3.12.2.6 Electron Microscopes

Question 1

What is resolving power?

Answer 1

A microscope’s ability to distinguish structures which are close to each other

Question 2

Why do electron microscopes have a much higher resolving power than light microscopes?

Answer 2

The wavelength of an electron beam is much smaller than that of light

Question 3

How is the wavelength of electrons related to the resolving power of the microscope?

Answer 3

As the wavelength of the electrons decreases the resolving power of the microscope increases

Question 4

How are images formed in a TEM?

Answer 4

• An electron beam produced by thermionic emission is accelerated using an anode
• The electrons are then focused by magnetic lenses, so that any electrons at the edge are deflected towards the centre
• The electrons are then focused on a sample by a condenser lens and pass though the sample
• The projector lens creates the final image on a fluorescent screen

Question 5

Why is the sample used in a TEM very thin?

Answer 5

So that the electrons passing through do not slow down and their wavelength remains unchanged

Question 6

What does the condenser lens do in a TEM?

Answer 6

First lens - deflects the electrons so that they form a wide parallel beam

Question 7

What does the objective lens do in a TEM?

Answer 7

Forms an image of the sample

Question 8

What does the projector lens do in a TEM?

Answer 8

Magnifies the image made by the objective lens and projects it onto the fluorescent screen

Question 9

How does increasing the accelerating voltage of the electron gun increase the resolving power of the microscope?

Answer 9

The speed of the electrons increases, decreasing their wavelength

Question 10

What are the 2 ways in which the resolving power of a TEM is limited by?

Answer 10

Sample thickness - as electrons pass through a thick sample they slow down, increasing their wavelength and decreasing the resolving power

Electrons travelling at different speeds - some electrons may lose kinetic energy after being emitted through thermionic emission from the electron gun due to collisions, leading to them travelling at different speeds, having different wavelengths and therefore being diffracted by different amounts, blurring the image (aberration)

Question 11

What is aberration?

Answer 11

Electrons travelling at different speeds causing blurring of the image as they have different wavelengths and are therefore diffracted by different amounts

Question 12

What will the diameter of the de Broglie wavelength of the electron beam have to be in order to resolve details around the size of an atom?

Answer 12

Around 0.1nm (the diameter of an atom)

Question 13

How does an STM form an image?

Answer 13

Quantum tunnelling of electrons - a fine tipped probe at a constant potential moves across the surface of a material to create a tunnelling current between the probe and the surface so electrons cross the gap from positive to negative

Question 14

What does STM stand for?

Answer 14

Scanning tunnelling microscope

Question 15

What does TEM stand for?

Answer 15

Transmission electron microscope

Question 16

How does quantum tunnelling occur?

Answer 16

Due to the wave nature of electrons, if the barrier/gap they are trying to cross is small enough, they can move across just like light waves would be able to (the smaller the barrier/gap the more likely it is that tunnelling will occur)

Question 17

Why is the probe/tip of an STM kept at a constant potential?

Answer 17

So that electrons can only travel in one direction

Question 18

What is the tunnelling current?

Answer 18

The movement of electrons across the gap between the tip of the probe and the surface of the object

Question 19

What is the relationship between the size of the gap between the probe and the surface, and the tunnelling current?

Answer 19

As the gap becomes larger, tunnelling current decreases
As the gap becomes smaller, tunnelling current increases

Question 20

What are the 2 ways an STM can operate?

Answer 20

Constant height mode - probe is kept at a constant height as it moves across the surface, tunnelling current is recorded and used to image the surface of the object

Constant current mode - current is constantly monitored and fed back to the microscope allowing it to adjust the probe height so the current is kept constant, movement of probe is used to image the surface