YiKoan Hong

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Ch 3. Optical properties
2001. 4 . 16. Yi-Koan Hong Department of
Metallurgy and Materials Engineering Hanyang
University, Ansan, Korea
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Outline

• Introduction
• - Optical and Electron Microscopy
• i ) The Optical Microscope-resolving power
  
• ii ) The Transmission Electron Microscope
• iii ) The Scanning Electron Microscope
• iv ) Dark - field Microscopy the
  ultramicroscope
• - Light Scattering
• i ) The Tyndall effect - turbidity
• ii ) Measurement of scattered light
• iii ) Light Scattering theory
• iv) Dynamic light scattering

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The Optical microscope resolving power

• d ? / 2n sin a
• ( a angular aperture, n refractive index, n
  sin a numerical aperture of the objective lens
  for a given immersion medium )
• d K1? / NA ( for increased resolution Small
  ?, Large NA )
• DOF K2? / NA2

Better resolution
Decreased DOF
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The observation of particles located at a defined
level in the electrophoresis cell
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The Electron Microscope vs Dark field Microscope

• Two techniques for overcoming the limitations of
  optical microscopy are of particular value in the
  study of colloidal system

• The Transmission Electron Microscope
• The Scanning Electron Microscope

(The limit of resolution is greatly extended)

• The Dark - field Microscopy

(The minimum observable contrast is greatly
reduced)
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The Transmission Electron Microscope

• To increase the resolving power of a microscope
  so that matter of colloidal dimensions may be
  observed directly, the wavelength of the
  radiation used must be reduced considerably below
  that of visible light.
• Electron beams can be produced with wavelengths
  of the order of 0.01nm and focused by electric or
  magnetic fields.
• The useful range of the TEM for particle size
  measurement is c.1nm-5µm diameter.
• From the angle of shadowing and the length of
  shadows, a 3D picture of the specimen can be
  built up.
• A most useful technique for examining surface
  structure is that of replication

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The Transmission Electron Microscope
Fiq 3.1
Fiq 3.2
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The Scanning Electron Microscope (I)

• In the scanning electron microscope a fine beam
  of medium-energy electrons scans across the
  sample in a series of parallel tracks.
• These interact with the sample to produce
  various signals.
• - SEE (Secondary electron emission)
• - BSE (Back-Scattered electrons)
• In the SEE mode The particles appears to be
  diffusely illuminated, particle size can be
  measured and aggregation behavior can be studied,
  but there is little indication of height.
• In the BSE mode The particles appears to be
  illuminated from a point source and resulting
  shadows lead to good impression of height.

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The Scanning Electron Microscope (II)

• The magnification achieved in a SEM(resolution
  limit of c.5nm) is less than in a TEM, but the
  major advantage of the technique(low numerical
  aperture) is the great DOF which can be achieved.
• SEM can give a DOF several hundred times greater
  than that of the optical microscope.
• In colloid and surface science this large DOF is
  extremely valuable in the study of the contours
  of solid surfaces and in the study of the
  particle shape and orientation.

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The Dark - field Microscopy

• Dark field illumination is a particularly
  useful technique for detecting the presence of,
  counting and investigating the motion of
  suspended colloidal particles.
• The two principal techniques of dark-field
  illumination
• - Slit
  - Cardioid
• Dark-field methods do not help to improve the
  resolving power of a microscope and are,
  neverthless, an extremely useful for studying
  colloidal dispersions and sedimentation,
  electrophoretic mobility, the progress of
  particle aggregation, number-average particle
  size. Etc.

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The Light scattering (I)

• Light scattering result from the electric field
  associated with the incident light inducing
  periodic oscillations of the electron clouds of
  the atoms of the material in question these
  then act as secondary sources and radiate
  scattered light.

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The Light scattering (II)

• The Tyndall effect - turbidity The noticeable
  turbidity associated with many colloidal
  dispersions is a consequence of intense light
  scattering. For example, a beam of sunlight is
  often visible from the side because of light
  scattered by dust particles.
• It/I0 exp-tl
  It I0 exp-tl
• It the intensity of the transmitted light beam
  ,
• I0 the intensity of the incident light
• t turbidity,
• lthe length of the sample

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The Light scattering (III)

• Measurement of scattering light
• Intensity, polarisation and angular distribution
  of the light scattered from a colloidal system
  depend on the size and shape of the scattering
  particles, the interactions between them, and the
  difference between the refractive indices of the
  particles and the dispersion medium.
• Light - scattering measurements are, therefore,
  of great value for estimating particle size,
  shape, and interactions, and have found wide
  application in the study of colloidal
  dispersions, association colloids.
• Light scattering offers the advantage of
  particle size analysis

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The Light scattering (IV)

• Light Scattering theory

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The Light scattering (V)

• Scattering by small particles (by Rayleigh)
  
• When electromagnetic wave of intensity I0 and
  wave-length ? falls on a small particles(ltc.
  ?/20) of polarisability a, oscillating dipoles
  are induced in the particles.
• The particle then serves as a secondary source
  for the emission of scattered radiation of the
  same wavelength as the incident light.
• Scattering by large particles ( by Debye and
  Mie)
• The theory of light scattering is more
  complicated when one or more of particle
  dimensions exceeds c. ?/20. Such particles cannot
  be considered as point source of scattered light,
  destructive interference between scattered light
  waves originating from different locations on the
  same particle must be taken into account.

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The Light scattering (VI)

• Dynamic light scattering
• The defined frequencies associated with laser
  sources makes it possible to exploit light
  scattering to study the motion of colloidal
  particles.
• Light scattered by moving particle will
  experience a doppler shift to slightly higher or
  lower frequency depending on whether the particle
  is moving towards or away from the observer.
• The measurement allows the diffusion coefficient
  of the particles to be calculated.

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The Light scattering (VII)

• Small angle neutron scattering
• Neutron scattering is useful for the study of
  adsorbed material on hydrosol particles. the
  neutron scattering length densities of hydrogen
  and deutrium differ considerably.
• By preparing a hydrosol in an appropriate H2O/D2O
  mixture, it is possible to match the neutron
  scattering length densities of the dispersion
  medium and the core particles.
• The neutron beam thus sees only the adsorbed
  layer, the thickness of which can be estimated.
  Alternatively, the dispersion medium can be
  matched to the adsorbed layer to permit
  estimation of the core-particle size.