Semiquantitative Submolecular Barrier Height Measurements of 4-Aza-8-fluorotryptanthrin Monolayers on HOPG: Orbital-Mediated Tunneling

Abstract

4-Aza-8-fluoroindolo[2,1-b]quinazoline-6,12-dione (4-aza-8-fluorotryptanthrin) was imaged via scanning tunneling microscopy and barrier height tunneling spectroscopy at the graphite–solution interface. Submolecular resolution was achieved for the apparent barrier height (ϕap) images and revealed that ϕap displays significant variation over the area of the molecule. Topographic effects and electrostatic potential effects were found to be insufficient to explain the behavior of the measured ϕap variations. Specifically, where barrier height images were collected over regions of large molecular orbital (MO) wave function amplitudes, the measured ϕap did not agree with either affecting phenomenon. The authors attribute the disparity to the fact that electron tunneling is orbital-mediated for these systems. Thus, the ϕap is dominated by the spatial disposition of the mediating MO in locations of large wave function amplitude. This hypothesis is corroborated by excellent agreement with a computational treatment...