STM

Question 1

SPM Pros + Cons

Answer 1

Pros:

• not limited by diffraction limit
• capable of atomic resolution
• very little sample prep
• ambient conditions
• 3D data set
• mechanical, chemical, electrical nanosurface characterisation all possible

Cons

• Slow scan speeds
• Small scan size
• resolution limited to tip size
• Tip can give rise to artefacts

Question 2

SPM vs SEM

Answer 2

SEM is better for depth of field for rougher sample and gives better chemical analysis
SPM is better for flatter and bio samples

Question 3

Resolution STM

Answer 3

0. 1 nm laterally

0. 1nm depth

Question 4

How does it work? STM

Answer 4

1. Bias applied between sample and tip
2. Tip rastered along surface
3. Tunneling current measured
4. Signal sent to control
5. Feedback loop sent to piezoelectric controller controlling tip

Question 5

Key components of STM

Answer 5

tip
piezoelectric controlling tip
controller/computer

Question 6

How to prepare tips for STM

Answer 6

pt-Ir / W wire - cutting and electrochemical etching using NaOH

Question 7

What does STM produce

Answer 7

map of electronic density of sample - interpreted as atomic structure

Question 8

What does tunneling depend on

Answer 8

distance, conductivity and bias between tip and sample

length and height of potential barrier

Question 9

possible causes of change in line profile

Answer 9

1. change in electron density e.g. area of different material
2. change in topography

Question 10

Sample requirements

Answer 10

flat and electrically conductive

Question 11

Environment requirements

Answer 11

quiet, free of vibration, can be done at range of pressures

Question 12

Operation modes of STM

Answer 12

1. Constant height -
   changes in tunnelling density recorded
   usually only preformed on small samples at low T due to high risk of tip collision
2. Constant current
   Tunnelling value kept constant and change in height measured

Question 13

what is voltage dependent imaging

Answer 13

image obtained is very dependent on bias between sample and tip

Question 14

what is STS

Answer 14

Scanning Tunnelling Spectroscopy
Measures I-V curves locally at defined locations. There is a fixed tip-sample prep.
Can start to probe homo-lumo gaps

Question 15

Applications

Answer 15

1. surface reconstructions
   surfaces are energetically unfavourable due to have dangling bonds
   some surfaces spontaneously reconstruct reducing number of said bonds and reducing energy of surface
   STM can resolve those reconstructions (eludes diffraction based approaches - TEM can’t resolve)
2. metal on metal deposition
   STM tip can be used to manipulate atoms on surface
   tunneling current can be adjusted to ‘pick up’ individual atoms and move them to new positions in surface

Question 16

Methods of metal on metal deposition

Answer 16

alloying/thin film growth - layer by layer

island growth