Doctor of Philosophy (PhD)
Porphyrins and metalloporphyrins have unique chemical and electronic properties and thus provide useful model structures for nanoscale studies of the role of chemical structure for electronic properties. Porphyrins have been proposed as viable materials for molecular-based information-storage devices, gas sensors, photovoltaic cells, organic light-emitting diodes and molecular wires. The function and efficiency of porphyrins in devices is largely attributable to molecular architecture and how the molecules are self-organized. Modifications of the porphyrin macrocycle, peripheral groups or bound metal ions can generate a range of electrical, photoelectrical or magnetic properties. The conductive properties are greatly influenced at the molecular level by the organization of porphyrins into supramolecular arrays, aggregates, and nanocrystals on surfaces. Conductive-probe atomic force microscopy (CP-AFM) has been used extensively for studies of alkanes, phenylalkanes and arenethiols; however, the conductive properties of porphyrins have not been studied as rigorously. Characterizations with CP-AFM are becoming prevalent for molecular electronics studies because of the dual capabilities for obtaining physical measurements and structural information with unprecedented sensitivity. For CP-AFM, the tip is placed directly on the sample surface, at a designated force. To acquire current-voltage (I-V) spectra, a conductive tip is grounded, and a bias is applied to the substrate. For this dissertation, cobaltcarborane porphyrins were synthesized using a ring-opening zwitterionic reaction to produce isomers with different numbers of carborane clusters per macrocycle. Particle lithography was used to prepare regular arrangements of well-defined nanopatterns of porphyrin nanocrystals on conductive substrates. Nanopatterned SAMs of alkanethiols and organosilanes were used successfully to direct the nanocrystals of porphyrins on the surface and characterized with contact and tapping mode imaging of AFM. Our goals were to elucidate the role of molecular structure, packing and orientation for the conductive properties of porphyrins. Understanding how the self-organization and surface assembly influence electrical properties and reliable measurements of conductive properties when these molecules are coordinated to different metals and surfaces will provide information for developing predictive models.
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Lyles, Venetia DeNae, "Surface studies of organic thin films using scanning probe microscopy and nanofabrication" (2013). LSU Doctoral Dissertations. 382.