Vardhan Bajpai

Carbon Nanotubes: Measuring Dispersion and Length

Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.

Anisotropic polarizability of isolated semiconducting single-wall carbon nanotubes in alternating electric fields

We measure the linear dichroism for aqueous suspensions of isolated semiconducting single-wall carbon nanotubes (SWCNTs) in alternating electric fields. The field-induced alignment of length-purified SWCNTs is determined from the anisotropy of the first and second interband optical transitions for the (6,5) semiconducting species. At 3kHz, the effective anisotropic polarizability of the DNA-wrapped SWCNTs is of order 10−28Fm2, comparable to that of a high-aspect-ratio conducting rod, but a factor of 5 larger than that of gold colloidal rods and an order of magnitude larger than that of tobacco mosaic virus.

Chromatographic Separation of Single Wall Carbon Nanotubes

Size exclusion chromatography (SEC) is shown to be an effective method to characterize single wall carbon nanotube (SWNT) dispersions. SEC separates nanotube dispersions by size, and measures the intrinsic viscosity on-line as a function of hydrodynamic size as is determined by Universal Calibration. This scaling contains information about the shape of the dispersed particles. This characterization method was tested on three representative dispersions: octadecyl amine functionalization in tetrahydrofuran (THF), butyl group functionalization in THF, and DNA wrapping in aqueous solution. Significant differences between the dispersions were found. Small angle neutron scattering (SANS) and atomic force microscopy (AFM) produced results consistent with the SEC method.