Ab initio study of the single molecule conductance of TCNQ and F₄TCNQ using NEGF-DFT

By investigating the conductance of single molecules we gain insight into the fundamental physics of electron transport. Such an understanding can lead to molecular-based electronic components and sensors. We study the transport properties of tetracyanoquinodimethane (TCNQ) and tetrafluoro-TCNQ (F₄TCNQ) at the level of density functional theory within the non-equilibrium Green's function formalism (NEGF-DFT). Experiments show at least three distinct conductance values for a single molecule of TCNQ and F₄TCNQ. We construct a model system which consists of two gold electrodes connected via a single molecule arranged in different orientations. Four orientations are defined: bidentate-bidentate (bi-bi), mono-bi, mono-mono, and flat. We find that conductance depends on the molecule's orientation between the electrodes which ranges over an order of magnitude. In both molecules, the bi-bi and mono-bi produce lower conductance (0.02 - 0.1 G₀) while the flat orientation gives a slightly higher conductance of 0.2 G₀. Surprisingly, the mono-mono shows the largest conductance at 0.4 and 0.6 G₀ for F₄TCNQ and TCNQ respectively. The results are in qualitative agreement with scanning tunneling microscopy break junction (STM-BJ) experiments, which find two low and one high conductance value.