Nanotube Metrology

Our goal is to develop methods to produce well-characterized fractions of carbon nanotube suspensions with controlled parameters (length, type, charge, concentration and impurities) and to utilize them for measuring intrinsic nanotube properties and for addressing emerging EHS concerns. By providing measurements that underlie this new class of materials, we allow vendors and buyers to trade with confidence and provide the final users with sound knowledge of the properties of the material.

In every single-walled carbon nanotube (SWCNT) production technique many different types (called chiralities) and lengths of SWCNTs are produced. The properties of the SWCNTs are linked strongly to both of these parameters. Thus to characterize the intrinsic properties of any specific SWCNT type or length, an initially polydisperse nanotube sample must be first purified of non-SWCNT carbon and metallic catalyst, and then separated by diameter or length to achieve robust results. Characterization by optical methods (absorption, fluorescence, Raman scattering), conductivity measurements, microscopy or other techniques then yields specific results.

Individual dispersion of the SWCNTs with DNA or small molecule surfactants such as sodium deoxycholate enables this purification and separation via non-destructive solution-phase separation techniques. We use a variety of different techniques to separate SWCNTs by chirality or length depending on the requirements of the characterization we wish to perform. Using techniques such as size exclusion chromatography, field-flow fractionation and ultracentrifugation (for either length or type separation) highly resolved SWCNT fractions are generated to allow for measurement development and individual property determination.

Our systematic focus on controlling the nanotube population in a sample through the utilization and development of separation technique differentiates the NIST effort in SWCNTs. Using well controlled populations ensures that we can accurately develop measurement techniques and characterize intrinsic SWCNT properties resulting in greater confidence in measurements by nanotube manufacturers, device makers and academic researchers.

Controlling the SWCNT population in a sample begins with the dispersion process.In the dispersion process the SWCNTs are brought into an aqueous or organic solvent through the use of small molecules called surfactants.As it is advantageous to individualize the SWCNTs as much as possible in the dispersion process, to allow for separations, the first research undertaken was to determine the most profitable methods for dispersion and dispersion metrology.

Utilizing small angle neutron scattering (SANS) to directly measure dispersion, we compared measurements from additional techniques such as multiple angle light scattering, dynamic light scattering, UV-visible-near infrared (UV-Vis-NIR) absorption and NIR fluorescence on the best dispersed surfactant – SWCNT systems, to develop the metrology of dispersion.This work identified the best dispersants as small single stranded DNA molecules, such as (GT)₁₅, and the small molecule surfactant sodium deoxycholate.

Length Separation

We advanced three different techniques to perform length separation: size exclusion chromatography (SEC), field-flow fractionation (FFF), and centrifugation. Each technique offers a different, complementary, balance between size resolution, mass throughput, and metrology development. The centrifugation technique allows for the largest scale separations, while the FFF and SEC both process smaller amounts of material but allow for higher resolution of the separated fractions, and for online measurement of the separation process. With both FFF and SEC we made significant advances in the separation science and metrology of SWCNTs within the last year. SWCNTs separated into different length fractions ranging from 1 micron to less than 50 nm were then subjected to intense characterization; our most prominent accomplishment in 2007 was the publication of the previously unrecognized dependence of SWCNT optical properties with length.

The development of SWCNT samples with different lengths, and the metrology of those samples, are enabling significant new investigations and collaborations. In concert with the biomaterials group, these samples were used to for the first determination of the effect of length on the uptake of SWCNT by multiple cell lines, including human lung fibroblasts. In this study we found that only the shortest SWCNTs, those less than 200 nm, were uptaken at a significant rate by the cells. Thus the potential toxicity of these materials can likely be significantly reduced by the use of SWCNTs longer than this threshold.

• The global market for nanotubes has the potential to reach $5 billion by 2012 in electrical, mechanical, health and medical applications.We are developing the measurements infrastructure that underlies this new material.
• Three biannual workshops on single-walled carbon nanotubes (SWCNTs), jointly sponsored by NIST and NASA, have led the way for dissemination of the NIST effort to both the industrial and academic communities.The output was a Recommended Practice Guide which launched the extensive ISO activities in SWCNTs. Eleven documentary standards are in preparation by five countries.
• 2009 will see the release of three SWCNT Reference Materials (RMs), each designed to address measurement needs reported by the SWCNT community. A SWCNT soot RM, a purified SWCNT "bucky paper", and a set of length separated SWCNTs in aqueous dispersion will be released.
• We collaborate with industrial, academic and government researchers including, Rice University,NASA and SouthWest Nanotechnologies Inc. by providing them with analysis of samples of unprecedented quality, by sorting samples into their component parts and by providing optical data.