Aldilene Saraiva-Souza

Molecular Spintronics: Destructive Quantum Interference Controlled by a Gate

The ability to control the spin-transport properties of a molecule bridging conducting electrodes is of paramount importance to molecular spintronics. Quantum interference can play an important role in allowing or forbidding electrons from passing through a system. In this work, the spin-transport properties of a polyacetylene chain bridging zigzag graphene nanoribbons (ZGNRs) are studied with nonequilibrium Greens function calculations performed within the density functional theory framework (NEGF-DFT). ZGNR electrodes have inherent spin polarization along their edges, which causes a splitting between the properties of spin-up and spin-down electrons in these systems. Upon adding an imidazole donor group and a pyridine acceptor group to the polyacetylene chain, this causes destructive interference features in the electron transmission spectrum. Particularly, the donor group causes a large antiresonance dip in transmission at the Fermi energy EF of the electrodes. The application of a gate is investigated and found to provide control over the energy position of this feature making it possible to turn this phenomenon on and off. The current-voltage (I-V) characteristics of this system are also calculated, showing near ohmic scaling for spin-up but negative differential resistance (NDR) for spin-down.

A single molecule rectifier with strong push-pull coupling

We theoretically investigate the electronic charge transport in a molecular system composed of a donor group (dinitrobenzene) coupled to an acceptor group (dihydrophenazine) via a polyenic chain (unsaturated carbon bridge). Ab initio calculations based on the Hartree-Fock approximations are performed to investigate the distribution of electron states over the molecule in the presence of an external electric field. For small bridge lengths (n=0-3) we find a homogeneous distribution of the frontier molecular orbitals, while for n>3 a strong localization of the lowest unoccupied molecular orbital is found. The localized orbitals in between the donor and acceptor groups act as conduction channels when an external electric field is applied. We also calculate the rectification behavior of this system by evaluating the charge accumulated in the donor and acceptor groups as a function of the external electric field. Finally, we propose a phenomenological model based on nonequilibrium Greens function to rationalize the ab initio findings.

Tuning the electronic and quantum transport properties of nitrogenated holey graphene nanoribbons

Recently, a new semiconductor two-dimensional (2D) material, namely, holey nitrogenated graphene 2D crystal (C2N-h2D), has been fabricated by using a bottom-up wet-chemical reaction. Using first-principles density functional theory (DFT) combined with the non-equilibrium Greens function (NEGF) technique, we investigate the atomic, electronic and quantum transport properties of porous C2N nanoribbons having both zigzag- and armchair-terminated edges. The zigzag C2N-h nanoribbons (ZC2N-hNRs) are semiconductors with an indirect band gap that decreases as the ribbon width increases. Meanwhile, the armchair C2N-h nanoribbons (AC2N-hNRs) show a metallic behavior for all ribbon widths, except for one of the candidates considered in this study, which presents a small band gap (0.14 eV). Interestingly, non-equilibrium calculations suggest that these structures display edge-dependent electronic transport properties where the armchair C2N-hNRs show a strong negative differential resistance (NDR) behavior with current peak-to-valley ratios that remarkably increase with increasing ribbon width, and non-linear current–voltage characteristics were found for the zigzag C2N-hNRs.

Light emission and current rectification in a molecular device: Experiment and theory

We have investigated optical and transport properties of the molecular structure 2,3,4,5-tetraphenyl-1-phenylethynyl-cyclopenta-2,4-dienol experimentally and theoretically. The optical spectrum was calculated using Hartree-Fock-intermediate neglect of differential overlap-configuration interaction model. The experimental photoluminescence spectrum showed a peak around 470 nm which was very well described by the modeling. Electronic transport measurements showed a diode-like effect with a strong current rectification. A phenomenological microscopic model based on non-equilibrium Greens function technique was proposed and a very good description electronic transport was obtained.