Self-Assembled Monolayers as Wettability Modifiers

This dissertation investigates the development of new nanoscale coatings in the form of self-assembled monolayers (SAMs) used as surface wettability modifiers. The aim of this research is to use SAMs to mimick polymer surfaces (PE and PTFE), alter the interfacial properties of gold surfaces, and microstructure GaAs substrates for transistors and solar cells. In the first study, the synthesis of two cyclohexyl-terminated alkanethiols (HCyHnSH; n = 10 and 11) and their fluorinated analogs C6F11(CH2)nSH (n = 10 and 11; FCyHnSH) was performed. These ring-terminated adsorbates were used to generate self-assembled monolayers (SAMs) on gold surfaces to serve as model polymeric interfaces on metal substrates. Comparison of the contact angles of a wide range of contacting liquids on these SAMs and their polymeric analogs, polyethylene (PE) and polytetrafluoroethylene (PTFE), found that these liquids exhibited similar wettability on the HCyHnSH SAMs and PE surfaces, and separately on the FCyHnSH SAMs and PTFE surfaces. The second study investigates two types of mixed self-assembled monolayers (SAMs) derived from adsorbates having cyclohexyl and phenyl tailgroups mixed with their perfluorinated analogs, respectively. The XPS results suggest that the relative solubility and steric bulkiness of the tail group moiety are two major contributions to the observed preferential adsorption. Moreover, the homogeneously mixed surfaces and precise-controlled surface composition were achieved by the mixture of adsorbates terminated with the phenyl tailgroup and its perfluorinated analog, which show a linear relationship between the mole fraction on the surface and the mole fraction in solution. In the third study, we investigate six different monodentate and bidentate alkanethiols on GaAs surfaces, used for surface passivation. The results show the bidentate alkanethiols exhibit excellent stability and can be used as a new type of material for the surface passivation of GaAs. Finally, we developed a new microstructuring method for GaAs substrates, reverse patterning lithography (RPL), which combines the use of microcontact printing (µCP) of a custom-designed fluorinated adsorbate on GaAs and the deposition of a polymeric resin as a wet-etching resist. Positive pattern formation on GaAs wafers of various designed shapes with sharp edges were obtained using the RPL technique. The RPL method benefits from being cost-effective and time-efficient compared to conventional photolithography and has the potential for use in the fabrication of various GaAs devices, including photovoltaics, light-emitting diodes, and microwave and radio-frequency transistors.