Current rectification by simple molecular quantum dots: An<i>ab initio</i>study

Abstract

We calculate current rectification by molecules containing a conjugated molecular group sandwiched between two saturated (insulating) molecular groups of different lengths (molecular quantum dot) using an ab initio nonequilibrium Green's function method. In particular, we study $\mathrm{S}({\mathrm{CH}}_{2}{)}_{m}---{\mathrm{C}}_{10}{\mathrm{H}}_{6}---({\mathrm{CH}}_{2}{)}_{n}\mathrm{S}$ with naphthalene as the conjugated central group. The rectification current ratio $\ensuremath{\sim}35$ has been observed at $m=2$ and $n=10,$ due to resonant tunneling through the molecular orbital (MO) closest to the electrode Fermi level (lowest unoccupied MO in the present case). The rectification is limited by the interference of other conducting orbitals, but can be improved by, e.g., adding an electron withdrawing group to the naphthalene.