Page 138 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

124 Principles and Methods

intensity distribution of their interference pattern constitutes the
HRTEM image.
With the HAADF-STEM techniques, images are formed by collecting
electrons that have forward scatter at high angles, typically a few degrees
or more, using high angle annular dark field (HAADF) scanning trans-
mission electron microscopy (STEM). Unlike normal dark-field imaging,
where the signal comes from elastic (Bragg) scattering of electrons typi-
cally to smaller angles, the HAADF-STEM signal is the result of inelas-
tic electron scattering typically to larger angles. At high angles, elastic and
inelastic interactions between the incident electrons and the columns of
atoms within the specimen produce the image contrast. Since inelastic scat-
tering depends on the number of electrons in an atom, the strength of scat-
tering varies with atomic number. Spatial resolution is determined by the
size of the focused incident electron probe. With electron beam sizes of less
than 3 angstroms, imaging at atomic level resolution is possible. In a
HAADF image, brighter spots represent the heavier atomic columns while
the less intense spots indicate the lighter atomic columns.
Coupled to EDS (energy dispersive spectrometry) or to EELS (energy
electron loss spectroscopy), TEM can provide information about the ele-
mental distribution at a very low spatial scale (several nanometers). The
energy resolution of EELS is 0.2 eV, while for EDX it is > 140 eV at
6 keV. In addition to elemental information, EELS can be used to deter-
mine the electronic structure, bonding, and nearest neighbor distribu-
tion of the specimen atoms. The high-loss energy is related to electrons
that have interacted with inner-shell or core electrons of the specimen
atoms. Thus, the information obtained is similar to that given by XAS
(see the above discussion)—that is, K-, L-, M-, and such ionization edges
of the elements present in the sample appear in the EELS spectrum
(near edge and extended energy-loss fine structure, ELNES and
EXELFS, regions being defined within them). Therefore, EELS can pro-
vide information on the speciation of elements. However, the very low
intensity of the EELS signal at energies higher than 1500–2000 eV is
a strong limitation to study the K edge of elements with an atomic
number higher than the silicon. Therefore EELS is well adapted to
study elements in a 100–1500 eV energy range corresponding to the K
edge of low atomic number elements like carbon and oxygen or the l edge
of transition metals like Fe or Cu.
Samples preparation. For conventional SEM measurements the sample
must be dry and conductive. If the sample material is not conductive it
must be coated with some material, usually carbon or gold, using a
sputter coating device. In some instances it may also be advantageous
to coat already conductive materials to improve image contrast. The con-
ductive coatings are usually applied at a thickness of about 20 nm,
which is too thin to interfere with dimensions of surface features. Prior