Page 173 - Science at the nanoscale

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June 5, 2009
8.2. Electron Microscopy
Scanning Electron Microscopy
8.2.1
Efforts put into the development of electron microscopy have paid
off as the electron microscopes can achieve better resolution and
greater depth of focus. Why does an electron microscope achieve
better resolution? Scanning electron microscope uses electrons
with energies of a few thousand electron volts (eV). The de Broglie
wavelength of an electron is given by λ = h/p, where h is Planck’s
constant and p is the momentum of the electron. For electrons
with energy of 3600eV, the wavelength is 0.02 nm. Hence the elec-
tron microscope would be able to achieve much better resolution
than ordinary optical microscopes. Figure 8.3 shows some exam-
ples of SEM images.
In practice, the resolving power of ordinary electron micro-
scopes is about one nanometre. This number is larger than the
de Broglie wavelength because instrument geometry and electron
scattering in the specimen are factors that influence the resolving
power. Furthermore, during the operation of a scanning elec-
tron microscope, the profile of the focused beam of electrons
Figure 8.3. SEM images of (a) compound eyes of an ant, (b) compound 163 ch08
eyes of a mosquito, (c) a strand of human hair and (d) the surface of the
wings of a housefly (from author’s lab).