Fullerene-based single molecule diodes with huge rectification ratios: a DFT-NEGF study†

A high-performance single molecule rectifier is one of the most important components in the field of molecular electronics. According to the rules proposed by Van Dyck and Ratner, we designed S-C₆H₄-(–CC-C₆H₄–)_n-N-C₆₀ composite devices (n = 1–5), and their transport properties and rectification ratios (RRs) were investigated using nonequilibrium Green's functions (NEGFs) in combination with density functional theory (DFT). Our results show that the device with n = 3 exhibits a huge RR of 2.2 × 10⁴, and the device with n = 4 exhibits a comparable RR. In addition, a significant negative differential resistance (NDR) effect is also observed. The rectification effect mainly comes from the alignment between the LUMO of the central molecule and the Fermi level of the electrode induced by the pinning effect, rather than the alignment between the HOMO and the LUMO. It is also found that the ON currents undergo an exponential decay as the molecular length increases (I ∝ e^−1.7n), while the OFF currents exhibit irregular variations, which lead to abnormal enhancement of the rectification effect. Our analysis suggests that the coupling between fullerene and the gold tip electrode, along with coplanar phenylacetylenes are the preconditions for achieving high RRs in our devices. As a result, besides the previous rules focusing on enhancing the ON current, enhancing the suppression of the OFF current is also very important, which may serve as further guidance for designing molecular rectifiers. In general, we not only designed molecular devices with RRs larger than 10⁴, but also proposed another rule to design molecular rectifiers.