Turning Points - Electron Microscopes

Question 1

Why are electron microscopes preferred over optical microscopes?

Answer 1

Electron microscopes get better resolution than normal microscopes because they can interact with (be diffracted by) much smaller objects. This is because their deBroglie wavelength is so small.

Question 2

What is the basic principle of operation for electron microscopes?

Answer 2

Electron microscopes fire electrons at a sample and see how they react with it

Question 3

How is the DeBroglie wavelength of the electrons changed?

Answer 3

By changing the anode potential that the electrons are accelerated by

Question 4

How can you find the anode potential for resolving detail around an atom?

Answer 4

use the equation lambda = h/sqrt(2meV), solve for V

The lambda value is 0.1nm or 1x10^/-10 m

Question 5

Explain the principles of operation for an Transmission Electron Microscope (TEM)

Answer 5

• Electrons are accelerated through a positive electric potential (such as an electron gun)
• The magnetic field of the condenser lens focuses the electrons into a thin, straight beam that passes through the thin sample
• The structure of the sample causes electrons to diffract
• The magnetic fields of the objective and magnifier lens deflect the electrons so that they eventually form a magnified image on a fluorescent screen
• The objective lens forms the image, whilst the magnifier lens magnifies the image.
• The smaller the DeBroglie wavelength, the better the resolution (i.e. the faster the electrons move)

Question 6

Why should a thin sample be used?

Answer 6

The thickness of the sample impacts the wavelength of the electrons. As electrons pass through the sample they will slow down, causing their wavelength to increase and so the resolving power is decreased

Question 7

What is Quantum Tunnelling?

Answer 7

Quantum Tunnelling is a phenomena that occurs due to the wave nature of electrons. Meaning that if the barrier they are trying to cross (which could be physical or potential) is small enough electrons can move across it just like light waves would be able to.

The smaller the barrier/gap, the more likely it is that tunnelling will occur.

Question 8

Describe the principles of operation of a Scanning Tunnelling Microscope (STM)

Answer 8

• A very fine probe is placed very close (around 1nm) to the surface of the sample
• A high voltage is applied between the probe and the surface, making the probe negatively charged in relation to the sample
• Electrons tunnel from the probe to the surface, resulting in a weak electric current
• The smaller the distance between the probe and the surface, the greater the current
• By scanning the probe over the surface and measuring the current, a 3D image of the surface of the sample is produced

Question 9

What are the two ways an STM can operate?

Answer 9

1. Constant distance mode: keep the probe at the same height and measure the changes in the current.
2. Constant current mode: Keep the current the same by adjusting the probe height

Question 10

How is the position of the probe adjusted?

Answer 10

The position of the probe is controlled by 3 piezoelectric transducers made from materials which experience a tiny change in length when a p.d. is applied to them.

This tiny change in length allows the probe to be moved by tiny distances when scanning the sample and altering the distance of the probe from the sample.

Question 11

Why should the electrons in the electron gun beam have the same speed?

Answer 11

Electrons travelling at different speeds have different deBroglie wavelengths so their resolutions will be reduced