Systematic Assessment of Benzenethiol Self-Assembled Monolayers

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Systematic Assessment of Benzenethiol Self-Assembled Monolayers on Au(111) as a Standard Sample for Electrochemical Tip-Enhanced Raman Spectroscopy Yasuyuki Yokota, Norihiko Hayazawa, Bo Yang, Emiko Kazuma, Francesca Celine Inserto Catalan, and Yousoo Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10829 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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The Journal of Physical Chemistry

Systematic Assessment of Benzenethiol SelfAssembled Monolayers on Au(111) as a Standard Sample for Electrochemical Tip-Enhanced Raman Spectroscopy

Yasuyuki Yokota,* Norihiko Hayazawa,* Bo Yang,† Emiko Kazuma, Francesca Celine I. Catalan, and Yousoo Kim*

Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.

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ABSTRACT

A molecular-scale understanding of electrolyte/electrode interfaces has long been a challenging issue in electrochemistry. Spectroscopic tools with high spatial resolution are required for advancing beyond conventional electrochemical measurements such as cyclic voltammetry (CV). In this study, we developed tip-enhanced Raman spectroscopy (TERS), which is based on an electrochemical scanning tunneling microscope (EC-STM), and demonstrated electrochemical TERS (EC-TERS) measurements of benzenethiol self-assembled monolayers (SAMs) on Au(111). A specially-designed cell enables us to carry out reproducible CV, EC-STM, and ECTERS measurements, which indicates consistent results among these techniques for the oxidative desorption of the SAMs. We also present direct evidence that the measured EC-TERS signals originate from molecules adsorbed on Au(111) and not from those on the STM tip.


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1. Introduction Microscopic and spectroscopic studies on electrolyte/electrode interfaces provide the most fundamental information not only for understanding the data from electrochemical (EC) measurements but also for designing novel electrochemical devices. Various types of in situ techniques, performed without taking the electrode out of electrolyte solutions, have become indispensable tools.1 Among them, scanning probe microscopy (SPM) under an EC environment, namely EC scanning tunneling microscopy (EC-STM) and EC atomic force microscopy (ECAFM), has been used for atomic resolution imaging of electrode surfaces,2-3 while the structural and chemical properties of interface species have been elucidated by EC vibrational spectroscopies, such as EC surface-enhanced Raman spectroscopy (EC-SERS) and EC infrared absorption spectroscopy.4-5 To realize simultaneous measurements of SPM and vibrational spectroscopy, tip-enhanced Raman spectroscopy (TERS) has been carried out under ambient conditions, where Au or Ag SPM tips near the sample surface enhance the incident and scattered electric fields, which enables the measurement of Raman spectra of molecules underneath the SPM tips.6-9 Since the pioneering report by Schmid et al.,10 several groups have developed TERS systems operating under liquid environments without electrochemical potential control.11-16 Finally, in 2015, ECTERS techniques based on EC-SPM were independently reported by Zeng et al. and Kurouski et al., revealing the potential-dependent structural changes of self-assembled monolayers (SAMs)17 and redox state changes of the molecules under the SPM tip,18 respectively. While TERS measurements under various environments have been widely reported in recent years,10-24 increasing the stability and reproducibility of TERS techniques has long been a challenging issue even under ambient conditions,25-28 as demonstrated by an interlaboratory


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project in 2014.29 In this interlaboratory comparison study, benzenethiol (PhSH) SAMs on Au(111) were used as a standard sample for TERS measurements under ambient conditions, where all the spectra measured by individual research groups showed similar spectral patterns, whatever the setup or tips that were used. In this study, we developed an EC-TERS setup with a specially designed cell based on a commercially available EC-STM system and verified its ability for microscopic and spectroscopic characterization using PhSH SAMs on Au(111) as a standard sample. Prior to the EC-TERS measurements, we performed a series of STM, SERS and TERS measurements in air, followed by cyclic voltammetry (CV), EC-STM and EC-SERS measurements under EC environments. A comparison of the Raman spectra measured by different techniques (SERS, TERS, EC-SERS, and EC-TERS) reveals good consistency between the characteristic Raman peaks regardless of electrochemical potential control. In addition, EC-TERS measurements were stable enough to perform potential dependence of the system. We demonstrate an EC treatment for removing contaminants on the Au tip by oxidation/reduction cycles and demonstrated a practical protocol for EC-TERS measurements with high reliability and stability. Finally, we discuss the remaining issues of EC-TERS measurements as a quantitative analysis tool.

2. Experimental 2.1 Materials Benzenethiol (PhSH) was purchased from Aldrich and used as received.

A ferrocene

derivative, 11-ferrocenyl-1-undecanethiol (FcC11H22SH), used for determining the roughness factor of Au(111) crystals was purchased from Dojindo Laboratories. The electrolyte solutions for electrochemical measurements were prepared using ultrapure grade HClO4 and H2SO4 (Cica-


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Merck) with Milli-Q water (Nihon Millipore). All other chemicals were of reagent grade or better and used without further purification. 2.2 Monolayer Preparation An Au(111) single crystal with 5N purity was purchased from MaTecK. The crystal has a modified-hat shape (Figure 1(a)) and the top side (a diameter of 6 mm) was polished to a roughness of less than 10 nm and orientation accuracy better than 0.1°. The Au(111) surface was annealed in a butane flame and immersed in 1 mM PhSH ethanol solution for longer than 12 h. The Au(111) surface, fully covered with SAMs, was rinsed with pure ethanol and dried with N2 gas just before each measurement.

Figure 1. Photographs of the EC-TERS cell in (a) front and (b) perspective views. WE, RE, and CE represent the working, reference, and counter electrodes, respectively. The dashed lines in (a) indicate the hat-shaped single crystal. (c) Optical micrographs of the laser spot focused on the gap between the paraffin-coated Au tip and Au(111) surface; the left and right images were captured using the same configuration with and without external illumination, respectively.

2.3 Electrochemical Measurements


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The electrode potential was controlled using a potentiostat (HZ-7000, Hokuto Denko) or a bipotentiostat (Bruker). CV measurements were performed in the EC-TERS cell with two Au wires as the quasi-reference and counter electrodes. Estimation of the roughness factor of the Au(111) surface is described in the main text. All potentials in this paper are quoted with respect to the Au electrode (0.45 ± 0.05 V vs. Ag/AgCl (3.0 M NaCl)). 2.4 EC-STM EC-STM measurements were carried out using a MS-10 STM (Bruker) controlled by NanoScope V (Bruker).

A bipotentiostat (Bruker) was used to independently control the

potentials of the tip (Etip) and sample (Esample). The sample bias of the EC-STM measurements is defined as Esample bias = Esample − Etip, but is not explicitly described in the following sections. In order to avoid confusion, the value of Etip is not described in the main text (except when necessary) but is described only in the figure captions. Electrochemically-etched Au tips were fabricated using a three-electrode electrochemical setup, as described elsewhere,30 and coated with paraffin wax (m.p. 58~62 °C, Aldrich) to minimize the residual Faradaic current and tip current noise.31-32 The typical residual current was less than 50 pA. The measurements were carried out in a custom designed Kel-F cell for EC-TERS measurements (Figure 1). Other experimental conditions were similar to those employed for electrochemical measurements. 2.5 EC-TERS Our Raman setup for EC-TERS measurements was constructed by combining the optical setup for TERS in ambient conditions with the EC-STM configuration (see Figure S1).33 The incident laser, a 633 nm laser diode (Lambda Beam, 633-70 WL, RGB Photonics), is focused by a long working distance objective lens (WD = 17 mm, NA = 0.45, Nikon) at an angle of 80° from the sample surface normal through a thin glass window (thickness = 0.2 mm, N-BK7, Edmund


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Optics). The incident laser power is typically 0.10~0.15 mW, as measured at the sample position, and its polarization is parallel to the tip. The position of the gap between the tip and sample can be observed through a thin glass window, as shown in Figure 1(a). Using the objective lens and a CCD camera as the guide, the gap is easily illuminated by incident laser light (Figure 1(b)(c)). Back-scattered light is collected by the same objective lens and passed through a dichroic beamsplitter (LPD02-633RU, RazorEdge, Semrock) and edge filter (632.8 US LPF, Iridian) to cut off the Rayleigh scattered light. Then the Raman spectra were obtained with a CzernyTurner monochromator equipped with a 600 grooves/mm grating (SP-2358, Princeton Instruments) and a thermoelectrically-cooled charge coupled device (CCD) camera (PIXIS100BReX, Princeton Instruments). The acquisition conditions of each experiment are indicated in the main text. Regulation of the tip height was carried out in the same manner as in the ECSTM measurements. All the spectra are shown without background correction unless otherwise stated. The reference TERS spectra in air were obtained under similar experimental conditions without using the EC-TERS cell. 2.6. EC-SERS SERS active Au wires (0.25 mm diameter) were prepared by 40 oxidation/reduction cycles in a 0.1 M KCl solution.

The potential was consecutively swept between −0.2 and 1.2 V vs.

Ag/AgCl (3.0 M NaCl) at 0.1 V/s.

After rinsing with Milli-Q water, PhSH SAMs were

fabricated on the activated Au wires in a similar manner as the SAMs on Au(111). EC-SERS measurements were performed with the EC cell similar to the EC-TERS cell (no holes for accommodating the Au(111) crystal, see Figure S1).

Reference SERS spectra in air were

obtained under similar experimental conditions without using the EC-SERS cell. 2.7. Calculated Vibrational Spectra


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The optimized structures and vibrational spectra of PhSH, PhSAu2, and PhSAu19 were calculated using Gaussian 16.34 Details of the simulation are published elsewhere.35 Briefly, density functional theory (DFT) calculations using the B3LYP hybrid functional were performed initially to optimize the structure and obtain the vibrational frequencies.36 In the computation, triple-ζ valence-polarization basis sets (Def2-TZVP)37 are utilized for the molecule while Au atoms are described with the Def-SVP basis set38 and effective core potentials39. The Raman scattering strength was calculated by the static polarizability and convoluted using Lorentzian broadening with a full width at half maximum (FWHM) of 16 cm−1.

To correct for

anharmonicity effects, the wavenumbers were scaled by a factor of 0.98.

3. Results and Discussion 3.1. Evaluation of the EC-TERS Cell First, we mention the design guidelines for our EC-TERS cell (Figure 1). There was a basic design reported by Zeng et al. previously,17 but we made several modifications as described below. All the cell parts in contact with the electrolyte solutions are thoroughly washed by soaking in concentrated H2SO4 for more than 12 h, followed by extensive rinsing with hot MilliQ water, and then quickly assembled after drying in a N2 flow. The most important aspect in designing our EC-TERS cell is that the electrode surface exposed to the electrolyte solutions is definitely a (111) surface and its area is regulated by the hat-shaped single crystal to perform reliable EC measurements without any leakage (Figure 1(a)). The EC-TERS cell made of Kel-F was fabricated with perfect alignment with the crystal, which enables us to quickly and smoothly assemble the cell after sample preparation. The grooves and marks on the collar of the hat-


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shaped crystal facilitate easy disassembly of the cell and indicate the crystallographic direction, respectively. To confirm that reliable and quantitative EC measurements are possible using our EC-TERS cell, CV measurements of clean Au(111) were performed in a 0.1 M H2SO4 solution, as shown in Figure 2(a). In addition to the sharp peaks at ~0.75 V and ~0.3 V corresponding to the oxidation and reduction of the Au surface, respectively, peaks assigned to the reversible phase change of adsorbed HSO4− were observed within the double layer region (inset, ~0.2 V), indicating that the surface exposed to the electrolyte solutions is considerably (111)-enriched. The roughness factor of the Au(111) single crystal, determined from the charge required for the reduction of gold oxide (cf. theoretical value is 444 µC/cm2) and the geometry of the (111) surface (φ = 6 mm: 0.283 cm2), was ~1.3;this value is in good agreement with the literature value using a standard EC cell.40 This result indicates that the electrolyte solutions do not permeate into the side of the crystal even though the Au(111) surface used is hydrophilic.

Figure 2. Cyclic voltammograms of (a) bare Au(111) in 0.1 M H2SO4 and (b) FcC11H22SH SAM on Au(111) in 0.1 M HClO4 using the EC-TERS cell. The inset in (a) shows a double layer region.


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Furthermore, we have also evaluated the exposed area quantitatively using relatively hydrophobic ferrocene-terminated alkanethiol (FcC11H22SH) SAMs; the surface density of the molecules is known to be 4.5 × 10−10 mol/cm2 and the ferrocene moieties undergo one-electron redox reactions.41-42 Figure 2(b) shows the CVs of FcC11H22SH SAMs on Au(111) in 0.1 M HClO4. The overall shape and peak positions of the CVs are in good agreement with those reported in previous studies.43-44 The roughness factor evaluated from the oxidation peak area is 1.01, indicating that reliable and quantitative CV measurements are possible by using our ECTERS cell without any leakage, irrespective of the hydrophobicity of the substrate.

3.2. Evaluation of the Paraffin-Coated Au Tip In order to perform EC-TERS measurements based on EC-STM, Au STM tips must be coated by insulators except for the very end of the tip apex to reduce residual Faradaic current and electrical noise which disturb the stable detection of tunneling current flowing between the tip and sample.31-32 Since the pioneering studies on EC-STM in the 1980s,45-51 various insulating materials and methods have been employed for reproducible and easy coating of the STM tips.5255

In this study, we chose paraffin wax54 as the coating material owing to its low luminescence

when illuminated by a 633 nm laser (Figure S2); the electrochemical and thermal stabilities of the coated tips are described below. Figure 3(a)(b) shows photographs of the electrochemically-etched Au tips before and after coating with paraffin wax, respectively. The low melting point of paraffin wax (~60 °C) enabled us to coat the Au tips easily by precisely controlling the temperature using a water bath. The exposed area of the coated Au tips was determined by CV measurements of K4Fe(CN)6, a


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common redox species used for this purpose.17,31,50 While the control experiment using Au wire provides textbook CV shapes of a one-dimensional diffusion system (Figure 3(c)), the coated Au tips portray a radial diffusion regime (Figure 3(d)).56 We assume the exposed tip to be a hemispherical microelectrode and estimate the tip radius r as ~100 nm using the following equation,56 I = 2πFDCr

(1)

where I, F, C, and D are the steady-state current, Faraday’s constant (9.65 × 104 C/mol), reactant concentration (0.01 mol/L), and diffusion coefficient (7.6 × 10−6 cm2/s), respectively. Note that this estimation is just a demonstration of determining the exposed area and we usually evaluate the quality of the tip coating by the offset current and noise level of EC-STM measurements.


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Figure 3. Optical micrographs of electrochemically-etched Au STM tips (a) before and (b) after paraffin coating. Cyclic voltammograms of the (c) Au wire and (d) paraffin-coated Au STM tip in 0.1 M KCl + 10 mM K4Fe(CN)6. The vertical dotted lines indicate the formal potential of K4Fe(CN)6. Cyclic voltammograms of the (e) Au wire and (f) paraffin-coated Au STM tip in 0.1 M HClO4. The inset in (e) shows a double layer region. The current-potential curve in (f) was taken by a STM preamplifier in order to measure small currents.

To evaluate the electrochemical stability of the coating material, we performed oxidation/reduction cycles on the paraffin-coated Au tips in 0.1 M HClO4 in which all of the following experiments were performed. Compared to the same measurements using uncoated Au wire (Figure 3(e)), the oxidation and reduction peaks of the coated Au tips were not easily discerned due to the small exposed area (Figure 3(f)). We note that the offset current and noise level do not change before and after the potential cycle (cf. blue and red curves at −0.1~0.2 V), indicating that the paraffin coating is electrochemically stable. The thermal stability of the coating material subjected to laser irradiation will be discussed in a later section.

3.3. CV and EC-STM of PhSH SAMs To act as a basis for EC-TERS, we performed CV and EC-STM measurements. Figure 4(a) shows the CV results in the double layer region of the PhSH SAMs on Au(111) in 0.1 M HClO4 using the EC-TERS cell. The rectangular shapes, whose heights are proportional to the potential scan rate, indicate that the PhSH SAMs can be regarded as a static dielectric independent of the applied potential.42 The value of the double layer capacitance (7.6 ± 0.5 µF/cm2), which is in good agreement with previous studies (7.2 µF/cm2).57-58 At a higher potential (> −0.1 V), a


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slightly larger charging current flowed for both positive- and negative-going scans and a much larger and asymmetric current with respect to the current axis was observed at more positive potentials (dotted line of inset in Figure 4(b)). We consider that the observed current response deviated from the simple dielectric model originates from the slow but reversible structural changes of the PhSH SAMs because the initial CV shape of Figure 4(a) completely recovered after the application of a high potential (up to 0.4 V).59-60

Figure 4. (a) Cyclic voltammograms of PhSH SAMs on Au(111) in 0.1 M HClO4 using the ECTERS cell. (b) Sequence of cyclic voltammograms for the oxidative desorption of PhSH SAMs on Au(111). The inset shows double layer regions before and after oxidative desorption. (c)−(e) EC-STM images (400 × 400 nm2) of the PhSH SAMs on Au(111) in 0.1 M HClO4. Esample = 0.0 V except that a 0.4 V was applied in the region sandwiched by the dotted lines in (d). Image (e)


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was taken after applying Esample = 0.9 V to desorb the PhSH molecules during a tentative tip retraction. The tunneling current and Etip were 0.4 nA and −0.1 V, respectively.

Figure 4(b) shows the sequence of CV curves beyond the double layer region. The large oxidation peak observed at 0.75 V for the first scan and a lower current flow in the following scans indicate the oxidative desorption of PhSH SAMs, as in the cases of n-alkanethiol and ωsubstituted alkanethiol SAMs.59-60 The large increase in the double-layer charging current after the application of 0.9 V (see inset) supports the desorption of most of the PhSH molecules. In order to reveal the homogeneity of the PhSH SAMs, we performed EC-STM imaging of the surface using the EC-TERS cell (Figure 1) and the paraffin-coated Au tip (Figure 3(b)). In the EC-STM image taken at 0 V (Figure 4(c)), we observed many protrusions on the flat surface and jagged step edges, as in the case of the STM image taken in air (see Figure S3). These characteristic features have been attributed to the structural changes in the Au(111) surface upon the adsorption of PhSH molecules.61-62 We revealed that the changes in potential within the double layer region (up to 0.4 V) do not affect these structures as shown in Figure 4(d). Note that we never observed long-range ordered molecular features by both STM in air and EC-STM, as in the case of most of the reports using similar sample preparation procedures.61-63 These observations revealed that the PhSH molecules are homogeneously distributed on the surface from the viewpoint of EC-TERS measurements. The typical spatial resolution of the nonresonant TERS technique is a few tens of nanometers.25-28 We have also confirmed reproducible tip positioning after tip retraction by a piezo scanner (~200 nm), allowing for any Esample and Etip to be applied to electrochemical reactions without damaging the tip and the sample. The EC-STM image shown in Figure 4(e) was obtained after


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retracting the tip and applying 0.9 V at the sample to oxidatively desorb the PhSH molecules. It is evident that the scanning area is almost the same as that before oxidative desorption (Figure 4(d)), while protrusions disappear and the jagged step edge becomes straight due to the desorption of molecules. Note that applying 0.9 V to the sample with the feedback-regulated tip at the tunneling distance inevitably deteriorates the tip because of the readsorption of the desorbed PhSH molecules onto the tip surface, instability of the tip position due to an electrochemical current upon readsorption, and appearance of insulating AuOx layers at the sample surface.

3.4. EC-SERS of PhSH SAMs In order to obtain the reference spectra of EC-TERS, EC-SERS measurements of PhSH SAMs on an electrochemically-activated Au wire were performed using an EC-SERS cell similar to the EC-TERS cell (Figure S1). The CV shapes of the PhSH SAMs in the double layer region were trapezoidal (Figure 5(a)) rather than rectangular for Au(111) (Figure 4(a)), indicating that less ordered structures of PhSH molecules are present on the activated Au wire. The EC-SERS spectra after briefly taking SERS in air are shown in Figure 5(b) with the calculated Raman spectrum of PhS-Au2 cluster for comparison (for details, see Figure S4).

The potential

dependence of the EC-SERS spectra was almost negligible in this potential range (−0.4 ~ 0 V) and all the peaks observed can be assigned by comparison with SERS in air and the calculated spectrum (Figure S4). While the observed spectra, including the broad background, were in good agreement with previous studies using a 633 nm laser, the relative intensity of the characteristic peaks at 1000 ~ 1100 cm−1 (see Figure 5(c)) is different for each paper.64-69 Because it is known that these peaks are sensitive to the structure and charge transfer between


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the molecule and substrate,67,70-74 we consider that the variation in the relative intensity originates from the differences in the Au substrates used, which affects the adsorption site and surface density of the molecules. The disordered structure of the molecules is not reflected in the ECSERS spectra because of the localized measurements of SERS; PhSH molecules adsorbed on the so-called “hot spot” may be highly ordered and act as simple dielectrics upon potential change.75

Figure 5. Cyclic voltammograms of PhSH SAMs on an activated Au wire in 0.1 M HClO4. (a) Double layer regions and (d) sequence for the oxidative desorption. The inset in (d) shows double layer regions before and after the oxidative desorption. EC-SERS spectra of the PhSH SAMs on an activated Au wire taken in 0.1 M HClO4 (b)(c) within and (e) beyond double layer regions. The arrows indicate the sequence of measurements. SERS spectra taken in air and the calculated Raman spectra of the PhS-Au2 cluster are also shown in (b) and (c) for comparison. Each spectrum is vertically shifted for clarity.


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At a more positive potential, the PhSH molecules on the activated Au wire showed similar CV shapes (Figure 5(d)) as those on Au(111) except for the less sharp desorption peak at 0.7 V for the first cycle and oxidation curves similar to Au(poly) for the following cycles (cf. Figure 3(e)).68,76 As shown in Figure 5(e), the EC-SERS spectra drastically changed beyond 0.8 V, where the EC-SERS peaks assigned to the adsorbed PhSH molecules completely disappeared; a broad peak at ~600 cm−1, which was previously assigned to the AuO stretching mode,77 appeared and the background intensity decreased. As expected, the application of an initial potential did not recover the original spectrum (cf. black and purple curves). We note that (i) at a potential close to that of oxidative desorption, the characteristic peaks at 1000~1100 cm−1 change their relative intensity and peak positions (see vertical dotted and dashed lines, respectively, in Figure S5(a)) and (ii) the background intensity after molecular desorption is more than twice that of the original spectrum (Figure 5(e)). Based on the knowledge obtained from CV, EC-STM, and ECSERS analyses, we performed EC-TERS measurements as described below.

3.5. EC-TERS of PhSH SAMs First of all, the TERS spectra of PhSH SAMs on Au(111) taken in air are shown in Figure 6(a). Compared with the spectrum taken with the retracted tip (~200 nm), the TERS spectrum of the tip within tunneling distance from the sample exhibits a large background signal as well as a Raman signal originating from the PhSH SAMs, indicating that both the signals are a result of the gap-mode plasmon formed between the tip and the sample.78-85

The peak of a broad

background was observed at ~1000 cm−1, similar to the (EC-)SERS spectrum shown in Figure 5. As previously reported,12,86-87 Raman signals were never observed for PhSH SAMs on Au(111) without the tip even when we increased the laser power and exposure time. Figure 6(b) shows


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the magnified TERS spectrum with the corresponding SERS spectrum of PhSH SAMs on the activated Au wire. While both spectra exhibit the same peaks at almost the same positions, the relative intensity of the characteristic peaks at 1000~1100 cm−1 was not identical, indicating that the microscopic structures of the PhSH SAMs on Au(111) are different from those on the activated Au wire.88

Figure 6. (a) TERS spectra of PhSH SAMs on Au(111) taken in air. The sample bias and tunneling current were 0.4 V and 0.4 nA, respectively. (b) Comparison of the TERS and SERS spectra taken in air. (c) EC-TERS spectra of PhSH SAMs on Au(111) taken in 0.1 M HClO4. The Esample and Etip values were −0.2 and −0.1 V, respectively. The tunneling current was 0.4 nA. (d) Comparison of the EC-TERS and EC-SERS spectra taken at Esample of −0.2 V. The peak


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marked by an asterisk originates from the bulk Raman signal of ClO4−. The SERS and EC-SERS spectra are scaled and vertically shifted for clarity.

Figure 6(c) shows the typical EC-TERS spectra of PhSH SAMs on Au(111) in 0.1 M HClO4 at Esample of −0.2 V. The background spectrum taken with the retracted tip was quite different from that taken in air and was found to be similar to the bulk Raman spectrum of 0.1 M HClO4, as shown in Figure S6 (for example, translational and librational modes of water (< 1000 cm−1), OH bending mode (~1600 cm−1), and ClO stretching mode (935 cm−1)).89-90 In contrast to TERS in air, the difference in the background intensity between the retracted and approached tips is not obvious, but an increase in the signal at 500~2500 cm−1 indicates the presence of a gap-mode plasmon in 0.1 M HClO4. Note that the Raman signal derived from the coating material (Figure S2(c)) was not observed even at long exposure times. Figure 6(d) shows the magnified ECTERS spectrum along with the corresponding EC-SERS spectrum taken at −0.2 V. These spectra are similar except for the relative peak intensities, as in the case of TERS and SERS taken in air. To reveal the stability and reproducibility of our EC-TERS measurements, we performed the following control experiments. First, the green spectrum in Figure 7(a) was obtained with a long exposure time (100 s) after taking the blue spectrum (30 s, reproduced from Figure 6(c)(d)). An increase in the signal to noise (S/N) ratio without the disappearance and appearance of Raman peaks indicates that our experimental conditions enable us to perform stable measurements without damaging the tip and sample.91


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Figure 7. Comparison of the EC-TERS spectra of PhSH SAMs on Au(111) taken in 0.1 M HClO4. (a) The exposure time and incident laser power were changed as indicated in the figure. The blue spectrum is reproduced from Figure 6(d) and the same tip and sample were used for taking the green spectrum, while a different set up was used for the orange spectrum. The Esample and Etip values were −0.2 and −0.1 V, respectively. The tunneling current was 0.4 nA. (b) Stability of the EC-TERS spectra before and after tip retraction. The experimental conditions were the same as those used for the blue spectrum in (a). (c) Sequence of the EC-TERS spectra to reveal the effect of potential changes. Oxidation of the tip and sample were performed by a potential cycle to 0.9 V, as in the cases of Figures 4(b) and 5(d), respectively. The experimental conditions for EC-TERS measurements were the same as (b) except for Esample = −0.4 V for the blue spectrum. Any potential changes were performed during tip retraction. Independent tips and samples were used for taking the sequences of (b) and (c). The arrows indicate the sequence


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of measurements. The asterisks indicate the bulk Raman signal of ClO4−. Each spectrum except for the bottom is vertically shifted for clarity.

Secondly, we have also confirmed that the S/N ratio increases with laser power up to 1 mW (orange spectrum). Even though we used a different set of tip and sample, the spectral shape is quite similar to the green and blue ones, except for the low relative intensity of the ClO stretching mode. This observation was surprising because many groups reported an increase in the local temperature upon laser irradiation, requiring a careful check for damages to the tip and sample.92-94 In addition to the above concerns, we need to ensure the thermal stability of the coating material because the melting point of paraffin wax is as low as ~60 °C. While we cannot discard the possibility that melted paraffin wax improves tip insulation when compared to the initial state, the increase in temperature during EC-TERS measurements is expected to be much lower than that of TERS in air due to the high thermal conductivity of water.95-96 Murakoshi et al. estimated the temperature coefficient of Au nano-dimers upon laser irradiation as 125 K mW−1 µm2 and 40.8 K mW−1 µm2 in air and aqueous solutions, respectively.95 Although these values are not directly applicable to our study due to the different laser wavelengths and experimental configurations, we believe that the local temperature of the gap region would be less than 60 °C, judging from the quality of the spectrum. Elucidation of the damages due to irradiation will be a future challenge for quick and stable EC-TERS measurements. Figure 7(b) shows the sequence of EC-TERS measurements for repeated cycles of tip retraction and approach while keeping the experimental conditions similar to those used in Figure 6(c)(d), but using a different tip and sample set. The obtained spectra are vertically shifted for clarity. Because of the differences in background intensity, these spectra are not


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equally spaced. While the spectra taken with the retracted tip showed only the bulk ClO stretching mode, the EC-TERS signals of the PhSH SAMs were consistently observed with the approached tip even after several retraction-approach cycles, indicating the mechanical stability of our EC-TERS measurements.97 The above experiments revealed that stable and reproducible EC-TERS spectra can be obtained using our EC-TERS system. In general, however, the obtained TERS signals for homogeneous samples such as PhSH SAMs cannot eliminate the possibility of molecules being accidentally transferred to the tip surface provide the enhanced Raman signal by the gap-mode plasmon. In the case of TERS in air or liquid environments, the cleanliness of the tip surface is often examined in terms of the absence of a TERS signal on clean substrates, but it is not easy to obtain direct evidence. Fortunately, we observed that the electrochemical stability of paraffincoated Au tips in conjunction with the mechanical stability during retraction-approach cycles enables us to clean the tip surface of contaminants by the oxidation/reduction cycle (see Figures 3(f) and 5(e)). Spectrum (i) in Figure 7(c) was obtained under the same experimental conditions as Figure 7(b) but using another tip and sample set. The tip was then oxidized while being retracted, followed by EC-TERS measurements, as shown in spectrum (ii). The almost constant spectrum after tip oxidation is proof that the EC-TERS signal indeed originated from the PhSH molecules adsorbed on the Au(111) surface. Furthermore, we continued with the EC-TERS measurements to demonstrate the effect of changes in the potential. Spectrum (iii) in Figure 7(c), obtained at Esample = −0.4 V, showed a weakened Raman signal for the PhSH SAMs. This change was not caused by tip or sample damage as the spectral shape was completely recovered after applying the initial potential (Esample = −0.2 V), as shown in spectrum (iv). One of the possible reasons for the potential dependence


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of the signal is that an increase in the tip-sample distance, caused by the increased bias voltage (Esample bias = Esample − Etip), weakens plasmonic coupling98 (for example, energy shift of the gapmode plasmon and/or decrease of an effective electric field)78-85. While the bias voltage can be kept constant by changing Etip at the same time, we fixed Etip at −0.1 V in this study because differences in the potential inevitably change the microscopic environment around the tip surface. This issue with regard to the optimal experimental conditions is discussed below. Finally, the Raman

signal

derived

from

the

PhSH

SAMs

completely

disappeared

after

the

oxidation/reduction cycle of the sample (spectrum (v)), as expected from the EC-SERS spectra shown in Figures 5(e) and S5(a). The above demonstrations revealed that PhSH SAMs on Au(111) can be used as a standard sample for EC-TERS, similar to SERS and TERS in air.12,29,64-74 In contrast to TERS in air and liquid environments, it is easy to prove that the obtained Raman signal in EC-TERS is definitely derived from the sample rather than molecules accidentally adsorbed on the tip surface by virtue of the electrochemical stability of the paraffin-coated Au tips and the mechanical stability of the system during retraction-approach cycles. Here, we discuss the remaining issues for maximizing the potential of EC-TERS measurements. First, for quantitative measurements concerning peak intensities, a precise control of the tip height in terms of the experimental conditions of EC-STM, such as tunneling current, Esample and Etip, is important because an enhancement of the electric field by the gap-mode plasmon strongly depends on the distance between the tip and sample.78-85 Compared to STM in air and ultrahigh vacuum environments, the relationship between tip height and experimental conditions of EC-STM is quite complicated due to the presence of a solution at the tunneling gap.99-101 We believe that the electrochemical stability of our paraffin-coated Au


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tips enables us to investigate the relationship between the tip height and experimental conditions for EC-STM. Secondly, the relationship between the gap-mode plasmon and the applied Esample and Etip is also unclear at present. We found that the background intensity of the EC-SERS after the desorption of molecules increased at negative potentials, as shown in Figure S5(b), indicating that plasmon resonance is modulated by the surface charge of the Au substrate. While the potential dependence of the background intensity was insignificant before oxidative desorption due to the small double-layer capacitance (~1/5, see inset in Figure 5(d)), the surface charge of the EC-TERS tip is considered to be changed with the applied Etip. Even though the origin of background emission upon irradiation with a 633 nm laser is still being debated,66,69,102-106 we believe that the background intensity is related to plasmon coupling, and thus an enhanced Raman signal, as presented in Figure 6(a)(c). A systematic investigation of the relationship between the applied potentials and the EC-TERS intensity on not only the Raman but also the background signals at a precisely-controlled tip height is necessary for quantitative measurements. Finally, but most importantly, the signal intensity of EC-TERS must be enhanced for better spectral resolution of the small peak shifts caused by different adsorption sites, and for spatial mapping of chemical information through multipoint measurements within a realistic time scale. In addition to optimizing the laser power while maintaining system stability (Figure 7(a)), knowledge obtained from systematic studies addressing the first and second issues will allow us to find the optimum experimental conditions for EC-TERS measurements that might be different from those used for conventional EC-STM imaging. These issues are currently being explored by our group.


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4. Summary In summary, we developed an EC-TERS system suitable for quantitative EC measurements and thoroughly characterized PhSH SAMs on Au(111) using CV, EC-STM, and EC-TERS techniques. We found that these techniques provide consistent results regarding the stability within the double layer region and oxidative desorption at high potentials, which implies that the PhSH SAMs can be used as a standard sample for EC-TERS measurements, as in the cases of SERS and TERS in air. Due to the electrochemical stability of paraffin-coated Au tips and the mechanical stability over tip retraction-approach cycles, we could prove that the EC-TERS signal is derived from the sample rather than molecules that were accidentally adsorbed on the tip surface. We found that various issues remain to be resolved before using EC-TERS as a quantitative analysis tool, such as the applied potential dependence on the gap distance and plasmon coupling. We have also shown that PhSH SAMs are suitable for elucidating these issues as a standard sample for EC-TERS measurements.


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ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. Details of the optical setup, Raman spectra of the coating materials, STM images taken in air, details of DFT calculations, details of EC-SERS, and Raman spectra of 0.1 M HClO4 and pure water (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected], [email protected], [email protected] Present Addresses † School of Science, Xijing University, 1 Xijing Road, Xi'an, Shaanxi 710123, China.

ACKNOWLEDGMENT This work is supported (in part) by the Japan Science and Technology Agency (JST) under the ACCEL project entitled, “Fundamentals and Applications of Diamond Electrodes”. We thank Prof. Yasuaki Einaga (Keio University) and Dr. Rafael Jaculbia (RIKEN) for fruitful discussions.


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Table of Contents Image


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Fig. 1 169x43mm (300 x 300 DPI)


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Fig. 2 117x49mm (300 x 300 DPI)



Fig. 3 163x108mm (300 x 300 DPI)


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Fig. 4 174x102mm (300 x 300 DPI)



Fig. 5 197x103mm (300 x 300 DPI)


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Fig. 6 121x103mm (300 x 300 DPI)



Fig. 7 121x101mm (300 x 300 DPI)


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Systematic Assessment of Benzenethiol Self-Assembled Monolayers

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