Electrical transport properties of molecular junctions are fundamentally affected by the energy alignment between molecular frontier orbitals (highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO)) and Fermi level (or work function) of electrode metals. Dithiafulvene (DTF) is used as substituent group to the oligo(phenylene ethynylene) (OPE) molecular wires and different molecular structures based on OPE3 backbone (with linear to cruciform framework) are achieved, with viable molecular orbitals and HOMO–LUMO energy gaps. OPE3, OPE3–DTF, and OPE3–tetrathiafulvalene (TTF) can form good self‐assembled monolayers (SAMs) on Au substrates. Molecular heterojunctions based on these SAMs are investigated using conducting probe–atomic force microscopy with different tips (Ag, Au, and Pt) and Fermi levels. The calibrated conductance values follow the sequence OPE3–TTF > OPE3–DTF > OPE3 irrespective of the tip metal. Rectification properties (or diode behavior) are observed in case of the Ag tip for which the work function is furthest from the HOMO levels of the OPE3s. Quantum chemical calculations of the transmission qualitatively agree with the experimental data and reproduce the substituent effect of DTF. Zero‐bias conductance, and symmetric or asymmetric couplings to the electrodes are investigated. The results indicate that improved fidelity of molecular transport measurements may be achieved by systematic studies of homologues series of molecular wires applying several different metal electrodes.
We investigate the electron transmission through molecules with multiple connections to the leads and compare this with the transmission through the same molecules where only select connections have been made. This enables us to probe the transmission through the individual pathways through the molecules and investigate their interaction. Generally, we see that the transmission of the multiconnected molecules differs from those obtained from a sum of their parts because of coherence effects between the paths through the molecules. The only exception to this trend is a case where the molecule can be considered as two separate parts, isolated electronically from each other via meta connections. We also explore the local currents though these molecules and separate these into channels, which reveals how this coherence comes into play.
We present theoretical methods and computations of the effects of nanoparticles on nonlinear optical properties of symmetric molecules. We utilize quantum mechanical/molecular mechanics (QM/MM) response methods for calculating electromagnetic properties of molecules interacting with nanoparticles, and we report calculations of the frequency-dependent first hyperpolarizability. The frequency-dependent first hyperpolarizability of (p)benzenedithiol in the presence of zero, one, or two gold nanoparticles of varying sizes and distances is calculated by DFT/MM. The hyperpolarizability of the molecule depends strongly on the distance between the nanoparticles and the molecule, whereas the size of the nanoparticle is of little importance. We clearly show that metal nanoparticles are able to induce a first hyperpolarizability in symmetric molecules.
In order to identify the location of an inelastic event and to distinguish between situations that are before or after this event, we derive equations for the interatomic inelastic transmission as a perturbation series in the electron-phonon interaction. This series contains both even and odd ordered corrections, and while the even ordered corrections can be thought as a Dyson’s expansion of the interatomic elastic transmission in the electron-phonon self-energy, the odd ordered corrections represent something new. We explicitly derive expressions for the interatomic inelastic transmission up to second order and the 1st order correction represents the lowest order term of this new family of terms. We apply this to three model systems and are able to distinguish between situations before and after the inelastic event as steps in the 2nd order transmission. We also see that when the transmission is evaluated between atoms that are coupled by the electron-phonon interaction, the 1st and 2nd order terms must be added together to form a meaningful transmission. Within the limited scope of the models considered here, the 1st order term appears to be the signature of the inelastic event.
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