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CHARACTERIZATION TECHNIQUES OF NANOMATERIALS

Dr. Sajini Vadukumpully

COURSE STRUCTUREWeek 1 Jan 21 - 26 Week 14 April 22 - 27

Week 2 Jan 28 – Feb 2 Week 15 April 29 – May 4

Week 3 Feb 4 - 9

Week 4 Feb 11 - 15

Week 5 Feb 18 - 23 Total hours 30 h

Week 6 Feb 25 – March 2 Weeks 1 - 3 SEM, TEM

Week 7 March 4 - 9 Weeks 4 -8 Chemical characterization techniques

Week 8 March 11 - 16 Weeks 9 - 12 Structural characterization - XRD

Week 9 March 18 - 23 Weeks 12 - 15 SPM

Week 10 March 25 – 30

Week 11 April 1 - 6

Week 12 April 8 - 13

Week 13 April 15 - 20

04/11/2023

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Characterization level as a function of known properties.

The characterization of nanomaterials - bottleneck

CHARACTERIZATION OF NANOMATERIALS

04/11/2023

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Electron microscopy is generally used – But it is not capable of providing insights into non-crystalline surface matter or ligands

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Scanning Electron Microscope (SEM): scattering of electrons, secondary and backscattered electrons, electron gun, lenses and apertures, and imaging modes in SEM.

One type of electron microscopic techniques – makes use of electron beams

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SCANNING ELECTRON MICROSCOPE (SEM)

What is SEM? Basic working principle. Major components and functions Electron beam – specimen interactions Different imaging modes in SEM Energy Dispersive X-ray spectroscopy (EDS)

http://virtual.itg.uiuc.edu/training/EM_tutorial/

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SCANNING ELECTRON MICROSCOPE Gives information about topography, chemical

composition, grain size and thickness. Uses electron beam (not light) to form an image. SEM have a magnifications ranging from 10 -

500000x and high resolution ( 2 nm).

Cost: $0.8-2.4M

Magnification Depth of field Resolution

Optical microscope

4 - 1000x 60 – 0.2 m 0.2 m

SEM 10 - 500000x 4 mm – 400 nm. 1 – 10 nm

OM SEMhttp://www.mse.iastate.edu/microscopy/

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MAGNIFICATION vs RESOLUTIONMagnification – Simple enlargement of an object.

Resolution – Capability of making the individual parts of an object, distinguishable.

J Nanopart Res (2010) 12:1777–1786

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MAGNIFICATION vs RESOLUTION

Depth of field

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SCANNING ELECTRON MICROSCOPE

Column

11Source: L. Reimer, “Scanning Electron Microscope”, 2nd Ed., Springer-Verlag, 1998, p.2

Electron Gun

A more closer look!!

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MAJOR COMPONENTS OF SEMo Electron optical column

o Electron gun to produce electrons (thermionic/field emission)

o Magnetic lenses –de-magnifies the beamo Magnetic coils – modify/control the beamo Apertures

o Vacuum systemo Chamber which holds vacuum (pumps to

produce vacuum)o Valves and gauges to control and monitor

vacuumo Signal detection & display

o Detectors which collect the signalo Electronics which produce an image from the

signal

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ELECTRON BEAM SOURCE

Source type : Thermionic, Schottky field emission and cold field emission

http://www.ammrf.org.au/myscope/sem/practice/principles/gun.php

Source Brightness Stability(%) Size Energy spread(eV) Vacuum W 3X105 ~1 50mm 3.0(eV)

10-5

LaB6 3x106 ~2 5mm 1.510-6

C-FEG 109 ~5 5nm 0.3 10-10

T-FEG 109 <1 20nm 0.7 10-9

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THERMIONIC ELECTRON GUNo The electrons boil off

(thermionic emission) the sharply bent tip of the filament and are attracted to the anode.

o The filament is either W or LaB6

o The anode is maintained at a positive voltage ( 5 – 30 kV).

o Vacuum is required to prevent the oxidation of the filament.

o The Wehnelt cylinder is biased negatively relative to the filament. It acts as a grid that repels the emitted electrons and focuses them into a spot of diameter d and divergence half angle, º.

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FIELD EMISSION GUN

http://fel.web.psi.ch/public/gun/index.html

• Electrons are drawn from the filament tip by an intense potential field set up by an anode that lies beneath the tip of the filament.

• Ultra-high vacuum (better than 10-6 Pa) is needed to avoid ion bombardment to the tip from the residual gas.

• Because of the smaller initial spot size (< 2 nm), and lower accelerating voltage (2-5 kV) a much smaller primary excitation zone is produced. Ultimately this results in much greater resolution than is capable with a conventional SEM using a tungsten filament or LaB6 crystal.

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LENSES Condenser lens – focusingdetermines the beam current which impinges on the

sample. Objective lens – final probe forming determines the final spot size of the electron beam,

i.e., the resolution of a SEM. Note: When the instrument is operational, the electron – optical column and sample chamber must be under vacuum.

If the column is filled with any gas, electrons will be scattered by gas molecules, which will reduce beam intensity and stability.

Other gas molecules (may be from sample or microscope) might condense on the sample. This will lower the contrast and clarity of the image.

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HOW THE ELECTRON BEAM IS FOCUSED

A magnetic lens is a solenoid designed to produce a specific magnetic flux distribution.

Lens formula: 1/f = 1/p + 1/q

f Bo2

Demagnification: M = q/p

f can be adjusted by changing Bo, i.e., changing the current through coil.

Condenser lenses are added to demagnify or to reduce spot size

Objective lens controls the focus of the electron beam by changing the magnetic field strength.

If the aperture is wider, electrons may not be focused to the same plane to form a sharp spot. Hence lences with narrow aperture is preferred.

The stigmator, which consist of two pairs of pole-pieces arranged in the X and Y directions, is added to correct the minor imperfections in the objective lens.

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ELECTRON BEAM – SPECIMEN INTERACTIONS

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TYPES OF DETECTORS A detector placed within the column is known as an

“in-lens” detector and produces a very different image compared to a conventionally located detector.

Side Mounted

In-Lens

SE detectors

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SECONDARY ELECTRONS They are produced by the inelastic scattering of

high energy electrons from the conduction/valence electrons in the atom of the specimen.

This causes ejection of electrons from the surface. The energy of emitted electrons is very less and hence, only SE’s from the surface exit the sample and can be examined.

Bright

Dark

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TYPES OF DETECTORS /IMAGING MODES

Everhart-Thornley SE Detector

http://emalwww.engin.umich.edu/newemal/courses/SEM_lectureCW/SEM_se2.html

The incident electrons do not go along a straight line in the specimen, but a zig-zag path instead.The penetration or, more precisely, the interaction volume depends on the acceleration voltage (energy of electron) and the atomic number of the specimen.

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BACKSCATTERED ELECTRONS (BSE)

•BSE are produced by elastic interactions of beam electrons with nuclei of atoms in the specimen and they have high energy and large escape depth.•BSE yield: h=nBS/nB ~ function of atomic number, Z•BSE images show characteristics of atomic number contrast, i.e., high average Z appear brighter than those of low average Z. h increases with tilt.

The image shows the different minerals within the thin section as variations in greyscale contrast (brightest to darkest: zircon, garnet, biotite, K-feldspar, palgioclase and sillimanite).

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BSE DETECTORS

BSE Robinson detector Semiconductor detector

A large scintillator collects the BSE and guides the beam to a photomultiplier

Large collection angle Works at TV frequency

A silicon diode with a p-n junction close to the surface collects the BSE.

Large collection angle Slow (poor at TV frequency)

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If the diameter of primary electron beam is ~5nm- Dimensions of escape zone of

ESCAPE VOLUMES OF VARIOUS SIGNALS

•Secondary electron: diameter ~10nm; depth~10nm

•Backscattered electron: diameter~1m; depth~1m•X-ray: from the whole interaction volume, i.e., ~5m in diameter and depth

This escape volume limits the resolution in images produced with BSE and x-ray (element mapping) to a value that is of the order of size of the escape volume.

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IMAGE FORMATION IN SEMSCAN COIL AND RASTER PATTERN

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Two sets of coils are used for scanning the electron beam across the specimen surface in a raster pattern similar to that on a TV screen.

This effectively samples the specimen surface point by point over the scanned area.

Blanking means interrupting the electron beam briefly and periodically

o Beam is scanned over specimen in a raster pattern in synchronization with beam in CRT.

o Intensity at A on CRT is proportional to signal detected from A on specimen and signal is modulated by amplifier.

beame-

A

A

Detector

Amplifier

10cm

10cm

M= = 10cm/x

C = length of the scan on CRT

x = length of the scan on specimen

HOW FINE WE CAN SEE WITH SEM

Need small electron beam probe to achieve high magnification.

Changing magnification does not involve changing any lens current, only changing the current in the scan coils, and so:• focus does not change as

magnification is changed • the image does not rotate with

magnification change (as in TEM) 28

Low MLarge x40mm

2500x

1.2mm

e-

High Msmall x7mm

x

15000x

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RESOLUTION IN SEMo Ultimate resolution depends on

the electron-optical specifications.

o Electron Optical limitationso Specimen Contrast Limitationso Sampling Volume Limitations o If we can scan an area with width

10 nm (10,000,000×) we are supposed to see atoms in SEM!! But, can we?

o Image on the CRT consists of spots called pixels which are the basic units in the image.

You cannot have details finer than one pixel!

http://emalwww.engin.umich.edu/newemal/courses/SEM_lectureCW/SEM_SpotSize2.html

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RESOLUTION IN SEMSmall beam size is needed for high resolution. Decrease the beam size by:

1.Increasing the current on condenser lens 2.Decreasing the working distance

Decreasing the beam size also decreases the beam current and therefore the signal to noise ratio gets worse.

P=D/Mag = 100m/Mag

P - pixel diameter on specimen surfaceD - diameter of an image point on the CRT

Resolution is the pixel diameter on specimen surface.

Mag P(m) Mag P(nm)10x 10 10kx 10 1kx 0.1 100kx 1

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RESOLUTION IN SEM

The optimum condition for imaging is when the escape volume of the signal concerned equals to the pixel size.

Signal will be weak if escape volume, is smaller than pixel size, but the resolution is still achieved.(Image is ‘noisy’)

Signal from different pixel will overlap if escape volume is larger than the pixel size. The image will appeared out of focus (Resolution decreased)

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IMAGE CONTRASTImage contrast, Cis defined by

SA-SB SC= ________ = ____

SA SA

SA, SB Represent signals generated from two points, e.g., A and B, in the scanned area.

In order to detect objects of small size and low contrast in an SEM it is necessary to use a high beam current and a slow scan speed (i.e., improve signal to noise ratio).

SE-topographic and BSE-atomic number contrast

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SEI vs BSI

Atomic number contrast

Topographic contrast

Field contrastVoltage Charging effect

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ENERGY DISPERSIVE SPECTROSCOPY (EDS)EDS systems include a sensitive x-ray detector, a liquid nitrogen dewar for cooling, and software to collect and analyze energy spectra. The most common detectors are made of Si(Li) crystals that operate at low voltages to improve sensitivity (latest - silicon drift detectors).An EDS detector contains a crystal that absorbs the energy of incoming x-rays by ionization, yielding free electrons in the crystal that become conductive and produce an electrical charge bias. The x-ray absorption thus converts the energy of individual x-rays into electrical voltages of proportional size; the electrical pulses correspond to the characteristic x-rays of the element.

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EDS DETECTOR COMPONENTS

FET – field effect transistor

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STEPS INVOLVED IN EDS

X-ray generation Pre amplification by FET Pulse processing

FET pre-amplificationPulse processor

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X-RAYS GENERATION

Two basic types of X-rays are produced on inelastic interaction of the electron beam with the specimen atoms in the SEM:

Characteristic X-rays result when the beam electrons eject inner shell electrons of the specimen atoms.

Continuum (Bremsstrahlung) X-rays result when the beam electrons interact with the nucleus of the specimen atoms.

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Characteristic X-ray Nomenclature

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MORE ABOUT X-RAYS IN EDS

The most probable transition, when a K-shell vacancy is created is the L to K transition.

Therefore Kα radiation will always be more intense than Kβ radiation.

It also follows that Kβ radiation will be of higher energy than Kα radiation.

For a given atom, Mα radiation will be of lower energy than Lα radiation, which in turn will be of lower energy than Kα radiation.

To ionize an atom, the incoming electron or ionizing radiation must possess a minimum amount of energy. That energy is the binding energy of the particular inner shell electron, which is a specific, characteristic energy for each electron in the atom.

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MOSELEY’S LAW The energy of the characteristic radiation within a

given series of lines varies monotonically with atomic number. This is Moseley’s Law:

E = C1 (Z- C2)2 where:

E = energy of the emission line for a given X-ray series (e.g. Kα)

Z = atomic number of the emitter C1

and C2 are constants

Moseley’s Law is the basis for elemental analysis with EDS.

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CRITICAL PARAMETERS FOR EDS For higher resolution analyses – requires high acceleration voltage Current – Typical EDS beam currents will be in nA range Working distance – Use a working distance ~ 9 mm (depends on the

instrument) Productivity depends on the rate of counts measured, called the output count

(acquisition) rate, rather than the input count rate into the detector As the input rate increases so will the acquisition rate, but an increasing

number of events are rejected because they arrive in a shorter time period than the TP. This phenomenon is termed “pulse pileup”. The way to avoid/minimize this is to ensure a certain percentage of “dead time” – time during which pulses are not measured.

Deadtime = (1 – Output rate/Input rate) x 100.

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Spot 2

Spot 1

Adapted from “Characterization Facility, University of Minnesota—Twin Cities”

Spacial Resolution : Low atomic number (Z): 1-5 um3; High Z: 0.2 – 1 um3

Acceleration voltage : Should be high as compared to normal imaging.

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ELEMENTAL MAPPING IN EDS

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(A) SEM image of a single Au/Ni MF. (B,C) Corresponding Au MR- and Ni KR-based EDAX images collected from the sample shown in panel A. (D) Single particle SEM image of Au/Co MF. A Au M- and Co K-based EDAX image of Au/Co MF (D) is shown in panels E,F, respectively

(A, B) FESEM and EDAX images, respectively, of a single Au/Ag bimetallic MF. (C) Magnified FESEM image of thestems of the Au/Ag MF and (D) corresponding EDAX image. Inset of (C) shows the line profile of the elements taken along the direction shown by the arrow. The elemental profiles are shown in different colors.

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EDS analysis to prove the biphasic nature of the dumbbell shaped nanowires. (A) TEM image of the dumbbell shaped nanowire chosen for EDS analysis. (B) Combined intensity maps for Ag and Te for the nanowire. (C) Te La intensity map across the length of the dumbbell shaped nanowire. Note that the intensity in the middle region is higher than from the tips. (D) Ag La intensity map showing that silver is limited only to both the ends. (E) EDS spectrum collected from area 1 in A showing the presence of both Ag and Te in the atomic ratio of 2 : 1. (F) EDS spectrum from area 2 in A, showing the presence of only Te in the middle section

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CONDITIONS FOR SEM SAMPLE PREPARATION

Must be made conducting to prevent charging (normally, Pt or Au is sputter coated).

Must be vacuum compatible.

Hard/soft materials can be tested.

If the particle size is small (< 100 nm), ultrasonicate to diperse a small amount of powder in a suitable solvent and place a drop on a flat surface and coat using Au or Pt.

If the particle size is large, sprinkle powder on carbon table, blow off excess sample and then coat

Dispersion

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TRANSMISSION ELECTRON MICROSCOPE

Interaction of electrons with matter, elastic and inelastic scattering.

Electron sources, lenses, apertures and resolution.

TEM instrument. Forming diffraction patterns and images, Selected area and convergent beam electron

diffraction patterns, Kikuchi diffraction Imaging and contrast in TEM, HRTEM

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HISTORY OF TEM

The electron microscope built by Ruska and Knoll in early 1930s

http://www.iopb.res.in/~tem_iopb/temuse.html

JEOL 2010, 200 keV

• In 1920’s Louis de Broglie discovered wave like nature of electrons.

• In 1926, H. Busch proved that e-beams can be focused by magnetic field (similar to that of light in optical lens)

• Ernst Ruska (Nobel Price in 1986!!), developed a lens system to magnify the specimen by 16 times and first used the term “electron microscope” in 1931.

• In 1939, the first TEM was manufactured by Siemens (Ruedernberg).

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Difference between optical and TE Microscope

• Do not observe the image directly in TEM.

• The inside of TEM is under vacuum.

• Instrument weight, sample preparation, cost, resolution etc are all different.

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WHY DO WE NEED VACUUM IN TEM• The electron microscope is built like a series of vessels

connected by pipes and valves separate all the vessels from each other.

• The vacuum around the specimen is around 10-7 Torr. The vacuum in the gun depends on the type of gun, either around 10-7 Torr (the tungsten or LaB6 gun) or 10-9 Torr (for the Field

Emission Gun).

• The pressure in the projection chamber is usually only 10-5 Torr (and often worse).

• The electrons will be hit with other molecules in the air and after a certain distance there will not be any electrons in the beam.

• Several hundred kilovolts are being applied across a small distance between the cathode and anode. It would result in enormous series of discharges (instead of a steady beam, it will be like lighting)

• Air contains contain large amount of hydrocarbons. They stick to the specimens and fall apart when hit by electron beam. The residual carbon as contamination.

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ROLE OF SCATTERING IN TEM

Electron scattering is the underlying physics of TEM

Diffraction: elastic scattering Imaging: elastic & inelastic scatteringSpectroscopy: inelastic scattering

Elastic - Direction of electron beam changes, but not magnitude.

Inelastic - Direction and magnitude of electron beam changes.

Inelastic scattering• Energy transferred to target atoms• Kinetic energy of electron beam decreasesNote: Lower electron energy will now increase the probability of elastic scattering of that electron.

• Secondary electron emission• X-ray generation• Plasmon excitation• Phonon excitation

Different processes

Beam Damage : High Voltage, high current density electron beam can cause damage to the materials.

Radiolysis and “knock-on” are two different types

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BEAM DAMAGE IN TEM• Beam heating is known to be a

problem in the TEM at high incident currents, for example if a large (or no) condenser-lens aperture is used.

• Even at low current densities, heating effects are worrisome for organic materials such as polymers, where thermal conductivity is quite low.

• To produce atomic displacement (loss of crystallinity or mass loss), there must be some mechanism for converting the energy acquired by atomic electrons to kinetic energy and momentum of atomic nuclei.

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COMPONENTS OF TEM – ELECTRON GUN

W hairpin

LaB6 crystal

Heated tungsten – A heated filament made from tungsten. When high voltage is fed through the filament, electrons are kicked off from it. The amount of voltage required is known as work function.

Lanthanum hexaboride (LaB6) - Thermal filament. Work function is lower than W and hence more efficient.

Tungsten field emission gun (FEG) – Electrons are expelled by applying a very powerful electric field close to the filament tip. The size and proximity of the electric field to the electron reservoir in the filament causes the electrons to tunnel out of the reservoir.

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Brightness : Current density per unit solid angle of the source

ie = Cathode emission currentd0 = diameter of the electron beam0 = angle at which electrons diverge from the source

Current density

Brightness is directly proportional to the accelerating voltage

Other important properties of electron sources : Spatial coherency

Energy spread Source size Stability

Accelerating Voltage of TEM – 100 – 400 kV

Reasons for choosing the highest kV – The gun is brightest Shorter the wavelength, the better

is the resolution Heating effect is smaller

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COMPONENTS OF TEM – LENSES

Objective lens : focus image. Magnification lens : Determines the magnification of the microscope. Whenever the magnification is changed, current through the lens also changes

Condenser lenses : control how strongly beam is focused (condensed) onto specimen. At low Mag. Spread beam to illuminate a large area, at high Mag. strongly condense beam.

C1 controls the spot size

C2 changes the convergence of the beam

Condenser-lens system

The condenser aperture must be centered

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Magnification in TEM

Depending on the magnification, some lens may not be used

Mob × Mint × Mproj = Total Mag

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EXTRACTING THE PHASE

In Focus

Under focus

Over focus

Sample

Objective Lens

Focal Plane

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LENSES AND SOME OF THE PROBLEMS

1.Spherical aberration

rs = spherical abberation radius in the gaussian image planeCs = spherical aberration coefficient = collection semi angle

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2. Chromatic aberration

In addition to the energy spread caused by the power source, electrons with different wavelengths are generated during the interaction of the electron beam with the sample. This leads to a distortion in the image known as chromatic aberration

A B

E = Energy loss of the electrons

E0 = Initial beam energy

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3. Astigmatism

Occurs when electrons sense a non-uniform magnetic field as they spiral around the optic axis. This imperfection is caused by machining errors, inhomogeneities in the iron of the lens, asymmetry in the windings, dirty apertures.

f = difference in focus induced by astigmatism

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VIEWING SCREENS/DETECTORS IN TEM

Semiconductor detectors Scintillator photomultiplier detectors

The viewing screen (lead glass) is coated with modified ZnS, which emits light with a wavelength of 450 nm, Modified ZnS give off green light at closer to 550 nm, hence screen will be of different shades of green, which is most relaxing to eyes.

Image recording:

1.Photographic emulsions2.Computer and charge coupled devices

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DETECTORSSemiconductor detectors (p-n junction)

Pros & Cons

Cheap and easy to replace

Easy to fabricate

´ Insensitive to low energy electrons

´ Electron beam might damage the detector

´ They have a large dark current

´ Noise is inherent

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Scintillator photomultiplier detectors

Pros & Cons

Gain is very high. Of the order of 10n

, n is the number of dynodes in PM

Noise level is low.

´ Susceptible to radiation damage

´ More expensive and bulky

´ They have a large dark current

´ Noise is inherent

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SPECIMEN PREPARATION Specimens are loaded on

Grids (3mm in diameter) Dispersible samples• Disperse in suitable

solvent• Drop cast a dilute solution

on grid• Allow it to dry

Solid samples Ultramicrotome (soft

samples) Ion milling Electropolishing Preferential chemical

etching Cleaving

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IMAGING MODES IN TEMo When we form images in

TEM, we either form an image using the central spot or use the scattered electrons.

o This can be achieved by inserting an aperture into the back focal plane of the objective lens.

o This allows either the direct beam or the scattered ones to pass through.

o If the direct beam is chosen, it is a BF image

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BRIGHT FIELD AND DARK FIELD IMAGES

Centered dark field image

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Carbon coated iron oxide nanoparticle

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IMAGING MODES IN TEM

If you form an image without the aperture, the contrast will be very poor because, then many atoms contribute to the image

Choice of aperture size determines, which electrons to contribute to the image and thus the contrast.

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MASS THICKNESS CONTRAST (BF IMAGING) Very important for biological / organic samples Arises from incoherent inelastic scattering of

electrons

Thicker areas of the specimen will scatter more electrons off axis than lower mass areas. Thus fewer electrons from the darker region fall on the area of the image plane, and therefore appears darker in BF images

The opposite will be true for DF images

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Lattice images (HRTEM)

IMAGING MODES IN TEM

• The image is formed by the interference of the diffracted beams with the direct beam (phase contrast).

La0.7Sr0.3Mn03(LSMO)-SrTiO3 interface coherently grown by pulsed laser deposition

CdSe

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Dark Field Imaging– Advantages and Disadvantages

Advantages Disadvantages

Provides high contrast for examining molecules with very low contrast such as DNA

More difficult to focus and correct for astigmatism since phase contrast is not present.

For crystalline objects, specific diffraction spots can be selected in the back focal plane of the objective lens in order to form a dark field image only from the electrons scattered by a chosen set of crystal planes.

Image brightness is low, since the objective aperture transmits only a small fraction of the scattered beam.

Longer exposure times needed to get good images.

Specimens are subjected to more radiation damage

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DYNAMIC LIGHT SCATTERING (DLS)

• Also known as Photon Correlation Spectroscopy.

• Measures the Brownian motion of particles and connects it with the particle size.

• DLS is used for the measurement of particle size of particles suspended in liquid.

• When light impinges on matter, the electric field of the light induces an oscillating polarization of electrons in the molecules and causes scattering of light. The frequency shifts, the angular distribution, the polarization, and the intensity of the scatter light are determined by the size, shape and molecular interactions in the scattering material.

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• Brownian movement causes the particles to diffuse through the medium. This diffusion depends on the particle size and shape

Stoke – Einstein Equation

d (H) = hydrodynamic radiusT = Temperatureh = viscosityD = translational diffusion co-efficientk = Boltzmann’s constant

Factors that affect “D”

• Ionic strength of the medium : Ionic concentration will affect the electrical double layer, low conductivity medium produce an extended electrical double layer and hence hydrodynamic radius will be more, where as high conductivity medium will suppress the double layer thickness and hence lesser hydrodynamic radius.

• Surface structure and shape: Any change to the surface structure will affect the diffusion speed.

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• The scattered light that is observed comes from a collection of scattering elements within a scattering volume that is defined by the scattering angle and detection apertures.

• The observed intensity of the scattered light at any instant will be a result of the interference of light scattered by each element; and thus, will depend on the relative positions of the elements.

• Because particles in Brownian motion move about randomly, the scattered intensity fluctuations are random. The fluctuations will occur rapidly for smaller, faster moving particles and more slowly for larger, slower moving particles.

Digital correlator

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HOW DLS WORKS??

• In DLS, the speed at which the particles are diffusing due to Brownian motion is measured.

• This is done by measuring the rate at which the intensity of the scattered light fluctuates when detected using a suitable optical arrangement.

• The rate at which these intensity fluctuations occur will depend on the size of the particles.

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DIGITAL CORRELATOR IN DLS

software uses algorithms to extract the decay rates for a number of size classes to produce a size distribution

Number, volume and intensity distributions of 5 and 50 nm particles (1:1) in a solution

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ZETA POTENTIAL ANALYZEROn the assumption that DLVO theory, zeta potential, factors affecting zeta potential is known

DLVO theory

VT = VR + VAA = Hamaker constantD = Particle separationa = Particle radius, p = solvent permeability, κ = a function of the ionic composition aζ = the zeta potential.

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ZETA POTENTIALThe liquid layer surrounding the particle exists as two parts; • An inner region (Stern layer)

where the ions are strongly bound

• An outer (diffuse) region where they are less firmly associated.

• Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. When a particle moves (e.g. due to gravity), ions within the boundary move it. Those ions beyond the boundary stay with the bulk dispersant. The potential at this boundary (surface of hydrodynamic shear) is the zeta potential.

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FACTORS AFFECTING ZETA POTENTIAL

• pH

• Conductivity• Concentration of a formulation

component

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CONFIGURATION OF A ZETA ANALYZER

UE = electrophoretic mobility, Z = zeta potential, ε = dielectric constant, η = viscosity and f(κa) =Henry’s functionκa = the ratio of the particle radius to electrical double layer thickness