Fundamentals of Nanoscience I 635 100Name:

Lab 5Electron Beam Specimen Interactions20 points Name:

Equipment: Atom game cards, subatomic particles, dice.

Procedure

In this lab we will simulate an electron beam scanning across a sample. Which phenomenon occurs will be determined randomly by rolling dice. Obtain two atom cards and construct the nucleus and electron configuration for both titanium and aluminum.

The following grid represents our sample:

1
Ti / 2
Ti / 3
Ti / 4
Ti
5
Ti / 6
Ti / 7
Ti / 8
Ti
9
Ti / 10
Ti / 11
Al / 12
Al
13
Al / 14
Al / 15
Al / 16
Al

Secondary electrons Backscattered electrons

SEIBEI

For each of the 16 cellseach person in the group rolls the dice. Use the following chart to determine the event which takes place:

1-3Secondary electron Ti = 2, Al = 1 electrons Outer shell electron removed

4Backscattered electron Ti = 2, Al = 1 electronsIncident electron bounces back

5Characteristic X-ray Inner electron ejected outer electron drops & emits x-ray

6Auger electron Same as above but outer shell electron absorbs X-ray and is emitted

Demonstrate the appropriate action using the paper subatomic particles for each event. Then reset the atom for the next player. When every player has taken a turn move on to the next cell. When you have completed all 16 cells then shade the SE Image and the BE Image following the gray scale for each electron detected.

Questions

  1. If the size of each of the 16 cells were 100 nm across how many Ti atoms would be needed to span the cell? (Hint: The distance between titanium atoms is about .2896 nm)
  1. If the 4x4 image were generated on a screen 30 cm x 30 cm what would be the magnification?
  1. List two ways the image could be improved.
  1. Count the cells which are labeled Ti and those labeled Al. What is the % titanium and % aluminum in the sample?
  1. What is the % titanium and % aluminum in the sample based on your X-ray counts?
  1. How could the X-ray estimate of the % titanium and % aluminum be improved?

Electron Beam Specimen Interactions

An electron beam in an electron microscope can interact with a sample to result in several different phenomena:

Incident electrons: These are the electrons in the electron beam.

Backscattered electrons: These are incident electrons which “bounce off” the nucleus. They are less numerous than the secondary electrons. The more protons an element has the more backscattered electrons will be produced.

Secondary electrons: These electrons are generated from the incident electrons “knocking off” some of the loosely held outer shell electrons from an atom. Each incident electron results in many secondary electrons being generated. Secondary electrons provide a topographical view of the sample. These are the primary electrons used to form images in a scanning electron microscope (SEM).

Transmitted electrons: These are electrons which pass completely through the sample. The sample must be very thin (electron transparent) in order for these electrons to be generated. These electrons are used to form images in a transmission electron microscope (TEM).

Characteristic X-Rays: When an incident electron “knocks out” a tightly held inner shell electron (1s) then an outer shell electron drops down to fill the empty orbital and emits a high energy photon called an X-ray. Since the energy of the orbitals for each element are unique these X-rays can be used to determine which elements are present in the sample. These X-rays are generated throughout a volume of approximately 1 micron in diameter.

Auger electrons: The most complicated phenomenon we will consider is the Auger electron. We begin with an incident electron “knocking out” an inner shell electron. An outer shell electron drops down to fill the vacant orbital. This produces a characteristic X-ray. This X-ray can be absorbed by an outer shell electron which is then emitted. Since the X-ray has a characteristic energy this emitted electron will also have a characteristic energy. Auger electrons can not travel very far without interacting with other electrons so the only Auger electrons which escape from the sample are those generated very near the surface of the sample (up to ~4 nm below the surface)