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Title: YiKoan Hong

1
Ch 3. Optical properties
2001. 4 . 16. Yi-Koan Hong Department of
Metallurgy and Materials Engineering Hanyang
University, Ansan, Korea
2
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

3
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
4
The observation of particles located at a defined
level in the electrophoresis cell
5
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)
6
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

7
The Transmission Electron Microscope
Fiq 3.1
Fiq 3.2
8
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.

9
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.

10
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.

11
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.

12
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

13
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

14
The Light scattering (IV)

Light Scattering theory

15
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.

16
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.

17
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.