[Ifeffit] EXAFS mean free path in small particles

[Frenkel, Anatoly]

Matt,

In their paper, Zhao and Montano studied Al particles that showed surface
plasmon activity for sizes as small as 10 A. I had no idea that Al does it,
and it is very peculiar. However, I have been dealing most heavily with gold
nanoparticles. According to literature (here is another article: Sarathy et
al, J.Phys.Chem. B, 1997, v. 101 pp. 9876-9880, and ref. 14 within), Au
nanoparticles show no surface plasmon activity below 20 A. Our recent
experiment, where we have taken UV-vis. measurements of several samples of
Au nanoparticles that were very small (13 to 100 atoms) and monodisperse,
did not show any surface plasmon band around 525 nm where it appears at
larger particles. In fact, Zhao and Montano say in p. 3403: "we would like
to point out that based on the experimental results for noble metals [Cu,
Au, and Ag] no evidence of a substantial change in the electron MFP value
between bulk and small particles was found". That, to my mind, is an
indirect but a fair reason to reuse bulk amplitudes, at least for Cu, Au and
Ag. 

I also think, if HRTEM and EXAFS agree, it says for itself (that bulk FEFF
amplitudes are correct). If they do not agree, AND if other reasons for
discrepancy can be ruled out (e.g., broad size distribution, or
electron-beam caused growth of particles), AND if the discrepancy is in the
right direction (EXAFS-determined size is smaller than the HRTEM-determined
size), then surface plasmon and other losses should be looked into. And, I
agree, it is an important issue to be aware about.

Anatoly


-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov]On Behalf Of Matt
Newville
Sent: Tuesday, June 14, 2005 9:47 PM
To: XAFS Analysis using Ifeffit
Subject: RE: [Ifeffit] EXAFS mean free path in small particles


Anatoly,

> The point is, to my opinion, that since In the most
> interesting range, below 2nm, the plasmon activity decreases,
> therefore its effect on EXAFS interpretation, if any, should
> be negligible.

I think I may not understand your point.  The Zhao and Montano
paper shows very pronounced changes in the plasmon spectra
between each of 10,3,2, and 1 nm for Al.  At 1 nm, the surface
plasmon is definitely dominating -- not going away.  So their
conclusion, that lambda changes substantially below 10 nm due to
the changing relative importance of surface-to-bulk plasmons
seems well-argued to me (not to say its correct).

I agree (I think) with Matthew that the Zhao and Montano work
shouldn't be too easily dismissed, though it might not be
completely correct.  I don't know of any other work on how
lambda might depend on particle size, and never really thought
about this.  The Rao paper seems broader in scope and not
necessarily addressing plasmons and mean free path, but I only
skimmed it, so maybe I missed something.  Their fig 8 definitely
shows a size dependence of O K-edge, though I didn't immediately
see a clear interpretation for this -- it could have several
explanations. Again, I only skimmed this paper, so maybe I
missed something: perhaps the electron diffraction literature
has more information??

> ...  There are other evidences that small particles have DOS
> similar to molecules, not typical metals. Most notable is the
> evidence that HOMO-LUMO band grows to more than 1 eV at sizes
> of 1 nm range.

Hmm, I'm not sure I see that becoming more molecular and less
metallic indicates that using the bulk mean-free-path is OK. .   
The 'universal mean free path' that Feff uses and the fact that
S02 is still has to be a fitting parameter even for excellent
data with Feff8 suggests to me that there may be many hidden
sins in Feff's mean-free-path calculation.

Sadly, we seem to be a little short on theorists these days.

--Matt


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