Dr. Alagiriswamy A A (Sr. Grade) [ M.Sc., PhD, PDF] Department of Physics and Nanotechnology

Dr. Alagiriswamy A A (Sr. Grade) [ M.Sc., PhD, PDF]Dr. Alagiriswamy A A (Sr. Grade) [ M.Sc., PhD, PDF]Department of Physics and Nanotechnology Faculty of Eng. & Technology, SRM UNIVERSITY Main campus, SRM Nagar,Kattankulathur,Chennai – 603203, TamilnaduVoice : - 91-9791512150 91- 9444128695Tel : 91-44 -27452270 Extn. 1325

Friday, April 21, 2023

Lecture 4Lecture 4


Outline of the Presentation

Friday, April 21, 2023 3

Bottom-up approaches

Sol-gel processes involve

colloidal particles dispersed in a liquid (sol)

deposition of such particles by spraying, dipping/spinning

they could polymerize (to form a continuous network)

final heat treatments (pyrolyze) to form dense structures

Perhaps this could be “ Bottom – top (up) approach”

Tetra ethyl orthosilicate (TEOS)


Fabrication procedures;The SOL is made of solid particles of a diameter of few hundred

of nm suspended in a liquid phase.

Liquid solution of organometallic precursors (TMOS, TEOS, Zr(IV)-Propoxide, Ti(IV)-Butoxide, etc. ), which, by means of hydrolysis and condensation reactions, lead to the formation of a new phase (SOL).

Then the particles condense in a new phase (GEL) in which a solid macromolecule is immersed in a liquid phase (solvent). Drying the GEL by means of low temperature treatments (25-100 C), it is possible to obtain porous solid matrices (XEROGELs).

The fundamental property of the solgel process is that it is possible to generate ceramic material at a temperature close to

room temperature.

Therefore such a procedure opened the possibility of incorporating in these glasses soft dopants, such as fluorescent dye molecules and organic chromophores.


Fabrication components/details

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Traditional Sol-Gel Methodology

http://www.solgel.com

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Typical Starting Materials

The sol-gel process: (a) sol; (b) gel.

Applications of Aerogel

Thermal InsulationAcoustic InsulationCatalyst SupportOptical applicationsNuclear Waste Storage Filler for paints or otherslow dielectric constant materialsBatteriesUltrafilteration, reverse osmosis Friday, April 21, 2023

Key advantages over other techniques

high purity products could be obtained

low temperature processing facilities

Simple, economic methodsEasily shape materials into complex geometries (gel phase)

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MICROWAVE SYNTHESIS OF MATERIALS


Microwaves are a form of electromagnetic energy.

Microwaves, like all electromagnetic radiation, have an electrical component as well as a magnetic component.

The microwave portion of the electromagnetic spectrum is characterized by wavelengths between 1 mm and 1 m, and corresponds to frequencies between 100 and 5,000 MHz

absorb the energy, they can reflect the energy, or they can simply pass the energy

Microwave interaction with matter is characterized by a penetration depth and its frequency

What are microwaves ???

MICROWAVE SYNTHESIS: - Modern Technology


Comparison of conventional heating with microwaves

Dipole interactions occur

polar ends of a molecule tend to align themselves

1) dipole interactions 2) ionic conduction

loss tangent is the measurable quantity


Conventional heating (thermal) conduction process mono-directional low uniformity low quality films attainable

Microwaves heating (non-conventional; Advanced)

convection process multi-directional high uniformity so good quality films (Al2O3, Fe2O3, Ti2O3……… attainable

A close comparison


Material Synthesis

The discovery of new materials requires the development of a diversity of synthetic techniques.

Microwave methods offer the opportunity to synthesize and modify the composition, structure and morphology of materials, particularly composites via differential heating.

Microwave-induced plasmas (MIPs) allow any solid mixture to be heated, and can promote direct microwave heating at elevated temperature, greatly expanding the use of microwaves for reactions between solids and gas–solid mixtures.

Microwave-assisted synthesis is generally much faster, cleaner, and more economical than the conventional methods. A variety of materials such as carbides, nitrides, complex oxides, silicides, zeolites, apatite, etc. have been synthesized using microwaves.


• Dipole interactions occur with polar molecules.

• The polar ends of a molecule tend to align themselves and oscillate in step with the oscillating electrical field of the microwaves.

• Collisions and friction between the moving molecules result in heating.

• Broadly, the more polar a molecule, the more effectively it will couple with (and be influenced by) the microwave field.

Principle of microwave synthesis;

On how


Dissipation factor (often called the loss tangent); a ratio of the dielectric loss (loss factor) to the dielectric constant.

The dielectric loss (a measure of how material absorbs)

How to quantify this µws procedure ?


SEM/TEM

SFM (AFM/STM and others)

Surface characterization (sophisticated) techniques


• The transmission electron microscope (TEM) was the first type of Electron Microscope to be developed and is patterned exactly on the light transmission microscope except that a focused beam of electrons is used instead of light to "see through" the specimen. It was developed by Max Knoll and Ernst Ruska in Germany in 1931.

• The first scanning electron microscope (SEM) debuted in 1938 (Von Ardenne) with the first commercial instruments around 1965. Its late development was due to the electronics involved in "scanning" the beam of electrons across the sample.

Dates

Max Knoll(1867 -1968)

Von Ardenne(1907 -1996)


Electron Microscopy Techniques Electron Microscopes are scientific instruments that use a beam of highly energetic electrons to examine objects on a very fine scale. The main advantage of Electron Microscopy is the unusual short wavelength of the electron beams, substituted for light energy ( = h/p) The wavelengths of about 0.005 nm increases the resolving power of the instrument to fractions Topography The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties (hardness, reflectivity...etc.)

Features of Electron microscopesMorphology

The shape and size of the particles making up the object; direct relation between these structures and materials properties (ductility, strength, reactivity...etc.)

Composition

The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties (melting point, reactivity, hardness...etc.)

Crystallographic Information. How the atoms are arranged in the object; direct relation between these arrangements and materials properties (conductivity, electrical properties, strength...etc.)


Could you name them all

Wings of butterfly

Red platelet cells

Fibrillar hair

A close comparison between

Digital microscope• Photon beam• 2D projection• No need of vacuum• Difficult to obtain in-situ images• Low magnification• Inexpensive

Electron microscope• Electron beam• 3D projection• need of high/ultrahigh

vacuum• Possible in-situ images• high (× 106)

magnification• Very expensive ; 50

lacs


Types

Scanning electron microscopy, which looks at the surface of a solid object.

Transmission electron microscopy, which essentially looks through a thin slice of a specimen.


The "Virtual Source" - the electron gun, produces a stream of monochromatic electrons.

This stream is focused to a small, thin, coherent beam by the use of condenser lenses 1 and 2. The first lens (usually controlled by the "spot size knob") largely determines the "spot size"; the general size range of the final spot that strikes the sample.

The second lens (usually controlled by the "intensity or brightness knob" actually changes the size of the spot on the sample; changing it from a wide dispersed spot to a pinpoint beam.

The beam is restricted by the condenser aperture (usually user selectable), knocking out high angle electrons (those far from the optic axis, the dotted line down the center)

The beam strikes the specimen and parts of it are transmitted

Source : - Inelastically Scattered Electrons Bragg’s lawKakuchi Bands: - Bands of alternating light and dark lines that are formed by inelastic scattering interactions that are related to atomic spacings in the specimen


Electrons-solid interactions


Scanning Electron Microscope (SEM)

Working Concept

SEM allows surfaces of objects to be seen in their natural state without staining.

The specimen is put into the vacuum chamber and covered with a thin coating of gold to increase electrical conductivity and thus forms a less blurred image.

The electron beam then sweeps across the object building an image line by line as in a TV Camera.

As electrons strike the object, they knock loose showers of electrons that are captured by a detector to form the image.

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Transmission Electron Microscope (TEM)

Working Concept TEM works much like a slide projector. A projector shines a beam of light

through (transmits) the slide, as the light passes through it is affected by the structures and objects on the slide.

These effects result in only certain parts of the light beam being transmitted through certain parts of the slide.

This transmitted beam is then projected onto the viewing screen, forming an enlarged image of the slide.

TEMs work the same way except that they shine a beam of electrons (like the light) through the specimen (like the slide).

Whatever part is transmitted is projected onto a phosphor screen for the user to see.

34Lanthanum hexaboride (LaB6)


Bone tissue

SEM Images of


SFM (AFM/STM and others)

Surface characterization (sophisticated) techniques

“Seeing” at the nanoscale Vertical resolution 1 Å level

Lateral resolution depends on tip sharpness

Branches of Scanning Probe Microscopy

http://spm.phy.bris.ac.uk/

Materials Investigated: Thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors.

Used to study phenomena of: Abrasion, adhesion, cleaning, corrosion, etching, friction, lubricating, plating, and polishing.

AFM can image surface of material in atomic resolution and also measure force at the nano-Newton scale.

General Applications

Scanning Probe Microscopes (SPMs)

Monitor the interactions between a probe and a sample surface

What we “see” is really an imageTwo types of microscopy we will look at:

Scanning Tunneling Microscope (STM)Atomic Force Microscope (AFM)

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Digital Instruments Multi-Mode head, scanner

and base

April 21, 2023

SPM system overview

The human hand cannot precisely manipulate at the nanoscale level

Therefore, specialized materials are used to control the movement of the tip

Putting It All Together

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Scanning Tunneling Microscopes (STMs)

• Monitors the electron tunneling current between a probe and a sample surface

• What is electron tunneling?– Classical versus

quantum mechanical model

– Occurs over very short distances

Scanning Probe

Tip and surface and electron tunneling

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Challenges of the STM

• Works primarily with conducting materials

• Vibrational interference

• Contamination

– Physical (dust and other pollutants in the air)

– Chemical (chemical reactivity)

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Atomic Force Microscopes (AFMs)

• Monitors the forces of attraction and repulsion between a probe and a sample surface

• The tip is attached to a cantilever which moves up and down in response to forces of attraction or repulsion with the sample surface– Movement of the

cantilever is detected by a laser and photodetector

Source: http://www.nanoscience.com/education/AFM.html

Laser and position detector used to measure cantilever movement

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SPM Tips

• The size of an AFM tip must be carefully chosen

Source: http://mechmat.caltech.edu/~kaushik/park/3-3-0.htm

•Interatomic interaction for STM (top) and AFM (bottom).

•Shading shows interaction strength.

STM tip

AFM tip

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Mode of Operation Force of Interaction

Contact mode strong (repulsive) - constant force or constant distance

Non-contact mode weak (attractive) - vibrating probe

Tapping mode strong (repulsive) - vibrating probe

Lateral force mode frictional forces exert a torque on the scanning cantilever

Modes of SFM


Different modes of SFM

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A group of grains exchange their neighbors

during deformation.

A compelling competition between crystallization

and disordering

A Nanomechanism

Superplasticity – the ability of a material

to sustain large plastic deformation – has

been demonstrated in a number of

metallic, intermetallic and ceramic

systems. Conditions considered necessary

for superplasticity are a stable fine-

grained microstructure and a

temperature higher than 0.5 Tm (where

Tm is the melting point of the matrix).

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Xenon atoms

Carbon monoxide molecules

Source: http://www.almaden.ibm.com/vis/stm/atomo.html

And What Can We Do?

• Using STMs and AFMs in Nanoscience – Allows atom by atom (or clumps of atoms by

clumps of atoms) manipulation as shown by the images below

Advantages of AFM

• AFM versus STM (scanning tunneling microscope): both conductors and insulators

• AFM versus SEM (scanning electron microscope): greater topographic contrast/3D projection)

AFM versus TEM (transmission electron microscope):• no expensive sample preparation• complicated steps• No need of vacuum (AFM images in air too)

Limitations of AFM

• AFM versus STM (scanning tunneling microscope):– relatively lower resolution

• AFM versus EM (Electron microscopes): • indirect imaging technique (relies on deflection of laser)• limited scan size/slow scanning speed• Hysteresis effects (artifacts)

TThhaannkk YYoouu!!

Why/how do we asses the true nature of…..

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To form a product matrix and find out the balancing amongst the teaching

and learning skills

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Email: [email protected]

HAVE A NICE DAY

Dr. Alagiriswamy A A (Sr. Grade) [ M.Sc., PhD, PDF] Department of Physics and Nanotechnology

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Transcript of Dr. Alagiriswamy A A (Sr. Grade) [ M.Sc., PhD, PDF] Department of Physics and Nanotechnology