Solid-State Electrochemical Oxide Pattern Formation on Au Surface Utilizing Polyelectrolyete Membrane Stamps
Authors: Fujii Tatsuya , Tsuji Atsuki , Murata Junji 
Published: December 2024

Nanoscale patterned gold (Au) surfaces are promising for a wide range of applications such as bio/chemical sensors and high-performance electrodes. However, pattern formation by conventional methods, which typically involves the use of resist, is complex, cost-consuming, and poses a high environmental burden. In this study, we report an investigation into the interfacial chemical reaction occurring between the polymer electrolyte membrane (PEM) and Au surfaces and its application for microscale pattern formation. Introducing a PEM instead of a liquid electrolyte can realize solid-state electrochemical surface processing. Room temperature and highly efficient electrochemical oxidation of Si, Ti [1], GaN [2], and SiC [3] can be achieved without the use of harsh liquid chemicals. The proposed method can be used for the rapid modification and etching of metal and semiconductor surfaces on the micro and nanoscale. This simple and direct pattern formation method, leading to the absence of resist film, liquid chemicals, and high-temperature treatment, offers significant advantages in terms of processing cost and environmental impact. As shown in Fig. 1, patterning was performed using an electrochemical system where the PEM was sandwiched between the Au (anode) and a cathode. The PEM can transport ions through nanoscale water channels within the membrane, effectively functioning as a solid-state electrolyte. The PEM stamps were prepared via a hot embossing process: The pattern structure of a master mold was transferred onto the PEM surface by plastic deformation of the membrane. A PEM stamp with a pattern on its surface was used, and the electrochemical reaction proceeded only at the contact area of the PEM stamp, thereby transferring the pattern onto the Au surface. Electrochemical oxidation using the wet PEM stamp under a bias of 3V effectively oxidized the Au surface and formed dot patterns with diameters and heights of approximately 2 µm and 400 nm, respectively. The time-dependent change in the morphology of the patterned Au surface was investigated. The patterned Au surface, after 30 days of exposure in air, still maintained the dot pattern structure. However, the pattern height decreased to approximately 250–300 nm after 30 days, which was 70% of the height of the pattern on the as-oxidized sample. This decrease in pattern height was likely caused by the reduction of the oxide to metallic Au after air exposure, as observed in the electrochemical oxidation of the Au surface using liquid electrolyte as reported by Nishio et al. [4]. According to a report by Takimoto et al. [5], an LSPR tip containing Au dots of approximately 60 nm in height can be applied for toxic gas sensors. The present patterned Au surface maintained a height of several hundred nanometers even after 30 days of air exposure, which is significantly larger than that of the previously reported LSPR tip, and therefore, this process can be applied for the preparation of LSPR templates. Chemical composition analysis by XPS confirmed that the Au3+ peak, which is associated with Au2O3 or Au(OH)3, was observed on the Au surface treated by solid-state electrolysis using PEM, while only metallic Au peaks exist in the spectrum from the untreated surface. After the treated sample was exposed to the air atmosphere for 12 weeks, the intensity of the Au3+ peaks significantly decreased, indicating that the oxidation film was reduced to form metallic Au by air exposure. Although further investigation of the patterning characteristics, such as the lifetime of the PEM stamps and the limitation of the pattern resolution, should be clarified, the present imprinting method on the Au surface based on solid-state electrochemical oxidation is promising as a fast and facile approach to preparing micro and nanoscale Au patterns that can potentially be applied for localized surface plasmon resonance templates for bio/chemical sensors.
Figure 1. Schematics of the electrochemical imprint lithography method using PEM stamp.
References
[1] P. Jia, R. Umezaki, J. Murata, Microelectron. Eng. 257 (2022) 111752.
[2] J. Murata, Y. Nishiguchi, T. Iwasaki, Electrochem. Commun. 97 (2018) 110–113.
[3] S. Zulkifle, K. Hayama, J. Murata, Diam. Relat. Mater. 120 (2021) 108700.
[4] K. Nishio, H. Masuda, Bull. Chem. Soc. Jpn. 86 (2013) 1144–1150.
[5] Y. Takimoto et al., Plasmonics. 15 (2020) 805–811.

Figure 1