Injecting spins into nonmagnetic molecular devices has attracted much attention in molecular spintronics. Herein, we propose a novel strategy to introduce magnetism into a single benzene molecule coupled with two armchair graphene nanoribbons (AGNR) electrodes, where the ends of two AGNR electrodes are cut into zigzag-edge triangular graphenes (ZTGs). The spin-dependent transport properties of the molecular junction are investigated by using the density functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) method. The analyses of the spin-dependent projected density of states and the net spin density distribution of the scattering region reveal that the intrinsic magnetism of the ZTGs is weakened, owing to spin transfer from ZTGs to AGNR electrodes and the benzene molecule. More interestingly, the attenuated intrinsic magnetism of the ZTGs can still contribute to a significant spin transport of the molecular junction. Transport calculations show that in the parallel spin configuration, a large spin polarization of nearly 90% current is obtained. However, the spin polarization of current is reversed in antiparallel spin configuration. Positive or negative tunneling magnetoresistance (TMR) can be modulated by bias voltage. A TMR up to 53% is obtained in the device. The results are further analyzed from the transmission spectra and local density of states. This work presents a promising potential applications of the ZTGs in the field of molecular spintronics, which can contribute to the design of graphene nanoribbons based molecular spintronic devices.
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.
The relationship between the molecular structure and the electronic transport properties of molecular junctions based on thiol-terminated oligoethers, which are obtained by replacing every third methylene unit in the corresponding alkanethiols with an oxygen atom, is investigated by employing the non-equilibrium Green’s function formalism combined with density functional theory. Our calculations show that the low-bias conductance depends strongly on the conformation of the oligoethers in the junction. Specifically, in the cases of trans-extended conformation, the oxygen-dominated transmission peaks are very sharp and well below the Fermi energy, EF, thus hardly affect the transmission around EF; the Au–S interface hybrid states couple with σ-bonds in the molecular backbone forming the conduction channel at EF, resulting in a conductance decay against the molecular length close to that for alkanethiols. By contrast, for junctions with oligoethers in helical conformations, some π-type oxygen orbitals coupling with the Au–S interface hybrid states contribute to the transmission around EF. The molecule-electrode electronic coupling is also enhanced at the non-thiol side due to the specific spatial orientation introduced by the twist of the molecular backbone. This leads to a much smaller conductance decay constant. Our findings highlight the important role of the molecular conformation of oligoethers in their electronic transport properties and are also helpful for the design of molecular wires with heteroatom-substituted alkanethiols.
The structure-property relationship of diarylethene (DAE)-derivative molecular isomers, which involve ring-closed and ring-open forms, is investigated by employing the non-equilibrium Green’s function formalism combined with density functional theory. Molecular junctions are formed by the isomers connecting to Au(111) electrodes through flanked pyridine groups. The difference in electronic structures caused by different geometry structures for the two isomers, particularly the interatomic alternative single bond and double bond of the ring-closed molecule, contributes to the vastly different low-bias conductance values. The lowest unoccupied molecular orbital (LUMO) of the isomers is the main channel for electron transport. In addition, more electrons transferred to the ring-closed molecular junction in the equilibrium condition, thereby decreasing the LUMO energy to near the Fermi energy, which may contribute to a larger conductance value at the Fermi level. Our findings are helpful for understanding the mechanism of low-bias conductance and are conducive to the design of high-performance molecular switching based on diarylethene or diarylethene-derivative molecules.
Single-molecule spin logic gates provide fundamental functions and are of importance in the field of molecular spintronics. Here, by using the firstprinciples method, the effects of ambient gas molecules (CO2, O2, N2, or H2O) on the functional stability of the investigated single-molecule spin logic gate consisting of two serially connected cobalt dibenzotetraaza[14]annulene (CoDBTAA) molecules between single-walled carbon nanotubes (SWCNTs) electrodes, have been theoretically investigated. The calculated results suggest that the investigated spin logic gate can realize AND, NOR, or XNOR logic functions depending on the definition of the input and output signals. It is found that these logic functions are not affected by CO2 adsorption. On the contrary, these logic functions are no longer retained upon O2, N2, or H2O adsorption. Further analysis reveals that the interaction between the CoDBTAA molecule and the CO2 adsorbate is very weak while it is strong for O2, N2, or H2O molecules. Therefore, the electronic states of the logic gate around Fermi energy (EF) are almost unchanged for CO2 adsorption. While the adsorption of O2, N2, or H2O obviously modifies the electronic states around EF. The strong interaction between CoDBTAA and these three gas adsorbates drives the conductive electronic states to move far away from EF, resulting in the blocking of both spin-up and spin-down currents and further voiding the logic functions. This work suggests that ambient air has an important effect on the functional stability of single-molecule devices and should be carefully evaluated in the future design of functional single-molecule devices.
Based on ab initio theory, the interfacial spin polarization of a benzene-dithiolate molecule vertically adsorbed on a nickel surface is investigated by adopting different microscopic contact configurations. The results demonstrate a strong dependence of the interfacial spin polarization on the contact configuration, where the sign of spin polarization may vary from positive to negative with the change of contact configuration. By analyzing the projected density of states, an interfacial orbital hybridization between the 3d orbital of the nickel atom and the sp3 hybridized orbital of the sulfur atom is observed. We also simulated the interfacial adsorption in mechanically controllable break junction experiments. The magnetoresistance obtained from Julliere model is about 27% based on the calculated interfacial spin polarization, which is consistent with experimental measurement.
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