CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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2.6. Transmission Electron Microscopy (TEM)
2.6. 1. Introduction
Most characterization techniques discussed previously are optical characterization. In
typical synthesis of nanomaterials, a distribution of particles sizes or presence of multiple
phases or amorphous phases are present. Before carrying out the carrier dynamics it is
necessary to obtain this information as all of the above factors mentioned earlier can
influence charge carrier dynamics. One of the versatile techniques that give all this
information is a transmission electron microscope. Since the features that are to be studied
are in nanometer size regime, imaging is only possible if wavelength of probe used is close to
the size of the features that is to be imaged. In a TEM, electrons are used as probes since
their wavelength can be tuned according to energy [2.10, 2.11]. In modern TEMs
accelerating voltage of ~100-1500keV are used. The wavelength of electrons for these
voltages is in Pico meters; therefore TEM can in principle image lattice planes. However due
to aberrations involved, the resolution of a TEM instrument is limited. However state of the
art TEM machines have overcome these problems in aberration corrected TEM. Additionally
one can perform electron diffraction and obtain crystallinity and crystal structure. Additional
information can also be obtained from inelastic scattering of electrons. These inelastic
scattering events can arise from a number of processes like phonon scattering, plasmon
scattering, ionization etc. These energies can be used for chemical characterization of the
sample. In a TEM, there are two common modes of imaging: bright field and dark field
imaging. Bright field image is collected by monitoring un-scattered electrons after passing
through the sample and objective aperture. Therefore regions where sample is not present