TEM and STM

Question 1

what does the condenser lens do

Answer 1

• magnetic field deflects electrons into a wide beam travelleing parallel to axis of microscope
• electron beam is uniformly incident on the sample

Question 2

what eoes objective lens do

Answer 2

• forms image of sample
  -deflects outer electrons towards central axis
• electrons travelling along microscopes axis are not deflected

Question 3

what does protector lens do

Answer 3

• causes beams from objective lens to spread out, magnifying the image
• directed onto fluorescent screen emitting light

Question 4

what are three drawbacks of TEM

Answer 4

1. electrons must pass through sample
   ( reduces speed, increases wavelength)
2. electrons in beam have a range of speeds

( different deflections in B field )

Question 5

how does constant height mode work STM

Answer 5

• tip doesnt move vertically up or down while scanning areas
• changes in surface increase/decrease gap size
• tunnelling current varies
• used to produce an image

Question 6

how does constant current mode work in STM

Answer 6

• when change in TC detected. tip moves up or down to keep current constant
• gap size always the same, maps an image of the surface