Asymmetry and rectification in the tunnel current of a nanometer-sized metal-conjugated polymer-metal junction

Electron transport processes of a nanometer metal-conjugated polymer-metal tunnel junction have been probed using a scanning tunneling microscope. The tunnel current of the junction shows two effects. The appearance of an asy mmetry in the tunnel current indicates that the junction transport mechanis m is different from that for which tunneling occurs directly between two me tallic electrodes. Thus, understanding of the asymmetry and hence the trans port mechanism demands a detailed description of the metal-polymer interfac e. By applying the theories of the metal-semiconductor interface to the tun nel junction, we show the presence of an asymmetric electrostatic potential -energy profile, which, together with the metal-induced gap states in the p olymer, gives rise to the observed asymmetry in the tunnel current. In some cases, a threshold of anomalously large currents enhances the current asym metry to give rise to rectification, indicating carrier excitations and car rier multiplication processes in the junction. Our results show that a deta iled description of the interface electronic structure is essential to unde rstanding electron transport in devices based on organic molecules. (C) 200 0 American Institute of Physics. [S0021-9606(00)70215-7].