afm and sem

8/6/2019 afm and sem

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Atomic force microscope &

Scanning electron microscope

Presented by: Dhwani(08EC061) Varuna(08EC068)


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Atomic force microscopy

It is also known as scanningforce microscopy.

AFM is a very high resolutiontype of scanning probemicroscopy , with resolution of order of a fraction of nanometer.

AFM is one of the foremosttools for imaging , measuring andmanipulating matter at thenanoscale.


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Bl ock Diagram of AFM


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Modes of AFM

There are three modes in which AFM can be operateddepending on application:-1. Static mode (contact).2. Dynamic mode(non contact).3. Tapping mode.

Static mode

Feedback-static tip deflection.Attractive forces-strong,causing the tip to snap in to the

surface.Force is repusive.


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Dynamic modeI n the dynamic mode, the cantilever is externally

oscillated at or close to its fundamental resonancefrequency or a harmonic . The oscillation amplitude,

phase and resonance frequency are modified by tip-sample interaction forces.

Non-contact mode AFM does not suffer from tip or sample degradation effects that are sometimes observedafter taking numerous scans with contact AFM. This

makes non-contact AFM preferable to contact AFM for measuring soft samples as shown in fig.


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Schemes for dynamic mode operation includefrequency modulation and

the more common amplitude modulation .T apping mode

I n tapping mode , the cantilever is driven to oscillate up anddown at near its resonance frequency by a small piezoelectricelement mounted in the AFM tip holder similar to non-contactmode.

Due to the interaction of forces acting on the cantilever whenthe tip comes close to the surface, Van der Waals force, dipole-dipole interaction, electrostatic forces, etc cause the amplitudeof this oscillation to decrease as the tip gets closer to thesample.


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A tapping AFM image is therefore produced by imaging the force of the intermittent contacts of the tip with the sample surface.

The advantage of TappingMode with respect to contact mode is thatit eliminates the lateral, shear forces present in contact mode. Thisenables TappingMode to image soft, fragile, and adhesive surfaceswithout damaging them, which can be a drawback of contact mode

AFM.


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Force spectroscopy

One of the application of AFM, the direct measurementof tip-sample interaction forces as a function of the gap

between the tip and sample.

Used to measure nanoscale contacts, atomic bonding,

Van der Waals forces, and Casimir forces, dissolutionforces in liquids and single molecule stretching andrupture forces.


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Advantages of AFMAFM provides a three-dimensional surface profile.

AFM do not require any special treatments (such as metal/carboncoatings )

AFM modes can work perfectly well in ambient air or even aliquid environment

AFM can provide higher resolution than SEM

High resolution AFM is comparable in resolution to scanningtunneling microscopy and transmission electron microscopy.


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Disadvantages of AFM

AFM can only image a maximum height on the order of 10-20micrometers and a maximum scanning area of about 150150micrometers.

The relatively slow rate of scanning during AFM imaging

AFM images can also be affected by hysteresis of the piezoelectric material and cross-talk between the x , y , z axes

AFM cannot normally measure steep walls or overhangs.


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Scanning E lectron Microscope

Scanning electron microscope is a type of electronmicroscope that images the sample surface by scanning itwith a high energy beam of electrons in a raster scan

pattern.The electrons interact with the atoms that make up the

sample producing signals that contain information aboutthe sample's surface topography, composition and other

properties such as electrical conductivity.


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W orking of SEM


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When beam hits the sample electrons and X-rays areejected from the sample


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Samp l e Preparation in SEMWater must be removed from the samples.Metals require no preparation before being used.

Non metals need to cover with a thin layer of conductivematerial called as sputter coater.

Sputter coater operation

Uses an electric field and argon gas.A sample is placed in a vacuum.Argon gas and an electric field makes the atoms positively charged.The argon ions then become attracted to a negatively charged gold

foil.The argon ions knock gold atoms from the surface of the gold foil.These gold atoms fall and settle onto the surface of the sample thus

producing a thin gold coating.


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G a ll ery of SEM images

Co loured SEM image of soybean cyst nematodeand egg. Th e co lourmakes t h e image easierfor non-specia l ists toview and understand t h estructures and surfacesrevea led inmicrograp h s.

SEM image of norma l circu latingh uman b lood. Th is isan o lder and noisymicrograp h of acommon subject forSEM micrograp h s:red b lood ce ll s.

SEM image of t h ecorrosion layer on t h esurface of an ancientg lass fragment; noteth e laminar structureof t h e corrosion layer


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Advantages of SEM

The SEM has a large depth of field , which allows more of aspecimen to be in focus at one time .

The SEM also has much higher resolution , so clearly spacedspecimens can be magnified at much higher levels .

Because the SEM uses electro magnets rather the lenses, theresearcher has much more control in the degree of magnification.


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These two instruments are complementary, the advantages of one

compensating for the drawbacks of the other.The combination of these two techniques in the same instrument

opens the way towards the construction of a multi-dimensional dataspace corresponding to the same place on the sample surface: theSEM gives access to information coming from secondary and

backscattered electrons and allows X ray analysis to be performed,whereas the SFM, besides 3-D morphological analysis, allowsnanotri-bological investigations (friction, wear, adhesion) andstudies of some physical properties (viscoelastic, electric and

magnetic) to be made.

CONCLUSION


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THANK YOU

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afm and sem