A simplified density functional theory method for charged adsorbates on an ultrathin, insulating film supported by a metal substrate

摘要

A simplified density functional theory (DFT) method for charged adsorbates onan ultrathin, insulating film supported by a metal substrate is developed andpresented. This new method is based on a previous DFT development that uses aperfect conductor (PC) model to approximate the electrostatic response of themetal substrate, while the film and the adsorbate are both treated fully withinDFT [I. Scivetti and M. Persson, Journal of Physics: Condensed Matter 25,355006 (2013)]. The missing interactions between the metal substrate and theinsulating film in the PC approximation are modelled by a simple force field(FF). The parameters of the PC model and the force field are obtained from DFTcalculations of the film and the substrate, here shown explicitly for a NaClbilayer supported by a Cu(100) surface. In order to obtain some of theseparameters and the polarisability of the force field, we have to include anexternal, uniformly charged plane in the DFT calculations, which has requiredthe development of a periodic DFT formalism to include such a charged plane inthe presence of a metal substrate. This extension and implementation should beof more general interest and applicable to other challenging problems, forinstance, in electrochemistry. As illustrated for the gold atom on the NaClbilayer supported by a Cu(100) surface, our new DFT-PC-FF method allows us tohandle different charge states of adsorbates in a controlled and accuratemanner with a considerable reduction of the computational time. In addition, itis now possible to calculate vertical transition and reorganisation energiesfor charging and discharging of adsorbates that cannot be obtained by currentDFT methodologies that include the metal substrate. We find that the computedvertical transition energy for charging of the gold adatom is in good agreementwith experiments.