Combined Electrochemical Impedance Spectroscopy, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy study of the distinct behavior of L-cysteine-thiolate formed at p- and n-GaAs(100) electrodes in Sodium Dodecyl Sulfate solution

Electrochemical impedance spectroscopy (EIS), X-ray spectroscopy (XPS), and atomic force microscopy (AFM) have been used to investigate the effects of the sodium dodecyl sulfate (SDS) interaction with the L-cysteine-thiolate covered p- and n-GaAs(100) electrodes in H2SO4 solution. The potential-induced reversible proton transfer found to occur within the L-cysteine-thiolate layer in the simple acid solution during the cathodic as well as the anodic potential scans at the p-doped semiconductor electrode and only in the anodic scan at the n-doped one is suppressed in both cases. XPS data revealed that the anionic surfactant has a complex interaction with the L-cysteine-thiolate covered GaAs(100) electrodes, which implicates both electrostatic and chemical forces. The SO42− head group interacts electrostatically with the NH3+ group of the cysteine-thiolate but it is also chemically involved in a chemisorption bond with the available Ga atoms. The amount of Ga-bound SDS species is much more pronounced at the L-cysteine-thiolate/p-GaAs(100) than at the L-cysteine thiolate/n-doped GaAs(100) electrodes meaning that cysteine-thiolate is mainly formed at As sites in the first case and at Ga sites in the latter one. This outcome strengthens the previous conclusions concerning the role of the dopant in determining the bonding way of the L-cysteine on GaAs(100) surfaces.