Wenkai Zhao

Electronic transport properties of indolyl spirooxazine/merooxazine-based light-driven molecular switch: The effect of amino/nitro substituents

Abstract By applying non-equilibrium Greens function formulation combined with first-principles density functional theory, we explore the electronic transport properties of indolinospironaphthoxazine (SO)/indolinomeronaphthoxazine (MO). The results indicate that the MO allows a far larger current than the SO. The substituent group can cause shifts of the energy levels. Higher ON/OFF current ratio can be obtained if either amino or nitro substituent is placed at the position of naphthalene moiety. Our results suggest that such molecular wires can generally display switching function and the efficiency can be increased by adding certain substituent groups to the molecules.

Enhanced rectifying performance by asymmetrical gate voltage for BDC20 molecular devices

By applying the asymmetrical gate voltage on the 1,4-bis (fullero[c]pyrrolidin-1-yl) benzene BDC20 molecule, we investigate theoretically its electronic transport properties using the density functional theory and nonequilibrium Greens function formalism for a unimolecule device with metal electrodes. Interestingly, the rectifying characteristic with very high rectification ratio, 91.7 and 24.0, can be obtained when the gate voltage is asymmetrically applied on the BDC20 molecular device. The rectification direction can be tuned by the different gate voltage applying regions. The rectification behavior is understood in terms of the evolution of the transmission spectrum and projected density of states spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analyses. Our finding implies that to realize and greatly promote rectifying performance of the BDC20 molecule the variable gate voltage applying position might be a key issue.

Effect of geometrical torsion on the rectification properties of diblock conjugated molecular diodes

We model several Au/conjugated molecule/Au junctions in the presence of molecular geometrical torsions. A rectification ratio of around 10 in the twisty diphenyldipyrimidinyl system is obtained, which is in good agreement with experiment. Deeper insight into the rectification mechanism of the conjugated molecular diodes is presented on the basis of simulations in a set of simpler but similar junctions. The rectification effect (the ratio) is significantly improved with increasing the molecular twist, while the conductance is reduced accordingly. Our results suggest that the rectification can be enhanced by the geometrical-torsion-induced reduction in the conjugation length of organic molecules.

Vacancy-induced spin polarization in graphene and B–N nanoribbon heterojunctions

By using nonequilibrium Greens functions (NEGF) and density functional theory (DFT), we investigate the spin-separated electronic transport properties in heterojunctions constructed by zigzag graphene and boron nitride nanoribbons. The results show that the heterojunctions exhibit a strong spin polarization and ferromagnetic state when there is a vacant position in the boron nitride nanoribbons (BNNRs). The spin-filter effect can be significantly tuned and improved by the species of the nitrogen and boron vacancy and the location of the vacancy in the boron nitride nanoribbons with the spin-filter efficiency (SFE) up to nearly 100%. The spin negative differential resistance (SNDR) properties at low bias can also be found in the proposed molecular spin devices. Mechanisms for the results are suggested, and these findings open up new possibilities for developing nano-spintronic devices.

Electronic transport properties of a dithienylethene-based polymer with different metallic contacts

We have studied the electronic transport behaviors of a dithienylethene-based polymer between two metal surfaces using nonequilibrium Greens functions combined with density functional theory. The present computational results show that the polymer with closed and open configurations really demonstrates switching behavior which confirms the experimental observation. It is also found that the switching behavior depends on the electronic properties of two configurations of polymer instead of the contact modes. The on–off ratios of conductance between the closed and open configurations reach up to two orders of magnitude. Negative differential resistance and rectification phenomena are also observed in such systems.