All-Carbon Molecular Electronics: From Flexible Fabrication to Multi-Step Transport and Molecular Optoelectronics

• Author / Creator

  Morteza Najarian, Amin

• The central idea of molecular electronics is to incorporate individual or ensemble molecules into the electronic circuit units in the hope of creating miniaturized functional devices with unique properties. This dissertation aims to present my efforts to pursue this ultimate goal, covering the fabrication of reliable all-carbon molecular junctions, and realization of structure-function characteristics in thick molecular layer (>4 nm). I addition, interaction of UV-Vis light with molecular layer and consequential photo-response in the molecular junction is discussed. Chapter 2 demonstrates the fabrication of large area molecular junctions on electron-beam deposited carbon (eC) surfaces with molecular layers in the range of 2 - 5.5 nm between conducting, amorphous carbon contacts. Incorporating eC as an interconnect between Au and the molecular layer improves substrate roughness, prevents electromigration and uses well-known electrochemistry to form a covalent C-C bond to the molecular layer. Au/eC/anthraquinone/eC/Au junctions were fabricated on Si/SiOx with high yield and reproducibility, and were unchanged by 107 current-voltage cycles and temperatures between 80 and 450 K. Au/eC/AQ/eC/Au devices fabricated on plastic films were unchanged by 107 JV cycles and repeated bending of the entire assembled junction. The low sheet resistance of Au/eC substrates permitted junctions with sufficiently transparent electrodes to conduct Raman or UV-Vis absorption spectroscopy in either reflection or transmission geometries. Collectively, eC on Au provides a platform for fabrication and operation of chemically stable, optically and electrically functional molecules on rigid or flexible materials. The relative ease of processing and the robustness of molecular junctions incorporating eC layers should help address the challenge of economic fabrication of practical, flexible molecular junctions for a potentially wide range of applications. Chapter 3 discusses that how carbon-based molecular junctions consisting of aromatic oligomers exhibit structure dependent current densities (J) when the molecular layer thickness (d) exceeds ~5 nm. All four of the molecular structures examined exhibit an unusual, nonlinear ln J vs bias voltage (V) dependence which is not expected for conventional coherent tunneling or activated hopping mechanisms. All molecules exhibit a weak temperature dependence, with J increasing typically a factor of two over the range of 200-440 K. The observed current densities for four examined molecules with d = 7-10 nm show no correlation with occupied (HOMO) or unoccupied (LUMO) molecular orbital energies, contrary to expectations for transport mechanisms based on the offset between orbital energies and the electrode Fermi level. UV-Vis absorption spectroscopy of molecular layers bonded to carbon electrodes revealed internal energy levels of the chemisorbed films, and also indicated limited delocalization in the film interior. The observed current densities correlate well with the observed UV-Vis absorption maxima for the molecular layers, implying a transport mechanism determined by the HOMO-LUMO energy gap. We concluded that transport in carbon-based aromatic molecular junctions is consistent with multistep tunneling through a barrier defined by the HOMO-LUMO gap, and not by charge transport at the electrode interfaces. In effect, interfacial “injection” at the molecule/electrode interfaces is not rate limiting due to relatively strong electronic coupling, and transport is controlled by the “bulk” properties of the molecular layer interior. In Chapter 4, photocurrent generated by illumination of carbon-based molecular junctions were investigated as diagnostics of how molecular structure and orbital energies control electronic behavior. Illumination through either the top or bottom partially transparent electrodes produced both an open circuit potential (OCP) and a photocurrent (PC), and the polarity and spectrum of the photocurrent depended directly on the relative positions of the frontier orbitals and the electrode Fermi level (EF). Electron donors with relatively high HOMO energies yielded positive OCP and PC, and electron acceptors with LUMO energies closer to EF than the HOMO energy produced negative OCP and PC. In all cases, the PC spectrum and the absorption spectrum of the oligomer in the molecular junction had very similar shapes and wavelength maxima. Asymmetry of electronic coupling at the top and bottom electrodes due to differences in bonding and contact area cause an internal potential gradient which controls PC and OCP polarities. The results provide a direct indication of which orbital energies are closest to EF and also indicate that transport in molecular junctions thicker than 5 nm is controlled by the difference in energy of the HOMO and LUMO orbitals.

• Subjects / Keywords

  • Covalantly bonded molecular layer
  • Multistep tunneling
  • Molecular junction
  • Charge transport mechanism
  • Solid-state electronics
  • Sequential tunneling
  • Electronic coupling
  • Molecular orbital
  • Energy level alignment
  • Fabrication
  • Open circuit potential
  • Photocurrent
  • Electronic circuit unit
  • Surface modification
  • Electrochemistry
  • Molecular electronics
  • Diazonium chemistry
  • Carbon electrode
  • Structure function correlation
  • Fermi-level of contacts

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3S75712M

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.