Liu De-Sheng

Electronic Transport Properties of an Anthraquinone-Based Molecular Switch with Carbon Nanotube Electrodes

Based on the nonequilibrium Greens function method and density functional theory calculations, we theoretically investigate the electronic transport properties of an anthraquinone-based molecular switch with carbon nanotube electrodes. The molecules that comprise the switch can convert between reduced hydroquinone (HQ) and oxidized anthraquinne (AQ) states via redox reactions. Our results show that the on-off ratio is increased one order of magnitude when compared to the case of gold electrodes. Moreover, an obvious negative differential resistance behavior at much low bias (0.07 V) is observed in the HQ form.

Electronic Transport Properties of a Naphthopyran-Based Optical Molecular Switch: an ab initio Study

The electronic transport properties of a naphthopyran-based molecular optical switch are investigated by using the nonequilibrium Greens function formalism combined with first-principles density functional theory. The molecule that comprises the switch can convert between its open and closed forms upon photoexcitation. Theoretical results show that the current through the open form is significantly larger than that through the closed form, which is different from other optical switches based on ring-opening reactions of the molecular bridge. The maximum on-off ratio (about 90) can be obtained at 1.4 V. The physical origin of the switching behavior is interpreted based on the spatial distributions of molecular orbitals and the HOMO-LUMO gap. Our result shows that the naphthopyran-based molecule is a good candidate for optical molecular switches and will be useful in the near future.

Effect of Temperature on Polaron Stability in a One-Dimensional Organic Lattice

Effect of temperature on the polaron stability in a one-dimensional organic lattice is investigated within the Su-Schrieffer-Heeger model. The temperature effect is simulated by introducing random forces to the equation of the lattice motion. It is found that the localized polaron state becomes delocalized even at low temperatures. The time of polaron keeping localized depends on the magnitude of temperatures. By taking into account the thermal effect, we find that the dissociation field is weaker as compared with earlier works.

Rectifying Properties of a Nitrogen/Boron-Doped Capped-Carbon-Nanotube-Based Molecular Junction

Based on the non-equilibrium Greens function method and first-principles density functional theory calculations, we investigate the electronic transport properties of a nitrogen/boron-doped capped-single-walled carbon-nanotube-based molecular junction. Obvious rectifying behavior is observed and it is strongly dependent on the doping site. The best rectifying performance can be carried out when the nitrogen/boron atom dopes at a carbon site in the second layer. Moreover, the rectifying performance can be further improved by adjusting the distance between the C60 nanotube caps.

Effect of Electric Field on Spin Polarized Current in Ferromagnetic/Organic Semiconductor Systems

Considering the special carriers in organic semiconductors, the spin polarized current under electric field in a ferromagnetic/organic semiconductor system is theoretically studied. Based on the spin-diffusion theory, the current spin polarization under the electric field is obtained. It is found that electric field can enhance the current spin polarization.

Low Bias Negative Differential Resistance Behavior in Carbon/Boron Nitride Nanotube Heterostructures

Based on the non-equilibrium Greens method and density functional theory, we investigate the electronic transport properties of ternary heterostructures based on carbon nanotubes and boron nitride nanotubes, with different atomic compositions, coupled to gold electrodes. Negative differential resistance (NDR) behavior can be observed due to suppression of the conduction channel at a certain bias. More importantly, the position of NDR can be tuned into the bias range as low as tens of meV by increasing the length of boron nitride nanotube. The peak-to-valley ratio, which is a typical character of NDR behavior, is also sensitive to the atomic compositions.

Ground state and polaron and bipolaron excited states in polydiacetylene

The electronic properties of ground state and charged excited states of non-degenerate polydiacetylene were investigated by means of a tight-binding model. The parameters of the model were obtained by comparison of the experimental and other theoretical results. It was found that there is a stable dimerized structure of polydiacetylene in ground state and the doping induces the nonlinear excitations, such as polarons and bipolarons. In order to compare the stability of polaron and bipolaron, the creation energy and binding energy were separately defined. By neglecting the electron-electron Coulomb interaction, a bipolaron is more stable than two independent polarons.

Reverse polarization in conjugated heterocycle polythiophene

Reverse polarization in polythiophene under an applied electric field has been studied in the framework of the tight-binding model. It is found that the applied electronic field has a great influence on the excited states of polythiophene. The effect of the heteroatoms on the polarization has been calculated and analysed carefully. It is indicated that a reverse polarization of biexcitons in polythiophene will be observed more easily. The heteroatoms increase this reversed polarization strength apparently.

Perfect Spin-Filtering in 4H-TAHDI-Based Molecular Devices: the Effect of N-Substitution

Based on the non-equilibrium Greens function formalism and spin-polarized density functional theory calculations, we investigate the spin transport properties of HDI and terahydrotetraazahexacene diimide (4H-TAHDI) with two ferromagnetic zigzag-edge graphene nanoribbon electrodes. Compared with HDI, four carbon atoms in the hexacene part of 4H-TAHDI are substituted by nitrogen atoms. The results show that the nitrogen substitution can improve significantly the spin-filtering performance and 4H-TAHDI can be used as a perfect spin filter. Our study indicates that suitable chemical substitution is a possible way to realize high-efficiency spin filters.

A First-principles Study of Spin-polarized Transport Properties of a Co-coordination Complex

Based on non-equilibrium Greens function formalism and density functional theory calculations, we investigate the spin-polarized transport properties of a Co-coordination complex between two gold electrodes, in which a Co ion is trapped between two 4-mercaptopyridine molecules. Our results demonstrate that the transmission spectra of the system show distinctive features in the spin-up and spin-down channels. Moreover, the current-voltage curves confirm that the system can exhibit robust spin-filtering effect at finite bias voltage, giving the system potential in molecular spintronics applications.