Quantum transport through conjugated organic molecular junctions

Algethami, Norah and Sadeghi, Hatef and Sangtarash, Sara (2019) Quantum transport through conjugated organic molecular junctions. PhD thesis, Lancaster University.

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Understanding the electronic transport properties of junctions consisting of a scattering region such as a nanoscale object or molecule connected to electrodes is the central basis for future nano and molecular scale applications. In this thesis, I shall discuss the theoretical methods needed to describe such junctions and the present three studies of the electronic properties of molecular junction. High electrical conductance molecular nanowires are highly desirable components for future molecular-scale circuitry, but typically molecular wires act as tunnel barriers and their conductance decays exponentially with length. In chapter 4, I demonstrate that the conductance of fused-oligo-porphyrin nanowires can be either length independent or increase with length at room temperature. I show that this negative attenuation is an intrinsic property of fused-oligo-porphyrin nanowires, but its manifestation depends on the electrode material or anchor groups. This highly-desirable, non-classical behaviour signals the quantum nature of transport through such wires. It arises, because with increasing length, the tendency for electrical conductance to decay is compensated by a decrease in their HOMO-LUMO gap. This study reveals the potential of these molecular wires as interconnects in future molecular-scale circuitry. Identification of structure-property relationships that govern single-molecule conductance is key to the continued development of molecular electronics. To realise new quantum-interference-based molecular junction, there is a need to establish simple and intuitive rules for synthesizing molecules with flexible and controllable chemical structures. In chapter 5, I demonstrate methoxyl groups (-OMe) induce destructive quantum interference (DQI) tuning in meta-phenylene ethylene-type oligomers (mOPE). My calculation reveals that the conductance of single molecules with -OMe pendant groups is sensitive to the position of the –OMe. This result is in agreement with recently developed magic ratio and orbital product rules and demonstrates that destructive QI can be tuned by changing the –OMe position. This novel method of DQI tuning provides a new design strategy for creating single-molecule junctions with desirable functions. The design and development of metal/single-molecule/metal junctions with a conductance response to external stimuli has been a strong driving force in molecular electronics community. Reproducible conductance increase (or decrease) of a junction in response to external stimuli have been exploited. Mechanoresistive metal-moleculemetal junctions, whose electrical conductance depends on the mechanical separation of the two electrodes, allow further control, which could be exploited to fabricate junctions responsive to multiple stimuli (e.g. electrochemical potential and electrode separation). Furthermore, knowledge of the structure-property relationships of mechanosensitive junctions provides a wealth of information about the nature, strength and configuration of metal-molecule interactions at the contact interface, which can be applied to fundamental studies of surface science and can be exploited to improve the design of molecular junction. In chapter 6, I demonstrate the metal/singlemolecule/metal junctions give a mechanoresistive behaviour with enhanced sensitivity, based on (methylthio)thiophene contacting groups. The effect arises from localised interactions between the thienyl sulfurs and the electrodes, which allows the junction to transition from a monodentate to a bidentate contact configuration as the junction is compressed, resulting in a up to two order of magnitude in the (methylthio)thiopheneterminated molecule higher conductance compared with the (methylthio)benzene counterpart.

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Thesis (PhD)
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18 Mar 2019 13:00
Last Modified:
04 Jun 2024 23:35