transmitted through the thin sample without any interaction occurring inside the sample. The
thicker areas will have fewer transmitted unscattered electrons and will therefore appear darker
than the thinner areas. The unscattered electrons strike a screen where light is generated,
allowing the user to see the sample image. Staining agents such as osmium tetroxide, ruthenium
tetroxide, or uranyl acetate can also be used to increase the contrast of thin samples that do not
sufficiently scatter the electron beam.
Quasi elastic light scattering (QELS, also called dynamic light scattering, DLS) is used to
determine the hydrodynamic radius or diameter of nano-sized particles suspended in a solution.
These particles are subject to Brownian motion, with small particles diffusing faster than large
particles according to the Stokes-Einstein equation (2-6) relating diffusion coefficient (D) and
particle radius (R):
k,T
D = (2-6)
6zy~rR
where kB is the Boltzmann constant, Tis the temperature, and rl is the solution viscosity. A beam
of laser light is first focused on the sample, the particles scatter the incoming light in all
directions, and the scattered photons are measured by a photomultiplier tube. The time variation
of the scattered intensity is then analyzed with a digital correlator used to compute the following
autocorrelation function C(r):
C(r) = Ae-2r "+ B (2-7)
where A and B are instrumental constants. The diffusion coefficient D is then determined from
the following 2 equations:
4mizsin( )
q = (2-8)