Stable surface coating of gallium arsenide with octadecylthiol monolayers

In this paper, we describe the deposition of octadecylthiol (ODT) monolayer s on highly doped n-GaAs electrode surfaces, which showed high stability bo th in air and in aqueous electrolytes. In the first part of this study, fou r different wet chemical etching procedures were investigated to optimize s urface treatment before ODT deposition. The chemical composition of the sur face was evaluated by X-ray photoelectron spectroscopy (XPS), demonstrating that the photochemical etching procedure (called "etch P" in this study) c an generate a surface enriched with arsenides, which can serve as the bindi ng sites for sulfides. In the second part of this study, the surface prepar ed by etch P was coated with an ODT monolayer. The monolayer showed high st ability in air, as indicated by the constant ellipsometric thickness. In el ectrolytes, the stability of the monolayer-coated surface was verified by i mpedance spectroscopy under zero-current potential (U-j=0 = -360 mV) for mo re than 10 h; then the stability of the interface was monitored under diffe rent bias potentials. Electrochemical passivation of the GaAs surface has b een demonstrated for the first time under physiological conditions tin aque ous electrolyte, near neutral pH), which allows for the application of GaAs electrodes to biological systems.