Unit 2, Part 3: Characterizing Nanostructure Size Dr. Brian Grady-Lecturer [e-mail]

1
Unit 2, Part 3 Characterizing Nanostructure
SizeDr. Brian Grady-Lecturerbpgrady_at_ou.edu
2
Outline

• Discrete Nanostructures
• Size and Shape
• Spheres
• Size from microscopy and light scattering
• Not Spheres
• Size from microscopy
• Composition
• ICP/AAS for bulk concentration
• XPS for surface concentration
• Arrangement of atoms
• Bonds-Raman, IR, NMR
• X-ray diffraction

3
Nanostructure Size

• If discrete particles, then in general particles
  are all the same general shape (all spheres or
  all ellipsoids or all cubes etc.)
• If particles are not spheres, then unless the
  particles are all the same size it is almost
  impossible to determine anything about size
  characteristics without microscopy

4
How to use microscopy to determine nanoparticle
size

• How electron microscopy works third unit
• Typically, use a dilute suspension of the solids
  in a solvent and directly deposit the
  nanoparticles in the solvent on the grid that
  will go in the microscope
• Trial and error to get particles not sitting on
  top of one another, yet having enough particles
  to get a representative image
• Take picture, and measure size either with a
  ruler or with image analysis software
• Figure 100 or so particles require to be
  quantified for decent statistics.

5
vv
vv
vv
vv
vv
vv
vv
vv
v
v
v
v
vv
vv
Too dilute
Too concentrated
About correct
6
Problems with Microscopy

• Very labor intensive
• Trial and error to concentration of nanoparticles
  and drop volume
• Quantifying image is difficult
• Image analysis programs are difficult to work
  correctly, typically have to confirm results
• No image analysis program a lot of measuring
  with a magnifying glass
• Tends to undercount really small particles
• Can you get single particle dispersion
• Does particle size get altered during drying
  (electron microscopy requires dry samples)

7
Non-spherical particles

• If non-spherical particles are uniform in size,
  x-ray or light scattering (depending on particle
  size) can be used to quantify size distribution
  for some shapes
• Probably the most common thing to do is to show
  an electron micrograph, and say this is the
  general shape and size of these non-spherical
  particles, and leave it at that. Work-benefit
  ratio to be more quantitative, with the inherent
  error especially with small particles, isnt
  usually high enough.

8
Spherical Particles

• There is one other commonly used method to
  determine particle size, and the particle size
  distribution, for spherical nanoparticles light
  scattering
• Requires that the material be dispersed in a
  solvent and that all nanoparticles exist
  individually
• The machine automatically figures out the correct
  concentration to have the best signal to noise
  assumes individually dispersed particles
• Can check assumption of individual dispersion by
  looking at particle size distribution or by
  changing concentration manually and making sure
  the result stays the same (always a good idea for
  an unknown material).

9
Particle size distribution

• Most spherical nanoparticle size distributions
  are fit well by the log normal distribution
  (slightly asymmetric)
• YAexp(-0.5(log(R)-log(Ro))2/b2)
• Two parameters b (the width of the distribution)
  and Ro (the average radius). A is set by the
  fact that the area under the curve is 1.

10
Two types of light scattering

• Static light scattering (also called Mie
  Scattering or just light scattering) and dynamic
  light scattering (also called photon correlation
  spectroscopy or quasi-elastic light scattering)
• Static light scattering measures scattered
  intensity as a function of angle (just like xrd)
  while dynamic light scattering measures variation
  in time of scattered intensity at a fixed
  scattering angle.
• Difficult to tell which machine is which (cant
  without reading the manual!)

11
Light Scattering (also called Static Light
Scattering)

• Light scattering measurement of intensity as a
  function of angle.
• Use a laser
• Scattering from a dispersed set of spheres is
  called Mie Scattering (Mie figured out the
  mathematics that relate size and refractive index
  to observed scattering pattern)
• Very complicated mathematics. Why?
• Light is polarized from a laser, x-rays are not
• Light can be transmitted at an interface but
  change direction (index of refraction), x-rays
  cannot (in a practical sense anyway).
• Light can be reflected at an interface, x-rays
  cannot in a practical sense
• Light scattering is due to the spheres
  themselves, not the arrangement of spheres in
  space (i.e. atoms dont cause peaks in x-ray
  scattering, the way in which they are arranged
  do!)
• Multiple scattering is a problem
• How get a distribution? Have to assume some
  functional form of the distribution and then
  compare the calculated scattering pattern with
  the measured pattern. If all the sizes are the
  same (monodisperse system), then the problem is
  trivial

12
Dynamic Light Scattering

• Measures the variation of light intensity with
  time (pick an angle, typically either 90 or 175
  degrees) and measure the variation in intensity
  with time (time increment is pretty short
  usually microseconds)
• Variation in time is caused by diffusion of
  particles which changes scattering angle slightly

13
Dynamic Light Scattering (cont.)

• A particular shape of the distribution does not
  need to be assumed can calculate in theory the
  distribution from first principles
• You are NOT actually measuring particle size you
  are actually measuring diffusion constant. Hence
  you must use some relationship between diffusion
  constant and particle size (Stokes-Einstein law)
• There is a great deal of mathematics that goes
  into converting that variation in intensity to
  diffusion constant.

(k is the Boltzmanns constant, T is temperature,
r is the radius of the sphere and h is the
viscosity of the fluid)
14
Why nothing scientific to read on DLS

• The mathematics are just too complicated
• I have given you two things both written for the
  new person (i.e. not really at a high enough
  level for a college course)

15
Light Scattering vs. DLS

• You will be doing light scattering in the lab
  however you will also be presented with results
  from a DLS system (http//www.bic.com/90Plus.html)
  (you cant do it yourself since it is at the
  Health Sciences Center)
• Light scattering has higher upper limit to
  minimum particle size (20-50 nm, depending on
  particle).
• Without the ability to change the polarization of
  incoming light, the upper limit on light
  scattering is around 500 nm.
• DLS machines lower limit is 0.5 nm to 5 nm,
  depending on particle.

16
Problems with Light Scattering

• Machines are essentially a black box, and will
  give you an answer no matter what almost. If you
  have non-spherical particles, this machine will
  still give you an answer. Can have multiple
  scattering, that can be an issue.
• Materials that settle quickly cannot be measured
  easily (although our light scattering machine
  does pump the material in a circle to reduce this
  problem)
• Works with colored materials but not as well.