RE: [Ifeffit] EXAFS mean free path in small particles
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@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@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 _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Hi Anatoly,
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 think this is a little unfair. That experimental work done prior to 1986 could not detect a change in MFP between bulk and nanoparticles does not mean that there is not such an effect. Zhao and Montano show (fig 1) large changes in the bulk plasmons below 10 nm -- well before the surface plasmon dominates. These will also affect the MFP, as they show in fig 2, where the MFP for 10nm and 3 nm are shown to be significantly different, even though the surface plasmons are barely noticeable. I'm not saying that Zhao and Montano are right, but I don't see that it's obviously wrong. And I sure don't like the idea of saying that using the bulk MFP from Feff is OK for nano-Au, Cu, and Ag, but not for nano-Al!
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.
I don't think the direction of the change in the MFP matters: if it changes with particle size, it will effect the EXAFS. Also, just to be clear: we know that the bulk Feff amplitudes are NOT correct to better than 5%. Since a lot of the interest in nanoparticles is precisely because of their unique electronic properties and the idea that "everything is different" for nanoparticles, it seems dangerous to assert that some electronic properties can be modelled as bulk properties without thinking about it. And some of those who did think about it (Zhao and Montano) concluded that the MFP is different between bulk and nano particles. I should say that the only analysis I've done on "nano" particles (10 years ago) showed slight differences in the Cu K-edge EXAFS amplitude between bulk and 13nm (ie, huge!!) Cu particles. At the time we argued that the 13nm particles were slightly more disordered, but still had an average coordination of 12 (which was in dispute even if it seems obvious now). But we did not try to adjust the MFP.... --Matt
participants (2)
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Frenkel, Anatoly -
Matt Newville