E0 for bulk crystals and nanoparticles
Dear all, could you please let me know your thoughts on this question that has been consuming much of my time lately? It goes like this: When we perform first shell analysis of elemental crystals and nanoparticles (like bulk Ge and Ge NPs, or bulk Pt and Pt NPs, for example), should both bulk and NP samples have the same E0 shift relative to the theory (or, fit-wise, the same deltaE0 when fitting in Artemis)? And, even more important to know, why so? I've been reading some papers and I can find people who do set the NPs deltaE0 to be the same as the crystalline foil as well as people who report different deltaE0s for NPs relative to the foil. This confuses me because by setting (or not) deltaE0 we directly change the correlated variables, delR and C3. Intuitively I would think that for Ge, in particular, NPs could have a different deltaE0 from the bulk since they present smaller band gaps and different electronic structure, causing differences in the occupied/unoccupied states (that's why they show some PL while the bulk doesn't). Would you say this is reasonable? I have used Athena to calibrate (maximum of energy derivative = atomic value), align edges, background subtract (with same parameters) and Fourier transform (with same parameters) both bulk and NP spectra (at the same temperature, from same run/measurement conditions), which I then fit in Artemis to the FEFF8-generated crystalline Ge standard. And I do see a significant change in the odd cumulants (delR and C3) when I let deltaE0 run free or when I set it to the bulk value (the difference is between 1 and 2 eV). Would you say this is physically reasonable or more likely to be a fit artifact? Thanks, Leandro
Hi Leandro, I suspect you will get several answers to your excellent questions. It's late as I type this and I am looking forward to seeing my pillow, so I am going to respond to just one part of your email. On Thursday 22 February 2007 20:59, Leandro Langie Araujo wrote:
I've been reading some papers and I can find people who do set the [nanoparticle]'s deltaE0 to be the same as the crystalline foil as well as people who report different deltaE0s for NPs relative to the foil. This confuses me because by setting (or not) deltaE0 we directly change the correlated variables, delR and C3.
This really gets at the heart of why exafs analysis is difficult. You are certainly correct that the deltaE0 is highly correlated with delR and C3. That correlation is hard to avoid in a non-linear analysis as all three terms effect the phase of the calculated chi(k). Most of the tricks that we so-called experts yammer on about on this mailing list have to do with ways of dealing with those strong correlations that are physically sound and statistically defensible. To continue with your example of comparing nanoparticles their bulk analogs, one strategy taken to deal with these correlation is to assert that some parameter will be the same in the bulk as at the nanoscale. By making that assertion -- or, if your prefer, by using prior knowledge of the system -- we reduce the correlation by supplying more information (perhaps in the form of a multiple data set fit) or we remove the correlation by promoting that parameter from "guessed parameter" to "parameter set according to prior knwoledge". To be more specific, we might assert that the changes to the electronic state of the nanoparticles compared to their bulk analog are relatively small -- small enough that we can neglect the effect. Saying that requires that one believes it. It may be true for metallic nanoparticles, but may not be true for, say, semiconductor nanoparticles. If you believe it to be true that the electronic state doesn't change much, then you are well justified in asserting that the E0 for carefully aligned data should be the same. Otherwise, you should lift that constraint and investigate how e0 might be different for your nanoparticles. So what should you do if you are not sure? Well, one strategy might be to do a multiple data set fit using carefully aligned bulk and nano data. Compare the fit using a constrained e0 to the fit where the two e0s float freely. If the free floating e0s make sense given what you know about the electronics of the nanoparticles and they provide a statistically significant improvement to the fit, then probably you should have free floating e0s. If you float the two e0s and they come out the same (within their error bars, of course) then you probably should reintroduce the constraint. The whole time you are doing that, you need to be mindful that, as you say, e0 is correlated to other parameters. The fitting results need to be both reasonable and defensible regardless of how you decide to cast the problem. "Reasonable and defensible" applied to all the parameters, not just e0. Hope that helps, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/
Hi Bruce, thank you very much for taking the time to write your helpful reply. As you mentioned,
This really gets at the heart of why exafs analysis is difficult. You are certainly correct that the deltaE0 is highly correlated with delR and C3. That correlation is hard to avoid in a non-linear analysis as all three terms effect the phase of the calculated chi(k).
That's exactly why I decided to post the question on the list asking for the advice of more experienced people who can certainly evaluate the question much better than myself. Also, I've always been trying to follow your "reasonable and defensible applied to all the parameters, not just e0" motto; another strong motivation for posting this question on the list. I don't know if it is just me, but in my short time working with EXAFS I got the strong impression that it is very easy to fall into the temptation of sophistry to get to the "expected" results from a fit. In this respect, I consider the mailing list an invaluable resource to help overcoming such a temptation. But coming back to the original question, would you say that analyzing a temperature-dependent set of the very same NC-containing sample (eight measurements between 10K and 300K) to get an E0 value would be a sound approach? Or should we expect E0 to vary with the temperature as well? Cheers, Leandro
Leandro - depending on the details of your system and treatment, electronic structure of the nanoparticle may or may not change at elevated temperatures. It may change as a result of hydrogen chemisorption, charge exchange with support, ligand desorption, oxidation, etc. As a result, you may observe changes in XANES that may be due to the change in core hole screening or Fermi energy change or both (depending on the degree of "metallicity" of the nanoparticle). There are many references in literature demonsrating each effect that I mentioned. Thus, keeping E0 the same at all temperatures may or may not be wise, depending on the magnitude of the edge shift that can, in principle, be observed at different temperatures. If that happens, you may want to compare the resuls obtained by keeping E0 the same vs those when E0 was allowed to vary independently for each scan. In short, there is no "one fits all" answer. Anatoly ________________________________ From: ifeffit-bounces@millenia.cars.aps.anl.gov on behalf of Leandro Araujo Sent: Sun 2/25/2007 9:44 PM To: Bruce Ravel; XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] E0 for bulk crystals and nanoparticles Hi Bruce, thank you very much for taking the time to write your helpful reply. As you mentioned, This really gets at the heart of why exafs analysis is difficult. You are certainly correct that the deltaE0 is highly correlated with delR and C3. That correlation is hard to avoid in a non-linear analysis as all three terms effect the phase of the calculated chi(k). That's exactly why I decided to post the question on the list asking for the advice of more experienced people who can certainly evaluate the question much better than myself. Also, I've always been trying to follow your "reasonable and defensible applied to all the parameters, not just e0" motto; another strong motivation for posting this question on the list. I don't know if it is just me, but in my short time working with EXAFS I got the strong impression that it is very easy to fall into the temptation of sophistry to get to the "expected" results from a fit. In this respect, I consider the mailing list an invaluable resource to help overcoming such a temptation. But coming back to the original question, would you say that analyzing a temperature-dependent set of the very same NC-containing sample (eight measurements between 10K and 300K) to get an E0 value would be a sound approach? Or should we expect E0 to vary with the temperature as well? Cheers, Leandro
Hi again, Hmm...Anatoly makes good points here. But I'd say that most of those kinds of things should give changes in XANES. If you don't see changes in XANES as you change the temperature, I'd still feel pretty comfortable constraining the E0 to be the same for all temperatures between 10 and 300 K. And that lets you decouple E0 from lattice expansion and the third cumulant, which was your goal. --Scott Calvin Sarah Lawrence College At 10:06 PM 2/25/2007, Anatoly wrote:
Leandro - depending on the details of your system and treatment, electronic structure of the nanoparticle may or may not change at elevated temperatures. It may change as a result of hydrogen chemisorption, charge exchange with support, ligand desorption, oxidation, etc. As a result, you may observe changes in XANES that may be due to the change in core hole screening or Fermi energy change or both (depending on the degree of "metallicity" of the nanoparticle).
There are many references in literature demonsrating each effect that I mentioned.
Thus, keeping E0 the same at all temperatures may or may not be wise, depending on the magnitude of the edge shift that can, in principle, be observed at different temperatures. If that happens, you may want to compare the resuls obtained by keeping E0 the same vs those when E0 was allowed to vary independently for each scan.
In short, there is no "one fits all" answer.
That's true - if there are no "shift-like" changes in XANES, E0 may be constrained. Anatoly ________________________________ From: ifeffit-bounces@millenia.cars.aps.anl.gov on behalf of Scott Calvin Sent: Sun 2/25/2007 10:20 PM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] E0 for bulk crystals and nanoparticles Hi again, Hmm...Anatoly makes good points here. But I'd say that most of those kinds of things should give changes in XANES. If you don't see changes in XANES as you change the temperature, I'd still feel pretty comfortable constraining the E0 to be the same for all temperatures between 10 and 300 K. And that lets you decouple E0 from lattice expansion and the third cumulant, which was your goal. --Scott Calvin Sarah Lawrence College At 10:06 PM 2/25/2007, Anatoly wrote:
Leandro - depending on the details of your system and treatment, electronic structure of the nanoparticle may or may not change at elevated temperatures. It may change as a result of hydrogen chemisorption, charge exchange with support, ligand desorption, oxidation, etc. As a result, you may observe changes in XANES that may be due to the change in core hole screening or Fermi energy change or both (depending on the degree of "metallicity" of the nanoparticle).
There are many references in literature demonsrating each effect that I mentioned.
Thus, keeping E0 the same at all temperatures may or may not be wise, depending on the magnitude of the edge shift that can, in principle, be observed at different temperatures. If that happens, you may want to compare the resuls obtained by keeping E0 the same vs those when E0 was allowed to vary independently for each scan.
In short, there is no "one fits all" answer.
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Anatoly and Scott, thanks for the quick and valuable answers! Sorry I didn't specify the system in more detail; this is a point that Matt always stresses when replying messages in this list (i.e., there is no "one fits all" answer, so a detailed description of the system is needed) and by now I should have assimilated it already... Anyway, I see no changes whatsoever in the XANES spectra of the T-dep measurements, so I think it will be worth the try. Since I'm dealing with Ge NCs buried about 1micron deep into a SiO2 matrix, I reckon effects like hydrogen chemisorption, charge exchange with support, ligand desorption and oxidation will not take place, specially at such low T's. Thanks once more for the help, Leandro
Hi Leandro, I would not expect E0 to vary with temperature between 10 K and 300K. Just to be sure nothing untoward happened, I would recommend taking a spectrum at 300 K at the end of the series as well as at the beginning. That's generally a good policy with a series like that just to make sure the sample didn't suffer an irreversible change from oxidation, condensation, beam damage, or whatever. --Scott Calvin Sarah Lawrence College At 09:44 PM 2/25/2007, you wrote:
But coming back to the original question, would you say that analyzing a temperature-dependent set of the very same NC-containing sample (eight measurements between 10K and 300K) to get an E0 value would be a sound approach? Or should we expect E0 to vary with the temperature as well?
participants (5)
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Bruce Ravel -
Frenkel, Anatoly -
Leandro Araujo -
Leandro Langie Araujo
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Scott Calvin