Challenges for Metrology and Nanotechnology
Donald R. Baer, Pacific Northwest National Laboratory
Opportunity and Challenge - Although some areas of nanotechnology, such as the
semiconductor industry, have current and well defined near term measurement needs,
many areas of nanotechnology, and the related metrology needs, are much less
developed. At a 2001 National Academy of Science Arthur Sackler Colloquium on
Nanoscience, George Whitesides observed that those of us doing nanoscience don’t really
know what it is and we certainly don’t know what nanotechnology is. My understanding
is that this statement is not an expression of pessimism but a statement of the nearly
unlimited potential for scientific and technological impact. The impacts may be
incremental at first, but dramatic and revolutionary changes will occur. At this time it is
not possible to clearly identify the long term instrumental and metrological needs. None
the less, it is essential to begin to establish the framework and approach to meet long term
needs and to address areas identified as important.
Nomenclature - NASA’s Dr. Meyyappan, Director of the Center for Nanotechnology,
Ames Research Center, says that nanotechnology is like ice cream, it comes in many
flavors. This seemingly simple statement has very significant implications. There are
many different things that we call nanoscience and nanotechnology for a variety of
different reasons. These cover different disciplines and a wide variety of technologies as
well as different physical, chemical, and biological phenomena. As stated by Prof.
Alexandra Navrotsky in the book Nanoparticles and the Environment,1 a “consistent
nomenclature” is needed. Well thought out terminology is an essential element for
identification and clarification of the concepts associated with nanoscience and
nanotechnology. It is also a critical component in the development of accurate,
reproducible and meaningful metrology. Most ASTM (American Society for Testing and
Materials) and ISO (International Organization for Standards) activities start with efforts
to define the critical terms.2
Nanoparticle Characterization – Challenges with accurate and useful characterization
of nanoparticles highlight a few topics of importance. Several different methods are used
to measure and report the size and size distribution of nanoparticles. Well defined and
generally accepted standard methods will be essential if nanoparticles are to be widely
used and distributed as “industrial” materials, particularly for size dependent applications.
However, the information needs for nanoparticles do not even begin to be satisfied by
knowing the size. At a September 2003 Nanoparticles and the Environment workshop,
Bob Hwang, director of the Center for Functional Nanomaterials (CFN) at Brookhaven
National Laboratory noted that the nature and properties of nanoparticles require a
multidimensional analysis because the properties can vary with time and size. Jill
Banfield’s research group3 has observed that the structure of nanoparticles( depend upon
the environment. Any useful reporting of nanoparticle properties must include
information related to production process, time after production, environmental
conditions and handling history. There are also complications associated with the
collection of nanoparticles for analysis or storage and the movement or transport of
nanoparticles from one environment to another for analysis (or use) without
contamination or unwanted alteration.
Descriptions of the structure and composition of nanoparticles are no less challenging
than measurements of size. Although traditional measurements of structure such as
transmission electron microscopy (TEM) and x-ray diffraction (XRD) are critically
important, it appears that they do not always provide an adequate picture of the detailed
structures of nanoparticles. Synchrotron based small angle scattering measurements
provide an indication of structure not apparent in the TEM or XRD data. Contamination
and surface structure will play a major role for chemical applications of nanoparticles and
our ability to accurately obtain such information on very small particle surfaces is very
limited. For nano-structured materials, it may be essential to interactively combine
multiple types of data to enhance the information obtained. For example, X-ray
Photoelectron Spectroscopy (XPS) can be used to obtain information about the elemental
distribution of contaminants on nanoparticles when information about the particle size is
known from other methods. Such combined approaches to data collection and analysis
are not well know, not well tested and not widely applied.
Distributions, Statistics and other Challenges - It will ultimately be both unfeasible
and undesirable to characterize each nano-component of any system or assembly.
However, as already noted for nanoparticles, it is essential to have an accurate sense of
the structure and size distribution of the components of nano-structured materials. This is
an immediate issue for current and near future generations of integrated circuits. The
importance of variations in geometry and time sensitive response can be demonstrated by
single molecule Raman data collected by H. Peter Lu’s group.4 Time resolved single
molecule spectra show a significant variation with time. The relative magnitudes of
various spectral components reflect many factors including flexing of the molecular
structure. Although the time averaged signal is identical to the spectrum of bulk material,
a series of time resolved single molecule spectra contain significant additional
information. In some situations it may be that the statistical distribution of data as
collected can be analyzed to provide information about the distributions, sizes, motions
and even function of nano-structured materials.
In most cases nano-sized components will need to be integrated into larger meso- to
macro-sized systems. There are significant challenges in designing, assembling
(including registry and electrical contacts) and verifying the nature of the structures
achieved. In addition to contamination issues, there are issues related to the stability of
nano-structures when exposed to photon or particle irradiation. Methods of confirming
the robustness of the final material and functional properties will need to be developed.
1
Nanoparticles and the Environment, Reviews in Mineralogy and Geochemistry 44, ed J. Banfield and A.
Navrosky, Mineralogical Society of America 2001.
2
ISO TC 201 on Surface Chemistry is initiating a subcommittee on Scanning Probe Microscopy that will
be involved with terminology and standards relevant to nanoscience and nanotechnology.
3
HENGZHONG ZHANG*, BENJAMIN GILBERT*, FENG HUANG & JILLIAN F. BANFIELD Nature
424, 1025 - 1029 (28 August 2003);
4
Yung Doug Suh, Gregory K. Schenter, Leyun Zhu, H. Peter Lu, Ultramicroscopy 97 (2003) 89–102