[Ifeffit] EXAFS mean free path in small particles

Laurent Menard lmenard at uiuc.edu
Tue Jun 21 10:54:16 CDT 2005


Disclaimer: I'm an experimentalist and not a theorist.  
 
I'm not sure that Zhao and Montano's calculations are correct.  I think
their calculation for the transition probability assumes that the
electron density is constant when, in actuality, there is a spatial
dependence resulting from quantum confinement effects.  As a result, the
energy of the surface plasmon peak as calculated by Zhao and Montano is
constant for all cluster sizes at approximately the classical value of
omega(bulk plasmon)/3^0.5 and shows an increase with decreasing particle
size.  Including quantum confinement results in a blue shift of the
surface plasmon resonance with decreasing particle size and a decrease
in transition probability for decreasing particle sizes (as suggested by
Anatoly).  This change in the calculation will also impact the bulk
plasmon and I don't know what that trend will be.
 
As far as their experimental demonstration with Fe nanoparticles is
concerned, I really can't make an assessment about the experiment since
they don't indicate how the nanoparticle size was determined and I'm not
familiar with their preparation of the nanoparticles.
 
I'm not sure from this paper what the impact on the MFP will be although
there very well could be some effect on the EXAFS.  Personally, when I
began working on EXAFS of nanoparticles and small clusters (Au13), I was
leery of using the bulk phase and amplitudes to fit the data.  My
experience to this point has been that there have not been observable
deviations of coordination numbers for nanoparticle systems using these
FEFF inputs.  That being said, I tend to agree with Matt that this is
something that people working in this area need to think about a bit
more than it seems they have judging by the literature.  I would like to
see some (correct) theoretical assessments that support the assertion
that bulk phases and amplitudes are ok for nanoparticles that I think
has been demonstrated by validating EXAFS results with other techniques
(mostly electron microscopy).
 
-Laurent
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