Alligator clips to molecular dimensions

Abstract

Techniques for fabricating nanospaced electrodes suitable for studying electron tunnelingthrough metal–molecule–metal junctions are described. In one approach, top contacts aredeposited/placed on a self-assembled monolayer or Langmuir–Blodgett film resting on aconducting substrate, the bottom contact. The molecular component serves as a permanentspacer that controls and limits the electrode separations. The top contact can be athermally deposited metal film, liquid mercury drop, scanning probe tip, metallic wire orparticle. Introduction of the top contact can greatly affect the electrical conductance ofthe intervening molecular film by chemical reaction, exerting pressure, or simplymigrating through the organic layer. Alternatively, vacant nanogaps can be fabricatedand the molecular component subsequently inserted. Strategies for constructingvacant nanogaps include mechanical break junction, electromigration, shadowmask lithography, focused ion beam deposition, chemical and electrochemicalplating techniques, electron-beam lithography, and molecular and atomic rulers.The size of the nanogaps must be small enough to allow the molecule to connectboth leads and large enough to keep the molecules in a relaxed and undistortedstate. A significant advantage of using vacant nanogaps in the construction ofmetal–molecule–metal devices is that the junction can be characterized with and withoutthe molecule in place. Any electrical artifacts introduced by the electrode fabricationprocess are more easily deconvoluted from the intrinsic properties of the molecule.