We need to differentiate two things: a) How one removes any offsets experimentally with an insulator. Often cleaning the sample is not an option -- this may also destroy it. b) What reference level one should use in a Wien calculation: the T=0 Fermi energy, the T=Room Temperature Fermi Energy (TEMP/TEMPS), a core level well away from the target atom (for a supercell), the vacuum level (if available)?
One has to be careful of possible artifacts due to a change in E_F in the cell because there is not a large enough bulk reservoire to set it correctly, artifacts in the total energy due to charged cell issues (which I think the Slater method avoids), artifacts due to interactions between core-holes (or half holes) in adjacent supercells, plus probably others I've not thought of yet. On Thu, Sep 18, 2008 at 8:17 AM, Pierre-Emmanuel Lippens <lippens at univ-montp2.fr> wrote: > At 11:19 18/09/2008, you wrote: >>Realy not a good idea, >> if you have C on the surface, what C is it ? CO, hydrocarbon, >> graphite ? All have a different binding energy of the C 1s ! >>I suggest, if youre sample is not to be supposed to contain C, you >>should clean it. > > > This is not my own choice (I am not XPS experimentalist) but what I > have read in some XPS papers. > > The main point: experimental calibration of the binding energy scale > requires a reference energy. > > C1s is surely not the most accurate choice. From discussions with XPS > users: this peak is used because, if you don't clean your C free > sample, you have C that may originate from contamination in the > analysis chamber (vacuum pump oil) and this C is not expected to form > chemical bonds with the sample. > > But as you said in your email you can have other C on the surface and > the reference is not expected to be very accurate. > > >>XPS energies have to be calibrated using the Au (or Ag) Fermi energy !!!!! > > A better choice than C1s but unfortunately not used by all the XPS > experimentalists... > > > >>and the other question: And for an insulator when the extra 1/2 (or >>1) electron goes into the >>conduction band and E_F changes....? >>An insulater will most probably charge if you do XPS, and the >>energies do not tell you much. > > Can we consider that charge mainly shifts the energy scale? What is > the effect on the binding energy differences for a given sample? > > Pierre > > > > _______________________________________________ > Wien mailing list > Wien at zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > -- Laurence Marks Department of Materials Science and Engineering MSE Rm 2036 Cook Hall 2220 N Campus Drive Northwestern University Evanston, IL 60208, USA Tel: (847) 491-3996 Fax: (847) 491-7820 email: L-marks at northwestern dot edu Web: www.numis.northwestern.edu Chair, Commission on Electron Crystallography of IUCR www.numis.northwestern.edu/IUCR_CED