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Abstract

The field of nanomaterials is ever expanding, populated with a wealth of nanostructures that have been effectively integrated into almost every field there are successful applications ranging from photovoltaics, biosensing and stimulation, photocatalytic water reduction, cellular stimulation, infrared detectors, gas sensing and monitoring to address a few. The source of this variety stems from the ability to capitalize on the well explored bulk properties of the base materials while the size scale confers an entirely new set of mostly unexplored properties. This duality dominates the nano-regime. Nanomaterials promise to provide better insight into fundamental studies and simultaneously guarantees advancements for practical applications and technologies. We will use this technology to develop advancements for traditional roles while opening up new unexplored platforms. The source of this vast potential arises from the shrinking length scale paired with increased surface area. As sizes are reduced the structure, morphology, and surfaces are precisely controlled through the synthesis process. Despite the range of nanomaterials that have been developed there still exists a tremendous number of properties, effects, and functionalities that have yet to be explored. Here we set out to explore and characterize the variation in a cross section of silicon nanomaterials and exploit our depth of knowledge in this region to employ targeted nanomaterial fabrication to specific silicon interfaces. The silicon choice for base structural competent here is strategic as silicon nanomaterials integrate the indirect band gap of silicon, synthetic control over morphology, chemical composition, doping profile, biodegradable and nontoxic functionality, and the potential for widely accessible silicon chemistry for surface modifications. Most significantly, the indirect band gap of silicon provides optical absorption over a wide range of wavelengths in the visible and near infrared (IR) portion of the electromagnetic spectrum provide the potential for photothermal and photoelectric processes for optical stimulation at the silicon interface. Within the scope of this work we will characterize the properties dictating silicon interfaces. In the first part we will address both bulk and surface modifications to (1) the bottom up CVD fabrication process of silicon nanowires (SiNWs) and (2) post synthetic modifications to surface structure. From this analysis of the range of modifications we will develop an understanding of the impacts of these processes on factors such as surface chemical variation and surface electrical structure. In this study, all modifications will look at new approaches to tailor the surfaces of silicon nanostructures. This opens up new methods to implement the bottom up synthesis to precisely arrange materials down to submicron or nanoscale dimensions. We then demonstrate several applications of this analysis protocol to direct fabrication of novel materials and implement these creative designs to tailor materials to unique silicon interfaces. In our second approach, we employ the same analysis procedures to guide post synthetic surface modifications including silicon functionalization, etching, and silicide coating. We analyze these range of synthesis procedures for tailoring the silicon nanostructure interfaces to understand how they make materials more or less effective in specific applications. With this fundamental study we establish an understanding of the vast range of surface properties that arise from a small cross section of materials and then explore the possible silicon interfaces that can be controlled from this point. In the second part of this work I will leverage the guidelines which arose from part one to establish a method of using this analysis to apply well characterized silicon interfaces for specific applications. In this portion we explore the interaction of silicon nanostructure interfacing with polymers, cells, tissues, and water surfaces. At silicon nanostructure – polymer interface we demonstrate the potential to imbed previously inaccessible functionality to well explored polymers. As silicon has biocompatibility and optical stimulation properties ideal for integration with biointerfaces, we next explore the photoelectric stimulation at cardiomyocyte cell interfaces. In a third approach, we expand on this stimulation at biointerfaces to explore the SiNW tissue interface. In a final approach, we examine the impact of silicon nanostructures assembled into hierarchal nanostructured porous structures for modulating water interfaces in unique biomimetic approaches. Through this work we produce a platform for surface characterization able to directly analyze the changes in surface chemical composition, surface band structure, and work function. We demonstrate the use of this analysis method for identifying changes to the silicon interface resulting from both CVD synthesis and post synthesis modifications. From this fundamental study we can develop a guiding design more precisely synthesize nanomaterials for a range of applications. We then integrate this analysis with materials applications to utilize the targeted synthesis of silicon for specified material interfaces.

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