Surface states mediated charge transfer in redox behavior of hemin at GaAs(100) electrodes.

EIS and XPS investigations on the interaction of hemin with p- and n-doped GaAs(100) electrodes in PBS solution revealed significant differences concerning both the adsorbed species and the mechanism of the redox process caused by dopant nature. XPS data show that hemin is adsorbed on p-GaAs(100) by its carboxyl groups and adopts a vertical position favorable to a polymeric film formation whereas on n-GaAs(100), the adsorbed hemin is monomeric and has a rather planar configuration involving mainly the OH groups of the organic molecule. Hemin gives rise to a reversible redox process at the p-GaAs(100) electrode whereas at n-GaAs(100), there is only one reduction wave of a considerably lower current density appearing at a more negative potential.

Hemin interaction with bare and 4,4'-thio-bis-benzene-thiolate covered n-GaAs (110) electrodes.

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) investigations on the redox behavior of hemin at bare and 4,4'-thio-bis-benzene-thiolate (TBBT) covered n-GaAs (110) electrodes in dimethylsulfoxide (DMSO) revealed the high irreversibility of the electroreduction process, which appeared to be closely related to the stable adsorbed species strongly interfering with the electronic properties of the semiconducting substrate. The subsequent exploration of the hemin-modified electrodes by second harmonic generation (SHG), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements pointed to significant differences between the iron protoporphyrin species adsorbed on the bare- and TBBT-GaAs (110) electrodes. Only Fe(2+) species having a flat configuration with the porphyrin plane oriented parallel to the surface were detected on GaAs, unlike the TBBT-GaAs, where Fe(2+) and Fe(3+) species having both flat and vertical adsorption positions could be observed.

Potential-induced conformational changes in an alpha-CN-terthiophene thiolate film on GaAs(110).

Second harmonic generation (SHG) investigations on alpha-CN-terthiophene-thiolate-covered GaAs(110) electrodes in 1 N H2SO4 solution revealed significant changes in the rotational anisotropy of the SH response. The enhancement of the 1- and the 3-fold contributions around -250 mV suggests changes in the symmetry properties of the delocalized electron system due to an alteration of the adsorption geometry induced by the applied potentials. The analysis of the EIS data showed that in the potential region where the SH signal exhibits the more important changes the Mott-Schottky plot undergoes a pronounced shift to more negative potentials as a result of the charging of the surface states grouped about 1.

Surface states- and field-effects at p- and n-doped GaAs(111)A/solution interface.

We have investigated the behaviour of n- and p-doped GaAs(111)A electrodes in sulfuric acid solution by electrochemical impedance spectroscopy (EIS) and second harmonic generation (SHG) measurements. The potential dependence of the SHG response was found to be closely related to the changes in the surface state population, as revealed by analysis of the impedance data. The nature of the majority of carriers turned out to be a key factor in shaping the surface state- and field-effect on the second harmonic generation process.