Interface Core-Level Shift

The XPS BE shift is uniquely proportional to the crystal potential at equifibrium, i.e., cohesive energy per bond. Any perturbation to the crystal potential by the quantum entrapment or polarization at the interface will lead to the core-level shift  

---

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. 

The interaction of a Pd4 (C4v) cluster with the oxide surface was analyzed in more detail with the help of electron density difference plots and other theoretical tools, such as population analysis, core level shifts as well as induced and dynamic dipole moments [175]. Three interaction mechanisms were found to contribute to different extent metal polarization with the subsequent electrostatic attraction, Pauli repulsion, and covalent orbital interactions. Electrostatic interactions make up a sizeable fraction of the adhesion energy the polarization of the metal adsorbate by the surface electric field provides an important bonding mechanism. For the adsorption of Pd on-top or in the vicinity of the surface Mg " cations this electrostatic interaction accounts for almost the entire adsorption energy, albeit counteracted by Pauli repulsion. For adsorption on-top 0 , on the other hand, mixing of adsorbate and substrate orbitals becomes noticeable. This hybridization or covalent bonding at the interface with the oxide anions is complemented by electrostatic polarization. Further work is required to establish in a more quantitative way the relative importance of electrostatic and chemical bonding contributions. However, in line with our other studies of... 

Patrick CE, Giustino F (2011) O Is core-level shifts at the anatase Ti02 (101)/N3 photovoltaic interface signature of H-bonded supramolecular assembly. Phys Rev B 84 085330... 

A theoretical study of Si 2p core-level shifts at the Si(001)/SiO2 interface serves as a good example of current capabilities. Experimentally, it is very difficult to obtain... 

The interfacial potential V z) shifts the core levels of the atoms located close to the interface, as well as their outer orbitals. Yet, most analyses of core-level shifts neglect the band-bending effects and take the vacuum level as the reference energy (Quiu et al, 1987). Such an assumption is justified only for highly ionic substrates. 

W. F. Egelhoff Jr., Thermochemical Values for Cu-Ni Surface and Interface Segregation Deduced from Core-Level Binding Energy Shifts, Phys. Rev. Lett. 50 587 (1983)... 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

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. 

In order to monitor the progress of interfacial reactions occurring during the metallization of cured polyimide, x-ray photoemission spectroscopy (XPS or ESCA) was used to reveal electronic core-levels indicative of the environment at the interface and adjacent regions. Evidence of chemical reaction would include the appearance of new peaks with characteristic binding energies (chemical shifts) representative of new or altered chemical states of the element. We can thus ascertain the formation of metal-oxygen chelate complexes (1). 

We use an ab-initio local spin density method to investigate the aluminum/polyimide interface at low coverage. We found in agreement with XPS and EELS experiments, that the aluminum atom bonds to the carbonyl group. Our calculations suggest a formation of a linear C-O-AI complex. We calculated core levels chemical shifts and vibrational frequencies in the vicinity of the carbonyl group. 

---