G.P. Tang

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 behaviors induced by BN-doping in trigonal graphene with zigzag edges

Based on nonequilibrium Green’s functions in combination with density-function theory, the transport properties of trigonal graphenes, with the vertex carbon atom substituted by one phosphorus or boron atom and bounded through a B-N pair, coupled to gold electrodes are investigated. The rectification behavior can be observed because a potential barrier similar to the p-n junction is formed in the B-N region of central molecule. When the size of a central molecule is enlarged, rectification ratio is improved greatly since the barrier height in it is enhanced as well.

All-carbon sp-sp2 hybrid structures: Geometrical properties, current rectification, and current amplification

All-carbon sp-sp2 hybrid structures comprised of a zigzag-edged trigonal graphene (ZTG)and carbon chains are proposed and constructed as nanojunctions. It has been found that such simple hybrid structures possess very intriguing propertiesapp:addword:intriguing. The high-performance rectifying behaviors similar to macroscopic p-n junction diodes, such as a nearly linear positive-bias I-V curve (metallic behavior), a very small leakage current under negative bias (insulating behavior), a rather low threshold voltage, and a large bias region contributed to a rectification, can be predicted. And also, a transistor can be built by such a hybrid structure, which can show an extremely high current amplification. This is because a sp-hybrid carbon chain has a special electronic structure which can limit the electronic resonant tunneling of the ZTG to a unique and favorable situation. These results suggest that these hybrid structures might promise importantly potential applications for developing nano-scale integrated circuits.

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.

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.

Edge contact dependent spin transport for n-type doping zigzag-graphene with asymmetric edge hydrogenation

Spin transport features of the n-type doping zigzag graphene nanoribbons (ZGNRs) with an edge contact are investigated by first principle methods, where ZGNRs are C–H2 bonded at one edge while C–H bonded at the other to form an asymmetric edge hydrogenation. The results show that a perfect spin filtering effect (100%) in such ZGNR nanojunctions can be achieved in a very large bias region for the unchanged spin states regardless of bias polarities, and the nanojunction with a contact of two C–H2 bonded edges has larger spin polarized current than that with a contact of two C–H bonded edges. The transmission pathways and the projected density of states (PDOS) demonstrate that the edge of C-H2 bonds play a crucial role for the spin magnetism and spin-dependent transport properties. Moreover, the negative differential resistance (NDR) effect is also observed in the spin-polarized current.

Controllable low-bias negative differential resistance and rectifying behaviors induced by symmetry breaking

Incorporating the characteristic of pyramidal electrode and symmetry breaking of molecular structure, we theoretically design a molecular device to perform negative differential resistance and rectifying behaviors simultaneously. The calculated results reveal that low-bias negative differential resistance behaviors can appear symmetrically when tetraphenyl molecule connects to pyramidal gold electrodes. However, as one phenyl of tetraphenyl molecule is replaced by a pyrimidyl, the symmetry breaking on the molecule will break the symmetry of negative differential resistance behavior. The peak-to-valley ratio on negative bias region is larger than that on positive bias region to perform a low-bias rectifying behavior. More importantly, increasing the symmetry breaking can further weaken these two behaviors which propose an effective way to modulate them.

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.

Magnetic structure and magnetic transport characteristics of nanostructures based on armchair-edged graphene nanoribbons

Exploring half-metallic nanostructures with a high Curie temperature and a wide half-metallic gap is a crucial solution for developing high-performance spintronic devices. Using the first-principles method, we design a new magnetic structure based on edge modification of armchair-edged graphene nanoribbons by Mn and F atoms (AGNR–Mn–F2). It is found that such a structure is an excellent half-metal with a wide bandgap (∼1.2 eV) and a stable magnetic ordering by a very high Curie temperature (Tc > 700 K) as well as being predicted to stably exist in a very large chemical potential range in experiment by the Gibbs free energy. And it is also shown that it possesses an outstanding magnetic device nature, such as a spin polarization of 100% in a very large bias region, a dual spin diode-like rectification ratio up to 105, and a spin-valve feature with a giant magnetoresistance approaching 108%, indicating a promising application for developing spintronic devices.

Altering regularities of electronic transport properties in twisted graphene nanoribbons

Based on density-function theory combined with nonequilibrium Green’s function method, the electronic transport properties of twisted armchair- and zigzag-edge graphene nanoribbons (AGNRs and ZGNRs) are investigated. Results show that electronic transport properties are sensitive to twisting deformations for semiconductor-type AGNRs, but are robust against twisting deformations for quasi-metallic AGNRs and ZGNRs. The electronic conduction becomes weaker gradually for moderate-gap semiconductor-type AGNRs, but gets stronger for wide-gap semiconductor-type AGNRs when the twisted angle increases to 120°. While for quasi-metallic AGNRs and ZGNRs, the electronic conduction is strong and obeys Ohm’s law of resistance strictly. Mechanisms for such results are suggested.