Drying droplets create nanotube films

Researchers at Rice University and the University of Houston, both in the US, have made nanotube films by evaporating droplets of water containing surfactant and single-walled carbon nanotubes. The process formed films about 100 nm thick. Pattern formed by the single-walled carbon nanotube/pluronics dispersion as it dries. Nanotube pattern "We can make thin transparent films of single-walled carbon nanotubes," Matteo Pasquali told nanotechweb.org. "There are still very few methods for making such films." Pasquali and colleagues placed 1 µL droplets of an aqueous solution of F68 Pluronic surfactant containing a dispersion of single-walled carbon nanotubes onto a glass slide. F68 Pluronic contains two poly(ethylene oxide) (PEO) chains, which are hydrophilic, linked by a hydrophobic poly(propylene oxide) (PPO) chain. The PPO chains tended to arrange themselves at the air–water interface, while the PEO chains remained fully inside the droplet. The nanotubes, meanwhile, self-assembled into a crust at the droplet surface. The researchers believe that the F68 molecules formed an entangled network at the surface that trapped the nanotubes. As water continued to evaporate, this crust then collapsed onto the substrate beneath. "We understand the mechanism, and we have shown that the system is very similar to the drying of a polymer solution," said Pasquali. "Because of this understanding, the process could be controlled, adapted, and scaled-up to other situations, e.g. continuous coatings." Repeating the process using a sodium dodecylsulphate (SDS) surfactant produced different results: the nanotubes deposited in the centre of the droplet rather than forming a crust. The researchers believe this is because the SDS molecules were not large enough to form an entangled network at the droplet surface that trapped the nanotubes. "Not all surfactants work," said Pasquali. "We had to understand the mechanism of crust formation, which explained why only some of the surfactants work." The researchers believe that the technique's most promising application is in making thin coatings of single-walled carbon nanotubes by liquid film-flow methods. Creating conductive films could require heating the material to remove surfactant molecules or using an increased concentration of nanotubes in the droplet to boost the number of nanotube contacts. Now the team is investigating how individual nanotubes move in liquids and how factors such as surfactant type or confinement by neighbouring walls affect this motion. "Source":[ http://nanotechweb.org/articles/news/5/1/16/1]

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