Question

Please use the data in this table as needed to answer the questions below Binding Energy (eV) Elementoe el| Scofield Photoemission Cross-section Cts 285 1.00 400 1.80 533 2.93 Fts 687 4.43 S2 230 1.43 Spl S2p82 165 0.57 164 1.11 Cup12 Cu32 952 8.66 932 16.73 1. (10%) Give two reasons why XPS is typically performed under ultrahigh vacuum. 2. (10%) Explain why XPS is selective to the top few nanometers of a surface. . (106) Write the equation in XPS that relates binding energy to photoelectron energy. 4. (10%) Why are monochromatic X-ray sources prefered for XPS analysis of organic, polymeric, and biologically derived samples? SP32 Cu Paz (SP (c) 5. The Al-Kamonochromatic X-ray photoelectron core level spectra of Cu,S, sample shown at right is from Y. Wang, et al., CrystEngComm 14 (2012) 7560. Use these spectra to answer these four questions Cu Pu2 (a) (159%) Write out the equation used to calculate the stoichiometry of Cu,S Define all variables and insert values that you know. (b) (15%) Explain the physical phenomenon that causes the baseline to rise in both spectra on the higher binding energy side of a peak (e) (1596) Describe and explain two characteristics of the Cu 2p XP spectrum (at left) that would appear different if you recorded it with a Mg-Ka achromatic X-ray source 960 950 940 930 166164162160158 Binding Energy (eV) (d) (159%) Assume that the Cu,S, spectra above are collected from a solid film with a large carbon and oxygen surface contamination. You have also prepared oleic acid terminated Cu,S, nanopartic les, which you then cast as a film and analyze by XPS. What type(s) of XPS measurements would you perform to distinguish these two different types of samples? Do not discuss any experiments besides XPS.

Answer 1

Solution1)

x-ray photo electron spectroscopy is performed in ultrahigh vacuum as Ultra high vacuum ensures that there would be almost negligible amount of gas molecule are present which give us

i) correct energy of the photo electron (ii) To enhance the intensity of the photo-electrons at the detector

Solution2)

As X-ray can penetrate more than 10nm in the sample but the photo-electrons which come from the depth larger than 10nm can collide with the other electrons and will go back .

Solution3)

Binding energy of the photo-electron come in x-ray photo electron spectroscopy is $E_B = h\nu - E_k - \phi$

where $E_B, E_k$ are binding and kinetic energy and $h\nu, \phi$ are energy of x-rays and work-function of the material respectively.

Solution4)

If Monochromatic source is not used, then it may happen that photo-electrons belongs to different element present in the sample get out of the samples using different energies of the beam and lead to the same kinetic energy which would create confusion about the type of element from which they are coming.