Revealing Intermolecular Interaction and Surface Restructuring of an

Dec 3, 2015 - We observed that the aromatic C═C bond stretching vibration can be a marker to monitor the strength of the intermolecular interaction ...
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Revealing Intermolecular Interaction and Surface Restructuring of an Aromatic Thiol Assembling on Au(111) by Tip-Enhanced Raman Spectroscopy Xiang Wang, Jin-Hui Zhong, Meng Zhang, Zheng Liu, De-Yin Wu, and Bin Ren* State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), The MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: Controlling the packing structure and revealing the intermolecular interaction of self-assembled monolayers (SAMs) on solid surfaces are crucial for manipulating its properties. We utilized tip-enhanced Raman spectroscopy (TERS) to address the challenge in probing the subtle change of the intermolecular interaction during the assembly of a pyridine-terminated aromatic thiol on the single crystal Au(111) surface that cannot produce enhanced Raman signal, together with electrochemical methods to study the charge transfer properties of SAM. We observed that the aromatic CC bond stretching vibration can be a marker to monitor the strength of the intermolecular interaction of SAMs, because this Raman peak is very sensitive to the intermolecular π−π stacking. Our results indicate that the SAM experiences a surface restructuring after the formation of a densely packed monolayer. We propose that the intermolecular electrostatic repulsion governs the restructuring when the packing density is high. The correlated TERS and electrochemical studies also suggest that the intermolecular interaction may have some impact on the charge transfer properties of SAM. This study provides a molecular-level insight into understanding and exploiting the intermolecular interactions toward better control over the assembling process and tuning the electrical properties of aromatic thiols.

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interaction, and van der Waals interaction, are relatively weak. Therefore, revealing the subtle change of the intermolecular interaction and its influence on the electrical properties of SAM is still challenging, and requires the use of techniques that can provide the molecular-level information with a high sensitivity. Among the methods that have been used to characterize the SAM layer, Raman spectroscopy appears to be a very powerful technique, because it can be readily applied to study the solid− liquid and solid−gas interfaces. More importantly, Raman spectroscopy provides abundant chemical fingerprint information on the molecular vibrations, which is sensitive to the forces affecting the vibration modes and can potentially reveal the intermolecular interactions. As there are only monolayer species on the surface, surface-enhanced Raman spectroscopy (SERS) has to be utilized to enhance the detection sensitivity with the use of nanostructured substrates.6−8 However, it is well-known that the assembly behavior will be different for molecules assembled on rough/nanostructured surfaces compared with that of the smooth surfaces, since the long-

elf-assembled monolayers (SAMs) provide a feasible bottom-up approach to tailor the interfacial properties of solid surfaces. Because of the flexibility to tune their chemical and physical properties, SAMs can be rationally functionalized to be compatible with inorganic, organic, and biological applications.1,2 There has been a strong interest to design different backbone and terminal groups to tailor the properties of SAMs. While the chemical structure of the backbone and terminal groups largely determines the electrical properties of a SAM, the packing structure and especially the intermolecular interaction of SAMs also play an important role.3 Molecular junctions based on the aromatic coupling between adjacent molecules4 and the hydrogen bonds between DNA bases5 have been demonstrated, highlighting the important effect of intermolecular interactions on molecular electronic devices. Despite the efforts in studying the structure and properties of various SAM, a clear understanding of the structure evolution, especially revealing the change of the intermolecular interactions during assembly, is still lacking, which is important for controlling the structure and properties of SAM. Compared with the covalent bonds in the molecular backbone and molecule−substrate interaction, the noncovalent intermolecular interactions, including hydrogen bonds, π−π © 2015 American Chemical Society

Received: September 21, 2015 Accepted: December 3, 2015 Published: December 3, 2015 915

DOI: 10.1021/acs.analchem.5b03588 Anal. Chem. 2016, 88, 915−921

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Figure 1. (a) Cyclic voltammograms and (b) Nyquist plots of the bare and 4-PBT modified Au(111) electrodes in 2.5 mM K4Fe(CN)6 + 2.5 mM K3Fe(CN)6 + 0.1 M KNO3 solution for different adsorption time (indicated in the figure). The dots in part b are experimental results, and the black curves are fitting results. (c) Variation of the double layer capacitance (Cdl) and charge transfer resistance (Rct) with the increasing adsorption time.

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range order can only be preserved on the smooth surfaces.9 Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) has partially solved the problem.10,11 However, the introduction of nanoparticles on the surface may perturb the SAM structure. Furthermore, it cannot identify the defects in the ordered regions as the spatial resolution of SHINERS is still limited by the diffraction of light. Tip-enhanced Raman spectroscopy (TERS) is a technique that combines scanning probe microscopy (SPM) and SERS. It not only retains the subnanometer spatial resolution of SPM to obtain the surface morphology, but also offers chemical fingerprint information on the surface simultaneously. Because the enhancement is highly localized underneath the nanometer-sized tip, it can provide fingerprint spectral information with nanometer spatial resolution12−15 and can also be applied to study various substrates, including single crystals.12,16−18 It appears to be a promising method for studying SAM on ideally flat single crystal surfaces with molecular-level information. Recently, the pyridine-terminated aromatic thiol, for example, 4-mercapto-pyridine (4−Mpy), has received considerable interest because of the interesting properties of the pyridine unit. The pyridine is an important anchoring group to form molecule−metal contact for application in molecular electronics.16,19 In addition, the pyridine group can be protonated/deprotonated, which makes it sensitive to environment pH and has found important applications in pH sensing for chemical and biological systems.20 In this study, we chose 4′-(pyridin-4-yl)biphenyl-4-yl)methanethiol (abbreviated as 4-PBT),21 a new class of pyridine-terminated thiol with multiple aromatic and aliphatic units, as a model molecule. We investigated the structure evolution of 4-PBT SAM on Au(111) surfaces with different adsorption (assembly) time by combining the electrochemical methods and TERS. Electrochemical methods were used to study the charge transfer (CT) properties of the SAM, and TERS was employed to obtain the molecular-level information on the SAM layer on the single crystal surface. Both the electrochemical and TERS results suggest that the SAM experiences a surface restructuring after forming a densely packed monolayer. The repulsive intermolecular interaction is proposed to drive the surface restructuring when the packing density is high. Furthermore, we found that the CT properties of 4-PBT SAM may be influenced by the strength of the intermolecular interactions. These findings will be helpful for a better control of the structure and the electrical properties of SAM.

EXPERIMENTAL SECTION SAM Preparation. The Au(111) single crystal bead was fabricated by Clavilier’s method22 and was electrochemically polished and flame-annealed before use. The as-prepared bead was readily used for scanning tunneling microscope (STM) and TERS measurements. For the electrochemical study, the gold was cut and polished to be a hemispherical bead having (111) orientation (surface area of 0.0267 cm2). The gold substrate was immersed in 25 μM ethanol solution of 4-PBT for different times: 10 s, 2 h, 6 h, 14 h, 22 h, and 144 h. Then, the Au(111) surface was washed by ethanol to remove the physically adsorbed molecules. Electrochemical Experiments. All electrochemical measurements were performed in a three-electrode configuration on Autolab PGSTAT128N (Metrohm, The Netherlands). The working electrode was the cut hemispherical Au bead. The counter electrode was a Pt wire. The reference electrode was a saturated calomel electrode (SCE) that was connected to the cell by a salt bridge. The electrolyte consisted of 2.5 mM K4Fe(CN)6, 2.5 mM K3Fe(CN)6, and 0.1 M KNO3. The working electrode was brought into contact with the electrolyte in a hanging meniscus configuration. The electrochemical impedance spectroscopy (EIS) measurement was performed at 0.19 V versus SCE in the frequency range 10 000−0.01 Hz with a 10 mV ac perturbation. The data were presented in Nyquist plots. STM and TERS Measurement. The morphology of the 4PBT SAM on Au(111) was obtained with Nanoscope E STM (Veeco, Japan) using mechanically cut Pt−Ir tips. The TERS signals were obtained with electrochemically etched Au tips in a home-built TERS setup.16,23 The Au tips were cleaned with concentrated sulfuric acid before each measurement. The TERS spectra were collected under 632.8 nm laser excitation with a power of 1 mW for 1 s. The cleanliness of the Au tip was checked with a freshly annealed Au(111) surface after measurement to ensure that the obtained signal was not from the molecules adsorbed on the tip.

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RESULTS AND DISCUSSION Electrochemical Studies of the CT Properties of 4-PBT SAM-Modified Au(111) Electrodes. Electrochemical studies can provide information on the electron transfer properties of SAM-modified electrodes, from which the molecular packing structure may be extracted. Figure 1a shows the cyclic voltammograms (CVs) of the bare and 4-PBT-modified Au(111) electrodes (named as 4-PBT(t)/Au in the following discussion, where t represents different adsorption time), using 2.5 mM Fe(CN)63−/4− as the redox probe. For the bare gold 916

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Table 1. Charge Transfer Resistance and the Double-Layer Capacitance Extracted from the EIS Data, from Which Rate Constant (k0), Thickness (d), and Coverage (θ) of the Molecular Layer Can Be Obtained Rct/Ω k0/cm s−1 d/Å Cdl/μF/cm2 θ

bare gold

10 s

2h

6h

14 h

22 h

144 h

1.2 × 102 3.3 × 10−2 0 27.2 0

3.47 × 103 1.1 × 10−3 5.6 7.9 0.81

2.98 × 105 1.3 × 10−5 13.0 3.7 0.99

6.39 × 105 6.2 × 10−6 14.3 3.5 1

4.60 × 105 8.7 × 10−6 13.7 3.5 1

1.67 × 105 2.4 × 10−5 12.1 4.2 0.97

9.20 × 104 4.3 × 10−5 11.1 4.1 0.97

where Cd is the measured capacitance for 4-PBT(t)/Au electrode, C0d is the capacitance of the bare gold, and Csd is the capacitance of the SAM covered gold. The coverage reaches about 0.81 after 10 s adsorption, indicative of fast adsorption kinetics of the molecule at the early stage of assembly.2,26−28 From the STM image (Figure 2a), it

surface, the peak-shaped CV indicates a mass transfer controlled kinetics. The peak separation is about 90 mV, indicative of a quasireversible reaction kinetics of Fe(CN)63−/4− on gold. For the 4-PBT(10 s)/Au electrode, the peak current decreases remarkably, and the peak separation becomes larger. This phenomenon indicates that the gold has been covered by 4-PBT molecules and the accessible gold area for the direct electrochemical redox reaction has been decreased. Nevertheless, the similar peak-shaped feature indicates linear diffusion behavior observed in macroelectrodes, as will be further discussed in the EIS study. Increasing the adsorption time to 2 and 6 h leads to more significant suppression of the CV current, suggesting that the direct electron transfer between the gold and redox couple is strongly inhibited. Interestingly, the CV current gradually increases, rather than decreases, for the adsorption time of 14, 22, and 144 h (Figure 1a). This reverse of the CV current may be attributed to desorption of molecules on the surface (i.e., increase of the accessible area of the gold electrode) or the variation of the packing structure and orientation of SAM. However, much more information on the electrode/electrolyte interface is needed to elucidate this variation. Such an interfacial structure may be determined by the EIS method from the obtained double layer capacitance (Cdl) and CT kinetics. For this purpose, we carried out EIS studies for bare and 4-PBT(t)/Au electrodes at the equilibrium potential of Fe(CN)63−/4−, and the EIS results were presented as Nyquist plots in Figure 1b. The curves could be well-fitted with an equivalent Randles circuit (Figure S1), and the fitting results are shown as black curves in Figure 1b. The fitted values of Cdl and the CT resistance (Rct) at different adsorption time are summarized in Table 1 and plotted in Figure 1c. For the bare gold surface, Cdl is about 27 μF/cm2, which is a typical value for metal surfaces.24 It decreases with the increasing adsorption time and reaches a minimum value of 3.5 μF/cm2 for the 4PBT(6 h)/Au electrode. This minimum Cdl value of 3.5 μF/ cm2 agrees well with the reported value for 4-PBT SAMcovered Au(111).25 After the SAM coating, the effective surface dielectric constant (ε) is decreased, and the distance of the capacitance (l) is increased, which results in the decreased capacitance in the coated area in considering Cdl = εε0/l (ε0 is the vacuum permittivity). In fact, the obtained Cdl is a result of two parallel capacitances of the SAM-modified area and the bare gold surface. Therefore, the decreased Cdl can be attributed to the increased coverage of the 4-PBT molecules on the surface. With the minimum capacitance as an indication of the saturated adsorption (i.e., the coverage is considered as 1), the fractional coverage (θ) of 4-PBT with different adsorption time can be estimated by eq 124 Cd = (1 − θ )Cd0 + θ Cds

Figure 2. STM images of 4-PBT adsorbed Au(111) surface with different adsorption time as indicated in the image. The scanning area is 200 × 200 nm2.

can be seen that a large fraction of the surface is covered by the molecules. The coverage is further increased to 0.99 for the adsorption time of 2 h, and remains almost constant (saturated, θ ∼ 1) for longer adsorption time. The STM images (Figure 2b−f) reveal that the Au(111) is almost fully covered by 4-PBT molecules. Note that, compared with the bare Au(111) surface (Figure S2), there are some vacancies on the SAM-covered surface, which are formed due to the strong Au−S bond and the lifting of the gold atom.29,30 More interesting interfacial information can be obtained by careful analysis of the evolution of Rct and diffusion impedance at different adsorption time. The Nyquist plot of the bare gold surface (Figure 1b) shows a small semicircle and a 45° straight line, suggesting a fast, diffusion controlled electrochemical kinetics. For 4-PBT(10 s)/Au, a well-defined semicircle appears, and Rct increases compared with the bare gold. On such a surface, there may be a large amount of defects, and the redox reaction may occur via the following two channels, i.e., the direct electron transfer on the gold surface and electron tunneling via the SAM layer. The defect sites allow the redox species to approach the gold surface for a fast direct electron transfer. The electron tunneling distance (i.e., the thickness of the SAM film, d) is estimated by considering that the electron transfer rate constant (k0) decays exponentially with increasing d (see Supporting Information for details). The results were shown in Table 1. The 4-PBT(10 s)/Au has a d value of 5.6 Å, indicating that the molecules are lying flat on the surface (note

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Figure 3. (a) TER spectra of 4-PBT adsorbed on Au(111) surface for different adsorption time (indicated in the figure). (b) An expanded view in the region 1500−1700 cm−1. The spectra were acquired for 1 s under 633 nm excitation at 1 mW.

that a vertical oriented 4-PBT SAM would have a d value of 16.02 Å, see Supporting Information for details). Therefore, the barrier for the electron tunneling through the SAM layer is relatively low, leading to a fast electrochemical redox kinetics. In the low frequency region of the Nyquist plot, there is a clear feature of the Warburg impedance, indicating the semi-infinite linear diffusion, in accordance with the peak-shaped feature of CV. Such a behavior may be a result of the overlap of the diffusion layer of the defect sites during the redox reaction. For the case of adsorption time longer than 2 h, a densely packed SAM layer was formed (θ ∼ 1). At such a high coverage, the 4-PBT molecules tend to be adsorbed perpendicular to the surface, leading to a high electron tunneling barrier. Indeed, a remarkable increase of Rct was observed for 4-PBT(2 h)/Au compared with that of 4-PBT(10 s)/Au, and the reaction is essentially controlled by the charge transfer process. There is only a rare amount of defects in the SAM layer at such a high coverage. Each defect can be considered as an independent microelectrode without overlap of the diffusion layer. Therefore, in the low frequency region, the diffusion impedance appears as a small semicircle approaching the real axis, characteristic of the radial diffusion of microelectrodes.31,32 At highest coverage, i.e., 4-PBT(6 h)/Au, Rct reaches its maximum, and the electrochemical kinetics is completely charge transfer controlled, as indicated by the large semicircle in the high frequency region and a horizontal line in the low frequency region. These results indicate the formation of a densely packed and defect-free monolayer. With further increase of the adsorption time to 14, 22, and 144 h, Rct progressively decreases, and diffusion impedance reappears, which agrees well with the CV results. Since θ stays almost unchanged, this reverse trend after 6 h of adsorption time cannot be simply attributed to the desorption of the molecules but rather to the variation of the molecular packing structure of the SAM. Intermolecular Interaction of 4-PBT SAM on Au(111) Surface as Probed by TERS. To gain a better insight into the evolution of the molecular structure with the adsorption time, we utilized TERS to investigate the SAM layer on the Au(111) surface, because TERS can provide chemical fingerprint information on substrate−molecule and intermolecular interactions. We focus on the SAM structure with an adsorption time longer than 2 h, in which the molecules have the similar tilted orientation and the SAM has a saturated coverage. It is particularly interesting to see the reverse of the Rct with

increasing adsorption time. We propose that the 4-PBT SAM might experience a restructuring after the formation of a densely packed monolayer. Figure 3a shows the TER spectra of the 4-PBT SAM on the Au(111) surface with different adsorption time. The normal Raman spectrum of the 4-PBT powder is shown in Figure S3. The TER spectra have the same spectral features as the powder spectrum. The assignment of the peaks is assisted by the density functional theory (DFT) calculation and detailed in Table S1. The most intense peak at about 1600 cm−1 is the aromatic CC bond stretching of the pyridine and benzene rings. As can be seen from Figure 3b, this peak red shifts from 1600 to 1596 cm−1 for 2 and 6 h adsorption time, respectively. At a longer adsorption time, instead of further red shift, the frequency blue shifts back to 1602 cm−1 for 144 h adsorption time. As the frequency shift is small, and to ensure the reliability of peak positions, the G peak of highly ordered pyrolytic graphite (HOPG), located at 1582 cm−1 and close to the peak of 4-PBT, was used to calibrate the Raman shift of the instrument for every measurement. Although the frequency shift is small, the reverse trend is clearly observed (Figure 3b). It should be pointed out that all of the CV, EIS, and TERS results report the same transition trends, and they agree well with each other. The singular point is at 6 h. With the chemical fingerprint information, TERS can provide much detailed molecular-level information that is not possible from the electrochemical methods. It has been found that when aromatic molecules, like 4-PBT, are assembled on the surface at a high packing density, the neighboring molecules may interact with each other via π−π stacking, van der Waals, or electrostatic interaction.2,28 The π−π interaction leads to a shift of the frequency to a lower wavenumber since the π electrons of the aromatic ring participate in the π−π stacking, which will reduce the force constant of the CC bond.33,34 Therefore, the frequency of this peak could be an indicator of the strength of the intermolecular interaction. The lower the wavenumber, the stronger the intermolecular interaction. A red shift of the CC stretching vibration of the aromatic ring in TER spectra clearly indicates a more compact assembling structure from 4-PBT(2 h)/Au to 4-PBT(6 h)/Au. The reason for the blue shift at a long adsorption time will be discussed in detail below. We notice that the other peaks do not show obvious frequency shift. This phenomenon can be understood by the fact that the structural change during this assembly period (high coverage case) is subtle. Only the vibrational mode that is very sensitive 918

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Analytical Chemistry to the intermolecular interaction, such as the aromatic CC bond stretching associated with the π electrons, can delicately reflect this kind of subtle structural changes. Impact of Intermolecular Interaction on the CT Properties of 4-PBT SAM. It has been pointed out that the lateral interactions between molecular backbones play an important role on the electron transfer properties of the monolayers.3,35,36 The main reason is that the electron transfer is associated with some molecular motion, and the restriction of the vibrational mode of the molecules leads to the suppression of the electron transfer rate.37,38 Indeed, it was found that a subtle change of the molecular packing structure can result in a different van der Waals interaction and consequently has a significant influence on the performance of organic diodes.3,39 As discussed above, the red shift of the 1600 cm−1 peak in the TERS result indicates that the SAM structure becomes more compact, and the intermolecular interaction strengthens with the increasing adsorption time from 2 to 6 h, during which period Rct increases. Thereafter, the blue shift of this peak suggests that the SAM structure becomes relatively loose and the intermolecular interaction becomes relatively weak, giving a higher degree of freedom and thus leading to the decrease of Rct. Although we cannot completely exclude some other influencing factors, the well-correlated TERS and electrochemical results suggest that the intermolecular interaction may have some impact on the CT properties of SAM. We notice that there is a slight increase of Cdl after 6 h of adsorption time, which can also be understood by the relatively loose packing structure and the resultant change of the surface dielectric constant. Possible Driving Forces for Surface Restructuring. The assembly of molecules on a surface is a dynamic process that involves the interplay of substrate−molecule and intermolecular interactions (including electrostatic, van der Waals, and π−π stacking) and the interaction between SAM and the environment, which altogether determines the packing structure and orientation of the monolayer.35 The strong Au−S bond is the major driving force that leads to the formation of a compact assembling structure in the first 6 h (Figure 4a,b). At a high packing density, the intermolecular interaction is expected to play a crucial role in determining the SAM structure.1 In this case, the intermolecular repulsive forces should be considered, which are able to drive surface restructuring.40−42 As seen in Table 1, the molecules are almost vertical to the surface for 4PBT SAM(6 h) (an estimated film thickness of 14.3 Å), which suggests that the packing density of molecules is very high. The TERS results also indicate the strengthening of intermolecular interaction for the 6-h adsorption time. These results suggest that the lateral distance between adjacent molecules would be very small. Since both the nitrogen lone pair of the pyridine ring and the center part of a benzene ring are electron-rich,43 the small distance would increase the electrostatic repulsion between adjacent molecules. Therefore, the 4-PBT SAM(6 h)/ Au has an energy-unfavorable configuration due to the intermolecular repulsion (Figure 4b). It was also suggested that a larger molecular dipole moment results in a more repulsive intermolecular (dipole−dipole) interaction (because all the molecules have the same dipole moment direction when standing upright on surface) and slower assembly rate constant.44 Different from common aromatic thiols with benzene ring only, the molecule with the introduction of the pyridine ring with nitrogen lone pair may

Figure 4. Schematic illustration showing the phase transition during assembly, from (a) low coverage to (b) high coverage, in which the repulsive intermolecular interaction and the stress induced by the unfavorable Au−S−C angle lead to the (c) restructuring. The blue rectangle represents the benzene and pyridine ring.

lead to a larger dipole moment, and, thus, a stronger repulsive interaction between adjacent 4-PBT molecules. On the basis of the above analysis, we speculate that the intermolecular repulsion is one of the driving forces for the restructuring (Figure 4c). Indeed, a similar intermolecular (CO−CO) repulsion was suggested to account for the restructuring of Pt(557) and Pt(332) surfaces under a high coverage of the adsorbed CO.42 Furthermore, some other factors such as the energy related to the bending of Au−S−C bond also contribute to the total energy of the SAM system.45 The Au−S−C bond angle may differ from the optimum value when molecules are standing upright (for the 6-h adsorption time, Figure 4b),45,46 which leads to the increased stress of the SAM layer and provides additional driving force for the restructuring.46 It should be pointed out that only when the surface coverage is high can the energy related to the Au−S−C angle and the repulsive intermolecular interaction become dominant and lead to the surface restructuring. The surface restructuring should be associated with the rearrangement of the substrate−adsorbate interface (Figure 4c).42,46 As shown in Figure 2d−f, the surface morphology continues to change, especially in that some adislands are observed in the case of 4-PBT(144 h)/Au (Figure 2f), similar to previous reports that nanoclusters were formed on the Pt stepped surface after restructuring.42 The atomic roughness induced by the ad-islands will increase the available surface sites for the adsorbates, leading to the relaxation of the Au−S−C bond angle and the intermolecular repulsive forces. Lastly, for such a molecule like 4-PBT consisting of multiple pyridine and benzene rings and a methylene spacer group, we may expect complex intermolecular interactions. We believe that all the above-mentioned factors could influence the assembly process in a competitive way,45 so that the SAM cannot form an energy-favorable configuration right away but it 919

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experiences a restructuring process. The molecular vibrational information provided by TERS unambiguously elucidates the change of the intermolecular interaction during assembly, which is weak and cannot be readily observed by other methods. Nevertheless, further experimental and theoretical approaches are still desired to fully unravel the nature of surface restructuring.

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CONCLUSION To summarize, we have utilized TERS and electrochemical methods to study the structure evolution during the assembly of 4-PBT on Au(111) surfaces. The electrochemical results show a reverse of the CT resistance with the increasing adsorption time, with the maximum appearing at 6 h. TERS results reveal that the frequency of the aromatic CC bond stretching red shifts in the first 6 h of adsorption followed by a blue shift for a longer adsorption time, suggesting that the intermolecular interaction first strengthens and then weakens during assembly. The correlated electrochemical and TERS results reveal that the intermolecular interaction may have some impact on the CT properties of SAM. Furthermore, the SAM was found to restructure to a relatively loose structure after a compact monolayer is formed. The intermolecular repulsive force was considered to be the main driving force that leads to the restructuring. The TERS results provided unambiguous molecular vibrational information revealing the subtle change of the intermolecular interaction during assembly, which deepens our understanding of the assembly of 4-PBT at a molecular level. While further theoretical and experimental efforts are required to unravel the nature of the surface restructuring, the present work will have implications for the control of assembly and the electrical properties of aromatic SAMs, especially highlighting the importance of intermolecular interaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b03588. Randles circuit, additional STM image, normal Raman spectrum of 4-PBT powder, calculated Raman spectrum, and peak assignments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

X.W. and J.-H.Z. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from the MOST (2013CB933703 and 2011YQ03012406), NSFC (21227004, 21321062, and J1310024), and MOE (IRT13036). We acknowledge Prof. Andreas Terfort and Dr. Jinxuan Liu for kindly providing us the 4-PBT sample and fruitful discussion.

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