Structure and stability of silicon nanosheets and nanocrystals




Shah, Tushti

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Silicon has been an element of interest for decades. The synthesis of silicon nanomaterials: nanocrystals, nanowires and nanosheets has further expanded the range of its applications from the semiconductor industry to lithium-ion batteries and biological imaging. As these applications are being explored, understanding the structure of these materials and their stability under different environmental conditions is essential because it dictates their ability to be employed in these. Silicon nanosheets have been explored in the recent decade and despite several reports on their synthesis, a method to synthesize them in significant quantities has not been identified. Using liquid-assisted exfoliation and liquid cascade centrifugation, size-selected nanosheets can be generated from siloxene, a layered silicon compound. These nanosheets show emission in the blue with some size-dependence in their optical properties. While this synthesis can produce significant quantities of nanosheets, the presence of air partially oxidizes and amorphizes them. Synthesizing nanosheets in an inert environment starting from siloxene produces crystalline nanosheets. Studying the Si L₂₃ edge using Electron Energy Loss Spectroscopy, shows that siloxene nanosheets have a Si L₂₃ edge distinct from Si and SiO₂. These nanosheets are also sensitive to damage due to the electron beam and undergo amorphization. Silicon nanocrystals have potential applications in biological imaging because of their biocompatibility and emission in the red-infrared range. A lot of bioconjugation strategies require the use of amines and the impact of amines on the photoluminescence properties and structure of ligand-stabilized silicon nanocrystals is studied. Amines progressively quench the photoluminescence of these nanocrystals and degrade them through the attack of the nitrogen lone pair. Silicon Nanocrystals were also studied using Electron Energy Loss Spectroscopy. EELS was used to understand the structure and surface of silicon nanocrystals. Si L₂₃ shows a change in the oxidation state of Si from the surface to core indicating the presence of Si-C bonds on the surface arising from surface passivation. Low-loss EELS shows the dependence of volume plasmon energy on the size of SiNCs arising from quantum confinement.


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