Molecular Wire Junctions Research Articles

Mode Tracking of Preselected Vibrations of One-Dimensional Molecular Wires

The mode-tracking principle (J. Chem. Phys. 2003, 118, 1634) for the direct quantum chemical calculation of preselected, characteristic molecular vibrations makes vibrational analyses of molecular wire junctions feasible.Characteristic vibrational parameters of molecular bridges such as vibrational frequencies and force constants can be of importance for subsequent treatments in physical model theories of electron transfer and conductance. We investigate how efficient the mode-tracking protocol can be applied to determine such vibrational parameters for a particular type of normal modes of dinuclear polyynediyl rhenium complexes. The frequencies of vibrations of the carbon chain in complexes of the type [Re]-(C≡C) n -[Re] with [Re] = (η 5 -C5Me5)Re(NO)(PPh 3 ) have been studied as a function of the chain length 2n, leading to molecules with up to 144 atoms, for which harmonic wavenumbers are determined. The harmonic approximation for the potential-energy surface is compared to explicitly calculated electronic energy values along normal coordinates in order to get an estimate for the role of anharmonicity effects. A possible vibration-induced rupture of molecular bridges is discussed. Different bond-breaking positions for a rupture process in the carbon chain of the rhenium complexes are investigated.

Nanowire-Based Molecular Monolayer Junctions: Synthesis, Assembly, and Electrical Characterization

Nanowire metal−molecule−metal junctions containing dithoilated molecules of dodecane (C12), oligo(phenylene ethynylene) (OPE), and oligo(phenylene vinylene) (OPV) were prepared by replicating the pores of sub-40 nm diameter polycarbonate track etched membranes. Bottom Au−S or Pd−S contacts were made by potential-assisted molecule assembly onto the tips of the first segment of the electrochemically deposited nanowires. Top S−Ag or S−Pd contacts were formed by depositing Ag or Pd nanoparticles, which also served as a thin seed layer for electrodeposition of the second nanowire segment. Room-temperature current−voltage (I−V) measurements of individual nanowires show that the conductance of junctions formed with π-conjugated oligomers are several orders of magnitude larger than the saturated alkanes, with the OPV junctions having the highest conductance. The molecular wire junction conductance was also found to be dependent on the metal contacts with symmetric Pd−Pd junctions yielding the best metal−molecule ...

Electron transport in molecular wire junctions.

Molecular conductance junctions are structures in which single molecules or small groups of molecules conduct electrical current between two electrodes. In such junctions, the connection between the molecule and the electrodes greatly affects the current-voltage characteristics. Despite several experimental and theoretical advances, including the understanding of simple systems, there is still limited correspondence between experimental and theoretical studies of these systems.

Molecular Wire Junctions: Tuning the Conductance

Junctions consisting of two electrodes and a single molecule acting as a wire can be investigated by scanning probe conductance measurements. The conductance for low temperature, short wire structures can be characterizedby the elastic scattering of the electrons, as described by general Landauer formulas. Tuning the bridge energy levels either by field effect transistor (FET)-like behavior (charging) or by chemical bond formation can raise or lower the conductance by moving the poles of the molecular Green's function. For small molecular structures with particular HOMO/LUMO characteristics, functionalization might be a more effective means than Coulomb charging by external gates.