Molecular Resonance Imaging and Manipulation of Hexabenzocoronene on NaCl(001) and KBr(001) on Ag(111)

The adsorption of hexa-peri-hexabenzocoronene (HBC) on NaCl and KBr bilayers deposited on Ag(111) is studied by scanning tunneling microscopy (STM) and spectroscopy at low temperature (5 K). HBC tends to move under the influence of the STM tip on NaCl/Ag(111), even in the mildest imaging conditions, preventing the imaging of its molecular electronic resonances (MERs). It is more stable on KBr, due to a higher diffusion barrier, as confirmed by a force-field based calculation of its adsorption on both surfaces. The MER associated with the lowest unoccupied molecular orbital (LUMO) of HBC is imaged and analyzed in detail on KBr/Ag(111). Assemblies of two to four HBC could be built on NaCl by lateral manipulations with the STM tip. These objects present a higher stability than single molecules making them more amenable to MER imaging in a large bias voltage range. While the constituting molecules are too far apart to interact chemically, their electronic clouds overlap, producing in some cases complex images of the MERs that are difficult to disentangle to extract the single molecule contributions. This problem is examined by comparing the images of a two HBC assembly to those of a single molecule on KBr. A combination rule is proposed that could be extended to extract single molecule contributions from larger assemblies.