2. Introduction
Invented by Binnig and Rohrer at IBM in 1981 (Nobel Prize
in Physics in 1986).
Binnig also invented the Atomic Force Microscope(AFM) at
Stanford University in 1986.
4. Introduction
Scanning tunneling microscopy is a microscopical
technique that allows the investigation of electrically
conducting surfaces down to the atomic scale.
Atomic resolution, several orders of magnitude better than
the best electron microscope.
Quantum mechanical tunnel-effect of electron.
Material science, physics, semiconductor science,
metallurgy, electrochemistry, and molecular biology.
5. Working Principle of STM
In the scanning tunneling microscope the sample is
scanned by a very fine metallic tip.
The tip is mechanically connected to the scanner, an
XYZ positioning device realized by means of
piezoelectric materials.
The sample is positively or negatively biased so that a
small current, the "tunneling current" flows if the tip is in
contact to the sample.
6. Working Principle of STM
A sharp conductive tip is brought to within a few
Angstroms of the surface of a conductor (sample).
The surface is applied a bias voltage, Fermi levels shift.
The wave functions of the electrons in the tip overlap
those of the sample surface.
Electrons tunnel from one surface to the other of lower
potential.
The tunneling system can be described as the model of
quantum mechanical electron tunneling between two
infinite, parallel, plane metal surfaces
7.
8. Working Principle of STM
This feeble tunneling current is amplified and measured.
With the help of the tunneling current the feedback
electronic keeps the distance between tip and sample
constant.
If the tunneling current exceeds its preset value, the
distance between tip and sample is decreased, if it falls
below this value, the feedback increases the distance.
The tip is scanned line by line above the sample surface
following the topography of the sample.
9. Experimental methods
the sample you want to study
a sharp tip mounted on a
piezoelectric crystal tube to
be placed in very close
proximity to the sample
a mechanism to control the
location of the tip in the x-y
plane parallel to the sample
surface
a feedback loop to control
the height of the tip above
the sample (the z-axis)
Basic Set-up
10. Tunneling Tips
Cut platinum – iridium wires
Tungsten wire electrochemically etched
Tungsten sharpened with ion milling
Best tips have a point a few
hundred nm wide
In reality is relatively easy to obtain such tips by etching or tearing a thin
metal wire.
Very small changes in the tip-sample separation induce large changes in
the tunneling current.
11. How to operate?
Raster the tip across the
surface, and using the
current as a feedback
signal.
The tip-surface
separation is controlled
to be constant by
keeping the tunneling
current at a constant
value.
The voltage necessary
to keep the tip at a
constant separation is
used to produce a
computer image of the
surface.
12. Two Modes of Scanning
Constant
Height Mode
Constant
Current Mode
Usually, constant current mode is superior.
13. Tunneling Current
The reason for the extreme magnification capabilities of
the STM down to the atomic scale can be found in the
physical behavior of the tunneling current.
The tunneling current flows across the small gap that
separates the tip from the sample,in better approach of
quantum mechanics the electrons are "tunneling" across
the gap.
The tunneling current I has a very important
characteristic: it exhibits an exponentially decay with an
increase of the gap d.
14. I= K*U*e -(k*d) k and K are constants, U is the
tunneling bias
Tip-sample tunneling contact
Exponential behavior of the
tunneling current I with distance d
15. STM Tips
Tunneling current
depends on the distance
between the STM probe
and the sample
Surface
Tunneling current depends on
distance between tip and surface
16. Tunneling Current
• It shows a cross section of a
sample surface with two surface
atoms being replaced by foreign
atoms, for instance adsorbates
(black).
• While at low bias (red) the tip may
follow the "actual" topography,
there may also be a bias where no
contrast is obtained (green) or a
bump is seen above the
adsorbates (blue).
• This bias dependent imaging is
used to create the color images:
three individual STM images of the
same sample area are obtained at
different tunneling bias.
17. Advantages
No damage to the sample
Vertical resolution superior
to SEM
Spectroscopy of individual
atoms
Relatively Low
Cost
Disadvantages
Samples limited to conductors
and semiconductors
Limited Biological Applications:
AFM
Generally a difficult
technique to perform
Figures of Merit
Maximum Field of View: 100 μm
Maximum Lateral
Resolution: 1 Å
Maximum Vertical
Resolution: .1 Å
23. Basic Principles of STM
Electrons tunnel between the tip and sample, a small current I is
generated (10 pA to 1 nA).
I proportional to e-2κd
, I decreases by a factor of 10 when d is
increased by 1 Å.
d ~ 6
Å
Bias voltage:
24. Instrumental Design: Controlling the Tip
Raster scanning
Precise tip control is achieved with
Piezoelectrics
Displacement accurate to ± .05 Å