Adsorption and dissociation of iron phthalocyanine on H/Si(111): Impact of van der Waals interactions and perspectives for subsurface doping

The adsorption of iron phthalocyanine (FePc) on the passivated H/Si(111) surface is explored from first principles. We find that the organic molecule is predominantly physisorbed with a distance to the surface of $2.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.16em}{0ex}}\AA{}$ and an adsorption energy of $1.55\ifmmode\pm\else\textpm\fi{}0.1$ eV but also exhibits sizable resonance with the underlying substrate. This establishes the present system as interesting mixed covalent--van der Waals--bound test case, which we use to compare the impact of different approaches to van der Waals interactions. Spin-polarized scanning tunneling microscopy (SP STM) images are simulated, selectively accessing different molecular orbitals via the applied bias voltage in the spirit of scanning tunneling spectroscopy. Comparison with experimental STM images reveals very good agreement. We report a significant magnetic contrast exceeding $\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}\AA{}$ in the SP STM images for $\ensuremath{-}2$ and $+1.5$ V. Aiming for a magnetic functionalization of Si for possible spintronics applications, magnetic moments and binding energies of different (transition-metal) atoms in the center of the Pc ring are presented, which particularly show that Fe is strongly bound in the molecule (about 9.6 eV). Finally, we discuss different mechanisms for subsurface Fe doping by room-temperature FePc deposition and point out two feasible reactions. Concomitantly, we identify the crucial role of a preceding destabilization of FePc, for instance, by preadsorbed H atoms, which subsequently strongly stabilize the final state of the reaction.