Electron Microscopes Flashcards
(17 cards)
What is meant by the resolving power of a microscope?
Resolving power is the microscope’s ability to distinguish between two structures that are close together.
Why do electron microscopes have a higher resolving power than light microscopes?
Electrons have a much smaller wavelength than visible light, allowing electron microscopes to resolve much smaller structures.
How does electron wavelength affect resolving power?
As electron wavelength decreases, the resolving power increases.
What are the two types of electron microscope?
Transmission Electron Microscope (TEM) and Scanning Tunnelling Microscope (STM).
How does a TEM work?
Electrons are accelerated by an electron gun, pass through a set of magnetic lenses, travel through an extremely thin sample, and the image is projected onto a fluorescent screen.
What is the purpose of the condenser lens in a TEM?
It forms a wide, parallel electron beam and directs it at the sample.
What does the objective lens do in a TEM?
It forms the initial image of the sample.
What is the function of the projector lens in a TEM?
It magnifies the image from the objective lens and projects it onto the screen.
What happens when you increase the accelerating voltage in a TEM?
Electron speed increases, wavelength decreases, and resolving power increases.
What are the two limitations of TEM resolving power?
Sample thickness slows electrons, increasing their wavelength; electron speed variation due to energy loss during thermionic emission causes aberration (image blurring).
What must the de Broglie wavelength of electrons be to resolve atomic structures?
Approximately 0.1 nm (the diameter of an atom).
How do you calculate the required accelerating voltage for a given de Broglie wavelength?
Use: λ = h / (2meV) ⇒ V = (h^2) / (2meλ^2)
Example: For λ = 1.00 × 10−10 m, V ≈ 150.8 V.
What principle allows an STM to work?
Quantum tunnelling, which occurs due to the wave nature of electrons – electrons can pass through tiny gaps even if they don’t have enough energy to overcome them classically.
What is the structure of an STM?
A very fine probe tip that moves across a surface at a constant potential; measures the tunnelling current.
What happens to the tunnelling current as the probe moves?
Smaller gap → Higher tunnelling current; Larger gap → Lower tunnelling current.
What are the two operating modes of an STM?
Constant height mode – probe height is fixed, current is recorded to map the surface; Constant current mode – current is kept constant by adjusting height; height data used to image surface.
Why does current only flow in one direction in an STM?
Because the probe is kept at a constant potential (positive or negative), ensuring unidirectional electron flow.
