Electric transport through nanostructures: Quantum dots, molecular nanowires and carbon nanotubes

The electric transport properties from the various nanometer scale objects are studied.;The scanning tunneling spectroscopy technique is tested on various semiconducting surfaces. On low temperature grown GaAs, mid-gap states are observed 0.3 eV above the valene band edge. Metal quantum dots on the MOS2 surface reveals semiconductor-to-metal transition depending on its size.;Nanometer size Au quantum dots are successfully fabricated from the multiple expansion cluster source. For the Au clusters smaller than 2 nm diameter, single electron tunneling effect can be observed at room temperature, while bigger ones show metallic properties.;Au clusters encapsulated with insulating molecules such as dodecanethiol form an hexagonal array when deposited on an atomically flat surface. The cluster arrays can be imaged with UHV STM under very low current conditions. Electric transport study through the cluster array reveals that the encapsulating molecules are electrically separating the clusters from the substrate. This is different from an isolated cluster which is found to be in close contact with the surface. From this observation, we can conclude that the encapsulating layers play a very important role in forming 2 dimensional cluster arrays.;The conductance spectra through phenyl based molecular wires are measured and fit by the relatively simple theory. To understand the transport mechanism correctly, the potential profile in the molecular layers must be included properly.;A single walled nanotube, attached at the end of the Pt/Ir tip, is used on a Au(111) surface as an STM tip. The measured energy band gap is consistent with a corresponding to (10,10) single walled nanotube. The Z-dependence of the tunneling current is found to have an exponential current decay. Though it shows thermal vibration, the high work function measured reveals the possibility of using a nanotube tip as a STM tip with extremely high aspect ratio.