Dear all,
In specialized literature, it is usually reported that asymmetry due electron-hole pair creation becomes more significant for transition metals when the cluster size decreases. However, there are also numerous examples in the literature where symmetric line shapes (G/L) have been used to fit the M(0) component of metal nanoparticles, especially when the spectral resolution is low.
On the other hand, for metal carbides, I found people tend to use asymmetric line shapes when the crystallite size of the carbide becomes larger than 2-3 nm.
In summary:
1) I was wondering how critical it is to consider the asymmetry in transition metal NPs when acquiring low-resolution XPS spectra. Is the asymmetry affected by the pass energy and the fact of having low metal loadings?
2) Is there a reliable method for predicting the degree of asymmetry, apart from using standards that replicate a transition metal nanoparticle in a complete zero-valent state in an analogous environment?
3) Are there truly general rules for predicting how asymmetry changes with cluster size in metal and metal carbides particles?
Hello Jaime,
The asymmetry in the M(0) originates from the possibility of a additional energy loss due to inelastic scattering. As metallic samples have no bandgap the shake-up process manifests as a continuous asymmetric shoulder rather than a defined shake-up peak seen in i.e. oxides.
The degree of this varies and depend on the details of a materials band structure. Particularly for small clusters they can change significantly, typically acting less metallic with hence less asymmetry.
Regarding 1) low resolution spectra taken at larger pass energies will show less of an asymmetry as the instrumental broadening, which is Gaussian will dominate (all sources of broadening add up in square sums).
Regarding 2) Reliable prediction would require detailed band structure calculations and are not easily done. For example see: Article Predicting X-ray Photoelectron Peak Shapes: the Effect of El...
Regarding 3) Just from very generic size confinement rules I'd suspect the smaller the cluster the less asymmetry. from my own experience even in thin metal films this can be seen with thin films showing less than i.e. single crystal bulk metals.
Hope that helps
Karsten
Dear Karsten,
Many thanks for your elaborate reply.
From your response and some papers I've been reading these days:Article X-ray photoemission of small platinum and palladium clusters
,Article XPS insights: Asymmetric peak shapes in XPS
,Article Core-electron binding energies in free and supported metal clusters
I understand that i) as cluster size decreases, the line-shape asymmetry reduces, and the line-width broadens. Nonetheless, for clusters showing broad size distributions, there could also be a line asymmetry, ii) Recording the valence band spectra for pure metals may be a good indication of whether one should expect a significant asymmetry (but not for accurate asymmetry prediction). iii) Low-resolution spectra at large pass energies (or in samples with very low metal loadings) will show less of an asymmetry.
In conclusion, one can guess if the asymmetry is expected to be significant or not, but for the exact degree, it will always be safer to reproduce as close as possible your sample problem in a pure zerovalent state for the metal.
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