460 Final 1, 2013
Note: while “Yes” can sometimes be a reasonable answer, you of course need to provide more than just this.
Section 1: Basic Facts (Explain/Justify with 1-2 paragraph answers and/or a short diagram, 4 pts each)
1. Why do you normally use a microscope at the eucentric height?
It minimizes focus change on tilting
Minimizes translation on tilting (perhaps)
Is often the best height for use of the microscope, although it does not have to be
2. What role does the source size play in how small an illuminated region you can obtain in the microscope?
The illuminated region is a demagnified image of the source. Hence the smaller the source size, the smaller
the illuminated region.
3. Why is it easier to detect low levels of an element using EDS than with EELS (in most cases)?
EDS has a much lower background level so it is easier to detect low levels of an element which could be
buried in the noise with EELS.
4. What are the possible sources of streaks along a particular direction in a diffraction pattern?
Either planes normal to the direction of the streak, or lines also normal. These correspond to the Ewald
sphere cutting either a plane or line respectively
5. Using BF, in some cases voids appear white, in other black. Why?
This depends upon both the diffraction condition and the depth of the voids in the sample – in general their
contrast oscillates with depth, and (in addition) if you change the diffraction conditions what is white can
become black and vica versa.
Section 2: Logic (10 pts)
6. Comment on the following, 2-4 paragraphs total (taken from eHow)
“The scanning transmission electron microscope (STEM) is an advanced transmission electron microscope used
for viewing unstained biological specimens. They are preferred to other types of microscopes, because of the
increased crispness.”
What a wrong quote!
a) STEM is used for a lot more than just biological specimens
b) It does not give increased crispness, indeed it is often less crisp (crispiness is hardly a scientific term)
c) STEM is not really more advanced than TEM
d) STEM has signals that TEM does not (at least easily)
Section 3: Diffraction (10 pts)
7. For an fcc sample with a lattice parameter of 4 Angstroms
a) Sketch the diffraction pattern along [110]
Close to but not exactly hexagonal with (002) and two (111) type
b) What are the angles between the innermost diffraction spots with
respect to the origin, i.e. the angle between (0->g) and (0->g’) where “0”
is the origin, g one diffraction spot and g’ another?
This is (111).(002)/mod(111)mod(002) =
2/sqrt(12)=1/sqrt(3)=cos(theta)=54.7 degrees, which gives 70.5 degrees
for the other angle.
c) With an electron wavelength of 0.025 Angstroms, how much would
you need to tilt off the zone axis so that (008) is in a 2-beam condition?
You can exploit the Braggs-law relationship (!), sin(theta)=lambda*g, g=8/4=2 A-1
2/gSin(theta)=lambda ; Sin(theta)=theta=lambda=0.025=25mRad
Section 4: Research Strategy - 20 points
8. A student wants to image Pd particles ranging from single atoms to 1nm clusters on silica overcoated with
TiO2. What advice to you have for him on how to prepare the samples, and what type of imaging should he
attempt? (Yes, this is all the information he proves initially, but you can ask for more.)
This requires thought. To image the sample you want to use either or both HREM & Z-contrast. However, you
need to have a thin sample in order to be able to see the Pd; if the sample has (for instance) 50nm thick SiO2
then everything is impossible.
The critical question is what sort of silica he has. If it is very small grains then crushing (gently) in some solvent
should work. If it is a macroscopic piece then he has to somehow thin it and hope that he does not destroy
everything. If he has made the wrong sample then everything becomes impossible – this is a case where the
sample should be designed to make the microscopy work.
Of course he should start with the obvious, BF, but then unless he finds something unexpected (e.g. 100nm
particles) he should move rather quickly to either or both HREM & Z-contrast. There is no point in doing EDS
or EELS except perhaps later on if more information is needed.
Section 5: Micrograph - 20 points
9. For the attached micrograph, explaining/justifying briefly your answers
1) BF, DF, Diffraction?
DF, nice vacuum region above the sample
2) What is the difference between a) and b)?
In a) the excitation error is large (sz), i.e. it is weak-beam. Hence the features are sharper.
3) What is the line running across as arrowed
Since the thickness fringes and dislocations run across it, this cannot be a grain boundary (except perhaps a very
low angle one). This suggests that this is one or more stacking fault or similar; there are other types of planar
defects (e.g. antiphase boundaries) which look similar. It is certainly not a twin (because the thickness fringes
go across)
4) What are the features at A and the near vertical lines (e.g. B)?
At A they are thickness fringes
At B those are dislocations running in the foil, i.e. not really changing their depth.
Note: the sample is GaN and the line across 3) is probably due to a deliberate change in the growth conditions
to try and stop threading dislocations.