Scanning Tunneling Microscopy
09TT20 CHARACTERIZATION OF TEXTILE
S.Dhandapani – 11MT62
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.
Topographic (real space) images
Spectroscopic (electronic structure, density of states)
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.
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
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.
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
The tunneling system can be described as the model of
quantum mechanical electron tunneling between two
infinite, parallel, plane metal surfaces
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
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.
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
a feedback loop to control
the height of the tip above
the sample (the z-axis)
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
Very small changes in the tip-sample separation induce large changes in
the tunneling current.
How to operate?
Raster the tip across the
surface, and using the
current as a feedback
separation is controlled
to be constant by
keeping the tunneling
current at a constant
The voltage necessary
to keep the tip at a
constant separation is
used to produce a
computer image of the
Two Modes of Scanning
Usually, constant current mode is superior.
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 tunneling current I has a very important
characteristic: it exhibits an exponentially decay with an
increase of the gap d.
I= K*U*e -(k*d) k and K are constants, U is the
Tip-sample tunneling contact
Exponential behavior of the
tunneling current I with distance d
depends on the distance
between the STM probe
and the sample
Tunneling current depends on
distance between tip and surface
• It shows a cross section of a
sample surface with two surface
atoms being replaced by foreign
atoms, for instance adsorbates
• 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
• 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.
No damage to the sample
Vertical resolution superior
Spectroscopy of individual
Samples limited to conductors
Limited Biological Applications:
Generally a difficult
technique to perform
Figures of Merit
Maximum Field of View: 100 μm
Resolution: 1 Å
Resolution: .1 Å
Interesting Images with STM
Xenon on Nickel
Iron on Copper
Imaging the standing wave created by interaction of species
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
Instrumental Design: Controlling the Tip
Precise tip control is achieved with
Displacement accurate to ± .05 Å
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