Modeling of gold nanocrystal assemblies in superlattices and vesicles, and the synthesis of nanocrystals for low-temperature solar cell fabrication
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Recently, nanocrystal (NC) research has grown substantially, due to their unique and diverse properties. Their flexibility has led to a wide set of proposed applications, such as contrast agents in biomedical fields, ordered nanostructures for microelectronics/plasmonics, and as a cheaper alternative to chemical vapor deposition (CVD) methods. While great advancements have been made in utilizing NCs, three challenges often arise – difficulty in characterizing complex nano-systems, a lack of theoretical exploration as to how or why nanocrystals assemble, and challenges in exploiting the benefits of nanocrystals while minimizing disadvantageous properties. This dissertation will address each of these issues in specific systems. First, thorough work has been done suggesting that hydrophobic gold nanocrystals can be encapsulated in vesicle bilayers. However, the primary characterization method for this system is Cryo-Transmission Electron Microscopy, which cannot provide adequate resolution and contrast to fully characterize nanostructures. Here, small angle x-ray scattering is explored as a method for revealing detailed information regarding the bilayer structure. vii Next gold nanocrystal superlattices are explored through molecular dynamics (MD) simulations. While many works have shown crystal structure transitions in a variety of systems, a detailed explanation as to why certain crystal structures are preferred has yet to be provided. This work offers detailed MD simulations to reveal details regarding the packing density of various crystal structures and to estimate diffusion coefficients in various packings. Furthermore, the free energy difference between BCC and FCC configurations for a small set of gold nanocrystals is explored by thermodynamic integration. The simulated properties are also compared to a small set of real systems. The second half of this dissertation addresses practical applications of NCs for photovoltaics. Despite manufacturing benefits, it is well known that the small NC grains and insulating capping ligands make it difficult to produce efficient solar cells. Therefore, two approaches to removing these ligands and growing nanocrystal grains are explored. The first approach focuses on further studying CuInSe2 synthesis as a window into grain growth. The second offers an example of a material with favorable properties for grain growth – Cu3BiS3 – and addresses difficulties in producing it.