Negative differential resistance of Styrene on an ideal Si[111] surface: dependence of the I-V characteristics on geometry, surface doping and shape of the STM-tip

We study the electron transport properties through a supported organic molecule styrene (C8H8) on an ideal silicon surface Si[111] and probed by a STM-tip. The I-V characteristics and the differential conductance of the molecule are calculated using a self consistent approach based on non equilibrium Green's functions. Two different adsorption configurations for the molecule on the surface were considered which corresponds to a global and a local minimum of the total energy. In both cases we find a negative differential resistance (NDR) in a given interval of bias voltages. This effect is controlled by the states available close to the Fermi level of the surface and can be manipulated by properly doping the substrate. We also analyze the influence of the tip-shape on the I-V characteristics.