Near-field Optical Metrology and Nanocharacterization
L. Novotny, University of Rochester
Progress in science and technology was often triggered by the invention of new
instrumentation. Due to the availability of new kinds of microscopes and spectroscopic
techniques we have developed a thorough understanding of physical phenomena, ranging
from the atomic structure to the structure of biological cells. In the words of Nobel
Laureate Rosalyn Yalow "New Truths become evident when new tools become
available." Also the rapid advance of nanoscience is largely due to our new
instrumentation that allow us to manipulate and measure structures on the nanometer
scale.
In nanoscience one often first attempts to understand the nanoscale building blocks in
isolated form before assembling them into a functional device (bottom-up approach).
However, the properties of the building blocks can change once they are embedded into a
macroscopic structure. This change is due to interactions between the building blocks and
also interactions with the environment. In fact, one of the most interesting aspects of
nanoscale systems involves properties dominated by collective phenomena. In some
cases, collective phenomena can bring about a large response to a small stimulus. In order
to study nanoscale systems in a complex environment it is necessary to develop
spectroscopic techniques with high spatial resolution.
Important instruments for the characterization of nanoscale materials are electron
microscopy and scanning probe microscopy. However, without any prior knowledge
about the specimen it is often difficult and challenging to identify the constituent parts of
the specimen and thus to learn about its functionality and the underlying physics. This is
mainly because electron microscopy and most scanning probe techniques render high
resolution topographical images with poor molecular (chemical) specificity.
Figure 1: Chemical specificity versus spatial resolution for di®erent microscopic techniques. Near-field
scanning optical microscopy (NSOM) achieves an excellent product of the two factors. Courtesy of S.
Stranick (NIST, Gaithersburg).
On the other hand, optical spectroscopy provides a wealth of information on structural
and dynamical properties of materials. This is due to the fact that the energy of light
quanta (photons) are in the energy range of electronic and vibrational transitions in
matter. Combining optical spectroscopy with microscopy is especially desirable because
the spectral features can be spatially resolved. Unfortunately, the diffraction limit has
been preventing researchers from resolving features smaller than half a wavelength of the
applied radiation. However, in recent years a novel technique, called near-field optical
microscopy, has extended the range of optical measurements beyond the diffraction limit
and stimulated interests in many disciplines. With near-field optical microscopy,
resolutions of 100nm are nowadays routinely achieved. Pushing the resolution of
optical microscopy down to 10nm would benefit both biological and materials science
since an instrument with 10nm would allow us to directly image and characterize
individual biological proteins in their native membranes and it would enable us to image
quantum wave functions (orbitals) in semiconductor nanostructures.
It is also important to realize that future nanoscale devices need to be protected from interactions with the environment. For example, capping layers are routinely applied to
semiconductor nanocrystals to prevent the escape of electron-hole pairs. Therefore,
instrumentation is required that is capable of subsurface imaging and characterization.
Optical radiation can penetrate through matter and is therefore well suited for subsurface
characterization. In short, optical spectroscopic methods with high spatial resolution are
essential for the understanding of the physical and chemical properties of nanoscale
materials including biological proteins, semiconductor quantum structures, and
nanocomposite materials.
In my presentation, I will review the ideas behind near-field optical microscopy and
spectroscopy. I will present results from our own research and I will discuss challenges
and prospects.
Figure 2: (a) Confocal and (b) near-field Raman scattering images of the same area of a carbon nanotube
sample acquired at º = 2615cm¡1 (G0-band). Near-field Raman imaging increases spatial resolution by a
factor 10-20. (c) Raman scattering spectrum recorded on an individual single-walled carbon nanotube. This
spectrum uniquely identifies the chemical nature of the sample.