Synthesis of single-walled carbon nanotubes and characterization of nanotube-based field-effect transistors

Very long single-walled carbon nanotubes (SWNTs) (lengths up to 0.6 millimeters) were synthesized by using a mixed methane and ethylene carbon source in chemical vapor deposition (CVD). Interesting loop and closed ring structures similar to those of fullerene "crop circles" are observed on these as-grown ultralong tubes. Surveying the electronic properties of individual SWNTs by transport measurements reveals that approximately 2/3 of individual SWNTs grown from isolated nanoparticles (derived from Ferritin) are semiconductors exhibiting field effect transistor (FET) characteristics. The distribution of CVD-grown SWNT chirality is elucidated for the first time, and implications to array-based nanotube electronics and sensors are discussed.; Carbon nanotube field-effect transistors commonly comprise nanotubes lying on SiO2 surfaces exposed to the ambient environment. The transistors exhibit hysteresis in their electrical characteristics because of charge trapping by water molecules around the nanotubes, including SiO 2 surface-bound water proximal to the nanotubes. Hysteresis persists for the transistors in vacuum, since the SiO2-bound water does not completely desorb in vacuum at room temperature, a known phenomenon in SiO2 surface chemistry. Heating under dry conditions significantly removes water and reduces hysteresis in the transistors. Nearly hysteresis-free transistors are obtainable by passivating the devices with polymers that hydrogen-bond with silanol groups on SiO2, (e.g., with poly(methyl methacrylate) (PMMA)). However, nanotube humidity sensors could be explored with suitable water-sensitive coatings. The results may have implications to field-effect transistors made from other chemically derived materials.; Finally, metal/SWNT contact issues will be discussed. Rhodium (Rh) is found similar to Palladium (Pd) in making near-ohmic electrical contacts to single-walled carbon nanotubes (SWNTs) with diameters d > ∼1.6 nm. Non-negligible positive Schottky barriers (SBs) exist between Rh or Pd and semiconducting SWNTs (S-SWNTs) with d < ∼1.6 nm. With Rh and Pd contacts, the characteristics of SWNT field-effect transistors (FETs) and SB heights at the contacts are largely predictable based on the SWNT diameters, without random variations among devices. Surprisingly, electrical contacts to metallic SWNTs (M-SWNTs) also appear to be diameter dependent especially for small SWNTs. Ohmic contacts are difficult for M-SWNTs with diameters ≤ ∼1.0 nm, possibly due to tunnel barriers.