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As project leader for the Particles, Tubes and Colloids project at NIST I seek to develop the metrology for, and characterize the fundamental properties of dispersed nanoparticles using solution phase methods. A primary target of my project's research is single-wall carbon nanotubes (SWCNTs), both as pristine materials, and as dispersed in liquid media.
SWCNTs are an exciting nanomaterial class with significant potential for use in a wide range of applications from solar energy to nano-medicine, to digital electronics and more. However, development of SWCNTs for almost all applications has been hindered by a general uncertainty in the fundamental properties and metrics of the material class, as well as materials quality issues and appropriate measurement techniques. In general, even defining the purity of a SWCNT sample has been a substantial challenge, and one that has delayed more rapid development of commerce in these materials.
Complicating matters is that SWCNTs are actually a class of many different chemical species, often called chiralities, defined by how the carbon sheet is (euphemistically) rolled up to generate the tube structure. Each SWCNT has its own unique diameter and physical properties. Depending on the species, SWCNTs can be metallic, semi-metallic or semi-conducting, and can have optical band gaps in different parts of the visible and infrared spectrum.
We pursue an iterative strategy based on separation of pure(r) populations to develop the basis of measurement science for SWCNTs and to measure fundamental nanotube properties. To produce purified fractions we use and develop dispersed-phase separation techniques (i.e. on nanotubes individualized in liquids) taken or extrapolated from the biosciences, colloids, and polymers fields. We have developed techniques in chromatographic, field-flow based, ultracentrifugation based, and utilizing multiphase separating polymer solutions to accomplish a range of technical capabilities. These include the ability to separate SWCNTs by their species to the single species level, by length with high resolution, and even by enatiomeric handedness. By producing better enriched samples with resolved parameters, we enable better (and cleaner) measurements of the fundamental properties of the SWCNTs, which in turn can drive improvement in the purification/separation science. Additionally, these improvements enable us to develop meaningful documentary standards by which SWCNT properties should be measured, and to produce reference materials with certified property values.My personal research efforts revolve around the interrogation of the nanotube-solution interface, and in the use of colloid science based techniques for separations. Key focuses are in the use of analytical ultracentrifugation to probe the adsorbed interfacial layer on the nanotube in complex environments, and on controlling that layer to drive separations and properties.
An example of a key technology in use in my lab is ultracentrifugation. Once I have dispersed SWCNTs in a centrifuge, through proper experimental design, I can separate them into fractions with different lengths, diameter distributions, metallics from semiconducting, empty ones from water-filled ones, and even enrich the enantiomeric handedness. This can all be controlled through altering the balance of the forces acting on the nanotubes in the centrifuge, and is effectively accomplished by changing the speed, temperature, amount/kind of surfactant, and-or the liquid density used in the processing.
I also use ultracentrifugation as a powerful characterization tool in my project. Analytical ultracentrifugation, in which we can directly observe the motion of particles during the centrifugation, allows for the characterization of size distributions and interfacial binding characteristics of nanotubes (and other nanoparticles) in their native environments. Recent efforts in AUC have demonstrated the power of small changes in the structure of adsorbed molecules at the nanotube interface to make significant differences to the amount of molecules that adsorb, and the nanotube properties. By careful experimentation, the effects of single-point structural modifications to these molecules were able to be linked to the resulting properties,.
Other Recent efforts in my lab include characterization through AUC of the composition and structure of water that can enter into the inside of open-ended nanotubes. This is important, because ingested material significantly affects the properties of the nanotubes and their behavior during separation processing. From these effects, nanotube species can be misidentified or rendered useless for a particular application. This work builds upon our separation science strengths and is directly made possible by a bulk separation methodology for empty and water-filled nanotubes published by my group in 2011.
Potential Post Doc Project Areas
1) Protein and Surfactant Corona Measurement on Dispersed Carbon Nanotubes
The shell of protein or surfactant molecules that surround a dispersed nanotube, whether in biological fluids such as serum, or non-biological dispersion dominates many of the interactions of the nanotube with its environment. Measuring the structure and nature of the dispersing molecules will allow for predictions of properties such as the zeta potential, and will provide important information for addressing the effects and potential accumulation sites of dispersed nanotubes in the human body and in the environment. Recent work at NIST and elsewhere are just now beginning to produce nanotube samples with known length, purity and chirality; these purified materials will be the starting point for measuring the structure and nature of the surrounding shell of adsorbed molecules.
2)Purified Nanotubes for Intrinsic Property Measurement
Carbon nanotubes are the subject of intense scientific interest for advanced applications such as biological sensors and flexible conductive coatings because of their remarkable strength and electrical properties. Developing and selecting nanotubes for these advanced applications will require the specific knowledge and demonstration of achievable properties beyond the theoretical predictions. In our recent efforts we and others have made progress towards isolating single length and diameter populations of nanotubes. In this project continued advancement of the separation technology and the optical characterization through multiple metrologies will be pursued.
3) Characterization of Complex Colloids via Analytical Ultracentrifugation
With the development of nanoparticle and colloidal technologies that include processing in the dispersed phase (or are fundamentally liquid phase processes) there is a need for measurement methods and applications of theory to adequately describe distributions of particles in complex fluid environments. In this project, the goal will be to develop analytical ultracentrifugation methods, with other polymer and colloidal science tools, to characterize particle or polymer distributions that occur in complex environments. Possible ideas for projects include characterization of Pickering (particle stabilized) emulsions for particle-interface adhesion strength, MW characterization of extended or structured polymers for organic electronics in organic solvents, characterization of interfacial coatings and interparticle interactions between single-wall carbon nanotubes in complex environments such as depleting polymers, or particle transport in nearly jammed suspensions.
NIST-Sigma Xi Katharine B. Gebbie Young Investigator Award, 2013
Presidential Early Career Award for Scientists and Engineers (PECASE), 2010
NIST Bronze Medal Award, 2009
Complex Fluids Group
2005-present: Materials Science and Engineering Division (formerly Polymers Division), NIST