No abstract

We present a study which reveals that the switching speed for the redox reaction of polyaniline is very fast. With a 1200Å film in 2M sulfuric acid, switching speeds of up to 100 μs with current densities of 100 A/cm2 are observed with the appropriate compensation for IR loss. The true switching speed is higher still and has not been measured. The oxidation and reduction processes have different electrical behaviors. For the reduction process, the reaction is controlled by the RC constant of the cell given by the resistance of the solution and the capacitance of the double layer. The oxidation process is more complicated and a full description is lacking.

Two layers of molecular oligomers were deposited on flat carbon electrode surfaces by electrochemical reduction of diazonium reagents, then a top contact applied to complete a solid-state molecular junction containing a molecular bilayer. The structures and energy levels of the molecular layers included donor molecules with relatively high energy occupied orbitals and acceptors with low energy unoccupied orbitals. When the energy levels of the two molecular layers were similar, the device had electronic characteristics similar to a thick layer of a single molecule, but if the energy levels differed, the current voltage behavior exhibited pronounced rectification. Higher current was observed when the acceptor molecule was biased negatively in eight different bilayer combinations, and the direction of rectification was reversed if the molecular layers were also reversed. Rectification persisted at very low temperature (7 K), and was activationless between 7 and 100 K. The results are a clear example of a "molecular signature" in which electronic behavior is directly affected by molecular structure and orbital energies. The rectification mechanism is discussed, and may provide a basis for rational design of electronic properties by variation of molecular structure.