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)