Scanning Tunneling Microscopy Imaging and Spectroscopy of a Single Viologen Molecule

A low-temperature ultrahigh-vacuum scanning tunneling microscope (UHV-STM) was used to image viologen (N-methyl-N′-di (8-mercaptooctyl)-4,4′-bipyridinium; HSC₈VC₈SH) molecules and to perform local spectroscopic measurements on these molecules. Self-assembly of viologen molecules was conducted on Au (111), which had been thermally deposited onto freshly cleaved, heated mica. Here, we demonstrate a novel SAM matrix appropriate for the isolation of viologen molecules composed of octanethiol (C₈) in which HSC₈VC₈SH was inserted at defects in the molecular lattice. The isolated single molecules of viologen inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. STM images at 298 K showed protrusions with a topographic height of about 2.71 nm (HSC₈VC₈SH) with viologen molecules that self-assembled on the substrate. The current–voltage (I–V) characteristics were measured while the electrical properties of the formed monolayer were scanned using scanning tunneling spectroscopy (STS). We found the high peak current-like rectification at +1.14 V (HSC₈VC₈SH). The rectification ratios, RR = J (at +2.5 V)/J (at −2.5 V), are in the range of 4.47.