Stm 07.08.13

Scanning Tunneling Microscopy
09TT20 CHARACTERIZATION OF TEXTILE
POLYMERS
Presentation I
S.Dhandapani – 11MT62

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.

Introduction
 Topographic (real space) images
 Spectroscopic (electronic structure, density of states)
images

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.

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.

 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

 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.

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

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.

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.

Two Modes of Scanning
Constant
Height Mode
Constant
Current Mode
Usually, constant current mode is superior.

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.

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

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

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.

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

Interesting Images with STM
Copper Surface
Xenon on Nickel
Single atom
lithography

Iron on Copper
Quantum Corrals
Imaging the standing wave created by interaction of species

Carbon Monoxide Man: CO on Platinum

Graphite is a good example!
 STM images of graphite
 Structure of graphite
 Overlay of structure shows only
every other atom is imaged

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:

Instrumental Design: Controlling the Tip
Raster scanning
Precise tip control is achieved with
Piezoelectrics
Displacement accurate to ± .05 Å

Stm 07.08.13

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Stm 07.08.13