Self-Assembly and Electronic States of an Electron Acceptor Molecule on a Metal Substrate.

Self-assembled monolayers and bilayers of the electron acceptor molecule tetracyanoquinodimethane (TCNQ) were investigated on Pb(100) using low-temperature scanning tunneling microscopy (STM) at 4.5 K and density functional theory (DFT) calculations. The monolayer exhibits an unusually low molecular density. In contrast, both the first and second layers in the bilayer adopt a packing density twice that of the monolayer. Our modeling reveals that Pb atoms, apparently released from step edges, stabilize the bilayer structure. The structural transition is accompanied by a pronounced change in the electronic structure: the bilayer hosts a singly occupied molecular orbital (SOMO) and its corresponding unoccupied partner (SUMO), arising from a spin-splitting of the lowest unoccupied molecular orbital (LUMO) observed in the monolayer.