CHARACTERIZATION OF SELF-ASSEMBLED MONOLAYERS BY SUM FREQUENCY GENERATION SPECTROSCOPY AND IMAGING MICROSCOPY

This dissertation reports the characterization of self-assembled monolayers utilizing sum frequency generation (SFG) vibrational spectroscopy and imaging microscopy. Several variables were studied for their effect on the conformational order of the formed monolayers. In the analysis of octadecanoxy-phenylethanethiolate monolayers on gold, it was determined that the presence of an aromatic ring near the gold substrate caused no significant loss of the conformational order of the monolayer. Gauche and kink defects were observed by SFG spectroscopy, caused by the cross-sectional difference of the alkoxy chains and the aromatic ring, forcing the chains to tilt and kink until the methylene groups further above the chains can align to form the all-trans conformation. Additional octadecyl chains attached to the aromatic ring led to an increase in gauche defects, implying the decrease of conformational order is also due to steric effects. SFG imaging microscopy was used to determine the effect of the grain size of the gold substrate on the quality of monolayers produced. Analysis of the resulting images revealed that dodecanethiolate monolayers grown on surfaces with a large average grain size have fewer gauche defects within the alkyl chain assembly and thus are more conformationally ordered than those deposited on smaller-grained substrates. This variance between surfaces was not observed with octadecanethiolate monolayers, possibly due to the increased conformational ordering associated with their longer chain length enables the these thiolates to reduce any chain disordering induced by the grain size of the substrate. Finally, the in situ SFG spectroscopy was used to monitor self-assembled monolayers as they reductively desorb in alkaline solution.