TY - BOOK

T1 - Self-Assembly of semiconductor nanoplatelets into stacks for applications in nanoelectronics

AU - Graf, Rebecca Tatjana

PY - 2024/8/21

Y1 - 2024/8/21

N2 - Nanocrystals (NCs) proved to be a highly versatile material class, with the potential to miniaturize devices and with new properties different from their bulk material. Furthermore, new collective properties arise, when the NCs are in close proximity to each other, which is often the case in applications. This thesis focuses on the stacking of cadmium chalcogenide nanoplatelets (NPLs) as interesting model system for 2D semiconductor NC with beneficial structural and optical properties. Due to their anisotropic form, they showed a great affinity to self-assemble into ordered stacks, e.g. through the addition of an antisolvent, leading to multiple collective properties, such as polarised light emission, or charge carrier transport through the stacks. As many interactions depend on the distance between the NCs, the inter-NPL distance was modified through ligand exchanges. The NPLs were destabilized with an antisolvent and encapsulated with an polymer to stabilize the stacks, before being investigated with different optical, elemental, and structural characterisation methods. Not only could the distance between the NPLs be reduced by half, but also enhanced charge carrier transport through the stack was shown for the smaller distances. To further enhance the charge separation by elongating the charge carrier lifetime, metal domains (palladium and platinum) were grown on the NPLs prior to the stacking and polymer encapsulation. The optimization indeed lead to metal decorated NPL stacks which showed a fast charge separation and charge carrier transport through the stacks. As in those experiments dense layers of polymer-encapsulated stacks resulted in slow electrolyte diffusion and an insulation layer around the stacks, a method to stack the NPLs directly in aqueous medium was developed. This opens up different methods to process the NPL stacks in water for the use in nanoelectronics. In order to induce the highest possible degree of stacking in water, the stacking mechanism and the driving forces were evaluated. Thereby, the polarity, as well as amount of the antisolvent was found to be crucial. Furthermore, the ligand length needs to be sufficiently long to stabilize the stacks. As this newly developed stacking procedure allowed the stacking and subsequent processing of the NPLs in aqueous medium, the system could be tuned further e.g. to optimize the charge carrier transport or allow sensing. Here, the beneficial features of NC-based gel networks, such as their high porosity and large active surface, were implemented into the stack system. Thereby, the cryogelation proved to lead to macroporous gel networks with retained stacking in which the high porosity indeed lead to fast electrolyte diffusion and measurable charge carrier transport through the stack-gel. This thesis thus introduced multiple factors to optimize the properties of the stacked NPLs and gives possibilities to investigate other structure-property correlations in aqueous medium in the future.

AB - Nanocrystals (NCs) proved to be a highly versatile material class, with the potential to miniaturize devices and with new properties different from their bulk material. Furthermore, new collective properties arise, when the NCs are in close proximity to each other, which is often the case in applications. This thesis focuses on the stacking of cadmium chalcogenide nanoplatelets (NPLs) as interesting model system for 2D semiconductor NC with beneficial structural and optical properties. Due to their anisotropic form, they showed a great affinity to self-assemble into ordered stacks, e.g. through the addition of an antisolvent, leading to multiple collective properties, such as polarised light emission, or charge carrier transport through the stacks. As many interactions depend on the distance between the NCs, the inter-NPL distance was modified through ligand exchanges. The NPLs were destabilized with an antisolvent and encapsulated with an polymer to stabilize the stacks, before being investigated with different optical, elemental, and structural characterisation methods. Not only could the distance between the NPLs be reduced by half, but also enhanced charge carrier transport through the stack was shown for the smaller distances. To further enhance the charge separation by elongating the charge carrier lifetime, metal domains (palladium and platinum) were grown on the NPLs prior to the stacking and polymer encapsulation. The optimization indeed lead to metal decorated NPL stacks which showed a fast charge separation and charge carrier transport through the stacks. As in those experiments dense layers of polymer-encapsulated stacks resulted in slow electrolyte diffusion and an insulation layer around the stacks, a method to stack the NPLs directly in aqueous medium was developed. This opens up different methods to process the NPL stacks in water for the use in nanoelectronics. In order to induce the highest possible degree of stacking in water, the stacking mechanism and the driving forces were evaluated. Thereby, the polarity, as well as amount of the antisolvent was found to be crucial. Furthermore, the ligand length needs to be sufficiently long to stabilize the stacks. As this newly developed stacking procedure allowed the stacking and subsequent processing of the NPLs in aqueous medium, the system could be tuned further e.g. to optimize the charge carrier transport or allow sensing. Here, the beneficial features of NC-based gel networks, such as their high porosity and large active surface, were implemented into the stack system. Thereby, the cryogelation proved to lead to macroporous gel networks with retained stacking in which the high porosity indeed lead to fast electrolyte diffusion and measurable charge carrier transport through the stack-gel. This thesis thus introduced multiple factors to optimize the properties of the stacked NPLs and gives possibilities to investigate other structure-property correlations in aqueous medium in the future.

U2 - 10.15488/17853

DO - 10.15488/17853

M3 - Doctoral thesis

CY - Hannover

ER -