I've prepared gold Nps over gelatin and I've recorded two XPS Au4f doublets with for example BE 4f7/2 of 84.0eV (metal) and 83.2eV (?). Actually I do not expect interaction between gelatin and Au Nps after doing different other tests.
It is well known that BE shifts accompanied with changes in FWHM can be caused by size effects (cluster size effect). Negative shifts, as in your case, might be explained based on surface effect (the increase in the ratio of surface/bulk atoms in the nano sized particles). On the contrary to this, it should be mentioned that a number of studies reported positive shifts.
Perhaps, the Au4f peaks splitting at the low binding energy side may be associated to differential charge effects.
The component at 83.2 may be associated to Au atoms in a Au conducting clusters and the component at 84.0 eV may be associated with the presence of similar Au atoms on the more isolating surface (gelatin)
Thank you for the responses. Actually these assumptions have been already envisaged in the team. We are then pleased to get these confirmations. Thank you so much
The work of Wertheim et al is very clear on this aspect (PRL 51:2310, 1983). Band narowing as initial state effect cause negative shifts, and the unit positive charge created that remains on the cluster during photoemission cause positive shifts (final state effects). I`ve seen in some papers the term "differential charge effects", which I don´t understand very well
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