Although molecular rectifiers were proposed over four decades ago^1,2, until recently reported rectification ratios (RR) were rather moderate^2–11 (RR ~ 10¹). This ceiling was convincingly broken using a eutectic GaIn top contact¹² to probe molecular monolayers of coupled ferrocene groups (RR ~ 10⁵), as well as using scanning tunnelling microscopy-break junctions^13–16 and mechanically controlled break junctions¹⁷ to probe single molecules (RR ~ 10²–10³). Here, we demonstrate a device based on a molecular monolayer in which the RR can be switched by more than three orders of magnitude (between RR ~ 10⁰ and RR ≥ 10³) in response to humidity. As the relative humidity is toggled between 5% and 60%, the current–voltage (I–V) characteristics of a monolayer of di-nuclear Ru-complex molecules reversibly change from symmetric to strongly asymmetric (diode-like). Key to this behaviour is the presence of two localized molecular orbitals in series, which are nearly degenerate in dry circumstances but become misaligned under high humidity conditions, due to the displacement of counter ions (PF₆ ^–). This asymmetric gating of the two relevant localized molecular orbital levels results in humidity-controlled diode-like behaviour.

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A method is presented for predicting one-particle energies for a molecule in a junction with one metal electrode, using density functional theory methods. In contrast to previous studies, in which restricted spin configurations were analyzed, we take spin polarization into account. Furthermore, in addition to junctions in which the molecule is weakly coupled, our method is also capable of describing junctions in which the molecule is chemisorbed to the metal contact. We implemented a fully self-consistent scissor operator to correct the highest occupied molecular orbital-lowest unoccupied molecular orbital gap in transport calculations for single molecule junctions. We present results for various systems and compare our results with those obtained by other groups.

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Molecular devices are planned as alternative solutions for heat dissipation problems and reliable fabrication of nano-scale devices. However, it also opens up possibilities of combining many other degrees of freedom into functional device design. While they introduce interesting opportunities for study, they also demand a versatile, scalable toolset. In this thesis we calculate the electronic transport through molecular devices using the DFT+NEGF technique. We model the interaction of the molecule with the electrodes surfaces taking into account different facts such as the gap reduction produced by the charge polarization on metallic surfaces, the spin states of the molecule and the hydrophilicity of the leads. We hope our contribution helps to improve the functional single molecule devices design. @en

We analyze the non-local shot noise in a multi-terminal junction formed by two Normal metal leads connected to one superconductor. Using the cross Fano factor and the shot noise, we calculate the efficiency of the Cooper pair splitting. The method is applied to d-wave and iron based superconductors. We determine that the contributions to the noise cross-correlation are due to crossed Andreev reflections (CAR), elastic cotunneling, quasiparticles transmission and local Andreev reflections. In the tunneling limit, the CAR contribute positively to the noise cross-correlation whereas the other processes contribute negatively. Depending on the pair potential symmetry, the CAR are the dominant processes, giving as a result a high efficiency for Cooper pair split. We propose the use of the Fano factor to test the efficiency of a Cooper pair splitter device.

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We investigate transport through mechanically triggered single-molecule switches that are based on the coordination sphere-dependent spin state of Fe^II-species. In these molecules, in certain junction configurations the relative arrangement of two terpyridine ligands within homoleptic Fe^II-complexes can be mechanically controlled. Mechanical pulling may thus distort the Fe^II coordination sphere and eventually modify their spin state. Using the movable nanoelectrodes in a mechanically controlled break-junction at low temperature, current-voltage measurements at cryogenic temperatures support the hypothesized switching mechanism based on the spin-crossover behavior. A large fraction of molecular junctions formed with the spin-crossover-active Fe^II-complex displays a conductance increase for increasing electrode separation and this increase can reach 1-2 orders of magnitude. Theoretical calculations predict a stretching-induced spin transition in the Fe^II-complex and a larger transmission for the high-spin configuration.

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