Quantum transport through conjugated organic molecular junctions
Creators: name: Algethami, Norah
Publisher: name: Lancaster University
Year: 2019

description: 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.