First-Principles Study Of Electronic Transport Properties In Molecular Devices

Based on the review of the progress of molecular device, the electron-transport properties ofsome molecular junctions are investigated systemically through density functional theory (DFT)combined with nonequilibrium Green’s function (NEGF) method. The effects of substituent sidegroups, heteroatoms, electrode molecule contacts, electrode materials, and intermolecularinteraction on the electron transport in the different molecular junctions are studied emphatically.The electron-transport properties of the various substituted molecules based on the thiol endedthiophene dimer (2Th1DT) are investigated. The current-voltage (I-V) curves of all theAu/2Th1DT/Au systems in this work display similar steplike feature. While their equilibriumconductances show a large difference and some of these I-V curves are asymmetric distinctly. Theresults reveal the dependence of the conductance on the energy level of the substituted2Th1DTmolecules. The rectifying behavior in the2Th1DT molecule containing the amino group close to themolecular end is more prominent than that in the other molecules. The electron-transport propertiesof the different2Th1DT molecules containing different heteroatoms are also investigated. Theresults show that the current in the molecules containing the heteroatom is larger than that in theirpristine analog, and the lighter heteroatom is more favorable than the heavier heteroatom for theelectronic transport of the thiophene dimer. The molecular conductances containing the heteroatomshow dependence on the HOMO energy level. The spatial distribution of the molecular projectedself-consistent Hamiltonian states indicates that the heteroatom plays a role of “bridge” for theelectron transport between two thiophene moieties.The electron-transport properties and rectifying behaviors of several molecular junctions basedon the bis-2-(5-ethynylthienyl)ethyne (BETE) molecule are investigated. In order to examine theroles of different rectification factors, asymmetric electrode molecule contacts and donor acceptorsubstituent groups are introduced into the BETE-based molecular junction. The asymmetric I-Vcharacteristics are obtained for the molecular junctions containing asymmetric contacts anddonor acceptor groups. In our models, the computed rectification ratios show that the mode ofelectrode molecule contacts plays a crucial role for the rectification, and the rectifying effect is notenhanced significantly by introducing the additional donor acceptor components for the molecularrectifier with asymmetric electrode molecule contacts.The electron-transport properties of C59N molecular junctions with different electrodes (Au, Al, and CNT) are investigated. The I-V characteristics of all the models are calculated. The results showthat both electrode species and nitrogen-atom location may affect the transport properties of theC59N molecular junction. When the nitrogen atom of the C59N molecule is located close to one sideof the junction, the rectifying behavior can be found in CNT-electrode models, while there is noobservable rectification in metal-electrode models. The NDR may be observed in the C59Nmolecular junction using CNT electrodes when the nitrogen atom is at a certain location.Several models of bimolecular junctions based on two different-lengthed andmonothiol-anchored carbon chains are designed and their electron-transport properties areinvestigated systematically. The results show that the shorter carbon chain moiety makes acontribution to the conductance of molecular junctions. The comparison of transport propertiesamong these junctions shows that the difference in lengths of two molecules can influence therectification effect. It indicates the modulation from the shorter carbon chain moiety. NDRbehaviors occur in all of the bimolecular junctions. The monomolecular and bimolecular junctions,which are based on cis-polyacetylene (PA) or fused pyrrole trimer (FP) molecule, are alsoinvestigated. The results show that the bimolecular conductance is usually greater than themonomolecular conductance. However, the monomolecular and bimolecular conductances areequivalent at the lower bias. The conductance of bimolecular junction with intermolecularinteraction is less than that without intermolecular interaction. The currents of PA bimolecularjunction with “face-to-face”(FF) arrangement are greater than those with “face-to-face reversal”(FFR) arrangement. However, the currents of FP bimolecular junction with FF arrangement are lessthan those with FFR arrangement obviously. The conductive performance of bimolecular junctionswith different arrangements shows good correlation with respective HOMO-LUMO gap.