Nanotube metrology improvement is a significant effort in the Particles, Tubes and Colloids project dedicated to improving the measurement methods for, and fundamental understanding of, single-wall carbon nanotubes (SWCNTs). Our strategy is based on the identification and improvement of separation methods for isolating sub-populations of nanotubes with selected attributes, such as the nanotube length, chemical species (i.e. chirality), enatiomeric handedness, or purity (chemical or morphological), followed by intensive characterization of these isolated materials. Utilizing resolved sub-populations allows for clear determination of the nanotube intrinsic properties and extrinsic dependencies on physical parameters, and how those properties are captured by various measurement methodologies. Our results are used to produce research publications, documentary standards on how to measure important characteristics, and NIST reference materials.
Exciting postdoctoral research opportunities are available in this project through National Research Council Fellowships. Please contact us for more details.
Single-wall carbon nanotubes (SWCNTs) are a tubular form of carbon consisting of a single shell of sp2 bonded carbon with a nanometer scale diameter that have many predicted properties superior to other available materials. However, every production technique for SWCNTs produces many different species of SWCNTs (called chiralities, and defined by the vector with which a graphene sheet would be rolled to form the tube structure), with different lengths of those SWCNTs agglomerated in varied nanoscale to mesocale structures containing impurities both carbonaceous and from the catalyst. This fundamental polydispersity of SWCNT materials from production has vastly complicated the application of SWCNT materials for industrial use. To underlie technology development, measurement techniques and standards are necessary (so that everyone is speaking the same language), and technology for the isolation of purified nanotubes by the structural parameters that define their properties. This effort pursues both of these directions.
We achieve separation and purification of SWCNTs thorough the use of dispersants such as small molecule surfactants or biomolecules such as DNA, that modify the surface of the nanotubes and enable them to be individually isolated in an aqueous environment. We can then use a variety of different methods to separate SWCNTs non-destructively, utilizing handles such as differential bouyancy or differential adsorption determined by the dispersant-SWCNT combination, or fundamental colloidal physics. Recently, we have pioneered the use of aqueous two-phase polymer extraction methods to enable a great leap in ease and scale of separation. The separated populations can then be used for fundamental studies.
Additional Technical Details:
Our systematic focus on controlling the nanotube population in a sample through use and development of separation techniques differentiates the NIST effort in SWCNTs. Using well controlled populations ensures that we can accurately develop measurement techniques and characterize intrinsic SWCNT properties with greater confidence. Three main classes of separations are currently being used in the project, multi-stage aqueous two-phase extraction, chromatography based SWCNT species or hydrodynamic separations, and ultracentrifugation techniques exploiting density and hydrodynamic differences.
Aqueous Two-Polymer Phase Extraction
Our most recent efforts in SWCNT separation technologies have been in utilizing aqueous two-phase extraction (ATPE) methods to improve both diameter driven (species separation) and electronic type (semiconducting from metallic SWCNTs) separations. Results in both these areas and utilizing either DNA dispersed or surfactant dispersed SWCNTs were published in 2013 and 2014 in JACS and Advanced Materials. Continuing work is improving the quality of these separation and the increasing the scope and scale of their applicability. Licensing opportunities can be discussed through the NIST Technology Transfer Office.
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(Materials Science and Engineering Division)