Electronic properties of nano and molecular

Albeydani, Ohood and Lambert, Colin (2021) Electronic properties of nano and molecular. PhD thesis, Lancaster University.

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Abstract

In this thesis I will investigate the theoretical electronic and thermal properties of two different types of two-terminal molecular-scale junctions: one attached to gold electrodes and the other attached carbon graphene electrodes. The methods used in this work are based on density functional theory (DFT), implemented using the SIESTA code, and non- equilibrium Green’s function theory. The main results of this thesis are as follows: The solvent medium namely propylene carbonate (PC) is shown to have a strong impact on the electrical conductance of ferrocene-based junctions. Chapter 4 presents a theoretical investigation of the properties of single-molecule junctions formed from ferrocene with thioacetate anchors (FcSAc2) or with pyridine anchors (FcPy2). These calculations were stimulated b y measurements carried out in Xiamen University, on molecules synthesized in the University of Western Australia, Perth. These are found to have significantly increased conductances in the presence of a polar medium. This study provides new insight into how the polar solvent propylene carbonate (PC) can influence charge transport in ferrocene-based, single-molecule junctions. In chapter 5, quantum interference effects are studied in molecular junctions formed from a bipyridine-based series of molecules attached to gold leads. These effects are revealed by changing the intra-molecular couplings through the series of para- para, para-meta, and metameta conductivities. The conductances of these molecules are measured by colleagues in Liverpool University, using a mechanically controlled break junction. A combined experimental and theoretical study is described, in which my work provides the theoretical understanding of the experiment. We used density functional theory method to investigate these junctions properties, and the results demonstrate thatfor bipyridine with single bound in between a consistently higher conductance for meta-meta linked bipyridine compared to the para-para linked of the same molecule. In the other hand, for bipyridine with triple bonds in between, para coupling has the highest conductance. This difference is due to the presence of a significant contribution to electron transport from sigma-orbitals when rings are coupled by single bonds, which masks the effects of quantum interference within the pi system. In chapter 6, we investigate the properties of a series of metallo-porphyrins placed in parallel and sandwiched between top and bottom graphene electrodes. The electrical conductance of Fe (III)-porphyrin is found to be enhanced by placing iodine (퐼ିଷ) in between the electrodes. In addition it is found that the conductance of Fe(III)- porphyrin coupled to iodine, can be turned by changing the position of the iodine.