Core-level studies

As mentioned earlier, the existence of surface shifted core levels has been questioned.6 Calculated results for TiC(lOO) using the full potential linearized augmented plane wave method (FLAPW) predicted6 no surface core level shift in the C Is level but a surface shift of about +0.05 eV for the Tis levels. The absence of a shift in the C Is level was attributed to a similar electrostatic potential for the surface and bulk atoms in TiC. The same result was predicted for TiN because its ionicity is close to that of TiC. This cast doubts on earlier interpretations of the surface states observed on the (100) surface of TiN and ZrN which were thought to be Tamm states (see references given in Reference 4), i.e. states pulled out of the bulk band by a shift in the surface layer potential. High resolution core level studies could possibly resolve this issue, since the presence of surface shifted C Is and N Is levels could imply an overall electrostatic shift in the surface potential, as suggested for the formation of the surface states. 

Demonstration of surface shifts in the nonmetal levels, i.e. in the C Is and N Is levels, was not feasible in these first efforts. These levels are located considerably deeper in energy and were inaccessible. Recent developments in synchroton radiation instrumentation,15 however, allow high resolution core level studies to be performed involving these levels.16-20 The first such study was carried out on the (100) and (111) surfaces of VC16 and a surface shifted C Is level was clearly observed (Figure 25.2). A C Is... 

Results from Core-Level Studies of Phthalimide-on-Copper. 

Tery L. Barr, Mengping Yin and Shikha Varma, Detailed x-ray photoelectron spectroscopy valence band and core level studied of select metals oxidations, J. Vac. Set. Technol. A, 10, 2383-2390 (1992). 

XPS core-level studies of the interaction between PHeTH and some vapour-deposited transition metals suggest that both Cu and Ag react exclusively with S atoms with no C atom participation, while Au does not appear to react at all [280], The interfacial CT results in the appearance of a low-BE S2p component which is shifted by about —2.2 eV from the main peak. The reactivity of the coinage metals with PHeTH was found... 

Hakansson, K. L., Johansson, H. I. R, Johansson, L. I. (1993). High-resolution core-level study of ZrC( 100) and its reaction with oxygen. Physical Review B Condensed Matter and Materials Physics, 48, 2623. doi 10.1103/PhysRevB.48.2623 PMID 10008658. 

The ability of XPD and AED to measure the short-range order of materials on a very short time scale opens the door for surface order—disorder transition studies, such as the surface solid-to- liquid transition temperature, as has already been done for Pb and Ge. In the caseofbulkGe, a melting temperature of 1210 K was found. While monitoring core-level XPD photoelectron azimuthal scans as a function of increasing temperature, the surface was found to show an order—disorder temperature 160° below that of the bulk. 

As stated earlier, the major use of UPS is not for materials analysis purposes but for electronic structure studies. There are analysis capabilities, however. We will consider these in two parts those involving the electron valence energy levels and those involving low-lying core levels accessible to UPS photon energies (including synchrotron sources). Then we will answer the question why use UPS if XPS is available ... 

As was mentioned previously, photoemission has proved to be a valuable tool for measurement of the electronic structure of metal cluster particles. The information measured includes mapping the cluster DOS, ionization threshold, core-level positions, and adsorbate structure. These studies have been directed mainly toward elucidation of the convergence of these electronic properties towards their bulk analogues. Although we will explore several studies in detail, we can say that studies from different laboratories support the view that particles of 150 atoms or more are required to attain nearly bulk-like photoemission properties of transition and noble metal clusters. This result is probably one of the most firmly established findings in the area of small particles. 

Further studies were carried out on the Pd/Mo(l 1 0), Pd/Ru(0001), and Cu/Mo(l 10) systems. The shifts in core-level binding energies indicate that adatoms in a monolayer of Cu or Pd are electronically perturbed with respect to surface atoms of Cu(lOO) or Pd(lOO). By comparing these results with those previously presented in the literature for adlayers of Pd or Cu, a simple theory is developed that explains the nature of electron donor-electron acceptor interactions in metal overlayer formation of surface metal-metal bonds leads to a gain in electrons by the element initially having the larger fraction of empty states in its valence band. This behavior indicates that the electro-negativities of the surface atoms are substantially different from those of the bulk [65]. 

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