ECE III-2 NanoTechnology Elective

7/28/2019 ECE III-2 NanoTechnology Elective

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Syllabus--Contd

Unit 5:

Nanoscale characterization Techniques:

Scanning Probe Techniques:

AFM, MFM, STM, SEM, TEM and XRD

Unit 6: Nanodevices and Nanomedicine

Lab on chip for bioanalysis,

Core / Shell Nanoparticles in drug delivery systems(Site specific and targeteddrug delivery)-----

Cancer Treatment and Bone tissue treatment.


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Syllabus-contd

Unit 7:

Nano and Molecular Electronics:

Resonant-tunneling structures,

Single electron tunneling.Single electron transistors,

coulomb blockade, Giant Magneto Resistance and Tunneling Magneto Resistance

Unit 8:

Nanolithography and Nanomanipulation:

E-beam lithography and SEM basd nanolithography and nanomanipulation. Ion beam lithography, Oxidation and Metallization

Mask and its application

Deep UV lithography and X-ray based lithography.


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Books

1 Introduction to Nanotechnology

by Charles P. Poole jr. and Frank J.Owens

John Wiley &SonsWiley Student edition

2. Principles of Nanotechnology By Phani Kumar

Scitech Publications(India) pvt.Ltd.

3. NanotechnologyA Gentle introduction to the Next Big IdeaByMark Ratner & Daniel Ratner

Pearson Education Low priced edition

.


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Unit-5Characterisation of Nano particles

After producing the nano particles we have tocharacterise them by finding the

Particle size

Crystal structure

Surface nature

Magnetic properties Optical properties etc.


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Unit 5-CharacterisationTechniques XRD

TEM

Scanning Probe Techniques:

SEM

STM

AFM

MFM


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Crystal structure andparticle size determination

Three techniques are used for the crystalstructure determination:

X Ray Diffraction---XRD

Electon Difraction ED

Neutron Diffraction--ND


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XRD, ED and ND

Collimated beams of :

X rays, electrons or neutrons are directed atthe crystal, and the angles at which the beamis diffracted are measured.

The methods are similar and all theconsiderations that apply to X-rays also applyto electrons and neutrons.


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XRD--Reflection of X-ray beam incident at an angle off twoparallel planes seperated by the distance d . The difference in path

length for the two planes is 2d sin .


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XRD ---Wave length of X-ray(and the Braggs condition)


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h, k, l indices


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XRD


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higher h,k,l have smaller d and larger

Planes with higher indices are closer togetherie smaller d.

And hence they have larger Braggangles(n=2dsin)

Amplitudes of diffraction lines depend on h,k,l

Some planes have zero amplitude. The relativeamplitudes help us to determine crystal structure

type.


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BCC and FCC structures

For eg:

ForBCC monoatomic lattice the only planesthat produce observed diffraction peaks are forthose h+k+l = n, n only even integer. And for

FCC lattice the only observed diffraction lineseitherhave all odd or all even integers


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FCC Structurehkl all odd or alleven


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Procedure--XRD

To obtain complete crystal structure

X-ray spectra are recorded for rotationsaround three mutually perpendicular planes ofthe crystal.

Using Fourier Transformation, the data is

converted into positions of atoms in the unitcell.

The lattice constants a,b,c of the unit cell andthe values of the angles ,, between them aredetermined.


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Procedure -- XRD

An example of TiN is shown:

The fact that all the planes have either all oddor all even indices identifies it as FCC

structure.And the lattice constant was found as

a = 0.42417nm.


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Grain size determination byXRD


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Average grain size in XRD


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TEM histogram


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Debye-Scherrer Powdermethod


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Debye-Scherrer

A monochromatic X-ray is used. Sample is in the form ofpowder in a glass tube.

The tube is rotated to smooth out the recorded pattern.

The conical pattern of X rays emerging for each angle 2,with satisfying Bragg condition is incident on the film strip inarcs.

From the fig. we can write = S/4R

Where S is the distance between the two corresponding reflectionson the film and

R is the radius of the film cylinder.


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JCPDS

Advantage of Powder method:

A single exposure gives all the Bragg angles atthe same time. To facilitate identification:JCPDS powder diffraction file

contains

results of over 20,000 compounds alreadystudied by Debye-Scerrer method and can beused to compare and obtain the crystalstructures.


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Microscopy

A microscope is in principle nothing else thana simple lens system for magnifying small

objects.


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Optical Microscopy

Optical Microscopy Principles

It has a lens system consisting of an Objective lens and an Eyepiece

The first lens, called the objective, has a short focal length (a fewmm), and creates an image of the object in the intermediate imageplane.

This image in turn can be looked at with another lens,

the eye-piece, which can provide further magnification.

The objective lens has a much shorter focal length than the eye-piece, in order to magnify the intermediate image (usually by afactor 40-100).


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Principle of

optical microscope.


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TEM


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Optical Microscopy andElectron Microscopy

Optical microscopy (OM) uses visible light as a source ofillumination and optical (glass) lenses to magnify specimensin the range between approximately 10 to 1,000 times theiroriginal size,

Electron Microscopy (EM) is operated in the vacuum andfocuses the electron beam and magnifies images with thehelp ofelectromagnetic lenses.

The electron microscope takes advantage of the much shorter wavelength of the electron (e.g., = 0.005 nm at an accelerating voltage of 50 kV) when

compared to the wavelengths of visible light ( = 400 nm to 700 nm) .


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OM vs EM

When the accelerating voltage is increased inEM,

the wavelength decreases and resolution

decreases. In other words, increasing the velocity of

electrons

results in a shorter wavelength and increasedresolving power


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Types of Electron Microscopes

TEM Transmission Electron Microscope

SEM Scanning Electron Microscope

(STEM- Scanning Transmission ElectronMicroscope)

STM Scanning Tunneling Microscope


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The Electron Microscope---TransmissionType

The Transmission Electron Microscope ---TEM

The transmission electron microscope (TEM)

operates on the same basic principles as thelight microscope but uses electrons instead oflight.

What you can see with a light microscope is

limited by the wavelength of light. TEMs use electrons as "light source" and their

much lower wavelength makes it possible toget a resolution a thousand times betterthan

with a light microscope


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Conventional TEM


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Magnetic Lenses Guide the Electrons

Magnetic Lenses Guide the Electrons

A "light source" at the top of the microscope emitsthe electrons that travel through vacuum in the

column of the microscope. Instead of glass lenses focusing the light in the

light microscope, the TEM uses electromagneticlenses to focus the electrons into a very thinbeam.

The electron beam then travels through thespecimen you want to study. Depending on thedensity of the material present, some of theelectrons are scattered and disappear from the

beam.


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TEM-image formation

At the bottom of the microscope theunscattered electrons hit a fluorescent screen,which gives rise to a "shadow image" of the

specimen with its different parts displayed invaried darkness according to their density.

The image can be studied directly by theoperator or photographed with a camera.


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High magnifications of TEM

You can see objects to the order of a fewangstrom (10-10 m).

For example, you can study small details in

the Biological cell or different materials downto near atomic levels.

The possibility for high magnifications has

made the TEM a valuable tool in both medical,biological and materials research.

El t


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ElectronMicroscope(Transmission)


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TEMResolving Power

The Transmission Electron Microscope (TEM) The ray of electrons is produced by a pin-shaped cathode

heated up by current. The electrons are vacumed up by ahigh voltage at the anode.

The acceleration voltage is between 50 and 150 kV. The

higher it is, the shorter are the electron waves and the higheris the power of resolution. But this factor is hardly everlimiting. The power of resolutionof electron microscopy isusually restrained by the quality of the lens-systems andespecially by the technique with which the preparation of thesample has been achieved.

Modern gadgets have powers of resolution that range from0,2 - 0,3 nm. The useful resolution is therefore around 500,000 x.


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TEM

The accelerated ray of electrons passes a drill-hole at the bottom of the anode. Its followingway is analogous to that of a ray of light in alight microscope. The lens-systems consist ofelectronic coils generating an electromagneticfield. The ray is first focused by a condenser. Itthen passes through the object, where it ispartially deflected. The degree of deflectiondepends on the electron density of the object.

.


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TEM

The greater the mass of the atoms, the greater isthe degree of deflection. Biological objects haveonly weak contrasts since they consist mainly ofatoms with low atomic numbers (C, H, N, O).

Consequently it is necessary to treat the samplewith special contrast enhancing chemicals (heavymetals) to get at least some contrast.

Additionally they are not to be thicker than 100nm, because the temperature raises due to

electron absorption. This again can lead todestruction of the prepared sample It is generally impossible to examine living

objects.


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TEM

After passing the object the scatteredelectrons are collected by an objective.Thereby an image is formed, that issubsequently enlarged by an additional lens-system (called projective with electronmicroscopes). The thus formed image is madevisible on a fluorescent screen or it isdocumented on photographic material. Photostaken with electron microscopes are alwaysblack and white. The degree of darknesscorresponds to the electron density (=differences in atom masses) of the sample.


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Detector positioning in Electron Microscope


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TEM

Electrons are better probes than neutrons orX-rays. They interact with matter more than X-rays or neutrons.

The thicknesses of specimens comparable tothe mean free path of the electrons gives thebest results.

Much thinner films exhibit too litle scatteringand thicker films have multiple scatteringmaking the image blurred or difficult tointerpret.


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TEM

The transmitted electron beam consists of 1. elctrons which have not undergone any

scattering.

2. Electrons which have lost energy throughinelestic scattering but not deviated from

their paths

3. electrons reflected by various h,k,l

crystallographic planes.


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TEM

To produce what is called a bright fieldimage(BF)

SAED- (Selected Area Electron Diffraction)aperture is inserted. This allows only mainundeviated transmitted electrons.

To produce what is called a dark fieldimage(DF)

The aperture is positioned in such a way thatonly one of the beams that is reflected by theparticulr hkl planes hits the screen.

Fast Fourier Transform of the


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Fast Fourier Transform of theimage The information can be increased from the

image by

Fast Fourier Transform

It can provide information similar to that in thedirect diffraction pattern.


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TEM


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Magnification

SEM Scanning Electron


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SEMScanning ElectronMicroscope

D bl d fl i f


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Double deflection system of aScanning electron Microscope


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SEM

The Scanning electron microscope (SEM) The path of the electron beam within the scanning electron

microscope differs from that of the TEM. The technology used is based on television techniques. The method is suitable for the

depiction of the Samples with conductive surfaces. Biological objects have thus to be made conductive by

coating with a thin layer of heavy metal (usually gold istaken).

The power of resolution is normally smaller than intransmission electron microscopes, but the depth of focusis several orders of magnitude greater.

Scanning electron microscopy is therefore also well-suited forvery low magnifications.


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SEM

The surface of the object is scanned with theelectron beam point by point whereby secondaryelectrons are set free.

The intensity of this secondary radiation isdependent on the angle of inclination of theobject's surface.

The secondary electrons are collected by a

detectorthat sits at an angle at the side above theobject. The signal is then enhanced electronically.The magnification can be chosen smoothly(depending on the model) and the image appears

a little later on a viewing screen.


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The scanning transmission electron microscope(STEM)

STEM: In this development of the SEM do the electrons passthrough the Sample and the secondary radiationthusgenerated is used for image formation. Here, too, theexpenditure is large, but it is still worthwhile, since

large molecules like nucleic acids orproteins or molecular

complexes like viruses can be depicted much better andgentler than with the TEM. . The interpretation of images gained with electron microscopy

is increasingly done with computerized interpretationprograms. But they are usually only suitable for thereconstruction of regularly recurring patterns and these,again, are found more often on a molecular level than on acellular one.

Differences Between TEM and


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Differences Between TEM andSEM Summary: 1.Both SEM and TEM are two types of electron microscopes and

are tools to view and examine small samples. Both instruments use electrons or electron beams. The images produced in both tools are highly magnified and offer

high resolution.

2.How each microscope works is very different from another. SEM scans the surface of the sample by releasing electrons and

making the electrons bounce or scatterupon impact. The machinecollects the scattered electrons and produces an image. The image

is visualized on a television-like screen. On the other hand, TEM processes the sample by directing an

electron beam through the sample. The result is seen using afluorescent screen.


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TEM vs SEM

Images are also a point of differencebetween two tools.

TEM pictures are two-dimensional and might

require a little bit of interpretation. SEM images are three-dimensional and are

accurate representations.

In terms ofresolution and magnification,TEM has more advantages compared toSEM.


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TEM vs SEM

specimens can be magnified roughly

between 500 to 5000,000 times in TEMs

between 10 and 2000,000 times in SEMs and

The resolution is 0.5 Angstroms in TEM

while it is 4 Angstroms in SEM

Though the power of resolution of SEM isnormally smaller than in TEM, the depth offocus is several orders of magnitude greater.

High Voltage Electron


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High Voltage ElectronMicroscope .The high voltage electron microscope: it operates with an

accelerating voltage of 700 - 3000 kV.

Its power of resolution is greater, the Sample can be thicker, the strain on the

preparation is smaller.

But the enormous technical expenditure isdisadvantageous.

The Scanning Tunneling Microscope


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The Scanning Tunneling MicroscopeSTM

A scanning tunneling microscope (STM) is aninstrument for imaging surfaces at the atomic level.

Its development in 1981 earned its inventors,

Gerd Binnig and Heinrich Rohrer(at IBM Zrich), theNobel Prize in Physics in 1986.

The scanning tunneling microscope (STM) is a type ofelectron microscope that shows three-dimensionalimages of a sample.

In the STM, the structure of a surface is studied using astylus that scans the surface at a fixed distance from it.

Scanning Tunneling


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Scanning TunnelingMicroscopy. The STM is based on three principles.

One is the quantum mechanical effect of tunneling. Itis this effect that allows us to see the surface.

Another principle is the piezoelectric effect. It is thiseffect that allows us to precisely scan the tip withangstrom-level control.

Lastly, a feedback loop is required, which monitors thetunneling current and coordinates the current and thepositioning of the tip.


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Tunneling

Tunneling is a quantum mechanical effect. A tunnelingcurrent occurs when electrons move through a barrierthat they classically shouldn't be able to move though.

In classical terms, if you don't have enough energy tomove "over" a barrier, you won't be able to cross it.

However, in the quantum mechanical world, electronshave wavelike properties. These waves dont endabruptly at a wall or barrier, but taper off quite quickly.


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Tunneling

If the barrier is thin enough, the probabilityfunction may extend into the next region,through the barrier!

Because of the small probability of anelectron being on the other side of thebarrier, given enough electrons, some willindeed move through and appear on the

other side. When an electron movesthrough the barrier in this fashion, it iscalled tunneling.


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Wave mechanical Tunneling


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Wave mechanical Tunneling

Quantum mechanics tells us that electrons have both wave andparticle like properties. Tunneling is an effect of the wavelike nature. The top image shows us that when an electron (the wave) hits a

barrier, the wave doesn't abruptly end, but tapers off very quickly -exponentially.

Fora thick barrier, the wave doesn't get past. The bottom image shows the senario if the barrier is quite thin (about a nanometer). Part of the wave

does get through, and therefore some electrons may appear on theother side of the barrier..

Because of the sharp decay of the probability function through thebarrier, the number of electrons that will actually do this is verydependent upon the thickness of the barrier.

The actual current through the barrier drops off exponentially withthe barrier thickness.


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Tunneling in STM

To extend this description to the STM: The starting point of the electron is either the tip or

sample (depending on the setup of the

instrument)

The barrier is the gap (air, vacuum, liquid),

and the second region is the other side tip orsample, again, depending on the experimentalsetup.

By monitoring the current through the gap, we

have very good control of the tip-sample distance.

piezoelectric effect discovered in


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piezoelectric effect discovered in1880 Piezoelectric Materials The piezoelectric effect was discovered by Pierre Curie.

The effect is created by squeezing the sides of certaincrystals, such as quartz or barium titanate.

The result is the creation of opposite charges on thesides.

The effect can be reversed as well; by applying avoltage across a piezoelectric crystal, it will elongate or

compress.These materials are used to scan the tip in an STM, andmost other scanning probe techniques.

A typical piezoelectric material used in STMs is PZT(Lead Zirconium Titanate).

Electronics and the Feedback


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Electronics and the FeedbackLoop

Obviously, you need electronics to measure thecurrent, scan the tip, and translate this informationinto a form that we can use.

A feedback loop constantly monitors the tunneling

current and makes adjustments to the tip tomaintain a constant tunneling current. These adjustments are recorded by the computer

and presented as an image in the STM software.Such a setup is called a constant current image.

In addition, for very flat surfaces, the feedback loop can be turned off and only the

current is displayed. This is a constant heightimage.

C C l h S f


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Currents Control the Surface

An extremely fine conducting probe is held close to thesample. Electrons tunnel between the surface and thestylus, producing an electrical signal.The stylus is extremely sharp, the tip being formed byone single atom. It slowly scans across the surfaceat a distance of only an atom's diameter.

The stylus is raised and lowered inorder to keep thesignal constant and maintain the distance. This enablesit to follow even the smallest details of the surface it isscanning.Recording the vertical movement of the stylus makes itpossible to study the structure of the surface atom byatom. A profile of the surface is created, and from that acomputer-generated contour map of the surface isproduced.

Scanning Tunneling Microscopy


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Scanning Tunneling MicroscopyBasics

.. Changing the position of the tip in the lateral (x,y) planeallows to scan continuously across the sample surface andchanging the vertical (z) position allows to maintain desiredtip-sample distance.

If that distance becomes small enough (100.1 Angstroms)and voltage is applied between the sample and the tip, thetunneling current (10.1 nA) can be observed.

Basic STM setup. An atomically-sharp tip is mounted on 3

piezo crystals that allow precise positioning in 3 directions.Moving in (x,y) plane scans the tip across the sample, z piezodetermines the tip-sample distance.

Actual STM image of Si(111) surface is used to represent thesample.

STM B i


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STM Basics

STM


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STM

STM Diff t S


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STMDifferent Scans

B i STM


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Basics--STM

The tunneling current depends exponentially onthe tip-sample separation. Therefore if a feedbackloop is used to adjust the vertical position to keepthe current constant (constant current scanning

mode) tip-sample separation can be kept constantwith great precision. Alternatively, the z-coordinate can be held

constant and the tunneling current recorded

during an (x,y) scan. Since the current is proportional to the density of

electronic states in the sample, the first methodmaps constant density of states contours and the

second method maps the actual density of states.

Scanning Tunneling


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Scanning TunnelingMicroscope For an STM, good resolution is considered to be 0.1 nm

lateral resolution and 0.01 nm depth resolution. With thisresolution, individual atoms within materials are routinelyimaged and manipulated.

The STM can be used not only in ultra-high vacuum butalso in air, water, and various other liquid or gasambients,

and at temperatures ranging from near zero kelvin to a

few hundred degrees Celsius.

STM


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STM

.The tunneling currentis a function of tipposition, applied voltage, and the local densityof states (LDOS) of the sample.

STM can be a challenging technique, as itrequires extremely clean and stable surfaces,

sharp tips, excellent vibration control, andsophisticated electronics, but nonethelessmany hobbyists have built their own.

STM Picture


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STM PictureAtomic resolution Au (100) surface.


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I t t ti


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Instrumentation

The components of an STM include scanning tip,

piezoelectric controlled height and x,y scanner,

coarse sample-to-tip control, vibration isolation system, and

computer.

P d


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Procedure

First, a voltage bias is applied and the tip is brought close to thesample by coarse sample-to-tip control, which is turned off when thetip and sample are sufficiently close.

At close range, fine control of the tip in all three dimensions when

near the sample is typically piezoelectric, maintaining tip-sampleseparation W typically in the 4-7 (0.4-0.7 nm) range, which is theequilibrium position between attractive (3


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Scanning

If the tip is moved across the sample in the x-y plane,the changes in surface height and density of statescause changes in current. These changes aremapped in images. This change in current withrespect to position can be measured itself, or the

height, z, of the tip corresponding to a constantcurrent can be measured.These two modes are calledconstant height mode and constant current mode,respectively.

In constant current mode, feedback electronics adjust

the height by a voltage to the piezoelectric heightcontrol mechanism.This leads to a height variationand thus the image comes from the tip topographyacross the sample and gives a constant chargedensity surface; this means contrast on the image is

due to variations in charge density.

Scanning


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Scanning

In constant height mode, the voltage and heightare both held constant while the current changesto keep the voltage from changing; this leads toan image made of current changes over thesurface, which can be related to charge density.The benefit to using a constant height mode isthat it is faster, as the piezoelectric movementsrequire more time to register the height change inconstant current mode than the current change inconstant height mode.

All images produced by STM are grayscale, withcolor optionally added in post-processing in orderto visually emphasize important features.

STM


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STM

In addition to scanning across the sample, information on theelectronic structure at a given location in the sample can beobtained by sweeping voltage and measuring current at aspecific location.

This type of measurement is called scanning tunnelingspectroscopy (STS) and typically results in a plot of the localdensity of states as a function of energy within the sample.

The advantage of STM over other measurements of thedensity of states lies in its ability to make extremely localmeasurements: for example, the density of states at animpurity site can be compared to the density of states far

from impurities. Framerates of at least 1 Hz enable so called Video-STM (up

to 50 Hz is possible). This can be used to scan surfacediffusion

Instrumentation


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Instrumentation

Due to the extreme sensitivity of tunnel current toheight, proper vibration isolation or an extremely rigidSTM body is imperative for obtaining usable results.

In the first STM by Binnig and Rohrer, magneticlevitation was used to keep the STM free from

vibrations; now mechanical spring or gas springsystems are often used.Additionally, mechanisms forreducing eddy currents are sometimes implemented.

Maintaining the tip position with respect to the sample,scanning the sample and acquiring the data is

computer controlled.The computer may also be usedfor enhancing the image with the help of imageprocessing as well as performing quantitativemeasurements.


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Probe tips


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Probe tips

STM tips are usually made from W (tungsten)

metal or Pt/Ir alloy where at the very end of thetip (called apex), there is one atom of thematerial.

Scanning tunneling


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g gmicroscopy

The Scanning Tunneling Microscope (STM)allows to probe the local geometric and electronicstructure of surfaces on a mesoscopic scale down

to atomic distances. Figure 1: Principle of scanning tunnelingmicroscopy: Applying a negative sample voltageyields electron tunneling from occupied states atthe surface into unoccupied states of the tip.

Keeping the tunneling current constant whilescanning the tip over the surface, the tip heightfollows a contour of constant local density ofstates.

Principle of STM


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Principle of STM

STM Study


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STM Study

The principle of the STM is based on the strongdistance dependence of the quantum mechanicaltunneling effect (Fig. 1).

A thin metal tip is brought in close proximity of thesample surface. At a distance of only a few , theoverlap of tip and sample electron wavefunctionsis large enough for a tunneling current It to occurwhich is given by

It ~ e-2kd

where d denotes the tip-sample distance and k is a constant depending on the height of the

potential barrier .

STM study


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STM study

Formetals with typical work functions of 4 eV-5 eV,

k is of the order of 1 -1. Hence, an increase of the tunneling distance

of only 1 changes the tunneling currents byabout an order of magnitude.

If the tip is scanned over the sample surface whilean electronic feedback loop keeps the tunneling

current constant (constant current mode), the tipheight follows a contour of constant local densityof states and provides information on thetopography of the sample surface.

Important in Many Sciences


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Important in Many Sciences

The study of surfaces is important in theapplications in semiconductor physics andmicroelectronics.

In chemistry, surface reactions also play animportant part, for example in catalysis.

The STM works best with conducting materials,

but it is also possible to fix organic molecules on asurface and study their structures. For example,this technique has been used in the study ofDNA molecules.

AFM


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AFM

0 ne of the main limitations of STM is the requirementof sample conductivity.

In 1986 Gerd Binnig, Calvin Quate and ChristophGerber

proposed a new type of microscope which couldovercome this limitation.

Insteadof measuring tunneling currents between aprobing tip and sample, the authors suggestedmeasuring forces on an atomic scale.

The atomic force microscope (AFM) is a synthesis of themechanical profilometer, using mechanical springs tosense forces, and the STM, using piezoelectric

transducers for scanning.

an er aa s spers onforces & dipole dipole


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forces & dipole-dipoleattractions Intermolecular versus intramolecular

bonds

Intermolecularattractions are attractions

between one molecule and a neighbouringmolecule.

The forces of attraction which hold anindividual molecule together (for example, thecovalent bonds) are known as in tramolecularattractions.

Inter Molecular forces


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Inter Molecular forces

All molecules experience intermolecular attractions,although in some cases those attractions are veryweak.

Even in a gas like hydrogen, H2, if you slow themolecules down by cooling the gas, the attractions are

large enough for the molecules to stick togethereventually to form a liquid and then a solid.

In hydrogen's case the attractions are so weak thatthe molecules have to be cooled to 21 K (-252C)

before the attractions are enough to condense thehydrogen as a liquid.

Helium's intermolecular attractions are even weaker-the molecules won't stick together to form a liquid untilthe temperature drops to 4 K (-269C).

van der Waals forcesalso known


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also knownas "London forces"

van der Waals forces : dispersion forces The origin of van der Waals dispersion forces

is the phenamenon ofTemporary fluctuatingdipoles

Attractions are electrical in nature. In asymmetrical molecule like hydrogen, however,there doesn't seem to be any electrical distortionto produce positive or negative parts. But that'sonly true on average.

The lozenge-shaped diagram represents a smallsymmetrical molecule - H2,. The even shadingshows that on average there is no electrical

distortion.

Mobility of electrons


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can bring polarity to the molecule

But the electrons are mobile, and at any oneinstant they might find themselves towards oneend of the molecule, making that end - ve. The

other end will be temporarily short of electronsand so becomes +ve.

An instant later the electrons may well havemoved up to the other end, reversing thepolarity of the molecule.

All molecules


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behave like fluctuating dipoles

This constant "sloshing around" of theelectrons in the molecule causes rapidlyfluctuating dipoles even in the mostsymmetrical molecule.

It even happens in monatomic molecules -molecules of noble gases, like helium, whichconsist of a single atom.

If both the helium electrons happen to be onone side of the atom at the same time, thenucleus is no longer properly covered byelectrons for that instant.

How tempo rary d ipolesg ive r ise to intermo lecu lar


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g ive r ise to intermo lecu lar

attract ions Imagine a molecule which has a temporary polarity beingapproached by one which happens to be entirely non-polar

just at that moment.

As the right hand molecule approaches, its electrons will tendto be attracted by the slightly positive end of the left hand

one. This sets up an induced dipo lein the approaching molecule,

which is orientated in such a way that the +ve end of one isattracted to the -ve end of the other.

Molecular polarities keep reversing


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Molecular polarities keep reversing

An instant later the electrons in the left handmolecule may well have moved up the other end.In doing so, they will repel the electrons in theright hand one.

The polarity of both molecules reverses, but youstill have +ve attracting -ve. As long as themolecules stay close to each other the polaritieswill continue to fluctuate in synchronisation so thatthe attraction is always maintained.

e s reng o spers onforces


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forces

The strength of dispersion forces Dispersion forces between molecules are

much weaker than the covalent bonds within

molecules. the size of the attraction varies considerably

with the size of the molecule and its shape.

The more electrons you have, and the moredistance over which they can move, the biggerthe possible temporary dipoles and thereforethe bigger the dispersion forces.

Bigger molecules have higher boiling


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points

bigger molecules have higher boiling pointsthan small ones. Bigger molecules have moreelectrons and more distance over which

temporary dipoles can develop - and so thebigger molecules are "stickier".

The shapes of the molecules also matter. Longthin molecules can develop bigger temporary

dipoles due to electron movement than shortfat ones containing the same numbers ofelectrons.

Inter Molecular forces & Inter molecularbonding


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bondingVan der Waals forces

van der Waals forces:


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dipole-dipole interactions

van der Waals forces: dipole-dipoleinteractions

A molecule like HCl has a permanent dipolebecause chlorine is more electronegative thanhydrogen. These permanent, in-built dipoleswill cause the molecules to attract each otherrather more than they otherwise would if theyhad to rely only on dispersion forces.



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It's important to realise that all moleculesexperience dispersion forces. Dipole-dipoleinteractions are not an alternative to dispersionforces - they occur in addition to them. Molecules

which have permanent dipoles will therefore haveboiling points rather higher than molecules whichonly have temporary fluctuating dipoles.

Surprisingly dipole-dipole attractions are fairly

minor compared with dispersion forces, and theireffect can only really be seen if you compare twomolecules with the same number of electrons andthe same size


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Produced by: Imaging Technology Group

Beckman Institute for Advanced Science

and TechnologyUniversity of Illinois at Urbana-Champaign

AFMAtomic Force
http://www.itg.uiuc.edu/http://www.itg.uiuc.edu/


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Microscope.

AFM measures the the force between theatoms essentially the Van der Waals

dispersive force and the dipole-dipole forces.

AFM


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AFM

The fundamental difference between the STMand the AFM is that :

The STM measures the tunneling current.

The AFM measures the force exerted betweenthe surface and the probe tip.

AFM


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AFM

Both STM and AFM have two modes ofoperation.

AFM can operate in the close contact

mode in which the core-to-core repulsiveforces with the surface dominate or in agreater seperation

no-contact mode in which the relevant force

is the gradient of the Van der Waals potential.

Like in the STM, a piezoelectric scanner is

used


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The vertical motions of the tip are monitoredby the interference pattern of a light beam froman optical fibre or by the reflection of theLASER beam

The AFM is sensitive to the vertical componentof the

surface forces.

AFM


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AFM

AFM Cantilever and Lasert


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arrangement

Applications of AFM


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Applications of AFM

Insulators, such as photosensitive silver halides, could be characterized by AFM without exposure to radiation.

Organic materials could be imaged from microns downto the molecular scale. Biological macromolecules,

polymers, ceramics and glasses are other examplesinvestigated by AFM.

Apart from being applied to different materials, theinstrument

was improved continuously. New detection methods, microfabricating processes for

the sensor preparation and incorporation of themicroscope into different environments, such as liquids,

vacuum, and low temperature, are examples of the

MFM


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MFM

The basic principle of this microscope to measure forcesor to measure interactions between a sharp probing tipand sample surface led to the creation of a

variety of other scanning probe microscopes (SPM),

such as

the magnetic force microscope (MFM),

The dipping force microscope (DFM),

th e friction force microscope (FFM), and

the electrostatic force microsope (EFM).

Applications of AFM


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Applications of AFM

By these new developments the field becamefurther subdivided.

Concurrently, there is also an unifying tendency to

combine different methods. such as STM/AFM, AFM/MFM, AFM/FFM. This provides the unique opportunity

to characterize a single nm-sized spot by acombination of methods and therefore gain moreinformation than by the separate application of asingle method.

MFM


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The magnetic force microscope (MFM) is avariety of atomic force microscope, where a sharp magnetized tip scans a magnetic

sample; the tip-sample magnetic interactions are detected

and used to reconstruct the magnetic structure ofthe sample surface.

Many kinds of magnetic interactions are

measured by MFM, including magnetic dipoledipole interaction. MFM scanning often uses non-contact AFM (NC-AFM) mode.

MFM measurements


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In MFM measurements, the magnetic force between the sample and the tipcan be expressed as,

F = (m. ) H where m is the magnetic moment of the tip (approximated as a point

dipole), is the magnetic stray field from the sample surface, and 0 is themagnetic permeability of free space.Where m is the magnetic moment strayfield from the sample surface, and 0 is the magnetic permeabilityof freespace.

Because the stray magnetic field from the sample can affect the magneticstate of the tip, and vice versa, interpretation of the MFM measurement isnot straightforward. For instance, the geometry of the tip magnetization

must be known for quantitative analysis. Typical resolution of 30 nm can be achieved, although resolutions as low as

10 to 20 nm are attainable.

Unit VII- Nano and MolecularElectronics


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Electronics

Nano Electronics: There are two categories:

Fabrication of Electonic devices andcomponents using the Nanotechnologies(Nanoparticles)

Fabrication of devices and components in the

100 nm range.In the second category we encounter inter-atomic interactions and quantum mechanicaleffects.

90nm & 65nm technologiesdo not produce nano devices


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do not produce nano devices

Hence the present day transisitors andprocessors like the CMOS 90 and Pentium 4

processors cannot be considered into Nanoelectronic devices although they use the 90nmand 65nm technologies.

Nano material Electronics:


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Nano material Electronics:

packing more ICs per chip

higher electron mobilities, higher dielectric constants and a

SYMMETRICAL ELECTRON - HOLE

characteristicin these devices.

Quantum Point Contact: BreakJunction


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Junction

This is an electrical junction between two wiresformed by pulling the wires apart and create aseperation of a few atomic distances.

piezoelectric crystals are used to pull the wire.

This makes a controlled breaking upto aprecision of an angstrom or less.

The breaking of the wire is controlled bymonitoring the electrical current through the

junction.

Break junctionTwo terminaldevice


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device

Conductance Vs Time plot is made. It has two regimes.

1. First Regime:Quantum point contact.here conductance decreases in steps equal tothe

conductance quantum Gq = 2e2 / h= 7.75 x 10-5 Siemens and this corresponds toroughly 12.9 K of resistance.

Break Junctionan electricaldiode


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diode

As each single atom wide strands bridging thewire keeps breaking the conductance decreasesin conductance quanta.

The neck becomes thinner as the strands breakand the neck reconfigures to give a step likedecrease in the conductance.,

2.The second regime; As all the strands arepulled apart the conductance falls to less than the

conductance quantum. This is the tunnelingregime.where the electrons tunnel through the vacuumbetween the electrodes.

Nano wires or quantum wires


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q

nanowires or quantum wires also exhibit theabove properties.

Electrons in nanowires are quantum confinedlaterally and hence occupy energy levels that aredifferent from the energy levels found in bulkmaterials.

Hence they manifest the quantum conductance ofelectricity. Such discrete values arise from aquantum mechnaical restraint on the number ofelectrons that can travel through the wire at the

nano scale

QUANTUM WIRES- Quantization ofResistance


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Resistance

Due to confinement of conduction electrons in the transversedirection of the wire their transeverse energy is quantised intoa series of discrete values E0 , E1 etc.

A consequence of this is non validity of the classical formulafor calculating the electrical resistivity of the wire.

Thus R= L / A is not valid for quantum wires.

An exact calculation of the transverse energies of theconfined electrons has to be performed to calculate the wiresresistance. Following the quantization of the electronenergies the resistance is also found to be quantized.

Nanocircuits.


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Nano circuits have Transistors

Interconnects and

Architectureall in the nano scale.

nano electronics


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In nano electronics, Transistors might be Organic molecules or

nano scale inorganic structures. And

Interconnects arenanotubes and nanowires

Nano circuits


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Architecture combines circuits, that haveredundant logic gates and interconnects, withthe ability to reconfigure structures at severallevels on the chip.

The redundancy allows the circuit to identifythe problems and reconfigure itself so that the

circuit can avoid more problems and workproperly.

Carbon nanotube transistor


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The transistor has two different branches ofcarbon nano tubes that meet at an angle point

giving it aY shape.

Current can flow through both the branchesand is controlled by a third branch that turnsthe voltage

On or off.

MEMS-MicroElectroMechanicalSystems


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MicroElectroMechanicalSystems

MEMS are micro machines which are made up ofcomponents between 1 and 100 micrometers andthey range in size between 20 micrometers to amillimeter.MEMS consist of a Central processing unit that

processes the data and several microsensors whichinteract with outside world.Due to large surface area to volume ratio of MEMS,surface effects such as electrostatics and wettingdominate the volume effects such as inertia and

thermal mass.MEMS became practical after they could befabricatedby the semiconductor fabrication tchniques.

MEMS merges into NEMS at the nano scale.

MEMS--Materials


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Several different types of materials could be used for thefabrication of MEMS, Depending on the material usedthe process technology differs.

Silicon: Semiconductor technology used for fabrication

of electronics is used. This is very reliable and accuratedevices can be produced. However, silicon and thesemiconductor technology are very expensive.

Polymers: Cheaply produced polymers can be used as

an alternative. Polymers can be produced in largevolumes and with variety of material characteristics.

These devices are especially suited to microfluidicapplications such as disposable blood testing cartridges.

NEMS


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NEMS NEMS are similar to MEMS and are in nano

scale.

Smaller displacements and smaller forces thanMEMs can be measurd with these devices.

They can be produced either by top-down orbottom-up approaches .

NEMS are still in their inception and canbecome a force soon.

Single Electron Tunneling


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Let us couple the quantum Dots into the circuitand see how it works. The dot is coupled toelectrodes that can add or subtract elctronsfrom it.

The figure shows an isolated dot or Islandcoupled through tunneling to two leads :

a source lead that supplies electrons and

a drain lead that removes electrons for use inan external circuit.

Quantum Dot in a circuit


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Single Electron Tunneling


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The applied Voltage V sd causes the direct current I toflow with electrons tunneling in and out of the quantumdot.

If we assume that the Ohms law V = IR, is valid in thesystem, the current flow I through the circuit equals theapplied voltage, the source drain voltage, Vsd dividedby the resistance R.

Resistance R arises mainly from the process of elctrontunneling

From source to quantum dot and from quantum dot todrain.


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Quantum Dot in a circuit with applied voltages


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Device works as FET


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Vg controls the resistance of the active regionof the quantum dot.

Consequently, the current flow between the

source and the drain is regulated.Thus this device functions as voltagecontrolled or field effect controlled Transistor(FET).

Continuous flow of the current


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In bulk systems or in the macroscopic dimensions the currentflow is continuous.

The discreteness of the individual electronspassing through the device manifests itself only by thepresence of the current fluctuations or shot noise.

where as in the quantum systems , our interest is in thepassage of the electrons, one by one, through thenanostructures and this can be realised with the circuit wehave been describing.

Nano sized electrodesto be used for circuitry with the nanodots


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y

For the FET type nanostructures the quantumdots are in the low nanometer range

and the attached electrodes have crosssections comparable in size to the nano dot.

Small Capacitance of the nano dot


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For Disc shaped and Spherical shaped dots Having radius r the capacitance is given by

C = 8 ( / ) r for the disc

And C = 4 ( / ) r for the sphere

where ( / ) is a dimensionless dielectric const. of the

semiconducting material of the nano dot.and

= 8.8542 x 10-12 F / m is the dielectric const.of free space

For GaAs nano dot ( / ) = 13.2 and this gives a very small C

C = 1.47 x 10 -18 r Farads for the spherical shape


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Change in the potentialis large enough to impede the tunneling


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g g p g

For a nano structure of radius of r = 10 nm

the change in the potential is 11 mV which iseasily measurable.

It is large enough to impede the tunneling ofthe next electron

Conditions for single elecrontransfer


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transfer

Two conditions must be satisfied for the observation of singleelectron charge transfer.

1. e2 / 2C > KBT ie Single Electron charging energy must exceed the thermal Energy of

random lattice vibrations

2. Hisenberg uncertainity principle between the product of the capacitor energy, e2 / 2C , and the time of charging T = RTC where R T is

tunneling resistance of the potential Barrier. E .T = ( e 2/2C ) (RTC ) > or = h

Tunneling conditions


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these two tunneling conditions correspond to:e2 / 2C >> KBT andRT >> h / e

2 = 25.813 K is the quantum of

resistance.

When these conditions are met and as the voltage acrossquantum dot is scanned the current jumps every time thevoltage changes by V = e / C 0.109 / r

and resistance changes in increments of 25.813 K.And the I-V characteristics will be a step function(CoulombStaircase)This is called the Coulomb Blockade.

Magneto Resistance


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Magneto Resistance :

First Discovered by William Thomson in 1856

It is Change of electrical resistivity of a material

when an external magnetic field is applied.

Anisotropic MagnetoResistance(AMR)


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Anisotropic Magneto Resistance(AMR):In normal metals

Theresistance increases when the magnetic field isparallell to the direction of the current and

the resistance decreaseswhen thefield is Perpendicular.

This difference is called the Anisotropic MagnetoResistance(AMR)

Maximum effect observed in metals was about 5%

In a Semiconductor with a single charge carrier

type(eg.InSb) the MR can be upto 100%.(MR in Semiconductor = (1+ B)

-

Giant Magneto Resistance (GMR)

spintronics- Generation I


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Observed in thin film structures of alternateFerromagnetic and Non magnetic layers. FM / NM

/ FM

Manifests as a significant DECREASE in electricalresistance in the presence of a magnetic field.

Needs structures consisting of layers of a few atomthick.

Hence GMR is considered as one of thefirst real applications ofNanotechnology

SuperlatticesShowing proper maching is necessary betweenlayers ofFM/NM/FM


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Giant Magneto ResistanceSPIN VALVE


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GMR is a very important property inNano structuresEg. THIN FILMS a few atoms

thick

When two ferromagnetic thin film layers areseparated by A non-magnetic thin layer theresistivity decreases in the presence of magneticfield.

MR = (RHRO)/RH isve and much larger effect(~10%) than AMR..

Such an arrangement is called the SPIN VALVE

GMR SPIN VALVESensitivity ~10%


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Magnetic

Magnetic

Substrate

Non Magnetic


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Documents

ECE III-2 NanoTechnology Elective