Nanotechnology-Enabled Sensors

5.7 Electron Microscopy 249
an electron beam at grazing incidence angle to the surface. He obtained
images of copper and gold surfaces at a magnification of only 10 times!
Later in 1938, Albert Prebus and James Hillier at the University of Toronto
developed the first practical transmission electron microscope. Von Borries
(1940) was much more successful with his grazing incidence method for
the development of the transmission electron microscope (TEM). He
placed the sample surface at a few degrees both to the viewing direction
and to the illuminating beam.
Unlike the characterization techniques discussed so far, electron microscopy
concerns the interaction between electrons and matter. Electron
microscopes utilize a highly energetic beam of electrons that interacts with
a material. From the interaction, information regarding topography, chemical
composition, morphology and crystallographic structure can be obtained.
Such instruments are capable of examining features in the nanoscale and
are perhaps the most routinely employed characterization instruments in
nanotechnology.
Electron microscopes are suitable for the characterization of both organic
and inorganic materials. However, as they employ high-energy electron
beams, prolonged exposure to the beam may cause certain materials,
particularly organic polymers and biological materials, to be damaged, deformed,
or destroyed. Furthermore, electron microscopes generally operate
at high vacuum and consequently not suitable for samples that contain a
liquid component or outgas.
Electron microscopes operate similarly to optical microscopes, as they
both have an illumination source and magnifying lenses. However, the illumination
source is a high-energy electron beam. Optical microscopes use
glass lenses, whereas the electron beam is deflected and focused by electromagnetic
fields, with what are called electron optics or electromagnetic
lenses. This is because it not practically possible to fabricate materialsbased
lenses for electrons. The beam is then focused onto the sample,
where electro-matter interactions give rise to measurable signals.
Electron microscopes have a much better resolution than their optical
counterparts because of the interaction of an electron’s matter wave with
the sample. From Bragg’s law, the minimum separation, dmin, which can be
resolved by any microscope, is given by:
λ
min
2sinθ
= d . (5.15)
The resolution can be improved by using shorter wavelengths. The
wavelength associated with an electron is given by the de Broglie relation:
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