Electrical resistivity of monolayers and bilayers of alkanethiols in tunnel junction with gate electrode

Effect of headgroup on electrical conductivity of self-assembled monolayers on mercury: n-alkanethiols versus n-alkaneselenols

The relative efficiencies of electron tunneling across self-assembled monolayers (SAMs) of n-alkanethiols and n-alkaneselenols, CH(3)-(CH(2))(n)-XH, where n = 8, 9, 11, and X = S or Se, deposited on mercury electrodes were measured via electroreduction of Ru(NH(3))(6)(3+) in aqueous solutions. Electron tunneling rates across the monolayer films decay exponentially with the monolayer thickness with a tunneling coefficient, beta = 1.1 +/- 0.1 per CH(2) irrespective of the identity of the -XH headgroup. Electron tunneling rates across n-alkanethiol monolayers are ca. 4-fold larger than the rates measured across n-alkaneselenol monolayers containing the same number of carbon atoms, signifying the importance of headgroup/metal contact resistance in electron transfer through SAMs on mercury.

Alligator clips to molecular dimensions

Techniques for fabricating nanospaced electrodes suitable for studying electron tunneling through metal-molecule-metal junctions are described. In one approach, top contacts are deposited/placed on a self-assembled monolayer or Langmuir-Blodgett film resting on a conducting substrate, the bottom contact. The molecular component serves as a permanent spacer that controls and limits the electrode separations. The top contact can be a thermally deposited metal film, liquid mercury drop, scanning probe tip, metallic wire or particle. Introduction of the top contact can greatly affect the electrical conductance of the intervening molecular film by chemical reaction, exerting pressure, or simply migrating through the organic layer. Alternatively, vacant nanogaps can be fabricated and the molecular component subsequently inserted. Strategies for constructing vacant nanogaps include mechanical break junction, electromigration, shadow mask lithography, focused ion beam deposition, chemical and electrochemical plating techniques, electron-beam lithography, and molecular and atomic rulers. The size of the nanogaps must be small enough to allow the molecule to connect both leads and large enough to keep the molecules in a relaxed and undistorted state. A significant advantage of using vacant nanogaps in the construction of metal-molecule-metal devices is that the junction can be characterized with and without the molecule in place. Any electrical artifacts introduced by the electrode fabrication process are more easily deconvoluted from the intrinsic properties of the molecule.