Surface atom core-level shift

The model of Citrin and Wertheim [65], developed to explain the surface atom core level shifts in the XPS spectra of metallic systems, has already been used to explain the extremely low value of the I.S. of the central 13 atoms in the AU55 cluster [44]. 

In order to understand this effective loss of 6s electron density by the core site atoms, use has been made [44] of the model introduced by Citrin and Wertheim [65] to explain the negative surface atom core level shifts, relative to the bulk, observed in the XPS spectra of metals surfaces [127]. Their model used... 

In a recent investigation31 of TaC(lll) the surface shifted Ta 4f levels were investigated before and after depositing a monolayer of graphite on the surface. A dense carbon overlayer on the polar TaC(lll) surface might be expected to quench the surface Ta core level and to leave only a bulklike core level. The spectrum recorded after deposition of a graphite monolayer, however, showed an attenuated and shifted surface Ta core level, shifted about 0.3 eV closer to the bulk peak. It thus appears that not even a dense graphite monolayer is sufficient to produce a bulk-like electronic environment for the surface Ta atoms on TaC(lll). 

The same sort of treatment as for ordinary metallic bulk matter can also be applied to surface core-level shifts. The surface atoms experience a different potential compared to the layers below because of the lower coordination number. This results in somewhat different core level binding energies. One can extend the previous Born-Haber cycle model to account for the surface-bulk core level shift. Empirically, the surface cohesive energy is approximately 80% of the bulk value. The impurity term can then be written as... 

Note that in core-level photoelectron spectroscopy, it is often found that the surface atoms have a different binding energy than the bulk atoms. These are called surface core-level shifts (SCLS), and should not be confiised with intrinsic surface states. Au SCLS is observed because the atom is in a chemically different enviromuent than the bulk atoms, but the core-level state that is being monitored is one that is present in all of the atoms in the material. A surface state, on the other hand, exists only at the particular surface. 

X-ray photoelectron spectroscopy also provides information on the chemical composition of a surface. An incoming photon causes electrons to be emitted from atomic core levels, which are then analyzed as a function of kinetic energy. The shifts of these core-level energies provide information about the chemical environment surrounding the excited atom. This information also includes changes in the oxidation state of the sample. 

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. 

The surface core level shift is defined as the shift in the core level binding energy for a surface atom relative to that of a bulk atom. Different theoretical approaches have been used to calculate surface shifts5,9 and for metals it has recently been shown21 that both initial- and final-state effects have to be included in ab initio calculations to obtain consistent agreement between experimental and calculated results. The basic assumption in this theoretical approach is that the final state is completely screened so the... 

This difference is taken for a compound in which the atoms of the element investigated (a Z-element) are changed to atoms of a Z + 1 element. The surface core level shift for the metal (M) and nonmetal (Y) level can then be expressed as,... 

X-ray photoelectron spectroscopy of atomic core levels (XPS or ESCA) is a very powerful tool for characterization of the chemical surrounding of atoms in molecules. In particular, since the method is very surface sensitive, it is possible to monitor the first stages of the interface formation, i.e., in our case the interaction between individual metal atoms and the polymer. Standard core level bonding energies are well known for common materials. However, in our case, we are studying new combinations of atoms and new types of structures for which there are no reference data available. In order to interpret the experimental chemical shifts it is useful to compare with theoretical estimates of the shifts. 

Even though photoelectron spectroscopy has not yet shown evidence for core level shifts corresponding to differences between atoms of a clean surface and atoms in the interior of a solid8), both single particle and collective excitations specific to valence electrons of surface atoms have been observed. 

Ultra-violet photoemission spectroscopy (UPS) probes the density of states, and ion neutralization spectroscopy (INS) and surface Penning ionization (SPI) provide similar information with probes of ions and metastable atoms, respectively. Angle-resolved UPS can determine the valence band structure. X-ray Photoelectron Spectroscopy (XPS) provides information on chemical shifts of the atomic core levels, and this can also help in understanding chemical bonding at the surface. 

Since most "surface-sensitive" techniques sample at least a few atomic planes into the sample, it is difficult to experimentally separate the electronic structure of the outermost plane of atoms from that of the planes below. Theoretical calculations are able to clearly separate surface from bulk electronic structure, of course it is common to calculate a separate electronic density-of-states for each plane in the crystal structure ("layer density-of-states"). Significant changes from the bulk electronic structure are sometimes found for the surface planes in calculations. However, it is difficult to confirm those results experimentally [1]. In some oxides, the bandgap at the surface has been observed to narrow compared to that of the bulk. The measured core-level binding energies of partially coordinated surface atoms are often shifted, by as much as an eV, from their bulk values [32] these are referred to as "surface core-level shifts". However, the experimental separation of surface from bulk electronic structure is at present far from satisfactory. 

Similar effects can also occur in surface electronic structure when a moiety is weakly physisorbed onto the surface. The surface core-level shifts measured at the vacuum interface are reduced when atoms or molecules are physisorbed onto the surface. Changes may also occur in the valence electronic structure upon physisorption, such as the disappearance of intrinsic surface states on metals and semiconductors. 

In most cases the experimental techniques used to study surface phenomena do not seem to yield consistent values for the surface segregation energies. One important exception is the special case of an atom of atomic number Z+1 in a host of atoms of atomic number Z, where the surface segregation energy may in fact be extracted with a high degree of accuracy from X-ray photoemission spectroscopy (XPS) measurements of surface core-level shifts (SCLS) [39]. In contrast they may be calculated quite accurately by modern first-principles methods [18,25,40]. 

Fig. 26. Si 2p core level shifts related to oxide formation (a) a clean silicon surface (b) adsorption of atomic oxygen (c) first indication of Si02 formation following exposure to excited oxygen (d) continued exposure of (c) to non-excited 02 (after Garner et al. [232]).

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