Asymmetric electron transport through a conjugated–saturated hydrocarbon molecular wire

Theoretical estimation of size effects on the electronic transport in tailored graphene nanoribbons

Focusing on the potential applications of tailored graphene nanoribbons (t-GNRs), in this work, we systematically study size effects on the electronic transport in t-GNR-based molecular junctions. As a result of the manufacturing error generated during the processing or synthesis of t-GNRs using techniques such as ion beam lithography, the final dimensions of the as-fabricated devices often deviate from the design values, giving rise to a size distribution around the mean value which could considerably affect the device performance. To simulate the effects of the manufacturing error, a series of t-GNR-based junctions with various dimensions have been modelled and systematically investigated using density functional theory (DFT) coupled with the non-equilibrium Green's function (NEGF). For junctions that consist of an acene chain connected with two graphene nanosheets, it is found that the chain length has little influence on the electronic transport and that, on the other hand, the junction conductivity is significantly altered by its width due to the different number and nature of the electron transfer pathways. Furthermore, increasing the width of the junction leads to a clear odd-even variation of decreasing amplitude in its transport behavior. These findings underpin further fundamental and device-based studies of t-GNRs.

A theoretical study of asymmetric electron transport through linearly aromatic molecules

Electron transport through a series of polyacene molecules connected via a π-conjugated bridge (an anthracene molecule) was investigated theoretically by using the nonequilibrium Green's function formalism combined with density functional theory. The results have shown that the asymmetric current-voltage characteristics can be achieved by tuning the position of a side π-bridge linked to the main conjugated backbone. The detailed analyses of the spatial distribution of molecular orbitals as well as the current density interpret how the location of π-bridge strongly affects the intramolecular electronic coupling. The rectification in the molecular junction arises from the localization of the molecular orbitals near the Fermi level and the asymmetric shift of molecule orbital energy levels under positive and negative bias. The rectification ratio decreases with increasing the length of the π-bridge which improves intramolecular electronic coupling between aromatic rings. Furthermore, the rectification properties of conjugated molecules are just slightly affected by the anchoring positions of thiol groups. These results demonstrated that the location and the length of π-bridge, which induce the asymmetric intramolecular coupling, play key roles in the rectification of the linearly aromatic molecules.

Molecular rectification in triangularly shaped graphene nanoribbons

We present a theoretical study of electron transport in tailored zigzag graphene nanoribbons (ZGNRs) with triangular structure using density functional theory together with the nonequilibrium Green's function formalism. We find significant rectification with a favorite electron transfer direction from the vertex to the right edge. The triangular ZGNR connecting to the electrode with one thiol group at each terminal shows an average rectification ratio of 8.4 over the bias range from -1.0 to 1.0 V. This asymmetric electron transport property originates from nearly zero band gap of triangular ZGNR under negative bias, whereas a band gap opens under positive bias. When the molecule is connected to the electrode by multithiol groups, the current is enhanced due to strong interfacial coupling; however, the rectification ratio decreases. The simulation results indicate that the unique electronic states of triangular ZGNR are responsible for rectification, rather than the asymmetric anchoring groups.