SCANNING TUNNELING MICROSCOPY

.Azhar AnsariHaris Ahmed

Ashutosh ChauhanMohd. Nauman

Nasir

Need for a STM

• In 1927, Werner Heisenberg stated, the more precisely position of particle is measured, lesser is the precision in determining its momentum

The Basics

• A scanning tunneling microscope ( referred to as STM) is a device used for imaging surfaces at the atomic level.

• Developed in 1981 by Gerd Binning & Heinrich Rohrer (IBM,Zurich)

• Used not only in vacuum but, also in air,water & various liquids.

Visualization of tunneling

Low energy particle is confined to stay in its potential well.

Higher energy particle has a possibility of passing through the wall and continuing with less energy than

before.

• PRINCIPLE:Based precisely on the concept of TUNNELING of

electrons.Bias applied between the device & surface

allows electrons to tunnel even in vacuum.The current hence, produced would depend on:Voltage(bias)Position of device wrt sample

How this is different than arcing

High Voltage

Let’s abandon quantum mechanics and our microscope for a second and look at the slightly different concept of arc/lightning.

This is drastically different than tunneling because the extremely high voltages are physically changing the transfer medium (ionizing) so that the

electrons can conduct. As such the arcing could be continuous (if the materials weren’t normally destroyed) but the current would change since the medium is constantly changing. Only at a certain distance is the voltage going to arc through the media (air). This could be repeated and as long as

the environment hasn’t changed it is very repeatable, but not practical.

Some engineering concerns

Let’s look at some engineering challenges. We need an extremely accurate way to bring a probing material to our sample, that is stable and is capable of

very fine adjustments. We need to be able to bring our probe to within Angstroms of the surface and then be able to move in Angstrom increments.

Sample Probe

Angstroms

Design

Instrumentation

• Components:Scanning tip( of W-metal)Piezoelectric tubeScannerCoarse-sample-to-tip controlComputer

A perfect probe

It is clear that we should have a probe that is extremely small, or ideally on the atomic scale. If we had a probe that had exactly one atom on the end then the subsequent

atoms behind it would contribute almost nothing to our current.

Sample

6Å

3Å

Current

~e-2(6)k

~2e-2(9)k

As it turns out atomic probes are easily made with either mechanical or chemical processes.

Accurate distance control

Angstrom level distance control is required which would be impossible for any mechanical gear workings.

A piezoelectric is a crystal that creates potential differences (voltages) when mechanical stresses are imposed on it.

Voltage

0

World of piezoelectrics

• Piezoelectricity is the electric charge that accumulates in certain solid materials.

• Discovered by Jacques & Pierre Curie in1880.• Observed in cermaics, bones & DNA (&

various protiens).• It is a linear electromechanical interaction

between the mechanical & the electrical state in crystalline materials.

Into the dynamics of piezoelectricity• Related to electric dipoles in

solids.• Dipoles near each tend to

be aligned in regions called domains.

• Applied mechanical stress changes polarization & piezoelectricity manifests.

Applications

• Generation and Detection of sound• Cigarette lighter: Pressing the button causes a spring-

loaded hammer to hit a piezoelectric crystal, causing a sufficiently high voltage current that ignites the gas

• Energy harvesting : A project of US Army that aims to power battlefield equipment by piezoelectric generators embedded in soldier’s boots.

• Trivia: The global demand for piezoelectric devices was valued at approximately US $14.8 billion in 2010.

Probing a surface

With all of the elements in place we can start probing our metallic surface.

We could either probe at a constant height and look at current

variations as we pass.

PROBE

Curr

ent

Distance

Or we could vary the height in search for constant current.

Heig

ht

Distance

Images of an STM

The Nobel prize winning first STM.

A more current example.

Beyond STM

• PSTM: Photon scanning microscopy, uses an optical tip to tunnel photons.

• STP: Scanning tunneling potentiometry, which measures electric potential across a surface.

• SPSTM: Spin polarized STM, uses a ferromagnetic tip to tunnel spin- polarized electrons into magnetic sample.

• Other methods involve manipulating tip to change topography of sample.

• Atomically precise positioning system allows atomic- scale manipulation.

• E.g: IBM developed a way to manipulate Xenon atoms.

• Recently bond manipulation has become possible employing STM

Trivia...

In 1989, at the Almaden IBM Research Facility scientists found that an STM could be used to lift atoms off a surface of metal and placed back in a different location, at low temperatures that is.

References

• wikipedia.com• electronicstutorials.com• www-03.ibm.com• nanoscience.com

THANK YOU!