAc molecule, which would explain the significantly slower SAM formation at higher coverages (see above). It is unclear at this point how the acetyl group becomes removed from the surface, but the participation of a solvent molecule (under formation of ethyl acetate) seems to be the most likely scenario. During this process, the chemisorbed thiolate might be able to adjust both conformationally and with respect to the adsorption site, resulting in the (2√3 × 4) rect structure that is not observed upon the direct chemisorption of the thiol.

ethanol. As anticipated, the layer formation from TP2-SAc proceeds significantly slower than from the thiol. In addition, a generally different kinetic behavior can be deduced from the shape of the curves. While the adsorption of the thiols seems to follow the well-established Langmuir kinetics, the formation of the SAM from the thioacetate is significantly delayed in the later stages of monolayer formation. This hints of a hampered completion of the monolayer, which can be attributed either to reduced adsorption energy or to a substitution mechanism, where weaker bound adsorbates are replaced by stronger bound ones. The corresponding energetics can be described by either a Tempkin or a Freundlich isotherm, depending on the relation between surface coverage and adsorption energy (linear vs logarithmic decrease). To become independent of the coverage-related adsorption energy, the initial rates of the adsorption processes dθ/dt|t→0 were determined. At this time interval, the adsorbate coverage is small, so the desorption processes can be neglected. Thus, the kinetics can be simplified to dθ = kac(A)n dt

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with θ being the surface coverage, ka the reaction constant, c(A) the concentration of the SAM precursor in the solution, and n the reaction order. Taking the common logarithm of this equation, one gets a linear relation

DISCUSSION The structure and morphology of the TP2-SAc derived SAMs described above should be compared with those for the reference system of the TP2-SH derived monolayers. According to the literature33 and control measurements performed in the present study, TP2-SH form on Au(111) the (2√3 × √3)

⎛ dθ ⎞ lg⎜ ⎟ = lg ka + n lg[c(A)] ⎝ dt ⎠ H


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The Journal of Physical Chemistry C R30° structure at RT and c(5√3 × 3) arrangement at HT, with molecular footprints of 21.6 and 27 Å2, respectively. The c(5√3 × 3) structure is common for all terphenyl-substituted alkanethiolate SAMs with even number of the methylene units in the aliphatic linker. In contrast, the (2√3 × √3)R30° arrangement is a special case; it is rather typical of the films with odd number of the methylene units in the aliphatic linker, thus breaking the common odd−even behavior of the entire series of the terphenyl-substituted alkanethiolates.33 Significantly, a typical domain size of the above structures was about 40−50 nm in the case of HT preparation and even smaller (10−15 nm) when formed at RT.33 These structures exhibited high density of defects (vacancy islands) as demonstrated for the HT case in Figure 4a. At the same time, spectroscopic measurements, integrating over a macroscopic area, exhibit quite high orientational order in the TP2-SH derived SAMs prepared at both RT and HT.31,33 Obviously, spectroscopic measurements are not that sensitive to the film morphology, with the spectra dominated by the contributions from the ordered regions. In contrast to the TP2-SH case, TP2-SAc derived SAMs exhibit the distinctly different (2√3 × 4) rect structure with a molecular footprint of ∼28 Å2 when prepared at both RT and HT (see Table 1). There is only a certain admixture of the (2√3 × √3)R30° arrangement for the films fabricated at RT in the presence of TEA, but the respective contribution is minor compared to the dominating (2√3 × 4) rect structure (see Figure 1 and Table 1). The size of the crystalline domains is ∼10−15 nm for the films prepared at RT, which is comparable to the morphology of the TP2-SH derived monolayers. In contrast, the TP2-SAc derived SAMs fabricated at HT exhibited far superior homogeneity and domain size (>100 nm) as compared to the TP2-SH derived film prepared at the same conditions. This suggests a promoting effect of the protection group on the quality of the monolayer, observed for the first time to our knowledge. We suggest that the observed high structural order can be ascribed to a different reaction mechanism that not only is slower than the direct adsorption of TP2-SH but also involves the removal of intermediate adsorbates (presumably the acetyl group). Such slower kinetics allows efficient “annealing” and elimination of defects, yielding defect-free domains boundaries, the process which is less efficient for the case of TP2-SH SAMs. The domain boundary decoration by etch pits justifies clearly the distinct deprotection mechanism as evident in Figure 5a. This promoting effect is observed for the HT preparation only, which suggests a certain kinetic hindrance regarding the behavior (abstraction) of the acetyl protecting group at RT49 and/or certain kinetic traps for efficient self-assembly, which are hard to overcome at low temperature. Significantly, the better morphology of the TP2-SAc derived SAMs prepared at HT as compared to RT is well reflected by the spectroscopic measurements. Indeed, both XPS and HRXPS data (Table 2) suggest a higher effective packing density for the films prepared at HT compared to RT. Also, both IRRAS and NEXAFS spectroscopy data (Figures 7, 9, and 10; Table 3) imply smaller molecular inclination in the SAMs fabricated at HT compared to RT. In view of the large area character of the above spectroscopic techniques, the observed differences are not related to the specific properties of the ordered molecular films but rather reflect their different domain structure and defect density. This is an instructive example of the limits of spectroscopic techniques, related to their intrinsic constraints.

However, frequently, microscopic data alone are not sufficient to characterize a molecular film to the full extent, since molecular composition, chemistry, and orientation parameters are not accessible. The best choice is always a combination of spectroscopic and microscopic techniques, as far as this is possible within a particular study. As to the effect of the deprotecting agent, it is hardly perceptible in the case of HT preparation but well visible at RT. On one hand, it is reflected by the admixture of the (2√3 × √3)R30° phase in the film prepared with the addition of TEA (see Table 1). On the other hand, the latter film is characterized by the higher effective packing density and smaller molecular inclination as compared to that fabricated without TEA (see Tables 2 and 3). Most likely, the presence of the deprotecting agent allows a deprotection in solution, simplifying the subsequent adsorption and assembly of the deprotected SAM precursors. But the presumable kinetic traps hinder the molecular mobility and subsequent assembly, leading to the poor morphology of the resulting films. Obviously, these traps can be overcome at HT, resulting in formation of large crystalline domain. The deprotection can then occur efficiently both in solution (in the presence of TEA) and upon adsorption (in the absence of TEA). Finally, the effect of the scan direction upon the acquisition of the STM images should be discussed. It is really interesting and promising that certain subtle details of the molecular structure can be imaged and recognized by this means. Even though a herringbone arrangement is the most likely one, its existence is just an assumption in most cases, based on general logic and the parameters of the derived unit cell. Direct imaging is always of advantage as additional proof for the rightness of a structural model and as a tool to understand the molecular organization in more detail.

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CONCLUSIONS We studied the effect of the protection group on the structure and morphology of aromatic SAMs, taking the TP2-SAc and TP2-SH derived monolayers on Au(111) as test systems. The film preparation was performed at both RT and HT, at either presence or absence of the deprotecting agent (TEA). The films were characterized by a combination of complementary microscopic (STM) and spectroscopic (IRRAS, XPS, HRXPS, NEXAFS spectroscopy) techniques. The presence of the protecting group resulted in distinctly different crystallographic structures in all SAMs studied as compared to the TP2-SH case. The molecular arrangement is described by the (2√3 × 4) rect unit cell which is not observed in the films derived from TP2-SH. Most important and in contrast to previous studies of similar systems, the presence of the protection group led to significant improvement of the SAM morphology, resulting in formation of comparably large domains with dimensions exceeding 100 nm. This effect was observed at the HT preparation only, whereas the morphology of the TP2-SAc derived SAMs prepared at RT was similar to that of their thiol analogue. The effect of the deprotecting agent upon the preparation of the TP2-SAc derived monolayers was found to be minor and only perceptible in the case of the RT preparation. Finally, there are two important methodical findings. First, the spectroscopic data correlated well with the STM results but rather reflected the differences in the SAM morphology than the parameters of the crystalline domains. On one hand, this shows that in some cases morphology considerations can be of I


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importance for the interpretation of spectroscopic data. On the other hand, this underlines the importance of combining spectroscopic and microscopic tools to obtain objective information on the system studied. Second, herringbone arrangement in the TP2-SAc derived SAMs could be directly visualized by variation of the scan direction upon the acquisition of the STM images. This provides a nice tool to recognize subtle details of the molecular structure in molecular films.

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AUTHOR INFORMATION

Corresponding Authors

*A.B.: phone, +49 211 6792 315; e-mail, [email protected]. *M.Z.: phone, +49 6221 54 4921; fax, +49 6221 54 6199; email, [email protected]. Present Address ∥

S.M.J.: Ångström Laboratory, Department of Chemistry, Uppsala University, Box 523, SE 751 20 Uppsala, Sweden. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Max-IV staff and A. Preobrajenski in particular, for the technical support during the synchrotron-based experiments. This work was supported financially by the Deutsche Forschungsgemeinschaft (Grant ZH 63/14-2) and by funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) CALIPSO under Grant Agreement No. 312284.

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