GaAs is a promising material for building devices for chemical sensing. For this purpose, a molecular monolayer coating is used both to stabilize the surface towards the oxidation and to selectively recognize the analyte molecules or ions (the analyte). Therefore, the molecules for the coating need to be bifunctional organic molecules: one of the functionalities binds covalently to the surface of the device and the other one to the analyte. The study of the quality and the extension of the molecular adsorption to the surface is then crucial. In fact, the stability of the sensing surface depends on the quality of the molecular coverage. The number of adsorbed molecules is also important to assure a good sensitivity of the device to the analyte.
High-resolution electron energy loss (HREELS) is an analysis technique highly sensitive to the extreme surface (~1 nm) and to the molecular orientation. HREELS is here used to study the influence of solvent (acetonitrile) water content on the adsorption of phenylphosphonic acid on gallium arsenide. Both using the vibrational and the electronic energy loss range, it is shown that there is a poor molecular adsorption for water contents ranging from 0 to 4% in volume: HREELS spectrum is always a combination of the substrate and the adsorbed molecule spectra. For a water content of 5% in volume there is an abrupt jump in the HREELS spectra shape: they become typical of phenyl group in the electronic region. In the vibrational region, the typical C-H frequencies of aliphatic chains disappear showing that the extreme surface is exclusively covered by phenylgroups. Simultaneously, the arsenic oxide disappears and the gallium oxide grows suggesting that the adsorption sites are constituted by gallium oxide surface hydroxyl groups. Also for the samples where a large adsorption occurred, surfaces become negatively charged under electron irradiation attesting the existence of a large number of traps for the incident electrons.

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