Electronic structure of methoxy-, bromo-, and nitrobenzene grafted onto Si(111).

The properties of Si(111) surfaces grafted with benzene derivatives were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The investigated materials were nitro-, bromo-, and methoxybenzene layers (-C(6)H(4)-X, with X = NO(2), Br, O-CH(3)) deposited from diazonium salt solutions in a potentiostatic electrochemical process. The UPS spectra of the valence band region are governed by the molecular orbital density of states of the adsorbates, which is modified from the isolated state in the gas phase due to molecule-molecule and molecule-substrate interaction. Depending on the adsorbate, clearly different emission features are observed. The analysis of XPS intensities clearly proves multilayer formation for bromo- and nitrobenzene in agreement with the amount of charge transferred during the grafting process. Methoxybenzene forms only a sub-monolayer coverage. The detailed analysis of binding energy shifts of the XPS emissions for determining the band bending and the secondary electron onset in UPS spectra for determining the work function allow one to discriminate between surface dipole layers--changing the electron affinity--and band bending, affecting only the work function. Thus, complete energy band diagrams of the grafted Si(111) surfaces can be constructed. It was found that silicon surface engineering can be accomplished by the electrochemical grafting process using nitrobenzene and bromobenzene: silicon-derived interface gap states are chemically passivated, and the adsorbate-related surface dipole effects an increase of the electron affinity.

Electron binding energies of hydrated H3O+ and OH-: photoelectron spectroscopy of aqueous acid and base solutions combined with electronic structure calculations.

The electronic structure of hydrated H3O+ and OH- is probed in a water jet by photoelectron spectroscopy employing 100 eV photons. The first ionization potential for OH- at 9.2 eV and the second ionization potential for H3O+ at 20 eV are resolved, corresponding to the removal of an electron from the 2ppi highest occupied molecular orbital and from the 1e orbital, respectively. These assignments are supported by present computational results based on a combination of molecular dynamics and ab initio calculations.