Halstone's electron-microscope lab

This lab uses TEMs (tunneling electron microscopes)

Can't use glass lenses to focus electron beams; have to use magnetic lenses; this *is* a tradeoff

X-ray microscopes don't have the quality we want yet, gamma-ray microscopes are still atleast a few years down the road

Mosely's law - 1914: hit something w/ electrons and produce X-rays

* Radiation issues inherent
* Depending on the energy level of the X-rays, you can tell what element emitted it


Electron gun fires a beam down the main column
Tungsten filament is excited and

200,000 volts
200,000 kilo-electron volts

Moving at ~ 1/3 the speed of light, need to use Relativity-based equations

Specimen in center of column, on the end of a handle

Whole column has to be under vacuum

One magnetic lens controls spot size of beam; there are several other lenses aswell

Objective lens nearest the bottom of the column; it's what actually produces the image.

^ How do you *see* electrons?
At bottom of column, a screen covered with zinc sulfide...Electrons put some part(s) of that into an excited state, and they flouresce green.

Green-ness caught by film

X-ray detectors chilled to liquid-nitrogen temperatures.
This can be used to analyze what elements ar ein each section of the sample

Specimens extremely small (~3mm diameter, and very thin (<200nm, even thinner for heavy elements))

What is 2,000,000x magnification? A person magnified this much would be two thousand miles tall?
What is a nanometer versus a meter? Size of a golf ball versions the size of the Earth

Electron microscopes very sensitive. Even sound waves form talking, etc. can cause the image to 'jump' a bit

Electron diffraction - electron beam scattered by hitting a crystalline structure. Depending on how they scatter, you can determine what the crystal structure is. Often a ring pattern of some sort.

Lattice planes in asbestos are 9 angstroms (.9 nm) apart

Easy to detect dead space in a crystalline structure vs. the location of the atoms themselves

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*Scanning electron microscope*

Shorter column
20k eV up to 30k or 40k eV

Secondary electrons kicked off by the sample itself


Can vary distance between sample and electron beam

General good microscope idea: start at low magnification to get an overview, then zoom in

A high frame rate leads to more noise "static" (spend less time scanning a particular pixel)

Low frame rate - less "noise", but takes longer to produce the image

Low voltage; less penetration; should mean more surface detail [and vice versa]

SEM useful for building a picture of where exactly atoms of element X are located throughout the sample

H, He, Li, and Be can't be detected via this X-ray detector equipment

astigmatism: vertical image in separate focus from the horizontal image

Spehrical and chromatic aberation you need different equipment to correct
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Halstone uses a srtaightforward same (copper+aluminum disk) to calibrate the settings

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The material you used to prepare the specimen (gold coat, etc), can screw with the view