Introduction to Nanotechnology

33. MfCROSCOPY 49
Figure 3.12. Transmission electron microscope figures from %3-nm-diameter ;' precipitates of
Nia(Ti, AI) in iron-base 5UperalrOy, showing (a) [ 1001 FCC zone diffraction pallern displaying
large bnght spots frorn the superalloy and weak spots from the ;' preciptates, (b) bright-field
image showing 25-nm elastic main fields around marginally visible :' phase parlicles, and (c)
dark-lieu image obfarned using an SAED apeflure le pass the 7' diffraction spot marked with lhe
arrow in (a). Image (c) displays Ihe ' precipitate particles. [From T. J Headley. cited in
A. 0. Romig, Jr., chapler in R E Whan (1986), p. 442.1
pitates of Ni,(Ti, AI) with a FCC: stnicture. The diffraction pattern in Fig. 3.12a
obtained without the aid of the fiItcr cxhihits large, bright spots from the supcnlloy,
and very small, dim spok from thc y' nanoparticles. In the bright-field irnagc
displayed in Fig. 3.12b the ;" particlcs nrc barely visible, but the 25-nm-diamctcr
clastic strain fields generated by them arc dearly seen. If the 7' diffraction spot heam
indicated by the arrow in Fig. 3. Ira is sclccred for passage 'by the SAED aperture.
the resulting dark-field image prescntcd in Fig. 3.12~ shows very clearly thc
positions of the ;" precipitates.
A technique called image processing can he used to increase the infomation
obtainable from a TEM image, and enhance some features that are close to the noise
levcl. If the image is Fourier-tmsfornicd by a highly efficient technique called a,/uv
fittricv ~imrs/br.nr, then it provides information similar to that in the direct diffraction
panern. An example of the advantages ol' iniagc processing is given by the sequence
of images presented in Fig. 3+13 for a Ni nanopaiticlc supported on a SiOz substrate.
Figiirc 3.131 shows the original image, and Fig. 3.13b presents the fast Fourier
transform, which has the appearance of a diffrnction pattern. Figures 3. I3c-3. I3e
illustntc successive steps in the image processing, and Fig. 3.13f is an image ofthe
SQ suhstratc obtained by subtracting the particle irnagc. Finally Fig. 3.13g presents
the nanopnrticle reconstruction hrn the processed data.
In addition to the directly mansrni~ted and thc diffrnctcd electrons, there are other
electrons in the beam that undergo inelastic scattering and lose energy by creating
excitations in the specimen. This can occur hy inducing vibntianat motions in zhe
atonis near thcir path. and thereby crearing phonons or quantized lattice vibrations
that will propagate through the cysta!. IF the samplc is a metal, then the incoming
electron can scatter inelastically by producing a plasmon. which is a collective
excitation of zhc frcc-electron gas in the conduction hand. A third very important
source of incl;hStEC scattering occurs when the incorning clcctron induces a single-