Molecular Structure and Electronic Properties of para-Hexaphenyl Monolayer on Atomically Flat Rutile TiO2(110)

Abstract

The structure and stability of a molecular wetting layer of the rodlike organic semiconductor molecule para-hexaphenyl grown at room temperature on atomically flat titanium dioxide TiO2(110) surfaces are investigated. The inner structure of the wetting layer was imaged with molecular resolution by means of scanning tunneling microscopy. The formation of molecular stripes with a molecular tilt of the alternating molecules is found, which is also confirmed by low-energy electron diffraction measurements and density functional theory (DFT) calculations. In the wetting layer, the molecular short axes are tilted by 20° with respect to the substrate surface plane. At submonolayer coverages, the molecules are highly mobile but finally form a compact and stable monomolecular wetting layer. Such a complete monolayer can be a precursor for the growth of crystalline para-hexaphenyl molecular nanoneedles. DFT calculations yielded adsorption energies of EdesWL = 4.06 eV/molecule for the wetting layer and EdesN = 2.71 eV/molecule for the nanoneedles, which is in fairly good agreement with molecular desorption energies deduced from thermal desorption spectroscopy (EdesWL = 3.75 ± 0.41 eV for the wetting layer and EdesN = 2.27 ± 0.26 eV for the nanoneedles). DFT also revealed charge transfer from 6P to TiO2 of 0.26 e/molecule, accompanied by a change in work function, which is in qualitative agreement with Kelvin probe force microscopy measurements.