Junfeng Ren

Length-dependent inversion of rectification in diblock co-oligomer diodes

The rectifying direction of diblock co-oligomer molecular diodes is investigated theoretically by analyzing the asymmetric bias effects on the molecular orbitals. The results reveal two competitive mechanisms in determining the rectifying direction, asymmetric energy shift of eigenstates and asymmetric spatial localization of wave functions upon the reversal of bias voltage. It is demonstrated that the dominated mechanism may be converted between the two mechanisms by changing the molecular length, which induces an inversion of the rectification. This work indicates the relative orientation of the two moieties is not sufficient to decide the rectifying direction of co-oligomer diodes.

Multi-state magnetoresistance in ferromagnet/organic-ferromagnet/ferromagnet junctions

Spin-dependent transport through a ferromagnetic metal/organic-ferromagnet/ferromagnet metal junction is investigated theoretically. It is demonstrated that the current through the device strongly depends on the alignment of the magnetization orientations of the electrodes and interlayer. The spin-related electron tunnelling between the ferromagnetic electrodes suffers a further spin selection induced by the spin-polarized states of the central organic ferromagnet. This work indicates an intriguing prospect of organic ferromagnets in spintronic devices, such as four-state magnetoresistance manipulated by a magnetic field.

Length dependence of rectification in organic co-oligomer spin rectifiers*

The rectification ratio of organic magnetic co-oligomer diodes is investigated theoretically by changing the molecular length. The results reveal two distinct length dependences of the rectification ratio: for a short molecular diode, the charge-current rectification changes little with the increase of molecular length, while the spin-current rectification is weakened sharply by the length; for a long molecular diode, both the charge-current and spin-current rectification ratios increase quickly with the length. The two kinds of dependence switch at a specific length accompanied with an inversion of the rectifying direction. The molecular ortibals and spin-resolved transmission analysis indicate that the dominant mechanism of rectification suffers a change at this specific length, that is, from asymmetric shift of molecular eigenlevels to asymmetric spatial localization of wave functions upon the reversal of bias. This work demonstrates a feasible way to control the rectification in organic co-oligomer spin diodes by adjusting the molecular length.

Density Functional Theory Calculations of Charge-Induced Spin Polarization in Pentacene

Based on density functional theory (DFT) calculations, we investigate the spin-related properties of spinless-hole injected organic molecule pentacene (Pc). DFT calculations reveal that there is spontaneous spin polarization in Pc when spinless-hole is injected. The charge-induced magnetic moment of Pc increases linearly with the increasing of the extra hole charge amount and its maximum can be up to 1 μB per injected spinless-hole per Pc molecule. The magnetic moment is expected due to the injected unpaired charge. The injected hole will preferably fill the spin-splitted carbon p z orbitals, which makes the Pc molecule spin polarize.

Spin polarization properties at the spinterface of thiophene/Fe(100): First principles calculations

Based on density functional theory (DFT), the spin polarization properties of a thiophene molecule which is adsorbed at Fe (100) surface are discussed. A variety of horizontal and vertical adsorption configurations as well as their influences on the spin density distributions are studied in detail. The spin polarization comes from the p-d orbital coupling between the thiophene molecule and the electrode, which leads to the molecules’ energy level shifting and the density of states (DOS) broadening, so the two spin states near the Fermi level are exchange split. It is also found that the interfacial spin polarization is different under different contact configurations, and the biggest one will be obtained when the S atom is directly placed above the Fe atom at the horizontal direction. On the other hand, interface spin inversion can be obtained by adjusting the adsorption position, which will be helpful to build spin sensors.

Adsorption of methanol molecule on graphene: Experimental results and first-principles calculations

Adsorption properties of methanol molecule on graphene surface are studied both theoretically and experimentally. The adsorption geometrical structures, adsorption energies, band structures, density of states and the effective masses are obtained by means of first-principles calculations. It is found that the electronic characteristics and conductivity of graphene are sensitive to the methanol molecule adsorption. After adsorption of methanol molecule, bandgap appears. With the increasing of the adsorption distance, the bandgap, adsorption energy and effective mass of the adsorption system decreased, hence the resistivity of the system decreases gradually, these results are consistent with the experimental results. All these calculations and experiments indicate that the graphene-based sensors have a wide range of applications in detecting particular molecules.

AMPLIFICATION OF CURRENT SPIN POLARIZATION IN FERROMAGNETIC/ORGANIC SYSTEM WITH SPIN-RELATED INTERFACIAL RESISTANCES

Current spin polarization in ferromagnetic/organic system is theoretically studied based on the spin diffusion theory and Ohms law, and the effects of the spin-related interfacial resistances on the spin-polarized injection are discussed. It is shown that the current spin polarization can be higher than the spin polarization of the ferromagnetic contact and may reach 100% at the interface by modulating the spin-related interfacial resistances. The proposed amplification scheme provides an efficient way to generate a highly spin-polarized current in organic materials because of the easy fabrication of ordered spin-related tunnel barriers at contact structures.