No abstract

The appearance of tetraphenylporphyrins in scanning tunneling micrographs depends strongly on the applied bias voltage. Here, we report the observation and identification of certain features in scanning tunneling microscopy (STM) images of intermixed layers of tetraphenylporphyrin (2HTPP) and cobalt-tetraphenylporphyrin (CoTPP) on Ag(111). A significant fraction of an ordered monolayer of commercially available CoTPP appears as "pits" at negative bias voltages around -1 V. The obvious possibility that these pits are missing molecules within the ordered layer could be ruled out by imaging the molecules at reduced bias voltages, at which the contrast of the pits fades, and at positive bias voltages around +1 V, at which the image contrast is inverted. With the investigation of the electronic structure, in particular the density of states (DOS) close to the Fermi level, of CoTPP and 2HTPP layers by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS), the contrast mechanism was clarified. The correlation of the bias dependent contrast with the UPS data enabled us to interpret the "pits" as individual 2HTPP molecules. Additional evidence could be provided by imaging layers of different mixtures of 2HTPP and CoTPP and by high-resolution STM imaging of the features in CoTPP.

The molecular arrangement and the internal conformation of Co(II)-5,10,15,20-tetrakis-(3,5-di-tert-butylphenyl)porphyrin (Co(II)-TTBPP) on Ag(111) was studied by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Interestingly, four different molecular arrangements were found using different routes to prepare the porphyrin films. After the preparation of a film with a coverage slightly below one monolayer by thermal sublimation, three clearly distinguishable molecular arrangements could be observed. Alternatively, a monolayer was prepared by deposition of a multilayer followed by subsequent annealing to desorb the excess multilayer molecules. Thereby, an additional, very rigid herringbone structure formed via a thermally activated process, covering most of the surface. The four ordered phases go along with different conformations of the individual molecules. On the basis of STM images with submolecular resolution, models for the lateral arrangements of the different polymorphs including the internal conformation of the individual molecules are proposed. Thereby, as an extension to previous works, the rotation of the upper tert-butyl groups of Co-TTBPP was taken into account. The derived molecular arrangements in combination with the intramolecular conformations give insight into the intermolecular interactions within and the nature of the different molecular phases.