Self-Assembled Two-Dimensional Supramolecular Networks Characterized by Scanning Tunneling Microscopy and Spectroscopy in Air and under Vacuum

Direct Observation of Distinctive Electronic States of Ferrocene Moieties in Ferrocene-Bridged Trisporphyrin on Au(111) Using Scanning Tunneling Microscopy/Spectroscopy

The mechanical and electronic properties of ferrocene-bridged trisporphyrin (Fc2P3), which consists of three porphyrin units bridged via a ferrocene (Fc) moiety and adsorbed onto Au(111), were investigated by a pulse injection method using low-temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). STM images revealed that Fc2P3 adsorbed onto Au(111) adopts a linear arrangement with a herringbone reconstruction structure comprising both Fc and porphyrin moieties. Sequential submolecular-resolution STM images and conformational analyses using a simple molecular model revealed that the ferrocene moiety acts as a molecular ball bearing. The electronic states originating from the ferrocene group were clearly observed by bias voltage-dependent STM images, the dI/dV map, and STS. Distinct electronic states are found approximately 1 eV below E_f in the STS spectrum acquired of Fc, whereas these are absent in the porphyrin spectrum.

Assembly structures and electronic properties of truxene-porphyrin compounds studied by STM/STS

The self-assembly of functional molecules into uniform nanostructures with innovational properties has attracted extensive research interest. In the present work, the assembly structures and electronic properties of a novel type of truxene derivative, e.g. truxene-porphyrin derivatives, were studied, for the first time, on a highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM) images revealed that the truxene-porphyrin compounds could be parallelly arranged into long-ranged lamellar patterns. Density functional theory (DFT) calculations helped explain the assembly mechanisms further. Besides, order distribution of the smaller compound 1T1P in the 1,3,5-tris(10-carboxydecyloxy)-benzene (TCDB) host network was achieved, which is a reflection of the dimensional effect in the host-guest assembly. Furthermore, together with theoretical analyses, scanning tunneling spectroscopy (STS) measurements were conducted to investigate the electronic properties of truxene-porphyrin compounds. Results showed that the metalation of the porphyrin units could have a significant effect on the band gap and the position of the gap center. The study enhances our understanding of the assembly mechanism of truxene derivatives at the molecular level and paves the way towards fabricating truxene-based functional nanodevices.