Creating and Probing Molecular Assemblies for Single-Molecule Devices

Open Access
Moore, Amanda Michelle
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
June 20, 2008
Committee Members:
  • Paul S Weiss, Committee Chair
  • Thomas E Mallouk, Committee Member
  • David Lawrence Allara, Committee Member
  • Mary Jane Irwin, Committee Member
  • scanning tunneling microscopy
  • single-molecule devices
  • oligo(phenylene-ethynylene)
  • molecular switch
  • capture surfaces
We explored the creation and conductance of molecular assemblies by looking at both the larger body of work that has been performed to characterize molecular devices, and through probing isolated single-molecule assemblies and creating cluster tether schemes. We have characterized the bistable conductance switching exhibited by oligo(phenylene-ethynylene) molecules. This conductance switching has been hypothesized to occur through a variety of interactions including reduction, rotation, neighboring molecule interactions, bond fluctuations and changes in hybridization. Using molecular design, we tailored the switch molecules to enable the testing of each mechanism. The hypothesis most consistent with our and others’ data is that of hybridization change at the molecule-substrate interface. The oligo(phenylene-ethynylene) switches also exhibited motion within the host self-assembled monolayers at substrate step edges. This was observed as three apparent heights in our analyses of scanning tunneling microscope data. We have characterized the ‘third’ apparent height as arising from the switch molecules place-exchanging up and down substrate step edges. Only switches residing at substrate step edges have the ability to exhibit three apparent heights, as compared to those isolated at domain boundaries, which only exhibit two possible apparent heights. The oligo(phenylene-ethynylene)molecules were further characterized to show that the switching and motion events occur on time scales faster (ms) than those of scanning tunneling microscopy imaging (min)using height vs. time measurements. We have expanded the capabilities of scanning tunneling microscopy to include measurements using microwave frequencies. Using AC measurements, we were able to compare the polarizabilities of several self-assembled monolayers. We applied the microwave frequencies to the systems of the oligo(phenylene-ethynylene) switches and gained predicative abilities of which molecules were likely to exhibit conductance switching or motion. In addition to our studies of isolating and studying switch molecules, we have developed capture surfaces for superatom clusters, which have element-like properties. We used barium ions to model the capture for alkaline-earth-metal-like clusters. We are developing the captures surfaces both to identify the cluster properties and to tether them for measurement with scanning tunneling microscopy. These studies demonstrate our abilities to capture, to isolate and to measure properties of molecules on the single-molecule scale.