2nd Joint Workshop on Measurement Issues in Single Wall …

  2nd Joint Workshop on Measurement Issues in Single Wall
                     Carbon Nanotubes:
                Purity and Dispersion Part II
                                           NIST
                                  Gaithersburg, Maryland
                                   January 26-28, 2005

                                Workshop Summary

The 2nd Joint Workshop on Measurement Issues in Single Wall Carbon Nanotubes, organized by
the National Aeronautics and Space Administration, Lyndon B. Johnson Space Center (NASA-
JSC) and the National Institute of Standards and Technology (NIST), was held January 26-28,
2005, at NIST in Gaithersburg, MD. In attendance were over 80 participants, representing
private corporations, universities, and government laboratories. The primary purpose of the
workshop was to bring together technical and business leaders in the field of single wall carbon
nanotubes (SWCNTs) to discuss measurement priorities and aid in the development of
measurement protocols. The primary output of the meeting will be "Recommended Practice
Guides", authored by workshop participants, and edited and published by NIST, for use by
scientists, and engineers involved in R&D, processing, and the production of nanotube-
containing products. These Practice Guides will contain measurement protocols that will help
harmonize sample preparation, measurement procedures, data analysis, and reporting among the
nanotube manufacturers, researchers and end users.

It was decided to revisit the topics of purity and dispersion, which were the subjects of the first
NASA/NIST workshop held in May 2003. While progress in these measurements has been
made, improvements are still needed to accurately measure and describe the quality of nanotube-
containing materials and the dispersion of nanotubes in liquids or polymers, both of which are
considered crucial for the continued growth of applications incorporating SWCNTs. The
organizers recognized that there remains considerable confusion and ambiguity regarding which
techniques to use for a particular purpose and the relative accuracy which one should expect to
achieve. Significant differences in both methodology and interpretation continue to exist from
one measurement laboratory to another. For this reason, comparison and specification of the
quality of SWCNT materials is extremely difficult, as noted in "Nature", 16th Dec. 2004.

To address these challenges, the organizing committee invited 23 speakers and developed an
agenda that encouraged active participation from attendees. Breakout sessions addressing both
workshop topics were held to foster open discussion and to invite consensus regarding best
techniques and measurement methods. A half-day open discussion led to a consensus on those
measurements for which protocols were most urgently needed.
Presentations
The Acting Director of NIST, Dr. Hratch Semerjian, welcomed the participants and introduced
the Opening Speaker, the Honorable Phillip Bond, Under Secretary of Commerce for
Technology. In his presentation, Mr. Bond pointed out the continuing growth of commerce in
the area of nanotechnology and thanked the workshop participants for their willingness to help
ensure its success. Mr. Bond's presentation was followed by a talk given by Dr. Stephanie
Hooker of NIST who emphasized the need for agreed-upon measurement procedures and further
explained the purpose of the workshop.

There were two Keynote Presentations. The first was given by Dr. Robert Haddon from the
University of California at Riverside, who presented the latest findings regarding the use of Near
IR Spectroscopy (NIR) as a primary of means of assigning a quantitative value to the purity of a
SWCNT-containing sample. Significant progress has been made in the use of this technique
since it was discussed at the first workshop. There was general agreement that NIR may be a
viable technique to quantify purity, but there are several questions remaining, including effects
of tube diameter, type of processing, etc.

The second Keynote Address was given by Mr. James Von Ehr, the founder and president of
Zyvex Corporation, who focused on the potential commercial applications for carbon nanotubes
in a number of different areas. He called attention to the need for measurement techniques that
would lead to improved qualification procedures for nanotubes from a given supplier.

The remainder of the first day was devoted to a number of plenary presentations on the topic of
purity measurement, followed by breakout sessions whose goal was to identify the critical
measurement issues relative to the determination of nanotube purity. A poster session with 25
posters was held in the evening to allow workshop participants to present their latest findings,
and to hold informal discussions with their colleagues.

The primary topic on the second day of the workshop was the measurement of nanotube
dispersion. There were also several presentations on the characterization of isolated nanotubes.
The end of the second day consisted of breakout sessions devoted to dispersion measurements.

The following sections provide some details of the important issues relative to measurement of
purity and dispersion as discussed during the meeting.

Purity
The purity of single wall carbon nanotubes was loosely defined by the attendees at the first
workshop as the quantity of SWCNTs relative to other carbon-like materials present (amorphous,
graphitic, and C60 carbons) as well as metal impurities. The point was made however that not all
producers or users employed this definition in a way that allows for a quantitative assessment of
the quality of a sample. A number of measurement techniques used for purity determination
were discussed, with the general consensus that no single technique can describe the quality of a
sample of nanotubes. After much discussion it was agreed that the most extensively utilized
techniques are thermogravimetric analysis (TGA), scanning electron microscopy (SEM),
transmission electron microscopy (TEM), Raman spectroscopy, and near infrared spectroscopy.
The need for rapid, inexpensive measurement methods was emphasized.
Both TEM and SEM are used extensively for qualitative analysis of a sample containing
SWCNTs. There was general accord that a TEM image demonstrating the existence of a
significant quantity of SWCNTs is an important measure of the quality of the material. For
quantitative estimation, a combination of TGA, Raman, NIR, and ICP methods was
recommended.

Dispersion
Dispersion was divided into two categories, macrodispersion, defined as the distribution of
nanotube bundles, and nanodispersion, the splitting of the bundles into individual tubes. In
macrodispersion, the primary issue is the agglomeration of SWCNTs in solvents or polymers,
while in nanodispersion, the focus is on eliminating SWCNT ropes. The question of dispersion
stability over time was also viewed as important.

It was agreed that it is critical to have a consistent sample preparation for all the dispersion
characterization methods. The technique of choice to determine the degree of macrodispersion
appears to be optical microscopy. However, there is lack of agreement on what constitutes good
versus poor dispersion. It was suggested that Raman mapping techniques, SEM, and scanned
probe microscopy (SPM) may be useful complements to optical microscopy. Small angle
scattering (neutron and X-ray) was put forward as a possible fundamental method to quantify
dispersion.

Conclusions
The final session of the workshop was devoted to presentations of the synopses of each of the
breakout sessions on the topics of purity and dispersion, and a general discussion and debate
regarding priority needs for measurements. There was consensus that protocols for measurement
techniques would be valuable even if they were incomplete.

On the topic of purity, the strengths, limitations, and research needs for a number of
measurement techniques were discussed. A matrix showing techniques on one axis and the item
measured on the other axis was used to reach a consensus for the most critical techniques to be
addressed in the Practice Guide. It was eventually agreed that chapters would be written on four
primary procedures, TGA, TEM/ SEM/SPM, Raman spectroscopy, and NIR, and two secondary
techniques, inductively coupled plasma (ICP), and x-ray fluorescence. Volunteer authors were
found for each of the chapters.

The attendees agreed that the topic of dispersion needed a separate Practice Guide from that of
purity. The chapters for this Practice Guide were divided into the topics of macro- and nano-
dispersion, and authors were found to write them. There was also a general agreement that one
of the needs was an agreed-upon terminology, which will be included in the Practice Guide. The
chapter on macrodispersion would be primarily devoted to microscopy procedures, while that on
nanodispersion would include mention of a wide variety of techniques including small-angle-
neutron and x-ray scattering, atomic force microscopy, etc.

A discussion was held regarding the need for reference materials. While there was consensus
that such materials would be useful, it was felt that production of such materials should be
delayed until the protocols for the measurement procedures are more firmly established. A
number of participants also commented that inter-laboratory tests could be quite useful in
establishing laboratory-to-laboratory correlations on measurements.

Finally, the question was raised regarding the need for future workshops of this kind. There was
a general sense that future workshops would be valuable, but there was not consensus on the
particular topic. However, nanotube dimension measurements and determination of chirality
both received strong support.