TEM

Question 1

What samples is TEM for?

Answer 1

e- of energy 50 keV, much higher than LEED - penetrates deep into the bulk

Question 2

What does TEM stand for?

Answer 2

transmission electron microscope

Question 3

Describe TEM process

Answer 3

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

Question 4

How to interpret TEM images?

Answer 4

darker spots = higher e- density

Question 5

pros and cons of TEM

Answer 5

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

Question 6

what is a STEM?

Answer 6

scanning transmission electron microscope

Question 7

uses for STEM

Answer 7

1) can image interfaces between materials - gives atomic resolution
2) can be used for bio-imaging

Question 8

eq for average energy spacing between energy levels in bonding of polyatomics

Answer 8

average energy spacing = total energy spacing/ no. of levels

Question 9

what is the splitting like for an infinite chain?

Answer 9

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)

Question 10

what does the density of states (DOS) tell us?

Answer 10

how many and the variation of molecular orbital states there are at a given energy within a band

Question 11

why are DOS much higher for d e-?

Answer 11

bc they are more localised on atoms, the DOS of d-states DOES NOT equal zero at the fermi level

Question 12

Describe DOS for TM

Answer 12

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

Question 13

what gives a wider band?

Answer 13

interaction of the d band with the sp band

Question 14

what does the bandwidth depend on?

Answer 14

the number of neighbours, when surface atoms are underbonded, the band is narrower (compared to the bulk)

Question 15

how does the DOS at a surface atom compare to in the bulk?

Answer 15

slightly lower DOS in the bulk than surface, apart from at the lowest E the bulk has the higher DOS

Question 16

describe the DOS in terms of reactivity and occupation of the d band

Answer 16

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

Question 17

describe the DOS in terms of reactivity and occupation of the d band and energy of the d-band

Answer 17

a d-band higher in energy (further right) means its more empty = more reactive

Co is more reactive than Ni

Question 18

describe the DOS in terms of narrowness of the band

Answer 18

a broader band ie. Ti has a broad and almost empty d-band

means tight bonding and strong/hard material

Question 19

describe the band gap properties of insulators

Answer 19

full valence band
large band gap of >3 eV
sigma increases with T

Question 20

describe the band gap properties of semiconductors

Answer 20

full valence band
band gap 0.5 - 3 eV
sigma increases with T

Question 21

describe the band gap properties of conductors (metals)

Answer 21

partially filled band
no band gap
sigma decreases with T

Question 22

which surfaces are dangling bonds on?

Answer 22

insulator/semiconductors

Question 23

describe dangling bonds

Answer 23

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

Question 24

describe the process of how you get electronic structure from experiments?

Answer 24

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

Question 25

What technique do you use to get electronic structure from experiments?

Answer 25

XPS or UPS
but UPS is used more bc surface sensitive bc photons don’t penetrate far into the bulk