Theory of molecular-scale transport in graphene nanojunctions
Creators: name: Wu, Qingqing
Publisher: name: Lancaster University
Year: 2018

description: A molecular junction consists of a single molecule or self-assembled
      monolayer (SAM) placed between two electrodes. It has varieties of
      functionalities due to quantum interference in nanoscale. Although there
      exist issues, advantages could still appeal to scientists who wish to
      investigate transport properties in many aspects such as electronics,
      thermoelectronics, spintronics, and optotronics.
      
      Recent studies of single-molecule thermoelectricity have identified
      families of high-performance molecules. However, controlled scalability
      might be used to boost electrical and thermoelectric performance over the
      current single-junction paradigm. In order to translate this discovery
      into practical thin-film energyharvesting devices, there is a need for an
      understanding of the fundamental issues arising when such junctions are
      placed in parallel. As a first step in this direction, we investigate here
      the properties of two C60 molecules placed in parallel and sandwiched
      between top and bottom graphene electrodes. It is found that increasing
      the number of parallel junctions from one to two can cause the electrical
      conductance to increase by more than a factor of 2 and furthermore, the
      Seebeck coefficient is sensitive to the number of parallel molecules
      sandwiched between the electrodes, whereas classically it should be
      unchanged. This non-classical behaviour of the electrical conductance and
      Seebeck coefficient are due to interjunction quantum interference,
      mediated by the electrodes, which leads to an enhanced response in these
      vertical molecular devices.
      
      Except the study of thermoelectricity, on the other hand, stable,
      single-molecule switches with high on-off ratios are an essential
      component for future molecularscale circuitry. Unfortunately, devices
      using gold electrodes are neither complementary metal-oxide-semiconductor
      (CMOS) compatible nor stable at room temperature. To overcome these
      limitations, several groups are developing electroburnt graphene
      electrodes for single molecule electronics. Here, in anticipation of these
      developments, we examine how the electrical switching properties of a
      series of porphyrin molecules with pendant dipoles can be tuned by
      systematically increasing the number of spacer units between the porphyrin
      core and graphene electrodes. The porphyrin is sandwiched between a
      graphene source and drain and gated by a third electrode. The associated
      rotation of porphyrin referred to graphene plane leads to the breaking of
      conjugation and a decrease in electrical conductances. As the number of
      spacers is increased, the conductance ratio can increase from 100 with one
      spacer to 200 with four spacers, and further enhanced by decreasing the
      temperature, reaching approximately 2200 at 100K. This design for a
      molecular switch using graphene electrodes could be extended to other
      aromatic systems.
      
      As mentioned in the design of 퐶60 -based thermoelectric vertical junction
      with graphene layers as electrodes and porphyrin-based switch in graphene
      nanogap, graphene provides a two-dimensional platform for contacting
      individual molecules, which enables transport spectroscopy of molecular
      orbital, spin, and vibrational states. Next, we report single-electron
      tunnelling through a molecule that has been anchored to two graphene
      leads. It is found that quantum interference within the graphene leads
      gives rise to an energy-dependent transmission and fluctuations in the
      sequential tunnelling. The lead states are electrostatically tuned by a
      global back-gate due to the weak screening effect compared to the metal
      electrodes, resulting in a distinct pattern of varying intensity in the
      measured conductance maps. This pattern could potentially obscure
      transport features that are intrinsic to the molecule under investigation.
      Finally, using ensemble averaged magneto-conductance measurements, lead
      and molecule states are disentangled, enabling spectroscopic investigation
      of the single molecule.
      
      As the above describes, there are varieties of research on the charge
      transport properties of molecular devices. It is noticed that noise exists
      in all electronic devices, and the investigation on noise could help us
      understand more fundamental information of the device, i.e. the
      imperfections and configurational changes in the system, the correlation
      of the transmission conduction channels or even exploit the noise
      characteristics for biosensing. In electroburnt graphene nanogaps, our
      collaborators observe that 1/f noise and random telegraph noise at room
      temperature and 77K respectively. Here, I employ a simple one-dimensional
      tight binding model to gauge the effect of two-level fluctuations in the
      electrostatic environment in the tunnel junctions. Two types of models are
      investigated. Model I describes the case that the environmental traps
      drive the tunnel barrier locally and differently. Model II is the case
      that the collective effect of all the environmental traps drives the
      tunnel barrier synchronously. It is concluded that the 77 K data is best
      described by a single environmental fluctuator influencing the
      transmission through the tunnel barrier. This may either occur via a local
      perturbation of the barrier potential, or via an overall modulation of the
      barrier height. A 1/푓 signal emerges as more fluctuators with different
      lifetime 휏 are added to the environment, corresponding to the thermal
      activation of multiple random telegraph noises (RTNs) at room temperature.