Transcript Nanoparticle Optics Lab Part II

Nanoparticle Optics Lab
Part II
Light Scattering
Theory
• A collimated light source is the most basic tool for nanoparticle work.
Often called a Tyndall beam. Named after the 19th century scientist
John Tyndall who studied light scattering in detail.
• HOTS: Higher Order Tyndall Spectra
Theory: Scattering Angle
• How is the angle measured?
• Zero is the forward direction, the direction of the
undeviated rays
• 180° is backward, rays scattered directly back into the
source.
• Note that in the diagram to the
right the scattering angles are
129 ° (180° – 51°) and
139 ° (180° – 42°),
respectively.
Theory: Scattering Plane
• the scattering plane is defined by the two rays involved, the source-particle
ray and the particle-observer ray
• The scattering plane is determined by observation, it is not fixed in space.
• For example, if the observer moves, the scattering plane will move with the
observer
• The scattering plane is useful to define the direction of polarization of light
(parallel and perpendicular)
Theory: Rayleigh Scattering
(electric dipole)
vertical source
polarization
horizontal source
polarization
Theory: Rayleigh Scattering
(electric dipole)
unpolarized source
Note that 90° scattering
is polarized perpendicular
to the scattering plane.
Theory: Mie
Absorption and Scattering by a Sphere
(exact solution)
• Gustav Mie (1908)
motivation: The colors of colloidal gold.
• Multipole expansion (EM modes of a sphere)
–
–
–
–
electric dipole
magnetic dipole, electric quadrupole
magnetic quadrupole, electric octupole
etc.
• If d < λ/20 then only the first term (dipole) is
needed. In this limiting case, Mie’s theory reduces
to Rayleigh’s theory
small particle limit: Mie  Rayleigh
Objective
• Learn about the scattering plane and the
polarization of Rayleigh scattering.
• Learn about Mie scattering and the angular
dependence of scattering.
• Observe HOTS and angular scattering for
monodisperse sols.
Procedure: Rayleigh Scattering
• Shine Tyndall beam through colloidal silica
without polarizer.
• Observe beam from top and side of jar.
• Use polarized lens to check polarity of light
scattering from silica in jar.
Procedure: Rayleigh Scattering
• Place polarizer between Tyndall beam and jar.
• Observe light intensity from side of jar.
• Note difference in scattering intensity between
parallel and perpendicular polarized source.
Procedure: HOTS
• Replace jar of colloidal silica with colloidal sulfur.
• With source polarized perpendicular, observe
different colors of HOTS spectra.
• Rank particle size in the two jars by counting the
number of times a certain color repeats when
moving 180 degrees around the jar.
• Larger particles cause more repetitions.
• Use one eye and look for an easy color to see such
as red.
Procedure: Scattering Angle
• Using the procedure for colloidal
sulfur, rank three polystyrene
samples in order of size.
• Put one polystyrene sample in the
path of the 543.5 nm HeNe laser.
• Prop one side of the sample
container on a slide to point the
back surface reflection of the
container away from the laser.
• Line up laser beam emitted from
sample container with iris.
• Use crossed polarizers to adjust
laser beam intensity.
Procedure: Scattering Angle
• Find points of minimum scattering intensity.
• Use one eye to line up sight in the middle of the bottle at
angle of minimum intensity.
• Record angles for each bottle.
Results: Rayleigh Scattering
• With unpolarized source, light scattered at
90 degrees from the source was polarized
perpendicular.
• With source polarized perpendicular, light
scattered at 90 degrees was polarized
perpendicular. Moving 180 degrees around
the bottle produced changes in intensity
with a minimum at 90 degrees.
Results: HOTS
• Observed different number of color
repetitions for colloidal sulfur.
• For polystyrene observed one, five, and
three repetitions for bottles D, E, and F
respectively.
Results: Scattering Angle
• Saw different numbers of scattering intensity minimums
for bottles D, E, and F.
• Observed one, five, and two minimums for bottles D, E,
and F respectively. This led us to believe the largest
particles were in bottle E, and the smallest were in D.
• Different observers recorded slightly different angles of
minimums.
Analysis
Bottle
D
E
F
Angles of Minimum Intensity (degrees)
98
49
70
64
105
80
97
• Table lists averages of measured angles of minimum
intensity from three observers.
• Angles were compared to Mie Plot data to estimate
diameter of polystyrene.
142
Analysis
Mie Plot Data
5
4
log(Intensity)
3
2
350 nm
1160 nm
750 nm
1
0
-1
-2
-3
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
Angle (degrees)
• Graph shows best fit to observed data.
• Minimums above 160 degrees and below 20 degrees were not taken
into account.
Questions
• Size estimated for particles is: 350 nm, 1160 nm,
and 750 nm for bottles D, E, and F respectively.
• Polydispersed sol will cause light from different
wavelengths to overlap in HOTS. Colors will be
less distinct.
• A way to improve this experiment would be to use
a light detector to measure the scattered intensity
at different angles. Human error would be
reduced.