M. Qiu

The transport properties of D-σ-A molecules: A strikingly opposite directional rectification

We design the A-R rectifier based on the D-σ-A molecules to examine the rectifying performances by the first-principles method. The calculated results show that the electronic structures for all of our systems perfectly match the A-R rectifier, as expected, but their rectifying direction is very strikingly opposite and working mechanism is completely different. This behavior can be rationalized through an asymmetrical shift of molecular levels under bias of different polarities, which is because of always-existing intrinsic asymmetrical coupling effects of molecular levels to electrodes. Detailed analysis demonstrates that the rectifying direction induced by this mechanism is always in opposition to that induced by the A-R mechanism.

Electrode metal dependence of the rectifying performance for molecular devices: A density functional study

Carrying out theoretical calculations using the nonequilibrium Green’s function method combined with the density functional theory, the transport properties of the terphenyl molecule connected to the two Y (Y=Li, Al, or Au) metal electrodes are investigated. The results show that the electrode metals have a distinct influence on rectifying performance of such devices. For the Au electrode system, we can observe a best rectifying performance, next for the Al electrode system, and the rectifying effect can be nearly neglected for the Li electrode system. Our findings suggest that the rectifying characteristics are intimately related to electrode materials.

Rectifying performance of D-π-A molecules based on cyanovinyl aniline derivatives

Using the first-principles method, we investigate rectifying performances of D-π-A molecules based on cyanovinyl aniline derivatives. The calculated results show that different functional groups can change the location of molecular orbitals and thus change the rectifying properties of molecules. Interestingly, we find that although the electronic structure for our studied systems is in agreement with that proposed originally by Aviram and Ratner [Chem. Phys. Lett. 29, 277 (1974)], the rectifying direction is opposite from it due to the asymmetric shift of molecular levels under biases of different polarities. Only for model (M4), it shows a forward rectifying performance under larger bias.

Examinations into the contaminant-induced transport instabilities in a molecular device

We report first-principles calculations of transport behaviors for a molecular device whose electrode surface is contaminated by various diatomic groups. It has been found that such a device demonstrates less transport variations for the contamination of the group PO or SO in the whole bias range but it shows more transport variations for contamination of the group CN, HS, or NO only under low bias, which suggests that contamination of all diatomic groups studied here always affects high-bias transport properties of a device in an extremely gentle manner.Using first-principles calculations, we study the work function of single wall silicon carbide nanotube (SiCNT). The work function is found to be highly dependent on the tube chirality and diameter. It increases with decreasing the tube diameter. The work function of zigzag SiCNT is always larger than that of armchair SiCNT. We reveal that the difference between the work function of zigzag and armchair SiCNT comes from their different intrinsic electronic structures, for which the singly degenerate energy band above the Fermi level of zigzag SiCNT is specifically responsible. Our finding offers potential usages of SiCNT in field-emission devices.

The site effects of B or N doping on I-V characteristics of a single pyrene molecular device

Using the non-equilibrium Green’s function method combined with the density functional theory, the electronic transport properties of boron (B) or nitrogen (N) doped pyrene molecular devices are investigated. The results show that effects of B or N doping on I-V characteristics of a single pyrene molecular device are not constant and can be changed by varying doped sites. More importantly, significant negative differential resistance (NDR) behaviors are found in B-doped pyrene molecular devices. The peak-to-valley ratio which is a typical character of NDR behavior is also sensitive to the B doped site.Using the non-equilibrium Green’s function method combined with the density functional theory, the electronic transport properties of boron (B) or nitrogen (N) doped pyrene molecular devices are investigated. The results show that effects of B or N doping on I-V characteristics of a single pyrene molecular device are not constant and can be changed by varying doped sites. More importantly, significant negative differential resistance (NDR) behaviors are found in B-doped pyrene molecular devices. The peak-to-valley ratio which is a typical character of NDR behavior is also sensitive to the B doped site.

Conduction switching behaviors of a small molecular device

We calculate the current-voltage properties for a small organic molecule system based on the local atomic orbital density-functional theory. It has been found that our system has a distinctive conduction switching behavior with the “on/off” ratio on the order of 102 at a bias of 0.8 V and then up to more than the order of 103 in a bias range from 0.8 to 1.8 V, and its explicit steady state and metastable state can be converted to each other by thermal activation. These findings suggest that this small molecular system has obvious potential advantages for the realization of the miniaturized molecular switch.

End-group effects on negative differential resistance and rectifying performance of a polyyne-based molecular wire

Based on first-principles approach, the end-group effects on negative differential resistance (NDR) and rectifying performance of polyyne-based molecular wires are investigated. The NDR behaviors are observed when the polyyne is attached to asymmetric (–NO2 and –NH2) or symmetric (double –S) end groups, and rectifying performance emerges with the presence of asymmetric groups. The analysis on microscopic nature reveals the intrinsic origin of these phenomena. Our results show the possibility of a multifunctional molecular device design simultaneously with NDR and rectifying performances by using a technology of capping certain end groups to polyyne.

Electrode conformation-induced negative differential resistance and rectifying performance in a molecular device

Carrying out theoretical calculations using the nonequilibrium Green’s function method combined with the density functional theory, the transport properties of a carbon wire connected to two Au electrodes are investigated. The results show that the negative differential resistance and rectifying performance can be observed apparently when a pure carbon chain is connected to two asymmetric Au electrodes. The main origin of the negative differential resistance behavior is a suppression of the highest occupied molecular orbital resonance at certain bias voltage. Also shown is that it is possible to make the negative differential resistance disappear and rectifying performance be weakened only by adding side groups to a wire.

Electronic transport of unimolecular devices with a group coadsorbed on one electrode surface: A density functional study

The first-principles calculations based on the density functional theory are applied to investigate the effect of a chemical group coadsorbed on one electrode surface on the electronic transport of a molecular device. We find that the types of the coadsorbed groups and their sites on one electrode surface affect the electronic transport significantly, and the resulting shift of the molecular levels upon coadsorption depends jointly on various effects, such as the electrostatic interaction, shift of the Fermi level of the electrode, the chemical interaction, and so on. Among these factors, the chemical interaction-induced the charge transfer across metal-molecule interface is identified as a determining factor resulting in the variation of transport properties. Our findings suggest that the coadsorption may offer the novel possibility to modify the transport behaviors of a molecular device in a controlled way and can improve/add some particular functionalities or should be avoided in order to keep a stable transport for a molecular device.

Length and end group dependence of the electronic transport properties in carbon atomic molecular wires

Carrying out theoretical calculations using the nonequilibrium Greens function method combined with the density functional theory, the transport properties of functionalized atomic chains of carbon atoms with different lengths are investigated. The results show that the I-V evolution and rectifying performance can be affected by the length of wire when both ends of it is capped with the benzene-thiol attached with an amino group and the pyridine attached with nitro group. But when capped with the benzene-thiol attached with an amino group and the nitro group, we can observe a surprised result that different systems show similar I-V characteristics and their transport properties are almost independent of molecular length, which suggests that this is a favorable way to design more ideal molecular interconnecting wires with a high length-independent conductance behavior.