Theoretical investigation of electron transport modulation through benzenedithiol by substituent groups

Density functional theory combined with nonequilibrium Green’s function techniques was used to model the conduction through disubstituted benzenedithiol molecules bonded to leads composed of 3×3, 5×5 gold and 3×3 aluminum. For the disubstituted 3×3 Au-benzenedithiol-Au systems, the small lead cross section results in a region of nearly zero transmission from −0.4to−0.2eV, relative to EF, due to the absence of lead states. This feature results in negative differential resistance in the current-voltage curves and also causes the main peaks in the transmission spectra, which are dominated by the highest occupied molecular orbitals, to be centered near EF. The zero-bias transmissions for the disubstituted benzenedithiol, as well as currents at applied biases, correlate very well with the Hammett parameter σp, a quantity that relates the electron donating or withdrawing strength of a substituent. Calculations on disubstituted benzenedithiol connected to 5×5 Au leads produced transmission spectra that showed no gaps over the energy range considered and no negative differential resistance. The transmission in these cases also predominately involves the highest occupied molecular orbitals, and electron donating and withdrawing groups are able to increase and decrease current, respectively. However, there is no strong correlation between current and σp for this system. This suggests that the correlation observed in the 3×3 Au systems arises from the abrupt cutoff of the main transmission peaks near EF. The disubstituted 3×3 Al-benzenedithiol-Al systems displayed markedly different behavior from the Au analogs. Electron donating groups and H benzenedithiol-substituted systems display almost no transmission over the energy range considered. However, electron withdrawing group disubstituted benzenedithiol systems had significant peaks in the transmission spectra near EF, which are associated with the lowest-energy, unoccupied π-type molecular orbitals. Higher currents are calculated for cases where the substituents have π-type orbitals that are conjugated with the ring moiety of benzenedithiol. In all cases, the current through the 3×3 Al-benzenedithiol-Al systems is about a factor of 2 less than that through the analogous Au systems. These simulations reveal that the electrical conductance behavior through nanosystems of the type investigated in this work depends on the nature of the molecule as well as the size and composition of the leads to which it is connected. The results suggest that rational design of nanoelectronic systems might be possible under certain conditions but that structure-function relationships cannot be transferred from one system to another.