Transmission Electron Microscope (TEM)

Definition:

• Slide projector - equivalent

• A projector shines a beam of light through (transmits) the slide, as the light passes throughit is affected by the structures and objects on the slide

• TEM, same way, except that they shine a beam of electrons (like the light) through thespecimen (like the slide)

• Whatever part is transmitted is projected onto a phosphor screen

Working of TEM:

• Electron virtual source electron gun

• Condenser lens 1 − Spot size knob

• Condenser lens 2 − Intensity or brightness knob

• Condenser aperture

• Beam strikes specimen and part of it is transmitted

• Transmitted portion is focused by objective lens into an image

• Image through projector lenses, enlarged

• Phosphor image screen and light is generating a image using light

Figure 1: Working concept of TEM.

Specimen interactions and utilization:

1. Unscattered Electrons:

Formation:

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• Incident electrons transmitted through thin specimen

• Without any interaction occurring outside specimen

Utilization:

• Transmitted intensity proportional to specimen thickness

• Thickness of sample at some points will make them darker due to less unscattered electrons

• Thin areas appear brigher

2. Elastically scattered electrons:

Formation:

• Incident electrons scattered by atoms in specimen

• Elastic scattering, without any loss of energy

• Then transmitted through specimen

Utilization:

• Condition of Bragg’s law is satisfied

• Atomic layers act as diffracting grating, magnetic lenses

• Forms spots, where each spot corresponding to a specific atomic spacing

3. Inelastically scattered electrons:

Formation:

• Incident electrons scattered by atoms in specimen through interaction

• Inelastic scattering, with loss of energy

• Then transmitted through specimen

Utilization:

a. Electron Energy Loss Spectroscopy

• Inelastic loss of energy by incident electrons

• Characteristic of interaction with specific elements

• Energies unique to each bonding state of each element

• Information regarding compositional and bonding information on specific region of speci-men

b. Kakuchi Bands

• Bands of alternating light and dark lines

• Formed by inelastic scattering interactions

• Directly related to atomic spacing in specimen

• These bands, either measured or traced

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Chapter 4 Ver 1.0 Oct 2015.