Scanning tunneling current-voltage spectroscopy on poly(p-phenylene vinylene) films: A nanoscale probe for the electronic conduction

We have determined the electronic properties of an organic PPV film deposited on a $p^{+}$-doped GaAs substrate by scanning tunneling spectroscopy. The normalized dynamical conductance curves of the PPV chains show well-defined conduction onsets and/or slope changes corresponding to the electron conduction through the LUMO and HOMO levels under forward and reverse bias. A theoretical calculation of the electronic transmission coefficient is used to describe the tunneling current as a function of the sample bias and to understand the electronic properties of the polymer. From the comparison between the calculations and the experimental results we obtain the single particle energy gap, the Fermi level position and the charge-carrier type.