460 Final, 2007
Section 1: Basic Facts (1-2 paragraph answers and/or a short diagram, 5 pts each)
1. For the transmitted beam g=0, so g.b=g.R=0 – but you can still see contrast from
dislocations. Why?
In kinematical theory, in fact you do not see any contrast at all in the transmitted
beam – which indicates that you have to think about dynamical theory. In 2-beam the
intensity of the transmitted beam is complimentary to the intensity of the diffracted beam.
Hence if g.b is not zero for the relevant diffracted beam, you will see contrast in the
transmitted beam.
2. If the illumination in the microscope is elongated in one direction, what is this
probably due to in terms of the microscope alignment, and how do you fix it?
Three possible sources:
1) Misalignment of the gun, although this would have to be severe.
2) Misalignemnt of the condenser aperture in STEM mode (which we have not
really covered in detail).
3) Astigmatism of the condenser lens
The 3rd case is the 99% probable source.
3. Name three types of electron source that are used in the TEM, listing them in terms of
tolerance to vacuum conditions, source size and cost.
In terms of increasing cost, decreasing tolerance of the vacuum and decreasing
source size (they track):
Hairpin
Pointed filament
LaB6
Thermally assisted field-emmission
Cold Field Emmission
There are other sources, the most common is laser-stimulated emission used for
ultrafast electron microscopy (micro- to pico-second time resolution). In principle
carbon nanotubes could be used, but their lifetime is too short.
4. With 2-beam imaging, what will be the general character of the image of a stacking
fault?
Fringes, straight (unless there are some partial dislocations in the stacking fault)
with a period of 1/seff. A sketch is important. Note: stacking fault tetrahedral are not that
common, I just happen to have some cute pictures of them.
Section 2: Logic (10 pts)
5. The following is taken from Wikepedia, discussing selected area diffraction:
“.. some fraction of the electrons will be scattered to particular angles, determined by
the crystal structure of the sample, while others continue to pass through the sample
without deflection. As a result, the image on the screen of the TEM will be a series of
spots -- the selected area diffraction pattern, SADP, each spot corresponding to a
satisfied diffraction condition of the sample's crystal structure.”
Briefly (one page) comment on how correct this is or not.
Issues with this:
1) Electrons don’t know where they are going until they are detected, so saying
some are deflected and others are not is marginal.
2) Some part the wave is multiply scattered ending up along the transmitted beam
direction.
3) The term “satisfied diffraction condition of the sample’s crystal structure” is
really not particularly accurate. This implies that for every spot Bragg’s law is satisfied
which is veryN wrong.
4) Nothing is mentioned about Kikuchi lines in thicker samples, although it is
reasonable to avoid this in a simple source.
5) Amorphous samples are not included in this definition, and polycrystalline
samples with rings.
This article is an oversimplification. Not too bad, also not the greatest. (Whoever
wrote it should take 460.)
Section 3: Research Strategy - 20 points
The wife of your TA is working with TiO2 powders which she is dip-coating as
thin (several microns thick) coatings onto various supports which are porous on the scale
of several microns. These powders probably contain two different phases of TiO2, and
may also contain impurities. Using TEM, how would you work out for her what she has,
starting from sample preparation.
Note: there is no perfect answer for this question.
First, there are two ways to approach this either from the powder or from a
coated sample. There is a chance that the coated sample is different from the original
material so you will have to look at that. However, the most obvious thing to do is to start
with the powder since sample preparation of this will be simple.
Take some powder dispersed in some (non H2O) solvent and drop it onto a carbon
film. Look in straight BF – powder size etc. DF would not be so easy because you cannot
align to a given zone that easily unless the grains are large, in which case it may be hard
to get an electron beam through.
Take a DP, and index the powder rings. For something like TiO2 you can look up
the possible phases (anatase and rutile), and even get a rough idea of the fraction of each
(x-ray diffraction is more quantitative for this). If the particles are small (20nm) HREM
would be useful – not otherwise as yet.
Take an EDX spectrum from a broad region and look to see what you have – any
signs of impurities.
At this stage you probably are best to go to the coated samples. Depending upon
what the substrate is you may be able to scrape some of the coating off onto a microscope
grid. Alternatively you have to cut a piece and cross-sectionally thin. Note: since the
coating is several microns thick plan-view imaging will not be that useful. Similar to
before, do BF, diffraction and EDX.
There are many more techniques you can use, but this is probably where you
should stop and go and talk the results over with her. In addition, until you know things
like the overall particle size doing Z-contrast, HREM, EELS is not clever. Also, CBED is
not the simplest of techniques, so you don’t want to jump into this. Unless well justified
using these techniques make the answer worse, not better.
Note: Time is money; points will be deducted for grossly inefficient approaches
Section 4: Micrograph - 20 points
For the attached set of four micrographs, with diffraction patterns inset for three,
answer the following questions with a brief justification in each case:
a) Which are bright-field, dark-field or what? 5 pts
The key to this is noting that the bottom left and bottom right images are
approximately complimentary, suggesting that one is BF and the other is DF. Of
the two, the lower right shows higher noise and contrast, which suggests that this
is DF. That makes the other three BF. Note: the contrast from the stackfault ends
is also an indicator that the lower right is DF, lower left BF.
b) Why does the contrast and features visible vary? 5 pts
Slightly obvious: different 2-beam conditions are being used. Features for which
g.b=0 (dislocations) or g.R=0 (stacking faults) vanish.
c) What is the feature immediately below the diffraction pattern? 5 pts
A stacking fault with R normal to the g in the diffraction condition inset.
d) What is the central feature (three wider regions connected by narrower
regions)? 5 pts
In the top left image the fringes in the feature vanish; in the top right the lines
connecting them vanish and in the bottom left one set of lines vanish.
Top left suggests that the region in the center is a stacking fault
Vanishing suggests that these are dislocations. Two dislocations separated by a
stacking fault  partial dislocations.