Conjugated dithiols as model systems for molecular electronics: assembly, structure, and electrical response
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Molecular assemblies are promising candidates for nano-scale electronics due to their chemical and structural versatility. The successful fabrication of assembly-based nano-scale electronics, where molecular assemblies comprise the electrically-active components, requires the ability to reliably form molecular assemblies and the ability to 'wire them into electrical junctions. This dissertation focuses on the processing-structure relationships of model conjugated dithiols, the formation of electrical junctions with these molecular assemblies, and the characterization of these junctions. Biphenyldithiol (BPDT), terphenyldithiol (TPDT), and quaterphenyldithiol (QPDT) are assembled in solution from their thioacetyl precursors which are converted in-situ to thiolates using NH4OH. We elucidated how the type of substrate, the solvent quality, and the concentrations of NH4OH and the thioacetyl precursors affect the final structures of these assemblies. BPDT molecular assemblies are disordered on both gold (Au) and gallium arsenide (GaAs) at all conditions explored. TPDT and QPDT adopt the most upright molecular orientations on both Au and GaAs when the assembly is carried out from EtOH-rich solutions at low NH4OH and high precursor concentrations. At these conditions, the assembly formation process is dominated by the adsorption of thioacetylterminated molecules. When the assembly is carried with high NH4OH and low precursor concentrations, adsorption is dominated by thiolates; TPDT and QPDT are disordered on Au and GaAs. None of the molecules adsorb significantly on GaAs from THF. The presence of S-Au bonds at the molecular assembly -- top Au contact interface was directly probed by x-ray photoelectron spectroscopy. Depositing Au electrodes on QPDT assemblies by nTP in dichloroethane results in the reproducible formation of S-Au bonds at the molecule-Au interface. Finally, we measured the electrical response of the model conjugated molecular assemblies on GaAs through direct contact with galinstan. The current densities scale inversely with the tunneling distance, which is determined by factors including the length of the conjugated molecule and the molecular orientation of the assembly. We also examined the electrical response of GaAs--QPDT--Au junctions in which the Au electrodes were transferred using an elastomeric stamps. The electrical characteristics of these junctions were independent of orientation of the molecules and the presence of SAu bonds at the charge transfer nterface. Hydrocarbon contamination on the Au electrodes left by the elastomeric stamp during transfer masked any electrical response from QPDT. It is therefore crucial to ensure the pristine quality of the electrical contact in order to reliably measure the electrical response of the molecular assembly. The fabrication and testing of assembly-based electrical junctions is challenging in terms of both controlling the assembly structures and measuring their electrical response. Careful attention must therefore be paid to each aspect of molecular assemblybased junction formation and characterization.