Scanning Probe Microscopy of Graphene and Carbon Nanotubes

This dissertation presents research on scanning probe microscopy and spectroscopy of graphene and carbon nanotubes. In total three experiments will be discussed. The first experiment uses a scanning tunneling microscope (STM) to study the topographic and spectroscopic properties of graphene on hexagonal boron nitride (hBN). Graphene was first isolated and identified on SiO₂ substrates, which was later found to be the source of graphene quality degradation, e.g. large surface roughness, increased resistivity and random doping etc. Researchers have been trying to replace SiO₂ with other materials and hBN is by far the most successful one. Our STM study shows an order of magnitude reduction in surface roughness and electrostatic potential variation compared with graphene on SiO₂.The second experiment shows a novel quantum interference effect of electron waves in graphene, loosely referred to as "Friedel oscillations." These arise when incident electron waves interfere with waves scattered from defects in the sample. This interference pattern shows up as a spatial variation in the local density of states, which can be probed by the STM. We measured such Friedel oscillations in graphene near step edges of hBN. Due to its peculiar band structure, the oscillations in graphene have a faster decay rate and their wavelength is an order of magnitude longer than similar oscillations previously observed on noble metal surfaces. By measuring the dependence of the Friedel oscillations on electron energy, we map out the band structure of graphene. The last experiment studies a different system: carbon nanotube quantum dots. By combining scanning probe microscopy and transport measurements, we obtain spatial information about quantum dots formed in a carbon nanotube field effect transistor. We also demonstrate the ability to tune the coupling strength between two quantum dots in series