Molecular Templated Assembly of Single-Walled Carbon Nanotubes and Their Electrical Characterization

We have developed a method for rapid, massively-parallel assembly and alignment of single walled carbon nanotubes (SWCNT) on a solid-state substrate. The results opened the possibility of production of SWCNT-based integrated circuits. In this strategy called "surface-templated assembly", SWCNTs from a solvent suspension are directed toward molecular patterns on the substrate and self-assemble onto specific locations with precise orientations. Since the method does not rely on any external forces or slow serial patterning techniques, it can be done in a completely parallel manner and is suitable for high-throughput applications. We have demonstrated the assembly of millions of individual SWCNTs and SWCNT-based circuit structures over ~1cm2 size sample surface in a matter of minutes. The experiments were first carried out on patterned hybrid self-assembled monolayers (SAM) of polar molecules and nonpolar molecules. Polar molecules were patterned with SAM of nonpolar molecules, such as 1-octadecanethiol (ODT). The molecular templated substrates were used successfully to assemble SWCNT. Polar molecules with different tail groups, both positive and negative, were shown to be effective, in contrast to the prediction that only molecules with positive tails can be used to align SWCNTs. Furthermore, we observed that the interaction between SWCNTs and metal surfaces also can be used to align SWCNTs using only nonpolar molecular patterns. A series of controlled experiments showed that the number density of aligned SWCNTs depends upon the nature of polar molecules and metal surfaces. We have also assembled SWCNTs on patterns of Au nanoparticles. Au nanoparticle patterns were created on composite SAM templates of nonpolar (ODT) and dithiol (octanedithiol) molecules through self-assembly of Au nanopaticles onto the dithiol region. On such templates, we found very strong adhesion of SWCNTs on Au nanoparticles and no adhesion on the nonpolar regions. We also examined systematically the adhesion of SWCNT on nonpolar molecules with varying coverage of Au. We found no SWCNT attachment when Au coverage is significant but incomplete. Strong adhesion of SWCNT is observed only when the coverage of nonpolar regions by Au is almost complete. These results indicates that nonpolar molecules like ODT play an active role in the alignment of SWCNT on ODT/metal and polar SAMs/ODT hybrid structures. Metal nanoparticle patterns on SAM can also be created via simple metal deposition. With the deposition of a thin metal (Au, Ti, Cr, etc.) film, cluster formation was observed over microscale SAM of nonpolar molecules while for polar molecule patterns of comparable size no cluster formation was observed. Using this surface-templated assembly process we have successfully produced field effect transistors (FET) based on SWCNT. SWCNTs were directed to assemble across prepatterned source and drain electrodes (Au or Pd) on a doped-Si/SiO2 substrate. The electrical characteristics of these self-assembled SWCNT-FETs are comparable to those fabricated with traditional lithographic methods while the large hystereses observed in the FET action of those devices were significantly reduced. We attribute this to the molecular passivation of the SiO2 surface by octadecyltrichlorosilane (OTS). This observation could have significant implications in exploiting the potential of such devices for chemical and biological sensing.