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X-ray Reflectometer
Objective:
The objective of this lab is to become familiar with x-ray (and
neutron) reflectometery. A hydrophilic polymer grafted to
a silicon wafer will be used to demonstrate x-ray
reflectometry.
Instruments to be used:
X-ray reflectometer
Materials:
Thin film of PNIPAM (hydrophilic polymer) grafted onto a silicon
wafer (produced by Sandip Argekar in Prof. Schaefer's Group).
Procedure:
The procedure is detailed in the "Reflectivity SOP How to Run
Reflectometer.pdf" file in the background section.
Analysis:
Analysis will be done using the Igor Pro programs written by Jan
Ilavsky "Irena". Install as instructed on the linked web
page.
In the SAS menu, fourth block at the bottom is
"Reflectivity". This will open a control panel for data
fitting (this is really a calculation with minimization not a
fit).
Ilavsky's code has extensive help manuals. Please find
this manual and the section on reflectivity.
Questions:
1) Determine the critical edge and the surface composition that
this critical edge corresponds with.
2) Determine the film thickness from the Kiessig fringes.
3) Verify that a log-log plot of reflectivity versus q follows a
power-law decay of -4 slope (this may be compromised by
background signal and other factors).
*4a) Use the Ilavsky code to determine the diffusion
profile of the film, diffusion coefficient, integrated
concentration, and the film thickness.
c(x) = c(surface) {1 -
erf[x/(2sqrt(Dt))]}
where erf() is the error fumction, t is the time from the start
of diffusion, D is the diffusion coefficient, x is the distance
into the film, and c(surface) is the fixed surface concentration
related to the vapor composition (humidity) times the solubility
of water in the film.
4b) If the method of 4a) is not viable the diffusion coefficient
can be determined using the method of Vogt et al.
and Singh et
al.
5) Compare the value of the diffusion coefficient you have obtained with that of water in PNIPAM or of water in other hydrophilic polymers. Is your value reasonable?
*May not work with x-ray reflectivity due to low contrast
between water and polymer and narrow q range. Better for
neutron reflectivity using D2O.