The Different Observation Modes in Electron Microscopy (SEM, TEM, STEM) - Sample Preparation Topic for Transmission Electron Microscopy - page 50

TEM Bright Field Image TEM Dark Field Image

C2 Annular Aperture

STEM Bright Field Image STEM Dark Field Image

STEM BF Detector STEM ADF, HAADF Detector Annular Dark Field

Fig. 3.13 Different microscopic imaging modes: parallel beam and convergent beam

As long as the incident beam is static parallel or convergent scanning, we can
make bright-field and dark-field images in TEM or STEM mode (Fig. 3.13) . In
STEM mode and annular dark-field mode, we can produce chemical images whose
resolution reaches the atomic level when the size of the probe is smaller than the
interatomic distance: annular dark-field (ADF) and high-angle annular dark-field
(HAADF) modes. The introduction of an annular aperture at the C2 condenser level
(Fig. 3.13) or a triangular aperture at the level of the objective diffraction plane also
enables imaging in atomic-number contrast mode ( Z -contrast).

5.4 Spectroscopic Contrast Imaging Modes in TEM
and TEM/STEM

Transmitted electron energy-loss spectroscopy (EELS) is used to characterize chem-
ical elements. The threshold position indicates the depth of the excited level with
regard to the Fermi level. The peak amplitude is characteristic of the quantity of
atoms involved in this signal. The energy-loss threshold structure enables the deter-
mination of the chemical bond type for the element analyzed and the study of
fine structures that follow the threshold (EXELFS), enabling the calculation of the
number of this element's neighbors. In energy-filtered imaging mode (EFTEM),
bright-field images can be obtained from the zero-loss peak using the elastic signal
only; dark-field images can be obtained from inelastic peaks specific to the atoms
of the target.
There are two types of spectrometers (Fig. 3.14a , b) available for spectroscopic
analysis, enabling the parallel collection of elastic and inelastic electrons to per-
form the energy-filtered TEM imaging (EFTEM) and EELS spectroscopy. The first
type, the one most commonly used, is an electron energy-loss spectrometer (PEELS)
located outside the column, below the microscope's viewing screen. The second