Browsing by Author "Hakonen, Pertti, Prof., Department of Applied Physics, Aalto University, Finland"

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    Carbon nanotubes and graphene as nanomechanical and superconducting sensors
    (Aalto University, 2017) Häkkinen, Pasi; Teknillisen fysiikan laitos; Department of Applied Physics; Nano; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Department of Applied Physics, Aalto University, Finland
    This thesis deals with proof of concept experiments made at cryogenic temperatures on carbon nanotubes, graphene, and nanomechanical resonators. The work also addresses self assembly of molecules on graphene. The main emphasis of the thesis is on charge detection with carbon nanotube devices. Single electron transistors and quantum dots are the most sensitive charge detectors available at present. Their operation is based on Coulomb blockade, which causes the conductance of the device to be sensitive on subelectron changes in external charge. We studied how coupling between single electron tunneling and mechanical oscillation in a suspended single-walled nanotube quantum dot affects the charge sensitivity and found that mechanically induced conductance changes of the nanotube can improve its sensitivity as an electrometer. The result is comparable to the state of the art radio frequency single electron transistors, but at audio frequencies, where high sensitivities are harder to reach. In second carbon nanotube experiment, we coupled a multiwalled nanotube with superconducting electrodes and proximity induced superconductivity directly to a microwave transmission line. The device was found to work as a sensitive wideband electrometer. The study on mechanical resonators concentrates on nonlinear properties of single-walled carbon nanotube resonators and on readout of doubly clamped aluminium beam resonators. We found that because of the strong coupling between mechanical motion and single electron tunneling in carbon nanotubes, third order conservative Duffing non-linearity can be compensated out, causing the conservative fifth order term to dominate high amplitude mechanical motion. In the study of metallic beam resonators coupled to LC-circuits, we found that the sensitivity of the readout can be improved via focused ion beam processing that facilitates devices with gate capacitances that are larger than in previous devices used for similar purpose. The final part of the thesis deals with two experiments in graphene. A graphene flake coupled to superconducting electrodes was operated as a cryogenic thermometer. The proximity induced supercurrent of the sample could also be used for probing inelastic electron-electron interaction in graphene. The interaction strength was found to be two orders of magnitude larger than expected for comparable two-dimensional metallic systems. In the second experiment, we studied self organization of cobalt phthalocyanine (CoPc) molecules on graphene. Results suggest that CoPc/graphene/hexagonal boron nitride systems are suitable for creating graphene with controlled doping and superimposed molecule-scale periodic potential.