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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.
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
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.
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
SurfaceTunneling 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 NickelSingle 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
Thank you
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
