Why Did the Electron Cross the Road? A Scanning Tunneling Microscopy (STM) Study of Molecular Conductance for the Physical Chemistry Lab

• Published in: Journal of chemical education 2014-02, Vol.91 (2), p.283-290
• Main Authors: Ewers, Bradley W, Schuckman, Amanda E, Batteas, James D
• Format: Article
• Language: English
• Subjects: Alkanes; Charge efficiency; Charge transport; Chemistry; Conductance; Crystal structure; Electronic properties; Electronic structure; Gold; Graphite; Learner Engagement; Metal surfaces; Microscopy; Molecular structure; Molecules; Organic chemistry; Physical chemistry; Pyrolytic graphite; Quantum mechanics; Quantum physics; Resistance; Scanning tunneling microscopy; Self-assembled monolayers; Self-assembly; Spectroscopy; Students; Tunneling
• ISSN: 0021-9584; 1938-1328
• DOI: 10.1021/ed400418h

A series of experiments employing scanning tunneling microscopy (STM) have been developed for the physical chemistry laboratory. These experiments are designed to engage students in cutting edge research techniques while introducing and reinforcing topics in physical chemistry, quantum mechanics, solid-state chemistry, and the electronic structure of molecules and materials. In the first of three experiments, students are introduced to the basics of STM operation while imaging and conducting spectroscopy on the highly oriented pyrolytic graphite (HOPG) surface. Images of the surface are used to determine the crystal structure of the material, and scanning tunneling spectroscopy is used to determine the electronic properties of the material and study the tunneling phenomenon. In the second experiment, the students image the Au(111) surface as well as a series of alkanethiol self-assembled monolayers (SAMs) of different chains lengths on the Au(111) surface. They examine the structural and electronic properties of the metal surface and the adlattice structure of the film. Finally, in the third experiment, the students examine the conductance of molecules adsorbed onto the Au(111) surface, including the alkanethiol SAMs and a thiol-tethered porphyrin molecule or a dimercaptostilbene embedded into the SAM matrix. By measuring the tunneling efficiency and spectroscopic characteristics of these molecules, the students can explore the relationship between chemical structure and charge transport efficiency. The experiments provide advanced chemistry students an opportunity to view and study materials at the atomic and molecular length scales and provide an opportunity to apply their understanding of quantum mechanical concepts to real systems.