Sun Yin

Investigation on organic magnetoconductance based on polaron-bipolaron transition

We explore the magnetoresistance (MC) effect in an organic semiconductor device based on the magnetic field related bipolaron formation. By establishing a group of dynamic equations, we present the transition among spin-parallel, spin-antiparallel polaron pairs and bipolarons. The transition rates are adjusted by the external magnetic field as well as the hyperfine interaction of the hydrogen nuclei. The hyperfine interaction is addressed and treated in the frame work of quantum mechanics. By supposing the different mobility of polarons from that of bipolarons, we obtain the MC in an organic semiconductor device. The theoretical calculation is well consistent to the experimental data. It is predicated that a maximum MC appears at a suitable branching ratio of bipolarons. Our investigation reveals the important role of hyperfine interaction in organic magnetic effect.

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.

The inversely proportional relationship in the asymmetric transport of a diblock co-oligomer junction

We investigate electron transport properties and the mechanism of current rectification effect in a diblock co-oligomer (diprimidinyl-diphenyl) molecular junction under electron-lattice coupling effect, using an extended Su–Schrieffer–Heeger tight-binding model combined with nonequilibrium Green’s function formalism. We find an inversely relationship between the positive threshold bias and the molecular dipole moment under proper electron-lattice coupling.

Voltage Dependence of Magnetoconductance in Organic Semiconductor Devices

We study the voltage effect on magnetoconductance (MC) in an organic semiconductor device based on voltage-related carrier density and mobility. With the effects of magnetic field and hyperfine interaction, we present the transition between polaron and bipolarons, and provide the dependence of MC on total carrier density and mobility. The MC effect will become weak with increasing voltage in a unipolar polymer device and the sign of MC will change at a critical voltage. In a small molecular device, the situation is opposite: MC increases with increasing voltage. Furthermore, our theoretical calculations are well consistent with the experimental results.