Scanning Tunneling Microscopy

Scanning Tunneling Scanning Tunneling MicroscopyMicroscopy

By Lucas CarlsonBy Lucas Carlson

Reed CollegeReed College

March 2004March 2004

Image from an STM

Iron atoms on the surface of Cu(111)

The STM is an electron microscope thatuses a single atom tip to attain atomic resolution.

The Scanning Tunneling Microscope (STM)

HistoryHistoryThe 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.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Gerd Binning



Heinrich Rohrer

General OverviewGeneral Overview

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

This enables it to follow even the smallestdetails of the surface it is scanning.

The TipThe Tip

As we will see later, is very important that thetip 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.



150x Magnification

Quantum TunnelingQuantum Tunneling

Classically, when an object hits a potential thatit doesn’t 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 willbounce back at you.

ClassicalWave FunctionFor Finite SquareWell PotentialWhere E<V


In quantum mechanics when a particle hits apotential that it doesn’t have enough energyto pass, when inside the square well, the wavefunction dies off exponentially.

If the well is short enough, there will be a noticeableprobability of finding the particle on the other side.

QuantumWave FunctionFor Finite SquareWell PotentialWhere E<V

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

Quantum TunnelingQuantum TunnelingMore graphs of tunneling:

An electron tunneling from atom to atom:

n(r) is the probability of finding an electron

V(r) is the potential


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

Quantum TunnelingQuantum TunnelingThrough 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.


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!

Where:


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 the gap distance.


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.

NoteNote

A STM does not measure nuclear positiondirectly. Rather it measures the electrondensity clouds on the surface of the sample.In some cases, the electron clouds representthe atom locations pretty well, but notalways.

Small MovementsSmall MovementsTo 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 conversepiezoelectricity.

ServosServos



Servos are small devices with a shaft that can be precisely controlled with electrical signals.

Servos are used all the time in radio controlled cars, puppets, and robots.

Converse PiezoelectricityConverse PiezoelectricityPiezoelectricity 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.

The tip

PizosElectric Field

Problems and SolutionsProblems 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-IsolationVibration-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-IsolationVibration-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 TraceOriginal Trace

Si(111) trace taken in 1983.

Processed TraceProcessed Trace

Computer processed version of the same trace of Si(111)

How to Process a TraceHow to Process a TraceThe 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 STMUses of STMMeasuring 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.



1 micron

Uses of STMUses of STMBy 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.



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 IdeasDifferent STM IdeasYou 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 IdeasDifferent STM IdeasImagine 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 IdeasDifferent STM IdeasThe iron atoms were then dragged along the surface of to form a circle.



Different STM Ideas

Iron atoms on the surface of Cu(111)

Different STM IdeasDifferent STM Ideas



ReferencesReferences



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/scanning/index.html

Pictures from http://www.almaden.ibm.com/vis/stm/blue.html

Carbon Monoxide on Platinum (111)

CarbonMonoxideMan

Scanning Tunneling Microscopy

Documents

Transcript of Scanning Tunneling Microscopy