Download STM_TG

Teachers Guide
Scanning Tunneling Microscope
Overview
In this activity, students explore models that help explain how a scanning tunneling
microscope works. They explore how quantum tunneling is used as a method for
“seeing” atoms. The STM is able therefore to determine surface structures of objects
and students are challenged to describe how the distribution of tunneling current is
related to surface structure. The compare the two operational modes of an STM, the
constant-height mode and constant-current mode, as methods for detecting surface
structure. Finally they explore ways to manipulate atoms using and STM tip.
Learning Objectives
Students will be able to:
 Explain how distance between the tipoff an STM and the surface of an atom are
related to the tunneling current.
 Describe how and STM current reveals the surface structure of an object.
 Compare constant-height and constant-current operational modes in an STM.
 Manipulate atoms using a simulated STM.
Student Prerequisite Knowledge
 Familiar with quantum tunneling and how it is represented. (see ET “Quantum
Tunneling” activity).
 Understanding of Electron Wave and the meaning of its probability.
Background Resources
Approximate time for lesson completion: XX minutes
Activity Answer Guide
Page 1:
No questions.
Page 2:
1 At which of the following distances did
you observe the most significant
tunneling current?
(d)
2. Describe the relationship between the
tunneling current and the distance
between the tip and the atom.
The tip needs to be close enough to the
atom to overcome the vacuum gap between
the tip and the surface of the atom. When it
gets close enough the voltage is strong
enough to enable the electron wave to
escape the probe and generate a weak
electrical signal.
Page 3:
1At which of the following locations did
you observe significant tunneling
current?
(a) (d)
Page 5:
1. Try to line up the three blue atoms on
the surface like the way the IBM
scientists did. Take a snapshot to prove
that you have achieved it.
2. Describe how the distribution of the
tunneling current is related to the
surface structure.
When the tip of the STM is over an atom
that is closer to it the tunneling is increased
when it is further away from something it is
decreased.
Page 4:
1. Take a snapshot that shows your STM
result using the constant-height mode.
2. Describe what you needed to do in
order to move the blue atoms.
First I moved the tip close enough to the
blue atom to attract it, I then used the tip to
move towards the other blue atoms, and
used the release button when the blue
atoms were next to each other. I raised the
tip so it was no longer too close to attract
the atoms, moved it along I repeated this
until I got them lined up.
Page 6:
1. Scanning tunneling microscopes can
create an image of the smallest details of
a surface by recording variations in:
(a)
2. Take a snapshot that shows your STM
result using the constant-current mode.
I observed that the electrons could flow as
long as one of the transistors had no
voltage applied to its gate (set to 1). It was
only when both gates had voltage applied
(set to 0) that the electrons could not flow.
2. There are two scanning tunneling tips
in the model on the right. Under one of
them there is an atom on the surface of
the substrate, which is deliberately
hidden. Click the "Run" button below to
start the simulation. Can you tell where
the atom is located? Explain your
answer.
The atom appears to be underneath the tip
on the right. You can see significant
tunneling of the electron wave down into the
yellow substrate.
3 Which of the following are the two
operational modes of the STM? (Check
all that apply.)
(a) (c)
2. Which of the following thing is not
possible with the STM?
(c)
3 It is hard to make tips for STM
microscopes that are as small as singleatom. What would happen if the probe
looked like the image shown to the left?
The recorded current data would be difficult
to interpret because the tips are at different
heights and would interact with the surface
differently.
Further Extensions