Page 185 - Science at the nanoscale

RPS: PSP0007 - Science-at-Nanoscale
10:16
June 5, 2009
8.2. Electron Microscopy
The unscattered electrons are incident electrons that pass right
through the thin sample without any interaction with the sample
atoms. The transmission of such unscattered electrons is inversely
proportional to the thickness of the sample: The thicker the sam-
ple, the fewer the transmitted electrons and vice versa. Therefore
the thicker parts of the sample appear darker while the thinner
parts of the sample appear lighter. Figure 8.15 shows TEM images
of various nanostructured materials, and Figure 8.16 shows atom-
ically resolved lattice fringes of selected nanomaterials.
When the incident electrons are deflected due to their
interaction with the sample but with no loss of energy, this gives
rise to elastically scattered electrons. During the elastic inter-
action, incident electrons are scattered according to Bragg’s law
of diffraction by atoms with regular atomic spacing. The scat-
tered electrons are collated using magnetic lenses to form a pat-
tern of diffraction spots. These diffraction patterns can be used
to identify the crystalline order of the sample, giving information
Figure 8.15. Examples of TEM images (from author’s lab). 175 ch08