Tuning negative differential resistance in a molecular film

We have observed variable negative differential resistance (NDR) in scanning tunneling spectroscopy measurements of a double layer of C-60 molecules on a metallic surface. Minimum to maximum current ratios in the NDR region are tuned by changing the tunneling barrier width. The multilayer geometry is critical, as NDR is not observed when tunneling into a C-60 monolayer. Using a simple model, we show that the observed NDR behavior is explained by voltage-dependent changes in the tunneling barrier height. (c) 2005 American Institute of Physics.