Surface engineering of gallium arsenide with 4-mercaptobiphenyl monolayers

In this study, we propose a new method for engineering of stoichiometric GaAs [100] surfaces with self-assembled monolayers of 4-mercaptobiphenyl. In the first part, surface homogeneity and topography were studied by contact angle measurements and atomic force microscopy (AFM), where it was concluded that grafting of mercaptobiphenyl layers did not increase surface roughness. Thickness of the biphenyl layer was measured by ellipsometry, 10 +/- 2 Angstrom, suggesting that the surface was coated with a monolayer. After grafting, ellipsometric angles were stable for more than a week, which confirmed chemical stability of the coated surface in air. In the second part, electrochemical properties of GaAs with biphenyl monolayers were studied. Cyclic voltammograms revealed a significant suppression of electrochemistry by deposition of electrically insulating biphenyl monolayers. Indeed, impedance spectra measured at a cathodic potential (-350 mV) demonstrated that the interface resistance was remarkably increased by a factor of 50. Furthermore, both interface resistance (3.2 MOmega cm(2)) and capacitance (0.45 muF cm(-2)) of the coated GaAs electrodes were stable for 22 h. Chemical modification of GaAs surfaces with mercaptobiphenyl monolayers as established here includes has a large potential toward for the flexible functionalization of GaAs-based semiconductor nanostructures with bio-organic molecular assemblies both in air and in aqueous electrolytes.