Adsorption of TCNQ and F4-TCNQ molecules on hydrogen-terminated Si(1 1 1) surface: van der Waals interactions included DFT study of the molecular orientations

Abstract A study of the molecules TCNQ and F4-TCNQ surface doped on hydrogen-terminated Si(1 1 1) surface [H–Si(1 1 1)], with emphasis on the role of van der Waals interactions in stabilizing the molecular adsorption orientations and the binding energy, has been carried out using density functional theory (DFT). The dispersion energy in the adsorption process is estimated using the DFT-D2/D3 and vdW-DF methods. The most stable spatial configuration of the adsorbed molecules predicted with and without including the dispersion energy in a low concentration dopant system is same though the magnitude of the binding energy is much larger in the former case. On the other hand in the case of increased dopant concentration (doubling the number of molecules on the same surface) the adsorption orientations predicted with and without the inclusion of dispersion energy are different and so is the magnitude of the binding energy. We conclude that inclusion of dispersion energy term is important for correct prediction of the spatial configurations of the adsorbed molecules in surface transfer doping of semiconductors.