133 The Electrical Measurement of Molecular Junctions M.A. REED, C. ZHOU, M.R. DESHPANDE, AND C. J. MULLER Center for Microelectronic Materials and Structures, Yale University, P.O. Box 208284, New Haven, Connecticut 06520-8284, USA T. P. BURGIN, L. JONES II, AND J. M. TOUR Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, USA ABSTRACT: We present the investigation of the electrical transport of metal/(organic mol- ecule or monolayer)/metal junctions. Utilizing a novel mechanically controllable break junction to form a statically stable system, we have self-assembled molecules of benzene- 1,4-dithiol onto two facing gold electrodes allowing for direct observation of charge trans- port through the molecules. Current-voltage I(V) measurements provides a quantitative measure of the conductance of a junction containing a single molecule. We have also created a technique to form well-defined, stable, and reproducible metallic contacts to a self-assem- bled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measure- ments show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal the dominant transport mecha- nisms, such as thermionic emission for the Ti-organic system. These techniques demon- strate the capability of electrically characterizing and engineering conductive molecular systems for future potential device applications. The measurement of charge transport in single organic molecules, and the determi- nation of their conductance, is a long-sought goal. Such measurements are experi- mentally challenging and intriguing since one can test the validity of transport approximations at the molecular level. A conceptually simple configuration would be to connect a single molecule between metallic contacts. Such a metal-molecule- metal configuration would present the molecular embodiment of a system analogous to a quantum dot, 1–9 with the potential barriers of the semiconductor system re- placed by any existing contact barrier of the molecule/metal interface. Previous mea- surements on atomic and molecular systems have been done by scanning tunneling microscopes (STM), 10–12 and can yield conductivity information. 13–15 Experiments with an evaporated-metal-top contact/molecules/metallic-bottom-contact configura- tion, which has ten of thousands of parallel active molecules, have also been dem- onstrated. 16,17 One experiment on an organic system 18 reported evidence for Coulomb charging. We have performed measurements in the configuration of a sin- gle molecule between metallic contacts; specifically on benzene-1,4-dithiolate con- nected between stable proximal metallic gold contacts, at room temperature. This approach complements previous approaches by presenting statically stable contacts, and concurrently restricts the number of active molecule(s) to as few as one.