Transcript Scanning Tunneling Microscopy

Scanning Tunneling
Microscopy
By Lucas Carlson
Reed College
March 2004
Image from an STM
Iron atoms on the surface of Cu(111)
The Scanning Tunneling Microscope (STM)
The STM is an electron microscope that
uses a single atom tip to attain atomic resolution.
History
The scanning tunneling microscope was
developed at IBM Zürich in 1981 by Gerd
Binning and Heinrich Rohrer who shared the
Nobel Prize for physics in 1986 because of
the microscope.
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Gerd Binning
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Heinrich Rohrer
General Overview
An extremely fine conducting probe is held
about an atom’s diameter from the sample.
Electrons tunnel between the surface and the tip,
producing an electrical signal.
While it slowly scans across the surface,
the tip is raised and lowered in order to keep
the signal constant and maintain the distance.
This enables it to follow even the smallest
details of the surface it is scanning.
The Tip
150x Magnification
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As we will see later, is very important that the
tip of the probe be a single atom.
Tungsten is commonly used because you can use
Electro-chemical etching techniques to create
very sharp tips like the one above.
Quantum Tunneling
Classical
Wave Function
For Finite Square
Well Potential
Where E<V
Classically, when an object hits a potential that
it doesn’t have enough energy to pass, it will
never go though that potential wall, it always
bounces back.
In English, if you throw a ball at a wall, it will
bounce back at you.
Quantum Tunneling
Quantum
Wave Function
For Finite Square
Well Potential
Where E<V
In quantum mechanics when a particle hits a
potential that it doesn’t have enough energy
to pass, when inside the square well, the wave
function dies off exponentially.
If the well is short enough, there will be a noticeable
probability of finding the particle on the other side.
Quantum Tunneling
The finite square well potential is a good
approximation for looking at electrons on conducting
slabs with a gap between them.
Quantum Tunneling
More graphs of tunneling:
n(r) is the
probability of
finding an electron
V(r) is the potential
An electron tunneling from atom to atom:
Quantum Tunneling
Now looking more in depth at the case of tunneling
from one metal to another. EF represents the Fermi
energy. Creating a voltage drop between the two
metals allows current.
Sample
Tip
Quantum Tunneling
Through a barrier, quantum mechanics predicts that the
wave function dies off exponentially:
So the probability of finding an electron after a barrier of
width d is:
And:
Where f(V) is the Fermi function, which contains a weighted
joint local density of states. This a material property obtained
by measurements.
Quantum Tunneling
Where:
Plugging in typical values for m, d, and phi (where
phi is the average work function of the tip and the
sample), when d changes by 1 Å, the current
changes by a factor of about 10!
Quantum Tunneling
So if you bring the tip close enough to the surface,
you can create a tunneling current,
even though there is a break in the circuit.
The size of the gap in practice is on the order
of a couple of Angstroms (10-10 m)!
As you can see, the current is VERY sensitive to the
gap distance.
Quantum Tunneling
The second tip shown above is recessed by
about two atoms and thus carries about a
million times less current. That is why we
want such a fine tip. If we can get a single
atom at the tip, the vast majority of the
current will run through it and thus give us
atomic resolution.
Note
A STM does not measure nuclear position
directly. Rather it measures the electron
density clouds on the surface of the sample.
In some cases, the electron clouds represent
the atom locations pretty well, but not
always.
Small Movements
To get the distance between the tip and the
sample down to a couple of Angstroms
where the tunneling current is at a measurable
level, STMs use feedback servo loops and converse
piezoelectricity.
Servos
Servos are small devices
with a shaft that can be
precisely controlled with
electrical signals.
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Servos are used all the
time in radio controlled
cars, puppets, and
robots.
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Converse Piezoelectricity
Piezoelectricity is the ability of certain crystals to
produce a voltage when subjected to mechanical
stress.
When you apply an electric field to a piezoelectric
crystal, the crystal distorts. This is known as
converse piezoelectricity. The distortions of a
piezo is usually on the order of micrometers,
which is in the scale needed to keep the tip of the
STM a couple Angstroms from the surface.
Electric Field
Pizos
The tip
Problems and Solutions
• Bringing the tip close to the surface and scanning the surface
• Feedback Servo Loops
• Keeping the tip close to the surface
• Converse Piezoelectricity
• Creating a very fine tip
• Electro-chemical etching
• Forces between tip and sample
• Negligible in most cases
• Mechanical vibrations and acoustic noise
• Soft suspension of the microscope within an ultra high
vacuum chamber (10-11 Torr)
• Thermal length fluctuations of the sample and especially the tip
• Very low temperatures
• The sample has to be able to conduct electricity
• There is no way around this, try using an AFM
Vibration-Isolation
The original STM design had the tunnel unit with
permanent magnets levitated on a superconducting lead
bowl. They used 20 L of liquid helium per hour.
Vibration-Isolation
The simple and presently widely used vibration protection
with a stack of metal plates separated by viton - an ultra
high vacuum compatible rubber spacer.
Original Trace
Si(111) trace taken in 1983.
Processed Trace
Computer processed version
of the same trace of Si(111)
How to Process a Trace
The trace (1) can be interpreted as a grid which can be
shown as a grayscale picture (2).
1
2
3
4
The grayscale picture can be interpreted as a contour
map (3) which can then be averaged out to make
smooth (4) and finally colored (below).
Uses of STM
Measuring high precision optical components and disk
drive surface roughness of machined or ground surfaces
is a common use for STM.
Below is a trace of an individual turn mark on a
diamond-turned aluminum substrate to be used for
subsequent magnetic film deposition for a high capacity
hard disc drive.
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1 micron
Uses of STM
By measuring variations in current, voltage, tip/surface
separation, and their derivatives, the electronic properties of
different materials can be studied.
One such element studied was the bucky ball (C60). When
you press down on a bucky ball by 1/10th nm, it lowers the
resistance of the bucky ball by 100 times.
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C. Joachim J. K. Gimzewski,
"An electromechanical amplifier
using a single molecule”,
Chemical Physics Letters, Vol.
265, Nos. 3-5, page 353,
February 7, 1997.
Different STM Ideas
You could decide not to use piezoelectricity to keep the
distance between the tip and the surface equal at all times,
and instead use the current measurements to determine the
surface of a sample.
Pros:
• You can scan much faster
Cons:
• The surface must not have
cavities more than a few
Angstroms deep (an atom or two)
because of tunneling
Different STM Ideas
Imagine increasing the tunneling current when you are on
top of an atom by lowering the tip a little. The attractive
force between the tip and the atom would then increase,
allowing you to “drag” atoms around.
IBM imagined this. Iron atoms were first physisorbed
(stuck together using intermolecular forces, aka Van Der
Waals foces) on a Cu surface. The iron atoms show up as
bumps below.
Different STM Ideas
The iron atoms were then dragged along the surface of
to form a circle.
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Different STM Ideas
Iron atoms on the surface of Cu(111)
Different STM Ideas
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References
Carbon
Monoxide
Man
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G. Binnig and H. Rohrer. "Scanning Tunneling Microscopy",
IBM J Res. Develop., 30:355, 1986.
G. Binnig, H. Rohrer, “Scanning Tunneling Microscopy From Birth to Adolescence”, Nobel lecture, December 8,
1986.
Tit-Wah Hui, “Scanning Tunneling Microscopy - A Tutorial”,
http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/
stmtutor.htm
Wikipedia, “Scanning Tunneling Microscope”,
http://en.wikipedia.org/wiki/Scanning_tunneling_microscope
Carbon Monoxide on Platinum (111)
Nobel e-Museum, “The Scanning Tunneling Microscope”,
http://www.nobel.se/physics/educational/microscopes/scannin
g/index.html
Pictures from http://www.almaden.ibm.com/vis/stm/blue.html