Fabrication of Sharp Gold Tips by Three-Electrode ... - ACS Publications

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C: Physical Processes in Nanomaterials and Nanostructures

Fabrication of Sharp Gold Tips by Three-Electrode Electrochemical Etching With High Controllability and Reproducibility Bo Yang, Emiko Kazuma, Yasuyuki Yokota, and Yousoo Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04078 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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The Journal of Physical Chemistry

Fabrication of Sharp Gold Tips by Three-Electrode Electrochemical Etching with High Controllability and Reproducibility Bo Yang,1,2 Emiko Kazuma*,1 Yasuyuki Yokota,1 and Yousoo Kim*,1,2 1

Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako City, Saitama 3510198, Japan 2

Department of Materials Science, Saitama University, 255 Shimo-Okubo, Saitama City, Saitama 338-8570, Japan

ACS Paragon Plus Environment

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ABSTRACT. Gold (Au) tips have wide application in local spectroscopies not only because of their high chemical stability, but also their strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. The energy and intensity of LSPR strongly depend on the tip shape. However, the conventional fabrication method of Au tips using electrochemical etching with two electrodes has problems regarding both the controllability and reproducibility of the tip shape. Here, we demonstrate a novel three-electrode electrochemical etching method to fabricate the Au tips by precisely tuning the applied electrochemical potential. The sharpness of the tip is well controlled by the applied potential, with high reproducibility.

1. INTRODUCTION Au is among the most commonly used materials to fabricate tips for a variety of local spectroscopies such as scanning tunneling microscopy (STM),1–7 tip-enhanced Raman spectroscopy (TERS),8–15 scanning near-field optical spectroscopy16–19, electrochemical scanning tunneling microscopy20, and point-contact spectroscopy.21 It has high free electron density and exhibits strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions.22 In TERS, for example, the localized electric field generated near the tip surface due to LSPR can strongly enhance Raman signals. In addition, Au has higher chemical stability than other metals exhibiting LSPR such as alkali metals, silver, and copper in air or an aqueous environment, which allows Au tips to be used for several sets of experiments.23 Several methods to fabricate Au tips have been developed over decades, mainly including the followings: (1) electrochemical etching,1–13,16–21,24 (2) chemical polishing,25 (3) ion milling,26 (4) cathode sputtering,27 (5) whisker growth28–30, and (6) mechanical shaping.31–33 Among these, electrochemical etching, in which a metal wire is anodically dissolved in an electrolyte, is widely


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used because of its low cost and easy operation. In previous studies, a two-electrode electrochemical system consisting of a working electrode (WE) and a counter electrode (CE) has been applied to produce Au tips (Figure 1a). However, etching conditions, such as chemical species and concentration of the electrolyte, the applied potential, and the materials of the CE vary largely according to the electrochemical systems. In addition, although the plasmon resonance wavelength and intensity strongly depend on the tip shape,34–37 the relationship between the tip shape and etching parameters has not yet been well clarified. In other words, the conventional methods using the two-electrode electrochemical etching system have problems in terms of controllability and reproducibility. In this study, we applied a three-electrode electrochemical etching system consisting of a WE, CE, and a reference electrode (RE), to fabricate Au tips for the first time (Figure 1b). The Au tips were prepared by etching the Au wires at a constant anodic potential which was precisely controlled against the potential of the RE. The sharpness of the tips was well controlled and reproduced by controlling the applied potential.

Figure 1. Schematic of (a) two- and (b) three-electrode electrochemical etching systems for fabricating Au tips.


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2. EXPERIMENT SECTION 2.1. Materials. An Au wire of 0.25 mm in diameter (99.95%, Nilaco Co.) annealed at 700 °C for 2 h was used as a WE to prepare the tips. A 1 cm diameter Au ring composed of 0.50 mm diameter wire was employed as a CE. An Ag/AgCl/saturated KCl electrode (ALS Co., Ltd) was applied as a RE. The electrolyte was a 2.79 mol/L KCl (Wako Pure Chemical Industries, Ltd.) aqueous solution. A salt bridge was created from the saturated KCl aqueous solution and agar (Wako Pure Chemical Industries, Ltd.).

2.2. Fabrication of Au tips. The Au ring was immersed into the solution and then lifted to form a meniscus at the interface between the ring and solution. The Au wire was positioned at the center of the ring and immersed in the solution to ~1 mm in depth. The Au tips were fabricated using a three-electrode electrochemical etching system (Figure 1b). The applied DC potential to the WE was 1.1–1.5 V, which was precisely controlled against the RE. Using a potentiostat (VersaSTAT 4, Princeton Applied Research), the real-time change in the anodic current during the etching was monitored. The etching process was automatically terminated at an optimal set point to avoid over-etching.8 Following the etching process, the prepared tips were carefully washed with ultrapure water to remove residual impurities from their surfaces.

2.3. Cyclic voltammetry (CV) measurement. CV measurements were performed in the potential region between -0.4 V and 1.1–1.5 V with the same electrochemical set-up as that used for Au tip fabrication. Both the initial and end points for the scan were fixed at -0.4 V and the turning point was set at 1.1, 1.2, 1.3, 1.4, and 1.5 V, respectively. The scan rate was 0.01–1.0 V/s.


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2.4. Linear sweep voltammetry (LSV) measurement. LSV measurements were performed before and after the fabrication of Au tips, for both the two- and three-electrode electrochemical etching systems, to evaluate the stability of the applied system. The LSV curves were obtained using fresh Au wires which were not used for tip fabrication. LSV measurement and the tipformation process were performed in turn. The initial and end points of the potential sweep were 0 V (-0.4 V) and 3.5 V (1.5 V) using the two- (three-) electrode electrochemical system. The sweep rate was set at 0.1 V/s. Other conditions such as voltage source, electrolyte, and the CE were the same.

2.5. Scanning electron microscope. The Au tips were observed using a field emission scanning electron microscope (FE-SEM, JSM-6330F, JEOL) to evaluate the radius of curvature (R) and cone angle (θ). The accelerating voltage and working distance were 20 kV and 8 mm, respectively.

2.6. STM measurement. All the measurements were conducted at 5.0 K with a low-temperature STM (Omicron GmbH) under ultrahigh vacuum of less than 5.0×10-11 Torr. The Au(111) surface was cleaned via several cycles of Ar+-ion sputtering and annealing at ∼660 °C. The adsorbate molecules (dimethyl disulfide) were deposited by evaporation from a glass ampule at room temperature. The temperature of the substrate was 1.34 V results from competitive reactions between the dissolution of Au and the formation of Au oxides which act as a passivation layer to inhibit the dissolution process. In addition, a sudden drop at ~1.45 V was observed (Figure 2e). This was attributed to the surface of the Au wire being fully covered with the oxide film. In the negative potential sweep shown in Figure 2a–d, a cathodic peak appears at 0.4–0.45 V because of the reduction reaction of AuCl4- due to the dissolution of Au. The intensities and positions of the peaks depend on the amount of AuCl4- species. In contrast, once the Au surface


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was fully covered with the Au oxide film (Figure 2e), no current flowed until ~0.6 V and a broad cathodic peak was observed. In the case of the CV curve of the Au electrode obtained in the HClO4 solution containing Cl-, two cathodic peaks of the reduction of Au oxides and AuCl4-, respectively, were detected, and the peak at the more positive potential was identified as the dissolution of the Au oxides.38 Therefore, the broad peak consists of two overlapped peaks corresponding to the reduction reactions of the Au oxides and AuCl4-, and the reduction of the Au oxides occurs prior to the reduction of AuCl4-. The Au tips were prepared by applying a constant potential to the Au wire according to the CV curves. The intensity of the anodic current is proportional to the surface area of the Au wire immersed in the solution, and thus, the current trace reflects the progress of etching. Figure 3a shows the current trace obtained at different potentials. In addition, the morphological change in the Au wire was monitored simultaneously by video recording.

Figure 3. (a) Current trace measured under different potentials (1.1, 1.2, 1.3, 1.4, and 1.5 V, respectively). The inset shows the current changes for 1 s from the start. (b) Snapshots of the movie recording the etching process at the applied potential of 1.3 V. (c) Schematic illustrations


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of the etching mechanism. The blue solid lines indicate the interface between the solution and the air. When the Au wire was immersed in the electrolyte solution, a meniscus formed at the interface between the solution, air, and Au surface (Figure 3b, c). The etching reaction of the Au wire occurred under applied potentials at 1.1–1.4 V, and the current gradually decreased because of the reduction in the diameter of the immersed wire. The current suddenly decreased at the moment when the small piece at the end of the wire drops (Figure 3b, c) and the etching process stops. The etching time decreased as the applied potential increased (Figure 3a). Notably, only the dissolution reaction is induced at ≤1.3 V; however, both the dissolution and oxide formation reactions occur at 1.4 V as previously described. In addition, several spike peaks appeared in the current trace during etching at 1.4 V because of the formation of bubbles resulting from the generation of Cl2 gas on the Au wire (Figure S1). The sudden drop in the current immediately after applying potential at 1.5 V (Figure 3a) is attributed to a rapid formation of the passivation layer of the Au oxide. Typical FE-SEM images of the Au tips fabricated at 1.1, 1.2, 1.3, 1.4, and 1.5 V are shown in Figure 4. The whole shape of the Au tips depends on the applied potential, as shown in the SEM images of low magnification (Figure 4a-d). The length of the etched part became shorter as the applied potential increased from 1.1 to 1.4 V (Figure S4). According to the video recording the etching processes (Supporting Information), the final shape of the tips was not determined by the shape of the meniscus but by the reaction rate controlled by the applied potential, because the tip was still immersed in the solution after the etching was stopped (Figure 3b). The shape of the wire did not change at 1.5 V, which corresponds to the current change shown in Figure 3a.


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Figure 4. FE-SEM images of the Au tips fabricated under different applied potentials. (a, f) 1.1 V, (b, g) 1.2 V, (c, h) 1.3 V, (d, i) 1.4 V, and (e) 1.5 V. (a-e) Low- and (f-i) high-magnification images. The LSPR properties of plasmonic tips are evaluated by a radius of curvature (R) and a cone angle (θ) as described in Figure 5a,23 of which values are extracted from the high-magnification SEM images (Figures 4, S2, S3). Figure 5b shows the applied potential dependence of the average values of R and θ from six Au tips. The average value of R of the Au tips prepared at 1.1 V was (1.6 ± 0.1) µm, which is much greater than that of conventional Au tips applied using TERS.8-11 When the applied potential is 1.2 V, sharp tips with a small R ((51 ± 19) nm) were obtained. The sharpest tips were available at 1.3 V (R = (16 ± 3) nm). In contrast, the R obtained at 1.4 V is greater than that obtained at 1.3 V because of the competitive reactions between dissolution and oxide formation, which retard the flow of electrons. In addition, bubbles formed at the Au surface also affect tip morphology. Therefore, R is well controlled by the applied potential. On the other hand, the average values of θ obtained at 1.1, 1.2, 1.3 and 1.4 V are 29 ± 6, 27 ± 20, 41 ± 8, and 37 ± 13, respectively (Figures 5b and S6). These results indicate that the values of θ are almost constant and thus are not controlled by the applied potential.


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Figure 5. (a) Schematic model of the Au tip with a radius of curvature (R) and cone angle (θ). (b) Applied potential dependence of the averaged values of R and θ. Each point is averaged from six Au tips. High reproducibility of the tips is required for practical applications of optical nanospectroscopies such as TERS, because it is directly related to the reproducibility of the tipenhancement effect.39,40 Good controllability of the tip shape, as shown in Figure 5b, is mainly based on the high stability of the etching system, leading to good reproducibility of the etching process. To examine the stability of the electrochemical etching conditions, LSV measurements were

conducted,

comparing

the

three-electrode

to

the

conventional

two-electrode

electrochemical etching systems (Figure 6). The LSV curve obtained prior to tip formation (Figure 6a-1st) is identical to the CV curve in the positive potential sweep at 0.1 V/s. After the first Au tip was prepared at 1.3 V using a fresh Au wire, the LSV curve was measured using the same electrolyte solution but with a different fresh Au wire (Figure 6a-2nd). The LSV measurement and the tip formation process were performed in turn. Both the shape and potential range of the curve were nearly identical even after the preparation of nine tips. This indicates that the etching condition did not considerably change during the tip fabrication process, although both the wire of the CE and the concentration of Au ions in the electrolyte changed as shown in


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Figure S7. Note that the small differences among the peak heights were caused by the error in the immersed depths of the wire. The same analysis was performed using the two-electrode electrochemical etching system (Figure 1a) in the potential region near the anodic peak (Figure 6b). Although the initial shape of the curve was similar to the curves obtained using the three-electrode electrochemical system, a shoulder at ~2 V gradually appeared following the fabrication of few tips. Furthermore, a successive potential shift of the anodic peak was observed. Thus, the weak point of the twoelectrode system is the fluctuation of the electrochemical potential, causing instability in the etching condition. In other words, the preparation of the Au tips using the two-electrode electrochemical etching system has a critical problem regarding both stability and reproducibility, which diminishes the controllability of the tip shape. In contrast, the three-electrode electrochemical etching system exhibits high stability leading to good reproducibility because the electrochemical potential is well controlled against the RE.


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Figure 6. LSV curves in the positive potential sweep obtained using the (a) three- and (b) twoelectrode electrochemical etching systems in the same electrolyte solution. The curves were measured using fresh Au wires before and after the fabrication of tips. The 1st curve was obtained prior to tip formation. The 2nd curve was measured after the first Au tip was prepared at 1.3 V using a fresh Au wire. LSV measurement and tip formation were performed in turn. The scan rate was 0.1 V/s. Finally, we applied the Au tips prepared at 1.3 V using the three-electrode electrochemical etching system to STM imaging to evaluate the quality of the tips. Clear STM images of the intrinsic herringbone reconstruction of the Au(111) surface were obtained (Figure 7a) of the same quality of spatial resolution as those obtained using conventional tungsten tips41,42.


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Furthermore, STM images of dimethyl disulfide molecules adsorbed on the Au(111) surface were also obtained at a single-molecule level (Figure 7b). In the case of the conventional tungsten tips, cleaning process of the tip surface to remove contaminations by applying high bias voltage pulse during scanning is necessary to measure high resolution STM images. However, the cleaning process causes change in tip shape. In contrast, our Au tips with smooth surfaces are chemically stable and hardly deteriorated due to contamination, and therefore are applicable to STM imaging without geometrical changes resulted from applying high voltage pulse.

Figure 7. Typical STM images of (a) the Au(111) surface with the herringbone structure and (b) the dimethyl disulfide molecules adsorbed on the Au(111) surface. Scan areas are both 20×20 nm2. Vsample and Itunnel are (a) 2 V and 1 nA and (b) 30 mV and 0.2 nA, respectively.

4. CONCLUSIONS. We fabricated Au tips of high reproducibility and controllability by electrochemical etching at a constant anodic potential in a KCl aqueous solution using a threeelectrode electrochemical etching system. The curvature of the radius was controlled by the applied potential. Electrochemical analysis showed that the stability of the three-electrode electrochemical etching system is much higher than that of the conventional two-electrode electrochemical etching system. The Au tips with the sharpness obtained at 1.3 V would be applicable not only as STM tips with high stability but also as probes exhibiting strong field enhancement of the plasmon.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Snapshots of the movie at the potential of 1.4 V (PDF), FE-SEM images of Au tips prepared at 1.2 V and 1.3 V, all values of geometrical parameters, and photographs of the Au CE and the KCl electrolyte solution and AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The present work was supported in part by a Grant-in-Aid for Scientific Research (A) (15H02025), Grant-in-Aid for Young Scientists (B) [16K17862], RIKEN FY2015 Incentive Research Projects and RIKEN International Program Associate (IPA) program. We would also like to thank the Materials Characterization Support Unit of RIKEN in SEM measurements. REFERENCES 1. Baykul, M. C. Preparation of Sharp Gold Tips for STM by Using Electrochemical Etching Method. Mater. Sci. Eng.: B 2000, 74, 229-233.


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Fabrication of Sharp Gold Tips by Three-Electrode ... - ACS Publications

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