| The successful development of carbon-nanotube based nanoscale electronics during last decade has stimulated great interests in molecular electronics beyond nanotubes. This extension has been hindered by fabrication of molecular circuits with conventional e-beam lithography. We have developed a shadow mask evaporation (SME) method to fabricate sub-micrometer metallic electrodes and other structures without using lithographic resists (e.g. photoresist or electron beam resist). We have fabricated electrode pairs with gaps as small as 200 nm, and I will discuss a method that we believe will allow reproducible fabrication of gaps smaller than 10 nm. We have used these shadow masks in a resist-free process to contact DNA strands on mica, carbon nanotubes on Si, as well as to create an etch mask for fabricating 200 nm polymer nanowires. High bias transport measurement shows that the saturation current of metallic carbon nanotubes is always enlarged in a two-probe measurement and can be used to estimate contact resistance. Carbon nanotubes can endure high voltages as high as 40 V Nanotubes break down when the tube temperature reaches the burning temperature. For a micrometer-long nanotube, most of the heat generated by the current is dissipated through the substrate at high bias. The longer the nanotube, the higher voltage it can sustain. We also report fabrication of sub-30 nm conducting polymer nanofiber with tunable resistance. Characterization by scanning conductance microscopy shows the dependence of conductivity on nanofiber diameter. This is confirmed by transport measurements on nanofibers contacted by SME. Transport measurements also provide evidence for the formation of Schottky barriers at the fiber-electrode interface, leading to rectifying behavior in asymmetric fiber samples. |
