Tools for the Characterization of Biomass at the Nanometer Scale - The Nanoscience and Technology of Renewable Biomaterials - page 74

and polystyrene as it was heated (112). These rigid polymers become rubbery and flex-
ible above their glass transition temperatures. The different phases were readily imaged
while the Young's moduli of the components were measured. The loading forces are
estimated at a modest 10 nN with a few nanometer indentation of the specimen.

3.4.2.3 Probe Shape

In all scanning probe microscopies, the shape and size of the probe tip is important
because the SPM image is a convolution of the probe tip shape and the morphology
of the specimen. The tip of the probe is often described as a hemisphere having a
particular radius, though real probe tips are more complex and change by wear during
the experiment. Therefore characterizing the tip is an important step when obtaining
nanoscale morphological information (113).
Carbon nanotubes with diameters near 1 nm have been suggested as the ultimate high-
resolution probe. These probes are currently very expensive and fragile, and the data
can be difficult to analyze when the nanotube is not precisely perpendicular to the
specimen surface (114). Multiwalled carbon nanotubes used as probes in tapping mode
AFM are more rugged and therefore easier to use than the single-walled nanotubes, but
data interpretation is more difficult because of the complex mechanics of multiwalled
nanotubes (115).

3.4.2.4 Near-Field Scanning Optical Microscopy

The spatial resolution attainable with conventional optical techniques is limited to about
half the wavelength of the light source used. For visible radiation, this results in a
theoretical resolution limit of 200-300 nm. Higher resolution can be obtained with
near-field scanning optical microscopy ( NSOM ) by illuminating a specimen through a
small aperture (approximately 50 nm) positioned within one aperture diameter of the
specimen. An extensive and detailed review of NSOM is recommended (116).
NSOM tips are typically used in tapping AFM mode by synchronizing the detection
and tip vibration. Many examples involve illuminating specimens with NSOM and
recording fluorescent emission from the specimen. Spectra from single molecules have
been measured using this approach. One of the current limitations of NSOM is the high
temperature (up to 500 ◦ C) developed at the end of the scanning tip because most of the
radiant energy is absorbed by the conductive coating that defines the aperture.

3.4.2.5 Nanoindentation

Although AFM has been used to measure material properties of specimens, such as elastic
modulus, hardness, and the identification of phases, the method has serious limitations.
More rigorous and quantitative material property measurements can be obtained at a
sacrifice of speed and some spatial resolution using a nanoindenter (87, 117). With
a nanoindenter, the force on the probe and the position of the probe are measured
independently. This is not the case with an AFM probe, for which the force on the
stylus point is determined by the deflection of the cantilever, which is related to the
position relative to the specimen. The nanoindenter probe moves only vertically, whereas
an AFM stylus is always tilted from the vertical and often twisted by lateral force.