Jeffrey Fagan (Fed)

Research Interests

As project leader for the Particles, Tubes and Colloids project at NIST, I seek to develop and support measurement technologies (metrologies) to characterize the fundamental and applied properties of dispersed nanoparticles, polymers, and colloids. This includes both experimental efforts, often utilizing solution phase and spectroscopic methods, and documentary standards development.  I am also the technical working area (TWA) chair for TWA 34, Nanoparticle Populations, in the Versailles Project on Advanced Materials and Standards (VAMAS).

A major focus of my project and personal research are single-wall carbon nanotube (SWCNT) populations.  SWCNTs are an exciting family of nanoscale carbon materials with exceptional potential for use in a wide range of applications from semiconductor electronics to sensors and optoelectronics.  A primary issue with these materials is the difficulty in producing pure single structure populations. There are hundreds of distinct SWCNT species, each with unique properties, and many additional factors (filling, environment, enantiomeric twist, length) also affect their measured characteristics and suitability for applications.  To support commerce of such materials improved technologies to produce and isolate the desired SWCNTs, better knowledge of their intrinsic properties, and disseminatable methods to effectively measure and report each property are needed.

The other major area of my research is the development of analytical ultracentrifugation (AUC) methods for nanomaterials. This technique, which essentially involves a centrifuge one can “see” into while it is running, is mainly used in the development and characterization of proteins, nucleic acids, and other biological structures.  However, in my research I have shown it can be similarly well applied to the characterization of dispersed nanomaterials.  Most powerfully it can be used to measure distribution properties such as size and shape, and to measure adsorbed interfacial layers, all with the particles in their native environment. Several of our recent studies have specifically used the AUC to demonstrate and quantify the effect of small variations in the chemical structure of the surfactant we use on the quantity of those molecules that adsorb to various single SWCNT species. Such adsorbed layers govern how nanotubes interact with their environment and behave in separations but are poorly determinable by any other measurement techniques.

SWCNT research:

As a foundation for advancing SWCNT technology and applications, we pursue an iterative strategy of developing technologies to better separate pure(r) SWCNT populations.  Such populations enable technology demonstrations, fundamental nanotube property determination, and advancements in measurement science. Each of these in turn provide positive feedback for improved separation methods powering a virtuous cycle of continually better available materials. The separation techniques I develop are extending methods from the intersections of biosciences, colloids, and polymers fields such as chromatography, field-flow methods, ultracentrifugation, and most importantly liquid-liquid extraction.  In this last category, by utilizing aqueous two polymer phase extraction (ATPE) controlled by surfactants, we can produce many SWCNT populations separated to the single species level, and even by their enantiomeric handedness (i.e., whether their lattice has a left or right-handed twist).  Other processes enable orthogonal length and filling resolution with great fidelity.  In combination, such populations empower us to develop meaningful documentary standards for measuring SWCNT properties, and to produce reference materials with certified property values.

As an example of the feedback loop, ongoing work is using our highly sorted SWCNT samples to develop a rapid optical method for determining the surfactant concentrations and other parameters (e.g., new additives) necessary to isolate specific (n,m) species SWCNTs in an ATPE separation.  In this new methodology, we can screen the extremely large phase space of variables that affect the ATPE separation at vastly increased (> 100 X) throughput and resolution (> 10 X) as compared to conducting actual separations. The highly precise data emerging from these measurements is then used to both optimize our current ATPE processes and to search for new variations that may lead to improved selectivity.  Such advancements facilitate larger scale generation of purer SWCNT populations, feeding back into improved fundamental property measurements and our technology development efforts. 

In technology development, a primary direction of our efforts is in using our separated SWCNT populations to advance technologies for integration of the nanotubes into devices by controlled assembly.  Assembly and integration of SWCNTs from freely dispersed objects to precisely positioned and aligned structures is critically important for digital logic, sensor, optoelectronics and quantum material uses, and are affected by both SWCNT characteristics (species, length, coating) and the specifics of the assembly technology.  Current research projects in this realm include assembly of SWCNT pairs by DNA hybridization (Benjamin Barnes), and separately, formation of large area aligned nanotube films by slow vacuum filtration (Pavel Shapturenka).  In addition to improving the technologies themselves, these efforts are providing critical samples for advancing methods of measuring assembly success and uniformity. Metrology for such factors and their underlying measurands, including relative alignment, spacing, and preciseness of position, are a critically missing factor for enabling commercial applications. 

Preparation and characterization of SWCNTs with controlled filling of molecules or chemical elements ingested into their interior, "endohedral," volume is the last major thrust of my SWCNT research efforts.  Control over this endohedral filling affects the optical and physical properties of nanotubes in useful ways and can enable new applications and metrology. This work builds upon our long-standing interest in differences between empty and water-filled nanotubes.

Post-Doctoral Research Project Areas
If you are looking for a post-doctoral position and are interested in the above areas or related topics, please contact me.  A selection of my current opportunities available through the National Academies of Science, Engineering and Medicine (NASEM) National Research Council (NRC) fellowship process are listed below.

Physical and Chemical Mechanisms of Single-wall Carbon Nanotube Separations

The separation of single-wall carbon nanotubes in liquid phases can be achieved by a number of different processes, yet the molecular mechanism behind any one of the separation processes is not fully understood. This opportunity aims at revealing separation mechanisms by studying the modulating effect of pH, ionic strength, and redox on the separation outcome, paying special attention to the reorganization of dispersant layers triggered by the modulating factors.

Carbon Nanotubes for Optical Applications - Directed Assembly, Characterization, and Emergent Properties

Single-wall carbon nanotube (SWCNT) species as materials display exceptional nanotube-structure specific optical properties of interest to sensing devices, quantum material use as single photon sources, terahertz applications and photonic structures.  The directed assembly of individual SWCNTs onto device structures or into 2D and 3D crystalline arrays is required for such applications.  Both research into specific assembly methods of SWCNT structures (aligned films, arrays of individualized SWCNTs) and the characterization of assembly success and emergent or structure-dominated optical behaviors are of interest.

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. Using nanotube samples with known length, purity and chirality, we can measure the structure and nature of the surrounding shell of adsorbed molecules and their effects on useful properties.

Purified Nanotubes for Intrinsic Property Measurement
Although we have made significant advances in the separation of specific SWCNT species, continued advancement of our separation technologies is still desirable.  In particular we are advancing aqueous two-polymer phase extraction (ATPE) to enable extraction of yet larger diameter SWCNT species and each specific enantiomer of previously extracted populations. Using these samples we can advance characterization of fundamental optical process in nanomaterials and enable applications requiring unique materials properties.

Characterization of Complex Colloids via Analytical Ultracentrifugation
Processing of nanoparticles and colloids in the dispersed phase requires measurement methods that can adequately describe distributions of particles in complex fluid environments.  Possible ideas for projects include characterization of biomolecules and colloids in deep eutectic solvents, characterization of extended and/or structured polymers in organic solvents, characterization of interfacial coatings and interparticle interactions between single-wall carbon nanotubes in complex environments such as depleting polymers, and development of high temperature instrument capabilities.

Professional Background

• 2008-present: Technical Working Area Chair (TWA 34), VAMAS
• 2007-present: Chemical Engineer, Materials Science and Engineering Division (formerly Polymers Division), NIST
• 2005-2007: NRC Postdoctoral Fellow, Polymers Division, NIST

Awards

• NIST Silver Medal Award, 2014
  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