NCAFM2023 Programme Booklet

Thursday 1500 - 1520

MOLECULAR SCALE 3D IMAGING OF FORCE AND POTENTIAL DISTRIBUTIONS AT IONIC LIQUID AU INTERFACES WITH VARIABLE TIP SAMPLE BIAS VOLTAGES

Takeshi Fukuma 1-3,* , Takahiko Ikarashi 2 , Takashi Sumikama 1 , Ryo Sakakibara 2 , Takumi Yoshino 2 , Kazuki Miyata 1-3 , Keisuke Miyazawa 1-3 , Sunao Shimizu 5 , Yoshihiro Iwasa 6,7 1 Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, 920-1192 Kanazawa, Japan 2 Graduate School of Kanazawa University, 920-1192 Kanazawa, Japan 3 Division of Frontier Engineering, Kanazawa University, 920-1192 Kanazawa, Japan 4 Division of Frontier Engineering, Kanazawa University, 920-1192 Kanazawa, Japan 5 CRIEPI, Nagasaka 2-6-1, Yokosuka, Kanagawa 240-0196, Japan. 6 Depatment of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo 113-8656, Japan 7 RIKEN CEMS, Hirosawa 2-1, Wako, Saitama 351-0198, Japan. Email: fukuma@staff.kanazawa-u.ac.jp

3D scanning force microscopy (3D-SFM) has been used for imaging 3D hydration (or solvation) structures on various materials. In this method, a tip is scanned in a 3D interfacial space and the force applied to the tip is recorded to produce a 3D force map. While the force map is often considered to represent the water (or solvent) density distribution, it should be also affected by the electrostatic interaction, especially for the measurements in ionic liquids (ILs). However, this contribution has hardly been investigated. In this study, we investigated the tip bias dependence of the 3D-SFM images obtained at an IL - Au (111) interface. In addition, we used open-loop electric potential microscopy (OL-EPM) for visualizing 3D potential distribution inside the electric double layer formed at the interface to determine the ionic species corresponding to the layer-like structures observed in the force maps. Figure 1(a) shows 3D force maps obtained with a tip bias of +0.5 V and -0.5 V. While both images show layer-like contrasts, the positive bias seems to provide a clearer contrast. Figure 1(b) shows the force and potential Z profiles measured with variable Au substrate potential. Comparing the images, we found that most of the repulsive force peaks correspond to the anion layers. This is probably because the molecular weight of TFSI (280) is significantly larger than that of DEME (146). The only exception is the lowest force peak observed with a negative sample bias, which corresponds to the cation layer. This is reasonable as a rigid cation layer is formed on a negatively charged Au electrode according to our MD simulation. Similarly, the Au-coated tip apex with a positive/negative bias should be covered with a rigid anion/cation layer, respectively. Thus, the repulsive force peaks corresponding to the anion layers are enhanced or weakened by the positive or negative tip bias, respectively. As demonstrated here, the combination of 3D-SFM and OL-EPM is a powerful method for investigating the IL/electrode interfacial structures as well as 3D-SFM imaging mechanisms.

Fig.1 3D imaging of force and potential distributions at DEME-TFSI / Au(111) interfaces.

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