TEM Flashcards
What samples is TEM for?
e- of energy 50 keV, much higher than LEED - penetrates deep into the bulk
What does TEM stand for?
transmission electron microscope
Describe TEM process
1) e- beam focussed onto a spot on the sample
2) uses a magnetic lens (e- are fired through)
3) the incident electrons have so much energy they knock the electrons from the sample = secondary electrons, and these are detected
4) the beam or sample is scanned across to see the whole sample
How to interpret TEM images?
darker spots = higher e- density
pros and cons of TEM
pro - can be used for almost any sample as long as e- can get through it
cons - e- are harder to focus than light bc they repel each other
e- are diffraction limited
the e- are much higher energy than in STM so penetrate well into the bulk
what is a STEM?
scanning transmission electron microscope
uses for STEM
1) can image interfaces between materials - gives atomic resolution
2) can be used for bio-imaging
eq for average energy spacing between energy levels in bonding of polyatomics
average energy spacing = total energy spacing/ no. of levels
what is the splitting like for an infinite chain?
splitting betw the top (fully antibonding) and bottom (fully bonding) energy levels increases with more atoms, but but the max is 4ß (when infinite chain)
what does the density of states (DOS) tell us?
how many and the variation of molecular orbital states there are at a given energy within a band
why are DOS much higher for d e-?
bc they are more localised on atoms, the DOS of d-states DOES NOT equal zero at the fermi level
Describe DOS for TM
s e- peak at the lowest and then the highest energy
huge d e- peak right on/after the lowest energy s e- peak
p e- fluctuating along all energies
what gives a wider band?
interaction of the d band with the sp band
what does the bandwidth depend on?
the number of neighbours, when surface atoms are underbonded, the band is narrower (compared to the bulk)
how does the DOS at a surface atom compare to in the bulk?
slightly lower DOS in the bulk than surface, apart from at the lowest E the bulk has the higher DOS
describe the DOS in terms of reactivity and occupation of the d band
Cu has a full d-band so is lower down further away from the Fermi energy.
Ni d-band is unoccupied at Ef so Ni is more reactive than Cu
more unoccupied states at Ef = more reactive
describe the DOS in terms of reactivity and occupation of the d band and energy of the d-band
a d-band higher in energy (further right) means its more empty = more reactive
Co is more reactive than Ni
describe the DOS in terms of narrowness of the band
a broader band ie. Ti has a broad and almost empty d-band
means tight bonding and strong/hard material
describe the band gap properties of insulators
full valence band
large band gap of >3 eV
sigma increases with T
describe the band gap properties of semiconductors
full valence band
band gap 0.5 - 3 eV
sigma increases with T
describe the band gap properties of conductors (metals)
partially filled band
no band gap
sigma decreases with T
which surfaces are dangling bonds on?
insulator/semiconductors
describe dangling bonds
a surface with directional covalent bonds with sp3 bonding e.g. diamond has bonds missing
the states have energies outside of the full valence band, but in the gap
before reconstruction to increase the bonding
describe the process of how you get electronic structure from experiments?
1) UPS photon fired in, e- detected out
2) angle of incidence varied to see how electronic states vary with direction of travel of wave
3) plot intensity vs E of e- is
What technique do you use to get electronic structure from experiments?
XPS or UPS
but UPS is used more bc surface sensitive bc photons don’t penetrate far into the bulk
