Jatin,
Estimating the size from EXAFS data alone may be very risky because you will have to assume a certain shape and morphology (say, 3D such as cuboctahedral, or oblate, such as truncated cuboctahedral) in order to find the size from EXAFS coordination numbers. They CNs are "indices" that describe not just size but the entire architecture, and there may be more than one such architecture that fits those four indices.
The simplest architectures that you can consider for modeling purposes are closed shell clusters that can be constructed from a seed cluster of a given shape by adding more layers on it until your model CNs and experimental CNs match within the error bars. However, such analysis is predicated that you have excellent size and shape selectivity. Hence, you need an independent data such as TEM, to get statistical information about the size/shape distribution. Of course, you need to make synthesis also size-shape selective as well.
If you do not have good control over size/shape, your analysis using 4 shells is an overkill. You can get average size by estimating it using Scott Calvin's equation in J. Appl. Phys. 94:778–83.
If you do have a good control over size/shape you should first question your fitting process that gives you the CNs and their uncertainties. You have to check if you included the most important multiple scattering contributions, the stability of the fit results to the changes in your fitting scheme (e.g., the ways you constrain (or not) parameters of single and multiple scattering collinear paths). You may also decide to check your model on bulk Cu and see if you will obtain correct CNs if you vary them (by fixing the S02). If you find that the CNs in bulk Cu are not in agreement with the crystallography, you should not expect anything meaningful from the CNs in your nanoparticle if you obtain them using the same fitting scheme as for the bulk.
After all the above is done, then you can do some modeling using CNs from 1st through 4th shell.
Here are some references how to do that using EXAFS results:
J. Phys. Chem. B, 105, 12689-12703 (2001).
J. Synchrotron Rad., 6 , 293 (1999).
Zeitschrift fur Kristallographie, 222, 605-611 (2007).
Ann. Rev. Anal. Chem., 4, 23-39 (2011)
J. Am. Chem. Soc. 132, 8747-8756 (2010)
Anatoly
________________________________________
From: ifeffit-bounces@millenia.cars.aps.anl.gov [ifeffit-bounces@millenia.cars.aps.anl.gov] on behalf of Rana, Jatinkumar Kantilal [jatinkumar.rana@helmholtz-berlin.de]
Sent: Thursday, January 09, 2014 12:55 PM
To: ifeffit@millenia.cars.aps.anl.gov
Subject: [Ifeffit] correlating number of nearest neighborst to the particle size
Hi All,
I would like to estimate the size of Cu nanoparticles grown in my sample by EXAFS fitting. I can refine the number of nearest neighbors in these Cu nanoparticles up to 4 shells (say NN1, NN2, NN3 and NN4), by constraining S02 to the value obtained by fitting the data of Cu reference foil. But, how do I correlate the refined values of nearest neighbors to the actual size of Cu nanoparticles ? Any help in this regard would be much appreciated.
Regards,
Jatin
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