CHARGE TRANSPORT IN NITRO SUBSTITUTED OLIGO(PHENYLENE-ETHYNYLENE) MOLECULES

Open Access
Author:
Cabassi, Marco Alberto
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 23, 2007
Committee Members:
  • David Lawrence Allara, Committee Member
  • Joan Marie Redwing, Committee Member
  • Jerzy Ruzyllo, Committee Member
  • Theresa Stellwag Mayer, Committee Chair
Keywords:
  • inelastic electron tunneling spectroscopy
  • molecular electronics
  • IETS
Abstract:
This thesis presents research aimed at tackling two issues in the field of molecular electronics. The first issue is the large range of molecular conductance values reported by various research groups for identical molecules. This is addressed by studying the same molecule in dissimilar environments. The second issue is experimental uncertainty – whether the observed effects are inherent to the molecule or due to external causes. This is addressed by performing in-situ spectroscopy of the molecule as part of its electrical characterization. Oligo(phenylene-ethynylene)s are a well studied class of molecules in the field of molecular electronics, and this work focuses on charge transport through nitro substituted oligo(phenylene-ethynylene) molecules. The electrical characterization of these molecules was performed utilizing two testbeds. An electromigrated break-junction testbed was used to probe individual molecules, while a nanowire molecular junction testbed was used to probe self-assembled monolayers of the molecule. Experiments performed on individual molecules revealed a temperature dependent transition in the dominant charge transport mechanism. Above 50K, hopping is the dominant charge transport mechanism, while below 50K direct tunneling is the dominant charge transport mechanism. Experiments performed on self-assembled monolayers did not reveal any temperature dependent transitions. The dominant charge transport mechanism appears to be direct tunneling throughout the temperature range investigated. The results also indicate that molecules embedded in a self-assembled monolayer have significantly lower conductance than individual molecules. This is primarily due to a second charge transport mechanism (hopping) that opens up above 50K that is available only to individual molecules, and secondarily due to better potential screening properties of the self-assembled monolayers. Inelastic electron tunneling spectra obtained for the molecules in a self-assembled monolayer show a characteristic three-peak structure. Each of the peaks can be associated with a specific vibrational mode of an oligo(phenylene-ethynylene) molecule. The results presented here directly address two issues in the field of molecular electronics. Firstly, the discrepancy in the reported molecular conductance values was addressed by establishing the important role played by the molecular environment in determining molecular conductance. Secondly, confidence in the results was established by obtaining an in-situ inelastic tunneling spectrum of the molecule under study.