The Origins and the Making of Nanotechnologies - Molecular Computing: Origins and Promises - page 9

Fig. 1.5 Mechanisms of
interaction of the electron
beam with matter

primary electron beam

back scattering

X-ray radiation cathodoluminescence

secondary electrons

auger-electrons

elastic scattering

inelastic scattering

nonscattered electrons

1.2.1 More Details: Before Building Something
attheNano-level,OneMustLearntoSeeWhatOneDoes

In order to create micro- and nano-sized objects, it is of course necessary to be able
to determine their characteristics—shape and structure, the gross composition, and
the composition of their local areas. The capabilities of the common optical
microscopy are limited by its low resolution which is determined by the wavelength
of visible light. Therefore, the maximum resolution is ~0.0002 mm, and the
achievable magnification does not exceed 500-1,000 times.
In contrast to the light radiation, electron beam proved to be an effective means
of studying the structure of matter at the micro- and nano-level. Depending on the
electron energy its corresponding wavelength may amount to 10 2 -10 3 nm, with
not very high acceleration voltage of electrons (tens of thousands of volts). The
processes of interaction of electrons with matter are characterized by significant
variety. Let the beam of accelerated electrons fall on a thin layer of substance
(Fig. 1.5 ). Some of them are elastically scattered. Besides it, the interaction of
electrons with the atoms of the substance leads to luminescence in the visible
spectrum, X-ray radiation, and reflected secondary electrons knocked out from
the object's atoms. A part of the secondary electrons, the so-called Auger electrons,
provide an opportunity to determine the composition of surface layers and even to
identify the distribution of a particular chemical element on the surface of the
studied object. A significant number of electrons falling on the object pass through
it without scattering and without losing energy, while some of them undergo both
elastic and inelastic (i.e., with a loss of energy) scattering.
In the transmission electron microscope, the electron-optical system creates a
monochromatized and focused electron beam which passes through the studied
specimen (Fig. 1.6 ). The distribution of electrons obtained as a result of the
interaction between the electrons and the specimen is focused by the projection
system on a fluorescent screen or on a photographic plate. Sometimes it is said that