Tuning Negative Differential Resistance in a Molecular Film
  We have observed tunable negative differential resistance (NDR) in scanning tunneling spectroscopy measurements of a double layer of C60 molecules on a metallic surface. Using a simple model we show that the observed NDR behavior is explained by voltage-dependent changes in the tunneling barrier height. 
M. Grobis, A. Wachowiak, R. Yamachika, and M. F. Crommie