Desheng Liu

Spin-filtering, giant magnetoresistance, rectifying and negative differential resistance effects in planar four-coordinate Fe complex with graphene nanoribbon electrodes

By using the nonequilibrium Greens function formalism combined with the density functional theory, we have investigated the spin-polarized transport properties of a planar four-coordinate Fe complex sandwiched between two zigzag-edge graphene nanoribbon (ZGNR) electrodes, where the ZGNRs are modulated by external magnetic field. The results show that the system can exhibit perfect dual spin-filtering and spin-rectifying effects at a wide bias range, giant magnetoresistance effect with large magnetoresistance ratio at small bias, and obvious negative differential resistance behavior. The mechanisms are proposed for these phenomena.

Gated electronic currents modulation and designs of logic gates with single molecular field effect transistors

The electronic transport properties of a gated single 1,3-benzenedithiol molecular device are studied by using nonequilibrium Greens function in combination with density functional theory, which is hoped to complement the experiments. The results show that the external transverse gate electrodes can effectively tune the electronic transport properties of the molecular devices. Negative differential resistance behaviors are observed almost at the same source-drain bias when applied different gate voltages. Mechanisms are proposed for these phenomena. Designs of using one gated molecular device to realize five basic logic gates are also put forward.

Growth and characterization of Nd-doped Ca0.28Ba0.72Nb2O6 crystal

We report the growth of a Nd-doped calcium barium niobate (Nd:CBN-28) single crystal using the Czochralski method. The point group and space group of Nd:CBN-28 are determined to be tetragonal 4mm and P4bm, respectively, and the effective segregation coefficients of Nd, Ca, Ba, and Nb are 0.988, 1.029, 0.992, and 0.985, respectively. The density of the material is 5.321 g cm−3 at 19.6 °C, in good agreement with the theoretical value. Thermal expansion measurements reveal that during the heating cycle, the c-axis of Nd:CBN-28 undergoes both thermal contraction and thermal expansion, while at the same time there is only expansion along the a-axis. Specific heat measurements reveal that an anomaly appears at about 220 °C due to a phase transition. The thermal diffusivity increases slowly with increasing temperature. The thermal conductivity of the crystal has been calculated from these results, and the phonon mean free paths la and lc along the a- and c-axes of Nd:CBN-28 crystal are estimated. The transmissio...

Energy alignment induced large rectifying behavior in endoheral fullerene dimers

Using the nonequilibrium Greens function formalism combined with density functional theory for quantum transport calculation, we have investigated the electronic transport properties of three endofullerenes Na@C60C60[@F, Na@C60C60, and F@C60C60. The results show that the electronic transport properties of these endofullerenes are strongly dependent upon the species inside the fullerene. A large rectifying behavior is observed in Na@C60C60, while Na@C60C60[@F and F@C60C60 can only present very weak rectification. It is revealed that the alignment between the molecular levels of two C60s moieties with the applied bias is the main cause of the large rectification in Na@C60C60.

Effects of disordered interchain interactions on polaron dynamics in semiconducting polymers.

Polaron dynamics in a system of two randomly coupled polymer chains is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of the Su-Schrieffer-Heeger model modified to include disordered interchain interactions and an external electric field. By analysing the polaron velocity statistically, we find that the polaron motion is determined by the competition between the electric field and the disordered interchain interactions. Polaron dynamics are classified into two types, weak-coupling dynamics and strong-coupling dynamics. It is found that the strength of interchain interactions is the dominant factor controlling charge propagation in weak-coupling dynamics, whereas the effects of disorder are dominant in strong-coupling dynamics. The charge carriers tend to have higher mobility for stronger interchain coupling, and interchain coupling disorder can be favorable for charge transport depending on the coupling strength and the electric field.

Spin-dependent transport properties in a pyrene–graphene nanoribbon device

Based on first-principles density functional theory combined with the nonequilibrium Greens function method, we have investigated the spin-dependent transport properties of a pyrene–zigzag graphene nanoribbon (ZGNR) system. The results show that this system can exhibit high-performance spin filtering, spin rectifying, giant magnetoresistance and negative differential resistance effects, by tuning the magnetization configuration of ZGNR electrodes. By analyzing the spin-resolved transmission spectrum, the local density of states, the transmission pathways, the band structure and symmetry of ZGNR electrodes, as well as the spatial distribution of molecular orbitals within the bias window, we elucidate the mechanism for these intriguing properties. Our results suggest that the pyrene–ZGNR system is a potential candidate for developing high-performance multifunctional spintronic devices.

A first principle study on the spin transport properties in heterojunctions based on zigzag-edged graphene nanoribbons and graphitic carbon nitride nanoribbons

By using non-equilibrium Green’s functions (NEGF) and density functional theory (DFT), we investigate the spin-dependent electronic transport properties of two heterojunctions based on zigzag-edged graphene nanoribbons and graphitic carbon nitride nanoribbons. The only difference is the scattering region, i.e., one is zigzag-edged graphene nanoribbons (ZGNRs) and the other is graphitic carbon nitride (g-C3N4) nanoribbons. The I–V curves in the ferromagnetic and antiferromagnetic states for both devices are demonstrated. Our results show that the heterojunctions are promising multifunctional devices in molecular spintronics due to their nearly perfect spin-filtering efficiency (SFE) and high rectification ratio (RR). Spin negative differential resistance (SNDR) properties at low biases can also be found in the two devices. The mechanisms are proposed for these phenomena. The spin polarizations in the transmission spectra result in the nearly perfect SFE, the asymmetry in the structures gives rise to the high RR. Moreover, for the SNDR, the suppression of the transmission spectra is mainly caused by the localization in the total density of states.

Effect of different electrodes on Fano resonance in molecular devices

By using nonequilibrium Green’s function in combination with density functional theory, we study the electronic transport properties of two typical π-conjugated molecules (dithiol-benzene and C4S2), sandwiched between two metallic electrodes made of different metals. The presence of two different electrodes leads to Fano resonances at certain energy. As a consequence, electronic transport in future molecular electric circuits can be substantially affected when the molecular devices placed between electrodes with different chemical potentials. The Fano line shapes reveal that there is nonresonant channel when two asymmetric electrodes are employed.

Reverse polarization in charged π-conjugated oligomers

Single-photon excitation in a charged pi-conjugated oligomer is studied theoretically. An apparent reverse polarization is obtained through single-photon excitation, which is different from that obtained through a double-photon excitation. The polarizability is calculated and it is found that a maximum reverse polarization will appear at a suitable conjugation length. In addition, we indicate that the reverse polarization is a nonlinear behavior with the induced electric field. Effects of nondegenerate confinement and interchain interactions on the reverse polarization are also discussed.

Effect of atomic disorder or chain length on the stability of photoinduced polarization inversion

The effect of atomic disorder or chain length on the stability of photoinduced polarization inversion has been studied. The atomic disorder was simulated by square-random or Gaussian-random model. It was found that for the square-random distribution case, photoinduced polarization inversion remains steadily when atomic disorder is less than 18a (a is the lattice constant of polyacetylene), the reversion polarization disappears and gets into normal polarized state when the disorder is stronger than over 18a. The reason of a normal polarization resulted from the strong lattice disorder was discussed. The relationship between the reverse polarization of biexciton state and the confinement constant te as well as the variation of chain length was also studied.