Electron transport studies of chemical nanostructures

In this thesis, I present electron transport studies of chemical nanostructures: single-walled carbon nanotubes (SWNTs) and single molecules. In chemical nanostructures, coulomb blockade and electronic energy level quantization dominate electron transport properties. High order correlated transport processes also play an important role in those nanostructures that are strongly coupled to electrodes.; The electron transport spectra of SWNTs are investigated as a function of coupling strength of nanotube-electrode contacts. Measurements of nanotube devices at different coupling regimes showed distinct transport phenomena, including coulomb charging and electron level quantization, the experimental determination of all Hartree-Fock parameters that govern the electronic structure of metallic nanotubes and the demonstration of Fabry-Perot resonators based on the interference of electron waves.; The presence of defects is important in low dimensional materials, for instance 1D SWNTs. A scanned gate microscopy (SGM) is used to locate defect center on SWNTs and study electron resonant scattering by defects. The reflection coefficient at the peak of a scattering resonance is determined to be ∼0.5 at room temperature. An intra-tube quantum-dot device formed by two defects was demonstrated by low-temperature transport measurements.; Transport investigation of semiconducting SWNTs transistors shows large hysteresis effect upon gate voltage sweeping, which came from local charge redistribution around semiconducting SWNTs. A nonvolatile charge storage memory operating at room temperature was realized.; Single molecule transistors incorporating different molecule (divanadium molecule [(N,N',N ″-trimethyl-1,4,7-triazacyclononane)2V2(CN) 4(mu-C4N4)], ferrocene and nickelocene) molecule, were achieved utilizing electromigration-induced break junction technique. Transport spectrum of divanadium molecules showed strange Kondo resonance where individual divanadium molecule serves as spin impurity. Significantly, we find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV. The Kondo resonance evolution in magnetic field was studied and a deviation from long believed two times Zeeman energy splitting was observed when gmuB was comparable with kBTk (Tk is Kondo temperature), illustrating the competition of Kondo process with magnetic energy splitting.; Transport spectra of single ferrocene transistors and single nickelocene transistors shows coulomb charging and energy level quantization. The low energy excitations indicate molecular vibrational states could couple to electron transport processes. Specifically, excitation of the lowest internal vibrational mode, ring-torsion mode contribute to electron transport both in ferrocene and nickelocene, with excitation energy of 3meV and 5meV respectively. Devices with high conductance also show charge state dependent Kondo resonance.