Tools for the Characterization of Biomass at the Nanometer Scale - The Nanoscience and Technology of Renewable Biomaterials - page 77

backscattering is dependent upon atomic number. Helium ions can penetrate deeply
into materials, effectively creating another contrast mechanism. To date no studies of

biomass have been reported using a helium ion microscope.

3.4.3.3 X-ray Beam Probes

Scanning transmission X-ray microscopy STXM measures the absorption or deflection
of an X-ray beam by a specimen. STXM uses a synchrotron X-ray source to gener-
ate up to 10-nm, 0.3-eV resolution images of specimens 100-150 nm thick. Though
specimen degradation is still an issue, X-rays are less damaging than are the electrons
used in transmission electron microscopy. Contrast is developed by selecting X-rays
of different wavelengths. This technique has been used to describe the morphology of
polymer composites (140). Developing contrast between cellulose and hemicellulose
may be difficult, though a recent study of mixtures of ethylene-butene copolymer with
ethylene-octene copolymer distinguished the separate phases of these polymers differing
only in the length of the side chain (141).
X-ray photoelectron spectroscopy ( XPS ), originally known as ESCA, is a common
technique for measuring the presence and oxidation state of atoms at the specimen
surface. XPS uses a monochromatic X-ray to eject an electron in the specimen, whose
kinetic energy is determined by the difference between X-ray energy and binding energy
of the electron. Ejected electrons rarely escape from more than 10 nm below the surface.
XPS is an ultrahigh-vacuum technique that typically probes approximately a square
centimeter of surface but can have up to 20 nm resolution when coupled to a focused
X-ray source, such as a synchrotron (142). This method is sensitive enough to determine
the oxidation state of carbon, so it can distinguish lignin from carbohydrate, and has been
applied extensively in probing chemical modification of surfaces. XPS data should be
interpreted cautiously because the escape depth, and therefore sensitivity, may change
with chemical composition of the surface, and the incident X-rays and ejected electrons
may cause chemical reactions in the organic substrate. Also, localized areas can become
charged and therefore eject electrons with different energy than the rest of the specimen.

3.4.3.4 Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry ( SIMS ) produces good chemical specificity with a
resolution of 50 nm to 1 m. A focused ion beam is directed to a solid surface,

removing material in the form of neutral and ionized atoms and molecules. The ions
are accelerated into a mass spectrometer and separated according to their mass-to-charge
ratio. This is inherently a surface probe with 1- to 10-nm depth resolution. The sensitivity
to different chemical species varies greatly because only ions are detected, whereas
most of the products of sputtering are neutral fragments (143). SIMS is inherently an
ultrahigh vacuum technique requiring flat specimens and some means to eliminate charge
accumulation.
The use of cluster ions, that is, ions of high molecular weight such as Cs n ,SF 5 ,
C 60 ,andAu n , to bombard the specimen generally produces better SIMS spectra for
molecular solids than the original SIMS method, which uses gallium ions (144, 145).

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