Molecular layer deposition : fundamental mechanisms and applications of ultrathin organic films

In recent years, many technological advancements in medicine, renewable energy, water purification, and semiconductor processing have resulted from access to nanotechnology. Though we have many methods for creating nano-sized features, our current nanomaterials toolkit must continue to expand in order to meet the increasing demand for smaller features, more complex architectures, and reduced defect frequencies required by these applications. Molecular layer deposition (MLD) is a promising new method for expanding that toolkit, allowing for the incorporation of organic components into ultrathin materials and nanostructures through a vapor-phase, layer-by-layer synthesis approach. Although a decade and a half of development has already gone into MLD, there is still a significant gap in our understanding of the mechanisms behind MLD growth and the microscopic properties of the resulting films, such as their molecular-level structure. This dissertation presents work to better understand these fundamental properties of MLD and use that understanding to control the thermal, mechanical, and catalytic properties of these materials. In the first half of this work, a study of the structure and growth behavior of organic MLD films is performed. First, the properties of polyurea films are explored as a function of backbone flexibility. Our results suggest that changes in growth rate between the most rigid and most flexible backbones (4 Å/cycle vs 1 Å/cycle) are not caused by differences in the length of molecular precursors, chain orientation (~25° on average for each backbone), or film density (1.0 -- 1.2 g/cm3), but instead are caused by an increased frequency of terminations in the more flexible chemistries. Measurement of the crystallinity and growth angle further suggest that polyurea MLD films exhibit multiple domains, with some chains adopting horizontally packed structures and some chains growing more out-of-plane, leading to an average growth angle of 25°. Interestingly, the observed terminations do not result in the complete cessation of film growth, suggesting that precursors may be absorbing into the film through non-covalent linkages. To observe these absorptions events, MLD is performed on surfaces whose reaction sites have been intentionally eliminated. These terminations are shown to be effective at reducing the growth rate of MLD, suggesting that MLD growth rates are heavily dependent on the number of reaction sites. However, after several cycles, the film growth rate is able to recover, suggesting that monomers have absorbed into the films to reintroduce new reaction sites. A model of growth is developed based on a site balance which suggests that roughly 3% of the chains are terminated by double reactions every cycle. Taken as a whole, this work provides a new paradigm for the growth of MLD films, showing that the films do not adopt the simple layer-by-layer covalent network that is typically portrayed for MLD. MLD has many potential applications in energy and semiconductor manufacturing. In the second half of this thesis, two studies related to the development of MLD are explored. First, a relatively unstudied "manganicone" manganese hybrid MLD chemistry is synthesized using bis(ethylcyclopentadienyl)manganese and ethylene glycol for use as an electrochemically-relevant catalyst material. Characterization of the composition and crystal structure of these films shows them to grow as manganese alkoxides, which partially degrade upon exposure to air into manganese carboxylates. Annealing the hybrid films to remove the carbon is shown to eliminate any porosity introduced through the incorporation of the organic components. However, annealed hybrid films are shown to be less prone to restructuring than ALD-grown MnOx, making them potentially desirable materials for electrodes in thin film batteries. Second, an investigation of the self-assembly of dodecanethiols from the vapor phase onto copper oxide was performed. Dodecanethiols are often used as a blocking layer in area-selective ALD and MLD. The thiols are shown to etch the surface of the CuO to create well-ordered copper-thiolate multilayers several nanometers thick, with crystallites oriented parallel and perpendicular to the substrate surface. In addition, after exposure to air for several days, the multilayer films ripen into particles several microns wide and several hundred nanometers high over the course of several days. This ripening has never before been observed for thiols deposited on copper or copper oxide Finally, a conclusion is presented with several perspectives on the possible use of MLD in the future.

Keywords: Chemical Engineering.
Advisor: Bent Stacey 
Bao Zhenan 
Salleo Alberto