Theory of molecular-scale transport in nanojunctions

Hou, Songjun and Lambert, Colin (2019) Theory of molecular-scale transport in nanojunctions. PhD thesis, Lancaster University.

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Abstract

It remains a major challenge to identify and exploit room-temperature quantum interference (QI) effects in charge transport through molecular systems at the angstromscale, although extensive and intensive research has been carried out by experimentalists and theoreticians. In this thesis, I investigate charge transport properties, thermoelectricity and solvent influences at the molecular scale by meansof density functional theory (DFT) and equilibrium Green’sfunction theory. The charge transport properties of halide perovskite quantumdots( QDs) are first investigated. It is demonstrated that room-temperature quantum interference (QI) is observed based on the fact that the conductance decays exponentially with the increasing distance between the twogold-goldtips and also there is a distinct conductance “jump”at the end of the sliding process. These findings open the way to new conceptual designs for perovskite-based molecular devices by exploiting QI effects. As for the property of exponentially attenuating electrical conductance with the length, molecular wires with low decaying factor � (�~�]^_ ) are of significance to realize themolecular electronics. Here we measured and calculated the single-molecule conductances of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n = 1,2,3 and 5. The [n] cumulenes with n =3 and 5 have almost the same conductance,and they are both more conductive than the alkene (n = 1). The lack of length dependence in the conductance of [3] cumulene and [5] cumulene is attributed to the strong decrease in HOMO-LUMO gap with increasing length. The conductance of the allene (n = 2) is much lower, due to its twisted geometry. Therefore, I suggest the cumulene series as a good candidate for high conductance molecular wires. Additionally, and also significantly, seeking materials for harvesting energy is an urgent task facing the serious global energy shortage. Herein, I investigated the electrical and thermoelectrical properties of glycine chains with and without cysteine terminal groups. The electrical conductance of (Gly)g, (Gly)gCys and Cys(Gly)gCys molecules (where Gly, Cys represent glycine and cysteine and n=1-3) was found to decay exponentially with length � as �]^_ (β~1.0 Å]k) .Furthermore, it is shown the (Gly)kC�� and Cys(Gly)kCys systems show good thermoelectrical performance ( high Seebeck coefficients ~ 0.2mV/K). With the contributions of both electrons and phonons taken into consideration, a high figure of merit ZT=0.8 is obtained for (Gly)kCys at room temperature, suggesting that peptide-based SAM junctions are promising candidates for thermoelectric energy harvesting. In the investigations of charge transports above, it is realized that the functionalities, reproducibility, stability of molecular junctions not only depend on the functional-molecular cores, but also on other effects such as connecting anchors and solvents. Therefore the conductances of single-molecule junctions with different anchoring groups in a variety of solvent environments are studied. It is found that the conductance of single-molecule junctions can be manipulated by nearly an order of magnitude by varying the solvent, and the solvent gating effect depends significantly on the choice of anchor group. My work suggests that the solvent-molecule interaction can provide significant solvent gating effect for the weakly coupled (-SMe anchor) single-molecule junctions