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What is Quantum Tunneling?

At the quantum level, matter has

corpuscular and wave-like properties

Tunneling can only be explained by the

wave nature of matter as described byquantum mechanics

Classically, when a particle is incident

on a barrier of greater energy than the

particle, reflection occurs

When described as a wave, the

particle has a probability of existing

within the barrier region, and even on

the other side of it

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2

Tunnel effect

.

The above equation shows that T can be

nonzero i.e, there is a finite probability of finding

particle in region III.

)(2where

EUmC

CLeT 2_

An approximate expression for the transmission

coefficient, when T

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3

Quantum tunnelling was first used to explain

alpha decay. Alpha particles coming out of thenucleus have energy of 4-5 MeV, still they cantunnel out from the nucleus with bindingenergy of the order 25 MeV.

However the probability of emission of analpha particle is 1 in 1038events. In a tunnel

diode, the charge carriers tunnel through thepotential barrier (depletion layer). Quantumtunnelling has found many applications invarious fields.

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Quantum tunnelling (Tunnel effect)

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STM

The Scanning Tunneling Microscope was invented in 1981 byGerd Binnig and Heinrich Rohrer

When a metal tip, usually made of tungsten or platinum-

iridium, is brought within .4-.7nm of the sample, electrons

tunnel across the gap and create a current in the tip

This current is then fed to a computer and used to generate an

image of the atomic surface of the sample

At this distance the coulomb force between the tip and an

atom of the sample is actually enough to move the atom

This has allowed physicists to create images and structures on

the atomic level

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STM : Scanning tunnelling Microscope

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STM

1

Scanning tunnelling microscope: A scanningtunnelling microscope (STM) is an instrument for

imaging surfaces at the atomic level.

It was developed in 1981 by Gerd-Binnig andHeinrich-Rohrer (at IBM Zrich), and for their

invention they got the Nobel Prize in Physics in

1986.

The basic principle behind an STM is quantum

tunnelling.

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2 When a conducting tip is brought near to the

sample surface, a bias voltage applied betweenthe two can allow electrons to tunnel through

the vacuum between them.

The resulting tunnelling current is a function oftip position, applied voltage, and electronic

nature of the sample.

Information is acquired by monitoring the

current as the tip's position scans across the

surface, and is usually displayed in image form.

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STM requires extremely clean and stable

surfaces, sharp tips, excellent vibration control,and sophisticated electronics.

The tip is scanned over the surface using apiezoelectric probe. Using STM individual atoms

can be imaged and manipulated.

For a current to flow the substrate beingscanned must be conductive. Insulators cannot

be scanned through the STM. STM has become a very important tool in

research and technology development at thenano-scale.

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Scanning Tunnelling Microscope

Tungsten STM tip(photo taken with an SEM)

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STM Images

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AFM

1

Atomic force microscope (AFM):The atomicforce microscope (AFM) is a very powerful

microscope invented by Binnig, Quate and

Gerber in 1986.

Besides imaging at the atomic level it is also

one of the foremost tools for the manipulation

of matter at the nanoscale.

The AFM consists of a cantilever with a sharp

tip (probe) at its end that is used to scan the

specimen surface.

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AFM:The cantilever is typically silicon or silicon nitride

with a tip radius of curvature on the order of

nanometers.

When the tip is brought near the sample surface,

forces between the tip and the sample lead to a

deflection of the cantilever according to Hooke'slaw.

The deflection is measured using a laser spot

reflected from the top surface of the cantilever into

an array of photodiodes.

Currently AFM is the most common form of

scanning probe microscopy

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AFM

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AFM- Working Principle

The AFM brings a probe inclose proximity to thesurface

The force is detected by the

deflection of a spring,usually a cantilever (divingboard)

Forces between the probe

tip and the sample aresensed to control thedistance between the thetip and the sample.

van der Waals force curve

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Two Modes

Repulsive (contact)

At short probe-sample distances,

the forces are repulsive

Attractive Force (non-contact)

At large probe-sample distances,the forces are attractive

The AFM cantelever can be used to

measure both attractive force

mode and repulsive forces.

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AFM Examples

View of Silicon Surface Reconstruction

Carbon Nanotube Used as a Conducting

AFM Tip for Local Oxidation of Si.