How a Transmission Electron Microscope (TEM) Works and What Affects Image Quality

A Transmission Electron Microscope (TEM) is a powerful instrument used to study extremely small structures, even down to the level of atoms. It works by passing a beam of electrons through a very thin sample and forming an image using electromagnetic lenses.

To fully understand how a TEM works, we need to look at two key aspects:

  1. How the image is formed
  2. What affects the quality and level of detail in the image

1. Process of Image Formation

1.1 Behaviour of electrons in electromagnetic lenses

Electromagnetic lenses in a TEM use magnetic fields to control electron trajectories.

  • Electrons travelling through the centre of the lens:
    • Experience no sideways magnetic force
    • Travel straight without being deflected
  • Electrons travelling away from the centre (near the edges):
    • Experience a magnetic force
    • Are bent towards the central axis

This bending is what allows the lens to focus the electron beam, similar to how a glass lens focuses light.

This focusing effect is not unique to one lens. It is the basic principle of all electromagnetic lenses in the TEM:

  • Condenser lens
  • Objective lens
  • Projector lens

All of them use this same idea:
Electrons are bent toward the axis to achieve focusing

1.2 Role of the condenser lens

The condenser lens prepares the electron beam before it reaches the sample.

Its job is to:

  • Produce a wide beam
  • Make the electrons parallel
  • Ensure the beam is uniform across the sample

This is important because:

  • The whole sample is illuminated evenly
  • Any brightness variation in the image is due to the sample itself, not uneven illumination

1.3 Interaction with the sample

The sample must be extremely thin so electrons can pass through it.

As electrons pass through:

  • Some go straight through (unscattered)
  • Some are deflected slightly
  • Some are scattered strongly

These differences carry information about:

  • Density
  • Thickness
  • Atomic arrangement

1.4 Formation of the image by the objective lens

The objective lens is the most important lens in the TEM.

It takes the electrons emerging from the sample and:

  • Refocuses them
  • Forms the first real image

At first, it may seem like bringing electrons together would destroy information, especially since scattered electrons travel in different directions.

But this is not what happens.

The lens does not mix all electrons into one point.
Instead, it performs a precise mapping:

Electrons leaving different points of the sample are brought to different points in the image.

Understanding this with an analogy

Think of a camera lens:

  • Light from one point spreads out
  • The lens collects it and refocuses it to one point in the image

At the same time:

  • Light from different points → forms different image points

So the structure is preserved.

1.5 Magnification by the projector lens

The image formed by the objective lens is very small.

The projector lens:

  • Magnifies this image further
  • Projects it onto:
    • A fluorescent screen (which glows when hit by electrons), or
    • A digital detector

Final structure of image formation:

  • Condenser lens → prepares beam
  • Sample → modifies beam
  • Objective lens → forms image
  • Projector lens → magnifies image

2. Factors Affecting Image Quality and Detail

2.1 Electron wavelength and speed

Electrons behave as waves with a wavelength given by the de Broglie wavelength:

λ=hp\lambda = \frac{h}{p}λ=ph​

  • Faster electrons → higher momentum → smaller wavelength

2.2 Why smaller wavelength improves detail

  • Large wavelength → strong diffraction → more spreading
  • Small wavelength → less diffraction → sharper image

Therefore:

Smaller wavelength → greater resolution → more detail visible

This is why TEMs use very high accelerating voltages.

2.3 Speed distribution of electrons

Electrons are emitted from a heated cathode.

  • This is a thermal process
  • Electrons have a range of speeds, not all identical

So the beam is not perfectly uniform.

2.4 Aberration due to speed differences

Electromagnetic lenses focus electrons depending on their speed.

  • Faster electrons → bend less
  • Slower electrons → bend more

As a result:

  • They do not all meet at the same point
  • The image becomes blurred

This effect is called, Chromatic aberration.

2.5 Effect of sample thickness

As electrons pass through the sample:

  • They lose energy
  • Their speed decreases

Therefore:

  • Momentum decreases
  • Wavelength increases

Consequences:

  • Larger wavelength → more diffraction → poorer resolution
  • Thicker samples → more scattering → more blurring

So:

Thin samples are essential for high-quality TEM images.

Final Summary

A TEM forms images by:

  • Producing a beam of electrons
  • Focusing it using electromagnetic lenses
  • Passing it through a thin sample
  • Using differences in scattering to create contrast
  • Magnifying the resulting image