Training Activity: oxygen on Ir(111)

In the last decades, transition metal surfaces have been the object of an intense research activity, motivated by the perspective of their application in scientifically and technologically relevant fields, owing to their  outstanding morphological, catalytic and  magnetic  properties.
The paper reports on the results of an experimental study of the clean and oxygen-covered Ir(111) surface, in which the electronic structure of the substrate was investigated by means of  high energy resolution  core level photoemission spectroscopy (HR XPS), in combination  with state of the art DFT calculations.
 
From the analysis  of the Ir 4f7/2 core level spectra, some relevant information was derived on the electronic properties of the sample and on the oxygen-induced modifications of the electronic structure of bulk and surface atoms.
The analysis of the spectra of the clean surface revealed the presence of two core level components, separated by 550 meV,  which were attributed to photoemission from bulk and surface atoms. The observed larger  Gaussian contribution to the line shape of the bulk peak  is most likely due to the presence of unresolved  second and deeper layer components.
The temperature evolution of the Ir 4f7/2 core level was subsequently monitored in a series of time-resolved experiments: the spectra clearly show  a different Gaussian broadening of the two components at increasing T, along with a  non equivalent shift of both peaks to lower Binding Energies, which can be explained in light of the distinct thermal lattice expansion of bulk and surface atoms.
The application of the harmonic model for phonon broadening proposed by Hedin and Rosengren to the data analysis allowed to derive an estimate of the Debye temperature of bulk and surface Ir(111) (298±8 K and 181±3 K, respectively).
After the preliminary characterization of the clean Ir(111) surface, the interaction of oxygen with this substrate was investigated by monitoring the evolution of the Ir 4f7/2 core level upon oxygen exposure (see the sequence of spectra  on the right). The analysis of the spectra allowed to shed light on the adsorbate-induced changes of the local configuration of first layer atoms. Oxygen adsorption induces a modification of  the electronic structure of the sample, which is reflected in a BE shift of the core level components, and in the appearance of new spectral contributions. The CLSs of the new surface features were shown to obey the additivity rule for adsorption-induced CLSs, according to which the CLS of first layer components scales linearly with the number of O-Ir bonds.

The preferential adsorption site of oxygen on Ir(111) is the threefold hcp site, in which an oxygen atom is coordinated with three Ir atoms. The non-equivalent populations of surface Ir atoms bonded  to a different number of adsorbates are highlighted with different colours in the model cell on the right. The same colour key has been used to plot the intensities of the Ir 4f7/2 components as a function of the oxygencoverage. As can be clearly seen from the plot, the initial stage of the experiment is dominated by the intensity drop of the component due to zero-coordinated atoms, accompanied by the simultaneous growth of the new oxygen-induced features, originated by singly and doubly coordinated Ir atoms. The knowledge of the adsorption site was used to estimate the oxygen coverage at saturation, which turned out to be 0.38±0.04 ML.  

The students authors of the New Journal of Physics paper at the SuperESCA beamline.

Form bottom to top, left to right:
Enrico Golfetto, Riccardo Comin, Marco Bianchi, Alberto Cavallin, Luca Postregna, Fabrizio Orlando, Damiano Cassese together with Alessandro Baraldi.


Clean and oxygen induced surface core level shift on Ir(111);
M. Bianchi, D. Cassese, A. Cavallin, R. Comin, F. Orlando, L. Postregna, E. Golfetto, S. Lizzit and A. Baraldi;
New J. Phys. 11, 063002 (2009).

 


Last Updated on Sunday, 03 June 2012 17:14