Morphologic Instability of Graphene and its Potential Applications
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Graphene is a monolayer of graphite. The surge of interest in graphene, as epitomized by the Nobel Prize in Physics in 2010, is largely attributed to its exceptional properties. Ultra thin, mechanically tough, electrically conductive, and transparent graphene films promise to enable a wealth of possible applications ranging from thin-film solar cells, flexible displays, to biochemical sensing arrays. However, significant gaps remain to realize these potential applications, largely due to the difficulty of precisely controlling graphene properties. Graphene is intrinsically non-flat and tends to be randomly corrugated. The random graphene morphology can lead to unstable performance of graphene devices as the corrugating physics of graphene is closely tied to its electronic properties. Future success of graphene-based applications hinges upon precise control of the graphene morphology, a significant challenge largely unexplored so far. This dissertation aims to explore viable pathways to tailoring graphene morphology and leverage possible morphologic instability of graphene for novel nano-device applications.
Inspired by recent experiments, we propose and benchmark a strategy to precisely control the graphene morphology via extrinsic regulation (e.g., substrate surface features, patterned nanowires and nanoparticles). A general energetic framework is delineated to quantitatively determine the extrinsically regulated graphene morphology through energy minimization. Such a framework is benchmarked by determining the graphene morphology regulated by various types and dimensions of nanoscale extrinsic scafffolds, including two dimensional herringbone and checkerboard corrugations on substrate surfaces and one dimensional substrate surface grooves and patterned nanowires. The results reveal a snap-through instability of the graphene morphology, that is, depending on interfacial bonding energy and substrate surface roughness, the graphene morphology exhibits a sharp transition between two distinct states: (1) closely conforming to the substrate surface and (2) remaining nearly flat on the substrate surface. This snap-through instability of graphene holds potential to enable graphene-based functional nano-devices (e.g., ultrasensitive nano-switches).
Another type of morphologic instability of graphene is the spontaneous scrolling of graphene into a carbon nanoscroll (CNS). The spiral multilayer nanostructure of CNSs is topologically open and thus distinct from that of carbon nanotubes (CNTs). The unique topological structure of CNSs can enable an array of novel applications, e.g., hydrogen storage, water channels and ultrafast nano-oscillators. However, the realization of CNS-based applications is hindered by the lack of reliable approach to fabricating high quality CNSs. We propose a simple physical approach to fabricating CNSs via CNT-initiated scrolling of graphene on a substrate. The successful formation of a CNS depends on the CNT diameter, the carbon-carbon interaction strength and the graphene-substrate interaction strength. We further demonstrate that the resulting CNS/CNT nanostructure can be used as an ultrafast axial nano-oscillator that operates at 10s GHz. Such CNS-based nano-oscillators can be excited and driven by an external AC electric field, further illustrating their potential to enable nano-scale energy transduction, harnessing and storage.