[Ifeffit] Regarding calculations for scattering amplitude

Sun Jan 24 21:23:01 CST 2016

Dear Max and Scott,
     First of all sorry for late reply and thank you very much for you kind
help. That explains a lot in this aspect. So just to summarize,
technically, I am/was not doing anything wrong. and also, the amplitude
value that I was considering (by plotting the graph in R space, and taking
the peak values) were correct. however, since I do not have 'calibration
curve' for nanoparticles for Pd synthesized under control conditions, I
cannot really find out the diameter of the nanoparticles, in short?
Secondly, could you please guide me with the publications or methods by
which I can find out the diameter of nano particles more accurately?
Anatoly mentioned about some recent paper in the thread, does anybody knows
which one he is talking about?
Max, do you happen to remember how did you modified the atoms program in
order to get better/accurate results? as to what did you modify etc. I am
not really a programmer, so if it is fairly technical, I probably would
skip that part and instead focus on another method of particle size. But if
it is technically simple method and only few USER FRIENDLY things are
needed and I can get better insight, I would not mind applying them and I
can cite your work as well in the paper I am writing.
Thanks again to all for their prompt response and sorry again for my late
response.
Sincerely
Pushkar

On Fri, Jan 22, 2016 at 2:52 AM, Maxim Boyanov <mboyanov at nd.edu> wrote:

> Hi Pushkar,
>
>
>
> We recently discussed a bit measuring particle size by interpreting
> coordination numbers obtained from fitting EXAFS data, also referring to
> the paper you mention:
>
>
>
>
> http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2015-August/thread.html#start
>
>
>
> The basic idea is that surface atoms are not coordinated to other atoms on
> the outside (i.e., for your Pd foil the surface Pd atoms would have say 6-8
> neighbors instead of 12 for the atoms on the inside) so when the particle
> size becomes less than a few nm the number of surface atoms become a
> significant proportion of the total number of atoms and you can now observe
> this undercoordination as a smaller amplitude (assumed to be due to average
> coordination number) of some coordination peaks in the FT. How you relate
> this average coordination decrease to particle size depends a bit on your
> system. Some approaches have been laid out in the literature, including the
> paper you mention. See the thread above for other. Some time back I used a
> “brute force” approach to calculate the average coordination vs size for a
> small particle of a certain crystal structure (see fig EA-7 in the
> supplementary material of Geochimica et Cosmochimica Acta 71 (2007)
> 1898–1912). I took the crystallographic structure and created the list of
> atom coordinates for a 10 nm spherical particle using the ATOMS program. I
> then wrote a simple program (which I can’t find now :-) to go through the
> list and calculate the average coordination for a certain shell by going to
> all atoms within a particle radius and counting the neighbors in each
> coordination shell of interest. The result is a graph of average
> coordination for that coordination shell vs radius, which you can then
> compare to the coordination numbers obtained in your fits of the EXAFS data
> to “read off” a particle size. This brute force approach takes into account
> the crystal structure of your material of interest and the slightly
> different surface density of atoms when you cut along different planes to
> create the particle. However, the improved accuracy from that will probably
> give you a difference in coordination number that is within the uncertainty
> of the coordination numbers in an EXAFS fit. You can use a spherical
> particle which is probably OK for your case with Pd, but you can also
> create say rod-shaped particles that emphasize certain crystallographic
> planes, depending on your case.
>
>
>
> Be aware of some of the issues raised by Scott in his email and in the
> thread in the link above when comparing EXAFS and TEM determined sizes.
> Another source of discrepancy may be the assumption that the EXAFS peak
> amplitude reduction is due only to a coordination number decrease. It is
> also conceivable that the strain in a 2nm particle gives a contribution to
> the Debye-Waller factor, potentially affecting the determination of the
> average coordination number by EXAFS.
>
>
>
> Overall, use particle size determinations as an estimate, regardless of
> the experimental or interpretation method used :-). More importantly, be
> aware of the pitfalls and their effect on the certainty of the conclusions
> you present. Unfortunately there doesn’t seem to be a recipe-like approach
> to the problem yet…
>
>
>
> Best,
>
> Max.
>
>
>
>
>
>
>
>
>
>
>
> *From:* Ifeffit [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] *On
> Behalf Of *pushkar shejwalkar
> *Sent:* Thursday, January 21, 2016 3:02 AM
> *To:* XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> *Subject:* [Ifeffit] Regarding calculations for scattering amplitude
>
>
>
> Dear All,
>
>    I sent a related question in the month of Dec but it seems that the
> question did not appeared on list because of mailbox being full, as we all
> have received an email. Since mailing list is started again I am sending
> this question once more.
>
> My question is actually regarding finding out the nanoparticle size. In
> this respect, I am referring to Harris's work in which by using a simple
> equation we can roughly estimate the nanoparticle size
>
> J. Appl. Phys., Vol. 94, No. 1, 1 July 2003
>
>  Now the question is that in this paper and another paper I found online
> as well, it refers to Nnano and Nbulk. I am guessing that this is basically
> coordination number of nanoparticle and bulk. But my first question is how
> do I calculate it?? e.g. I am working on Pd K-edge. So should I fit Pd foil
> with Pd crystal data and get the CN? but that would be 12 anyways. So would
> Nbulk be 12 in that case? and what will be Nnano then?
>
> In another context, CN is proportional to amplitude, so in that case is it
> directly proportional? as in, can I simply replaced in formula, Nbulk/Nnano
> with Amplitudebluk/amplitudenano?
>
> Third thing is that, if above is true then how should/can I calculated the
> amplitude? isnt that amplitude is always calculated by Feff? if so could
> someone help me with the procedure to do so? How will I know amplitude for
> nanoparticles, because I cannot have FEFF for nanoparticles, right?
>
> I went through lot of material, however, so far my calculations give me a
> solution that is vaguely correlated to TEM images. The TEM value is about
> 2nm (rouhgly 20 angstrom). my current values I am getting by solving the
> equation by my current understanding is only 2Ao. about 10 times less. What
> I am doing is simply plotting the crude data in athena to get x(R) Vs
> radial distance graph, calculate A-3 for nanoparticles and bulk and using
> this as amplitudes. It is possible that I may be doing something totally
> irrelevant, so please help me in this aspect.
>
> Thank you
>
> Sincerely
>
> Pushkar
>
>
>
> --
>
> Best Regards,
> Pushkar Shejwalkar.
> Post-doctoral -Researcher,
>
> Tokyo Engineering University,
>
> Tokyo-to
>
> Japan
>
>
>
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-- 
Best Regards,
Pushkar Shejwalkar.
Post-doctoral -Researcher,
Tokyo Engineering University,
Tokyo-to
Japan
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