In the field of molecular electronics, few materials show as much promise as carbon nanotubes that comprise hollow cylinders of graphite that have a diameter of a few Angstroms. Carbon nanotubes have excellent electrical properties, which make them attractive for applications in nanotechnology.
Semiconducting carbon nanotubes, in particular, have received attention, due to their promising performance in electronic devices, such as diodes and transistors. For example, semiconducting carbon nanotubes can be used as channels in field effect transistors (FETs). Therefore, semiconducting carbon nanotubes are considered to be one of the most promising candidate materials for making nano-sized semiconductor circuits.
The most common prior art method of fabricating carbon nanotube FETs starts with depositing a carbon nanotube on a thin oxide film from a liquid suspension. Source and drain contacts are then formed lithographically on the nanotube to form a FET device.
An exemplary prior art carbon nanotube FET device 10 is illustratively shown in FIG. 1. Specifically, the bulk Si substrate 12 functions as a back gate. The thin oxide film 14, onto which the carbon nanotube 18 is deposited, functions as the gate dielectric. Source and drain contacts 16a and 16b are formed over the gate dielectric 14 at two terminal ends of the carbon nanotube 18. In this manner, the carbon nanotube 18 bridges between the source and drain contacts 16a and 16b, so it can function as the channel in the FET device 10.
The deposition of carbon nanotubes on an oxide surface, followed by lithographic patterning of the source and drain contacts, has been successfully used in the prior art for the construction of single carbon nanotube FETs. However, fabrication of integrated circuits from nanotubes requires the precise placement and alignment of large numbers of carbon nanotubes on a surface (e.g., spanning the source and drain contacts). E. Valentin, et al., “High-density selective placement methods for carbon nanotubes”, Microelectronic Engineering, 61-62 (2002), pp. 491-496 disclose a method in which the adhesion of carbon nanotubes onto a SiO2 surface is improved using aminopropyltriethoxysilane (APTS). In this prior art, APTS is employed to form a silanized surface on SiO2 which is then used to selectively place the carbon nanotubes.
As known to those skilled in the art, SiO2 and other oxides of non-metals are acidic oxides which form acids when combined with water. Such oxides are known to have low isoelectric points. The term “isoelectric point” is used throughout the present application to denote the pH at which the net charge on the oxide molecule is zero.
A drawback with the prior art process disclosed in the E. Valentin, et al. article is that the trialkoxysilane undergoes polymerization in solution and self-assembly must be carried out under controlled conditions excluding water. Additionally, APTS cannot be printed using conventional poly(dimethylsiloxane) (PDMS) stamps in contact printing because the solvents that are used for APTS could swell and destroy such stamps.
In view of the above, there is a continuing need for a method in which carbon nanotubes can be selectively placed on substrate surfaces, while avoiding the drawbacks of the above-described prior art placement process, in which APTS is employed.