Electron microscopy
 
Comparison between (HR)TEM and (HR)-HAADF-STEM Imaging
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Table 1437. Comparison between (HR)TEM and (HR)-HAADF-STEM imaging.

  (HR)TEM (HR)-HAADF-STEM
   ===============================  ===============================
Magnification
High magnification High magnification
   
Scattered electrons
Highly-coherent elastically-scattered electrons Incoherent elastically-scattered electrons: each atom is an independent scatterer since there is no constructive or destructive interference between them
   
Imageing principle
Interference of coherently scattered electron waves Incoherently elastic & quasi-elastical scattering of electrons
   
Imaging mode
TEM: parallel recording with a parallel beam STEM: serial recording with a narrow probe at high angle (75 - 150 mrad)
   
Point resolution
~2 Å (without) and < 0.5 Å (with aberration-correction) ~2 Å (without) and < 0.5 Å (with aberration-correction)
   
Spatial resolution

Assuming their lens properties are equivalent, the spatial resolution of HR-HAAD-STEM due to the incoherency is better and less disturbed by the microscope defects than that of HR-TEM due to the coherency (see page1436). The resolution in HAADF imaging is less affected by phase errors induced by objective lens than that in TEM imaging.

   
Recording time
0.5 – 5 s 5 – 30 s (More sample-stability- and drift-problems)
   
LaB6, tungsten, field-emission Field-emission
   
Obtainable information
Atomic positions (or plus elemental distribution) Atomic positions and elemental distribution
   
Contrast
Phase contrast: sum of the wavefunction amplitudes. The atom columns always appear dark at Scherzer defocus (see page4234). Z-contrast: sum of the intensities from individual atoms and thus atom columns are always bright
   
Contrast reverse
Can be reversed by defocusing or by specimen thickness Cannot be reversed
   
Advantages
Observation of (nano-)crystals in amorphous matrix Observation of chemical ordering in crystal matrix (e.g. quasicrystals) with Z-contrast function.
   
Data interpretation
Easier technique (e.g. more easily distinguish between crystal and amorphous phases due to "less" serious requirements of specimen preparation) Atom types and positions are more easily and directly interpreted due to the incoherent property.
 
Image examples

(a) HRTEM and (b) HAADF-STEM images of nanocrystalline Mg97Zn1Y2 alloy

Figure 1437a. (a) HRTEM and (b) HAADF-STEM images of nanocrystalline Mg97Zn1Y2 alloy. The inset in (a) is the corresponding electron diffraction pattern of the HRTEM image. (see page1604 for details) [1]
(a) HAADF-STEM image and (b) HRTEM image of an Al87Ni7Cu3Ce3 amorphous alloy
(a) HAADF-STEM image and (b) HRTEM image of an Al87Ni7Cu3Ce3 amorphous alloy
(a)
(a)

Figure 1437b. Microstructure of an Al87Ni7Cu3Ce3 amorphous alloy: (a) HAADF-STEM image and (b) HRTEM image of an Al87Ni7Cu3Ce3 amorphous alloy. In the HAADF-STEM (Z-contrast) image, some brightest spots in white circles represent heavy Ce atoms or its small clusters (Z=58), while some darkest areas in red circles represent fcc-Al (Z=13) nanostructures in the amorphous matrix. However, the HRTEM image does not present clearly the structures of fcc-Al nanocrystals. Adapted from [1]

 

 

 

 

[1] Eiji Abe, Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM, Materials Transactions, 44(10), 2035-2041, (2003).

 

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