Download STM Scanning tunneling microscope History

Molecular spectroscopy and reaction
dynamics
Group III
Sigtryggur Bjarki Sigtryggsson
Styrmir Svavarsson
Úlfar Þór Björnsson Árdal
STM
Scanning tunneling microscope
History
• The STM was invented 1981 by Gerd Binnig and
Heinrich Rohrer.
• They received the Nobel prize in physics in 1986 for the
STM
• The STM was first used in showing its atomic scale
resolution in a image of silicon 7x7 restructed (111)
surface.
Theory
• STM works on the principles of tunneling
• Electrons can not transfer between the tip of the STM
and the sample because there is an energy barrier.
• When a voltage is imposed between the two, the shape
of the energy barrier changes and electrons can move
from the sample to the tip wich results in a small current if
the distance is sufficiently small
The probability distribution is given by:
Ψ 2 𝑧 = 𝑒 −2𝜅𝑧
The current is proportional to:
2
𝐼∝Ψ
Where z is the distance and к is given by:
1
2𝑚(𝑈 − 𝐸) 2
𝜅=
ℎ
Where U is the potential energy, h is the planck‘s
constant and E is the energy of the electron .
• The tip used is usually made out of tungsten or
a palladium-iridium alloy where at the very end
of the tip there is only one atom.
• The positioning of the tip is changed using
array of piezoelectrics.
• Piezoelectric materials change shape when a
electric field is applied on them.
The STM can be operated in two different ways:
Constant current mode or constant voltage mode.
• In constant current mode the voltage is kept
unchanged and the vertical position of the tip is
changed to maintain a constant current while
scanning over the sample.
• In constant voltage mode the vertical position of
the tip is kept constant while the current is
changed by changing the voltage bias (difference)
• The constant voltage mode is faster.
• The constant current mode shows contrast
directrly related to electron charge density
profiles.
AFM
Atomic force microscope
• Contrary to the STM wich can only scan
elictricly conducting surfaces the AFM can scan
all surfaces.
• The AFM measures short distance forces using
a cantilever wich is monitored with laser beam
pointed at the tip of the cantilever as the
schematic picture shows.
• https://www.youtube.com/watch?v=4VW767Qh
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