AFM

Question 1

What does AFM detect?

Answer 1

Force (not current like STM)

Question 2

Does the cantilever touch the surface?

Answer 2

Yes

Question 3

Does the cantilever have to be made from a conducting material?

Answer 3

No

Question 4

When is the shape of the tip important and why?

Answer 4

When a sample has a large variation in height as the end of the tip may not touch the surface if the features are too steep, which would give rise to artefacts.

Question 5

What is the flexibility/rigidity of cantilevers characterised by and what factors affect it?

Answer 5

Force constant k, which increases with thickness and decreases with length.

Question 6

What 2 shapes do cantilevers come in and what is each one better for?

Answer 6

V-shaped/triangular: better for imaging surface topography
Rectangular: has more rotational freedom so better for probing lateral forces.

Question 7

How does the cantilever move along the surface without missing the surface or crashing into features?

Answer 7

Moves in response to interatomic forces: If tip near surface as it approaches/moving away from surface, repulsive force pushes it back but if tip v close to it, strong attractive force helps keep it close and follow the surface well.

Question 8

What are the 3 modes of AFM operation? Give details of each.

Answer 8

Contact Mode, Non-contact Mode, Tapping Mode

See notes for details

Question 9

Explain 3 further variations in AFM imaging

Answer 9

See notes

Question 10

Give 5 advantages of AFM

Answer 10

• 3D surface profile
• Don’t need conductive samples
• Samples don’t need any special treatments prior to scanning which may damage the sample
• AFM works well in ambient conditions at an air or liquid environment, therefore can study biological macromolecules and living organisms.
• High res images

Question 11

Give 5 disadvantages of AFM

Answer 11

• has a max height it can image
• slow scanning speed and this can cause drifting in the image so not ideal for measuring topographic distances
• Tips can break easily
• can have image artefacts from unsuitable tip, poor operating environment or from sample.
• Steep walls or overhangs cannot be imaged easily due to cantilever shapes.

Question 12

Give 4 benefits of using carbon nanotubes as AFM probes.

Answer 12

• provide high-aspect ratio
• they buckle elastically (therefore safe from crashes and no tip wear)
• mechanically stable
• good for chemical functionalisation

Question 13

Give 2 ways to make CNT AFM probes

Answer 13

1. Can grow CNTs directly on probe by: Depositing nanometre-sized metal catalyst particles, and then perform chemical vapour deposition.
2. Just pick up CNTs w/ probe and let them stick to the tip.