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Scanning Tunneling

MicroscopyBy Lucas Carlson

Reed CollegeMarch 2004

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Image from an STM

Iron atoms on the surface of Cu(111)

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The STM is an electron microscope that

uses a single atom tip to attain atomic resolution.

The Scanning Tunneling Microscope (STM)

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History

The scanning tunneling microscope wasdeveloped at IBM Zrich in 1981 by GerdBinning and Heinrich Rohrer who shared theNobel Prize for physics in 1986 because of

the microscope.

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Gerd Binning

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Heinrich Rohrer

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General Overview

An extremely fine conducting probe is heldabout an atoms 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 keepthe signal constant and maintain the distance.

This enables it to follow even the smallest

details of the surface it is scanning.

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The Tip

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 useElectro-chemical etching techniques to createvery sharp tips like the one above.

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150x Magnification

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Quantum Tunneling

Classically, when an object hits a potential thatit doesnt have enough energy to pass, it will

never go though that potential wall, it alwaysbounces back.

In English, if you throw a ball at a wall, it will

bounce back at you.

ClassicalWave Function

For Finite Square

Well Potential

Where E

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Quantum Tunneling

In quantum mechanics when a particle hits apotential that it doesnt have enough energyto pass, when inside the square well, the wavefunction dies off exponentially.

If the well is short enough, there will be a noticeable

probability of finding the particle on the other side.

QuantumWave Function

For Finite Square

Well Potential

Where E

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Quantum Tunneling

The finite square well potential is a goodapproximation for looking at electrons on conductingslabs with a gap between them.

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Quantum Tunneling

More graphs of tunneling:

An electron tunneling from atom to atom:

n(r) is the

probability of

finding an electron

V(r) is the potential

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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.

TipSample

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Quantum TunnelingThrough a barrier, quantum mechanics predicts that the

wave function dies off exponentially:

So the probability of finding an electron after a barrier ofwidth 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.

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Quantum Tunneling

Plugging in typical values for m, d, and phi (wherephi is the average work function of the tip and thesample), when d changes by 1 , the currentchanges by a factor of about 10!

Where:

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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 orderof a couple of Angstroms (10-10 m)!

As you can see, the current is VERY sensitive to thegap distance.

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Quantum Tunneling

The second tip shown above is recessed by

about two atoms and thus carries about amillion times less current. That is why wewant such a fine tip. If we can get a singleatom at the tip, the vast majority of thecurrent will run through it and thus give usatomic resolution.

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Note

A STM does not measure nuclear positiondirectly. Rather it measures the electron

density clouds on the surface of the sample.In some cases, the electron clouds representthe atom locations pretty well, but notalways.

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Small Movements

To get the distance between the tip and thesample down to a couple of Angstromswhere the tunneling current is at a measurablelevel, STMs use feedback servo loops and converse

piezoelectricity.

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Servos

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Servos are small deviceswith a shaft that can beprecisely controlled withelectrical signals.

Servos are used all thetime in radio controlledcars, puppets, androbots.

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Converse Piezoelectricity

Piezoelectricity is the ability of certain crystals toproduce 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.

The tip

Pizos

Electric Field

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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 highvacuum 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

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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.

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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.

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Original Trace

Si(111) trace taken in 1983.

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Processed Trace

Computer processed version

of the same trace of Si(111)

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How to Process a Trace

The trace (1) can be interpreted as a grid which can beshown 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).

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Uses of STM

Measuring high precision optical components and diskdrive surface roughness of machined or ground surfaces

is a common use for STM.

Below is a trace of an individual turn mark on adiamond-turned aluminum substrate to be used for

subsequent magnetic film deposition for a high capacity

hard disc drive.

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1 micron

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Uses of STM

By measuring variations in current, voltage, tip/surfaceseparation, 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.

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Different STM Ideas

You could decide not to use piezoelectricity to keep thedistance 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 havecavities more than a few

Angstroms deep (an atom or two)

because of tunneling

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Different STM Ideas

Imagine increasing the tunneling current when you are ontop 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.

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Different STM Ideas

The iron atoms were then dragged along the surface ofto form a circle.

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Different STM Ideas

Iron atoms on the surface of Cu(111)

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Different STM Ideas

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References

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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

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

Carbon Monoxide on Platinum (111)

Carbon

MonoxideMan
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