Origin of Negative Differential Resistance in a Strongly Coupled Single Molecule-Metal Junction Device

A new mechanism is proposed to explain the origin of negative differential resistance (NDR) in a strongly coupled single molecule-metal junction. A first-principles quantum transport calculation in a Fe-terpyridine linker molecule sandwiched between a pair of gold electrodes is presented. Upon increasing the applied bias, it is found that a new phase in the broken symmetry wave function of the molecule emerges from the mixing of occupied and unoccupied molecular orbitals. As a consequence, a nonlinear change in the coupling between the molecule and the lead is evolved resulting in NDR. This model can be used to explain NDR in other classes of metal-molecule junction devices.

Figure 1

Schematic of the strongly coupled single molecule (FETP)-gold junction.

Figure 2

Calculated current and conductance as a function of applied bias. $I_p$ refers to the peak value for current.

Figure 3

Bias dependent transmission as a function of injection energy $E$. Fermi energy is set to zero in the energy scale; dotted lines in each panel represent the chemical potential window.

Figure 4

(a) Occupied molecular orbital under applied bias: from right to left (column wise) the orbitals are HOMO, $\mathrm{HOMO}-1$, $\mathrm{HOMO}-2$, and $\mathrm{HOMO}-3$. (b) Unoccupied molecular orbitals under applied bias: from right to left (column wise) the orbitals are LUMO, $\mathrm{LUMO}+1$, $\mathrm{LUMO}+2$, and $\mathrm{LUMO}+3$. Bias changes row wise. Notation: dark gray (red) corresponds to negative lobe, and light gray (green) corresponds to the positive lobe.