STM

Question 1

what is correlated motion?

Answer 1

the e- get out of each other’s way

Question 2

what is screening?

Answer 2

the negative charge of the e- compensates for some of the ion charge to reduce the electric field around a positive ion

Question 3

why does a valence e- have favourable interactions with ion cores?

Answer 3

correlated motion and screening

Question 4

what is Ef?

Answer 4

fermi level, the highest E for an e-

Question 5

what is E∞?

Answer 5

vacuum level, the lowest E for an e-

Question 6

what is the highest E for an e-?

Answer 6

Ef

Question 7

What is the lowest E for an e-?

Answer 7

E∞

Question 8

where does an e- feel an average potential?

Answer 8

in the bulk, the energy is lower in the bulk so the e- wants to be in the bulk

Question 9

what is the jellium model?

Answer 9

where the electron goes out through the surface in the z (axis) direction , so a smear ion core charges out in a positive background

Question 10

Describe the jellium model

Answer 10

the electron density has Friedel oscillations at the surface

at first the charge is neutral, then as the electron density passes out of the surface and into the vacuum (tunnelling) , at the top by the surface there is electron deficiency (positive charge), creates a dipole at the surface

at the bottom under the curve outside of the surface there are excess electrons so negative charge

Question 11

why does electron density tunnel in the jellium model?

Answer 11

because the e- have E < Ef which is not enough to enter the surface barrier region unless by tunnelling

Question 12

when is the workfunction smaller?

Answer 12

on an open surface, charge can spread out across the surface, there is less spill out of charge in the z direction, smaller surface dipole, gives smaller wavefunction

Question 13

when is the wavefunction larger?

Answer 13

on a closed surface the atom density is too high, the charge can’t spread out, there is more spill out of charge in the z direction, higher surface dipole, gives larger wavefunction

Question 14

What is the main thing which changes the size of the workfunction?

Answer 14

lower atom density = lower wavefunction,
higher atom density = higher wavefunction

Question 15

Describe an STM

Answer 15

scanning tunnelling microscope (STM)

metal wire scans along the surface, electronics make the wire move up and down, current flows between the surface and tip of the wire, produces a microscope image

Question 16

Why does there have to be a barrier between the surface and the tip when they are made of the same materials? How does this affect the flow of the current?

Answer 16

so the tip doesn’t stick to the surface and the e- are confined on each side of the barrier, as the Ef (barrier) is the same for both,
the current then has to flow from surface to tip or tip to surface,
-> to get through the Ef barrier, tunnelling is needed

Question 17

what is the process of STM?

Answer 17

EITHER
1) a bias voltage is applied (U) between the tip and sample to make the surface neg and move up in energy
2) the tip is then lower in energy so the e- flow from the occupied surface states to the tip, the tip has Ef - eU

OR
1) a reverse bias voltage is applied which gives a positive surface
2) the e- flow from the tip to the unoccupied surface states, the tip has Ef + eU

Question 18

What is “seen” in STM when the e- flow from surface to tip?

Answer 18

a sum (integral) of of occupied surface electronic states in the range of Ef to Ef - eU

Question 19

What is “seen” in STM when the e- flow from tip to surface?

Answer 19

a sum (integral) of of unoccupied surface electronic states in the range of Ef to Ef + eU

Question 20

What is NOT seen in STM?

Answer 20

atoms/molecules aren’t seen directly, instead a sum of electronic states

Question 21

Why is STM tunnelling?

Answer 21

there is a small voltage applied to make the current flow, which is much smaller than the workfunction (barrier), so the e- must be moving by tunnelling

Question 22

what is tunnelling probability affected by?

Answer 22

the barrier width L (tip-surface distance)
the probability rapidly decreases with decreasing L

tip closer to the sample = current increases & tunnelling prob increases
tip further from sample = current v rapidly decreases

Question 23

What is the shadow effect?

Answer 23

when the tip is close to several atoms and there is a shadow zone seen above a certain frequency

Question 24

What is the current dominated by in STM?

Answer 24

by the outermost atom on the tip and the closest atom on the surface

Question 25

what do sharp peaks/states in the graph of density of states indicate?

Answer 25

molecule adsorbed on the surface, these occur at particular energies

Question 26

how do you interpret a STM graph?

Answer 26

tunnelling from the surface samples are below Ef, tunnelling from tip to surface samples above Ef

s electrons have a lower density of states bc they spread out more, d e- have higher

d electrons will be below the Ef energy if a TM

Question 27

how is the tip cleaned?

Answer 27

in controlled experiments it is done v carefully, but often just crash the tip into the surface at a non-interesting part until the tip has only one atom on it, and the it is used for imaging the desired surface part

Question 28

why might an image suddenly jump in the middle?

Answer 28

the tip has changed halfway through the scan (may have picked up an atom from the surface)

Question 29

What might clear, long rows of atoms in an STM image be?

Answer 29

pi-pi stacking

Question 30

advantages of STM?

Answer 30

1) can get atomic resolution - see electronic states
2) can move atoms/molecules into position at liq He temp as the atoms are fixed
3) can induce reactions on a small scale, e.g. dissociation
4) not limited to UHV, can work in air or liq
5) can measure properties e.g. conductance of individual molecules

Question 31

What can affect the appearance of an STM image?

Answer 31

• too high temperature - atoms are too mobile for the tip to scan them and an image to be seen, however decreasing temp might make the surface disordered
• the tip - molecules on the tip might give higher resolution, or not
• how clean the tip is
• coadsorbates with a known structure to compare to the surface atoms in the unknown