Microscopic characterization of the interface between aromatic isocyanides and Au(111): A first-principles investigation

Molecular assemblies comprising aromatic rings have received widespread attention as possible components of molecular electronic devices. An essential prerequisite to understand their stability and transport properties is the microscopic characterization of the interface formed with metallic leads. We present a comprehensive study, based on density functional theory (DFT) of the interface between the Au(111) surface and 1,4-phenylenediisocyanide, the simplest representative of aromatic isocyanides. We find that low coverage is favored at low temperature; this prediction is accessible to experimental validation as the metal work function shift upon adsorption has opposite signs, depending on the coverage. The computed binding energy between the isocyanide groups and the surface (similar to 0.5 eV) is smaller than that obtained for benzenethiols, but the presence Of Surface defects may considerably increase binding, by about 0.8 eV. The computed N C stretching frequencies on atop sites show a blue shift, compared with the gas phase, consistent with experiments. In addition, our calculations show that energy differences between geometries with straight and those with tilted molecules on the surface are small compared with room temperature, which may explain the disordered patterns recently inferred from spectroscopy measurements. This suggests that longer isocyanide chains may be better suited to produce ordered self-assembled monolayers (SAMs) on Au(111). Finally, we discuss the electronic properties at the organic/metal interface by including self-energy corrections through many-body perturbation theory and surface polarization effects. Our results indicate that electronic structure calculations beyond DFT are required to make a correct, qualitative assessment of energy level alignment between SAMs and the metallic leads.