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?
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?
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.
4. With 2-beam imaging, what will be the general character of the image of a stacking
fault?
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.
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: 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
b) Why does the contrast and features visible vary? 5 pts
c) What is the feature immediately below the diffraction pattern? 5 pts
d) What is the central feature (three wider regions connected by narrower
regions)? 5 pts