Characterization of Coated (Core/Shell) Particles Using
Polarized Extinction Spectroscopy
We performed numerical simulations to demonstrate the feasibility of
our approach to characterize coated particles (experimental measurements are currently
underway). A numerically simulation was performed, using Lorenz-Mie theory for coated
spheres, to obtain extinction spectrum. The following input were used in the simulations:
- Overall size of the particles: 200 nm
- Coating thickness variation: 2 to 20 nm (a log-normal distribution with mean of 10 nm
and standard deviation of 1.25). Note that this is not the variation within each particle,
but across the whole population of particles. We assumed a uniform coating thickness for
each individual particle.
- Refractive index of the core: 1.36 + 0.0i
- Refractive index of the coating: 1.48 + 0.5i
To represent experimental conditions, three different turbidity
spectral data sets were created with +/- 2%, +/- 5%, and +/- 10% maximum value random
noise added to the simulated spectra (the actual noise in the experiments would be less
than 1%). The simulated spectrums with and without noise (+/- 10%) are shown in Figure 1
as case B and A, respectively.
Figure 1 - Numerically Simulated Extinction Data with Noise
Inversion was performed to obtain the distribution of coating thickness
values by using modified Chahine iteration scheme. The results are depicted in Figure 2.
Although we obtained results for different random noise values, we present only the case
in which recovered distribution considerably deviated from the distribution used for
simulation. The symbols in the figure represent the values used as input to simulate the
turbidity spectra. Case B and A, respectively, represent the inversion performed with
simulated spectrum with and without noise added to it From the results, it is evident that
it is possible to obtain coating thickness variations in a particle suspension containing
monodispersed particles (with reasonable accuracy.)
Figure 2 - Coating thickness distribution after inversion
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