Nonlinear Electrical Properties of One-Dimensional Nanostructures

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A general method is first reported for reliably fabricating highly-ordered conventional superconductor nanowire arrays, with good control over nanowire cross section (down to 10 nm by 11 nm) and length (up to 200 microns). Nanowire size effects are systematically studied through electrical measurements and explained with theories. A comprehensive investigation of influence of nanowire length on superconductivity is reported for the first time. We further demonstrate the preparation and electrical properties of high-temperature superconductor nanowires. We find that high-temperature superconductivity can be retained in nanowires ~10 nm in width and >100 microns in length. All nanowires exhibit a superconducting transition above liquid nitrogen temperature, and a transition temperature width that depends strongly upon the nanowire dimensions. The experience gained from the above projects has allowed for the fabrication of superconductor films patterned with ultrahigh-density (pitch ~30 nm) two-dimensional arrays of nano-holes. Significantly enhanced critical currents are observed in such systems. We then describe a method for the assembly of nanoparticles into granular solids that can be tuned continuously from two dimensions to one dimension, and establish how electron transport evolves between these limits. We find that the energy barriers to transport increase in the one-dimensional limit, in both the variable-range-hopping and sequential-tunneling regimes. Furthermore, in the sequential-tunneling regime, we find an unexpected relationship that is peculiar to one-dimensional systems, between the temperature and the voltage at which the conductance becomes appreciable. These results are explained by extrapolating existing theories to one dimension. We also describe an approach to combine the geometric confinement of a Si nanowire and the electric field confinement from an array of ultrahigh-density top gates to form a concatenated array of coupled quantum dots. Reproducible confinement and coupling effects are observed. We have achieved single-atomic resolution in our scanning tunneling microscopy studies of graphene sheets on SiO2 substrates, from which we discovered significant changes in electronic states for bended regions in graphene sheets. We have also carried out the first systematic study on local conductance variations in graphene. Our results suggest large local variations in both the morphology and the electrical properties of graphene.