Low-dimensional carbon nanotube and graphene devices
Electronic devices in which the electrons are confined to fewer than three spatial dimensions are an important tool for physics research and future developments in computing technology. Recently discovered carbon nanotubes (1991) and graphene (2004) are intrinsically low-dimensional materials with remarkable electronic properties. Combined with semiconductor technologies they might be used to fabricate smaller devices with more complex functionality. This thesis addresses two routes towards this goal. The detection of charge transport through quantum dots using a GaAs point contact is a potential tool for quantum computation. This project aimed to fabricate and measure hybrid devices with carbon nanotube quantum dots on top of GaAs point contacts. Dispersion and AFM manipulations of nanotubes on GaAs were studied, revealing comparatively weak binding. Transport measurements indicated that GaAs induces disorder in nanotubes, creating multiple tunnel barriers. Preliminary attempts were made at CVD growth and ink-jet printing of nanotubes directly onto GaAs. Although only one atom thick, graphene is macroscopic in area and must be patterned to confine conduction; room temperature transistor behaviour requires graphene ribbons only a few nanometres wide. This work fabricated such structures using a charged AFM tip, achieving reliable cutting even on single layer graphene and feature sizes as small as 5 nm. The cutting mechanism was found to be chemical oxidation of carbon by a polarised water layer, with an activation energy determined by the energy of dissociation of water at the graphene surface. The critical variables were the voltage difference between the tip and graphene and the atmospheric humidity. An unstable solid oxide intermediate was also observed. Thermal annealing revealed the presence of a layer of water beneath flakes. Finally, EFM measurements were made of graphene at 20 mK, enabling estimates of the local carrier density and revealing spatial variations in the electronic structure on a scale consistent with electron and hole puddles.
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