Imaging Single-Electron Charging in Nanostructures by Low-Temperature Scanning Force Microscopy
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In this thesis we investigate Coulomb blockade phenomena and single-electron charging effects in two nanoscale structures: Long semiconducting carbon nanotubes (CNTs) and gold nanoparticles that are linked to a CNT by an organic molecule. While gold nanoparticles naturally exhibit single-electron charging at low temperature, it is disorder that causes the formation of quantum dots in long semiconducting CNTs at low carrier density. Our instrument of choice is a lowtemperature atomic force microscope (AFM) that is sensitive to electrostatic sample forces. A theory of the interactions between single-electron charging of a quantum dot and the AFM tip and cantilever is worked out in linear response. In semiconducting CNTs we resolve single-electron charging events in the resonance frequency of the AFM cantilever. The AFM?s spatial resolution allows us to locate the quantum dots and address them individually. We extract the size of the quantum dots, their gate couplings, and exemplify how to extract their charging energy from the AFM measurements. We frequently observe interaction between neighboring quantum dots and characterize their interdot coupling. The evolution of the quantum dots in CNTs with gate voltage reflects the underlying potential energy landscape for the carriers on the tube. We observe the CNT band structure and extract quantitative information about the disorder potential. On the gold nanoparticle sample, we combine dissipation and frequency shift measurements by our AFM. In addition to the electrostatic gate couplings and the charging energy, this combination allows us to characterize the tunnel coupling between the gold nanoparticle and the CNT, which is acting as a lead. The power of the demonstrated force probe techniques lies largely in the local nature of the measurement. Sensitive, spatially resolved information on electron transport is available even in the absence of device conduction. This advantage is apparent in the single-contact geometry of the gold nanoparticles, but also demonstrated on CNTs.
Carbon Nanotubes; Atomic Force Microscopy; Scanning Force Microscopy; Scanning Probe Microscopy; Nanoparticles
dissertation or thesis