Question

Transmission Electron Microscopy

a).Explain how bright field and dark field imaging is obtained in a TEM.

b).What is the difference between bright/dark field imaging and high resolution TEM?

c).Describe why you see contrast in TEM images, e.g. the origin of the contrast in bright field imaging, dark field imaging, mass-thickness contrast, z-contrast.

Answer 1

In the bright field (BF) mode the aperture is placed in the back focal plane of the objective lens which allows only the direct beam to pass.

In dark field (DF) images, the direct beam is blocked by the aperture hence only diffracted beams are allowed to pass through the objective aperture. Since diffracted beams comes through interaction with the specimen, very useful information can be derived from Dark field images.

Transmission electron microscopy works on the principle of electron diffraction however the high-resolution transmission electron microscopy (HRTEM) uses the principle of interference by using both the transmitted and the scattered beams. All electrons emerging from the specimen are combined at a point in the image plane to give the final HRTEM image There can be Phase contrast, Amplitutde contrast or Diffraction contrast. The contrast occurring in bright field and dark field imaging is diffraction contrast, this occurs due to diffraction on beams at special Bragg angles. Phase contrast occurs whenever multiple beams contribute to the image resulting in formation of fringes