Tour, James M.
Doctor of Philosophy
This dissertation describes meniscus-mask lithography (MML): a planar top-down method for the fabrication of precisely positioned narrow graphene nanoribbons (GNRs) and metallic and semiconducting nanowires. The method does not require demanding high resolution lithography tools. The mechanism behind the method involves masking by atmospheric water adsorbed at the edge of the lithography pattern written on top of the target material. Chapter 1 describes the fabrication of sub-10 nm GNR from graphene sheets using MML technique. The electronic properties of resulting GNRs depend on the graphene etching method with argon reactive ion etching yielding remarkably consistent results. The influence of the most common substrates (Si/SiO2 and BN) on the electronic properties of GNRs is demonstrated. The MML technique is also shown to be applicable for fabrication of narrow metallic wires, underscoring the generality of MML for narrow features on diverse materials. In chapter 2 the MML method is shown to be effective for fabrication of narrow wires in a variety of materials. Si, SiO2, Au, Cr, W, Ti, TiO2, Al nanowires are fabricated and characterized. A wide range of materials and etching processes are used and the generality of approach suggests possible applicability of MML to a majority of materials used in modern planar technology. High reproducibility of MML method is shown and some fabrication issues specific to MML are addressed. Crossbar structures produced by MML demonstrate that junctions of nanowires could be fabricated as well, providing all the building blocks required for fabrication of nanowire structures of any complex planar geometry. Chapter 3 is focused on nanoscale menisci behavior and provides additional insights into the mechanism of MML. The width of structures formed by the MML process in concave corners is found to be much more sensitive to changes in the process than the width of MML nanowires. The possibility of change in lateral dimensions of menisci and therefore the nanowire width through webbing formation and through the changes in surface roughness is demonstrated. The water-based meniscus theory is additionally supported. In chapter 4, the chemical modifications of MML-fabricated GNRs are targeted at improving of the GNR electronic properties. Oxygen-containing groups on GNRs edges, mostly carbonyls and hydroxyls, are supposedly generating charge traps and therefore they considerably reduce charge carrier mobilities in GNRs. Those groups are demonstrated to be efficiently removed from the edges of GNRs with lithium aluminum hydride (LAH) treatment combined with tosylation, treatment with Na/K alloy and annealing in H2 atmosphere at 900 C, which is supported by XPS spectroscopy of GNRs. Some alternative methods are shown to be inefficient. Overall, the work accomplished in this dissertation is a step forward toward integration of nanowires and graphene nanoribbons into modern planar semiconductor technology using the simple method of MML.
lithography; nanofabrication; dry etching; nanowires