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44158-AC5
The Structure of Dynamics of Wet Electrons at Metal Oxide Surfaces

Hrvoje Petek, University of Pittsburgh

Research was performed on the unoccupied electronic states of atoms and molecules on metal surfaces.  The unoccupied electronic structure was investigated by experiment and theory.  The PRF grant primarily supported the theoretical part of the research by Dr. Jin Zhao.

 The GW band structure of TiO₂.  In the 2007 report, we discussed the electronic structure of O atom vacancy defects on TiO₂ surfaces.  We extended these studies to the OH impurities, which also reduce the surfaces.  Both experiments show a delocalized structure of excess electrons that are introduced by the surface reduction.  Our theoretical calculations of the geometrical and electronic structure of reduced surfaces using the GGA method with the PBE functional is in excellent agreement with the experimental structures.  Different theoretical approaches that account for self-interaction through inclusion of the Hartree-Fock exchange and correlation, e.g. the B3LYP functional, give localized excess electron distributions and asymmetric distortion of TiO₂ lattice that contradict our experiments.  Because a significant fraction of the community believes hybrid functionals are superior for describing the electronic structure of metal oxides, we have had difficulty publishing our results.  In order to remedy the existing misconceptions on how to calculate the electronic structure of metal oxides, together with Prof. Angel Rubio, we performed a GW calculation of the electronic structure of TiO₂.  With a perturbation theoretical approach to self-interaction and accounting for the excitonic effects by solving the Bethe-Saltpeter equation, we could reproduce the optical band gap of TiO₂ better than is possible by either GGA or B3LYP approaches.  In the future, we will extend these calculations to the structure of reduced TiO₂ surfaces.  Developing a theoretical approach to treating defects in metal oxides, which do not rely on the choice of the functional, will represent a significant advance in our understanding of the electronic structure of these important materials.  Publications for the Journal of Chemical Physics and Physical Review B are in preparation.

The m=1 states of alkali atoms on metals.  We extended our studies on the electronic structure of alkali atoms on noble metal surfaces by exploring the unoccupied states derived from the p_x and p_y orbitals, which have p (m=1) symmetry with respect to the surface normal.  We observed such states for Cs and K on Ag(111) and Cu(111) surfaces.  With a wave packet propagation method, we determined the energies of m=1 states with respect to the previously known m=0 states that are derived from the valence s orbital.  The experimental discovery and theoretical interpretation of the m=1 states significantly adds to our knowledge of how metal surface modifies modify the electronic structure of simple, one-electron, adsorbates.  This research is under review at the Physical Review Letters.

Superatom states of hollow molecules.  We discovered a new kind of electronic state that is particular to hollow molecules such as fullerenes and nanotubes.  By spectroscopic imaging of single and variously aggregated C₆₀ molecules on copper surfaces, we mapped out the local density of states (LDOS) at different bias energies.  In previous STM studies, the LDOS of the LUMO states with the p orbital character have been extensively studied.  We discovered that above LUMO+2, the LDOS of the p orbitals on the C atom framework can no longer be resolved.  Rather, the images show much simper LDOS maps that appear as the s, p, and d orbitals of atoms (Fig. 1). In plane wave DFT calculations on single C₆₀ molecules, Dr. Jin Zhao found that ~3.5 eV above LUMO there exist orbitals, which are no longer bound to the individual C atoms, but rather appear as s, p, and d orbitals of atoms centered on the hollow molecular core.  Because of their appearance and properties, we have dubbed them the superatom molecular orbitals (SAMOs).  By experiment and theory we have shown that SAMOs of two adjacent C₆₀ molecules on Cu surfaces hybridize like the atomic orbitals of H atoms when they form an H₂ molecule.  Even more intriguing, 1D chains and 2D islands of C₆₀ molecules form bands with nearly free-electron like delocalization, such as found s-electron metals (e.g. alkali metals).  By analyzing the theoretical potential of a C₆₀ molecule, we concluded that SAMOs exist because of a weak central potential within the C₆₀ molecular core.  The origin of this potential is the screening of an external charge by the C atom sheet, which gives rise to a short-range exchange correlation (correlation hole) potential within the hollow core.  Because screening is a universal property, we expect that such states exist in all hollow materials. 

This research was published in Science, and will be the subject of a future article in the Accounts in Chemical Research.

Figure 1.  The dI/dV images of an isolated C₆₀ molecule on copper surface showing the LDOS of LUMO-2 and s-, p-, and d-SAMO states. 

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