Patterning applications via photochemical and intermolecular reactions within monolayers on metal oxide thin films
Soja, Gregory R.
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One of the challenges in materials assembly is fabricating patterned surfaces which have well-controlled short-range interconnectivity, long-range order and are prepared with high throughput at low cost. Many industrial patterning techniques involve high temperatures, extreme pressures, corrosive etchants, and/or high-energy patterning sources. These factors make for expensive processes. In addition, many substrates are unstable under these conditions. Given these challenges, new techniques for patterning, new methods for preparing templates for materials assembly, and an improved understanding of the properties of surfactant monolayers are required. The research presented in this dissertation involves two projects: the development of a novel patterning method involving substrate-catalyzed monolayer photolithography and site-selective nanoparticle deposition, and fundamental studies of the time-dependent compositional changes of mixed monolayers on metal oxide substrates. Chapter 2 focuses on a materials assembly technique for the site-selective deposition of CdSe nanoparticles onto 16-mercaptoalkanoic acid (MHDA)-derivatized nanocrystalline titania films. MHDA-derivatized films are patterned using a substrate-catalyzed mechanism utilizing the photoexcitation of the bandgap transition of TiO 2 . MHDA is oxidized by the valence-band hole created upon excitation of TiO 2 with 355-nm light. MHDA is removed from the titania surface by one of several photodegradation mechanisms, including oxidative decarboxylation and carbon-carbon bond cleavage. Immersion of the patterned MHDA-derivatized titania films in THF suspensions of CdSe nanoparticles leads to site-selective adsorption of the nanoparticles in only the unilluminated regions of the substrate surface. In proof-of-concept experiments, micropatterned, CdSe-functionalized films were fabricated. Chapter 3 focuses on applying the substrate-catalyzed photolithography technique to the fabrication of patterned, multi-component (MC), porphyrin-derivatized titania films. The photochemical mechanism of porphyrin degradation and removal differs from the mechanism of MHDA degradation discussed in Chapter 2. Porphyrins are removed via decarboxylation; however, photochemical mechanisms for the reduction and oxidation of the porphyrin ring are also active. Patterned, MC films were fabricated which consist of entirely separate regions of various porphyrin molecules. Mixed monolayers have been used as templates for materials assembly, but are systems which are not always straightforward. Chapter 4 focuses on unprecedented time-dependent compositional changes of mixed monolayers of mercaptoalkanoic acids (MAAs) and alkanoic acids (AAs) on titania films. The composition of the mixed monolayers evolved with time, even after saturation coverages were reached. A mechanism is presented, wherein dimerization occurs between neighboring MAA molecules through their terminal thiol group. These stable, dimerized MAA surfactants, which are coordinated to the titania surface through multiple functional groups, gradually displace the singly-bound AA molecules over time. A series of experiments were performed to characterize the mixed monolayers and to establish the dimerization-based mechanism. In addition, the influences of solution composition and surfactant chain length on the kinetics of the mechanism were characterized.