Figure 3 shows the main parts of a transmission electron microscope (TEM) - AQA - A-Level Physics - Question 3 - 2017 - Paper 7

To determine the resolution capability, we can use the formula for resolution, which is given by: $d = \frac{\lambda}{2 \sin(\theta)}$ Where:

• $\lambda$ is the wavelength of the electrons.
  • The wavelength is related to the kinetic energy, $E$, of the electrons by: $\lambda = \frac{h}{\sqrt{2me}}$ In this case, we can calculate the wavelength:
  • Using Planck's constant, $h \approx 6.63 \times 10^{-34} \text{ Js}$, and the electron mass, $m \approx 9.11 \times 10^{-31} \text{ kg}$, and substituting $E = 4.1 \times 10^{-16} \text{ J}$:

$\lambda = \frac{6.63 \times 10^{-34}}{\sqrt{2 \times 9.11 \times 10^{-31} \times 4.1 \times 10^{-16}}}$

Calculating this will reveal a wavelength on the order of picometers, which is much smaller than atomic dimensions (~0.1 nm). This means that individual atoms can theoretically be resolved in the TEM.

The image formation in a TEM involves several steps:

1. Electron Source: Electrons are generated by the thermionic emission from a heated filament or via field emission.
2. Acceleration of Electrons: The emitted electrons are accelerated by high voltages toward the specimen.
3. Condenser Lens: These electrons are focused onto a thin sample using condenser lenses, ensuring that the beam is appropriately directed.
4. Interaction with the Sample: As the electrons pass through the sample, they interact with the atomic structure, causing scattering. This provides contrast in the resulting image.
5. Projection Lens: The scattered electrons are then magnified by the projector lens onto a fluorescent screen or a camera, constructing the final image.

Several factors can influence image quality in a TEM:

• Electron Wavelength: The resolution is directly linked to the wavelength of the electrons; shorter wavelengths yield better resolution.
• Lens Aberrations: Imperfections in the optical lenses can distort the image, affecting sharpness and detail.
• Sample Thickness: Thicker samples may lead to greater scattering and loss of detail.
• Alignment of the Instrument: Properly aligned components ensure that the electron beam remains focused and correctly projected.
• Environmental Conditions: Factors such as vibrations and magnetic fields can disrupt the electron paths, leading to blurring or loss of resolution.