Hysteresis caused by water molecules in carbon nanotube field-effect transistors

Carbon nanotube field-effect transistors commonly comprise nanotubes lying on SiO₂ surfaces exposed to the ambient environment. It is shown here that the transistors exhibit hysteresis in their electrical characteristics because of charge trapping by water molecules around the nanotubes, including SiO₂ surface-bound water proximal to the nanotubes. Hysteresis persists for the transistors in vacuum since the SiO ₂-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 SiO ₂ (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.