[Ifeffit] idealized 95-atom Pt cluster

[Scott Calvin]

Hi Charles,

Here's what I would try for your situation:

1) Use Artemis to create FEFF files for bulk crystalline Pt. Also 
generate FEFF files for some plausible Pt-surface interactions.

2) Constrain the amplitude of the Pt-Pt paths to fall off with reff 
in the way expected for a homogenous sphere (I have a couple of 
publications on this, including a recent one in Physica Scripta). If 
you are confident you know the crystallite size, then use that for 
the size of the sphere; if not, you can leave it as a parameter for 
Artemis to fit. If you were really confident you knew the size of the 
crystallites, then it might be worth it to do something more precise 
(e.g. use a magic number cluster), but that doesn't sound like your 
situation. In clusters of this size that are "roughly" spherical, 
I've found that the best-fit results aren't all that different if you 
just use a sphere, although the uncertainties in the results may be 
considerably higher.

3) Use Artemis to guess the fraction of Pt's having a surface 
interaction and start trying the different candidate interactions.

In advance, and knowing very little about your system, I'd warn you 
that the problem will be much more tractable by the method I describe 
if the bonding to the surface is somewhat random and floppy; i.e. if 
there is not a fixed orientation and distance of the Pt particle from 
whatever the substrate is. Then you can be more confident that the 
high-R part of the FT is entirely due to the Pt-Pt interactions, and 
can thus gauge the relative contributions of Pt-Pt vs. surface 
interactions more easily.

--Scott Calvin
Sarah Lawrence College

>Anatoly,
>
>I did not consider magic numbers & don't know for certain what the 
>exact # is. The 95 atom cluster is only an estimate of the size of 
>the composition of particles that was observed in TEM images of Pt 
>tethered onto a a surface with -COH, -C=O groups; the exact nature 
>of the bonding is what I'm trying to determine. The Pt seems to be 
>stabilized by its surface interactions; the exact nature of the 
>binding is what I'm trying to figure out. The EXAFS data that I have 
>"sees" both the Pt-Pt interactions as well as the Pt-surface 
>interactions.  I realize that it would be best to simple scan a 
>sample containing Pt clusters of the same size as observed in the 
>TEM; but, unfortunately, Pt has a tendency to agglomerate and it is 
>difficult to get a monodispersed size in colloidal suspension in 
>solution.  Comparing with Pt foil doesn't work because of the 
>quenching problems in the fluorescence since there are so many atoms.
>
>I wanted to get a simulated EXAFS spectrum of something that just 
>represents the bulk Pt-Pt only to compare with the data that I have. 
>Differences in the spectra (in principle) would be attributed to 
>Pt-surface interactions present in the TEM.  I would like to try to 
>get an idea of the # of Pt interacting with the surface as opposed 
>to the Pt-Pt interactions.
>
>Charles
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