STM Flashcards
what is correlated motion?
the e- get out of each other’s way
what is screening?
the negative charge of the e- compensates for some of the ion charge to reduce the electric field around a positive ion
why does a valence e- have favourable interactions with ion cores?
correlated motion and screening
what is Ef?
fermi level, the highest E for an e-
what is E∞?
vacuum level, the lowest E for an e-
what is the highest E for an e-?
Ef
What is the lowest E for an e-?
E∞
where does an e- feel an average potential?
in the bulk, the energy is lower in the bulk so the e- wants to be in the bulk
what is the jellium model?
where the electron goes out through the surface in the z (axis) direction , so a smear ion core charges out in a positive background
Describe the jellium model
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
why does electron density tunnel in the jellium model?
because the e- have E < Ef which is not enough to enter the surface barrier region unless by tunnelling
when is the workfunction smaller?
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
when is the wavefunction larger?
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
What is the main thing which changes the size of the workfunction?
lower atom density = lower wavefunction,
higher atom density = higher wavefunction
Describe an STM
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
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?
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
what is the process of STM?
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
What is “seen” in STM when the e- flow from surface to tip?
a sum (integral) of of occupied surface electronic states in the range of Ef to Ef - eU
What is “seen” in STM when the e- flow from tip to surface?
a sum (integral) of of unoccupied surface electronic states in the range of Ef to Ef + eU
What is NOT seen in STM?
atoms/molecules aren’t seen directly, instead a sum of electronic states
Why is STM tunnelling?
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
what is tunnelling probability affected by?
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
What is the shadow effect?
when the tip is close to several atoms and there is a shadow zone seen above a certain frequency
What is the current dominated by in STM?
by the outermost atom on the tip and the closest atom on the surface
what do sharp peaks/states in the graph of density of states indicate?
molecule adsorbed on the surface, these occur at particular energies
how do you interpret a STM graph?
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
how is the tip cleaned?
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
why might an image suddenly jump in the middle?
the tip has changed halfway through the scan (may have picked up an atom from the surface)
What might clear, long rows of atoms in an STM image be?
pi-pi stacking
advantages of STM?
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
What can affect the appearance of an STM image?
- 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
